U.S. patent application number 11/930649 was filed with the patent office on 2009-10-01 for process for producing niobium suboxide.
This patent application is currently assigned to H.C. Starck GmbH. Invention is credited to Christoph Schnitter.
Application Number | 20090242853 11/930649 |
Document ID | / |
Family ID | 34042009 |
Filed Date | 2009-10-01 |
United States Patent
Application |
20090242853 |
Kind Code |
A1 |
Schnitter; Christoph |
October 1, 2009 |
Process For Producing Niobium Suboxide
Abstract
A method is described for preparing a niobium suboxide
represented by the formula, NbO.sub.x, in which
0.7<.times.<1.3. The method involves reacting NbOy (in which
y<1.8<2.1) with a stoichiomerc amount of niobium metal, in
the presence of hydrogen. The niobium suboxide produced by such
method may be used to fabricate anodes for solid electrolyte
capacitors.
Inventors: |
Schnitter; Christoph;
(Holle, DE) |
Correspondence
Address: |
Connolly Bove Lodge & Hutz LLP
1007 North Orange Street, P. O. Box 2207
Wilmington
DE
19899-2207
US
|
Assignee: |
H.C. Starck GmbH
|
Family ID: |
34042009 |
Appl. No.: |
11/930649 |
Filed: |
October 31, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10894279 |
Jul 19, 2004 |
7341705 |
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11930649 |
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Current U.S.
Class: |
252/519.1 |
Current CPC
Class: |
C04B 2235/608 20130101;
C01P 2004/61 20130101; C04B 2235/549 20130101; C04B 2235/528
20130101; C04B 2235/5481 20130101; C01P 2004/62 20130101; C01P
2006/20 20130101; C01G 33/00 20130101; C01P 2004/51 20130101; C04B
2235/5409 20130101; C04B 2235/3251 20130101; C01P 2006/12 20130101;
C04B 35/6265 20130101; C04B 2235/5427 20130101; C04B 35/6268
20130101; Y10T 428/31504 20150401; C01P 2006/40 20130101; C04B
2235/5445 20130101; C04B 2235/6581 20130101; H01G 9/0525 20130101;
C04B 2235/3253 20130101; C04B 35/495 20130101 |
Class at
Publication: |
252/519.1 |
International
Class: |
H01B 1/08 20060101
H01B001/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 22, 2003 |
DE |
103 33 156.5 |
Claims
1-9. (canceled)
10. A niobium suboxide powder represented by the following formula,
NbO.sub.x, wherein 0.7<.times.<1.3, further wherein said
niobium suboxide has a flow rate of at most 60 seconds/25 grams, as
determined in accordance with ASTM B 213.
11. A capacitor comprising an anode, wherein said anode comprises a
sintered powder of the niobium suboxide of claim 10.
12. The niobium suboxide powder as claimed 10, wherein the flow
rate is at most of 50 seconds/25 grams, as determined in accordance
with ASTM B 213.
13. The niobium suboxide powder as claimed 10, wherein the flow
rate is at most of 40 seconds/25 grams, as determined in accordance
with ASTM B 213.
14. The niobium suboxide powder as claimed 10, wherein x is between
0.95 and 1.1
15. The niobium suboxide powder as claimed 13, wherein x is between
0.95 and 1.1.
16. The niobium suboxide powder as claimed 10, wherein x is between
1 and 1.05
17. The niobium suboxide powder as claimed 13, wherein x is between
1 and 1.05.
18. A capacitor comprising an anode, wherein said anode comprises a
sintered powder of the niobium suboxide of claim 17.
Description
CROSS REFERENCE TO RELATED PATENT APPLICATION
[0001] The present patent application claims the right of priority
under 35 U.S.C. .sctn.119 (a)-(d) of German Patent Application No.
103 33 156.5, filed Jul. 22, 2003.
FIELD OF THE INVENTION
[0002] The present invention relates to a process for producing
niobium suboxide of the approximate composition NbO, the niobium
suboxide being suitable in particular for the production of anodes
for solid electrolyte capacitors.
