INERT ELECTRODE MATERIAL IN NANOCRYSTALLINE POWDER FORM

MATERIAU D'ELECTRODE INERTE SOUS FORME DE POUDRE NANOCRISTALLINE

English Abstract

The invention relates to an inert electrode material in powder form comprising
particles having an average particle size of 0.1 to 100 ~m and each formed of
an agglomerate of grains of a ceramic material and grains of a metal or alloy
with each grain of ceramic material comprising a nanocrystal of the ceramic
material and each grain of metal or alloy comprising a nanocrystal of the
metal or alloy. Alternatively, each particle can be formed of an agglomerate
of grains with each grain comprising a nanocrystal of a single phase ceramic
material, a metal or an alloy. The electrode material in powder form according
to the invention is useful for the manufacture of inert electrodes having
improved thermal shock and corrosion resistance properties.

French Abstract

La présente invention concerne un matériau d'électrode inerte, se présentant sous forme de poudre et comprenant des particules de taille moyenne allant de 0,1 à 100 µm, chacune constituée d'un agglomérat de grains d'une matière céramique et de grains d'un métal ou d'un alliage. Chaque grain de matière céramique comprend un nanocristal de la matière céramique et chaque grain de métal ou d'alliage comprend un nanocristal du métal ou de l'alliage. En variante, chaque particule peut être constituée d'un agglomérat de grains, chaque grain comprenant un nanocristal d'une matière céramique à une phase, d'un métal ou d'un alliage. Le matériau d'électrode sous forme de poudre selon cette invention est utilisé dans la production d'électrodes inertes présentant des propriétés de résistance au choc thermique et à la corrosion améliorées.

Claims

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CLAIMS:

1. An inert electrode material in powder form comprising particles
having an average particle size of 0.1 to 100 µm and each formed of an
agglomerate of grains of a ceramic material and grains of a metal or alloy
with
each grain of ceramic material comprising a nanocrystal of said ceramic
material and each grain of metal or alloy comprising a nanocrystal of said
metal or alloy.

2. An inert electrode material according to claim 1, wherein each
said particle is formed of an agglomerate of said grains of ceramic material
and
said grains of metal.

3. An inert electrode material according to claim 2, wherein said
ceramic material comprises an oxide, nitride or carbide of a metal selected
from the group consisting of transition metals, p-group metals, rare earth
metals and alkaline earth metals.

4. An inert electrode material according to claim 3, wherein said
ceramic material comprises an oxide, nitride or carbide of a transition metal
selected from the group consisting of Ag, Co, Cu, Cr, Fe, Ir, Mo, Mn, Nb, Ni,
Ru, Ta, Ti, V, W, Y, Zn and Zr.

5. An inert electrode material according to claim 3, wherein said
ceramic material comprises an oxide, nitride or carbide of a p-group metal
selected from the group consisting of Al, Ge, In, Pb, Sb, Si and Sn.

6. An inert electrode material according to claim 3, wherein said
ceramic material comprises an oxide, nitride or carbide of a rare earth metal
selected from the group consisting of Ce, La and Th.





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7. An inert electrode material according to claim 3, wherein said
ceramic material comprises an oxide, nitride or carbide of an alkaline earth
metal selected from the group consisting of Ca, Mg and Sr.

8. An inert electrode material according to claim 2, wherein said
metal is selected from the group consisting of chromium, cobalt, copper, gold,
iridium, iron, nickel, niobium, palladium, platinum, rubidium, ruthenium,
silicon, silver, titanium, yttrium and zirconium.

9. An inert electrode material according to claim 1, wherein each
said particle is formed of an agglomerate of said grains of ceramic material
and
said grains of alloy.

10. An inert electrode material according to claim 9, wherein said
ceramic material comprises an oxide, nitride or carbide of a metal selected
from the group consisting of transition metals, p-group metals, rare earth
metals and alkaline earth metals.

11. An inert electrode material according to claim 10, wherein said
ceramic material comprises an oxide, nitride or carbide of a transition metal
selected from the group consisting of Ag, Co, Cu, Cr, Fe, Ir, Mo, Mn, Nb, Ni,
Ru, Ta, Ti, V, W, Y, Zn and Zr.

12. An inert electrode material according to claim 10, wherein said
ceramic material comprises an oxide, nitride or carbide of a p-group metal
selected from the group consisting of Al, Ge, In, Pb, Sb, Si and Sn.





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13. An inert electrode material according to claim 10, wherein said
ceramic material comprises an oxide, nitride or carbide of a rare earth metal
selected from the group consisting of Ce, La and Th.

14. An inert electrode material according to claim 10, wherein said
ceramic material comprises an oxide, nitride or carbide of an alkaline earth
metal selected from the group consisting of Ca, Mg and Sr.

15. An inert electrode material according to claim 9, wherein said
alloy is selected from the group consisting of Cu-Ag, Cu-Ag-Ni, Cu-Ni, Cu-Ni-
Fe, Cu-Pd, Cu-Pt and Ni-Fe alloys.

16. An inert electrode material according to claim 15, wherein said
alloy is a Cu-Ag alloy.

17. An inert electrode material according to claim 16, wherein said
ceramic material comprises a NiFe2O4 spinel.

18. An inert electrode material in powder form comprising particles
having an average particle size of 0.1 to 100 µm and each formed of an
agglomerate of grains with each grain comprising a nanocrystal of a single
phase ceramic material, wherein ceramic materials selected from the group
consisting of CeO2, SiC, WC and carbides, nitrides and borides of Nb, Ti, V
and Zr are excluded.

19. An inert electrode material according to claim 18, wherein said
ceramic material comprises an oxide of a transition metal selected from the
group consisting of Ag, Co, Cu, Cr, Fe, Ir, Mo, Mn, Nb, Ni, Ru, Ta, Ti, V, W,
Y, Zn and Zr.





