U.S. patent application number 10/250499 was filed with the patent office on 2004-03-18 for refractory hard metals in powder form for use in the manufacture of electrodes.
Invention is credited to Blouin, Marco, Boily, Sabin.
Application Number | 20040052713 10/250499 |
Document ID | / |
Family ID | 4168043 |
Filed Date | 2004-03-18 |
United States Patent
Application |
20040052713 |
Kind Code |
A1 |
Boily, Sabin ; et
al. |
March 18, 2004 |
Refractory hard metals in powder form for use in the manufacture of
electrodes
Abstract
The invention relates to a refractory hard metal in powder form
comprising particles having an average particle size of 0.1 to 30
.mu.m and each formed of an agglomerate of grains with each grain
comprising a nanocrystal of a refractory hard metal of the formula
(I): A.sub.xB.sub.yX.sub.z wherein A is a transition metal, B is a
metal selected from the group consisting of zirconium, hafnium,
vanadium, niobium, tantalum, chromium, molybdenum, manganese,
tungsten and cobalt, X is boron or carbon, x ranges from 0.1 to 3,
y ranges from 0 to 3 and z ranges from 1 to 6. The refractory hard
metal in powder form according to the invention is suitable for use
in the manufacture of electrodes by thermal deposition or powder
metallurgy.
Inventors: |
Boily, Sabin; (Quebec,
CA) ; Blouin, Marco; (Quebec, CA) |
Correspondence
Address: |
FOLEY AND LARDNER
SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Family ID: |
4168043 |
Appl. No.: |
10/250499 |
Filed: |
July 3, 2003 |
PCT Filed: |
January 2, 2002 |
PCT NO: |
PCT/CA02/00013 |
Current U.S.
Class: |
423/276 ;
423/414 |
Current CPC
Class: |
C01P 2002/60 20130101;
B82Y 30/00 20130101; C01B 32/921 20170801; C01P 2004/62 20130101;
C01P 2004/64 20130101; C25C 3/12 20130101; C01B 32/90 20170801;
C01B 32/907 20170801; C01P 2002/72 20130101; C01B 32/914 20170801;
C01P 2004/50 20130101; C01P 2004/61 20130101; C01B 35/04
20130101 |
Class at
Publication: |
423/276 ;
423/414 |
International
Class: |
C22C 029/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 5, 2001 |
CA |
2,330,352 |
Claims
1. A refractory hard metal in powder form comprising particles
having an average particle size of 0.1 to 30 .mu.m and each formed
of an agglomerate of grains with each grain comprising a
nanocrystal of a refractory hard metal of the formula:
A.sub.xB.sub.yX.sub.z (I) wherein A is a transition metal, B is a
metal selected from the group consisting of zirconium, hafnium,
vanadium, niobium, tantalum, chromium, molybdenum, manganese,
tungsten and cobalt, X is boron or carbon, x ranges from 0.1 to 3,
y ranges from 0 to 3 and z ranges from 1 to 6.
2. A refractory hard metal in powder form according to claim 1,
wherein A is a transition metal selected from the group consisting
of titanium, chromium, zirconium and vanadium.
3. A refractory hard metal in powder form according to claim 2,
wherein A is titanium, X is boron and y is 0.
4. A refractory hard metal in powder form according to claim 3,
wherein x is 1 and z is 1.8.
5. A refractory hard metal in powder form according to claim 3,
wherein x is 1 and z is 2.
6. A refractory hard metal in powder form according to claim 3,
wherein x is 1 and z is 2.2.
7. A refractory hard metal in powder form according to claim 2,
wherein A is titanium, X is carbon and y is 0.
8. A refractory hard metal in powder form according to claim 7,
wherein x is 1 and z is 1.
9. A refractory hard metal in powder form according to claim 2,
wherein A is titanium, B is zirconium or hafnium, X is boron and y
is other than 0.
10. A refractory hard metal in powder form according to claim 9,
wherein B is zirconium, x is 0.5, y is 0.5 and z is 2.
11. A refractory hard metal in powder form according to claim 9,
wherein B is zirconium, x is 0.9, y is 0.1 and z is 2.
12. A refractory hard metal in powder form according to claim 2,
wherein B is hafnium, x is 0.5, y is 0.5 and z is 2.