BACKGROUND OF THE INVENTION
[0003] Solid electrolyte capacitors with a very large active
capacitor surface area and therefore a small overall construction
suitable for mobile communications electronics used are
predominantly capacitors with a niobium or tantalum pentoxide
barrier layer applied to a corresponding conductive substrate,
utilizing the stability of these compounds ("valve metals"), the
relatively high dielectric constants and the fact that the
insulating pentoxide layer can be produced with a very uniform
layer thickness by electrochemical means. The substrates used are
metallic or conductive lower oxide (suboxide) precursors of the
corresponding pentoxides. The substrate, which simultaneously forms
a capacitor electrode (anode) comprises a highly porous,
sponge-like structure which is produced by sintering extremely
fine-particle primary structures or secondary structures which are
already in sponge-like form. The surface of the substrate structure
is electrolytically oxidized ("formed") to produce the pentoxide,
with the thickness of the pentoxide layer being determined by the
maximum voltage of the electrolytic oxidation ("forming voltage").
The counterelectrode is produced by impregnating the sponge-like
structure with manganese nitrate, which is thermally converted into
manganese dioxide, or with a liquid precursor of a polymer
electrolyte followed by polymerization. The electrical contacts to
the electrodes are produced on one side by a tantalum or niobium
wire which is sintered in during production of the substrate
structure and on the other side by the metallic capacitor sheath,
which is insulated with respect to the wire.
[0004] The capacitance C of a capacitor is calculated using the
following formula:
C=(F.epsilon.)/(dV.sub.F)
where F denotes the capacitor surface area, s the dielectric
constant, d the thickness of the insulator layer per V of forming
voltage and V.sub.F the forming voltage. Since the dielectric
constant .epsilon. is 27.6 or 41 for tantalum pentoxide or niobium
pentoxide, respectively, but the growth in the layer thickness per
volt of forming voltage d is 16.6 or 25 .orgate./V, both pentoxides
have an almost identical quotient .epsilon./d=1.64 or 1.69,
respectively. Capacitors based on both pentoxides, with the same
geometry of the anode structures, therefore have the same
capacitance. Trivial differences in details concerning specific
weight-related capacitances result from the different densities of
Nb, NbO.sub.x (0.7<.times.<1.3; in particular
0.95<.times.<1.1) and Ta. Anode structures made from Nb and
NbO.sub.x therefore have the advantage of saving weight when used,
for example, in mobile telephones, in which every gram of weight
saving is a priority. With regard to cost aspects, NbO.sub.x is
more favourable than Nb, since some of the volume of the anode
structure is provided by oxygen.
[0005] The niobium suboxide powders are produced using the standard
metallurgical reaction and alloying processes, according to which a
mean oxide content is produced by exposing niobium pentoxide and
niobium metal, in the presence of hydrogen, to a temperature at
which an oxygen concentration balancing takes place, cf. for
example WO 00/15555 A1:
2Nb.sub.2O.sub.5+3Nb.fwdarw.5NbO (1)
[0006] The process therefore comprises the use of a high-purity
commercially available niobium pentoxide and mixing it with
high-purity niobium metal, both in powder form corresponding to the
stoichiometric proportions and treating them for several hours at a
temperature of from 800 to 1600.degree. C. in a hydrogen-containing
atmosphere, which should preferably contain up to 10% of hydrogen.
It is preferable for both the pentoxide and the metal to have
primary particle sizes which, after the oxygen balancing has taken
place, correspond to the desired primary particle size of less than
or slightly over 1 .mu.m (smallest) cross-sectional dimension.
[0007] In is process, crucibles made from niobium or tantalum which
have been filled with a mixture of niobium pentoxide and niobium
metal powders are heated to the reaction temperature in a furnace
under a hydrogen-containing atmosphere. The niobium metal required
for the oxygen exchange with niobium pentoxide is preferably
produced by reduction of high-purity niobium pentoxide to form the
metal.
[0008] This can be effected aluminothermically by igniting an
Nb.sub.2O.sub.5/A1 mixture and washing out the aluminium oxide
which is formed and then purifying the niobium metal ingot by means
of electron beams. The niobium metal ingot obtained after reduction
and electron beam melting can be embrittled using hydrogen in a
known way and milled, producing plateletlike powders.