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20. An inert electrode material according to claim 18, wherein said
ceramic material comprises a nitride or carbide of a transition metal selected
from the group consisting of Ag, Co, Cu, Cr, Fe, Ir, Mo, Mn, Ni, Ru, Ta, Y and
Zn.

21. An inert electrode material according to claim 18, wherein said
ceramic material comprises an oxide or nitride of a p-group metal selected
from the group consisting of Al, Ge, In, Pb, Sb, Si and Sn.

22. An inert electrode material according to claim 18, wherein said
ceramic material comprises a carbide of a p-group metal selected from the
group consisting of Al, Ge, In, Pb, Sb and Sn.

23. An inert electrode material according to claim 18, wherein said
ceramic material comprises an oxide of a rare earth metal selected from the
group consisting of La and Th.

24. An inert electrode material according to claim 18, wherein said
ceramic material comprises a nitride or carbide of a rare earth metal selected
from the group consisting of Ce, La and Th.

25. An inert electrode material according to claim 18, wherein said
ceramic material comprises an oxide, nitride or carbide of an alkaline earth
metal selected from the group consisting of Ca, Mg and Sr.

26. An inert electrode material according to claim 18, wherein said
ceramic material is zinc oxide.






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27. An inert electrode material according to claim 18, wherein said
ceramic material includes at least one dopant comprising an element selected
from the group consisting of Al, Co, Cr, Cu, Fe, Mo, Nb, Ni, Sb, Si, Sn, Ti,
V,
W, Y, Zn and Zr.

28. An inert electrode material according to claim 27, wherein said
dopant is present in an amount of about 0.002 to about 1 wt.%.

29. An inert electrode material according to claim 28, wherein the
amount of dopant ranges from about 0.005 to about 0.05 wt.%.

30. An inert electrode material according to claim 29, wherein the
amount of dopant is about 0.008 wt.%.

31. An inert electrode material according to claim 27, wherein said
ceramic material comprises zinc oxide doped with aluminum oxide.

32. An inert electrode material according to claim 31, wherein the
aluminum oxide is present in an amount of about 0.008 wt.%.

33. An inert electrode material in powder form comprising particles
having an average particle size of 0.1 to 100 µm and each formed of an
agglomerate of grains with each grain comprising a nanocrystal of a metal.

34. An inert electrode material according to claim 33, wherein said
metal is selected from the group consisting of chromium, cobalt, copper, gold,
iridium, iron, nickel, niobium, palladium, platinum, rubidium, ruthenium,
silicon, silver, titanium, yttrium and zirconium.





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35. An inert electrode material according to claim 34, wherein said
metal is copper.

36. An inert electrode material according to claim 1, 9, 18 or 33,
wherein said average particle size ranges from 1 to 10 µm.

37. An inert electrode material in powder form, for use in electrolytic
production of a metal by electrolytic reduction of a metal compound,
comprising particles having an average particle size of 1 to 30 µm and each
formed of an agglomerate of grains with each grain comprising a nanocrystal
of an alloy, wherein alloys selected from the group consisting of Cr2Nb,
CrSi2,
NbSi2 and alloys of formula (Mg1-x A x)D y in which A is an element selected
from the group consisting of Li, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, V,
Zr, Nb, Mo, In, Sn, O, Si, B, C and F, D is a metal selected from the group
consisting of Fe, Co, Ni, Ru, Rh, Pd, Ir and Pt, x is a number ranging from 0
to
0.3 and y is a number ranging from 0 to 0.15, are excluded.

38. An inert electrode material according to claim 37, wherein said
alloy is selected from the group consisting of Cu-Ag, Cu-Ag-Ni, Cu-Ni, Cu-Ni-
Fe, Cu-Pd, Cu-Pt and Ni-Fe alloys.

39. An inert electrode material according to claim 38, wherein said
alloy is a Cu-Ni alloy.

40. A process for producing an inert electrode material in powder
form as defined in claim 2, which comprises the steps of:

a) subjecting at least one metal oxide, nitride or carbide to high-
energy ball milling to form a first powder comprising particles having an





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average particle size of 0.1 to 100 µm and each formed of an agglomerate of
grains of a ceramic material;

b) subjecting a metal to high-energy ball milling to form a
second powder comprising particles having an average particle size of 0.1 to
100 µm and each formed of an agglomerate of grains with each grain
comprising a nanocrystal of said metal;

c) mixing said first and second powders to form a powder
mixture; and

d) subjecting the powder mixture obtained in step (c) to high-
energy ball milling to form a nanocrystalline powder comprising particles
having an average particle size of 0.1 to 100 µm and each formed of an
agglomerate of grains of said ceramic material and grains of said metal,
wherein each grain of ceramic material comprises a nanocrystal of said ceramic
material and each grain of metal comprises a nanocrystal of said metal.

41. A process according to claim 40, wherein said metal oxide,
nitride or carbide is an oxide, nitride or carbide of a metal selected from
the
group consisting of transition metals, p-group metals, rare earth metals and
alkaline earth metals.

42. A process according to claim 41, wherein said metal oxide,
nitride or carbide is an oxide, nitride or carbide of a transition metal
selected
from the group consisting of Ag, Co, Cu, Cr, Fe, Ir, Mo, Mn, Nb, Ni, Ru, Ta,
Ti, V, W, Y, Zn and Zr.

43. A process according to claim 41, wherein said metal oxide,
nitride or carbide is an oxide, nitride or carbide of a p-group metal selected
from the group consisting of Al, Ge, In, Pb, Sb, Si and Sn.





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44. A process according to claim 41, wherein said metal oxide,
nitride or carbide is an oxide, nitride or carbide of a rare earth metal
selected
from the group consisting of Ce, La and Th.

45. A process according to claim 41, wherein said metal oxide,
nitride or carbide is an oxide, nitride or carbide of an alkaline earth metal
selected from the group consisting of Ca, Mg and Sr.

