U.S. patent application number 13/337343 was filed with the patent office on 2012-05-03 for methods of making a niobium metal oxide and oxygen reduced niobium oxides.
This patent application is currently assigned to CABOT CORPORATION. Invention is credited to Heather L. Enman, Jeffrey A. Kerchner, Ricky W. Kitchell, Stephen J. Krause, David M. Reed, Dorran L. Schultz, Sridhar Venigalla.
Application Number | 20120107224 13/337343 |
Document ID | / |
Family ID | 33479866 |
Filed Date | 2012-05-03 |
United States Patent
Application |
20120107224 |
Kind Code |
A1 |
Reed; David M. ; et
al. |
May 3, 2012 |
Methods Of Making A Niobium Metal Oxide and Oxygen Reduced Niobium
Oxides
Abstract
Methods to at least partially reduce a niobium oxide are
described wherein the process includes mixing the niobium oxide and
niobium powder to form a powder mixture that is then heat treated
to form heat treated particles which then undergo reacting in an
atmosphere which permits the transfer of oxygen atoms from the
niobium oxide to the niobium powder, and at a temperature and for a
time sufficient to form an oxygen reduced niobium oxide. Oxygen
reduced niobium oxides having high porosity are also described as
well as capacitors containing anodes made from the oxygen reduced
niobium oxides.
Inventors: |
Reed; David M.;
(Douglassville, PA) ; Venigalla; Sridhar;
(Macungie, PA) ; Kitchell; Ricky W.;
(Douglassville, PA) ; Krause; Stephen J.;
(Phoenixville, PA) ; Enman; Heather L.;
(Bethlehem, PA) ; Schultz; Dorran L.;
(Perkiomenville, PA) ; Kerchner; Jeffrey A.;
(Fleetwood, PA) |
Assignee: |
CABOT CORPORATION
Boston
MA
|
Family ID: |
33479866 |
Appl. No.: |
13/337343 |
Filed: |
December 27, 2011 |
Related U.S. Patent Documents
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Application
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Patent Number |
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12419342 |
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8110172 |
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13337343 |
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10848970 |
May 19, 2004 |
7515397 |
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12419342 |
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60471649 |
May 19, 2003 |
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60471650 |
May 19, 2003 |
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60533931 |
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60534461 |
Jan 6, 2004 |
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60535603 |
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60568967 |
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Current U.S.
Class: |
423/594.17 ;
264/101; 264/109; 264/118; 264/43 |
Current CPC
Class: |
C01G 33/00 20130101;
C01P 2006/40 20130101; C01P 2004/03 20130101; C01P 2006/12
20130101; C01P 2006/14 20130101; C01P 2004/51 20130101; C01P
2004/62 20130101; Y10T 428/2982 20150115; C01P 2004/61 20130101;
C01P 2006/17 20130101 |
Class at
Publication: |
423/594.17 ;
264/43; 264/101; 264/118; 264/109 |
International
Class: |
C01G 33/00 20060101
C01G033/00; B29B 9/16 20060101 B29B009/16; B29B 9/08 20060101
B29B009/08; B29C 67/04 20060101 B29C067/04 |
Claims
1-66. (canceled)
67. A method to control porosity in pressed and sintered valve
metal sub-oxide powders comprising adjusting the granule size
and/or pre-heat treatment temperature of said valve metal sub-oxide
to obtain a pre-determined porosity.
68. A method of making valve metal oxide particles, comprising:
heat treating a starting valve metal oxide under vacuum or inert
conditions to form agglomerated particles; and optionally
deagglomerating said agglomerated particles.
69-91. (canceled)
92. The valve metal oxide particles formed by the method of claim
68.
93. A capacitor comprising the valve metal oxide particles of claim
92.
94. The capacitor of claim 93, wherein said capacitor has a
capacitance of from about 40,000 to about 300,000 CV/g.
95. The capacitor of claim 94, wherein said capacitor has a DC
leakage of from about 0.05 to about 5 nA/CV.
96. The method of claim 68, further comprising at least partially
reducing said valve metal oxide particles to form an oxygen reduced
valve metal oxide.
97. The oxygen reduced valve metal formed by the method of claim
96.
98. A capacitor comprising the oxygen reduced valve metal of claim
97.
99. The capacitor of claim 98, wherein said capacitor has a
capacitance of from about 40,000 to about 300,000 CV/g.
100. The capacitor of claim 98, wherein said capacitor has a DC
leakage of from about 0.05 to about 5 nA/CV.
101. (canceled)
102. A method of making agglomerated particles, comprising heat
treating a starting valve metal oxide to form agglomerated
particles, wherein said agglomerated particles have a pore size
distribution that is at least 10% greater than a pore size
distribution of said starting valve metal oxide, and wherein said
agglomerated particles have a BET surface area that is at least 90%
of a BET surface area of said starting valve metal oxide.
103-116. (canceled)
117. The agglomerated particles formed by the method of claim
102.
118. A capacitor comprising the agglomerated particles of claim
117.
119. The capacitor of claim 118, wherein said capacitor has a
capacitance of from about 40,000 to about 300,000 CV/g.
120. The capacitor of claim 118, wherein said capacitor has a DC
leakage of from about 0.05 to about 5 nA/CV.
121. The method of claim 102, further comprising at least partially
reducing said agglomerated particles to form an oxygen reduced
valve metal oxide.
122. The oxygen reduced valve metal formed by the method of claim
121.
123. A capacitor comprising the oxygen reduced valve metal of claim
122.
124. The capacitor of claim 123, wherein said capacitor has a
capacitance of from about 40,000 to about 300,000 CV/g.
125. The capacitor of claim 123, wherein said capacitor has a DC
leakage of from about 0.05 to about 5 nA/CV.
126-226. (canceled)
227. Oxygen reduced niobium oxide granules, wherein the oxygen
reduced niobium oxide granules have a multi-modal pore size
distribution of from about 0.1 to about 20 microns, after being
pressed and sintered, and are formed from oxygen reduced niobium
oxide having a BET surface area of from about 0.5 to about 8
m.sup.2/g, wherein said granules after being pressed and sintered
have a bimodal pore size distribution of from about 0.1 to about 10
microns.
Description
[0001] This application claims the benefit under 35 U.S.C.
.sctn.119(e) of prior U.S. Provisional Patent Application No.
60/471,649 filed May 19, 2003, U.S. Provisional Patent Application
No. 60/471,650 filed May 19, 2003, U.S. Provisional Patent
Application No. 60/533,931 filed Jan. 2, 2004, U.S. Provisional
Patent Application No. 60/534,461 filed Jan. 6, 2004, U.S.
Provisional Patent Application No. 60/535,603 filed Jan. 9, 2004,
and U.S. Provisional Patent Application No. 60/568,967 filed May 7,
2004, which are incorporated in their entireties by reference
herein.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to niobium and other valve
metals and oxides thereof and more particularly to niobium oxides
and methods to at least partially reduce niobium oxide and further
to oxygen reduced niobium oxides and other valve metal oxides. The
present invention relates to niobium oxides and other valve metal
oxides useful, for instance, in the production of capacitors,
sintered anode bodies, and the like. The present invention also
relates to methods of making niobium oxide particles and other
valve metal oxide particles. The present invention also relates to
niobium, niobium powder, hydrided forms thereof, and electrolytic
capacitors made therefrom. More particularly, the present invention
relates to methods of preparing niobium or hydrided niobium
feedstock to form suitable niobium powder or hydrided niobium for a
variety of uses.
[0003] Efforts are continually being made to improve the handling
of metal powder, such as niobium powder. In particular, fine
powders, for instance, having particle sizes of 0.1-200 microns,
can prove difficult to work with and thus, methods to agglomerate
or granulate fine metal powder have been developed. In addition to
developing methods to agglomerate fine metal powders, efforts have
also been made to agglomerate such powders in such a manner that
flow properties and/or other desirable properties such as
electrical characteristics are maintained or improved.
[0004] Development of metal powders suitable for making capacitors
has resulted from efforts by both capacitor producers and metal
powder processors to delineate the characteristics required for
metal powder to best serve in the production of quality capacitors.
Such characteristics include surface area, purity, shrinkage,
pressability, green strength, flowability, and stability.
[0005] Solid state electrolytic capacitors, and valve metal
capacitors in particular, have been a major contributor to the
miniaturization of electronic circuits. In addition to the reduced
size and higher frequencies of current electronic equipment and
electronic circuits, a growing demand exists for capacitors
offering higher capacitance and lower equivalent series resistance
(ESR). Valve metal capacitor anodes typically are manufactured by
compressing valve metal powder to less than half of the metal's
true density with an anode lead wire to form a pellet, sintering
the pellet to form a porous body (i.e., an anode), and then
anodizing the porous body by impregnation with a suitable
electrolyte to form a continuous dielectric oxide film on the
porous body. The anodized porous body is then impregnated with a
cathode material to form a uniform cathode coating, connected to a
cathode lead wire by soldering, for instance, and encapsulated with
a resin casing. Thus, open, uniform pores are important for the
steps of anodizing and impregnating the pellet to form the
cathode.
[0006] Valve metal oxides that have been oxygen reduced, in other
words, valve metal suboxides such as niobium monoxide (NbO), TaO,
and the like, have been recently recognized as a viable solid
electrolytic capacitor material that offers unique performance
advantages such as high dielectric stability, low leakage current,
and low flammability. These advantages, combined with lower costs,
make valve metal suboxides an attractive and economical alternative
to valve metal as an electrolytic capacitor material. Conventional
methods of preparing niobium suboxide include converting a mixture
of stoichiometric proportions of niobium pentoxide
(Nb.sub.2O.sub.5) powder and niobium metal powder (acting as an
oxygen getter) into NbO via heat treatment under vacuum. However,
these methods have the possibility that some incomplete reactions
may occur that yield a sub-optimal material such as NbO.sub.2 that
is interspersed within and difficult to separate from the NbO.
Thus, it is advantageous to eliminate the chances of any
semi-conducting material being present in the powder material used
to form the anodized porous body.
[0007] It is hypothesized that the ESR of a capacitor anode is
related to the cohesiveness of the primary particles used in
formation of the capacitor. The cohesiveness of the primary
particles can be related to the quantity and quality of connections
between primary particles achieved in agglomerating the primary
particles to form agglomerates (agglomerates can be described as
clusters of smaller primary particles). Thus, the ESR of a
capacitor anode can be decreased via the coarsening that results
from thermal agglomeration of the primary powder particles used in
forming the anodized porous body. However, coarsening of the
particles tends to be accompanied by densification of the particles
which reduces the surface area (reduced capacitor capacitance).
Thus, the production of agglomerated particles having a large
surface area, suitable cohesive strength, and uniform porosity that
enable the production of a valve metal capacitor having both high
capacitance (i.e., high volumetric efficiency) and low ESR is
considerably difficult using capacitor grade powders made by
conventional methods.
[0008] In addition, conventional methods of preparing a valve metal
suboxide powder typically produce powder particles having a
relatively rough, irregular surface texture. The rough particle
surface tends to retain significantly more organic material during
formation of capacitors from the valve metal suboxide powder.
Residual organics can result in high levels of residual carbon in
the finished capacitors, causing high DC leakage of the
capacitors.
[0009] With an ever increasing demand for capacitor materials such
as tantalum, alternatives to tantalum have become an important
priority in order to meet industry demands. Niobium is becoming one
of the alternatives to tantalum but as the industry has realized,
niobium is not a complete substitute for tantalum due to niobium
not providing the same electrical properties. Accordingly, further
developments in the niobium area continue today.
[0010] Another alternative to tantalum is niobium metal oxides that
have been oxygen reduced, in other words, niobium sub-oxides such
as NbO and the like. The oxygen reduced niobium oxides show
considerable promise as providing an additional material that can
be used in the formation of capacitor anodes. In order to further
satisfy industry demands, several properties of the oxygen reduced
niobium oxides can preferably be improved such as the crush
strength of the oxygen reduced niobium oxides as well as efforts to
reduce the amounts of contamination that occurs in the
manufacturing of the oxygen reduced niobium oxides. In addition,
acid leaching is commonly used to reduce the level of contamination
occurring when niobium is milled to achieve particular particle
sizes. The acid leaching step complicates the manufacturing process
and leads to the manufacturing process being more expensive. In
addition, the flow property of the oxygen reduced niobium oxides
could be further improved to better satisfy industry standards.
[0011] While there are various methods to make oxygen reduced
niobium oxides and other oxygen reduced valve metal oxides, there
is a constant need to improve upon the resulting properties of the
final product. In some processes, the treatment steps can cause a
loss of surface area and other treatment steps can cause a loss of
flow properties and a decline in other favorable properties
generally useful in the fabrication of capacitor anodes. For
instance, the sintered crush strength of the oxygen reduced niobium
oxide powders is a desirable property which can be generally low
with current oxygen reduced niobium oxides. However, in any effort
to improve upon this property, other properties can be affected
such as the surface area, pore structure, pore size distribution,
flow properties, and the like. Thus, there is a need in the
industry to provide a method, as well as to provide products which
provide a fine balance of properties, including a desirable
sintered crush strength and green strength, along with other
properties, such as BET surface area, capacitance capability,
particle size, and the like.
[0012] The metal powder should provide an adequate surface area
when formed into a porous body and sintered. The CV/g of metal
capacitors can be related to the specific surface area of the
sintered porous body produced by sintering a metal powder pellet.
The specific surface area of metal powder can contribute to the
maximum CV/g attainable in the sintered porous body.
[0013] Purity of the powder can be an important consideration as
well. Metallic and non-metallic contamination tends to degrade the
dielectric oxide film in metal capacitors. While high sintering
temperatures serve to remove some volatile contaminants, high
temperatures tend to shrink the porous body reducing its net
specific surface area and thus the capacitance of the resulting
capacitor. Minimizing the loss of specific surface area under
sintering conditions, i.e., shrinkage, is helpful in producing high
CV/g metal capacitors.
[0014] Flowability of the metal powder and green strength
(mechanical strength of pressed unsintered powder pellets) are also
important characteristics for the capacitor producer in order to
provide efficient production. The flowability of the agglomerated
metal powder can be important to proper operation of automatic
pellet presses. Sufficient green strength permits better handling
and transport of a pressed product, e.g., pellet, without excessive
breakage.
[0015] A "pellet," as the term is used herein, is a porous mass or
body comprised of metal particles or oxides thereof. Green strength
is a measure of a pellet's unsintered mechanical strength. The term
"pressability" describes the ability of a metal powder to be
pressed into a pellet. Metal powder or oxides thereof that forms
pellets that retain their shape and have sufficient green strength
to withstand ordinary processing/manufacturing conditions without
significant breakage have good pressability.
[0016] A desirable characteristic of metal powders or oxides
thereof of relatively fine particle size is stability. Stability of
metal powders can be achieved by surface passivation of the
particles with, for example, oxygen or an oxide layer. Surface
passivation is typically accomplished in a separate passivation
step.
[0017] Accordingly, a need exists to provide fine metal particles
such as niobium powder, not only to address the problems of fine
powders but also to lead to agglomerated metal particles that have
desirable properties such as good flow properties and improved pore
size distribution.
[0018] Ongoing efforts persist to develop superior niobium
materials and to refine niobium preparation processes to produce
capacitor grade metal material that can be formed into high
performance capacitors characterized by high capacitance (CV/g) and
low DC leakage. Examples of morphology and other observable or
measurable microstructure characteristics of capacitor grade
material that can affect the performance characteristics of
capacitors made therefrom, include primary particle size
(D.sub.50), granule size, flow, purity, degree of roundness,
specific (BET) surface area, particle size distribution (e.g.,
D.sub.10, D.sub.50, and D.sub.90), Scott density, pressability,
crush strength, porosity, stability, dopant content, alloy content,
and the like.
[0019] Niobium metal oxides that have been oxygen reduced, in other
words, niobium suboxides such as niobium monoxide (NbO) and the
like, have been recently recognized as a viable solid electrolytic
capacitor material that offers unique performance advantages such
as high dielectric stability, low leakage current, and low
flammability. These advantages, combined with lower costs, make
niobium suboxides an attractive and economical alternative to
tantalum as an electrolytic capacitor material. Conventional
methods of preparing niobium suboxides typically include converting
a mixture of stoichiometric proportions of Nb.sub.2O.sub.5 powder
and niobium metal which acts as an oxygen getter into niobium
suboxide(s) via heat treatment under vacuum. In addition to being
relatively expensive due to the historically high cost of niobium
metal, reduction methods using niobium metal offer significant
challenges in controlling morphology and microstructure of the
oxygen reduced final product (e.g., NbO) to obtain performance
characteristics, such as high CV/g.
[0020] In addition to limited control over final product
morphology, conventional methods of preparing niobium suboxide that
use solid getter materials other than niobium metal, such as
tantalum and magnesium, have other drawbacks. For instance, the
final niobium suboxide product can become contaminated in the
reduction process by unreacted or residual getter material and/or
oxidized getter material being mixed in with the niobium suboxide.
The likelihood of contamination is increased when the getter
material is physically contacted with the starting niobium oxide
such as by homogenizing, blending, mixing or the like. Also, getter
materials used in conventional reduction reactions typically have
high atomic weight and particles that provide a relatively low
surface area with which the oxygen can react. Thus, currently used
getter materials must be present in large quantities to reduce a
given amount of starting niobium oxide. As a result, the extent of
contamination of the reduced oxygen niobium oxide is greater
because the ratio of getter material to the niobium oxide is high.
Preparation of the final product by decontaminating the niobium
suboxide by screening or acid leaching the niobium suboxide, for
example, becomes more difficult and more waste is produced.
[0021] Accordingly, a need exists for an oxygen reduced niobium
oxide such as NbO that is less expensive to produce than niobium
suboxides produced by current methods that involve using niobium
metal to reduce niobium oxides. A further need exists for a process
that provides a greater degree of control over the morphology,
microstructure, and/or particle size distribution of the oxygen
reduced final product than what is presently possible using
conventional reduction methods. A need also exists for a method to
reduce a niobium oxide in which contamination of the oxygen reduced
niobium oxide during the reaction process is minimized by having a
comparatively low ratio of getter material to niobium oxide
relative to conventional methods. Also needed are capacitors that
have superior performance characteristics such as high capacitance
and low DC leakage made from niobium suboxides having superior
morphology.
[0022] A need also exists for a method of making primary and
agglomerated valve metal oxide particles useful in producing a
valve metal sintered body, that suitably controls the surface area,
the cohesive strength, the porosity, the crush strength, and other
properties of the valve metal oxide particles used as the
capacitor-grade material, as well as minimizes the presence of
semi-conducting valve metal oxides in the capacitor-grade
material.
[0023] Accordingly, a need exists to overcome one or more of the
above-described disadvantages.
SUMMARY OF THE PRESENT INVENTION
[0024] It is therefore a feature of the present invention to
provide oxygen reduced niobium oxides having a porous, multi-modal
or unimodal structure or a unimodal structure with an extended
shoulder.
[0025] Another feature of the present invention is to provide
oxygen reduced niobium oxides having improved sintered crush
strength, porosity, green strength, and flow characteristics.
[0026] A further feature of the present invention is to provide
high capacitance capacitors having low DC leakage made from oxygen
reduced niobium oxides.
[0027] A further, feature of the present invention to provide a
method of controlling the porosity and the BET surface area of
capacitor-grade valve metal oxide particles by a method that
includes thermal heat treatment.
[0028] Another feature of the present invention is to provide a
method of making valve metal oxide particles that produces
beneficial coarsening of the particles without sacrificing the
desired impregnability of the particles.
[0029] Another feature of the present invention is to provide a
method of preparing niobium pentoxide particles as well as other
valve metal oxide particles that can be deoxidized to form a
finished niobium suboxide (or other valve metal suboxide) powder
product having superior electrical properties.
[0030] A further feature of the present invention is to provide a
method of thermally agglomerating a valve metal oxide in which pore
size of the particles is maintained or increased without a
substantial loss of BET surface area of the particles.
[0031] Yet another feature of the present invention is to provide a
post-reaction, heat treated, oxygen reduced valve metal oxide in
which the particles have a reduced BET surface area and are
characterized as being less susceptible to retaining organic
material during capacitor formation.
[0032] A further feature of the present invention to provide
methods to form useful niobium powders or hydrided forms thereof
which are preferably capable of forming useful granules for a
variety of uses including capacitor applications and the formation
of oxygen reduced niobium sub-oxides.
[0033] Another feature of the present invention is to provide
niobium powders optionally in granule form which have a variety of
useful properties.
[0034] A further feature of the present invention is to provide
capacitor anodes having a variety of useful electrical properties
such as high capacitance and/or low leakage.
[0035] A further feature of the present invention is to provide a
method to at least partially reduce a niobium oxide using an oxygen
getter material that has a high accessible surface area to volume
ratio.
[0036] Another feature of the present invention is to reduce
production costs of oxygen reduced niobium oxides by using an
inexpensive getter material, decreasing the ratio of getter
material to niobium oxide, and/or limiting the extent of
contamination of the niobium oxide by the getter material and its
oxide.
[0037] A further feature of the present invention is to control
reaction conditions during oxygen reduction of niobium oxide to
produce niobium suboxides having a desired morphology.
[0038] Additional features and advantages of the present invention
will be set forth in part in the description that follows, and in
part will be apparent from the description, or may be learned by
practice of the present invention. The objectives and other
advantages of the present invention will be realized and attained
by means of the elements and combinations particularly pointed out
in the description and appended claims.
[0039] To achieve these and other advantages, and in accordance
with the purposes of the present invention, as embodied and broadly
described herein, the present invention relates to a method to at
least partially reduce a niobium oxide. The method includes mixing
a niobium powder and a starting niobium oxide together to form a
powder mixture; heat treating the powder mixture under vacuum to
form a heat treated powder; and reacting the heat treated powder in
an atmosphere which permits the transfer of oxygen atoms from the
starting niobium oxide to the niobium powder, wherein the reacting
occurs for a time and at a temperature sufficient to form an oxygen
reduced niobium oxide.
[0040] Preferably, the niobium powder and starting niobium oxide
are mixed together by co-milling. Preferably, the co-milling is
used to aggressively break down any hard aggregates and to
preferably reduce the particles to primary particles of the
starting niobium oxide and niobium powder. As a further option,
prior to heat treating the powder mixture, the powder mixture is
granulated to form a plurality of granules.
[0041] The present invention further relates to a method to at
least partially reduce a niobium oxide that includes heat treatment
of the starting materials that causes mass transfer between
particles characterized by necking of the particles.
[0042] The present invention further relates to a method to at
least partially reduce a niobium oxide that includes granulating
one or both of the starting materials either separately or as a
mixture, and/or the oxygen reduced niobium oxide.
