CA2341779A1 - Inert electrode material in nanocrystalline powder form - Google Patents
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- CA2341779A1 CA2341779A1 CA002341779A CA2341779A CA2341779A1 CA 2341779 A1 CA2341779 A1 CA 2341779A1 CA 002341779 A CA002341779 A CA 002341779A CA 2341779 A CA2341779 A CA 2341779A CA 2341779 A1 CA2341779 A1 CA 2341779A1
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- C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
- C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
- C04B35/26—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on ferrites
- C04B35/2666—Other ferrites containing nickel, copper or cobalt
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- B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
- B22F1/07—Metallic powder characterised by particles having a nanoscale microstructure
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- B22—CASTING; POWDER METALLURGY
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- B22F9/00—Making metallic powder or suspensions thereof
- B22F9/002—Making metallic powder or suspensions thereof amorphous or microcrystalline
- B22F9/004—Making metallic powder or suspensions thereof amorphous or microcrystalline by diffusion, e.g. solid state reaction
- B22F9/005—Transformation into amorphous state by milling
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- C04B35/453—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on zinc, tin, or bismuth oxides or solid solutions thereof with other oxides, e.g. zincates, stannates or bismuthates
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Abstract
The invention relates to an inert electrode material in powder form comprising particles having an average particle size of 0.1 to 50 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 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. 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.
Description
INERT ELECTRODE MATERIAL IN NANOCRYSTALLINE
POWDER FORM
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.
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-Herolalt 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 causes swelling and deformation of the cathode carbon blocks. The increase of voltage between the a ectrodes 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 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 large 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 low-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 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.
POWDER FORM
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.
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-Herolalt 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 causes swelling and deformation of the cathode carbon blocks. The increase of voltage between the a ectrodes 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 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 large 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 low-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 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.
It is therefore an obj ect 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 50 ~,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.
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 50 ~m and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a single phase ceramic material.
The teen "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 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.
Examples of suitable ceramic materials include oxides and nitrides of metals such as Al, Co, Cr, Fe, Mo, Nb, Ni, Sn, Ti, Zn, Zr, V and W.
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 copper, gold, iridium, palladium, platinum, rubidium, ruthenium or silver. 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 can be far example a Cu-Ag, Cu-Ag-Ni, Cu-Pd or Cu-Pt 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, Fe, Mo, Nb, Ni, Sn, Ti, Zn, Zr, V and W. The dopant is generally present in an amount of about 0.002 to about 0.5 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.
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) subjecting at least one metal oxide or nitride to high-energy ball milling to form a first nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 ~m and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a ceramic material;
b) subjecting a metal to high-energy ball milling to form a second nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 ~m and each formed of an agglomerate of grains with each grain comprising a nanocrystal of the metal;
c) mixing the first and second nanocrystalline powders to form a powder mixture; and d) subjecting the powder mixture obtained in step (c) to high-energy ball milling to form a third nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 ~m and each formed of an agglomerate of grains of the ceramic material and grains of the metal, wherein each grain of ceramic material comprises a nanocrystal of the ceramic material and each grain of metal comprises a nanocrysta.l of the metal.
It is therefore an obj ect 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 50 ~,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.
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 50 ~m and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a single phase ceramic material.
The teen "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 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.
Examples of suitable ceramic materials include oxides and nitrides of metals such as Al, Co, Cr, Fe, Mo, Nb, Ni, Sn, Ti, Zn, Zr, V and W.
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 copper, gold, iridium, palladium, platinum, rubidium, ruthenium or silver. 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 can be far example a Cu-Ag, Cu-Ag-Ni, Cu-Pd or Cu-Pt 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, Fe, Mo, Nb, Ni, Sn, Ti, Zn, Zr, V and W. The dopant is generally present in an amount of about 0.002 to about 0.5 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.
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) subjecting at least one metal oxide or nitride to high-energy ball milling to form a first nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 ~m and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a ceramic material;
b) subjecting a metal to high-energy ball milling to form a second nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 ~m and each formed of an agglomerate of grains with each grain comprising a nanocrystal of the metal;
c) mixing the first and second nanocrystalline powders to form a powder mixture; and d) subjecting the powder mixture obtained in step (c) to high-energy ball milling to form a third nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 ~m and each formed of an agglomerate of grains of the ceramic material and grains of the metal, wherein each grain of ceramic material comprises a nanocrystal of the ceramic material and each grain of metal comprises a nanocrysta.l of the metal.