46. A process according to claim 40, wherein said metal is selected
from the group consisting of chromium, cobalt, copper, gold, iridium, iron,
nickel, niobium, palladium, platinum, rubidium, ruthenium, silicon, silver,
titanium, yttrium and zirconium.

47. A process according to claim 40, wherein steps (a), (b) and (d)
are carried out in a vibratory ball mill operated at a frequency of 5 to 40
Hz.

48. A process according to claim 47, wherein said vibratory ball null
is operated at a frequency of about 17 Hz.

49. A process according to claim 40, wherein steps (a), (b) and (d)
are carried out in a rotary ball mill operated at a speed of 100 to 2000
r.p.m.

50. A process according to claim 49, wherein said rotary ball mill is
operated at a speed of about 1200 r.p.m.

51. A process according to claim 40, wherein steps (a) and (b) are
carried out under an inert gas atmosphere.





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52. A process according to claim 51, wherein said inert gas
atmosphere comprises argon.

53. A process according to claim 40, wherein steps (a) and (b) are
carried out for a period of time of about 5 to 10 hours.

54. A process for producing an inert electrode material in powder
form as defined in claim 9, which comprises the steps of:

a) subjecting at least one metal oxide, nitride or carbide to high-
energy ball milling to form a first powder comprising particles having an
average particle size of 0.1 to 100 µm and each formed of an agglomerate of
grains of a ceramic material;

b) subjecting at least two metals to high-energy ball milling to
form a second powder comprising particles having an average particle size of
0.1 to 100 µm and each formed of an agglomerate of grains with each grain
comprising a nanocrystal of an alloy of said metals;

c) mixing said first and second powders to form a powder
mixture; and

d) subjecting the powder mixture obtained in step (c) to high-
energy ball milling to form a nanocrystalline powder comprising particles
having an average particle size of 0.1 to 100 µm and each formed of an
agglomerate of grains of said ceramic material and grains of said alloy,
wherein each grain of ceramic material comprises a nanocrystal of said ceramic
material and each grain of alloy comprises a nanocrystal of said alloy.

55. A process according to claim 54, wherein said metal oxide,
nitride or carbide is an oxide, nitride or carbide of a metal selected from
the
group consisting of transition metals, p-group metals, rare earth metals and
alkaline earth metals.




-27-

56. A process according to claim 55, wherein said metal oxide,
nitride or carbide is an oxide, nitride or carbide of a transition metal
selected
from the group consisting of Ag, Co, Cu, Cr, Fe, Ir, Mo, Mn, Nb, Ni, Ru, Ta,
Ti, V, W, Y, Zn and Zr.

57. A process according to claim 55, wherein said metal oxide,
nitride or carbide is an oxide, nitride or carbide of a p-group metal selected
from the group consisting of Al, Ge, In, Pb, Sb, Si and Sn.

58. A process according to claim 55, wherein said metal oxide,
nitride or carbide is an oxide, nitride or carbide of a rare earth metal
selected
from the group consisting of Ce, La and Th.

59. A process according to claim 55, wherein said metal oxide,
nitride or carbide is an oxide, nitride or carbide of an alkaline earth metal
selected from the group consisting of Ca, Mg and Sr.

60. A process according to claim 54, wherein said metals are selected
from the group consisting of chromium, cobalt, copper, gold, iridium, iron,
nickel, niobium, palladium, platinum, rubidium, ruthenium, silicon, silver,
titanium, yttrium and zirconium.

61. A process according to claim 54, wherein ferric oxide and nickel
oxide are subjected to said high-energy ball milling in step (a), whereby said
first powder comprises particles having an average particle size of 0.1 to
100 µm and each formed of an agglomerate of grains of a NiFe2O4 spinet.



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62. A process according to claim 61, wherein copper and silver are
subjected to said high-energy ball milling in step (b), whereby said second
powder comprises particles having an average particle size of 0.1 to 100 µm
and each formed of an agglomerate of grains with each grain comprising a
nanocrystal of a Cu-Ag alloy.

63. A process according to claim 54, wherein steps (a), (b) and (d)
are carried out in a vibratory ball mill operated at a frequency of 5 to 40
Hz.

64. A process according to claim 63, wherein said vibratory ball mill
is operated at a frequency of about 17 Hz.

65. A process according to claim 54, wherein steps (a), (b) and (d)
are carried out in a rotary ball mill operated at a speed of 100 to 2000
r.p.m.

66. A process according to claim 65, wherein said rotary ball mill is
operated at a speed of about 1200 r.p.m.

67. A process according to claim 54, wherein steps (a) and (b) are
carried out under an inert gas atmosphere.

68. A process according to claim 67, wherein said inert gas
atmosphere comprises argon.

69. A process according to claim 54, wherein steps (a) and (b) are
carried out for a period of time of about 5 to 10 hours.

70. A process according to claim 54, wherein step (b) is carried out
in the presence of a lubricant.


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71. A process according to claim 70, wherein said lubricant is stearic
acid.

72. A process for producing an inert electrode material in powder
form, which comprises subjecting a starting material consisting of a metal
oxide, nitride or carbide to high-energy ball milling to form a
nanocrystalline
powder comprising particles having an average particle size of 0.1 to 100
µm
and each formed of an agglomerate of grains with each grain comprising a
nanocrystal of a single phase ceramic material, wherein starting materials
selected from the group consisting of WC and carbides, nitrides and borides of
Nb, Ti, V and Zr are excluded.

73. A process according to claim 72, wherein said starting material is
an oxide of a transition metal selected from the group consisting of Ag, Co,
Cu,
Cr, Fe, Ir, Mo, Mn, Nb, Ni, Ru, Ta, Ti, V, W, Y, Zn and Zr.