[0043] Furthermore, the present invention relates to valve metal
sub-oxide powders having a sintered crush strength of at least 35
pounds.
[0044] In addition, the present invention relates to a valve metal
sub-oxide powder having a granule strength, which is preferably
substantially independent of screen size. The present invention
further relates to valve metal sub-oxide powders having a green
crush strength that is desirable for capacitor anode formation.
[0045] Also, the present invention relates to valve metal sub-oxide
powders that have a porosity distribution such that a mono-modal
log differential intrusion peak forms between 0.5 and 0.8 .mu.m,
wherein the log differential intrusion peak has a width of from 0.3
to 1.1 .mu.m at 0.1 mL/g, and the mono-modal log differential
intrusion peak has a height greater than 0.6 mL/g.
[0046] Also, the present invention relates to valve metal sub-oxide
powders that have a porosity distribution such that a mono-modal
log differential intrusion peak is present wherein the mono-modal
log differential intrusion peak has a shoulder extending from 1.3
.mu.m (or less) to 10 .mu.m (or greater) with a shoulder height of
less than 0.1 mL/g.
[0047] In addition, the present invention relates to valve metal
sub-oxide powders that have a pore size distribution such that a
mono-modal log differential intrusion peak is present with a
shoulder, wherein the shoulder has a cumulative volume measured
from 1 to 10 .mu.m wherein the ratio is from 1 to 7.5.
[0048] Further, the present invention relates to valve metal
sub-oxide powders that have a pore size distribution and forms a
mono-modal log differential intrusion peak with a shoulder, wherein
the shoulder has a total porosity of from 4 to 13 percent at above
1 .mu.m and/or a total porosity of 1 to 4 percent at less than 10
.mu.m.
[0049] Also, the present invention relates to valve metal sub-oxide
powders having a pore size distribution to form a mono-modal log
differential intrusion peak at about 0.4 .mu.m, wherein the log
differential intrusion peak has a width or breadth of from 0.2 to
0.6 .mu.m at 0.1 mL/g, wherein the mono-modal log differential
intrusion peak has a height greater than 0.5 mL/g.
[0050] The present invention also relates to a method to control
porosity in valve metal sub-oxide materials which comprises forming
granules and adjusting the granule size to obtain desired porosity.
For instance, a pore size distribution can be achieved to form a
log differential intrusion peak which can have an adjustable log
differential intrusion peak height of from about 0.4 mL/g to about
0.75 mL/g. This can be adjusted, for instance, by screen size
and/or pre-heat treatment variations.
[0051] The present invention also relates to a method of making
valve metal oxide particles. The method generally includes heat
treating a starting valve metal oxide under vacuum or inert
conditions to form agglomerated particles, followed by
deagglomerating the agglomerated particles.
[0052] The present invention further relates to niobium oxide(s)
and other valve metal oxides that have superior impregnation
properties.
[0053] The present invention also relates to high capacitance
capacitors having low DC leakage made from valve metal oxide
particles.
[0054] The present invention further relates to a post reaction
heat treatment of an oxygen reduced valve metal oxide which reduces
its BET surface area while maintaining its crush strength, flow,
porosity, and/or other properties.
[0055] The present invention also relates to a method of making a
niobium powder, e.g., a surface-passivated niobium powder, or
hydrided forms thereof. The method includes a first milling (e.g.,
wet milling) of a niobium or hydrided niobium feedstock using a
first milling media to form a first milled niobium or hydrided
niobium powder; and a second milling (e.g., wet milling) of the
first milled niobium or hydrided niobium powder using a second
milling media after the first milling to preferably form a
surface-passivated niobium or hydrided niobium powder, preferably
having an oxygen content of at least 1,000 ppm, wherein the first
milling media has a size that is greater than a size of the second
or subsequent milling media.
[0056] The present invention further relates to niobium or hydrided
niobium powders, optionally in granule form, which have desirable
primary particle sizes, BET surface areas, flow rates, oxygen
contents, high capacitance capability, bimodal pore size
distribution, low metal impurities, or combinations thereof.
[0057] The present invention also relates to a method to at least
partially reduce a niobium oxide. The method generally includes
heat treating a starting niobium oxide in the presence of a getter
material and in an atmosphere which permits a transfer of oxygen
atoms from the starting niobium oxide to the getter material, for a
time and at a temperature sufficient to form an oxygen reduced
niobium oxide, wherein the getter material includes titanium.
Preferably, the getter material is a titanium sponge.
[0058] The present invention also relates to capacitor-grade
niobium suboxides having a variety of beneficial properties and
characteristics.
[0059] The present invention further relates to capacitors that
have superior performance characteristics such as high capacitance
and low DC leakage made from niobium suboxides having superior
morphology.
[0060] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only and are intended to provide a further
explanation of the present invention, as claimed.
[0061] The accompanying drawings, which are incorporated in and
constitute a part of this application, illustrate some of the
embodiments of the present invention and together with the
description, serve to explain the principles of the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0062] FIG. 1 is a graph of pore size distribution of various
oxygen reduced niobium oxide powders that are pressed and
sintered.
[0063] FIG. 2 is a graph of cumulative pore volume of various
oxygen reduced niobium oxide powders that are pressed and
sintered.
[0064] FIGS. 3-6 are graphs of pore size distributions and
cumulative pore volumes of various oxygen reduced niobium oxide
powders that are pressed and sintered.
[0065] FIG. 7 is a graph showing the granule strengths of various
oxygen reduced niobium oxide powders at various mesh screen
sizes.
[0066] FIGS. 8-9 are graphs of pore size distributions and
cumulative pore volumes of various oxygen reduced niobium oxide
powders that are pressed and sintered.
[0067] FIGS. 10 and 11 are SEMs of agglomerated niobium pentoxide
particles of the present invention at two different
magnifications.
[0068] FIG. 12 is a graph of porosimetry comparisons of niobium
suboxide anodes made via the present invention of various
sizes.
[0069] FIGS. 13 and 14 are graphs of particle size distribution for
a single step milling, and two or three staged millings of
different milling times.
[0070] FIG. 15 is a graph of BET surface areas versus milling times
for 1/32'' media for NbH powder.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0071] For purposes of the present invention, the preferred powder,
which is an oxygen reduced niobium oxide will be primarily
discussed for exemplary purposes. It is to be realized that the
scope of the present invention relates to oxygen reduced valve
metal oxides, such as oxygen reduced tantalum oxides, oxygen
reduced aluminum oxides, oxygen reduced titanium oxides, oxygen
reduced zirconium oxides, and alloys thereof. For purposes of the
present invention, examples of the starting valve metal oxides can
be, but are not limited to, at least one oxide of the metals in
Groups 4, 5, and 6 (IUPAC designations) of the Periodic Table,
aluminum, bismuth, antimony, and alloys thereof and combinations
thereof. Generally, the alloys of the valve metal oxides will have
the valve metal as the predominant metal present in the alloy
oxide. Specific examples of the starting valve metal oxides,
include, but are not limited to, Nb.sub.2O.sub.5, Ta.sub.2O.sub.5,
and Al.sub.2O.sub.3. Also, for purposes of the present invention,
oxygen reduced niobium oxides are used interchangeably with the
term "niobium sub-oxides" and have the same meaning. Likewise,
oxygen reduced valve metal oxides have the same meaning as "valve
metal sub-oxides."
[0072] A method of making oxygen reduced niobium oxides according
to the present invention generally includes the steps of mixing a
niobium powder and a starting niobium oxide together to form a
powder mixture; heat treating the powder mixture preferably under
inert or vacuum conditions to form a heat treated powder; and
reacting the heat treated powder in an atmosphere which permits the
transfer of oxygen atoms from the niobium oxide to the niobium
powder for a time and at a temperature sufficient to form an oxygen
reduced niobium oxide. The method optionally further includes
granulating the starting niobium oxide, the niobium powder, both
the starting niobium oxide and the niobium powder either separately
before the mixing or as the powder mixture, or the oxygen reduced
niobium oxide after formation. Preferably, the niobium powder and
starting niobium oxide are mixed together by co-milling, preferably
using high energy or aggressive milling so that the starting
niobium oxide and niobium powder are essentially primary particles
intimately mixed with one another. Furthermore, preferably the
mixture that is preferably co-milled is granulated prior to heat
treating.
[0073] Another method of making valve metal oxide particles
according to the present invention generally includes heat treating
a starting valve metal oxide or an oxygen reduced valve metal oxide
to form primary and/or agglomerated valve metal oxide particles.
The method preferably includes heat treating a starting valve metal
oxide under vacuum or inert conditions to form agglomerated
particles, and then deagglomerating the agglomerated particles.
[0074] A method of making niobium powder, e.g., surface-passivated
niobium powder, or hydrided forms thereof according to the present
invention includes milling, such as wet or dry milling, that
involves multiple milling steps or staged milling of a niobium or
hydrided niobium metal to form a surface-passivated niobium or
hydrided niobium powder. In staged milling, there are at least two
stages of milling, wherein in the first stage, the diameter of the
milling media is larger than the diameter of the milling media used
in the second and any subsequent stages of milling. The staged
milling of the niobium or hydrided niobium metal leads to a variety
of beneficial properties including a narrower particle distribution
range and/or a smaller mean particle size of the niobium or
hydrided niobium powder. In addition, surface-passivating during
staged milling of the niobium or hydrided niobium metal eliminates
the need for an additional passivation step which may be necessary
for particles of comparable size prepared by other methods. For
purposes of the present invention, the material can be niobium,
hydrided niobium (e.g., NbH), doped forms of niobium (e.g.,
nitrided Nb), and the like. While "niobium" powder is discussed in
detail below, it is understood that the other powders described
above apply equally to this process and any part of the present
invention, including, but not limited to, niobium hydride
powder.
[0075] Various steps can be incorporated into the method to form
the niobium powder, e.g., the surface-passivated niobium powder. In
general, the present invention can include reducing the particle
size of a niobium feedstock. In addition, any preliminary or
intermediate or final milling step can be used in addition to the
milling steps described herein. For example, a jet milling can be
used at any point.
[0076] According to an embodiment of the present invention, the
milling process is preferably a wet milling process. In wet
milling, the niobium metal is milled using a milling media in the
presence of an aqueous solution, preferably water. An example of a
suitable ratio for a wet milling process is 800 grams of niobium
powder to 300 ml of water. The milling media can occupy the
remaining volume in the mill. An example of a suitable ratio is
5,000 g Nb powder to 1,600 ml water to 1 gallon of media, plus or
minus 1-20% for each component.
[0077] For purposes of the present invention, surface passivation
of the niobium powder can be characterized by formation of a
niobium oxide on the surface of the niobium powder. Wet-milling of
the niobium feedstock or the first milled niobium powder (both of
which are hereinafter referred to as niobium metal) can produce
surface-passivation of the niobium powder, for instance, from a
reaction of the niobium metal with water as represented by the
reaction:
2Nb+5H.sub.2O.fwdarw.Nb.sub.2O.sub.5+5H.sub.2 (Eq. 1)
As can be seen by Eq. 1, wet milling niobium metal in the presence
of water can result in forming a niobium oxide layer, such as a
niobium pentoxide layer on the niobium metal. In more detail, the
method of preparing the niobium powder of the present invention
involves the use of an aqueous solution along with niobium
feedstock to form the milled niobium powder of the present
invention.
[0078] Preferably, the niobium powder of the present invention
preferably has an oxygen content of at least about 1,000 ppm, more
preferably, at least about 5,000 ppm, even more preferably, at
least about 15,000 ppm, even more preferably, at least about 22,000
ppm, and most preferably, at least about 28,000 ppm. Higher oxygen
contents are possible, especially using at least 3 wet millings,
such as from about 28,000 ppm to about 70,000 ppm or more. In one
embodiment, the surface passivated niobium can be used for
formation of niobium sub-oxides, like NbO.sub.0.17 to NbO.sub.0.55
upon heat treatment as described later. This is optional. The
oxygen content of the surface-passivated niobium powder can be
related to the BET surface area of the surface-passivated niobium
powder or particles. Preferably, a greater BET surface area is
characterized by a greater oxygen content of the surface-passivated
niobium powder preferably corresponding to a surface-passivation of
the niobium powder. Specific levels of oxygen content can be
achieved by increasing surface passivation or decreasing the oxygen
content by conventional de-ox processes.
[0079] In preparing the niobium feedstock, in one embodiment, a
niobium ingot can be subjected to a hydriding process in order to
embrittle the niobium metal for purposes of crushing the ingot into
feedstock in the form of powder, which is preferably then subjected
to a screen in order to obtain a uniform particle distribution,
which can be from about 5 to about 425 microns in size. If desired,
the niobium powder can be subjected to the crusher two or more
times in order to achieve the desired particle size and uniform
particle distribution. At this point, the hydrided niobium can be
maintained as hydrided niobium or can be converted to any other
form of niobium, such as non-hydrided niobium, nitrided niobium,
and the like.
[0080] The niobium feedstock used in the present invention can be
in any shape or size. Preferably, the niobium feedstock is in the
form of a powder or a plurality of particles. Examples of the type
of powder that can be used include, but are not limited to, flaked,
angular, nodular, spherical, and mixtures or variations thereof.
Preferably, the niobium feedstock is in a powder form that more
effectively leads to the passivation of the surface of the niobium
powder. Examples of such preferred niobium feedstock include
niobium powders having mesh sizes of from about 60/100 to about
100/325 mesh and from about 60/100 to about 200/325 mesh. Another
range of size is from -40 mesh to about +325 mesh, or a size of
-325 mesh.
[0081] Preferably, the niobium feedstock used in the present
invention is high purity niobium metal to minimize the introduction
of other impurities during the milling process. Accordingly, the
niobium metal in the niobium feedstock preferably has a purity of
at least about 98% and more preferably at least about 99%. Further,
it is preferred that impurities are not present or are present in
negligible amounts in the niobium feedstock, e.g., below about 500
ppm and preferably, below 100 ppm, excluding gases.
[0082] In one milling technique, the niobium metal can be milled
wherein all of the surfaces that come in contact with the niobium
metal are niobium, an alloy thereof, an oxide thereof, a nitride
thereof, doped niobium (e.g., dopants such as nitrogen, oxygen,
sulfur, phosphorus, boron, yttrium, and the like), or a substance
coated with any of these. In this process, the milling of the
niobium metal to form the niobium powder preferably occurs in a
mill wherein all of the surfaces that come in contact with the
niobium metal are niobium. In other words, preferably all of the
contact surfaces of the mill, arms, and grinding media used in the
mill have a niobium surface. The niobium surface on the contact
areas of the mill and grinding media can be accomplished by coating
the grinding media and internal surfaces of the mill with niobium
metal or plates of niobium metal can be placed (e.g., welded) in
the mill. The grinding media, such as balls, can be coated with
niobium or can be made entirely of niobium. By having all contact
surfaces of the mill and grinding media made of niobium, the amount
of contamination to the niobium metal is significantly reduced, and
preferably reduced to such a level that acid leaching is not
necessary and is preferably avoided. This is especially
advantageous since acid leaching can be inconsistent and lead to
varying levels of contamination from production lot to production
lot.
[0083] Preferably, the amount of niobium present on the contact
surfaces of the mill and grinding media is of a sufficient level
such that during the milling process, none of the non-niobium
underlying surfaces come in contact with the niobium metal.
Preferably, the thickness (e.g., about 1 mm or less to about 100 mm
or more) of the niobium coating on the contact surfaces of the mill
and grinding media is sufficient such that repeated milling can
occur from production lot to production lot. Preferably, the
milling of the niobium metal occurs in a wet mill, which leads to a
more uniform particle distribution size of the niobium powder. In
wet milling, the liquid used can be aqueous, such as water,
alcohol, and the like. Preferably, the liquid used is water.
Preferably, the milling is sufficient to reduce the niobium metal
particle size to a range of from about 0.5 micron or less to about
10 microns, and more preferably, from about 0.5 micron or less to
about 5 microns. In the alternative, dry milling (e.g., jet
milling) can be used in one or more or all of the milling
steps.
[0084] In an embodiment of the present invention, the staged
milling of the niobium metal achieves desired particle distribution
sizes. For instance, the particle distribution sizes preferably of
the first milled niobium powder are such that the mean particle
size or D.sub.50 of the first milled niobium powder is from about 3
to about 4 microns. Preferably, the size distribution of the first
milled niobium powder has both a D.sub.10 value of less than about
2 microns, and preferably from about 1.5 microns to about 1.9
microns and a D.sub.90 value of less than about 10 microns, and
preferably from about 5 microns to about 8 microns. In addition,
the particle distribution sizes preferably of the twice milled
niobium powder are such that the mean particle size or D.sub.50 of
the niobium powder is from about 0.5 micron or less to about 3
microns, and more preferably from about 2 microns to about 1 micron
or less. The D.sub.10 value is less than about 2 microns, and
preferably from about 0.1 micron or less to about 1.3 microns, and
the D.sub.90 value is less than about 4 microns, and preferably
from about 1.8 microns or less to about 3.8 microns. Furthermore,
the particle distribution sizes preferably of the three times
milled niobium powder is such that the mean particle size or
D.sub.50 of the niobium powder is from about 0.5 micron or less to
about 1.5 microns, and more preferably from about 0.75 micron to
about 0.9 micron or about 1 micron. The D.sub.10 value is less than
about 1 micron and preferably from about 0.1 micron or less to
about 0.75 micron, and the D.sub.90 value is less than about 3
microns, and preferably from about 1 micron or less to about 2
microns.
[0085] The oxygen reduced niobium oxide or niobium suboxide can
have any morphology, and preferably has an interconnected or
cellular morphology. The niobium suboxide can be in any shape or
size. Preferably, the niobium suboxide is in the form of a powder
or a plurality of particles. Examples of possible types of powder
include, but are not limited to, flaked, angular, nodular,
spherical, and mixtures or variations thereof. Examples of
preferred niobium suboxide powders include those having mesh sizes
of from about 60/100 to about 100/325 mesh and from about 60/100 to
about 200/325 mesh. Another range of size is from -40 mesh to about
+325 mesh, or a size of -325 mesh. Preferably, the niobium suboxide
powders have a D.sub.50 of from about 0.25 to about 5 microns, and
a BET surface area of from about 1 to about 8 m.sup.2/g. The
niobium suboxide preferably has a particle size distribution range
in which the D.sub.10, D.sub.90, or both is within about 300% of
the D.sub.50. The niobium suboxide preferably has an apparent
density of from about 0.5 to about 2.5 g/cc. The niobium suboxide
preferably has a porous microstructure having pores of from about
0.01 to about 100 micrometers. The niobium suboxide preferably has
a pore volume of from about 10 to about 90%. The niobium suboxide
can have a monomodal or a multimodal pore size distribution, and
more preferably a bimodal pore size distribution. Measurements
relating to the porosity of the niobium suboxide can be made as
described above. The niobium suboxide preferably has excellent flow
properties such as from about 100 mg/s to about 2000 mg/s or more,
and more preferably at least about 200 mg/s as measured by the
methods described above. The niobium suboxide can contain a range
of modifying agents or additives or dopants, including, but not
limited to, nitrogen, silicon, phosphorous, boron, carbon, sulfur,
yttrium, or combinations thereof. The niobium suboxide can be
nitrided and/or contain a nitride layer. Preferably, the niobium
suboxide is capable of a capacitance of at least 10,000 CV/g, when
formed into a capacitor anode.
[0086] In an embodiment of the present invention, the staged
milling of the niobium metal preferably achieves desired particle
BET surface areas. The primary particles can be about 1 micron or
less, such as 0.5 micron. The niobium powder of the present
invention can have a high specific surface area. Further, the
niobium powder of the present invention can be characterized as
having a preferred BET surface area. For instance, the BET surface
areas preferably of the first milled niobium powder are from about
1 to about 2 m.sup.2/g. In addition, the particle BET surface areas
preferably of the twice milled niobium powder are from about 2 to
about 8 m.sup.2/g, and more preferably from about 3 to about 4.5
m.sup.2/g or more. Higher BET surface areas above 5 m.sup.2/g, and
above 8 m.sup.2/g are obtainable with 2 or more milling stages as
described herein.
[0087] In an embodiment of the present invention, the first milling
of the niobium feedstock, the second milling of the first milled
niobium powder, and/or any subsequent milling step comprise ball
milling. The first milling, the second milling, and/or any
subsequent milling step occurs in an Attritor mill such as a 1 S
mill which is operated, for example, at about 350 rpm or higher.
Other operating rpms can be used. Other mill sizes can be used.
When the milling is completed, the mixture can optionally then be
subjected to the heat treatment as described in U.S. Pat. Nos.
6,416,730; 6,391,275; and 6,322,912, all incorporated in their
entirety by reference herein.
[0088] The milling of the niobium metal can be achieved
sequentially by milling with varying ball diameters. For instance,
the first milling can use 3/16'' dia. media (e.g., Nb spherical
media), the second milling can use 1/16'' dia. media (e.g., Nb
spherical media), the third milling can use 1/32'' dia. media
(e.g., Nb spherical media), and so on. The use of staged milling
dramatically reduces the time required to obtain a desired particle
size and size distribution. The milling of the niobium metal can
occur in stages in different mills or in the same mill. In the
preferred embodiment, faster milling is achieved early in the
process by using a large ball diameter to product diameter ratio.
When the milled product population increases, the ball diameter can
be reduced in order to increase the ratio of balls to product, and
thereby increasing the chances of the product being hit and
shattered. Preferably, the size of the niobium metal can be as
large as 1/10 of the size of the ball diameter. This niobium
metal-to-ball ratio can be used until the niobium metal-to-ball
ratio size is from about 1/1000 to about 1/500, more preferably,
until the niobium metal-to-ball ratio is about 1/200. The ball
diameter can then be changed so that the ratio of niobium
metal-to-ball diameter is about 1/10. This process can be continued
until the original feedstock reaches a size of from about 5 microns
to about 1 micron, and can further be used to make submicron
particles. The milling balls tend to reduce the larger niobium
particles faster than the smaller milling balls. Therefore, rather
than using very small ball diameter milling balls for the second
milling step, which would take more time and create a broad
distribution, a ball diameter is sequentially selected that takes
advantage of being relatively massive while still being numerous,
and yet follow the 1/10 ratio of niobium metal-to-ball diameter. As
an example, and in more detail, the first stage milling can use the
largest diameter milling media in this process, such as a 3/16''
diameter niobium spherical media. The niobium or hydrided niobium
feedstock is milled such as in a attritor mill, preferably in the
presence of a liquid such as water. The typical milling time for
this first stage of milling is from about 30 minutes to about 6
hours and more preferably about 3 hours. At the end of this first
stage of milling, the average particle size is preferably from
about 3 to about 4 microns. The surface area as measured by BET is
approximately from about 1 to about 2 m.sup.2/g. Then, in a second
stage milling, the material from the first stage milling is placed
in a mill such as an attritor mill using 1/16'' diameter niobium
spherical media. Milling is preformed, for instance, from about 30
minutes to about 24 hours or more and more preferably from about 6
hours to about 20 hours to produce niobium or hydrided niobium
particles having an average particle diameter of from about 0.5 to
about 3 microns and more preferably from about 1 to about 2 microns
with a surface area as measured by BET of from about 2 to about 8
m.sup.2/g and more preferably from about 2.5 to about 4 m.sup.2/g.
