U.S. patent application number 12/673559 was filed with the patent office on 2011-05-26 for nanosize structures composed of valve metals and valve metal suboxides and process for producing them.
This patent application is currently assigned to H.C. Starck GmbH. Invention is credited to Manfred Bobeth, Holger Brumm, Gerhard Gille, Helmut Haas, Robert Muller, Christoph Schnitter.
Application Number | 20110123822 12/673559 |
Document ID | / |
Family ID | 40139278 |
Filed Date | 2011-05-26 |
United States Patent
Application |
20110123822 |
Kind Code |
A1 |
Gille; Gerhard ; et
al. |
May 26, 2011 |
NANOSIZE STRUCTURES COMPOSED OF VALVE METALS AND VALVE METAL
SUBOXIDES AND PROCESS FOR PRODUCING THEM
Abstract
A strip-like or sheet-like valve metal or valve metal suboxide
structure which has a transverse dimension of from 5 to 100 nm.
Inventors: |
Gille; Gerhard; (Goslar,
DE) ; Schnitter; Christoph; (Holle-Sottrum, DE)
; Brumm; Holger; (Goslar, DE) ; Haas; Helmut;
(Achim, DE) ; Muller; Robert; (Dresden, DE)
; Bobeth; Manfred; (Dresden, DE) |
Assignee: |
H.C. Starck GmbH
Goslar
DE
|
Family ID: |
40139278 |
Appl. No.: |
12/673559 |
Filed: |
July 23, 2008 |
PCT Filed: |
July 23, 2008 |
PCT NO: |
PCT/EP08/59659 |
371 Date: |
February 9, 2011 |
Current U.S.
Class: |
428/607 ;
428/220; 428/544; 428/606; 75/710; 75/745 |
Current CPC
Class: |
B22F 1/0044 20130101;
C22B 34/1268 20130101; Y10T 428/12431 20150115; C22B 5/04 20130101;
H01G 9/0525 20130101; B22F 9/22 20130101; Y10T 428/12 20150115;
B22F 2998/00 20130101; H01G 9/052 20130101; Y10T 428/12438
20150115; B22F 2998/00 20130101; C22B 34/24 20130101; B22F 9/22
20130101; B22F 2201/40 20130101 |
Class at
Publication: |
428/607 ; 75/710;
75/745; 428/544; 428/606; 428/220 |
International
Class: |
C22B 9/14 20060101
C22B009/14; B21C 37/02 20060101 B21C037/02; B21C 37/00 20060101
B21C037/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 16, 2007 |
DE |
10 2007 038 581.3 |
Nov 30, 2007 |
DE |
10 2007 057 761.5 |
Claims
1.-14. (canceled)
15. A strip-like or sheet-like valve metal or valve metal suboxide
structure which comprises a transverse dimension of from 5 to 100
nm.
16. The valve metal or valve metal suboxide structure according to
claim 15 having a sheet- or layer-like primary structure in the
form of powders.
17. The valve metal or valve metal suboxide structure according to
claim 15 in the form of a surface strip structure.
18. The valve metal or valve metal suboxide structure according to
claim 17 in the form of foils or wires having strips having a width
of from 5 to 100 nm and a strip spacing of from one to 2 times the
strip width.
19. The valve metal or valve metal suboxide structure according to
claim 15, wherein the strips or lamellae are aligned parallel in
groups.
20. The valve metal or valve metal suboxide structure according to
claim 15, wherein the transverse dimension or strip width is from 8
to 50 nm.
21. The valve metal or valve metal suboxide structure according to
claim 15 comprising Ti, Zr, V, Nb, Ta, Mo, W, Hf or Al or alloys
thereof.
22. The valve metal or valve metal suboxide structure according to
claim 15 comprising Nb or Ta or alloys thereof.
23. The valve metal suboxide structure according to claim 15 having
the formula NbO.sub.x where 0.7<x<1.3.
24. The valve metal or valve metal suboxide structure according to
claim 15, having a content of at least one reducing metal in an
amount of from 10 to 500 ppm.
25. A process for the reduction of the valve metal oxides by means
of the vapor of reducing metals at a temperature sufficient for
reduction to form lamellar nanosize structures, wherein the reduced
valve metal oxide is frozen before thermal decomposition of the
lamellar structure and transformation into coarsened
structures.