13. A refractory hard metal in powder form according to claim 2,
wherein A is zirconium, B is vanadium, X is boron and y is other
than 0.
14. A refractory hard metal in powder form according to claim 13,
wherein x is 0.8, y is 0.2 and z is 2.
15. A refractory hard metal in powder form according to claim 1,
wherein said average particle size ranges from 1 to 5 .mu.m.
16. A process for producing a refractory hard metal in powder form
as defined in claim 1, comprising the steps of: a) providing a
first reagent selected from the group consisting of transition
metals and transition metal-containing compounds; b) providing a
second reagent selected from the group consisting of boron,
boron-containing compounds, carbon and carbon-containing compounds;
c) providing an optional third reagent selected from the group
consisting of zirconium, zirconium-containing compounds, hafnium,
hafnium-containing compounds, vanadium, vanadium-containing
compounds, niobium, niobium-containing compounds, tantalum,
tantalum-containing compounds, chromium, chromium-containing
compounds, molybdenum, molybdenum-containing compounds, manganese,
manganese-containing compounds, tungsten, tungsten-containing
compounds, cobalt and cobalt-containing compounds; and d)
subjecting said first, second and third reagents to high-energy
ball milling to cause solid state reaction therebetween and
formation of particles having an average particle size of 0.1 to 30
.mu.m, each particle being formed of an agglomerate of grains with
each grain comprising a nanocrystal of a refractory hard metal of
the formula (I) as defined in claim 1.
17. A process according to claim 16, wherein said first reagent
comprises a transition metal selected from the group consisting of
titanium, chromium, zirconium and vanadium.
18. A process according to claim 17, wherein said transition metal
is titanium.
19. A process according to claim 16, wherein said first reagent
comprises a titanium-containing compound selected from the group
TiH.sub.2, TiAl.sub.3, TiB and TiCl.sub.2.
20. A process according to claim 16, wherein said second reagent
comprises boron.
21. A process according to claim 16, wherein said second reagent
comprises a boron-containing compound selected from the group
consisting of AlB.sub.2, AlB.sub.12, BH.sub.3, BN, VB.sub.2,
H.sub.2BO.sub.3 and Na.sub.2B.sub.4O.sub.7.
22. A process according to claim 16, wherein said second reagent
comprises carbon.
23. A process according to claim 16, wherein said second reagent
comprises tetraboron carbide.
24. A process according to claim 16, wherein said third reagent is
a compound selected from the group consisting of HfB.sub.2,
VB.sub.2, NbB.sub.2, TaB.sub.2, CrB.sub.2, MoB.sub.2, MnB.sub.2,
Mo.sub.2B.sub.5, W.sub.2B.sub.5, CoB, ZrC, TaC, WC and HfC.
25. A process according to claim 16, wherein step (d) is carried
out in a vibratory ball mill operated at a frequency of 8 to 25
Hz.
26. A process according to claim 25, wherein said vibratory ball
mill is operated at a frequency of about 17 Hz.
27. A process according to claim 16, wherein step (d) is carried
out in a rotary ball mill operated at a speed of 150 to 1500
r.p.m.
28. A process according to claim 27, wherein said rotary ball mill
is operated at a speed of about 1000 r.p.m.
29. A process according to claim 16, wherein step (d) is carried
out under an inert gas atmosphere.
30. A process according to claim 29, wherein said inert gas
atmosphere comprises argon or helium.
31. A process according to claim 16, wherein step (d) is carried
out under a reactive gas atmosphere.
32. A process according to claim 31, wherein said reactive gas
atmosphere comprises hydrogen, ammonia or a hydrocarbon.
33. A process according to claim 16, wherein step (d) is carried
out for a period of time of about 5 hours.
34. A process according to claim 16, wherein a sintering aid is
added during step (d).
35. A process for producing a refractory hard metal in powder form
as defined in claim 5 or 8, comprising subjecting TiB.sub.2 or TiC
to high-energy ball milling to cause formation of particles having
an average particle size of 0.1 to 30 .mu.m, each particle being
formed of an agglomerate of grains with each grain comprising a
nanocrystal of TiB.sub.2 or TiC.
36. A process according to claim 35, wherein said high-energy ball
milling is carried out in a vibratory ball mill operated at a
frequency of 8 to 25 Hz.