Then, in a third stage milling, 1/32'' niobium media can be used to
mill the material resulting from the second stage milling. This
milling step can be preformed, for instance from about 30 minutes
to about 24 hours or more and more preferably about 20 hours to
produce very fine niobium or hydrided niobium particles having an
average particle size of from about 0.1 to about 1 micron and more
preferably about 0.5 to 1 micron with a BET surface area of from
about 4 to about 10 m.sup.2/g and more preferably from about 5 to
about 7 m.sup.2/g. When three stages or more than three stages of
milling are used, the milling time can be dramatically reduced to
achieve the same given particle size and surface area. For
instance, to mill a powder with a surface area of 5.5 m.sup.2/g, a
powder resulting from a two stage milling identified as a "fine"
particle in Table 9 can be milled to the desired BET surface area
in about 29 hours of total milling time or when using a feed which
is identified as a "superfine" powder in Table 9 (and thus milled
longer) would take approximately 43 hours in total milling
time.
[0089] In the staged milling embodiment of the present invention,
two or more stages of milling using progressively smaller diameters
are used. In other words, in the first milling of the niobium
feedstock, the diameter of the milling media is larger than the
diameter of the milling media used in the second milling.
Furthermore, if more than two stages are used, preferably each
subsequent milling stage uses a milling media that has a diameter
smaller than the previous milling stage. More than two milling
stages can be used depending upon the desired particle size of the
final product. For purposes of the present invention, at least two
stages or three stages of milling accomplish the desired result,
namely a product having a particle size of from about 0.5 micron to
about 5 microns. Submicron particles can be obtained by use of this
technique.
[0090] Using staged milling, the overall milling time can be
reduced by at least 10%, and more preferably, can be reduced by at
least 15%, and even more preferably, by at least 50%, compared to
milling having only one stage of milling using the same milling
media.
[0091] In the preferred embodiment, in each subsequent milling
stage, the ball diameter is smaller than the ball diameter of the
previous milling stage. The above process permits a more uniform
milling of the niobium metal, since smaller diameter balls permit a
more uniform milling. The advantage of using this preferred method
of the present invention is that the method can reduce the overall
milling time to achieve the preferred sizes of the niobium, e.g.,
from about 1 micron or less to about 5 microns. Reduced milling
time lowers production costs and the amount of exposure of the
niobium metal to contaminants. Moreover, to further reduce
contamination, each mill and its grinding ball can be made of
niobium metal or lined with niobium metal.
[0092] Any temperature can be used for the milling, such as
0.degree. C. to 100.degree. C. or higher. For purposes of the
present invention, any of the milling steps described in the
present application can be conducted under heat, such as described
in U.S. Pat. No. 6,402,066 B1, incorporated in its entirety by
reference herein. Also, other additives can be added during any
milling step, such as a binder, lubricant, surfactant, dispersant,
solvent, and the like.
[0093] In another embodiment of the present invention, prior to
subjecting the starting materials to heat treatment in order to
form the desired capacitor grade niobium powder, the milled niobium
powder can be granulated or agglomerated. The granulation can occur
by a number of techniques. For instance, wet screening or drum
agglomeration of wet material can be used. Other examples of
agglomeration techniques include a tilted dish agglomeration that
involves a rotating pan set on an angle to which fresh powder is
added and on which a fine water spray, optionally with binders, is
used. The agglomerate builds up as a spherical mass and eventually
rolls off the pan into a collector. Another example is dry drum
agglomeration that involves adding the niobium powder to a large
drum which turns fairly rapidly and has lifters. The showering
particles are brought in contact with pellets and lightly hammered
together. Another example is compactors which are devices that
press the powder plus recycle between two rolls and makes slabs
which are then milled to give feed to a screen set. Another example
is a pin pelletizer.
[0094] As indicated above, the granulation can occur in a dried
state or wet state. The liquid used can be water, water-based
liquids, alcohols, organic liquids, and the like. With respect to
screening, the granulation can occur by passing the powder over a
screen, such as 20 mesh with openings larger than the desired
granule size (for instance, less than 40 mesh). The majority of
granules have sizes smaller than the openings and a few fines (for
instance, less than 50 microns). This method works especially well
for powders with high surface area (for example, greater than 1.5
m.sup.2/g). The granulation can also be achieved by agitating the
niobium in water, such as by vibrating, blending and the like.
Another method imparts the tumbling motion of moist particles to
form spherical shaped granules. The water content in the powder,
primary particle size, the rotation speed, and the size of media
and tumbling time can be used to control the final granule size.
Typical water contents are less than 50% by weight of the total
ingredients and more preferably less than 30% by weight and
residence times are preferably less than an hour to form granules
greater than 50 microns in size (average). Screening operations to
classify the materials may also be used to remove excessively large
or small granules from the final product. The large and fine
granules may be recycled and again used as feed material. As
indicated above, the water content can be any amount such as
amounts from about 5% to about 40% by weight of the total materials
used in agglomeration and more preferably from about 10% to about
30% by weight. Tumbling speed during granulation when a tumbling
motion is used can be any rotational speed depending upon the size
of the tumbler. For a small lab tumbler, for instance, rotational
speeds can be from about 30 to about 60 rpm and more preferably
from about 40 to about 50 rpm. The amount of material granulated
can be any amount and of course depends upon the size of the device
being used to form granulation. Preferably, a media (e.g.,
1/16''-1/2'') can be also used during granulation such as media
balls made from the same metal that forms the niobium powder. The
media can be present in any amount, such as from about 1 to about
20% or from about 5 to 10% by volume. If wet granulating is used,
the liquid can be added at any rate, such as a slow continuous rate
or as a spray until the desired granule size is achieved. After
granulation, if a liquid is used, the powder can then be dried
using any drying technique such as drying under a vacuum oven or a
convection oven at relatively lower temperatures. For instance, the
drying can occur at temperatures of from about 85 to about
100.degree. C. for about 15 to about 60 minutes or more. The
granules can then be classified by screening the granules. The
screening operation can be preformed either before or after the
drying step. While any size can be achieved by this screening,
examples include -40 mesh (-425 microns) or -50 mesh (-300
microns). The screening allows the removal of coarse and fine
granules based on desired particle distribution.
[0095] The granulation methods of the present invention preferably
form granules that have excellent flow properties such as from
about 100 to about 1000 mg/s, and more preferably from about 280 to
about 320 mg/s as measured by ASTM B 213 using a 3 mm diameter
orifice. In one embodiment of the present invention, the present
invention relates to a niobium powder in granule form. The granules
preferably have a size of from about 30 to about 1,000 microns. The
niobium powder of the present invention can have a microporous
surface and can have a sponge-like structure. The niobium powder of
the present invention can also have a porous structure with
approximately 50% porosity. The granules have an oxygen content of
from about 5,000 to about 15,000 ppm and a mean particle size of
from about 3 to about 4 microns when said granules have a BET
surface area of from about 1 to about 2 m.sup.2/g, an oxygen
content of from about 15,000 to about 22,000 ppm and a mean
particle size of from about 2 to about 3 microns when said granules
have a BET surface area of from about 2 to about 3 m.sup.2/g, an
oxygen content of from about 22,000 to about 28,000 ppm and a mean
particle size of from about 1 to about 2 microns when said granules
have a BET surface area of from about 3 to about 4.5 m.sup.2/g, and
an oxygen content of at least about 28,000 ppm and a mean particle
size of less than about 1 micron when said granules have a BET
surface area of at least about 4.5 m.sup.2/g.
[0096] The granulated products of the present invention preferably
provide excellent physical properties with respect to bulk density,
flowability, green strength, and pressability of the powders.
Preferably, the pressed and sintered granules of the present
invention have a bimodal or multi-modal pore size distribution,
such as from about 0.1 to about 10 microns. Preferably, the pressed
granules of the present invention have a diametric shrinkage during
sintering of from about 1 to about 12%. Preferably, the granules of
the present invention have a combined amount of Fe/Ni/Cr of less
than about 100 ppm, more preferably less than 50 ppm, and even more
preferably, less than 30 ppm, such as from about 5 ppm to about 50
ppm, or from about 5 ppm to about 25 ppm. Preferably, the granules
of the present invention have a pressability of from about 2.4 to
about 3.5 g/cc, wherein pressability includes maintaining the
integrity of the pressed compact for handling purposes. With the
granulation techniques of the present invention, one can maintain
the desired microstructure and electrical properties of the fine
powders while retaining the physical properties during the forming
process.
[0097] The niobium powder, e.g., after milling or after
granulation, can optionally be subjected to one or more heat
treatments, such as an inert gas or vacuum heat treatment, at the
same or different temperatures, if more than one is used. The
temperature and the time of the heat treatment can depend on a
variety of factors. The heat treatment can be at any temperature
which is below the melting point of the niobium powder. Generally,
the heat treatment of the niobium powder is at a temperature of
from about 500.degree. C. or less to about 1900.degree. C., and
other temperatures can be from about 800.degree. C. to about
950.degree. C., such as about 850.degree. C., for a time of from
about 5 to about 100 minutes, and more preferably from about 30 to
about 60 minutes. Furthermore, during the heat treatment process, a
constant heat treatment temperature can be used during the entire
heat-treating process or variations in the temperature or the
temperature steps can be used. Variations of these steps can be
used to suit any preferences of the industry. Routine testing in
view of the present application will permit one skilled in the art
to readily control the times and the temperatures of the heat
treatment in order to obtain the desired properties of the niobium
powder. If more than one heat treatment is used, the same furnace
can be used and can be achieved in one run, and cooling can be
avoided. For instance, heat treatment can occur at 700.degree. C.
for two hours and go directly to 1,000.degree. C. or go directly to
500.degree. C. for two hours. Also, N.sub.2 can be used to effect
control of the N.sub.2 level of the niobium powder.
[0098] Once the heat treatment is complete and the desired niobium
powder is obtained, the powder can then be pressed into an anode
using conventional methods of forming anodes from valve metals. In
the present invention, the niobium powder has significantly
improved flow properties as well as crush strength and further has
low impurities which all lead to beneficial capacitor anode
properties, such as an extremely low leakage.
[0099] The starting niobium oxide can be at least one oxide of
niobium metal and/or alloys thereof. A specific example of a
starting niobium oxide is Nb.sub.2O.sub.5. The starting niobium
oxide used in the present invention can be in any shape or size.
Preferably, the niobium oxide is in the form of a powder or a
plurality of particles. For purposes of the present invention,
examples of starting valve metal oxides and/or oxygen reduced valve
metal particles can be, but are not limited to, at least one oxide
of the metals in Groups IV, V, and VI (IUPAC designations) of the
Periodic Table of the Elements, aluminum, bismuth, antimony, and
alloys thereof and combinations thereof. Preferably, the starting
valve metal oxide is an oxide of tantalum, aluminum, titanium,
zirconium, niobium, and/or alloys thereof, and most preferably is a
niobium oxide, or alloys thereof. Generally, the alloys of the
valve metal oxides will have the valve metal as the predominant
metal present in the alloy oxide. Specific examples of starting
valve metal oxides, include, but are not limited to
Nb.sub.2O.sub.5, Ta.sub.2O.sub.5, and Al.sub.2O.sub.3. Further, the
starting valve metal oxide can be a valve metal suboxide, such as
TaO or NbO. The starting valve metal oxide can also be a valve
metal oxide which is a semi-conductor as a lower oxide and which
converts to a higher oxide with high insulating properties and has
useful dielectric properties. Mixtures or combinations of oxides
can be used. Examples of the type of powder that can be used
include, but are not limited to, flaked, angular, nodular,
spherical, and mixtures or variations thereof. Preferably, the
starting niobium oxide is in the form of a powder that more
effectively leads to the oxygen reduced niobium oxide. Examples of
such preferred starting niobium oxide powders include those having
mesh sizes of from about 60/100 to about 100/325 mesh and from
about 60/100 to about 200/325 mesh. Another range of size is from
-40 mesh to about +325 mesh, or a size of -325 mesh. Preferably,
the starting niobium oxide powders have a primary particle size
(D.sub.50) of from about 0.1 to about 5 microns, and a BET surface
area of from about 1 to about 15 m.sup.2/g.
[0100] The starting valve metal oxide, the agglomerated particles,
and the final product (i.e., valve metal oxide particles) of the
present invention can be in any shape or size. The starting valve
metal oxide, the agglomerated particles, and/or the final product
(collectively, "the powders") can be, for example, in the form of a
powder or a plurality of particles. Examples of the type of powder
that can be used or formed include, but are not limited to, flaked,
angular, spherical, fibrous, nodular, and mixtures or variations
thereof. Examples of such preferred valve metal oxide powders
include those having mesh sizes of from about 60/100 to about
100/325 and from about 60/100 to about 200/325 mesh. Another range
of size is from about -40 mesh to about -325 mesh. Preferably, the
powders have a primary particle size (D.sub.50) of from about 0.25
to about 5 microns, and/or a BET surface area of from about 1 to
about 8 m.sup.2/g. The powders preferably include an apparent
density of from about 0.2 to about 1.5 g/cc. The powders preferably
include a porous microstructure (due to pore size, the number of
pores, and/or total pore value) having pores of from about 0.1 to
about 100 micrometers. The powders preferably include a pore volume
of from about 10 to about 90%. The powders can include a monomodal
or a multimodal pore size distribution, and more preferably, a
bimodal pore size distribution. Measurements relating to the
porosity of the valve metal oxide can be made, for example, as
described in U.S. Pat. Nos. 6,576,038 B1, and 6,479,012 B1, and
Published U.S. Patent Application Nos. 2003/0115985, and
2002/0033072, each of which is incorporated in its entirety herein
by reference. The powders preferably include excellent flow
properties, such as from about 100 to about 500 mg/s or more, and
more preferably, at least about 200 mg/s as measured by ASTM B 213
using a 3 mm diameter orifice. The powders can contain a range of
modifying agents or additives or dopants, including nitrogen,
silicon, phosphorous, boron, carbon, sulfur, yttrium, or
combinations thereof. The powders can be nitrided and/or contain a
nitride layer. Preferably, the powders are capable of a capacitance
of at least 40,000 CV/g, when formed into a capacitor anode.
Furthermore, with respect to the numerous beneficial properties
described above, such as the D.sub.50, BET surface area, flow
properties, oxygen contents, electrical properties, and the like,
it is important to appreciate that for purposes of the present
invention, the powders can have at least one of these
characteristics, or two or more of these characteristics, or all of
these characteristics. Any combination of properties and
characteristics is possible.
[0101] The starting niobium oxides can be prepared by calcining at
1000.degree. C. until removal of any volatile components. The
oxides can be sized by screening. Preheat treatment of the niobium
oxides can be used to create controlled porosity in the oxide
particles. Preheat treatment can be at temperatures from about 600
to about 1400.degree. C.
[0102] The niobium powder acts as a getter material for purposes of
the present invention and is capable of reducing the specific
starting niobium oxide to the oxygen reduced niobium oxide.
Preferably, the niobium powder becomes part of the final product of
the present invention, namely the oxygen reduced niobium oxide(s).
The niobium powder for purposes of the present invention is any
niobium powder that can remove or reduce at least partially the
oxygen in the niobium oxide. Thus, the niobium powder can be an
alloy or a material containing mixtures of niobium metal with other
ingredients. Preferably, the niobium powder is predominantly, if
not exclusively, niobium metal. The purity of the niobium metal is
not important but it is preferred that high purity niobium metal
comprise the niobium powder to, among other reasons, avoid the
introduction of other impurities during the heat treating and
reacting processes. Accordingly, the niobium metal in the niobium
powder preferably has a purity of at least about 98% and more
preferably at least about 99%.
[0103] The niobium can be in any shape or size. For instance, the
niobium can be in the form of a tray that contains the starting
niobium oxide to be reduced or can be in a particle or powder size.
Preferably, the niobium is in the form of a powder in order to have
the most efficient surface area for reducing the niobium oxide. The
niobium powder, thus, can be flaked, angular, nodular, spherical
and mixtures or variations thereof. Preferably, the niobium powder
has a primary particle size (D.sub.50) of from about 0.5 to about 5
microns, and a BET surface area of from about 1 to about 15
m.sup.2/g.
[0104] In general, the materials, processes, and various operating
parameters as described in U.S. Pat. Nos. 6,416,730; 6,391,275; and
6,322,912; U.S. patent application Ser. No. 09/533,430 filed Mar.
23, 2000; and U.S. Provisional Patent Application Nos. 60/100,629
filed Sep. 16, 1998; 60/229,668 filed Sep. 1, 2000; and 60/246,042
filed Nov. 6, 2000 can be used in the present invention and all of
these applications are incorporated herein by reference in their
entirety.
[0105] The oxygen reduced niobium oxide is any niobium oxide which
has a lower oxygen content in the metal oxide compared to the
starting niobium oxide. Typical reduced niobium oxides comprise
NbO, NbO.sub.0.7, NbO.sub.1.1, NbO.sub.2, and any combination
thereof with or without other oxides present. Generally, the
reduced niobium oxide of the present invention has an atomic ratio
of niobium to oxygen of about 1:less than 2.5, and preferably 1:2
or less, or 1:less than 1.5, and more preferably 1:1.1, 1:1, or
1:0.7. Put another way, the reduced niobium oxide preferably has
the formula Nb.sub.xO.sub.y, wherein Nb is niobium, x is 2 or less,
and y is less than 2.5x. More preferably x is 1 and y is less than
2, such as 1.1, 1.0, 0.7, and the like. Preferably, the niobium
sub-oxide of the present invention is an NbO or oxygen depleted NbO
or an aggregate or agglomerate which contains NbO and niobium metal
or niobium metal with a rich oxygen content. Unlike NbO, NbO.sub.2
is less desirable due to its resistive nature, whereas NbO is very
conductive. Accordingly, capacitor anodes which are formed from NbO
or oxygen depleted NbO or mixture of NbO with niobium metal are
desirable and preferred for purposes of the present invention.
[0106] In making the oxygen reduced niobium oxides of the present
invention, and preferably NbO or variations thereof, hydrogen gas
(or other carrier gases) is preferably used as the carrier wherein
oxygen is transferred from the starting niobium oxide, e.g.,
Nb.sub.2O.sub.5, to Nb with the use of the H.sub.2 gas as the
carrier. The preferred reaction scheme is as follows:
##STR00001##
As can be seen from Eq. 2, using a niobium metal as the getter
material, the getter material along with the starting niobium oxide
can all result into the final product of the present invention
which is preferably NbO. In more detail, there are typically at
least two processes involved in preparing the niobium sub-oxides of
the present invention. One process involves the preparation of the
getter material, i.e., niobium powder, and the other part of the
process involves the use of the niobium powder along with the
starting niobium oxide to form the niobium sub-oxide of the present
invention.
[0107] In preparing the niobium powder, a niobium ingot can be
subjected to a hydriding process in order to harden the niobium
metal for purposes of crushing the ingot into powder which is
subsequently subjected to a screen in order to obtain a uniform
particle distribution which is preferably from about 5 to about 425
microns in size. If needed, the powder can be subjected two or more
times to the crusher in order to achieve the desired uniform
particle distribution. Afterwards, the powder can then be subjected
to milling in order to preferably obtain a particle size that is
from about 0.5 to about 5 microns in size. In this process, the
milling of the niobium metal (or hydrided niobium metal) in order
to form the niobium getter powder (or hydrided niobium powder)
preferably occurs in a mill wherein all of the surfaces that come
in contact with the niobium (or hydrided niobium) getter material
are niobium. In other words, preferably all of the contact surfaces
of the mill, arms, and grinding media used in the mill have a
niobium surface. The niobium surface on the contact areas of the
mill and grinding media can be accomplished by coating the grinding
media and internal surfaces of the mill with niobium metal or
plates of niobium metal can be placed (e.g., welded) in the mill.
The grinding media, such as balls can be coated with niobium or can
be completely made of niobium. By having all contact surfaces of
the mill and grinding media made of niobium, the amount of
contamination to the niobium getter material is significantly
reduced and preferably reduced to such a level that acid leaching
is not necessary and is preferably avoided. This is especially
advantageous since acid leaching can be inconsistent and lead to
varying levels of contamination from production lot to production
lot. Preferably, the amount of niobium present on the contact
surfaces of the mill and grinding media is of a sufficient level
such that during the milling process, none of the non-niobium
underlying surfaces come in contact with the niobium getter
material. Preferably, the thickness of the niobium on the contact
surfaces of the mill and grinding media is sufficient such that
repeated milling can occur from lot to lot.
[0108] Preferably, the milling of the niobium getter powder occurs
in a wet mill that leads to a more uniform particle distribution
size of the getter material. In wet milling, the liquid used can be
aqueous or non-aqueous, such as water, alcohol, and the like.
Preferably, the milling is sufficient to reduce the size to a range
of from about 0.5 to about 10 microns, and more preferably, from
about 1 micron to about 5 microns.
[0109] Similarly, in the present invention, the starting niobium
oxide can be subjected to milling, e.g., wet milling, in order to
achieve a more uniform particle distribution (e.g., to the size
ranges as described for the getter powder). Typically, the milling
time required to achieve a similar particle distribution size of
the starting niobium oxide as compared to the niobium getter powder
requires less time. Preferably, the milling, and more preferably
the wet milling of the starting niobium oxide occurs in a similar
milling set-up used with respect to the niobium getter powder. In
other words, preferably, the contact surfaces of the mill, arms,
and grinding media are preferably niobium metal to again avoid
contamination of the starting niobium oxide. Preferably, the
milling is sufficient to reduce the size of the starting niobium
oxide to a size substantially similar to the niobium getter powder.
In one sense, the starting niobium oxide is de-agglomerated by the
milling.