26. The process according to claim 25, wherein the reduction is
carried out at an inert gas pressure of less than 0.2 bar and a
vapor pressure of the reducing metal of from 10.sup.-2 to 0.4
bar.
27. The process according to claim 25, wherein the reduction
product is cooled to below 100.degree. C. within a few minutes
immediately after the reduction is complete.
28. The process according to claim 25, wherein the reducing metal
is Li, Al, Mg or Ca or a mixture thereof.
29. The process according to claim 25, wherein the reducing metal
is Mg.
30. The process according to claim 25, wherein oxides of Al, Hf,
Ti, Zr, V, Nb, Ta, Mo and/or W or mixed oxides thereof, are used as
oxides to be reduced.
31. The process according to claim 25, wherein oxides of Nb or Ta
or mixed oxides thereof are used as oxides to be reduced.
Description
[0001] The present invention relates to novel lamellar structures
of valve metals and valve metal suboxides which have a dimension of
less than 100 mm in one direction and a process for producing
them.
[0002] Fine structures composed of metals and metal suboxides which
are present in powders or surface regions of larger metal
substrates have a wide variety of uses as catalysts, support
materials for catalysts, in the field of membrane and filter
technology, in the medical sector as implant material, as storage
materials in secondary batteries and as anode material of
capacitors because of their large specific surface area.
[0003] WO 00/67936 discloses a process for producing finely divided
valve metal powders by reduction of valve metal oxide powders by
means of gaseous reducing metals such as Mg, Al, Ca, Li and Ba.
Owing to the volume shrinkage in the reduction of the oxide to the
metal and the volume increase caused by the solid oxides of the
reducing metals which are formed, highly porous valve metal powders
having a high specific surface area which are suitable, in
particular, for producing solid electrolyte capacitors are
formed.
[0004] It has now been found that under particular reduction
conditions lamellar structures having transverse dimensions in the
nanometre range are formed, with the laminates initially comprising
alternate layers of the reduced valve metal oxide and of the
oxidized reducing metal.
[0005] Dissolution and leaching of the oxide of the reducing metal
in mineral acids enables the nanosize valve metal structures to be
freed of the oxide of the reducing metal.
[0006] Depending on the geometric structure of the starting valve
metal oxide, finely divided powders having a lamellar structure or
strip-like or lamellar surface structures on metal substrates
having relatively coarse/large structures are obtained, with the
metal and/or suboxide strips or lamellae having a width of less
than 100 nm and a spacing (intermediate space) which can be up to
twice the strip width, depending on the valve metal oxide and the
oxidation state which it attains.
[0007] Thus, when finely divided valve metal oxide powders having
average dimensions of the primary structure particle size of from
50 to 2000 nm, preferably less than 500 nm, more preferably less
than 300 nm, are used, finely divided metal or suboxide powders
having a lamellar structure and a width of the metal or suboxide
strips of from 5 to 100 nm, preferably from 8 to 50 nm,
particularly preferably up to 30 nm, and transverse dimensions of
from 40 to 500 nm and a specific surface area of above 20
m.sup.2/g, preferably above 50 m.sup.2/g, are obtained.
[0008] When relatively large valve metal oxide substrates having
dimensions above, for example, 10 .mu.m are used, metallic or
suboxidic strips having a width of up to 100 nm, preferably from 5
to 80 nm, particularly preferably from 8 to 50 nm, more preferably
up to 30 nm, and spacings of from one to two times the strip width
are obtained on these structures. The depth of the grooves between
the strips can be up to 1 .mu.m.
[0009] Relatively large metal structures or substrates, for example
wires or foils, having a strip-like surface can be obtained by
firstly oxidizing the surface chemically or anodically and then
reducing the surface according to the invention, with the strip
depth being determined by the thickness of the oxide layer
initially produced.
[0010] Furthermore, structures according to the invention can be
obtained by providing a substrate comprising, for example, another
metal or ceramic with a valve metal oxide layer, for example by
application of a valve metal layer by vapour deposition or
electrolytic deposition, oxidizing the coating and reducing it
according to the invention to the metal or suboxide.