According to still a further aspect of the invention, there is provided a process for producing an inert f;lectrode 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 or nitride to high-energy ball milling to form a first nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 ~m and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a ceramic material;
b) subjecting at least two metals to high-energy ball milling to form a second nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 ~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 nanocrystalline powders to form a powder mixture; and d) subjecting the powder mixture obtained in step (c) to high-energy ball milling to form a third nanocrystalline powder comprising particles having an average particle size of 0.1 to ~0 ~m and each formed of an 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 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 single phase ceramic material. The process of the invention comprises subjecting a metal oxide or nitride to high-energy ball milling to form a nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 ~,m and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a single phase ceramic material.
The expression "high-energy ball milling" as used herein refers to a ball milling process capable of forming the aforesaid particles comprising nanocrystalline grains, 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.
According to a preferred embodiment, the high-energy ball milling 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 carry out such a ball milling in a rotary ball mill operated at a speed of 1 SO to 1500 r.p.m., preferably about 1000 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 _g_ 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 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 spinel 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 spinel with excess NiCJ 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 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 Hg. 1 wt.% of stearic acid was added as a lubricant. After 10 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 NiFe204 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 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 Hg. After one hour of high-energy ball milling, a nanocrystalline powder comprising particles each formed of an agglomerated 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 coarse-grained Zn0 powder X99.9% pure) having an average grain size of 1 ~.m and a specific surface area of 3 m2/g was used as starting material. 0.008 wt.% A1203 and 2 wt:% PVA were added as dopant and binder, respectively. The powder mixture was ball milled in a tungsten crucible using a SPEX 8000 vibratory ball mill operated at a frequency of about 17 Hz. After hours of high-energy ball milling, a nanocrystalline Zn0 powder having a particle size between 1 and 5 ~m and an average grain size smaller than 100 10 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.
a) subjecting at least one metal oxide or nitride to high-energy ball milling to form a first nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 ~m and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a ceramic material;
b) subjecting at least two metals to high-energy ball milling to form a second nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 ~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 nanocrystalline powders to form a powder mixture; and d) subjecting the powder mixture obtained in step (c) to high-energy ball milling to form a third nanocrystalline powder comprising particles having an average particle size of 0.1 to ~0 ~m and each formed of an 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 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 single phase ceramic material. The process of the invention comprises subjecting a metal oxide or nitride to high-energy ball milling to form a nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 ~,m and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a single phase ceramic material.
The expression "high-energy ball milling" as used herein refers to a ball milling process capable of forming the aforesaid particles comprising nanocrystalline grains, 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.
According to a preferred embodiment, the high-energy ball milling 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 carry out such a ball milling in a rotary ball mill operated at a speed of 1 SO to 1500 r.p.m., preferably about 1000 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 _g_ 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 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 spinel 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 spinel with excess NiCJ 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 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 Hg. 1 wt.% of stearic acid was added as a lubricant. After 10 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 NiFe204 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 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 Hg. After one hour of high-energy ball milling, a nanocrystalline powder comprising particles each formed of an agglomerated 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 coarse-grained Zn0 powder X99.9% pure) having an average grain size of 1 ~.m and a specific surface area of 3 m2/g was used as starting material. 0.008 wt.% A1203 and 2 wt:% PVA were added as dopant and binder, respectively. The powder mixture was ball milled in a tungsten crucible using a SPEX 8000 vibratory ball mill operated at a frequency of about 17 Hz. After hours of high-energy ball milling, a nanocrystalline Zn0 powder having a particle size between 1 and 5 ~m and an average grain size smaller than 100 10 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.
Claims (60)
1. An inert electrode material in powder form comprising particles having an average particle size of 0.1 to 50 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 or nitride of a metal selected from the group consisting of Al, Co, Cr, Fe, Mo, Nb, Ni, Sn, Ti, Zn, Zr, V and W.
4. An inert electrode material according to claim 2, wherein said metal is selected from the group consisting of copper, gold, iridium, palladium, platinum, rubidium, ruthenium and silver.
5. An inert electrode material according to claim 3, wherein each said particle is formed of an agglomerate of said grains of ceramic material and said grains of alloy.
6. An inert electrode material according to claim 5, wherein said ceramic material comprises an oxide or nitride of a metal selected from the group consisting of Al, Co, Cr, Fe, Mo, Nb, Ni, Sn, Ti, Zn, Zr, V and W.
7. An inert electrode material according to claim 5, wherein said alloy is selected from the group consisting of Cu-Ag, Cu-Ag-Ni, Cu-Pd and Cu-Pt alloys.