37. A process according to claim 36, wherein said vibratory ball
mill is operated at a frequency of about 17 Hz.
38. A process according to claim 35, wherein said high-energy ball
milling is carried out in a rotary ball mill operated at a speed of
150 to 1500 r.p.m.
39. A process according to claim 38, wherein said rotary ball mill
is operated at a speed of about 1000 r.p.m.
40. A process according to claim 35, wherein said high-energy ball
milling is carried out under an inert gas atmosphere.
41. A process according to claim 40, wherein said inert gas
atmosphere comprises argon or helium.
42. A process according to claim 35, wherein said high-energy ball
milling is carried out under a reactive gas atmosphere.
43. A process according to claim 42, wherein said reactive gas
atmosphere comprises hydrogen, ammonia or a hydrocarbon.
44. A process according to claim 35, wherein said high-energy ball
milling is carried out for a period of time of about 20 hours.
45. A process according to claim 35, wherein a sintering aid is
added during said high-energy ball milling.
Description
FIELD OF THE INVENTION
[0001] The present invention pertains to improvements in the field
of electrodes for metal electrolysis. More particularly, the
invention relates to refractory hard metals in powder form for use
in the manufacture of such electrodes.
BACKGROUND ART
[0002] Aluminum is produced conventionally in a Hall-Hroult
reduction cell by the electrolysis of alumina dissolved in molten
cryolite (Na.sub.3AlF.sub.6) at temperatures of up to about
950.degree. C. A Hall-Hroult cell typically has a steel shell
provided with an insulating lining of refractory material, which in
turn has a lining made of prebaked carbon blocks contacting the
molten constituents of the electrolyte. The carbon lining acts as
the cathode substrate and the molten aluminum pool acts as the
cathode. The anode is a consumable carbon electrode, usually
prebaked carbon made by coke calcination.
[0003] During electrolysis in Hall-Hroult cells, the carbon anode
is consumed leading to the evolution of greenhouse gases such as CO
and CO.sub.2. The anode has to be periodically changed and the
erosion of the material modifies the anode-cathode distance, which
increases the voltage due to the electrolyte resistance. On the
cathode side, the carbon blocks are subjected to erosion and
electrolyte penetration. A sodium intercalation in the graphitic
structure occurs, which causes swelling and deformation of the
cathode carbon blocks. The increase of voltage between the
electrodes adversely affects the energy efficiency of the
process.
[0004] Extensive research has been carried out with refractory hard
metals such as TiB.sub.2, as electrode materials. TiB.sub.2 and
other refractory hard metals are practically insoluble in aluminum,
have a low electrical resistance and are wetted by aluminum.
However, the shaping of TiB.sub.2 and similar refractory hard
metals is difficult because these materials have high melting
temperatures and are highly covalent.
DISCLOSURE OF THE INVENTION
[0005] It is therefore an object of the present invention to
overcome the above drawbacks and to provide a refractory hard metal
in powder form suitable for the manufacture of electrodes by
thermal deposition or powder metallurgy.
[0006] According to one aspect of the invention, there is provided
a refractory hard metal in powder form comprising particles having
an average particle size of 0.1 to 30 .mu.m and each formed of an
agglomerate of grains with each grain comprising a nanocrystal of a
refractory hard metal of the formula:
A.sub.xB.sub.yX.sub.z (I)
[0007] wherein A is a transition metal, B is a metal selected from
the group consisting of zirconium, hafnium, vanadium, niobium,
tantalum, chromium, molybdenum, manganese, tungsten and cobalt, X
is boron or carbon, x ranges from 0.1 to 3, y ranges from 0 to 3
and z ranges from 1 to 6.
[0008] The term "nanocrystal" as used herein refers to a crystal
having a size of 100 nanometers or less.
[0009] The expression "thermal deposition" as used herein refers to
a technique in which powder particles are injected in a torch and
sprayed on a substrate. The particles acquire a high velocity and
are partially or totally melted during the flight path. The coating
is built by the solidification of the droplets on the substrate
surface. Examples of such techniques include plasma spray, arc
spray and high velocity oxy-fuel.
