Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02330352 2001-O1-OS
REFRACTORY HARD METALS IN POWDER FORM FOR
USE IN THE MANUFACTURE OF ELECTRODES
The present invention pertains to improvements in the field of
electrodes for metal electrolysis. More particularly, the invention relates to
a
refractory hard metals in powder form for use in the manufacture of such
electrodes.
Aluminum is produced conventionally in a Hall-Heroult
reduction cells by the electrolysis of alumina dissolved in molten cryolite
(Na3A1F6) at temperatures of up to about 950 °C. A Hall-Heroult 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.
During electrolysis, in Hall-Heroult cells, the carbon anode is
consumed leading to the evolution of greenhouse gases such as CO and C02.
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 cause swelling and deformation of the cathode carbon
blocks. The increase of voltage between the electrodes adversely affects the
energy efficiency of the process.
Extensive research has been carried out with refractory hard
metals such as TiBz, as electrode materials. TiB2 and other refractory hard
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metals are practically insoluble in aluminum, have a low electrical resistance
and are wetted by aluminum. However, the shaping of TiB2 and similar
refractory hard metals is difficult because these materials have high melting
temperatures and are highly covalent.
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 electrode by thermal deposition or powder
metallurgy.
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 dm and each formed of an agglomerate of grains with
each grain comprising a nanocrystal of a refractory hard metal of the formula:
AXByXZ ~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 from 1 to 6.
The term "nanocrystal" as used herein refers to a crystal having a
size of 100 nanometers or less.
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 budded by the solidification of
the
droplets on the substrate surface. Examples of such techniques include plasma
spray, arc spray and high velocity oxy-fuel.
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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.
Typical examples of refractory hard metals of the formula (I)
include TiBl.s, TiB2, TiC, Tio.sZro.sBz~ Tio.9Zro.~Bz~ Tio.sHfo.sBz and
Zro.sVo.2B2~
TiB2 is preferred.
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:
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, chromium, chromium-containing compounds,
molybdenum, molybdenum-containing compounds, manganese, manganese-
containing compounds, tungsten, tungsten-containing compounds, cobalt and
cobalt-containing compounds; and
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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 Vim, 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.
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.
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 TiH2, TiAl3, TiB and TiCl2.
Examples of suitable boron-containing compounds which may be
used as the aforesaid second reagent include A1B2, A1B,2, BH3, BN, VB, H2B03
and Na2B40~. It is also possible to use tetraboron carbide (B4C) as either a
boron-containing compound or a carbon-containing compound.
Examples of suitable compounds which may be used as the
aforesaid third reagent include HfB2, VBZ, NbB2, TaB2, CrB2, MoB2, MnB2,
Mo2B5, W2B5, CoB, ZrC, TaC, WC and HfC.
According to a preferred embodiment, step (d) 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.
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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 Y203 can be
added
during step (d).
In the particular case of TiB2 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 Vim, each particle being formed of an agglomerate of grains with
each
grain comprising a nanocrystal of TiB2 or TiC.
The high-energy ball milling described above enables one to
obtain refractory hard metals in powder form having either non-stoichiometric
or stoichiometric compositions.
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.
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The following non-limiting examples illustrate the invention,
reference being made to the accompanying drawing in which the sole figure
shows the X-ray diffraction of the refractory hard metal in powder form
obtained in Example 1.
EXAMPLE 1.
A TiB2 powder was produced by ball milling 3.45g of titanium
and I.SSg 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 TiB2
structure
was formed, as shown on the X-ray diffraction pattern in the accompanying
drawing. The structure of TiB2 is hexagonal with the space group P6/mmm
( 191 ). The particle size varied between 1 and 5 pm and the crystallite size,
measured by X-ray diffraction, was about 30 nm.
EXAMPLE 2.
A TiB2 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).
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EXAMPLE 3.
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.
A TiB2 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.
A TiB 1.g 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.
A TiB2.2 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.
A TiBo.SZro.5B2 powder was according to the same procedure as
described in Example 1 and under the same operating conditions, with the
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exception that 1.9 g of titanium, 3.1 g of zirconium diboride and 0.8 g of
boron
were milled.
EXAMPLE 8.
A TiBo.9Zro,lBz 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.
A TiBo.SHfo,5B2 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.
A Zro,gVo.2B2 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.
_g_