[0009] According to a preferred process for producing the niobium
metal in accordance with WO 00/67936 A1, the high-purity niobium
pentoxide powder is firstly reduced by means of hydrogen at 1000 to
1600.degree. C. to form the niobium dioxide of approximately the
formula NbO.sub.2, and is then reduced to the metal using magnesium
vapour at 750 to 1100.degree. C. Magnesium oxide which is formed in
the process is washed out by means of acids. The latter process is
preferred in particular on account of its considerably lower energy
demand, on account of the fact that the primary particle size of
the niobium pentoxide is in principle maintained and that there is
a lower risk of contamination with substances which are harmful to
the capacitor properties.
[0010] One drawback of the reaction in accordance with reaction
Equation (1) is that the volumetric shrinkage of the niobium
pentoxide during the transition to the niobium suboxide amounts to
approx. 50%, which causes a very loose crystal microstructure of
the suboxide which can only be densified by conditioning with a
risk of crystal defects being incorporated, and therefore may
ultimately have an adverse effect on the capacitor properties. The
poor crystal quality of the suboxide is evidently also a reason for
its inadequate flow properties.
[0011] Good flow properties of the capacitor powders represent a
significant process parameter in the production of the capacitors,
since the powders are pressed by means of automatic high-speed
pressers which are supplied with the powder to be pressed via
storage containers. Good flow properties represent a precondition
for a defined quantity of powder to flow into the press mould with
an accuracy which satisfies modern-day requirements, for example of
+/-0.5 mg.
SUMMARY OF THE INVENTION
[0012] It is an object of the invention to overcome the drawbacks
of the prior art, It is a further object of the invention to
provide a niobium suboxide powder with improved flow
properties.
[0013] A further object of the invention is to reduce the
consumption of high-purity magnesium and the production of
magnesium oxide, and at the same time to reduce the outlay involved
in washing out the magnesium oxide. Furthermore, it is an object of
the invention to increase the capacity of the furnaces
significantly.
[0014] Another object of the invention is to further reduce the
risk of contamination during the production of the niobium metal
required for the production of niobium suboxide.
[0015] In accordance with the present invention there is provided a
method of producing NbO.sub.x comprising [0016] reacting NbO.sub.y,
where 1.8<y<2.1, [0017] with a stoichiometric quantity of
niobium metal, in the presence of hydrogen, wherein for NbO.sub.x,
0.7<.times.<1.3.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Accordingly, according to the invention it is proposed that
a niobium dioxide of the approximate composition NbO.sub.2 be used
as starting oxide for the metallurgical oxygen balancing with the
niobium metal powder. The niobium dioxide is preferably produced by
reduction of niobium pentoxide under flowing hydrogen at a
temperature of from 1000 to 1600.degree. C.
[0019] The subject matter of the present invention is therefore a
process for producing NbO.sub.x where 0.7<.times.<1.3,
preferably 0.9<.times.<1.15, particularly preferably
1<.times.<1.05, by reacting NbO.sub.y where 1.8<y<2.1,
preferably 1.9<y<2, with a stoichiometric quantity of niobium
metal in the presence of hydrogen. The temperature and duration of
the reaction are to be determined in such a way that the reaction
takes place substantially completely.
[0020] A further subject of the invention is niobium suboxide
powders of the formula NbO.sub.x, where 0.7<.times.<1.3,
preferably 0.9<.times.<1.15, particularly preferably
1<.times.<1.05, which have ASTM B 213 flow properties of at
most 60 s/25 g, preferably at most 50 s/25 g, particularly
preferably at most 40 s/25 g.
[0021] A reaction temperature of from 900 to 1600.degree. C. is
preferred for the process according to the invention. The reaction
time can be selected to be between 0.5 and 4 hours, depending on
the reaction temperature and the composition and particle structure
of the starting substances and the composition of the end
product.