74. A process according to claim 72, wherein said starting material is
a nitride or carbide of a transition metal selected from the group consisting
of
Ag, Co, Cu, Cr, Fe, Ir, Mo, Mn, Ni, Ru, Ta, Y and Zn.

75. A process according to claim 72, wherein said starting material is
an oxide, nitride or carbide of a p-group metal selected from the group
consisting of Al, Ge, In, Pb, Sb, Si and Sn.

76. A process according to claim 72, wherein said starting material is
an oxide, nitride or carbide of a rare earth metal selected from the group
consisting of Ce, La and Th.


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77. A process according to claim 72, wherein said starting material is
an oxide, nitride or carbide of an alkaline earth metal selected from the
group
consisting of Ca, Mg and Sr.

78. A process according to claim 72, wherein zinc oxide is subjected
to said high-energy ball milling.

79. A process according to claim 72, wherein at least one dopant
comprising an element selected from the group consisting of Al, Co, Cr, Cu,
Fe, Mo, Nb, Ni, Sb, Si, Sn, Ti, V, W, Y, Zn and Zr is admixed with starting
material prior to ball milling.

80. A process according to claim 79, wherein said dopant is used in
an amount of about 0.002 to about 1 wt.%.

81. A process according to claim 80, wherein the amount of dopant
ranges from about 0.005 to about 0.05 wt.%.

82. A process according to claim 79, wherein said metal oxide is zinc
oxide and said dopant is aluminum oxide.

83. A process according to claim 82, wherein said dopant is used in
an amount of about 0.008 wt.%.

84. A process according to claim 72, wherein said high-energy ball
milling is carried in a vibratory ball mill operated at a frequency of 5 to 40
Hz.

85. A process according to claim 84, wherein said vibratory ball mill
is operated at a frequency of about 17 Hz.


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86. A process according to claim 72, wherein said high-energy ball
milling is carried out in a rotary ball mill operated at a speed of 100 to
2000
r.p.m.

87. A process according to claim 86, wherein said rotary ball mill is
operated at a speed of about 1200 r.p.m.

88. A process according to claim 72, wherein said high-energy ball
milling is carried out under an inert gas atmosphere.

89. A process according to claim 88, wherein said inert gas
atmosphere comprises argon.

90. A process far producing an inert electrode material in powder
form as defined in claim 33, which comprises subjecting a metal to high-
energy ball milling to form a nanocrystalline powder comprising particles
having an average particle size of 0.1 to 100 µm and each formed of an
agglomerate of grains with each grain comprising a nanocrystal of said metal.

91. A process according to claim 90, wherein said metal is selected
from the group consisting of chromium, cobalt, copper, gold, iridium, iron,
nickel, niobium, palladium, platinum, rubidium, ruthenium, silicon, silver,
titanium, yttrium and zirconium.

92. A process according to claim 91, wherein said metal is copper.

93. A process according to claim 90, wherein said high-energy ball
milling is carried in a vibratory ball mill operated at a frequency of 5 to 40
Hz.


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94. A process according to claim 93, wherein said vibratory ball mill
is operated at a frequency of about 17 Hz.

95. A process according to claim 90, wherein said high-energy ball
milling is carried out in a rotary ball mill operated at a speed of 100 to
2000
r.p.m.

96. A process according to claim 95, wherein said rotary ball mill is
operated at a speed of about 1200 r.p.m.

97. A process according to claim 90, wherein said high-energy ball
milling is carried out under an inert gas atmosphere.

98. A process according to claim 97, wherein said inert gas
atmosphere comprises argon.

99. A process for producing an inert electrode material in powder
form as defined in claim 37, which comprises subjecting at least two metals to
high-energy ball milling to form a nanocrystalline powder comprising particles
having an average particle of 1 to 30 µm and each formed of an agglomerate
of
grains with each grain comprising a nanocrystal of an alloy of the metals,
wherein alloys selected from the group consisting of Cr2Nb, CrSi2, NbSi2 and
alloys of formula (Mg1-x A x)D y in which A is an element selected from the
group consisting of Li, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Y, Zr, Nb,
Mo, In, Sn, O, Si, B, C and F, D is a metal selected from the group consisting
of Fe, Co, Ni, Ru, Rh, Pd, Ir and Pt, x is a number ranging from 0 to 0.3 and
y
is a number ranging from 0 to 0.15, are excluded.


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100. A process according to claim 99, wherein said metals are selected
from the group consisting of chromium, cobalt, copper, gold, iridium, iron,
nickel, niobium, palladium, platinum, rubidium, ruthenium, silicon, silver,
titanium, yttrium and zirconium.

101. A process according to claim 100, wherein copper and nickel are
subjected to said high-energy ball milling, whereby said nanocrystalline
powder comprises particles having an average particle size of 1 to 30 µm
and
each formed of an agglomerate of grains with each grain comprising a
nanocrystal of a Cu-Ni alloy.

102. A process according to claim 99, wherein said high-energy ball
milling is carried in a vibratory ball mill operated at a frequency of 5 to 40
Hz.

103. A process according to claim 102, wherein said vibratory ball
mill is operated at a frequency of about 17 Hz.

104. A process according to claim 99, wherein said high-energy ball
milling is carried out in a rotary ball mill operated at a speed of 100 to
2000
r.p.m.

105. A process according to claim 104, wherein said rotary ball mill is
operated at a speed of about 1200 r.p.m.

106. A process according to claim 99, wherein said high-energy ball
milling is carried out under an inert gas atmosphere.