[0110] In a preferred embodiment of the present invention, the
milling and preferably wet milling of the niobium getter powder and
the starting niobium oxide are milled to the extent that the two
components have similar particle distribution sizes. The advantage
of having similar particle distribution sizes leads to an improved
reacting rate or rate of forming the oxygen reduced starting
niobium oxide. In other words, when the sizes of the two components
are similar, the niobium getter powder more uniformly accepts
oxygen atoms from the starting niobium oxide and similarly, the
starting niobium oxide more readily is reduced. Thus, the final
product, which is the oxygen reduced niobium oxide, is more uniform
and includes as part of the final product, the niobium getter
powder that has been converted as well as the oxygen reduced
starting niobium oxides. The reaction rate is also increased due to
the shorter distance for the oxygen to defuse out of the starting
niobium oxide and to defuse into the niobium getter powder. This
shorter distance also minimizes oxygen gradients within the final
product resulting in a more stable product. By improving the
reaction kinetics, the processing temperature for the reaction may
be decreased to a temperature that is more favorable for the
formation of preferred oxygen reduced niobium oxides, such as
NbO.
[0111] The wet milling of the niobium powder and the starting
niobium oxide can occur together. In more detail, the wet milling
of the niobium powder and the starting niobium oxide can occur
simultaneously wherein both materials are introduced in a mill, as
described above, and uniformly mixed together in the mill for
purposes of achieving a uniform powder mixture as well as uniform
particle size. As an option, the niobium powder can be introduced
first and milled alone for a certain time, such as a particular
targeted size of, for instance, from about 1 to about 10 microns,
and then the starting niobium oxide can be introduced into the same
mill and the milling is continued with both components present
until a targeted primary particle size of both particles is
obtained which is preferably from about 0.1 to about 10 microns,
and more preferably, from about 0.25 to about 5 microns.
Alternatively, the niobium powder and starting niobium oxide can be
co-milled together from start to finish. In a preferred embodiment,
the milling occurs in an Attritor mill such as a 1 S, 5 S, or 30 S
mill that is operated at about 100 to 300 rpm or more, depending on
the size of the mill. When the milling is completed, the powder
mixture can then be subjected to granulation and/or heat treatment.
One advantage of the co-milling process is that the particle
coordination is controlled in each phase, namely the starting
niobium oxide phase and the niobium metal or hydrided niobium metal
phase. During the co-milling step, the two phases are preferably
aggressively milled, such as attritor milled in water or any
suitable liquid (aqueous or non-aqueous) to preferably break down
any existing hard aggregates that have formed in the starting
niobium oxide or niobium metal phase and to break each of these
phases, preferably into their respective primary particles. By
causing this size reduction, an intimate mixture of the primary
particles is achieved between each phase. By combining the two
phases on the primary particle size scale, the niobium metal (or
hydrided niobium) can form a rigid network that constrains the
sintering between the starting niobium oxide particles. Put another
way, in a preferred embodiment, the particle coordination (e.g.,
the niobium metal or hydrided niobium metal particles are
surrounded by the starting niobium oxide particles and vice versa)
will retain a finer microstructure at comparable heat treatment
temperatures. This permits higher capacitance capability values to
be obtained while maintaining other desirable electrical and
physical characteristics.
[0112] The milling of the niobium getter powder, the starting
niobium oxide, and/or the final product can occur sequentially by
milling with varying ball diameters. In other words, the milling of
the niobium powder, for instance, can occur in stages in different
mills or in the same mill, but in each instance using ball
diameters that have a lower ball diameter with each successive
milling step. This permits a more uniform milling of the
component(s) since smaller diameter balls permits a more uniform
milling. This staged milling can be applied to any of the milling
of the components used in the present invention and results in
reducing the overall milling time to achieve the target size. With
the reduction in milling time, the length of time that the material
is exposed to possible contamination is also reduced. Also, the
overall cost of production is also reduced. An example of a
suitable ratio for wet milling is 800 grams of powder to 300 ml of
water. The remaining volume in the mill is taken up by milling
media. In any of the milling, dry milling can be used in lieu of
wet milling, and an inert atmosphere can be used.
[0113] The heat treating of the starting valve metal oxide and/or
the oxygen reduced valve metal oxide can be achieved in any
treatment device or furnace commonly used in the heat treatment of
metals, such as niobium and tantalum. For instance, heat treating
can be conducted in any reaction system or reactor such as a
retort, vacuum chamber, vacuum furnace, or a vacuum kiln, as
described, for example, in U.S. Pat. Nos. 6,380,517 B2; 6,271,501
B1; and 6,105,272, each of which is incorporated in its entirety
herein by reference. The heat treatment of the starting valve metal
oxide is preferably at a temperature and for a time sufficient to
form the agglomerated particles and further promote a porous matrix
that exists with the agglomerated particles. The heat treatment can
occur at any temperature such as temperatures sufficient to
agglomerate the particles together but not melt the particles. Heat
treating can be any thermal cycle, and can include, but is not
limited to, calcining, sintering, annealing, or any combination
thereof. Heat treating can be used to create controlled porosity in
the heat-treated powder.
[0114] According to one embodiment, heat treating is preferably at
a temperature that is at least about 80% of a melting point
temperature of the starting valve metal oxide. Preferably, the heat
treating is at a temperature of from about 80 to 99% of a melting
point temperature of the valve metal oxide. For example, for NbO
and Nb.sub.2O.sub.5, which have melting points of about
1,810.degree. C. and about 1,510.degree. C., respectively, the heat
treating can preferably be at a temperature of from about 1,200 to
about 1,800.degree. C. The heat treating is preferably at a
temperature that is sufficient to cause coarsening of the
agglomerated particles. Also, heat treating preferably occurs at a
temperature that is sufficient to cause a pore size distribution of
the agglomerated particles to be at least 10% greater than a pore
size distribution of the starting valve metal oxide. The heat
treating can be at a temperature that causes densification or
shrinkage of the powder particles and that causes a reduction in
the BET surface area of the powder particles. Preferably, the heat
treating achieves controlled shrinkage of the BET surface area of
the powder.
[0115] Heat treating preferably occurs under vacuum or inert
conditions. For example, heat treating preferably occurs in an
atmosphere of from about 1.times.10.sup.-5 to about 1,000 torr.
Inert gas(es) can be present or absent from the atmosphere in which
the heat treating occurs. For example, the atmosphere can be a
hydrogen atmosphere. The heat treatment temperature and heat
treatment time can be dependent on a variety of factors such as the
type of starting valve metal oxide. During heat treatment, a
constant temperature can be used during the entire heat treating
process or variations in temperature or temperature steps can be
used. Routine testing in view of the present application will
permit one skilled in the art to readily control the duration,
atmospheres, and temperatures of the heat treatment to obtain the
proper or desired properties in the valve metal oxide particles
formed by the present invention. Optionally, multiple heat treating
steps can be performed on any of the powders of the present
invention.
[0116] Heat treating of starting valve metal oxide can be achieved
under dry or wet conditions. Under wet conditions, a volatilizable
or vaporizable liquid can be combined with or added to the starting
valve metal oxide in any conventional manner which includes methods
of simply mixing a solid with a liquid. For instance, simple
stirring can be used as well as more sophisticated methods of
blending and milling, such as with a mixer-muller. Alternatively,
the liquid can simply be poured on top of a container containing
the metal particles with or without mixing or stirring. The
volatilizable or vaporizable liquid can be any liquid which is
capable of these properties. Examples include, but are not limited
to, water, water-containing liquids, alcohols, non-aqueous liquids,
aqueous liquids, aromatic-containing liquids, alkane-containing
liquids, and the like. Preferably, the volatilizable or the
vaporizable liquid is aqueous in nature and more preferably is
water, and most preferably is deionized water. Any element/chemical
helpful in controlling the agglomerating kinetics of the powders at
high temperatures can be added to the water at the desired
proportions. The agglomerated particles can be agglomerated into a
cake. Heat treating the starting valve metal oxide to form
agglomerated particles can be achieved by any method to agglomerate
metal particles, as described, for example, in U.S. Pat. Nos.
6,576,038 B1, and 6,479,012 B2, each of which is incorporated in
its entirety herein by reference.
[0117] Heat treating of the starting valve metal oxide powder is
preferably at a temperature and for a time sufficient to cause
thermal agglomeration of the powder to form agglomerated particles.
The agglomerated particles can be fused together and/or can exhibit
a mass transfer between individual particles of the powder that can
be characterized by necking of the particles. Heat treating
preferably forms agglomerated particles that upon the agglomerating
and optional subsequent processing, forms a final product that has
a porous microstructure, a multimodal and preferably a bimodal pore
size distribution, and superior crushed strength. The term
"bimodal" denotes a distribution with two modes, i.e., the presence
of two distinct value ranges that are conspicuously more frequent
than neighboring values.
[0118] The agglomerated particles can be hard and/or soft
agglomerates. Soft agglomerates denote agglomerates that can easily
break up upon impact with a harder surface. Furthermore, when a
soft agglomerate does break, the powders return rather easily to
their primary particle make-up. In addition, when the soft
agglomerates are formed, the primary particles can easily be
identified by SEM techniques. In other words, there is generally no
necking of the primary particles with each other and the primary
particle shapes can be maintained even after agglomeration. Also,
heat treating the starting valve metal oxide can result in the
formation of a hard agglomerate. The primary particles even after
being heat treated can essentially maintain their pre-treatment
shape and/or exhibit necking of adjacent particles. Essentially,
the primary particles can maintain their structural shape and
integrity even though they are in contact with each other while
achieving the desired cohesiveness. Also, the agglomerated
particles can be fused together such that the individual primary
particles are indistinguishable. The soft and hard agglomerated
particles of the present invention are quite beneficial with
respect to creating a favorable pore distribution throughout the
agglomerated particles.
[0119] According to one embodiment, the agglomerated particles are
subjected to deagglomerating to form valve metal oxide particles.
The deagglomerating can be sufficient to cause the deagglomeration
of the agglomerated particles to their primary particle size or
finer, and/or to clusters of primary particles having a particle
size that is greater than the particle size of the starting valve
metal oxide. Deagglomerating can be achieved by any technique that
reduces the size of the agglomerated particles. For example, the
agglomerating can be achieved by crushing, jet-milling, attritor
milling, ball milling, classification, or any combination thereof.
Preferably, deagglomerating includes multi-stage milling.
Preferably, deagglomerating does not introduce a substantial amount
of contaminants into the powder. Milling can be dry or wet milling,
as described, for example, in U.S. Patent Application No.
60/471,650 filed on May 19, 2003, which is incorporated in its
entirety herein by reference. Milling can include or not include
one or more heating cycles. Milling can increase or decrease the
BET surface area of the agglomerated particles. Preferably,
deagglomerating is sufficient to cause the valve metal oxide
particles produced therefrom to have a BET surface area that is at
least 10% greater than a BET surface area of the agglomerated
particles. Optionally, the agglomerated particles and/or the other
powders can be can be subjected to multiple deagglomerating steps,
for example, two or more times in multi-stage milling.
[0120] According to one embodiment of the present invention, heat
treating can occur at or below a temperature of from about 40 to
about 78% of a melting point temperature of the starting valve
metal oxide. For example, for NbO or niobium pentoxide, a suitable
heat treatment temperature can be from about 600 to about
1400.degree. C. Heat treating can otherwise be achieved
substantially as described above. The agglomerated particles formed
according to this embodiment preferably have a pore size
distribution that is at least 1% greater than a pore size
distribution of the starting valve metal oxide, as well as a BET
surface area that is at least 40% of a BET surface area of the
starting valve metal oxide. That is, the agglomerated particles
formed according to this embodiment preferably retain the desired
porosity without substantial loss of BET surface area such that a
deagglomerating step is unnecessary.
[0121] According to one embodiment of the present invention, a
method to at least partially reduce a valve metal oxide includes
subjecting a starting valve metal oxide to a first heat treatment
in the presence of a getter material and in an atmosphere which
permits the transfer of oxygen atoms from the starting valve metal
oxide to the getter material, to form an oxygen reduced valve metal
oxide having a first BET surface area, and then subjecting the
oxygen reduced valve metal oxide to a second heat treatment under
vacuum or inert conditions to form a heat-treated oxygen reduced
valve metal oxide having a second BET surface area, wherein the
second BET surface area is less than the first BET surface area.
According to another embodiment of the present invention, a method
of making valve metal oxide particles includes providing an oxygen
reduced valve metal oxide, and heat treating the oxygen reduced
valve metal oxide under vacuum or inert conditions to form a
heat-treated oxygen reduced valve metal oxide having a BET surface
area that is less than a BET surface area of the oxygen reduced
valve metal oxide. For example, the BET surface area of a
heat-treated oxygen reduced valve metal oxide can be less than
about 90%, less than about 80%, less than about 70%, or less than
about 60% of the BET surface area of the oxygen reduced valve metal
oxide. For example, the oxygen reduced valve metal oxide can have a
BET surface area of at least about 2 m.sup.2/g, and preferably, at
least about 3 m.sup.2/g, while the heat-treated oxygen reduced
valve metal oxide can have a BET surface area of less than about 2
m.sup.2/g, and preferably, less than about 1.5 m.sup.2/g. In
addition, the surface of the heat-treated oxygen reduced valve
metal oxide is preferably smoother than the surface of the oxygen
reduced valve metal oxide. For example, the surface of the
heat-treated oxygen reduced valve metal can have fewer salient
points and/or edges, and/or less contour.
[0122] Although, in the above-described embodiment, the heat
treating reduces the BET surface area of the valve metal oxide
particles, preferably one or more physical properties (e.g., flow,
porosity, capacitance, crush strength, and the like) of the valve
metal oxide particles are substantially maintained. In other words,
the present invention relates to a controlled heat treating that
allows more control over the morphology of the heat treated
particles. For example, the heat-treated oxygen reduced valve metal
oxide can have a crush strength of at least about 70%, at least
about 80%, and preferably, at least about 90% or more of a crush
strength of the oxygen reduced valve metal oxide. As another
example, the heat-treated oxygen reduced valve metal oxide can be
capable of being formed into a capacitor having a capacitance of at
least about 70%, at least about 80%, and preferably, at least about
90% or more of a capacitance of which the oxygen reduced valve
metal oxide is capable when formed into a capacitor. As another
example, the heat-treated oxygen reduced valve metal oxide can have
a porosity of at least about 70%, at least about 80%, and
preferably, at least about 90% or more of a porosity of the oxygen
reduced valve metal oxide. Similarly, the heat-treated oxygen
reduced valve metal oxide has a flow of at least about 70%, at
least about 80%, and preferably, at least about 90% or more of a
flow of the oxygen reduced valve metal oxide. The heat-treated
particles can have one, two, three, or more of the above
properties.
[0123] In another embodiment of the present invention, a post heat
treatment of the metal sub-oxide powder is used. This post heat
treatment can be used for any metal sub-oxide powder irrespective
of how the metal sub-oxide powder was obtained. The post heat
treatment is most beneficial with metal sub-oxide powders prepared
in view of the above-described methods of making metal sub-oxide
powders. The post heat treatment is most useful with metal
sub-oxide powders and can be used with metal sub-oxide powders in
loose form, agglomerated form, pressed form, and the like.
[0124] In this embodiment, metal sub-oxide powder is subjected to a
post heat treatment which will generally reduce the BET surface
area of the powder and also reduce the capacitance capability of
the metal sub-oxide powder. In a preferred embodiment, the post
heat treatment occurs at a temperature of from about 800.degree. C.
to about 1300.degree. C. This post heat treatment can last from a
few minutes (e.g., about 5 minutes to about 60 minutes) to several
hours (e.g., about 1 hour to about 48 hours or more) depending upon
the desired amount of post heat treatment. The post heat treatment
preferably occurs in vacuum or under inert conditions. The post
heat treatment, for instance, can be achieved in a furnace or other
heating device. Other temperatures above and below the above
described range can be used.
[0125] Without wishing to be bound to any theory, the post heat
treatment, in certain embodiments, permits one to treat metal
sub-oxides that may contain very small blisters or cracks, such as
nanoscale cracks, wherein the post heat treatment will serve to
reduce or eliminate these nanoscale cracks. The removal of these
cracks or at the very least, the use of post heat treatment,
permits one to take metal sub-oxide powder and mix it with a binder
or lubricant, wherein the anode resulting from pressing this
anode/binder mixture together can then be sintered. Upon delubing
(or other means of removing the binder), the presence of nanoscale
cracks or blisters or some other physical reason prevents the
complete or nearly complete removal of the binder from the pressed
anode which is detrimental in the formation of the anode as a
component in a capacitor. Thus, the post heat treatment serves to
preferably reduce or eliminate the blisters or nanoscale cracks or
somehow treat the powder to prevent the retention of the binder in
significant amounts after delubing. Accordingly, in one embodiment
of the present invention, the post heat treatment is at a
temperature and is for a time sufficient to reduce or prevent the
significant retention of binder in a press anode after
delubing.
[0126] With the post heat treatment, the BET surface area of the
metal sub-oxide powder is generally reduced. In one embodiment, the
BET surface area will be reduced by at least 1% and may be reduced
by at least 5% or more or may be reduced by 10% or more depending
upon the amount of post heat treatment as well as the starting BET
surface area of the metal sub-oxide powder. Irrespective of whether
the BET is reduced by the post heat treatment, the capacitance
capability of the powder is not reduced more than 25%. In other
words, if one took a metal sub-oxide powder which did not receive
post heat treatment and compared that to the same powder which
received the above-described post heat treatment, the capacitance
capability is not reduced by more than 25% and preferably is not
reduced by more than 10% and even more preferably is not reduced by
more than 5%. The capacitance capability is the capacitance that
the powder is capable of when formed into a press anode with the
following conditions:
[0127] Sintered at 1400.degree. C. for 10 minutes. A formation
voltage of 30 volts. A DC bias voltage applied of 10 volts. A press
density of 2.8 g/cc.
[0128] If one formed a press anode with the above conditions and
measured the capacitance, this capacitance would be within 25% of
an anode formed under the same conditions receiving no post heat
treatment.
[0129] In one embodiment, the metal sub-oxide powder can have one
or more of the following characteristics:
[0130] a) BET surface area of powder ("BET"): about 1.4 to about
2.5 m.sup.2/g
[0131] b) Scott Density of powder ("Scott"): about 19 to about 28
g/in.sup.3
[0132] c) capacitance @10Vb ("CV/g"): 69,000-83,000 .mu.FV/g
[0133] d) CV/g.times.BET.times.Scott:
1.12.times.10.sup.11-3.55.times.10.sup.11 CV/(m*g)
[0134] e) CV/g.times.1/BET.times.Scott:
3.2.times.10.sup.10-10.1.times.10.sup.10 (CV*g)/m.sup.5
[0135] f) CV/g.times.1/BET.times.1/Scott:
1.62.times.10.sup.-2-5.11.times.10.sup.-2 (CV*m/g)
[0136] g) CV/g.times.1/BET: 33,000-57,000 CV/m.sup.2
[0137] h) CV/g.times.Scott: 1.10.times.10.sup.6-2.20.times.10.sup.6
CV/in.sup.3
[0138] The metal sub-oxide powder, preferably niobium sub-oxide
powder, has at least 2, 3, 4, 5, 6, 7, or all 8 characteristics as
set forth above. Preferably, the powder has at least a) through c)
as well as at least one, two, three, four, or all five of d)
through h).
[0139] Heat treating the starting valve metal oxide in the presence
of a getter material and in an atmosphere which permits the
transfer of oxygen atoms from the starting valve metal oxide to the
getter material can be achieved by any conventional method to at
least partially reduce a valve metal oxide. For example, in making
the oxygen reduced niobium oxides of the present invention, and
preferably NbO or variations thereof, hydrogen gas (or other
carrier gases) is preferably used as the carrier wherein oxygen is
transferred from the starting niobium oxide, e.g., Nb.sub.2O.sub.5,
to Nb with the use of the H.sub.2 gas as the carrier. The preferred
reaction scheme is as follows:
##STR00002##
As can be seen from Eq. 3, using a niobium metal as the getter
material, the getter material along with the starting niobium oxide
can all result into the oxygen reduced valve metal of the present
invention, here, NbO. Other oxygen active materials can be used as
the oxygen getter material, for instance, tantalum, magnesium, or
their hydrides, as well as hydrided niobium. The first heat
treatment can be, for example, at a temperature of from about 800
to about 1000.degree. C. Other temperatures are possible. The first
heat treatment can occur, for example, in an atmosphere of from
about 1.times.10.sup.-5 to about 1000 torr. The atmosphere is
preferably a hydrogen atmosphere. Other atmospheres are
possible.
[0140] The oxygen reduced valve metal oxide can be any oxygen
reduced valve metal oxide as described above, for example, a
niobium suboxide and/or a tantalum suboxide. Heat treating of the
oxygen reduced valve metal oxide can be achieved as described
above, and preferably is at a temperature of from about 1000 to
about 1200.degree. C. Other temperatures are possible. Heat
treating of the oxygen reduced valve metal oxide preferably occurs
in an atmosphere of from about 1.times.10.sup.-5 to about 1000
torr. Other atmospheres are possible, for example, from about 10
torr to about 1000 torr, from about 100 torr to about 1000 torr, or
from about 300 torr to about 1000 torr.
[0141] A variety of processing steps can be performed on any one or
more of the powders of the present invention, before, during,
and/or after any one or more of the above-described steps of the
present invention. For example, where a higher valve metal oxide,
e.g., niobium pentoxide, is used as the starting valve metal oxide,
the niobium pentoxide particles formed by the present invention can
be deoxidized by any known method or technique to form an oxygen
reduced niobium suboxide. As another example, any of the powders of
the present invention and/or oxygen getter material can be
granulated at any stage, as described, for example, in U.S. Patent
Application No. 60/450,536 filed on Feb. 26, 2003, which is
incorporated in its entirety herein by reference. The valve metal
oxide and/or the valve metal oxide-getter material mixture can be
pre-heat treated, as described, in U.S. Provisional Application No.
60/471,649 filed on May 19, 2003, which is incorporated in its
entirety herein by reference. In general, the materials, processes,
and various operating parameters as described in U.S. Pat. Nos.
6,563,695 B1; 6,527,937 B2; 6,517,645 B2; 6,462,934 B2; 6,432,161
B1; 6,420,043 B1; 6,416,730 B1; 6,402,066 B1; 6,391,275 B1;
6,338,832 B1; 6,338,816 B1; 6,322,912 B1; 6,312,642 B1; 6,231,689
B1; 6,165,623; 6,071,486; 6,051,044; 6,051,326; 5,993,513;
5,986,877, 5,954,856; 5,580,516; 5,284,531; 5,261,942; 5,242,481;
5,234,491; 5,171,379; 4,960,471; 4,722,756; 4,684,399; and
4,645,533, and Published U.S. Patent Application Nos. 2003/0115985
A1; 2003/0082097 A1; 2003/0057304 A1; 2003/0037847 A1; 2003/0026756
A1; 2003/0003044 A1; 2002/0172861 A1; 2002/0135973 A1; 2002/0072475
A1; 2002/0028175 A1; 2002/0026965 A1; and 2001/0036056 A1, can be
used in the present invention, and each are incorporated herein in
their entirety by reference.