[0022] The starting niobium dioxide to be used for the process
according to the invention is preferably produced by reduction of
niobium pentoxide in flowing hydrogen. It is preferable for the
reaction to take place at a hydrogen partial pressure of from 50 to
1100 mbar. It can be detected that the reaction has ended when the
flowing hydrogen is free of water vapour. After the reaction has
ended, it is preferable for the reaction product still to be held
for a certain time, for example 0.1 to 0.5 hours, at a temperature
of from 900 to 1600.degree. C., preferably from 1200 to
1600.degree. C., in order to stabilize and densify the NbO.sub.y
crystal lattice.
[0023] Furthermore, it is preferable for the temperature during the
reduction of the pentoxide to form the dioxide to be gradually
increased from a starting temperature in the range from 950 to
1100.degree. C. to a maximum temperature in the range from 1300 to
1600.degree. C., particularly preferably from a starting
temperature in the range from 1000 to 1050.degree. C. to a maximum
temperature in the range from 1350 to 1600.degree. C., and then for
the reduction to be continued with a gradually decreasing
temperature, if appropriate after a certain residence time at the
maximum temperature. On account of the decreasing oxygen
concentration in the first reduction phase, the reduction rate can
be substantially maintained by the increasing temperature, or
excessively quick lattice widening as a result of an excessively
fast reduction rate can be avoided by using a lower starting
temperature. The high final temperature in the range from 1300 to
1600.degree. C. is then held for a certain time, so that the
crystal lattice can density and lattice defects are largely
annealed,
[0024] On the other hand, it is possible to bring about initially
very rapid reduction and therefore very extensive widening of the
crystal lattice as early as during production of the dioxide, by
means of very rapid heating to a reduction temperature of from 1450
to 1600.degree. C., so that the lattice becomes highly unstable,
producing a relatively strong primary particle growth. This may be
desirable if a very fine-particle niobium pentoxide is used as
starting material, with the intention being to produce capacitors
with a medium capacitance in the range from 30 000 to 70 000
.mu.FV/g. In this case too, holding at a temperature of from 1200
to 1600.degree. C. in order to consolidate the dioxide crystal
lattice is advantageous.
[0025] The reduction times required are dependent on the particle
size of the niobium pentoxide used and on the reduction temperature
selected. With a pentoxide primary particle size of 0.3 to 0.5
.mu.m, a reduction time of from 20 to 40 minutes is generally
sufficient.
[0026] On account of the relatively high reduction temperatures
(including the maximum temperature in the first case), sintered
bridges with an advantageously extremely high strength even in the
niobium dioxide are formed.
[0027] Further reduction of the dioxide to form the metal by means
of magnesium vapour can be carried out at a relatively low
temperature, for example 900 to 1100.degree. C. At these low
temperatures, only minimal primary grain coarsening occurs. As a
result, it is possible for niobium dioxide from a single source on
the one hand in part to be reduced further to form the metal and on
the other hand to be mixed with the metal without further treatment
and then to carry out the oxygen balancing to form the suboxide,
since primary grain and agglomerate sizes of dioxide and metal are
no different, approximately matching one another in particular
after the oxygen balancing.
[0028] According to the invention, therefore, the niobium suboxide
is produced in accordance with the following formula;
NbO.sub.2+Nb.fwdarw.2NbO (2).
[0029] The volumetric shrinkage during the transition of the
NbO.sub.2 to the NbO is just 13%. Although the majority of the
volumetric shrinkage of the pentoxide of 42% has been shifted to
the production of the NbO.sub.2, is has no adverse effect, since it
is possible to effect intermediate stabilizing of the crystal
microstructure as NbO.sub.2 during the hydrogen reduction.
[0030] A further advantage is that the magnesium consumption, the
washing outlay and the proportion of magnesium oxide which has to
be processed for the production of the niobium metal are in each
case reduced by 20% by the process according to the invention
(based on the final yield of NbO).
[0031] A further advantage of the invention is the increase in the
capacity of the furnaces for the reaction to form the NbO. Whereas
according to reaction Equation (1) the volumetric shrinkage from
the starting mixture to the product is 23.5%, according to the
reaction equation of the invention there is an increase in volume
of (in theory) just 6%, which is practically compensated for by
sintering shrinkage. The crucible of the furnace, which according
to Equation (1) is initially 100% fill, after the reaction has
ended is (in theory) only 81% fill with NbO.