107. A process according to claim 106, wherein said inert gas
atmosphere comprises argon.

Description

CA 02441578 2003-09-19
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INERT ELECTRODE MATERIAL 1N NANOCRYSTALLINE
POWDER FORM
TECHNICAL FIELD
The present invention pertains to improvements in the field of
electrodes for metal electrolysis. More particularly, the invention relates to
an inert electrode material in nanocrystalline powder form for use in the
manufacture of such electrodes.
BACKGROUND ART
Aluminum is produced conventionally in a Hall-Heroult
reduction cell 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. Typically, for each ton of aluminum produced, 0.5 ton
of carbon anode is required.
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


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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.
Many attempts have been made to find a suitable material for
inert anodes and a number of materials have been proposed and tested. The
proposed materials include metals such as proposed in US Patent
No. 6,162,334, ceramics such as proposed in US Patent Nos. 3,960,678 and
4,399,008, and cermets such as proposed in US Patent No. 5,865,980. In
spite of intensive efforts of more than 20 years to produce inert anodes, to
date, no fully acceptable inert anode materials have been found. Ceramics
are generally brittle and do not resist to the thermal shocks during start-up
and operation of a Hall-Heroult cell. Metal oxide ceramics are generally
resistant to oxidation, but they are not good electrical conductors. Metals,
however, are very good conductors but the corrosion rate of metallic anodes
in cryolite is very high. Cermets, on the other hand, seem to be promising
materials for anode applications. Cermets combine the good properties of
metals (conductivity, toughness) with good properties of ceramics
(corrosion resistance).
US Patent No. 5,865,980 describes a cermet comprising a
ferrite, copper and silver which can be used as an inert anode. These cermet
anodes exhibit a good corrosion resistance due to the ceramic part and a
good electrical conductivity due to the metallic part. Fabrication process of
such a cermet is complex and consists of several steps. At least two metal
oxides, such as Ni0 and Fe203, are mixed and calcined at high temperatures
(1300-1400 °C) for a relatively long period of time (12 h) in order to
synthesize a nickel ferrite spinel with or without excess of NiO. The


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resulting material is grinded to reduce the average particle size to about 10
microns, mixed with a polymeric binder and water, spray dried, and mixed
with copper and silver powder. The powder mixture thus obtained is then
pressed and sintered at about° 1350 °C for 2-4 hours. The
resulting cermet
has ceramic phase portions and alloy phase portions.
Although the above-mentioned cermet seems to be a
promising material for inert anode applications, several disadvantages are
associated with its production and the characteristics of the final product.
The process is complex and requires several steps, which results in a
product having a high cost. The sintering and densification rates of ceramic
and metal powders having an average particle size of about 10 microns are
slow so that it is very difficult to obtain a highly dense cermet. A small
amount of porosity is present in the cermet obtained, resulting in a decrease
of mechanical properties. Thus, an anode made of such a cermet is easily
destroyed when subjected to repeated thermal shocks. In order to increase
the final density, the sintering temperature must be increased. Using high
sintering temperature results in an excessive grain growth and an increase
in the final cost of the product.
Segregation is a serious problem when powders having a
Iarge average particle size are mixed together. Segregation is more
pronounced when the difference between the densities of the particles or
their size is larger. Metal particles having a density greater than that of
ceramic particles tend to segregate from the Iow-density ceramic particles.
This results in a non-homogeneous powder mixture and, consequently, in a
non-homogeneous sintered anode. Since the conductivity of the ceramic
phase is much lower than that of the metal phase, any non-homogeneity


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results in a non-homogeneous current density during use of the anode. On
the other hand, the corrosion or erosion rates of the ceramic and metal
phase portions of the cermet in cryolite are not the same. Therefore, any
non-homogeneity results in an excessive local degradation of the anode.
The purpose of sintering is to obtain a solid product having
maximum density and homogeneity. During sintering, two phenomena are
particularly important: densification (pore elimination) and grain growth.
Higher sintering temperatures and longer sintering times generally lead to
high densification but, on the other hand, favor grain growth. When
powders having a large average particle size are used as starting material,
densification is slow and in order to obtain higher densities, the sintering
temperature and/or time must be increased. This results in a cermet with a
coarse microstructure which decreases the thermal shock resistance of the
cermet. Coarse structured cermets also exhibit low mechanical properties
and non-homogeneous corrosion rates.
DISCLOSURE OF INVENTION
It is therefore an object of the invention to overcome the
above drawbacks and to provide an electrode material in powder form for
use in the manufacture of inert electrodes having improved thermal shock
and corrosion resistance properties.
According to one aspect of the invention, there is provided an
inert electrode material in powder form comprising particles having an
average particle size of 0.1 to 100 ~,m and each formed of an agglomerate
of grains of a ceramic material and grains of ~ a metal or alloy with each


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grain of ceramic material comprising a nanocrystal of the ceramic material
and each grain of metal or alloy comprising a nanocrystal of the metal or
alloy.
According to another aspect of the invention, there is
provided an inert electrode material in powder form comprising particles
having an average particle size of 0.1 to 100 ~,m and each formed of an
agglomerate of grains with each grain comprising a nanocrystal of a single
phase ceramic material.
According to a further aspect of the invention, there is
provided an inert electrode material in powder form comprising particles
having an average particle size of 0.1 to 100 ~,m and each formed of an
agglomerate of grains with each grain comprising a nanocrystal of a metal.
According to still a further aspect of the invention, there is
provided an inert electrode material in powder form comprising particles
having an average particle size of 0.1 to 100 ~,m and each formed of an
agglomerate of grains with each grain comprising a nanocrystal of an alloy.
The term "nanocrystal" as used herein refers to a crystal
having a size of 100 nanometers or less. The nanocrystalline microstructure
considerably favors densification, even without sintering aids, when the
electrode material in powder form according to the invention is compacted
and sintered to produce dense electrodes. Nanocrystalline powders also
minimize grain growth ~ since sintering can be effected at lower
temperatures. The sintering time is also much shorter than that required for
densification of the conventional coarse-grained (about 10 ~.m) powder