[0010] The expression "powder metallurgy" as used herein refers to
a technique in which the bulk powders are transformed into preforms
of a desired shape by compaction or shaping followed by a sintering
step. Compaction refers to techniques where pressure is applied to
the powder, as, for example, cold uniaxial pressing, cold isostatic
pressing or hot isostatic pressing. Shaping refers to techniques
executed without the application of external pressure such as
powder filling or slurry casting.
[0011] The present invention also provides, in another aspect
thereof, a process for producing a refractory hard metal in powder
form as defined above. The process of the invention comprises the
steps of:
[0012] a) providing a first reagent selected from the group
consisting of transition metals and transition metal-containing
compounds;
[0013] b) providing a second reagent selected from the group
consisting of boron, boron-containing compounds, carbon and
carbon-containing compounds;
[0014] c) providing an optional third reagent selected from the
group consisting of zirconium, zirconium-containing compounds,
hafnium, hafnium-containing compounds, vanadium,
vanadium-containing compounds, niobium, niobium-containing
compounds, tantalum, tantalum-containing compounds, chromium,
chromium-containing compounds, molybdenum, molybdenum-containing
compounds, manganese, manganese-containing compounds, tungsten,
tungsten-containing compounds, cobalt and cobalt-containing
compounds; and
[0015] d) subjecting the first, second and third reagents to
high-energy ball milling to cause solid state reaction therebetween
and formation of particles having an average particle size of 0.1
to 30 .mu.m, each particle being formed of an agglomerate of grains
with each grain comprising a nanocrystal of a refractory hard metal
of formula (I) defined above.
[0016] The expression "high-energy ball milling" as used herein
refers to a ball milling process capable of forming the aforesaid
particles comprising nanocrystalline grains of the refractory hard
metal of formula (I), within a period of time of about 40
hours.
DESCRIPTION OF DRAWING
[0017] In the accompanying drawing, the sole FIGURE shows the X-ray
diffraction of the refractory hard metal in powder form obtained in
Example 1.
MODES FOR CARRYING OUT THE INVENTION
[0018] Typical examples of refractory hard metals of the formula
(I) include TiB.sub.1.8, TiB.sub.2, TiB.sub.2.2, TiC,
Ti.sub.0.5Zr.sub.0.5B.s- ub.2, Ti.sub.0.9Zr.sub.0.1B.sub.2,
Ti.sub.0.5Hf.sub.0.5B.sub.2 and Zr.sub.0.8V.sub.0.2B.sub.2.
TiB.sub.2 is preferred.
[0019] Examples of suitable transition metals which may be used as
the aforesaid first reagent include titanium, chromium, zirconium
and vanadium. Titanium is preferred. It is also possible to use a
titanium-containing compound such as TiH.sub.2, TiAl.sub.3, TiB and
TiCl.sub.2.
[0020] Examples of suitable boron-containing compounds which may be
used as the aforesaid second reagent include AlB.sub.2, AlB.sub.12,
BH.sub.3, BN, VB, H.sub.2BO.sub.3 and Na.sub.2B.sub.4O.sub.7. It is
also possible to use tetraboron carbide (B.sub.4C) as either a
boron-containing compound or a carbon-containing compound.
[0021] Examples of suitable compounds which may be used as the
aforesaid third reagent include HfB.sub.2, VB.sub.2, NbB.sub.2,
TaB.sub.2, CrB.sub.2, MoB.sub.2, MnB.sub.2, Mo.sub.2B.sub.5,
W.sub.2B.sub.5, CoB, ZrC, TaC, WC and HfC.
[0022] According to a preferred embodiment, step (d) of the process
according to the invention is carried out in a vibratory ball mill
operated at a frequency of 8 to 25 Hz, preferably about 17 Hz. It
is also possible to conduct step (d) in a rotary ball mill operated
at a speed of 150 to 1500 r.p.m., preferably about 1000 r.p.m.
[0023] According to another preferred embodiment, step (d) is
carried out under an inert gas atmosphere such as a gas atmosphere
comprising argon or helium, or under a reactive gas atmosphere such
as a gas atmosphere comprising hydrogen, ammonia or a hydrocarbon,
in order to saturate dangling bonds and thereby prevent oxidation
of the refractory hard metal. An atmosphere of argon, helium or
hydrogen is preferred. It is also possible to coat the particles
with a protective film or to admix a sacrificial element such as Mg
or Ca with the reagents. In addition, a sintering aid such as
Y.sub.2O.sub.3 can be added during step (d).