8. An inert electrode material according to claim 7, wherein said alloy is a Cu-Ag alloy.
9. An inert electrode material according to claim 8, wherein said ceramic material comprise a NiFe2O4 spinet.
10. An inert electrode material in powder form comprising particles having an average particle size of 0.1 to 50 µm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a single phase ceramic material.
11. An inert electrode material according to claim 10, wherein said ceramic material is an oxide or nitride of a metal selected from the group consisting of Al, Co, Cr, Fe, Mo, Nb, Ni, Sn, Ti, Zn, Zr, V and W.
12. An inert electrode material according to claim 11, wherein said ceramic material is zinc oxide.
13. An inert electrode material according to claim 10, wherein said ceramic material includes at least one dopant comprising an element selected from the group consisting of Al, Co, Cr, Fe, Mo, Nb, Ni, Sn, Ti, Zn, Zr, V and W.
14. An inert electrode material according to claim 13, wherein said dopant is present in an amount of about 0.002 to about 0.5 wt.%.
15. An inert electrode material according to claim 14, wherein the amount of dopant ranges from about 0.005 to about 0.05 wt.%.
16. An inert electrode material according to claim 15, wherein the amount of dopant is about 0.008 wt.%.
17. An inert electrode material according to claim 13, wherein said ceramic material comprises zinc oxide doped with aluminum oxide.
18. An inert electrode material according to claim 17, wherein the aluminum oxide is present in an amount of about 0.008 wt.%.
19. An inert electrode material according to claim 1, wherein said average particle size ranges from 1 to 10 µm.
20. 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 or nitride to high-energy ball milling to form a first nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 µm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a ceramic material;
b) subjecting a metal to high-energy ball milling to form a second nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 µm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of said metal;
c) mixing said first and second nanocrystalline powders to form a powder mixture; and d) subjecting the powder mixture obtained in step (c) to high-energy ball milling to form a third nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 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.
a) subjecting at least one metal oxide or nitride to high-energy ball milling to form a first nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 µm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a ceramic material;
b) subjecting a metal to high-energy ball milling to form a second nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 µm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of said metal;
c) mixing said first and second nanocrystalline powders to form a powder mixture; and d) subjecting the powder mixture obtained in step (c) to high-energy ball milling to form a third nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 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.
21. A process according to claim 20, wherein said metal oxide or nitride is an oxide or nitride of a metal selected from the group consisting of Al, Co, Cr, Fe, Mo, Nb, Ni, Sn, Ti, Zn, Zr, V and W.
22. A process according to claim 20, wherein said metal is selected from the group consisting of copper, gold, iridium, palladium, platinum, rubidium, ruthenium and silver.
23. A process according to claim 20, wherein steps (a), (b) and (d) are carried out in a vibratory ball mill operated at a frequency of 5 to 40 Hz.
24. A process according to claim 23, wherein said vibratory ball mill is operated at a frequency of about 17 Hz.
25. A process according to claim 20, wherein steps (a), (b) and (d) are carried out in a rotary ball mill operated at a speed of 150 to 1500 r.p.m.
26. A process according to claim 25, wherein said rotary ball mill is operated at a speed of about 1000 r.p.m.
27. A process according to claim 20, wherein steps (a) and (b) are carried out under an inert gas atmosphere.
28. A process according to claim 27, wherein said inert gas atmosphere comprises argon.
29. A process according to claim 20, wherein steps (a) and (b) are carried out for a period of time of about 10 hours.
30. A process according to claim 20, wherein said step (d) is carried out for a period of time of about one hour.
31. A process for producing an inert electrode material in powder form as defined in claim 5, which comprises the steps of:
a) subjecting at least one metal oxide or nitride to high-energy ball milling to form a first nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 µm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a ceramic material;
b) subjecting at least two metals to high-energy ball milling to form a second nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 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 nanocrystalline powders to form a powder mixture; and d) subjecting the powder mixture obtained in step (c) to high-energy ball milling to form a third nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 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.
a) subjecting at least one metal oxide or nitride to high-energy ball milling to form a first nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 µm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a ceramic material;
b) subjecting at least two metals to high-energy ball milling to form a second nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 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 nanocrystalline powders to form a powder mixture; and d) subjecting the powder mixture obtained in step (c) to high-energy ball milling to form a third nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 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.
32. A process according to claim 31, wherein said metal oxide or nitride is an oxide or nitride of a metal selected from the group consisting of Al, Co, Cr, Fe, Mo, Nb, Ni, Sn, Ti, Zn, Zr, V and W.