[0032] In the case of the reaction according to the invention
corresponding to Equation (2), therefore, the capacity can
(theoretically) be increased by (19%/81%=) 23%. In reality, taking
the sintering shrinkage into account, the increase in capacity is
even greater.
EXAMPLES
Example 1
[0033] a) Production of the niobium dioxide NbO.sub.y
[0034] A partially agglomerated, high-purity, spherical niobium
pentoxide, which has been sieved through a sieve of mesh width 300
.mu.m, with a primary grain size of approximately 0.7 .mu.m
diameter and a specific surface area, determined in accordance with
BET (ASTM D 3663), of 2.4 m.sup.2/g is used.
[0035] The pentoxide is reduced to the niobium dioxide under
flowing hydrogen at a temperature which rises over the course of 40
minutes from 950 to 1300.degree. C., is then held at the latter
temperature for 30 minutes and then lowered to 1200.degree. C. over
the course of 30 minutes and then held for 1 hour at this
temperature.
[0036] The niobium dioxide had a composition corresponding to the
formula NbO.sub.2.01. The primary grain size had been coarsened to
approximately 0.9 .mu.m (determined visually from SEM images), and
the BET surface area was 1.1 m.sup.2/g.
[0037] Measurement of the grain size distribution using a
Mastersizer S.mu. produced by Malvern (ASTM B 822, wetting agent
Daxad 11) after pushing through a sieve of 300 .mu.m mesh width,
gave a D10 value of 32 .mu.m, a D50 value of 164 .mu.m and a D90
value of 247.mu.m.
b) Production of the niobium metal
[0038] Part of the niobium dioxide obtained under a) was placed, in
a reactor, onto a mesh of niobium wire. 1.1 times the
stoichiometric quantity of magnesium, based on the oxygen content
of the dioxide, was placed beneath the mesh in a crucible. The
reactor was purged with argon from the bottom upwards. Then, the
reactor was heated to 1050.degree. C. After 8 hours, the reactor
was cooled and air was slowly admitted in order to passivate the
metal surface.
[0039] The niobium metal powder obtained had a primary grain size
of 0.85 .mu.m, a BET surface area of 1.32 m.sup.2/g and, after
being pushed through a sieve, with a mesh width of 300 .mu.m, had a
D10 value of 33 .mu.m, a D50 value of 176 .mu.m and a D90 value of
263 .mu.m.
c) Production of the niobium suboxide NbO.sub.x
[0040] 43 parts by weight of the niobium powder obtained under b)
and 57 parts by weight of the niobium dioxide powder obtained under
a) were mixed and introduced into a crucible which was filled up to
the brim. The crucible was then heated to 1380.degree. C. over a
period of 2.5 hours in a furnace which was purged with a gas
mixture comprising 85% by volume of argon and 15% by volume of
hydrogen.
[0041] After cooling, a niobium suboxide powder corresponding to
the formula NbO.sub.0.96 was obtained. The suboxide powder had a
primary grain size of 0.95 .mu.m and a BET surface area of 1.1
m.sup.2/g. After sieving through a sieve with mesh width 300 .mu.m,
the D10 value was 41 .mu.m, the D50 value was 182 .mu.m and the D90
value was 258 .mu.m.
d) Capacitor production
[0042] In each case 103 mg of the niobium suboxide powder in
accordance with c) were introduced into press moulds, so as to
surround a niobium contact wire, and then pressed to form pressed
bodies with a pressed density of 2.8 g/cm.sup.3.
[0043] The pressed bodies were sintered standing freely on a
niobium platform under high vacuum of 10.sup.-3 Pa for 20 minutes
at a temperature of 1450.degree. C. The anodes were formed in an
electrolyte comprising 0.1% strength phosphoric acid at a
temperature of 85.degree. C. and a forming current of 150 mA up to
a forming voltage of 30 V, which was maintained for 2 hours after
the current had decayed.
[0044] The capacitance and residual current of the anode bodies,
which had been provided with a barrier layer of niobium pentoxide
by the forming, were measured by the counterelectrode being
simulated by an 18% strength sulphuric acid at 25.degree. C. The
measurements were carried out at a voltage of 21 V (70% of the
forming voltage), a frequency of 120 Hz and a bias voltage of 10 V
after a charging time of 3 minutes. The mean specific capacitance
was determined as 75 158 .mu.FV/g and the residual current as 0.76
nA/.mu.FV.