[0011] Valve metal oxides used for the purposes of the present
invention can be oxides of the elements of transition groups 4 to 6
of the Periodic Table, e.g. Ti, Zr, V, Nb, Ta, Mo, W and Hf, and
also their alloys (mixed oxides) and Al, preferably Ti, Zr, Nb and
Ta, particularly preferably Nb and Ta. As starting oxides,
preference is given to, in particular, Nb.sub.2O.sub.5, NbO.sub.2
and Ta.sub.2O.sub.5. Preferred reaction products according to the
invention are the metals of the starting oxides. Lower oxides
(suboxides) of the starting valve metal oxides can also be obtained
as reduction products. A particularly preferred reduction product
is niobium suboxide having metallically conducting properties of
the formula NbO.sub.x where 0.7<x<1.3, which, in addition to
tantalum and niobium, is suitable as anode material for capacitors,
according to the invention particularly for use in the range of low
activation voltages up to 10 V, particularly preferably up to 5 V,
in particular up to 3 V.
[0012] As reducing metals, it is possible to use Li, Mg, Ca, B,
and/or Al and their alloys according to the invention. Preference
is given to Mg, Ca and Al, as long as these are less noble than the
metals of the starting oxides. Very particular preference is given
to Mg or a eutectic of Mg and Al.
[0013] A characteristic of the reduction products according to the
invention is their content of reducing metals in the range above 10
ppm, in particular from 50 to 500 ppm, owing to doping during
reduction.
[0014] The process of the invention by which the nanosize
structures can be produced is based on reduction of metal oxides by
reducing metals in vapour form as described in WO 00/67936. Here,
the valve metal oxide to be reduced in powder form is brought into
contact with the vapour of the reducing metal in a reactor. The
reducing metal is vaporized and conveyed by means of a carrier gas
stream such as argon over the valve metal oxide powder present on a
mesh or in a boat at elevated temperature, typically from 900 to
1200.degree. C., likewise typically for a period of from 30 minutes
to some hours. Since the molar volume of valve metal oxides is from
two to three times the volume of the corresponding valve metal, a
considerable decrease in volume takes place during reduction.
Sponge-like, highly porous structures in which the oxide of the
reducing metal is deposited are therefore formed in the reduction.
Since the molar volumes of the oxides of the reducing metals are
greater than the difference between the molar volumes of the valve
metal oxide and the valve metal, they are incorporated into the
pores with production of residual stresses. The structures can be
freed of the oxides of the reducing metals by dissolution of these
oxides, so as to obtain highly porous metal powders. Studies on the
mechanism of the reduction and the formation of the pores and their
distribution have shown the following: starting from small reaction
nuclei on the surface of the valve metal oxide particles or
substrates, layer-like structures having nanosize dimensions are
formed behind the valve metal/valve metal oxide reaction front in
the initial phase of the reaction. The layers are firstly oriented
perpendicular to the surface in regions of the particles/substrates
close to the surface. However, as the reaction front moves deeper
into the oxide particles/substrate, orientation and dimensions of
the lamellae are determined by the crystal orientation and
dimensions of the primary particles in the valve metal oxide and by
the reaction conditions. A certain number of lattice planes in a
valve metal oxide crystallite are replaced by a stoichiometrically
equivalent number of lattice planes of the valve metal and of the
oxide of the reducing metal. These nanosize layer structures, which
are actually very energetically unfavourable because of the high
interfacial stress, are nevertheless produced and become possible
since the reduction is strongly exothermic and at least part of the
excess energy is not dissipated as heat but "invested" in structure
formation which makes fast reaction kinetics possible. The many
flat interfaces of the layer structures act as "fast roads" for the
atoms of the reducing metals, i.e. they allow fast diffusion and
thus reaction kinetics which lead quickly and effectively to
reduction of the total energy of the reaction system. However, the
layer-like structures composed of valve metals and oxides of the
reducing metals are formed only in a metastable state which on
introduction of thermal energy leads to a structural state having
an even lower energy. In a reduction process carried out "normally"
with relatively long heat treatment times and constant reaction
conditions (temperature, vapour pressure of the reducing metal,
etc.), this structural transformation inevitably occurs, i.e. the
nanosize layer structures are converted into a greatly coarsened
and interpenetrating structure composed of valve metal regions and
regions of the reduction metal oxides.