[0142] The various valve metal oxide particles of the present
invention can be further characterized by the electrical properties
resulting from the formation of a capacitor anode using the niobium
suboxides of the present invention. In general, the valve metal
oxide particles of the present invention can be tested for
electrical properties by pressing powders of the valve metal oxide
particles into an anode and sintering the pressed powder at
appropriate temperatures and then anodizing the anode to produce an
electrolytic capacitor anode which can then be subsequently tested
for electrical properties.
[0143] Accordingly, another embodiment of the present invention
relates to anodes for capacitors formed from the valve metal oxide
particles of the present invention. The capacitor of the present
invention can be formed by any method, for example, as described in
U.S. Pat. Nos. 6,576,099 B2; 6,576,038 B1; 6,563,695 B1; 6,562,097
B1; 6,527,937 B2; 6,479,012 B1; 6,462,934 B2; 6,420,043 B1;
6,416,730 B1, 6,375,704 B1; 6,373,685 B1, 6,338,816 B1; 6,322,912
B1; 6,165,623; 6,051,044; 5,986,877, 5,580,367; 5,448,447;
5,412,533; 5,306,462; 5,245,514; 5,217,526; 5,211,741; 4,805,704;
and 4,940,490, and Published U.S. Patent Application Nos.
2003/0115985 A1; 2003/0026756 A1; 2003/0003044 A1; 2002/0179753 A1;
2002/0152842 A1; 2002/0135973 A1; 2002/0124687 A 1; 2002/0104404
A1; 2002/0088507 A1; 2002/0072475 A1; 2002/0069724 A1; 2002/0050185
A1; 2002/0028175 A1; and 2001/0048582 A1, each of which is
incorporated herein in its entirety by reference.
[0144] The capacitors can be used in a variety of end uses such as
automotive electronics, cellular phones, computers, such as
monitors, mother boards, and the like, consumer electronics
including TVs and CRTs, printers/copiers, power supplies, modems,
computer notebooks, disc drives, and the like. Anodes can be made
from the valve metal oxide particles in a similar process as used
for fabricating metal anodes, i.e., pressing porous pellets with
embedded lead wires or other connectors followed by optional
sintering and anodizing. The lead connector can be embedded or
attached at any time before anodizing. Anodes made from some of the
valve metal oxide particles of the present invention can have a
capacitance of from about 1,000 CV/g or lower to about 400,000 CV/g
or more, and other ranges of capacitance can be from about 20,000
to about 300,000 CV/g or from about 62,000 to about 200,000 CV/g,
and preferably from about 40,000 to about 400,000 CV/g. In forming
the capacitor anodes of the present invention, a sintering
temperature can be used that will permit the formation of a
capacitor anode having the desired properties. The sintering
temperature will be based on the particular valve metal oxide used.
For a niobium suboxide, for example, the sintering temperature is
preferably from about 1,200 to about 1,750.degree. C., more
preferably from about 1,200 to about 1,400.degree. C., and most
preferably from about 1,300 to about 1,400.degree. C.
[0145] The anodes formed from the niobium oxides of the present
invention are preferably formed at a voltage (V.sub.f) of about 30
volts, and preferably from about 6 volts to about 80 volts or more.
When niobium suboxide particles are used, preferably, the forming
voltages are from about 6 to about 50 V, and more preferably from
about 10 to about 40 volts. The valve metal oxide particles provide
excellent low DC leakage at high formation voltages. Also, the
anodes formed from the valve metal oxides of the present invention
preferably include a DC leakage of less than about 5 nA/CV, for
example, from about 5 nA/CV to about 0.05 nA/CV.
[0146] The previously described versions of the present invention
include many advantages, including superior capacitor-grade
materials and preparation processes to produce capacitor grade
metal material that can be formed into high performance capacitors
characterized by high capacitance and low DC leakage. Examples of
morphology and other observable or measurable microstructure
characteristics of the capacitor grade material of the present
invention that enhance performance characteristics of capacitors
made therefrom include, controlled primary particle size, high
flowability, high purity, high BET surface area, uniform particle
size distribution, Scott density, pressability, crush strength,
microporosity, stability, dopant content, and the like. The valve
metal oxide particles formed by deagglomerating the agglomerated
particles of the present invention have beneficial properties that
include, high BET surface area and uniform porosity with a minimal
proportion of closed pores and micropores.
[0147] One option of achieving higher surface area anodes is by
pressing greater amounts of valve metal oxide powder to form the
porous pellet before sintering. This approach is restricted,
however, because there is an inherent limit to the amount of powder
which can be compacted into a given pellet size. Pellets pressed
with higher than usual compression ratios result in anodes having
poor porosity distributions with closed and non-uniform pores. The
valve metal oxide powders of the present invention have high
specific surface areas and their use in anode formation is
preferable to increasing the quantity of valve metal oxide powder
used to produce the pellet. By using the high surface area powders
of the present invention, capacitor sizes can be reduced while
achieving the same level of capacitance. The ESR value of a
capacitor is related to the magnitude of heat generation
accompanying increased speeds of electronic circuits such that as
ESR increases, heat generation also increases. Thus, capacitors
used in the CPUs and power supply circuits of personal computers
preferably have a low ESR. In forming a high performance capacitor
anode, it is beneficial to form a uniform cathode material coating
on the valve metal sintered body. Manganese oxide is typically used
for the cathode material coating. In forming a cathode material
coating comprised of manganese oxide, a manganese nitrate solution
be impregnated into the sintered body followed by heating and
thermal decomposition of the manganese nitrate. To uniformly form a
cathode material coating on the valve metal sintered body, it is
preferable to use a valve metal sintered body having uniform
porosity with a minimal proportion of closed pores and micropores,
which is possible by use of the valve metal oxide particles of the
present invention.
[0148] In one embodiment of the present invention, prior to
subjecting the starting materials to heat treatment, one or both of
the starting materials, namely the starting niobium oxide and/or
the niobium powder, or the powder mixture comprising both can be
granulated or agglomerated. This granulation or agglomeration is
especially useful when the materials have been co-milled together.
The granulation can occur by a number of techniques. For instance,
wet screening or drum agglomeration of wet material can be used.
Other examples of agglomeration techniques include a tilted dish
agglomeration which involves a rotating pan set on an angle to
which fresh powder is added and on which a fine water spray,
optionally with binders, is used. The agglomerate builds up as a
spherical mass and eventually rolls off the pan into a collector.
Another example is dry drum agglomeration which involves taking a
powder and adding it to a large drum which turns fairly rapidly and
has lifters. The showering particles are brought in contact with
pellets and lightly hammered together. The granules can be formed
by agitation of the powder in water, such as vibrating, blending,
and the like. The granules can be formed by vacuum drying followed
by screening. The granules can be formed by tumbling the powders in
the presence of water. Another example is compactors which are
devices that press the powder plus recycle between two rolls and
makes slabs which are then milled to give feed to a screen set.
Another example is a pin pelletizer. Granulating can be
accomplished as described, for example, in U.S. Patent Application
Publication No. US 2002/0033072 A1, incorporated in its entirety
herein by reference.
[0149] As indicated above, the granulation can occur in a dried
state or wet state. The liquid used can be water, water-based
liquids, alcohols, organic liquids, and the like. With respect to
screening, the granulation can occur by passing the powder over a
screen, such as 20 mesh with openings larger than the desired
granule size (for instance, less than 40 mesh). The screening can
occur at any screen size, such as 20 mesh or lower (20 mesh to 200
mesh or lower). The majority of granules have sizes smaller than
the openings and a few fines (for instance, less than 50 microns).
This method works especially well for powders with high surface
area (for example, greater than 1.5 m.sup.2/g). Another method
imparts the tumbling motion of moist particles to form spherical
shaped granules. The water content in the powder, primary particle
size, the rotation speed, and the size of media and tumbling time
can be used to control the final granule size. Typical water
contents are less than 50% by weight of the total ingredients and
more preferably less than 30% by weight and residence times are
preferably less than an hour to form granules greater than 50
microns in size (average). Screening operations to classify the
materials may also be used to remove excessively large or small
granules from the final product. The large and fine granules may be
recycled and again used as feed material. As indicated above, the
water content can be any amount such as amounts from about 5% to
about 40% by weight of the total materials used in agglomeration
and more preferably from about 10% to about 30% by weight. Tumbling
speed during granulation when a tumbling motion is used can be any
rotational speed depending upon the size of the tumbler. For a
small lab tumbler, for instance, rotational speeds can be from
about 30 to about 60 rpm and more preferably from about 40 to about
50 rpm. The amount of material granulated can be any amount and of
course depends upon the size of the device being used to form
granulation. Preferably, a media (e.g., 1/16''-1/2'') can be also
used during granulation such as media balls made from or coated
with niobium. The media can be present in any amount, such as from
about 1% to about 20% or from about 5 to 10% by volume. If wet
granulating is used, the liquid can be added at any rate, such as a
slow continuous rate or as a spray until the desired granule size
is achieved. After granulation, if a liquid is used, the powder can
then be dried using any drying technique such as drying under a
vacuum oven or a convection oven at relatively lower temperatures.
For instance, the drying can occur at temperatures of from about 85
to about 100.degree. C. for about 15 minutes or less to about 60
minutes or more. The granules can then be classified by screening
the granules. The screening operation can be preformed either
before or after the drying step. While any size can be achieved by
this screening, examples include -40 mesh (-425 microns), -50 mesh
(-300 microns), -100 mesh (-150 microns), -140 mesh (-106 microns),
and the like. The screening allows the removal of coarse and fine
granules based on desired particle distribution.
[0150] If not already achieved, the niobium getter powder is mixed
with or blended with the starting niobium oxide, which is
preferably Nb.sub.2O.sub.5, to form a powder mixture. The powder
mixture can then be subjected to a heat treatment, for instance,
under inert or vacuum conditions, which can occur at a temperature
below the melting point of the powder mixture constituents and
preferably at a temperature of from about 600 to about
1,600.degree. C., and which preferably occurs under inert or vacuum
conditions. The heat treating that the powder mixture is subjected
to can be conducted in any heat treatment device or furnace
commonly used in the heat treatment of metals such as niobium and
tantalum. In a preferred embodiment, heat treating and subsequent
reacting are achieved using the same equipment, without removing
the powder. For example, after the heat treating of the powder
mixture, the powder mixture can be allowed or made to cool and the
reacting process can begin as described in detail below. More than
one heat treatment can occur, and multi-heat treatment steps can
occur at different temperatures, and without cooling in between.
With respect to the multi-heat treatment steps, optionally, other
steps can occur between heat treatment steps, such as, but not
limited to, screening one or more times between one or more heat
treatment steps.
[0151] Heat treating under vacuum of the powder mixture is
preferably at a temperature and for a time sufficient to cause mass
transfer between the particles of the powder mixture that can be
characterized by necking of the particles. Heat treating under
vacuum preferably forms a heat treated powder that, upon subsequent
reacting, forms a final product, i.e., oxygen reduced niobium oxide
that has a porous microstructure (due to mean pore size and/or
total pore volume), a unimodal or multi-modal pore size
distribution (e.g., a bimodal pore size distribution), and superior
crush strength. FIG. 1 is a graph of pore size distribution for
pressed and sintered powders at various heat treating temperatures.
The graph shows the bimodal pore size distribution associated with
heat treated powders versus the unimodal pore distribution of
non-heat treated powder. "Bimodal" means a distribution with two
modes (i.e., the presence of two distinct value ranges that are
conspicuously more frequent than neighboring values). FIG. 2 is a
graph of cumulative pore volume for pressed and sintered powders at
various heat treating temperatures. The graph shows the increased
porosity associated with heat treated powders versus non-heat
treated powder.
[0152] "Mono-modal" means a distribution with one mode (i.e., the
presence of one distinct value range that is conspicuously more
frequent than neighboring values). A "mono-modal log differential
intrusion peak with a shoulder or extended shoulder" means a
distribution with one mode (i.e., the presence of one distinct
value range that is conspicuously more frequent than neighboring
values with the values on one side of the log differential
intrusion peak being higher with respect to frequency than the
values on the other side of the peak, thus forming a shoulder, for
instance as shown in FIG. 3). FIG. 3 shows a mono-modal log
differential intrusion peak (black line) with no shoulder while the
gray line shows a mono-modal log differential intrusion peak with a
shoulder.
[0153] FIG. 3 is graph of pore size distribution for present
sintered powders wherein the powder identified by the black line
represents an oxygen reduced niobium oxide that was co-milled as
described above and the grey line represents an oxygen reduced
niobium oxide which formed a mixture of the starting niobium oxide
with the niobium metal or hydrided niobium metal without
co-milling. In each case, the material was sintered at
1,380.degree. C. for 10 minutes. As can be seen from FIG. 3, the
niobium sub-oxides of the present invention which have been
co-milled provided a unimodal pore size distribution without any
extended shoulder or bimodal log differential intrusion peaks. FIG.
4 is also a graph showing the pore size distribution associated
with heat treated powders using a smaller scale for the log
differential intrusion. Again, a different pore size distribution
can be seen with respect to the various niobium sub-oxide powders.
In FIG. 4, the same powders are represented as in FIG. 3. In FIG.
5, a graph representing pore size distribution is again shown
wherein the solid black line and the dotted black line represent
niobium sub-oxide powders which been sintered and pressed at
1,380.degree. C. for 10 minutes at a 2.8 g/cc pressed density. The
samples again show essentially a unimodal pore size distribution
whereas the sample represented by the grey line was prepared
without co-milling, but only combining the starting ingredients. As
can be seen, a bimodal distribution is achieved. The same powder
samples shown in FIG. 5 are shown in FIG. 6, except with respect to
a smaller scale with respect to the log differential intrusion. The
legend from FIG. 5 applies to FIG. 6 too.
[0154] The heat treated powder is preferably then subjected to
further heat treatment or reacting which preferably occurs at a
temperature of from about 800 to about 900.degree. C. in the
presence of hydrogen, for instance at a pressure of from about 50
to about 900 Torr. Preferably, the reacting occurs for a sufficient
time to achieve the reaction set forth above which is the full
conversion of the niobium powder and the starting niobium oxide to
the final product which is an oxygen reduced niobium oxide such as
NbO, NbO.sub.0.7, NbO.sub.1.1, or combinations thereof. Thus, in
this process, the niobium powder as well as the starting niobium
oxide become the final product.
[0155] The reacting that the powder mixture is subjected to can be
conducted in any heat treatment device or furnace commonly used in
the heat treatment of metals, such as niobium and tantalum. The
reacting of the powder mixture containing the starting niobium
oxide and the niobium powder is at a temperature and for a time
sufficient to form an oxygen reduced niobium oxide. The temperature
and time of the reacting can be dependent on a variety of factors
such as the amount of reduction of the niobium oxide, the amount of
the niobium powder, and the type of niobium powder as well as the
type of starting niobium oxide. Generally, the reacting of the
niobium oxide is at a temperature of from about 750.degree. C. or
less to about 1900.degree. C. or more, and preferably from about
800 to about 900.degree. C., and more preferably at about
850.degree. C. Reacting can be for a time of from about 5 minutes
to about 4 hours, and more preferably from about 1 to about 3
hours. Routine testing in view of the present application will
permit one skilled in the art to readily control the times and
temperatures of the reacting process in order to obtain the proper
or desired reduction of the niobium oxide.
[0156] As stated, after heat treating preferably under inert or
vacuum conditions, primary reacting occurs in an atmosphere which
permits the transfer of oxygen atoms from the niobium oxide to the
niobium powder. The reacting preferably occurs in a hydrogen
containing atmosphere which is preferably just hydrogen. Other
gases can also be present with the hydrogen, such as inert gases,
so long as the other gases do not react with the hydrogen.
Preferably, the hydrogen atmosphere is present during the reacting
at a pressure of from about 10 to about 2000 Torr, and more
preferably from about 100 to about 1000 Torr. Mixtures of H.sub.2
and an inert gas such as Ar can be used. Also, H.sub.2 in N.sub.2
can be used to effect control of the N.sub.2 level of the final
product.
[0157] During the reacting process, a constant heat treatment
temperature can be used during the entire reacting process or
variations in temperature or temperature steps can be used. For
instance, hydrogen can be initially admitted at 1,000.degree. C.
followed by increasing the temperature to 1,250.degree. C. for 30
minutes followed by reducing the temperature to 1,000.degree. C.
and held there until removal of the H.sub.2 gas. After the H.sub.2
or other atmosphere is removed, the furnace temperature can be
dropped. Variations of these steps can be used to suit any
preferences of the industry. As an option, the use of hydrogen can
be avoided, for instance by preferably using high reaction
temperatures, such as 1400.degree. C. or higher, and with uniform
mixing of the starting materials.
[0158] Set forth below is a discussion regarding the various
physical and chemical and electrical characteristics of the niobium
sub-oxide powders of the present invention. The niobium sub-oxide
powders of the present invention can have one or more or any
combination of the properties described herein. The discussion with
respect to these properties and characteristics is simply for
convenience sake and is not meant to limit the combination of
properties that the niobium sub-oxides of the present invention can
have.
[0159] Once the reaction is complete and the desired oxygen reduced
niobium oxide is obtained, the powder can then be pressed into an
anode using conventional methods of forming anodes from valve
metals. In the present invention, the oxygen reduced niobium oxide
has significantly improved flow properties as well as crush
strength and further has low impurities (e.g., Fe, Ni, and Cr),
which all lead to beneficial capacitor anode properties, such as an
extremely low leakage.
[0160] The oxygen reduced niobium oxides of the present invention,
in one embodiment, can be in granulated or granule form. These
granules can have a spherical shape though other shapes are
possible. Also, a combination of spherical and other shapes can be
present. The oxygen reduced niobium oxide preferably forms granules
that have excellent flow properties such as from about 100 to about
1000 mg/s or more, or from about 300 to about 700 mg/s, and more
preferably at least about 300 mg/s as measured by ASTM B 213 using
a 3 mm diameter orifice. In one embodiment of the present
invention, the present invention relates to a oxygen reduced valve
metal oxide that comprises granules preferably having a size of
from about 30 to about 1000 microns and more preferably from about
30 to about 300 microns. As indicated, in one embodiment, the
pressed and sintered oxygen reduced niobium oxides of the present
invention, including the granule form, independently of other
properties, can have a bimodal pore size distribution. In other
words, when the pore size distribution of the pressed and sintered
oxygen reduced niobium oxide granules is measured, and, for
instance, depicted by a graph, at least two major or primary log
differential intrusion peaks are detected with respect to the
primary pore size distribution. Preferably, the pore size
distribution is from about 0.1 to about 10 microns. This bimodal
pore size distribution is especially beneficial for
counterelectrode impregnation thus forming a suitable capacitor or
part thereof. With respect to pore volume, the pressed and sintered
oxygen reduced niobium oxides of the present invention have
excellent pore volumes. Alternatively, the pressed and sintered
granules can have a mono-modal pore size distribution of from about
0.1 to about 10 microns, or a mono-modal pore size distribution
with a shoulder (e.g., extended shoulder) on one side of the
mono-modal log differential intrusion peak with a distribution of
from about 0.1 to about 10 microns. For instance, the niobium
sub-oxides of one embodiment of the present invention with respect
to porosity, when pressed to a press density of 2.8 cc/g and
sintered at 1380.degree. C. for 10 minutes, can have a pore size
distribution with a mono-modal log differential intrusion peak at
0.4 microns, with the mono-modal log differential intrusion peak
having a breadth (or width) of from 0.2 to 0.6 microns at 0.1 mL/g
and the mono-modal log differential intrusion peak can have a
height greater than 0.5 mL/g. These various numbers are with
respect to the measurements, for instance, set forth in the
figures. As another example, the niobium sub-oxides of the present
invention can have a pore size distribution which has a mono-modal
log differential intrusion peak located at 0.5 to 0.8 microns, with
a breadth (or width) of from 0.3 to 1.1 microns at 0.1 mL/g and the
mono-modal log differential intrusion peak height is preferably
greater than 0.6 mL/g. As another example, the niobium sub-oxides
of the present invention can have a pore size distribution wherein
a mono-modal log differential intrusion peak is present with a
shoulder, for instance, extending from 1.3 microns or less to 10
microns or greater with a shoulder height of less than 0.1 mL/g. In
one embodiment, the shoulder that can be present in the pore size
distribution can have a ratio of the cumulative volume located
between 1 and 10 microns, wherein the ratio is from 1 to 7.5. In
another embodiment, the shoulder from the pore size distribution
can have a total porosity of from 4 to 13 percent at a pore size
above 1 micron and/or can have a total porosity of from 1 to 4
percent with the pore sizes of less than 10 microns. The pressed
and sintered granules can have a total pore volume of from about
0.1 to about 0.30 mL/g. Other pore volumes can be achieved.
[0161] The granulated products of the present invention preferably
provide excellent physical properties with respect to bulk density,
flowability, green strength, and pressability of the powders. With
the granulation techniques of the present invention, one can
maintain the desired microstructure and electrical properties of
the fine powders while retaining the physical properties during the
forming process.
[0162] The oxygen reduced niobium oxide granules, when they are
pressed and sintered, preferably have a diametric shrinkage that is
quite beneficial, such as from about 1 to about 12%. Furthermore,
the oxygen reduced niobium oxide granules, once they are pressed
and sintered and form as a body of granules, preferably have a BET
surface area of from about 0.5 to about 4.0 m.sup.2/g. Other BET
surface areas are possible. The niobium granules of the present
invention can preferably be pressed at a pressed density of from
about 2.4 to about 3.5 g/cc and provide sufficient crush
strength.
[0163] The oxygen reduced niobium oxides of the present invention,
in one embodiment, can have a sintered crush strength of at least
35 lbs. More preferably, the sintered crush strength is from 35
lbs. to about 75 lbs. In other embodiments, the sintered crush
strength can be 20 lbs. or greater, such as 25 lbs. or greater, for
instance, 25 lbs. to 75 lbs. The sintered crush strength is based
upon the oxygen reduced niobium oxide or other valve metal oxide
being sintered at 1,380.degree. C. for 10 minutes with a pressed
density of 2.8, an anode diameter of 0.197 inch, an anode height of
0.208 inch with the lead wire not present.