Example 2
[0045] a) Production of the niobium dioxide NbO.sub.y
[0046] The starting material used was a partially agglomerated,
high-purity, virtually spherical Nb.sub.2O.sub.5 after sieving to
<300 .mu.m with a specific surface area determined in accordance
with BET (ASTM D 3663) of 2.3 m.sup.2. Part of this Nb.sub.2O.sub.5
is reduced to an oxide of the composition NbO.sub.2.02 under
flowing hydrogen at a temperature which rises from 1000.degree. C.
to 1450.degree. C. over the course of 60 minutes and is then held
at 1450.degree. C. for 200 minutes. The specific surface area of
the dioxide was 0.32 m.sup.2/g, and the grain size distribution
determined by laser diffraction (ASTM B 822) had a D10 value of 67
.mu.m, a D50 value of 176 .mu.m and a D90 value of 284 .mu.m.
b) Production of the niobium metal.
[0047] Part of the niobium dioxide produced under a) was placed
onto a wire mesh in a reactor, and beneath the wire mesh there was
a crucible holding 1.2 times the stoichiometric quantity (based on
the O content of the niobium dioxide) of magnesium. The reactor was
then heated under flowing argon for 4 h to 900.degree. C., during
which period the magnesium evaporated and reduced the niobium
dioxide above it to the metal. After cooling and passivation, the
magnesium oxide formed was removed from the niobium metal formed by
washing repeatedly with sulphuric acid followed by water.
[0048] The niobium metal powder formed had a primary grain size of
from 0.4 to 0.6 .mu.m (determined visually from SEM images), a
specific surface area of 3.87 m.sup.2/g and a D10 value of 54
.mu.m, determined by laser diffraction (ASTM D 3663, Malvern
Mastersizer), a D50 value of 161 .mu.m and a D90 value of 272
.mu.m.
c) Production of the niobium suboxides NbO
[0049] 1. Procedure according to the prior art;
[0050] Half of the niobium metal produced under b) is mixed with
the Nb.sub.2O.sub.5 described under a) in a weight ratio of 1:0.95
and then heated in a furnace to 1400.degree. C. for 3 h under a
hydrogen partial pressure of 67 mbar absolute. Then, the powder was
pushed through a sieve of mesh width 300 .mu.m. The niobium
suboxide obtained in this way ("powder A") had a composition
NbO.sub.1.01 and a primary grain size of from 0.95 to 1.1 .mu.m
(determined visually from SEM images). The specific surface area
was 1.07 m.sup.2/g, and the D10 value determined by laser
diffraction was 71 .mu.m, the D50 value 165 .mu.m and the D90 value
263 .mu.m.
[0051] 2. Procedure according to the invention:
[0052] The other half of the niobium metal produced under b) is
mixed with part of the NbO.sub.2.02 produced under a) in a weight
ratio of 1:1.34 and then heated in a furnace for 2 h under a
hydrogen partial pressure of 67 mbar absolute to 1210.degree. C.
The niobium suboxide obtained ("Powder B") had a composition
NbO.sub.0.93 and a specific surface area of 1.13 m.sup.2/g. The
primary grain size, determined visually from SEM images, was on
average 1.0 .mu.m, and the grain size distribution determined from
laser diffraction resulted in a D10 value of 62 .mu.m, a D50 value
of 158 .mu.m and a D90 value of 269 .mu.m. The flow properties of
both powders were determined in accordance with ASTM B 213. The
following values resulted: [0053] Powder A: 65 s/25 g [0054] Powder
B; 26 s/25 g.
[0055] Accordingly, the procedure of the invention leads to niobium
suboxides which are distinguished by improved flow properties
compared to products obtained conventionally.
[0056] Although the invention has been described in detail in the
foregoing for the purpose of illustration, it is to be understood
that such detail is solely for that purpose and that variations can
be made therein by those skilled in the art without departing from
the spirit and scope of the invention except as it may be limited
by the claims.
* * * * *