[0015] It has now been found that the lamellar structures can be
frozen if care is taken to ensure that the reduction product is
cooled to a temperature at which the lamellar structures remain
stable before transformation of the structures occurs. According to
the invention, the reduction conditions are therefore set so that
the reduction can proceed very uniformly within a short time, i.e.
if a pulverulent starting oxide is used, within the powder bed of
the oxide and the reduction product is cooled as quickly as
possible immediately after the reduction is complete.
[0016] For this reason, preference is given to employing a low
thickness of the powder bed to ensure uniform permeation of the
vapour of the reducing metal through the bed. The thickness of the
powder bed is particularly preferably less than 1 cm, more
preferably less than 0.5 cm.
[0017] Furthermore, the uniform permeation of the vapour of the
reducing metal through the powder bed can be ensured by providing
for a large free path length of the vapour of the reducing metal.
According to the invention, the reduction is therefore preferably
carried out under reduced pressure, more preferably in the absence
of carrier gases. The reduction is particularly preferably carried
out at a vapour pressure of the reducing metal of from 10.sup.-2 to
0.4 bar, more preferably from 0.1 to 0.3 bar, in the absence of
oxygen. A low carrier gas pressure of up to 0.2 bar, preferably
less than 0.1 bar, can be accepted without disadvantages. Suitable
carrier gases are, in particular, noble gases such as argon and
helium and/or hydrogen.
[0018] The increase in depth of the lamellar structures decreases
with increasing depth as a result of the longer diffusion path
along the interface between reduced metallic lamellae and the oxide
of the reducing metal formed between the metallic lamellae. It has
been found that essentially no transformation of the lamellar
structure takes place during the reduction up to a depth in the
material of up to 1 .mu.m.
[0019] Preference is therefore given, according to the invention,
to using valve metal oxide powders whose smallest cross-sectional
dimension of the primary structure particle size (crystallite
dimension) does not exceed 2 .mu.m, preferably 1 .mu.m,
particularly preferably an average of 0.5 .mu.m. The valve metal
oxide powders can be used as porous sintered agglomerates if the
primary structures have appropriately small dimensions. It is also
advantageous for the primary particles to be sintered together
strongly but a hierarchically structured network of open pores to
be present between the agglomerated primary particles, so that the
pore size distribution of the open pores makes it possible for the
vapour of the reducing metal to directly reach and reduce a very
large proportion of the surfaces of the primary particles.
[0020] Even though they are significantly less effective than the
pore channels, grain boundaries between adjacent primary particles
can also accelerate diffusion. It is therefore advantageous for
very high proportions of grain boundaries between the primary
particles to be formed in addition to small primary particles and
an open porosity in the aggregated valve metal oxide particles.
This is achieved by optimization of the primary particle size and
sintering in the precipitation of oxide precursors as hydroxides
and the calcination of the hydroxides to form the valve metal
oxides. Calcination is preferably carried out at temperatures of
from 400 to 700.degree. C. The calcination temperatures are
particularly preferably from 500 to 600.degree. C.
[0021] In the production of metal foils or wires having a lamellar
surface structure, preference is given to using metal foils or
wires whose surface has an oxide layer having a thickness of less
than 1 .mu.m, preferably less than 0.5 .mu.m.
[0022] After the reduction under subatmospheric pressure, which can
take from a few minutes to some hours, preferably from about 10 to
90 minutes, depending on the reducing metal vapour or metal vapour
mixture used and its vapour pressure, the reduction is stopped by
interrupting the supply of the vapour of the reducing metal and the
reduced valve metal is quickly cooled to a temperature below
100.degree. C. in order to stabilize the nanosize lamellar
structure of layers of valve metal or valve metal suboxide and
oxide of the reducing metal. Sintering of adjacent lamellar
structures having different orientations with slight coarsening can
be accepted. Cooling can be brought about, for example, by means of
a fast pressure increase by introduction of protective gas (cooling
gas), preferably argon or helium. Preference is given to cooling to
300.degree. C. within 3 minutes, further to 200.degree. C. within a
further 3 minutes and further to 100.degree. C. within a further 5
minutes.