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mixtures for a same densification level. Thus, the overall cost of the
sintering process is considerably decreased.
Since the time and the temperature of the sintering are
considerably low, the resulting electrode has a fine microstructure. The
finer the microstructure, the higher the toughness and the resistance to
thermal shock, and consequently the longer the electrode life time.
In the case where each particle is formed of an agglomerate of
grains with each grain comprising a nanocrystal of a metal, the
nanocrystalline metals are more resistant to corrosion than polycrystalline
metals because of the growth of a passivation layer. This protective layer
grows faster at the surface of a nanocrystalline metal than in a
polycrystalline metal.
The present invention also provides, in a further aspect
thereof, a process for producing an inert electrode material in powder form
as previously defined, wherein each particle is formed of an agglomerate of
grains of a ceramic material and grains of a metal. The process of the
invention comprises the steps of
a) subj ecting at least one metal oxide, nitride or carbide to
high-energy ball milling to form a first powder comprising particles having
an average particle size of 0.1 to 100 ~,m and each formed of an
agglomerate of grains of a ceramic material;
b) subjecting a metal to high-energy ball milling to form a
second powder comprising particles having an average particle size of 0.1
to 100 ~,m and each formed of an agglomerate of grains with each grain
comprising a nanocrystal of the metal;


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c) mixing the first and second powders to form a powder
mixture; and
d) subjecting the powder mixture obtained in step (c) to high-
energy ball milling to form a nanocrystalline powder comprising particles
having an average particle size of 0.1 to 100 ~.m and each formed of an
agglomerate of grains of the ceramic material and g~'ains of the metal,
wherein each grain of ceramic material comprises a nanocrystal of the
ceramic material and each grain of metal comprises a nanocrystal of the
metal.
According to still a further aspect of the invention, there is
provided a process for producing an inert electrode material as previously
defined, wherein each particle is formed of an agglomerate of grains of a
ceramic material and grains of an alloy. The process of the invention
comprises the steps of:
a) subjecting ~at least one metal oxide, nitride or carbide to
high-energy ball milling to form a first powder comprising particles having
an average particle size of 0.1 to 100 ~,m and each formed of an
agglomerate of grains of a ceramic material;
b) subjecting at least two metals to high-energy ball milling to
form a second powder comprising particles having an average particle size
of 0.1 to 100 ~m and each formed of an agglomerate of grains with each
grain comprising a nanocrystal of an alloy of the metals;
c) mixing the first and second powders to form a powder
mixture; and
d) subjecting the powder mixture obtained in step (c) to high-
energy ball milling to form a nanocrystalline powder comprising particles
having an average particle size of 0.1 to 100 ~,m and each formed of an


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agglomerate of grains of the ceramic material and grains of the alloy,
wherein each grain of ceramic material comprises a nanocrystal of the
ceramic material and each grain of alloy comprises a nanocrystal of the
alloy.
According to another aspect of the invention, there is
provided a process for producing an inert electrode material in powder form
as previously defined, wherein each particle is formed of an agglomerate of
grains each comprising a nanocrystal of a single phase ceramic material.
The process of the invention comprises subjecting a metal oxide, nitride or
carbide to high-energy ball milling to form a nanocrystalline powder
comprising particles having an average particle size of 0.1 to 100 ~m and
each formed of an agglomerate of grains with each grain comprising a
nanocrystal of a single phase ceramic material.
According to yet another aspect of the invention, there is
provided a process for producing an inert electrode material in powder form
as previously defined, wherein each particle is formed of an agglomerate of
grains each comprising a nanocrystal of a metal. The process of the
invention comprises subjecting a metal to high-energy ball milling to form
a nanocrystalline powder comprising particles having an average particle
size of 0.1 to 100 ~.m and each formed of an agglomerate of grains with
each grain comprising a nanocrystal of the metal.
According to still another aspect of the invention, there is
provided a process for producing an inert electrode material in powder form
as previously defined, wherein each particle is formed of an agglomerate of
grains each comprising a nanocrystal of an alloy. The process of the


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invention comprises subjecting at least two metals to high-energy ball
milling to form a nanocrystalline powder comprising particles having an
average particle size of 0.1 to 100 ~,m and each formed of an agglomerate
of grains with each grain comprising a nanocrystal of an alloy of the metals.
The expression "high-energy ball milling" as used herein
refers to a ball milling process capable of forming the aforesaid particles
within a period of time of about 40 hours. In the aforementioned step (d),
the high-energy ball milling is carried out for a period of time sufficient to
break the agglomerates formed in steps (a) and (b), and to form new
agglomerates comprising nanocrystalline grains of the ceramic material and
nanocrystalline grains of the metal or alloy. Generally, such a period of
time is about one hour.
MODES FOR CARRYING OUT THE INVENTION
Examples of suitable ceramic materials include oxides,
nitrides and carbides of transition metals such as Ag, Co, Cu, Cr, Fe, Ir,
Mo, Mn, Nb, Ni, Ru, Ta, Ti, V, W, Y, Zn and Zr, p-group metals such as
Al, Ge, In, Pd, Sb, Si and Sn, rare earth metals such as Ce, La and Th, and
alkaline earth metals such as Ca, Mg and Sr.
In the case where each particle is formed of an agglomerate of
grains of ceramic material and grains of metal, the metal can be for example
chromium, cobalt, copper, gold, iridium, iron, nickel, niobium, palladium,
platinum, rubidium, ruthenium, silicon, silver, titanium, yttrium or
zirconium. On the other hand, in the case where each particle is formed of
an agglomerate of grains of ceramic material and grains of alloy, the alloy