[0024] In the particular case of TiB.sub.2 or TiC wherein titanium
and boron or carbon are present in stoichiometric quantities, these
two compounds can be used as starting material. Thus, they can be
directly subjected to high-energy ball milling to cause formation
of particles having an average particle size of 0.1 to 30 .mu.m,
each particle being formed of an agglomerate of grains with each
grain comprising a nanocrystal of TiB.sub.2 or TiC.
[0025] The high-energy ball milling described above enables one to
obtain refractory hard metals in powder form having either
non-stoichiometric or stoichiometric compositions.
[0026] The refractory hard metals in powder form according to the
invention are suitable for use in the manufacture of electrodes by
thermal deposition or powder metallurgy. Due to the properties of
refractory hard metals, the emission of toxic and greenhouse effect
gases during metal electrolysis is lowered and the lifetime of the
electrodes is increased, thus lowering maintenance costs. A lower
and constant inter-electrode distance is also possible, thereby
decreasing the electrolyte ohmic drop.
[0027] The following non-limiting examples illustrate the
invention.
EXAMPLE 1
[0028] A TiB.sub.2 powder was produced by ball milling 3.45 g of
titanium and 1.55 g of boron in a hardened steel crucible with a
ball-to-powder mass ratio of 4.5:1 using a SPEX 8000 (trademark)
vibratory ball mill operated at a frequency of about 17 Hz. The
operation was performed under a controlled argon atmosphere to
prevent oxidization. The crucible was closed and sealed with a
rubber O-ring. After 5 hours of high-energy ball milling, a
TiB.sub.2 structure was formed, as shown on the X-ray diffraction
pattern in the accompanying drawing. The structure of TiB.sub.2 is
hexagonal with the space group P6/mmm (191). The particle size
varied between 1 and 5 .mu.m and the crystallite size, measured by
X-ray diffraction, was about 30 nm.
EXAMPLE 2
[0029] A TiB.sub.2 powder was produced according to the same
procedure as described in Example 1 and under the same operating
conditions, with the exception that the ball milling was carried
out for 20 hours instead of 5 hours. The resulting powder was
similar to that obtained in Example 1. The crystallite size,
however, was lower (about 16 nm).
EXAMPLE 3
[0030] A TiC powder was produced according to the same procedure as
described in Example 1 and under the same operating conditions,
with the exception that titanium and graphite were milled.
EXAMPLE 4
[0031] A TiB.sub.2 powder was produced by ball milling titanium
diboride under the same operating conditions as in Example 1, with
the exception that the ball milling was carried out for 20 hours
instead of 5 hours. The starting structure was maintained, but the
crystallite size decreased to 15 nm.
EXAMPLE 5
[0032] A TiB.sub.1.8 powder was according to the same procedure as
described in Example 1 and under the same operating conditions,
with the exception that 3.6 g of titanium and 1.4 g of boron were
milled.
EXAMPLE 6
[0033] A TiB.sub.2.2 powder was according to the same procedure as
described in Example 1 and under the same operating conditions,
with the exception that 3.4 g of titanium and 1.7 g of boron were
milled.
EXAMPLE 7
[0034] A Ti.sub.0.5Zr.sub.0.5B.sub.2 powder was according to the
same procedure as described in Example 1 and under the same
operating conditions, with the exception that 1.9 g of titanium,
3.1 g of zirconium diboride and 0.8 g of boron were milled.
EXAMPLE 8
[0035] A Ti.sub.0.9Zr.sub.0.1B.sub.2 powder was according to the
same procedure as described in Example 1 and under the same
operating conditions, with the exception that 2.9 g of titanium,
0.6 g of zirconium and 1.5 g of boron were milled.
EXAMPLE 9
[0036] A Ti.sub.0.5Hf.sub.0.5B.sub.2 powder was according to the
same procedure as described in Example 1 and under the same
operating conditions, with the exception that 0.9 g of titanium,
3.3 g of hafnium and 0.8 g of boron were milled.
EXAMPLE 10
[0037] A Zr.sub.0.8V.sub.0.2B.sub.2 powder was according to the
same procedure as described in Example 1 and under the same
operating conditions, with the exception that 3.5 g of zirconium,
0.5 g of vanadium and 1.0 g of boron were milled.
* * * * *