[0023] According to the invention, the reduction is preferably
carried out at a comparatively low temperature to minimize
coarsening of the nanosize lamellar structures. A temperature of
the valve metal oxide to be reduced of from 500 to 850.degree. C.,
more preferably less than 750.degree. C., particularly preferably
less than 650.degree. C., is preferred. Here, the actual
temperature can be considerably exceeded at the beginning of the
reduction because of the exothermic nature of the reduction
reaction.
[0024] The various measures according to the invention for avoiding
disintegration and coarsening of the nanosize lamellar structures
composed of reaction product and oxidized reducing metal which are
initially formed in the reduction can be used as alternatives or in
combination.
[0025] For example, at a high reduction temperature it is
sufficient to ensure a short reduction time by providing for
effective, fast access of the vapour of the reducing metal, for
example by means of a small powder bed of the starting metal oxide
and/or a reduced carrier gas pressure, i.e. an increased free path
length for the atoms of the vapour of the reducing metal.
[0026] On the other hand, at a low reduction temperature longer
reduction times can be accepted.
[0027] Starting valve metal oxide powder agglomerates having an
advantageous open-pore structure require less stringent process
conditions to achieve the lamellar structure according to the
invention.
[0028] After the reduction is complete and the reduced valve metal
oxide has been cooled and made inert by gradual introduction of
oxygen or air, the enclosed oxide of the reducing metal can be
leached from the resulting nanosize structure, for example by means
of mineral acids such as sulphuric acid or hydrochloric acid or
mixtures thereof, washed with demineralized water until neutral and
dried.
[0029] In the case of the reduction of finely divided powders,
these comprise particles having a tabular primary structure which
are partly grown into one another in a dendrite-like fashion.
[0030] After the oxides of the reducing metals have been leached
out, the now free-standing lamellar structures of the valve metals
remain geometrically stable since they are sufficiently well
sintered to the adjacent, generally differently oriented lamellar
structures via the end parts of the individual layers. The original
(polycrystalline) valve metal oxide particle has thus been
converted into an aggregated valve metal particle whose primary
particles comprise layer structure groups of differing orientation
and which are sintered to one another. Overall, a stable
interpenetrating structure of metal and "flat" pores has thus been
formed.
[0031] FIG. 1 schematically shows an apparatus for carrying out the
process of the invention. The reactor which is generally denoted by
1 has a reduction chamber 2. Reference numeral 3 denotes the
temperature control which comprises heating coils and cooling
coils. Protective or flushing gas or cooling gas is introduced via
a valve into the reduction chamber in the direction of the arrow 4.
The reduction chamber is evacuated or gases are taken off in the
direction of the arrow 5. The reduction chamber 2 is joined by a
vaporization chamber 6 for the reducing metal which is provided
with separate heating 7. The thermal separation of vaporization
chamber and reduction chamber is effected by means of the valve
region 8. The valve metal oxide to be reduced is present as a thin
powder bed in the boat 10. If valve metal oxide foils or wires or
foils or wires having a surface composed of valve metal oxide are
used, these are preferably suspended vertically and parallel to the
flow of the vapour of the reducing metal in the reduction chamber.
The reducing metal in the boat 9 is heated to a temperature which
provides the desired vapour pressure.
[0032] The oxide powder is introduced as a bed having a height of 5
mm in a boat. A boat containing magnesium turnings is placed in the
vaporization chamber. The reactor is flushed with argon. The
reduction chamber is then heated to the reduction temperature and
evacuated to a pressure of 0.1 bar. The vaporization chamber is
subsequently heated to 800.degree. C. The magnesium vapour pressure
(static) is about 0.04 bar. After 30 minutes, the heating of
reduction chamber and vaporization chamber is switched off and
argon which has been cooled by depressurization from 200 bar is
introduced and passed through the reduction chamber for a further
period. The reduction chamber walls are at the same time cooled by
means of water.
[0033] FIGS. 2, 3 and 4 show transmission electron micrographs of
tantalum powder which has been reduced according to the invention
after focused ion beam preparation of the reduction product at
various magnifications. The dark stripes in the figures are
tantalum lamellae and the light-coloured stripes are magnesium
oxide lamellae. The different orientations of the lamellar
structures correspond to different crystallite orientations of the
starting tantalum pentoxide.
* * * * *