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can be for example a Cu-Ag, Cu-Ag-Ni, Cu-Ni, Cu-Ni-Fe, Cu-Pd, Cu-Pt or
Ni-Fe alloy. When these particles are sintered, they will form a cermet
material having ceramic phase portions and .metal or alloy phase portions.
In the case where each particle is formed of an agglomerate of
grains with each grain comprising a nanocrystal of a single phase ceramic
material, the ceramic material advantageously includes a dopant for
improving the sinterability of the powder and/or for increasing the
conductivity of the electrode eventually made from the ceramic powder.
Examples of suitable dopants include those comprising an element selected
from the group of Al, Co, Cr, Cu, Fe, Mo, Nb, Ni, Sb, Si, Sn, Ti, V, W, Y,
Zn and Zr. The dopant is generally present in an amount of about 0.002 to
about 1 wt.%, preferably between about 0.005 and about 0.05 wt.%. Since
the corrosion, erosion and thermal expansion of a single phase ceramic
material are uniform, electrodes produced from the nanocrystalline powder
according to the invention, comprising such a material, have a longer life
time.
In the case where each particle is formed of an agglomerate of
grains with each grain comprising a nanocrystal of a metal, the metal can be
for example chromium, cobalt, copper, gold, iridium, iron, nickel, niobium,
palladium, platinum, rubidium, ruthenium, silicon, silver, titanium, yttrium
or zirconium. Copper is preferred. On the other hand, in the case where each
particle is formed of an agglomerate of grains with each grain comprising a
nanocrystal of an alloy, the alloy can be for example a Cu-Ag, Cu-Ag-Ni,
Cu-Ni, Cu-Ni-Fe, Cu-Pd, Cu-Pt or Ni-Fe alloy. When these particles are
sintered, they form a dense metallic material.


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According to a preferred embodiment of the process of the
invention, the high-energy ball milling is carned out in a vibratory ball mill
operated at a frequency of ~ to 25 Hz, preferably about 17 Hz. It is also
possible to carry out such a ball milling in a rotary ball mill operated at a
speed of 100 to 2000 r.p.m., preferably about 1200 r.p.m.
According to another preferred embodiment, the high-energy
ball milling is carried out under an inert gas atmosphere such as a gas
atmosphere comprising argon or helium. An atmosphere of argon is
preferred.
The electrode material in powder form according to the
invention can be used to produce dense electrode by powder metallurgy. 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. The dense electrodes thus obtained
have improved thermal shock and corrosion resistance properties.
The electrode material in powder form according to the
invention can also be used to produce electrodes by thermal deposition
applications. The expression "thermal deposition" as used herein refers to a
technique in which powder particles are injected in a torch and sprayed on a
conductive substrate such as graphite or copper, to form thereon a highly
dense coating. The particles acquire a high velocity and are partially or


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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.
Since the electrodes produced from the nanocrystalline
powder according to the invention have a high density, the electrolyte does
not penetrate into the electrode via pores and, consequently, the degradation
of the electrode is minimized.
The following non-limiting examples illustrate the invention.
EXAMPLE 1
A NiFe204 spinal powder was produced by ball milling 51.7
wt.% Ni0 and 48.3 wt.% Fe203 in a tungsten carbide crucible with a ball-to-
powder mass ratio of 15:1 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. The crucible was closed and sealed
with a rubber O-ring. After 10 hours of high-energy ball milling, a
nanocrystalline structure comprising a NiFe204 spinal with excess Ni0 was
formed. The particle size varied between 0.1 and 5 ~.m and the crystallite
size, measured by X-ray diffraction, was about 30 nm.
A Cu-Ag alloy powder was also produced by ball milling 69.5
wt.% Cu and 29.5 wt.% Ag in a tungsten carbide crucible with a ball-to-
powder mass ratio of 10:1 using a SPEX 8000 vibratory ball mill operated at
a frequency of about 17 Hz. The operation was performed under a controlled
argon atmosphere. 1 wt.% of stearic acid was added as a lubricant. After 10


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hours of high-energy ball milling, a nanocrystalline structure comprising an
alloy of copper and silver was formed. The particle size varied between 10
and 30 ~m and.the crystallite size, measured by X-ray diffraction, was about
40 nm.
80 wt.% of the NiFea04 spinet powder and 20 wt.% of the Cu-
Ag alloy powder produced above were mixed and the resulting powder
mixture and the resulting powder mixture was ball milled in a tungsten
carbide crucible with a ball-to-powder mass ratio of 10:1 using a SPEX
8000 vibratory ball mill operated at a frequency of about 17 Hz. After one
hour of high-energy ball milling, a nanocrystalline powder comprising
particles each formed of an agglomerate of grains comprising nanocrystals
of the NiFe204 spinet and nanocrystals of the Cu-Ag alloy was obtained.
The particle size varied between 5 and 10 ~,m. This nanocrystalline powder
was then pressed uniaxially at a pressure of 400 MPa. The compacted
powder was then sintered at a temperature of 950°C for one hour to
produce
a dense electrode having excellent thermal shock and corrosion resistance
properties.
EXAMPLE 2
A NiFea04 ferrite powder was produced by ball milling 51.7 wt.% of
Ni0 and 48.3 wt.% Fe203 in a steel crucible with a ball-to-powder mass
ratio of 10:1 using a SIMOLOYER (trademark) rotary ball mill operated at
a speed of 1200 r.p.m.. The operation was performed under a controlled
argon atmosphere by continously flushing the crucible with argon. After
5 hours of high-energy ball milling, an amorphous NiFe204 spinet was