[0164] In one embodiment, the oxygen reduced niobium oxide or valve
metal sub-oxide powder can have a granule strength, which is
substantially independent of screen size. One means to test granule
strength is based on a D.sub.50(NU)/D.sub.50(120 S-U) ratio,
wherein NU means "no ultrasound" and 120 S-U means "120 seconds of
ultrasound." This test determines the strength of the granule when
an ultra sound is applied for 120 seconds and compares it to the
strength of the granule when no ultra sound is applied. By doing
this test, one can readily determine the granule strength of the
oxygen reduced niobium oxide or valve metal sub-oxide powder when
formed into a granule. When using this test, the granule strength
of various oxygen reduced valve metal sub-oxides of the present
invention show excellent consistent granule strength irrespective
of the size of the granule. As shown, for instance, in FIG. 7, the
granule strength of the metal sub-oxide powders was surprisingly
consistent throughout various screen sizes which is quite unusual.
More importantly, even though this granule strength is maintained
throughout various screen sizes, the remaining electrical
properties such as capacitance and DC leakage are also maintained.
Thus, the present invention provides a means as well as a powder
which provides excellent consistent granule strength irrespective
of the screen size which can be quite desirable for various
capacitor manufacturer specifications. In addition, in at least one
embodiment, the present invention relates to oxygen reduced niobium
oxide or valve metal sub-oxide powders that have a
D.sub.50(NU)/D.sub.50(120 S-U) ratio of from about 1.0 to about 3.5
and more preferably, from about 1 to about 3 wherein this granule
strength is substantially independent of granule size and
preferably this granule strength is substantially independent of
granule sizes of from -40 mesh to about -140 mesh. For purposes of
the present invention, an example of substantially independent
means that the granule strength D.sub.50 ratio does not alter by
more than .+-.3 and more preferably by no more than .+-.2 and even
more preferably no more than .+-.1. In addition, or as an option,
the oxygen reduced niobium oxide can have a green crush strength of
1 lb. or greater, more preferably from 1.6 lbs. to 6 lbs. or
greater. In some embodiments, the green crush strength can be 6
lbs. or greater.
[0165] The present invention also relates to a method to control
porosity in valve metal sub-oxide materials which comprises forming
granules and adjusting the granule size to obtain desired porosity.
For instance, a powder can be prepared to have a pore size
distribution to form a log differential intrusion peak which can
have an adjustable peak height of from about 0.4 mL/g to about 0.75
mL/g. This can be adjusted, for instance, by screen size and/or
pre-heat treatment variations. The log differential intrusion peak
height can also vary from about 0.5 mL/g to about 0.6 mL/g. The log
differential intrusion peak can be a mono-modal log differential
intrusion peak with or without a shoulder(s) or can be a part of a
multi-modal distribution.
[0166] Furthermore, as indicated above, the oxygen reduced niobium
oxides, e.g., oxygen reduced niobium oxide granules, preferably
have low metal impurities such as low Fe, Ni, Cr, C, and other low
metal impurities. Preferably, these impurities are less than 500
ppm and more preferably less than 100 ppm, and even more preferably
less than 50 ppm, excluding gases. The oxygen reduced niobium
oxides can also contain levels of nitrogen, e.g., from about 100 to
about 80,000 ppm N.sub.2 or to about 130,000 ppm N.sub.2. Suitable
ranges include from about 31,000 to about 130,000 ppm N.sub.2 and
from about 50,000 to about 80,000 N.sub.2.
[0167] The oxygen reduced niobium oxides of the present invention
have a variety of beneficial properties and characteristics. The
oxygen reduced niobium oxides can have a primary particle size
(D.sub.50) of from about 0.1 micron to about 5 microns. Preferably,
the primary particle size is from about 0.5 micron to about 5
microns. Other suitable ranges are possible. The oxygen reduced
niobium oxides of the present invention also preferably have a
microporous surface and preferably have a sponge-like structure, as
granules. The oxygen reduced niobium oxides of the present
invention can have high specific surface area, and/or a porous
structure having pores from about 0.1 to about 10 microns, and
total porosity of 50% or more. The oxygen reduced niobium oxides
can also have a variety of BET surface areas. The BET surface areas
are with respect to the primary particles. Suitable BET surface
areas include, but are not limited to, at least 0.5 m.sup.2/g or
higher. Other ranges include a BET surface area of from about 1
m.sup.2/g to about 15 m.sup.2/g or higher. Other suitable BET
ranges are possible. The oxygen reduced niobium oxides of the
present invention can be characterized as having a preferred BET
specific surface area of from about 0.5 to about 10.0 m.sup.2/g,
more preferably from about 0.5 to 2.0 m.sup.2/g, and even more
preferably from about 1.0 to about 1.5 m.sup.2/g. The preferred
apparent density of the powder of the oxygen reduced niobium oxides
is less than about 2.0 g/cc, more preferably, less than 1.5 g/cc
and more preferably, from about 0.5 to about 1.5 g/cc.
[0168] The various oxygen reduced niobium oxides of the present
invention can be further characterized by the electrical properties
resulting from the formation of a capacitor anode using the oxygen
reduced niobium oxides of the present invention. In general, the
oxygen reduced niobium oxides of the present invention can be
tested for electrical properties by pressing powders of the oxygen
reduced niobium oxide into an anode and sintering the pressed
powder at appropriate temperatures and then anodizing the anode to
produce an electrolytic capacitor anode which can then be
subsequently tested for electrical properties.
[0169] Accordingly, another embodiment of the present invention
relates to anodes for capacitors formed from the oxygen reduced
niobium oxides of the present invention. Anodes can be made from
the powdered form of the oxygen reduced niobium oxides in a similar
process as used for fabricating metal anodes, i.e., pressing porous
pellets with embedded lead wires or other connectors followed by
optional sintering and anodizing. The lead connector can be
embedded or attached at any time before anodizing. Anodes made from
some of the oxygen reduced niobium oxides of the present invention
can have a capacitance of from about 1,000 CV/g or lower to about
400,000 CV/g or more, and other ranges of capacitance can be from
about 20,000 to about 300,000 CV/g or from about 62,000 to about
200,000 CV/g and preferably from about 40,000 to about 400,000
CV/g. As a further example, anodes made from some of the oxygen
reduced niobium oxides of the present invention can have a
capacitance from about 55,000 to about 175,000 at a 10 V bias. In
forming the capacitor anodes of the present invention, a sintering
temperature can be used which will permit the formation of a
capacitor anode having the desired properties. The sintering
temperature will be based on the oxygen reduced niobium oxide used.
Preferably, the sintering temperature is from about 1,200 to about
1,750.degree. C. and more preferably from about 1,200 to about
1,400.degree. C. and most preferably from about 1,300 to about
1,400.degree. C.
[0170] The sintering temperature in the formation of a capacitor
anode of the present invention can be accomplished at a variety of
temperatures. For instance, the sintering temperature can be
conducted at about 800.degree. C. or lower to about 1,750.degree.
C. or higher. When lower temperatures are used such as on the order
of from about 900.degree. C. or lower to about 1,100.degree. C.,
sintering can occur for any sufficient time to result in a
capacitor anode that provides capacitance. When lower sintering
temperatures are used to form the capacitor anodes of the present
invention, the sintering time is preferably longer than
conventional times used for forming capacitor anodes in general.
For instance, the sintering times can be from about 1 hour to about
10 hours or more (e.g., 1 or more days). As a more specific
example, sintering times can be from about 1 hour to about 5 hours
or from about 2 hours to about 4 hours. These long sintering times
at low sintering temperatures preferably results in an acceptable
capacitance for the capacitor anode as well as a low DC leakage
such as below about 0.5 nanoampheres/CV. In addition, less
shrinkage occurs at these lower sintering temperatures that
preferably yield a more desirable pore structure. For example, with
lower sintering temperatures using the anodes of the present
invention, the number of pores is greater and the diameter of these
pores is larger which results in very beneficial properties in
using these capacitor anodes in electrical applications. For
example, these improved properties with respect to the number of
pores and size of the pores further results in achieving maximum
capacitance retention through the capacitor manufacturing process.
Accordingly, when the various preferred embodiments of the present
invention are used, such as the milling options described above as
well as using lower sintering temperatures, a whole host of
improved properties are achieved with respect to the powder and the
resulting capacitor anode as described herein. Generally, the lower
the sintering temperature, the longer the sintering time for
purposes of achieving the desirable properties such as capacitance,
low DC leakage, and other properties. Thus, if the sintering
temperature is more on the order of about 800.degree. C. the
sintering time will be much longer compared to a sintering
temperature of 1100.degree. C. or more. As stated above and shown
in the examples, the sintering time can be a variety of different
times pending upon the desired properties of the resulting
capacitor anode.
[0171] The anodes formed from the niobium oxides of the present
invention are preferably formed at a voltage (V.sub.f) of about 30
volts and preferably from about 6 to about 80 volts or more. When
an oxygen reduced niobium oxide is used, preferably, the forming
voltages are from about 6 to about 50 V, and more preferably from
about 10 to about 40 volts. The DC leakage achieved by the niobium
oxides of the present invention have provided excellent low leakage
at high formation voltages. Also, the anodes formed from the oxygen
reduced niobium oxides of the present invention preferably have a
DC leakage of less than about 5.0 nA/CV. In an embodiment of the
present invention, the anodes formed from some of the oxygen
reduced niobium oxides of the present invention have a DC leakage
of from about 5.0 nA/CV to about 0.50 nA/CV.
[0172] The oxygen reduced niobium oxides in granulated form or
non-granulated form can have beneficial electrical properties. For
instance, when the oxygen reduced niobium oxide granules of the
present invention are pressed and sintered, beneficial capacitance
and/or low leakage properties are exhibited. In more detail, when
the oxygen reduced niobium oxide granules of the present invention
are sintered at a temperature of 1380.degree. C. for 10 minutes and
formed at 30 volts, and at a formation temperature of 90.degree.
C., wherein the granules are pressed at 2.8 g/cc, the sintered
oxygen reduced niobium oxide granules exhibit a capacitance of from
about 40,000 to about 300,000 CV/g and/or have a leakage current of
less than 0.5 nA/CV. Other capacitance and low leakage values are
possible depending upon the sintering conditions, the sintering
time, the formation voltage, and the like.
[0173] The oxygen reduced niobium oxides, including the granule
form of the present invention, can be formed into capacitor parts
such as capacitor anodes using conventional capacitor formation
techniques which can include one or more of the techniques
mentioned herein. Thus, one embodiment of the present invention is
a capacitor anode containing the oxygen reduced niobium oxides of
the present invention, and having the remaining aspects of an anode
and capacitor, e.g., dielectric layer, and the like.
[0174] Furthermore, with respect to the numerous beneficial
properties described above, such as the primary particle size
(D.sub.50), BET surface area, flow properties, electrical
properties and the like, it is important to appreciate that for
purposes of the present invention, the oxygen reduced niobium
oxides can have at least one of these characteristics, or two or
more of these characteristics, or all of these characteristics. Any
combination of properties and characteristics is possible.
[0175] For instance, the oxygen reduced niobium oxides of the
present invention can have a primary particle size (D.sub.50) of
from about 0.1 to about 5 microns and a granule size, once
granulated, of from about 30 to about 1,000 microns. These oxygen
reduced niobium oxides can also have a BET surface area of from
about 1 to about 15 m.sup.2/g and optionally a flow rate of at
least about 300 mg/s.
[0176] The following paragraph relates to one preferred embodiment.
Other embodiments are possible. As indicated, the primary particles
of the oxygen reduced niobium oxides preferably have a spherical or
essentially a spherical shape. Furthermore, the granules of the
oxygen reduced niobium oxides of the present invention also
preferably have a spherical or substantially spherical shape. When
the powders of the present invention are granulated but not
sintered, the agglomerates are essentially soft agglomerates. In
other words, these agglomerates can easily break up upon hitting a
hard surface. Furthermore, when a soft agglomerate does break, the
powders return rather easily to their primary particle make-up. In
addition, when the soft agglomerates are formed, the primary
particles can easily be identified by SEM techniques. In other
words, there is generally no necking of the primary particles with
each other and the shapes which are essentially spherical are
maintained even when granulated. Also, when the granules are
sintered to form a sintered body, this results in a hard
agglomerate. Importantly, spherical primary particles even after
being sintered essentially maintain their spherical shape and there
is necking of particles with adjacent particles. Essentially, the
primary particles maintain their structural shape and integrity
even though they are in contact with each other. This is quite
beneficial with respect to creating a favorable pore distribution
throughout the sintered body. Again, the sintered body of granules,
even under an SEM analysis, essentially maintains their primary
particle shape. These properties and characteristics, which are
preferred, are especially preferred when the oxygen reduced niobium
oxide is NbO, NbO.sub.0.7, and/or NbO.sub.1.1.
[0177] The oxygen reduced niobium oxide formed from the
above-described preferred process of the present invention can be
combined with a sufficient amount of binder in order to form the
capacitor anode. Preferably, the amount of binder used is from
about 1% to about 5% by weight based on the weight of the capacitor
anode. Suitable binders include, but are not limited to, PEG and
Q-Pac. Other suitable binders are described in one of the earlier
referenced applications which are incorporated in their entireties
by reference herein.
[0178] With respect to the anodes formed from the niobium oxides of
the present invention, the oxygen reduced niobium oxide powder can
optionally be mixed with a binder and/or lubricant solution in an
amount sufficient to be able to form the niobium oxide powder into
an anode when pressed. The amount of the binder and/or lubricant in
the powder can range from about 1 to about 20 wt %, based on the wt
% of the combined ingredients. After mixing the niobium oxide
powder with the binder and/or lubricant solution, the solvent that
may be present as part of the binder/lubricant solution is removed
by evaporation or other drying techniques. Once the solvent, if
present, is removed, the niobium oxide powder is then pressed into
the shape of an anode, preferably with a tantalum, niobium, or
other conductive wire embedded in the anode. While a variety of
press densities can be used, preferably, the pressed density is
from about 2 to about 4 g/cc, and more preferably from about 2.4 to
about 3.5 g/cc. These ranges can also reflect pressability, meaning
the pressed compact maintains its structural integrity for handling
purposes. Once pressed into the anode, a de-binding or de-lube step
occurs to remove the binder and/or lubricant present in the pressed
anode. The removal of the binder and/or lubricant can occur a
number of ways including putting the anode in a vacuum furnace at
temperatures, for instance, of from about 250 to about 1200.degree.
C. to thermally decompose the binder and/or lubricant. The binder
and/or lubricant can also be removed by other steps, such as
repeated washings in appropriate solvents to dissolve and/or
solubilize or otherwise remove the binder and/or lubricant that may
be present. Once the de-binding/de-lube step is accomplished, the
anode is then sintered in a vacuum or under inert atmosphere at
appropriate sintering temperatures, such as from about 900 to about
1900.degree. C. The finished anode then preferably has reasonable
body and/or wire pull strength as well as low carbon residue. The
anodes of the present invention, which use the reduced oxygen
niobium oxides of the present invention, have numerous advantages
over tantalum and/or niobium powders which are formed into anodes.
Many organic binders and/or lubricants, which are used to improve
press performance in the formation of an anode, lead to high carbon
residues that are present after de-binding or de-lubing and
sintering. The full removal of the carbon residue can be extremely
difficult since carbon forms carbides with metals. The presence of
carbon/carbides leads to the formation of defective dielectrics and
thus an undesirable product. With the anodes of the present
invention, the micro-environment of the anode is oxygen-rich. Thus,
when the anode is sintered at high temperature, carbon residue in
the anodes can evaporate as carbon monoxide after reacting with
oxygen. Thus, the anodes of the present invention have a
"self-cleaning" property that is quite different from other anodes
formed from tantalum or niobium. Accordingly, the anodes of the
present invention have a high tolerance of organic impurities
during processing and handling and have the ability to use a wide
range of hydrocarbon containing binders and/or lubricants for
improved processability including improved powder flow, improved
anode green strength, and the like. Accordingly, the binders and/or
lubricants that can be used in the present invention include
organic binders and organic lubricants as well as binders and
lubricants that contain high amounts of hydrocarbons. Examples of
suitable binders that can be used in the formation of the pressed
anodes of the present invention, include, but are not limited to,
poly(propylene carbonates) such as QPAC-40 available from PAC
Polymers, Inc., Greenville, Del.; alkyd resin solutions, such as
GLYPTAL 1202 available from Glyptal Inc., Chelsea, Mass.;
polyethylene glycols, such as CARBOWAX, available from Union
Carbide, Houston, Tex.; polyvinylalcohols, stearic acids, and the
like. The procedures and additional examples of binders and/or
lubricants set forth in Publication Nos. WO 98/30348; WO 00/45472;
WO 00/44068; WO 00/28559; WO 00/46818; WO 00/19469; WO 00/14755; WO
00/14139; and WO 00/12783; and U.S. Pat. Nos. 6,072,694; 6,056,899;
and 6,001,281, all of which are incorporated in their entirety by
reference herein, can be used in the present invention.
[0179] Once the oxygen reduced niobium oxides are formed, as
indicated above, the particles can be mixed with a binder in the
amounts indicated above and then compacted. The compacted particles
can then be crushed sufficiently to form a particle distribution of
from about 100 microns to about 500 microns and more preferably
from about 100 microns to about 300 microns. These particles can
then be pressed into anodes and sintered for anode production using
conventional techniques known to those skilled in the art. As shown
in the Examples, the crush strength of the oxygen reduced niobium
powders of the present invention are significantly improved
compared to previous oxygen reduced niobium oxides and further have
significantly lower leakage.
[0180] The crush strength and other properties can be achieved by
taking the niobium powder formed from the above-described preferred
process of the present invention and combining it with a sufficient
amount of binder to form the capacitor anode. The use of binder is
optional. Preferably, the amount of binder used is from about 1% to
about 5% by weight based on the weight of the capacitor anode.
Suitable binders include, but are not limited to, PEG and Q-Pac.
Other suitable binders are described in one of the earlier
referenced applications, which are incorporated in their entireties
by reference herein. The flow properties of the oxygen reduced
niobium oxides of the present invention are preferably improved as
well as the impurity levels of the surface-passivated niobium
powder, as shown in the examples.
[0181] Once the niobium powder is formed, as indicated above, the
particles can be mixed with a binder in the amounts indicated above
and then optionally compacted. The particles, if desired, can then
be crushed sufficiently to form a particle distribution of from
about 100 microns to about 500 microns, and more preferably from
about 100 microns to about 300 microns. These particles can then be
pressed into anodes and sintered for anode production using
conventional techniques known to those skilled in the art. As shown
in the examples, the crush strength of the surface-passivated
niobium powder of the present invention are significantly improved
compared to previous niobium powders and further have significantly
lower leakage.
[0182] The present invention also relates to a capacitor in
accordance with the present invention having a niobium oxide film
on the surface of the capacitor. Preferably, the film is a niobium
pentoxide film. The capacitor of the present invention can be
formed by any method, for example, as described in U.S. Pat. Nos.
6,527,937 B2; 6,462,934 B2; 6,420,043 B1; 6,375,704 B1; 6,338,816
B1; 6,322,912 B1; 6,616,623; 6,051,044; 5,580,367; 5,448,447;
5,412,533; 5,306,462; 5,245,514; 5,217,526; 5,211,741; 4,805,704;
and 4,940,490, all of which are incorporated herein in their
entireties by reference. For instance, a capacitor can be formed
which is impregnated with an electrolyte, such as a polymer or
MnO.sub.2. As an example, certain oxygen reduced niobium oxides of
the present invention can be more useful with a polymer electrolyte
or other oxygen reduced niobium oxides of the present invention can
be favorable with MnO.sub.2. For instance, when oxygen reduced
niobium oxides of the present invention have a mono-modal porosity
curve which, for instance, can be formed by using a heat treatment
prior to reaction of less than or equal to about 1150.degree. C.,
these oxygen reduced niobium oxides may be more favorably used with
a polymer electrolyte because, for instance, of the high
capacitance, higher sinter crush, and mono-modal porosity curve.
Further, other oxygen reduced niobium oxides, for instance, having
a porosity curve with an extended shoulder out to 10 microns or
more may be more useful for impregnation with MnO.sub.2. These
oxygen reduced niobium oxides, for instance, can be formed at a
higher heat treatment temperature prior to reaction such as greater
than or equal to about 1,300.degree. C.
[0183] In the alternative, the niobium of the present invention can
be used as the starting niobium and mixed with a starting niobium
oxide, e.g., Nb.sub.2O.sub.5, to form oxygen reduced niobium
oxides, as described in U.S. Pat. Nos. 6,416,730; 6,391,275; and
6,322,912; U.S. patent application Ser. No. 09/533,430 filed Mar.
23, 2000; and U.S. Provisional Patent Application Nos. 60/100,629
filed Sep. 16, 1998; 60/229,668 filed Sep. 1, 2000; and 60/246,042
filed Nov. 6, 2000 and all of these applications are incorporated
herein by reference in their entirety.
[0184] Preferably, the granules of the present invention have a
capacitance of from about 35,000 to about 300,000 CV/g and a
leakage current of from about 0.2 to about 2 nA/CV when said
granules are sintered at a temperature of 1125.degree. C. for 10
minutes at a V.sub.f of 40 V. The niobium of the present invention
can be sintered at a range of different temperatures, such as from
about 1050.degree. C. to 1300.degree. C. Preferred formation
voltages include from about 20V to about 40V.
[0185] The niobium powders of the present invention have a variety
of beneficial properties and characteristics. The niobium powders
can have a primary particle size (D.sub.50) of less than 1 micron
(e.g., 0.75 micron to about 0.9 micron) to about 5 microns.
Preferably, the primary particle size is from about 1 micron to
about 4 microns. Other suitable ranges include from about 2 microns
to about 3 microns, from about 3 microns to about 4 microns, less
than 1 micron, or from about 1 micron to about 2 microns, and the
like.
[0186] The niobium powders of the present invention, in one
embodiment, can be in granulated or granule form. The granule size
can be a variety of sizes such as from about 30 microns to about
1,000 microns. These granules can have a spherical shape though
other shapes are possible. Also, a combination of spherical and
other shapes can be present. The niobium powders also can have an
oxygen content. The oxygen content is preferably at least 1,000 ppm
or higher. Preferred ranges include from about 5,000 ppm to about
28,000 ppm or higher. These oxygen contents are preferably with
respect to the primary particles of the niobium powder. Suitable
ranges of oxygen contents for the niobium powder include, but are
not limited to, from about 5,000 ppm to about 28,000 ppm, from
about 15,000 ppm to about 22,000 ppm, from about 22,000 ppm to
about 28,000 ppm, from about 35,000 ppm to 55,000 ppm, and from
about 5,000 ppm to about 15,000 ppm.