CA 02441578 2003-09-19
.~ ~~ . , , ,~ ~ , ~CA02Q0SJ
hPt'lt~t~t1 30 46 ~O3 ~ESCI="AMD
- 14-
produced with an excess of nanocrystalline NiO. The particle size varied
between 0.1 and 5 ~,m.
- A Cu-Ag alloy powder was also produced by ball milling 98 wt.%
Cu and 2 wt.% Ag in a steel crucible with a ball-to-powder mass ratio of
10:1 using a SIMOLOYER rotary ball mill operated at a speed of
1200 r.p.m. The operation was performed under a controlled argon
atmosphere. 1 wt.% stearic acid was added as a lubricant. After 5 hours of
high-energy ball milling, a nanocrystalline structure comprising an alloy of
copper and silver was formed. The particle size varied between 5 to 30 ~,m
and the crystallite size, measured by X-ray diffraction, was about 20 nm.
81.3 wt.% of the above NiFea04 spinal powder, 16.6 wt.% of the
above Cu-Ag alloy powder and 2 wt.% of CAPLUBE G (trademark) acting
as a lubricant arid binder were mixed and the resulting powder mixture was
ball milled in a steel crucible with a ball-to-powder mass ratio of 10:1 using
a SIMOLOYER rotary ball mill operated at 800 r.p.m.. After 1 S minutes of
high-energy ball milling, a nanocrystalline powder comprising particles
each formed of an agglomerate of grains comprising nanocrystals of the
NiFe20a spinal and nanocrystals of the Cu-Ag alloy was obtained. The
particle size varied between 5 and 10 p,m. This nanocrystalline powder was
then cold isostatically pressed at 138 Mpa. The compacted powder was then
sintered at a temperature of 1050°C for one hour to produce a dense
electrode having excellent thermal shock and corrosion resistance
properties.
1 °,' RECTIFIED SHEET (RULE 91 ) i~~~ a~.,~00~

' CA 02441578 2003-09-19
Pr~r'lte~ ,~30 06 20t~~ DESGI='.~1M~:! . CRO~t?Q395
-15-
EXAMPLE 3
A NiFe204 ferrite powder was produced by ball milling 51.7
wt.% of Ni0 and 48.3 wt.% Fe20~ in a steel crucible with a ball-to-powder
mass ratio of 10:1 using a SIMOLOYER rotary ball mill operated at a speed
of 1200 r.p.m.. The operation was performed under a controlled argon
atmosphere by continously flushing the crucible with argon. After 5 hours of
high-energy ball milling, an amorphous NiFe204 spinet was produced with
an excess of nanocrystalline NiO. The particle size varied between 0.1 and
S wm.
A Cu-Ag alloy powder was also produced by ball milling
98 wt.% Cu and 2 wt.% Ag in a steel crucible with a ball-to-powder mass
ratio of 10:1 using a SIMOLO'YER rotary ball mill operated at a speed of
1200 r.p.m.. The operation was performed under a controlled argon
atmosphere. 1 wt.% stearic acid was added as a lubricant. After 5 hours of
high-energy ball milling, a nanocrystalline structure comprising an alloy of
copper and silver was formed. The particle size varied between 5 to 30 ~,m
and the crystallite size, measured by X-ray diffraction, was about 20 nm.
81.3 wt.% of the above NiFe20~ spinet powder, 16.6 wt.% of the
above Cu-Ag alloy powder produced above and 2 wt.% of CAPLUBE G
acting as a lubricant and binder were mixed and the resulting powder
mixture was ball milled in a steel crucible with a ball-to-powder mass ratio
of 5:1 using a SPEX 8000 vibratory ball mill operated at 17 Hz. After 15
minutes of high-energy ball milling, a nanocrystalline powder comprising
particles each formed of an agglomerate of grains comprising nanocrystals
of the NiFe204 spinet and nanocrystals of the Cu-Ag alloy was obtained.
2r RECTIFIED SHEET (RULE 91 )
45 (l6-2002

' CA 02441578 2003-09-19
~'rlnted 30 00 20U3V! ~=DAESCPAM~Y: ~C~t~~0t139
-16-
The particle size varied between 5 and 10 ~.m. This nanocrystaIline powder
was then uniaxially pressed at 138 Mpa. The compacted powder was then
sintered at a temperature of lOSO°C for one hour to produce a dense
' electrode having excellent thermal shock and corrosion resistance
properties.
EXAMPLE 4
A coarse-grained Zn0 powder (99.9% pure) having an
average grain size of 1 ~,m and a specific surface area of 3 m~/g was used as
starting material. 0.008 wt.% A1z03 and ~ wt.% PVA were added as dopant
and binder, respectively. The powder mixture was ball milled in a tungsten
carbide crucible using a SPEX 8000 vibratory ball mill operated at a
frequency of about 17 Hz. After I S hours of high-energy ball milling, a
nanocrystalline Zn0 powder having a particle size between I and 5 ~,m and .
an average grain size smaller than I00 nm was obtained. The specific
surface area of the nanocrystalline grains was 40 m2/g. This nanocrystalline
powder was then pressed uniaxially at a pressure of 400 MPa. The
compacted powder was then sintered at a temperature of 1250°C for one
hour to produce a dense electrode having excellent thermal shock and
corrosion resistance properties.
EXAMPLE 5
A nanocrystalline Cu-Ni alloy powder was produced by ball
milling 70 wt.% Cu and 30 wt.% Ni in a steel crucible with a ball-to-
powder mass ratio of 10:1 using a SIMOLOYER rotary ball mill operated
at a speed of 1200 r.p.m.. 1 wt.% stearic acid was added as a lubricant.
~'' RECTIFIED SHEET (RULE 91) ,p~TO,Or~O(~~

' CA 02441578 2003-09-19
~Pr~t~t~C~ ~30 06 20E~3I ? C?ESt'rPAM~~G~tt?,'~G0039~"J
7 _
After 5 hours of high-energy ball milling, a nanocrystalline powder
comprising particles each formed of an agglomerate of grains comprising
nanocrystals of an alloy of copper and nickel was obtained. The particle
' size varied between 5 to 30 ~m and the crystallite size, measured by X-ray
diffraction, was about 20 nm. This nanocrystalline powder was mixed with
2 wt.% of CAPLUBE G and uniaxially pressed at 300 Mpa. The compacted
powder was then sintered at a temperature of 1000°C for one hour to
produce a dense electrode.
4, RECTIFIED SHEET (RULE 91) p~ a0.~~p0