[0187] The niobium powders can also have a variety of BET surface
areas. The BET surface areas are with respect to the primary
particles. Suitable BET surface areas include, but are not limited
to, at least 0.5 micron or higher m.sup.2/g. Other ranges include a
BET surface area of from about 1 m.sup.2/g to about 4.5 m.sup.2/g
or higher, from about 1 m.sup.2/g to about 2 m.sup.2/g, from about
2 m.sup.2/g to about 3 m.sup.2/g, from about 3 m.sup.2/g to about
4.5 m.sup.2/g, from about 5 m.sup.2/g to about 7 m.sup.2/g, and
various ranges in between these BET surface area sizes.
[0188] As indicated above, the niobium granules can have beneficial
flow properties such as greater than about 300 mg/s using the flow
test previously described.
[0189] The niobium powder in granulated form or non-granulated form
can have beneficial electrical properties. For instance, when the
niobium granules of the present invention are pressed and sintered,
beneficial capacitance and/or low leakage properties are exhibited.
In more detail, when the niobium granules of the present invention
are sintered at a temperature of from about 1050.degree. C. to
about 1250.degree. C. for 10 minutes and formed at 40 volts,
wherein the granules are pressed at 2.8 g/cc, the sintered niobium
granules exhibit a capacitance of from about 35,000 to about
300,000 CV/g and/or having leakage current of from about 0.2 to
about 2 nA/CV. Other capacitance and low leakage values are
possible depending upon the sintering conditions, the sintering
time, the formation voltage, and the like.
[0190] Also, the niobium granules of the present invention
independently of other properties preferably have a bimodal pore
size distribution. In other words, when the pore size distribution
of the pressed and sintered Nb granules is measured, and depicted
by a graph, at least two peaks are detected with respect to the
primary pore size distribution. Preferably, the pore size
distribution is from about 0.1 to about 10 microns. This bimodal
pore size distribution is especially beneficial for
counterelectrode impregnation thus forming a suitable capacitor or
part thereof.
[0191] The niobium granules when they are pressed and sintered
preferably have a diametric shrinkage that is quite beneficial such
as from about 1 to about 12%. Furthermore, the Nb granules once
they are pressed and sintered and form a body of granules
preferably have a BET surface area of from about 0.5 to about 4.0
m.sup.2/g. Other BET surface areas are possible. The niobium
granules of the present invention can preferably be pressed at a
pressed density of from about 2.4 to about 3.5 g/cc.
[0192] Furthermore, the niobium, e.g., niobium granules, preferably
have low metal and other impurities such as low Fe, Ni, Cr, C, and
other metal impurities. Preferably, these impurities are less than
500 ppm and more preferably less than 100 ppm, excluding gases.
[0193] The niobium powder including the niobium granules of the
present invention can be formed into capacitor parts such as
capacitor anodes using conventional capacitor formation techniques
which can include one or more of the techniques mentioned above.
Thus, one embodiment of the present invention is a capacitor anode
containing the niobium powders of the present invention, and having
the remaining aspects of an anode and capacitor, e.g., dielectric
layer and the like.
[0194] Furthermore, with respect to the numerous beneficial
properties described above, such as the primary particle size
(D.sub.50), BET surface area, flow properties, oxygen contents,
electrical properties, and the like, it is important to appreciate
that for purposes of the present invention, the niobium powder can
have at least one of these characteristics, or two or more of these
characteristics, or all of these characteristics. Any combination
of properties and characteristics is possible.
[0195] For instance, the niobium powder of the present invention
can have a primary particle size (D.sub.50) of from about 3 to
about 4 microns and a granule size, once granulated, of from about
30 to about 1,000 microns. These niobium powders can also have a
BET surface area of from about 1 to about 2 m.sup.2/g and
optionally an oxygen content of from about 5,000 ppm to about
15,000 ppm. In another embodiment, niobium powders can have a
primary particle size of from about 2 to about 3 microns and once
granulated, can have a granule size of from about 30 microns to
about 1,000 microns. These niobium powders can also have a BET
surface area of from about 2 to about 3 m.sup.2/g and optionally
oxygen contents of from about 5,000 ppm to about 22,000 ppm and
more preferably from about 15,000 ppm to about 22,000 ppm.
[0196] In another embodiment, niobium powders can have a primary
particle size (D.sub.50) of from about 1 micron to about 2 microns,
and once granulated, have a granule size of from about 30 microns
to about 1,000 microns. These niobium powders can also have a BET
surface area of from about 3 to about 4.5 m.sup.2/g and optionally
oxygen contents of from about 5,000 ppm to about 28,000 ppm or
more, and preferably from about 22,000 ppm to about 28,000 ppm.
[0197] In another embodiment, niobium powders of the present
invention can have a primary particle size (D.sub.50) of less than
1 micron and once granulated, can have a granule size of from about
30 to about 1,000 microns. These powders can also have a BET
surface area greater than 4.5 m.sup.2/g and can optionally have
oxygen contents of from about 5,000 ppm to 28,000 ppm or higher and
more preferably 8,000 ppm or higher.
[0198] The following paragraphs relate to one preferred embodiment.
As indicated, the primary particles of the niobium powder
preferably have a spherical or essentially a spherical shape.
Furthermore, the granules of the niobium of the present invention
also preferably have a spherical or substantially spherical shape.
When the powders of the present invention are granulated but not
sintered, the agglomerates are essentially soft agglomerates. In
other words, these agglomerates can easily break up upon hitting a
hard surface. Furthermore, when a soft agglomerate does break, the
powders return rather easily to their primary particle make-up. In
addition, when the soft agglomerates are formed, the primary
particles can easily be identified by SEM techniques. In other
words, there is generally no necking of the primary particles with
each other and the shapes which are essentially spherical are
maintained even when granulated. Also, when the granules are heat
treated and/or sintered, this results in a hard agglomerate.
Importantly, spherical primary particles even after being sintered
essentially maintain their spherical shape and there is necking of
particles with adjacent particles. Essentially, the primary
particles maintain their structural shape and integrity even though
they are in contact with each other. This is quite beneficial with
respect to creating a favorable pore distribution throughout the
sintered body. Again, the sintered body of granules, even under an
SEM analysis, essentially maintains their primary particle shape
and are easily identifiable as primary particles.
[0199] In one embodiment of the present invention, a method to at
least partially reduce a niobium oxide according to the present
invention includes heat treating a starting niobium oxide in the
presence of a getter material and in an atmosphere which permits a
transfer of oxygen atoms from the starting niobium oxide to the
getter material, for a time and at a temperature sufficient to form
an oxygen reduced niobium oxide, wherein the getter material is or
includes titanium. Preferably, the getter material is a titanium
sponge.
[0200] For purposes of the present invention, the term "morphology"
can denote the shape, microstructure, form, and/or other observable
or nonobservable characteristics of the starting niobium oxide,
intermediate niobium oxides, or niobium suboxides of the present
invention. Preferably, the morphology can be determined upon visual
inspection utilizing scanning electron microscope photographs
(SEMs) or micrographs as desired. Other properties may be
determined by procedures which are known and generally utilized in
the relevant art.
[0201] In detail, the starting niobium oxide used in the present
invention can be at least one oxide of niobium metal and/or alloys
thereof. A specific example of a starting niobium oxide is
Nb.sub.2O.sub.5. The starting niobium oxide can have any
morphology, and preferably has an interconnected or cellular
morphology. The starting niobium oxide can be in any shape or size.
Preferably, the starting niobium oxide is in the form of a powder
or a plurality of particles. Examples of the type of powder that
can be used include, but are not limited to, flaked, angular,
nodular, spherical, and mixtures or variations thereof. Preferably,
the starting niobium oxide is in the form of a powder that more
effectively leads to the niobium suboxide. Examples of such
preferred starting niobium oxide powders include those having mesh
sizes of from about 60/100 to about 100/325 mesh and from about
60/100 to about 200/325 mesh. Another range of size is from -40
mesh to about +325 mesh, or a size of -325 mesh. Preferably, the
starting niobium oxide has an average primary particle size
(D.sub.50) of from about 0.25 to about 5 microns, and a BET surface
area of at least about 0.5 m.sup.2/g, for example, from about 1 to
about 8 m.sup.2/g. The starting niobium oxide preferably has a
particle size distribution range in which the D.sub.10, D.sub.90,
or both is within 300% of the D.sub.50. The starting niobium oxide
preferably has an apparent density of from about 0.3 to about 2.0
g/cc. The starting niobium oxide preferably has a porous
microstructure (due to mean pore size, the number of pores, and/or
total pore volume) having pores of from about 0.1 to about 100
micrometers. The starting niobium oxide preferably has a pore
volume of from about 10 to about 90%. The starting niobium oxide
can have a monomodal or a multimodal pore size distribution, and
preferably has a bimodal pore size distribution. Measurements
relating to the porosity of the starting niobium oxide can be made,
for example, as described in U.S. Pat. Nos. 6,576,038 B1, and
6,479,012 B1, and Published U.S. Patent Application Nos.
2003/0115985, and 2002/0033072, each of which is incorporated in
its entirety herein by reference. The starting niobium oxide
preferably has excellent flow properties such as from about 100 to
about 2000 mg/s or more, and more preferably at least about 200
mg/s as measured by ASTM B 213 using a 3 mm diameter orifice. The
starting niobium oxide can contain a range of modifying agents or
additives or dopants, including nitrogen, silicon, phosphorous,
boron, carbon, sulfur, yttrium, or combinations thereof. The
starting niobium oxide can be nitrided and/or contain a nitride
layer.
[0202] The oxygen-active or getter material used in practicing the
present invention can be any solid or non-solid material containing
titanium that is capable of reducing the starting niobium oxide to
an oxygen reduced niobium oxide or niobium suboxide. The getter
material can be any material containing titanium that facilitates
the removal of oxygen atoms from the starting niobium oxide.
Preferably, the getter material is an oxygen-active titanium
material that has a greater affinity for oxygen than does the
starting niobium oxide. Preferably, the getter material is a
titanium-containing substance that allows the starting niobium
oxide to be at least partially reduced without substantially
alteration of the morphology of the starting niobium oxide in the
process. The getter material can be any commercially available
titanium material and/or titanium material that is prepared by any
known method.
[0203] The getter material for purposes of the present invention is
any material containing titanium metal that can remove or reduce at
least partially the oxygen in the niobium oxide. Thus, the getter
material can be an alloy (e.g., Ti--Zr), or a material containing
mixtures of titanium metal with other constituents. Preferably, the
getter material is predominantly, if not exclusively, titanium
metal. The purity of the getter material is not important but it is
preferred that the getter material comprise high purity titanium to
avoid the introduction of undesirable impurities during the heat
treating process. Accordingly, the titanium metal in the getter
material preferably has a purity of at least about 98%, and more
preferably at least about 99%. Preferably, impurities that affect
DC leakage, such as iron, nickel, chromium, and carbon, are below
about 100 ppm.
[0204] The getter material can be in any size, shape, or form such
as sheet, sponge, or powder material. Preferably, the getter
material is in a form having a superior surface area for reducing
the niobium oxide. For instance, the getter material can be in the
form of a tray which contains the starting niobium oxide or can be
in a particle or powder size. The getter material can be flaked,
spherical, angular, nodular, and mixtures or variations thereof,
e.g., coarse granular material having a particle size of from about
0.1 to about 10 mm that can be readily separated from the niobium
suboxide by screening or by acid leaching, for instance. Most
preferably, the getter material is a titanium sponge metal. The
titanium sponge can be any commercially available titanium sponge
and/or titanium sponge produced by any known method, as described,
for example, in U.S. Pat. No. 6,226,173, which is incorporated
herein in its entirety by reference.
[0205] Preferably, the titanium sponge has a morphology that leads
to efficient mass transfer kinetics in the reduction process, which
results in a smaller quantity of getter material necessary to
reduce a given amount of starting niobium oxide. Preferably, the
titanium sponge has a morphology characterized in that it readily
reacts with oxygen atoms from the starting niobium oxide. For
example, the titanium sponge preferably has a high accessible
surface area to volume ratio. The titanium sponge preferably has a
specific (BET) surface area, such as a BET of from about 0.01 to
about 2 m.sup.2/g and more preferably of from about 0.01 to about
0.1 m.sup.2/g. As another example, the titanium sponge preferably
has a microporous structure (due to mean pore size, the number of
pores, and/or total pore volume) having many accessible (i.e., not
enclosed), large pores. Preferably, the titanium sponge has pores
of from about 10 to about 100,000 nanometer (nm). The titanium
sponge preferably has a pore volume of from about 10 to about 80%.
Preferably, the titanium sponge has a high surface area as well as
an easily accessible open porosity.
[0206] Generally, a sufficient amount of getter material is present
to at least partially reduce the niobium oxide being heat treated.
Further, the amount of the getter material is dependent upon the
amount of reducing desired to the niobium oxide. For instance, if a
slight reduction in the niobium oxide is desired, then the getter
material will be present in a stoichiometric amount. Similarly, if
the niobium oxide is to be reduced substantially with respect to
its oxygen presence, then the getter material is present in an
amount of about 2 to 5 times stoichiometric amount. Generally, the
amount of getter material (e.g., based on the getter material being
100% titanium) can be present in a getter material to starting
niobium oxide ratio of from about 0.25 to 0.50 to about 1.5 to 2.0.
Preferably, the getter material and the starting niobium oxide are
present in a wt. ratio of about 1:less than 4.
[0207] An example of a preferred reduction of Nb.sub.2O.sub.5 to
NbO can be generally represented as follows:
2Nb.sub.2O.sub.5+3Ti=4NbO+3TiO.sub.2 (Eq. 4)
As can be seen from Eq. 4, the reduction process preferably
involves the transfer of oxygen atoms from the starting niobium
oxide, e.g., niobium pentoxide, to the getter material, e.g.,
titanium sponge, to form the desired niobium suboxide and a
titanium oxide. The niobium suboxide is any niobium oxide which has
a lower oxygen content in the metal oxide compared to the starting
niobium oxide. The niobium suboxide can have oxygen levels that are
less than stoichiometric for a fully oxidized niobium. Typical
niobium suboxides comprise NbO, NbO.sub.0.7, NbO.sub.1.1,
NbO.sub.2, and any combination thereof with or without other oxides
present. Generally, the niobium suboxide of the present invention
has an atomic ratio of niobium to oxygen of about 1:less than 2.5,
and preferably 1:2 or less, or 1:less than 1.5, and more preferably
1:1.1, 1:1, or 1:0.7. In other words, the niobium suboxide
preferably has the formula Nb.sub.xO.sub.y, wherein Nb is niobium,
x is 2 or less, and y is less than 2.5x. More preferably x is 1 and
y is less than 2, such as 1.1, 1.0, 0.7, and the like. Preferably,
the niobium suboxide of the present invention is an NbO, an oxygen
depleted NbO, an aggregate or agglomerate which contains NbO and
niobium metal, or a niobium metal with a rich oxygen content.
Unlike NbO, NbO.sub.2 is less desirable due to its resistive
nature, whereas NbO is very conductive. Accordingly, capacitor
anodes which are formed from NbO, oxygen depleted NbO, or a mixture
of NbO with niobium metal are desirable and preferred for purposes
of the present invention.
[0208] According to one embodiment, the reduction process of the
present invention provides the desired control over the morphology,
microstructure, particle size distribution, and the like of the
final product, i.e., the niobium suboxide. Preferably, the starting
niobium oxide and the niobium suboxide have substantially similar
morphologies, and more preferably, substantially indistinguishable
morphologies. Preferably, the niobium oxide remains in a solid
state throughout the reduction process. Preferably, the starting
niobium oxide has a cellular or porous microstructure, and the
niobium suboxide has a cellular or porous microstructure.
[0209] The heat treatment of the starting niobium oxide can be
achieved in any heat treatment device or furnace commonly used in
the heat treatment of metals, such as niobium and tantalum. For
instance, heat treating can be conducted in any reaction system or
reactor such as a retort, vacuum chamber, vacuum furnace, or a
vacuum kiln, as described, for example, in U.S. Pat. Nos. 6,380,517
B2; 6,271,501 B1; and 6,105,272 each of which is incorporated in
its entirety herein by reference. The heat treatment of the
starting niobium oxide in the presence of the getter material is at
a temperature and for a time sufficient to form the niobium
suboxide. The temperature and time of the heat treatment can be
dependent on a variety of factors such as the amount of reduction
of the niobium oxide, the amount and type of the getter material,
as well as the type of starting niobium oxide. Generally, heat
treating the niobium oxide can be at a temperature of from less
than or about 800 to about 1,900.degree. C., and more preferably is
from about 800 to about 1,400.degree. C., and most preferably is
from about 800 to about 1,100.degree. C., for a time of from about
5 to about 600 minutes, and more preferably of from about 30 to
about 120 minutes.
[0210] The getter material need not be in physical contact with the
starting niobium oxide, but is preferably disposed in close
proximity to the niobium oxide. The getter material can be blended
or mixed together with the starting niobium oxide prior to or
during the heat treatment. The titanium oxide formed in the
reaction process, and any residual titanium can be substantially
removed from the oxygen reduced niobium oxide by screening or
sieving techniques, for example, and/or by leaching in a
concentrated hydrofluoric, HCl, or nitric acid solution, i.e., acid
leaching, for instance.
[0211] Heat treatment is preferably achieved in an atmosphere that
permits the transfer of oxygen atoms from the niobium oxide to the
getter material (e.g., a hydrogen atmosphere), and preferably at a
temperature of from about 900 to about 1,100.degree. C. Preferably,
the atmosphere is under vacuum or is an inert atmosphere. Gas(es)
such as hydrogen and argon can be, but need not be present in the
atmosphere. Preferably, the atmosphere is present during the heat
treatment at a pressure of from about 1.times.10.sup.-5 to about
2,000 torr, and more preferably from about 100 to about 1,000 torr,
and most preferably from about 100 to about 930 torr. During the
reacting process, a constant heat treatment temperature can be used
during the entire reacting process or variations in temperature or
temperature steps can be used. For instance, the atmosphere can be
initially admitted at 900.degree. C., followed by an increase in
the temperature to 1,000.degree. C. for 30 minutes, followed by a
decrease in the temperature to 1,000.degree. C., and held there
until removal of the atmosphere. After the atmosphere is removed,
the furnace temperature can be dropped. Variations of the above
steps can be used to suit any preferences of the industry. Routine
testing in view of the present application will permit one skilled
in the art to readily control the times and temperatures of the
heat treatment to obtain the proper or desired reduction of the
niobium oxide.
[0212] In general, the materials, processes, and various operating
parameters as described in U.S. Pat. Nos. 6,563,695 B1; 6,527,937
B2; 6,517,645 B2; 6,462,934 B2; 6,432,161 B1; 6,420,043 B1;
6,416,730 B1; 6,402,066 B1; 6,391,275 B1; 6,338,832 B1; 6,338,816
B1; 6,322,912 B1; 6,312,642 B1; 6,231,689 B1; 6,165,623; 6,071,486;
6,051,044; 6,051,326; 5,993,513; 5,986,877, 5,954,856; 5,580,516;
5,284,531; 5,261,942; 5,242,481; 5,234,491; 5,171,379; 4,960,471;
4,722,756; 4,684,399; and 4,645,533, and Published U.S. Patent
Application Nos. 2003/0115985 A1; 2003/0082097 A1; 2003/0057304 A1;
2003/0037847 A1; 2003/0026756 A1; 2003/0003044 A1; 2002/0172861 A1;
2002/0135973 A1; 2002/0072475 A1; 2002/0028175 A1; 2002/0026965 A1;
and 2001/0036056 A1, can be used in the present invention, and each
are incorporated herein in their entirety by reference.
[0213] The various niobium suboxides of the present invention can
be further characterized by the electrical properties resulting
from the formation of a capacitor anode using the niobium suboxides
of the present invention. In general, the niobium suboxides of the
present invention can be tested for electrical properties by
pressing powders of the niobium suboxide into an anode and
sintering the pressed powder at appropriate temperatures and then
anodizing the anode to produce an electrolytic capacitor anode
which can then be subsequently tested for electrical
properties.
[0214] Accordingly, another embodiment of the present invention
relates to anodes for capacitors formed from the niobium suboxides
of the present invention. The capacitor of the present invention
can be formed by any method, for example, as described in U.S. Pat.
Nos. 6,576,099 B2; 6,576,038 B1; 6,563,695 B1; 6,562,097 B1;
6,527,937 B2; 6,479,012 B1; 6,462,934 B2; 6,420,043 B1; 6,416,730
B1, 6,375,704 B1; 6,373,685 B1, 6,338,816 B1; 6,322,912 B1;
6,165,623; 6,051,044; 5,986,877, 5,580,367; 5,448,447; 5,412,533;
5,306,462; 5,245,514; 5,217,526; 5,211,741; 4,805,704; and
4,940,490, and Published U.S. Patent Application Nos. 2003/0115985
A1; 2003/0026756 A1; 2003/0003044 A1; 2002/0179753 A1; 2002/0152842
A1; 2002/0135973 A1; 2002/0124687 A1; 2002/0104404 A1; 2002/0088507
A1; 2002/0072475 A1; 2002/0069724 A1; 2002/0050185 A1; 2002/0028175
A1; and 2001/0048582 A1, each of which is incorporated herein in
its entirety by reference. The capacitors can be used in a variety
of end uses such as automotive electronics, cellular phones,
computers, such as monitors, mother boards, and the like, consumer
electronics including TVs and CRTs, printers/copiers, power
supplies, modems, computer notebooks, disc drives, and the like.
Anodes can be made from the powdered form of the niobium suboxides
in a similar process as used for fabricating metal anodes, i.e.,
pressing porous pellets with embedded lead wires or other
connectors followed by optional sintering and anodizing. The lead
connector can be embedded or attached at any time before anodizing.
Anodes made from some of the niobium suboxides of the present
invention can have a capacitance of from about 1,000 CV/g or lower
to about 400,000 CV/g or more, and other ranges of capacitance can
be from about 20,000 to about 300,000 CV/g or from about 62,000 to
about 200,000 CV/g, and preferably from about 40,000 to about
400,000 CV/g. In forming the capacitor anodes of the present
invention, a sintering temperature can be used which will permit
the formation of a capacitor anode having the desired properties.
The sintering temperature will be based on the niobium suboxide
used. Preferably, the sintering temperature is from about 1,200 to
about 1,750.degree. C. and more preferably from about 1,200 to
about 1,400.degree. C. and most preferably from about 1,300 to
about 1,400.degree. C.
[0215] The sintering temperature in the formation of a capacitor
anode of the present invention can be accomplished at a variety of
temperatures. For instance, the sintering temperature can be
conducted at about 800.degree. C. or lower to about 1,750.degree.
C. or higher. When lower temperatures are used such as on the order
of from about 900.degree. C. or lower to about 1,100.degree. C.,
sintering can occur for any sufficient time to result in a
capacitor anode that provides capacitance. When lower sintering
temperatures are used to form the capacitor anodes of the present
invention, the sintering time is preferably longer than
conventional times used for forming capacitor anodes in general.
For instance, the sintering times can be from about 1 to about 10
hours or more (e.g., 1 or more days). As a more specific example,
sintering times can be from about 1 to about 5 hours or from about
2 to about 4 hours. These long sintering times at low sintering
temperatures preferably results in an acceptable capacitance for
the capacitor anode as well as a low DC leakage such as below about
0.5 nA/CV. In addition, less shrinkage occurs at these lower
sintering temperatures that preferably yield a more desirable pore
structure. For example, with lower sintering temperatures using the
anodes of the present invention, the number of pores is greater and
the diameter of these pores is larger which results in very
beneficial properties in using these capacitor anodes in electrical
applications. For example, these improved properties with respect
to the number of pores and size of the pores further results in
achieving maximum capacitance retention through the capacitor
manufacturing process. Accordingly, when the various preferred
embodiments of the present invention are used, such as the milling
options described above as well as using lower sintering
temperatures, a whole host of improved properties are achieved with
respect to the powder and the resulting capacitor anode as
described herein. Generally, the lower the sintering temperature,
the longer the sintering time for purposes of achieving the
desirable properties such as capacitance, low DC leakage, and other
properties. Thus, if the sintering temperature is more on the order
of about 800.degree. C. the sintering time will be much longer
compared to a sintering temperature of 1,100.degree. C. or more. As
stated above and shown in the examples, the sintering time can be a
variety of different times pending upon the desired properties of
the resulting capacitor anode.
[0216] The anodes formed from the niobium oxides of the present
invention are preferably formed at a voltage (V.sub.f) of from
about 6 to about 80 volts or more and preferably at about 10 to 40
volts. When a niobium suboxide is used, preferably, the forming
voltages are from about 6 to about 80 V, and more preferably from
about 10 to about 40 volts. The DC leakage achieved by the niobium
oxides of the present invention have provided excellent low leakage
at high formation voltages. Also, the anodes formed from the
niobium suboxides of the present invention preferably have a DC
leakage of less than about 5.0 nA/CV. In an embodiment of the
present invention, the anodes formed from some of the niobium
suboxides of the present invention have a DC leakage of from about
5.0 nA/CV to about 0.1 nA/CV.
[0217] The present application also can be applied to other valve
metal suboxides such as those described in U.S. Pat. No. 6,322,912,
incorporated in its entirety by reference herein.
[0218] While the above-described embodiments have been discussed
using niobium as one preferred material, the present invention
equally applies to other valve metals and hydrided forms thereof as
described, for example, in U.S. Pat. No. 6,322,912 B1, which is
incorporated in its entirety by reference herein.
[0219] The powder, granules, pressed and/or sintered versions, and
anodes thereof can be packaged using vacuum packing, such that the
oxygen permeability is less than 1.0 cc/100 in.sup.2/day and more
preferably is less than 0.01 cc/100 in.sup.2/24 hrs. Bags from
Fres-Co System USA can be used optionally in conjunction with a
Fres-Co vacuum packaging machine or other similar packaging
device.
[0220] The capacitors of the present invention can be used in a
variety of end uses such as automotive electronics, cellular
phones, computers, such as monitors, mother boards, and the like,
consumer electronics including TVs and CRTs, printers/copiers,
power supplies, modems, computer notebooks, disc drives, and the
like.
[0221] The powder, granules, pressed and/or sintered versions, and
anodes thereof can be packaged using vacuum packing, such that the
oxygen permeability is less than 1.0 cc/100 in.sup.2/day and more
preferably is less than 0.01 cc/100 in.sup.2/24 hrs. Bags from
Fres-Co System USA can be used optionally in conjunction with a
Fres-Co vacuum packaging machine or other similar packaging
device.
[0222] The present invention will be further clarified by the
following examples, which is intended to be exemplary of the
present invention.
EXAMPLES
Example 1
[0223] According to an embodiment of the invention, niobium powder
(200 g) having a BET surface area of 4.1 m.sup.2/g and starting
niobium oxide (161 g), i.e., niobium pentoxide, with a BET surface
area of 1.3 m.sup.2/g were mixed together to form a powder mixture.
The powder mixture was then granulated and screened to a granular
size of about -40 mesh. The granular powder mixture was then placed
into a vacuum heat treatment furnace and heated under vacuum to
about 700.degree. C. for about 2 hours. Hydrogen gas was then
admitted to the furnace to a pressure of about 960 Torr. The
temperature in the furnace was then brought to about 850.degree. C.
and held for about 1 hour. After formation, the oxygen reduced
niobium oxide was tested for certain properties which are set forth
in Table 1. Three more trials were made with three more sample lots
of niobium powder and starting niobium oxide wherein the above
described method was repeated with heat treatment temperatures of
900, 1,100, and 1,300.degree. C., respectively. In these trials,
after the heat treatment under vacuum, the powder mixture was
cooled to below 850.degree. C. before the hydrogen gas was admitted
to the furnace. The oxygen reduced niobium oxide formed in the
three trials were tested for same properties as for the final
product of the first trial, and the results are set forth in Table
1. Table 1 also includes the observed properties of an oxygen
reduced niobium oxide in which the step of heat treatment under
vacuum was omitted. The pore size distribution and cumulative pore
volumes for the pressed and sintered oxygen reduced niobium oxides
formed in the five trials were also determined and are graphed in
FIGS. 1 and 2, respectively. In addition to the particular
properties set forth in Table 1 and in FIGS. 1 and 2, it was
observed that the oxygen reduced niobium oxides could also be
pressed at lower press densities and still maintain acceptable
crush.
TABLE-US-00001 TABLE 1 Nb Nb2O5 NbO Nb + Nb2O5 Nb Final BET BET
Nb/O Trial # Mixing Method Type m2/g m2/g Mole Ratio 1 BMA SFG 4.16
1.34 0.98 2 BMA-HT-700 SFG 4.16 1.34 0.98 3 BMA-HT-900 SFG 4.16
1.34 0.98 4 BMA-HT-1100 SFG 4.16 1.34 0.98 5 BMA-HT-1300 SFG 4.16
1.34 0.98 POST REACTION TEST DATA React DCL @ Anode NbO BET Scott
Crush CV/g CV/g 180 sec Shrink BET Trial # m2/g g/in3 Lb 2.5 V 10 V
nA/CV % m2/g 1 3.42 14.6 4.65 86530 71111 0.54 8.78 1.184 2 3.16
18.8 3.87 80364 68199 0.15 8.5 1.116 3 3.01 18.8 3.62 87694 72610
0.15 6.5 1.164 4 2.59 20.1 1.94 78849 71429 0.78 3 1.074 5 1.40
21.2 0.87 74258 62722 0.14 1 0.931 HT = Heat Treatment under vacuum
at stated temp .degree. C. BMA = Ball Milled and Agglomerated with
water with agitation SFG = Super fine granular-nodular
Example 2
[0224] According to an embodiment of the invention, niobium hydride
powder (20 g) having a BET surface area of 3.9 m2/g and starting
niobium oxide (16.1 g), i.e. niobium pentoxide, with a BET surface
area of 3.9 m2/g were mixed together to form a powder mixture. The
powder mixture was then attritor milled (comilled) in water for 2
hours using 3/16'' Nb media. The powder slurry was vacuum dried at
120.degree. C. and granulated by passing the dried powder over a 50
mesh screen. The granular powder mixture was then placed into a
vacuum heat treatment furnace and heated to 1100.degree. C. for
about 2 hours. The temperature in the furnace was then brought to
about 850.degree. C. and hydrogen gas was admitted to the furnace
to a pressure of about 960 Torr and held for about 1 hour. After
formation, the oxygen reduced niobium oxide was tested for certain
properties which are set forth in Table 2. In addition, sample lots
of niobium hydride and niobium pentoxide were mixed as described
above without the attritor milling step. Powder mixtures were
granulated and heat treated as described above. The oxygen reduced
niobium oxide formed in this manner was tested in the same manner
and the results are set forth in Table 2. The pore size
distribution and cumulative pore volumes for the press and sintered
oxygen reduced niobium oxides at 2.8 g/cc are shown in FIGS. 3 and
4.
TABLE-US-00002 TABLE 2 Scott Flow Crush CV/g CV/g DCL Anode BET Lot
# g/in3 s/25 g lbs 1.5 V 10 V 180 s Shrink % m2/g Comilled
9026-91-B6 24 18 7.2 137500 94500 0.19 5.95 1.53 yes 9026-30-2A
23.5 23 2.81 129466 87000 0.15 4.07 1.46 no
Example 3
[0225] According to an embodiment of the invention, niobium hydride
powder (400 g) having a BET surface area of 3.9 m2/g and starting
niobium oxide (322 g), i.e. niobium pentoxide, with a BET surface
area of 3.9 m2/g were mixed together to form a powder mixture. The
powder mixture was then attritor milled (comilled) in water for 2
hours using 3/16'' Nb media. The powder slurry was vacuum dried at
120.degree. C. and granulated by passing the dried powder over a 50
mesh screen. In addition, some of the powder was passed over a 70
mesh, 100 mesh and 140 mesh screen. The sample passing through the
appropriate screen was collected. The granular powder mixture was
then placed into a vacuum heat treatment furnace and heated to
1400.degree. C. for about 2 hours. The temperature in the furnace
was then brought to about 850.degree. C. and hydrogen gas was
admitted to the furnace to a pressure of about 960 Torr and held
for about 1 hour. After formation, the oxygen reduced niobium oxide
powders were tested for certain properties which are set forth in
Table 3. The pore size and cumulative pore volumes for the pressed
and sintered oxygen reduced niobium oxides screened at different
mesh sizes were also determined and are graphed in FIGS. 5 and 6.
In addition to the tests performed in Table 3 and FIGS. 5 and 6, it
was observed that the granules had a high degree of strength.
Granules that were heat treated were subjected to particle size
measurement that included with and without sonication of an
ultrasonic probe. A ratio of the d.sub.50 values without and with
sonication for two minutes was determined and plotted in FIG. 7. As
shown in FIG. 7 granulated samples had ratios of .about.2 or less
and the granule strengths were independent of granule size.
TABLE-US-00003 TABLE 3 granule size Scott Crush Flow CV/g CV/g DCL
Lot # mesh g/in3 lbs s/25 g 1.5 V 10 V 180 s Shrink % d50/d50
9030-30-46-36 E -50 23 0.89 20.4 111990 80200 0.15 0.41 1.36
9030-30-46-36 E1 -70 22.3 0.8 21 112100 80300 0.12 0.43 2.02
9030-30-46-36 E2 -100 21 1.39 23.3 112500 80100 0.11 0.6 1.79
9030-30-46-36 E3 -140 20.4 1.29 34 111500 79900 0.12 0.56 2.1
Example 4
[0226] According to an embodiment of the invention, niobium hydride
powder (140 g) having a BET surface area of 3.9 m2/g and starting
niobium oxide (91.5 g), i.e. niobium pentoxide, with a BET surface
area of 3.9 m2/g were mixed together to form a powder mixture. The
powder mixture was then attritor milled (comilled) in water for 2
hours using 3/16'' Nb media. The powder slurry was vacuum dried at
120.degree. C. and granulated by passing the dried powder over a 50
mesh screen. In addition, some of the powder was passed over a 70
mesh, and 100 mesh. The samples passing through the appropriate
screen was collected. The granular powder mixture was then placed
into a vacuum heat treatment furnace and heated to 1400.degree. C.
for about 2 hours. The temperature in the furnace was then brought
to about 850.degree. C. and hydrogen gas was admitted to the
furnace to a pressure of about 960 Torr and held for about 1 hour.
After formation, the oxygen reduced niobium oxide powders were
tested for certain properties which are set forth in Table 4.
TABLE-US-00004 TABLE 4 granule size Scott Crush Flow CV/g CV/g DCL
Lot # mesh g/in3 lbs s/25 g 1.5 V 10 V 180 s Shrink % 9030-46-38D
-50 22 4.38 22.4 153400 100100 0.12 3.99 9030-46-38D1 -70 21.5 3.84
23.2 153470 100060 0.12 4.34 9030-46-38D2 -100 20.4 4.2 29.8 153700
99450 0.12 4.18
Example 5
[0227] Samples in Example 4 were also sintered at two different
temperatures and tested for certain properties outlined in Table
5.
TABLE-US-00005 TABLE 5 Sinter Scott Crush Flow CV/g CV/g DCL Lot #
Temp .degree. C. g/in3 Lbs s/25 g 1.5 V 10 V 180 s Shrink %
9030-46-38D 1380 22 4.38 22.4 153400 100100 0.12 3.99 9030-46-38D
1280 22 4.38 22.4 181700 109800 0.12 0.52
Example 6
[0228] According to an embodiment of the invention, niobium hydride
powder (20 g) having a BET surface area of 3.9 m2/g and starting
niobium oxide (16.1 g), i.e. niobium pentoxide, with a BET surface
area of 3.9 m2/g were mixed together to form a powder mixture. The
powder mixture was then attritor milled (comilled) in water for 2
hours using 3/16'' Nb media. The powder slurry was vacuum dried at
120.degree. C. and granulated by passing the dried powder over a
100 mesh screen. The granular powder mixture was then placed into a
vacuum heat treatment furnace and heated to 1400.degree. C. for
about 2 hours. The temperature in the furnace was then brought to
about 850.degree. C. and hydrogen gas was admitted to the furnace
to a pressure of about 960 Torr and held for about 1 hour. After
formation, the oxygen reduced niobium oxide was tested for certain
properties which are set forth in Table 6. In addition, sample lots
of niobium hydride and niobium pentoxide were mixed as described
above without the attritor milling step. Powder mixtures were
granulated and heat treated to 1100.degree. C. followed by reaction
in hydrogen as described above. The oxygen reduced niobium oxide
formed in this manner was tested in the same manner and the results
are set forth in Table 6. The pore size distribution and cumulative
pore volumes for the press and sintered oxygen reduced niobium
oxides at 2.8 g/cc are shown in FIGS. 8 and 9.
TABLE-US-00006 TABLE 6 granule size Scott Crush Flow CV/g DCL Lot #
mesh g/in3 Lbs s/25 g 10 V 180 s Shrink % comilled 9030-46-36E2
-100 21 1.39 23.3 80100 0.11 0.6 yes 16-11 -50 21.9 1.65 41 82824
0.22 4.5 no 16-10 -50 19.8 2.7 23 76751 0.17 3.5 no
Example 7
[0229] Four samples of niobium pentoxide were placed in separate
crucibles and placed into a vacuum heat treatment furnace. The
furnace was evacuated to a pressure of 1 torr. Argon gas was
admitted to the furnace to a pressure of 0.11 torr. The temperature
in the furnace was ramped up to 1450.degree. C. and held for 60
minutes. At this point, argon was readmitted to the furnace and the
furnace cooled to a temperature of 50.degree. C. The heat treated
samples were then passivated with air by incremental increases in
the pressure of the furnace to atmosphere.
[0230] Samples of the agglomerated niobium pentoxide were
deagglomerated using an attritor mill. 1000 grams of the
agglomerated Nb.sub.2O.sub.5 was mixed with one liter of water, to
which a number of 3/16'' media were added to form a slurry. The
slurry was milled for separate milling runs of 20, 30, and 10
minutes, each with the mill operating at 350 rpm. Four additional
milling runs of 10 minutes were carried out at 350 rpm. The 3/16''
media were removed from the slurry, and 1/16'' media added. Several
20 minute milling runs were then made with the mill operating at
200 rpm.
[0231] FIGS. 10 and 11 are microphotographs obtained by use of a
scanning electron microscope (SEM) showing niobium pentoxide
particles of the present invention taken at 500.times. and
2,000.times. magnification, respectively. FIG. 12 is graph of
porosimetry comparisons of various niobium suboxide anodes made via
the present invention.
Example 8
[0232] According to an embodiment of the invention, a niobium
feedstock was a highly purified crushed niobium hydride screened to
-40 mesh. The feedstock was milled in 1 S Attritor mill to obtain
the desired size reduction. The mill itself was lined with Nb and
was outfitted with Nb arms to minimize contamination. The milling
was accomplished by stirring 3/16'' Nb balls in water and then
adding the 40 mesh Nb powder to create a slurry. The mill was
operated at about 450 rpm for a time from between 3 to 6 hours to
reduce the size of the first milled niobium powder to between 3 to
4 microns, a BET surface area of about 1.5 m.sup.2/g, and an oxygen
content of about 10,000 ppm.
[0233] The first milled niobium metal as prepared in this Example
was removed from the mill and separated from the 3/16 inch milling
media using a screen. 1/16'' Nb media was placed in the mill and
the slurry returned to the mill. The mill was then operated at
about 450 rpm for an additional 4-8 hours with the smaller media to
obtain the surface-passivated niobium powder having a size of
between 2-3 microns, a BET surface area of about 2.6 m.sup.2/g, and
an oxygen content of about 20,000 ppm.
[0234] The first milled niobium metal as prepared was removed from
the mill and separated from the 3/16 inch milling media using a
screen. 1/16'' Nb media was placed in the mill and the slurry
returned to the mill. The mill was then operated at about 450 rpm
for an additional 15-24 hours to obtain the surface-passivated
niobium powder having a size of between 1-2 microns, a BET surface
area of about 4.0 m.sup.2/g, and an oxygen content of about 27,000
ppm.
[0235] The particle size distribution of the niobium powders
obtained are shown in FIG. 13 as coarse grind, fine grind, and
super fine grind, respectively. The results are also shown in Table
7.
TABLE-US-00007 TABLE 7 Media Milling BET Size Milling Time: Surface
Oxygen Fe/Ni/Cr Degree of Tip Speed Diam. Time: Stage I Stage II
D10 D50 D90 Area Content Content Milling (in/min) (inch) (hours)
(hours) (.mu.m) (.mu.m) (.mu.m) (m.sup.2/g) (ppm) (ppm) Coarse
5k-9k 3/16 3-6 N/A 1.7 3.5 7.0 1.5 10,000 35 Fine 5k-9k 1/16 3-6
4-8 1.3 2.3 3.8 2.6 20,000 45 Super Fine 5k-9k 1/16 3-6 15-24 0.8
1.3 1.8 4.0 27,000 60
[0236] The milled slurry was decanted to remove any excess water,
and placed in a vacuum oven at 100.degree. C. to dry. After the
powder was dry it was screened through a 20 mesh screen to produce
spherical granules. An additional screening step using a 40 mesh
screen was used to prepare granules that were less than 425 microns
in size. The granules were then placed in a vacuum heat treated
furnace and heated under vacuum between 600 and 1000.degree. C.
and/or combinations of these heat treatment temperatures. The heat
treated powders had flow values greater than 300 mg/s as measured
by ASTM B 213. Heat treated granulated powders were then pressed
and sintered at 1125.degree. C. After formation at 40 V, the Nb
samples (fine and super fine) were tested for certain tests which
are set forth in Table 8.
TABLE-US-00008 TABLE 8 1st Heat 2nd Heat nA/CV Sample Type Treat
(C.) Treat (C.) 0 V CV/g 1.5 V CV/g 2.5 V CV/g 10 V CV/g 180 s 10 V
Shrink % 1 Fine 600 0 152033 97165 84647 66927 1.59 8 2 Fine 600
800 163060 102093 88334 68665 1.58 6 3 Fine 700 0 154851 98294
85364 67235 1.72 7.5 4 Fine 800 0 154904 97793 85102 66776 1.81 6 5
Super Fine 600 0 127855 89609 80476 67377 0.56 9.98 6 Super Fine
700 0 134940 93705 83828 69753 0.45 9.65 7 Super Fine 700 800
154410 103194 91173 74579 0.51 7.97 8 Super Fine 800 0 144850 98322
87281 71949 0.61 8.3 9 Super Fine 900 0 151414 99804 88310 72987
0.68 5.5 10 Super Fine 1000 0 145564 95238 84498 70362 0.5 2.79
Example 9
[0237] In this Example, Example 8 was essentially repeated except
as shown in Table 9, and except an extra stage of milling was used
for some of the samples. Thus, in this Example, a first stage
milling was compared to a two stage milling and to a three stage
milling wherein the second stage and third stage had varying
milling times as shown in Table 9. The various physical parameters
of the resulting hydrided niobium are also set forth in Table
9.
TABLE-US-00009 TABLE 9 Milling Milling Milling Time: Time: Time BET
Tip Stage I Stage II Stage III Surface Scott Oxygen Speed ( 3/16'')
( 1/16'') ( 1/32'') d.sub.10 d.sub.50 d.sub.90 Area Density Content
Type (ft/min) Hours Hours Hours .mu.m .mu.m .mu.m m.sup.2/g
(lb/ft.sup.3) ppm Coarse 5000-9000 3 0 0 1.7 3.5 7.0 1.5 25.3
~10000 Fine 5000-9000 3 6 0 1.3 2.3 3.8 2.6 24 ~20000 Super
5000-9000 3 20 0 0.8 1.2 1.8 3.8-4.0 22 ~27000 Fine Ultra 5000-9000
3 6 20 0.6 0.9 1.3 5.5 19 ~50,000 Fine Ultra 5000-9000 3 20 20 0.6
0.9 1.3 5.5 19 ~50,000 Fine
[0238] FIG. 15 shows a comparison of BET surface area versus
milling time for 1/32'' media. The various meaning of the
feedstocks are set forth in Table 9. In addition, FIG. 14 shows a
particle size distribution for the various powders milled in
accordance with Example 9. As further shown in the Examples, the
milled powders had varying oxygen amounts. In these Examples, all
powders had some level of oxygen associated with the passivated
surface. Therefore, the powders milled were a hydrided niobium with
a thin shell of niobium pentoxide on the surface (e.g.,
approximately 8 nm thick). Even after removing the hydrogen by
heating, for instance, the niobium metal with this niobium
pentoxide shell would still exist.
[0239] Other embodiments of the present invention will be apparent
to those skilled in the art from consideration of the present
specification and practice of the present invention disclosed
herein. It is intended that the present specification and examples
be considered as exemplary only with a true scope and spirit of the
invention being indicated by the following claims and equivalents
thereof.
* * * * *