33. A process according to claim 31, wherein said metals are selected from the group consisting of copper, nickel, palladium, platinum and silver.
34. A process according to claim 31, wherein ferric oxide and nickel oxide are subjected to said high-energy ball milling in step (a), whereby said first nanocrystalline powder comprises particles having an average particle size of 0.1 to 50 µm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a NiFe2O4 spinel.
35. A process according to claim 34, wherein copper and silver are subjected to said high-energy ball milling in step (b), whereby said second nanocrystalline powder comprises particles having an average particle size of 0.1 to 50 µm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a Cu-Ag alloy.
36. A process according to claim 31, wherein steps (a), (b) and (d) are carried out in a vibratory ball mill operated at a frequency of 5 to 40 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 31, wherein steps (a), (b) and (d) are 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 31, wherein steps (a) and (b) are carried out under an inert gas atmosphere.
41. A process according to claim 40, wherein said inert gas atmosphere comprises argon.
42. A process according to claim 31, wherein steps (a) and (b) are carried out for a period of time of about 10 hours.
43. A process according to claim 31, wherein said step (d) is carried out for a period of time of about one hour.
44. A process according to claim 31, wherein step (b) is carried out in the presence of a lubricant.
45. A process according to claim 44, wherein said lubricant is stearic acid.
46. A process for producing an inert electrode material in powder form as defined in claim 10, which comprises subjecting a metal oxide or nitride to high-energy ball milling to form a nanocrystalline powder comprising particles having an average particle size of 0.1 to 50 µm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a single-phase ceramic material.
47. A process according to claim 46, wherein said metal oxide or nitride is an oxide or nitride of a metal selected from the group consisting of Al, Co, Cr, Fe, Mo, Nb, Ni, Sn, Ti, Zn, Zr, V and W.
48. A process according to claim 46, wherein zinc oxide is subjected to said high-energy ball milling.
49. A process according to claim 46, wherein at least one dopant comprising an element selected from the group consisting of Al, Co, Cr, Fe, Mo, Nb, Ni, Sn, Ti, Zn, Zr, V and W is admixed with said metal oxide or nitride prior to ball milling.
50. A process according to claim 49, wherein said dopant is used in an amount of about 0.002 to about 0.5 wt.%.
51. A process according to claim 50, wherein the amount of dopant ranges from about 0.005 to about 0.05 wt.%.
52. A process according to claim 49, wherein said metal oxide is zinc oxide and said dopant is aluminum oxide.
53. A process according to claim 52, wherein said dopant is used in an amount of about 0.008 wt.%.
54. A process according to claim 46, wherein said high-energy ball milling is carried in a vibratory ball mill operated at a frequency of 5 to 40 Hz.
55. A process according to claim 54, wherein said vibratory ball mill is operated at a frequency of about 17 Hz.
56. A process according to claim 46, wherein said high-energy ball milling is carried out in a rotary ball mill operated at a speed of 150 to r.p.m.
57. A process according to claim 56, wherein said rotary ball mill is operated at a speed of about 1000 r.p.m.
58. A process according to claim 46, wherein said high-energy ball milling is carried out under an inert gas atmosphere.
59. A process according to claim 58, wherein said inert gas atmosphere comprises argon.
60. A process according to claim 46, wherein said high-energy ball milling is carried out for a period of time of about 15 hours.
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| CNA028070186A CN1498287A (en) | 2001-03-20 | 2002-03-20 | Inert electrode material in nanocrystalline powder form |
| JP2002574408A JP2004531644A (en) | 2001-03-20 | 2002-03-20 | Inert electrode material in nanocrystalline powder form |
| EP02706576A EP1466039A2 (en) | 2001-03-20 | 2002-03-20 | Inert electrode material in nanocrystalline powder form |
| CA002441578A CA2441578A1 (en) | 2001-03-20 | 2002-03-20 | Inert electrode material in nanocrystalline powder form |
| US10/472,590 US20040045402A1 (en) | 2001-03-20 | 2002-03-20 | Inert electrode material in nanocrystalline powder form |
| BR0208273-0A BR0208273A (en) | 2001-03-20 | 2002-03-20 | Nanocrystalline pulverized inert electrode material |
| PCT/CA2002/000395 WO2002075023A2 (en) | 2001-03-20 | 2002-03-20 | Inert electrode material in nanocrystalline powder form |
| NO20034198A NO20034198L (en) | 2001-03-20 | 2003-09-19 | Inert electrode material in nanocrystalline powder form |
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| WO2007000014A1 (en) * | 2005-06-29 | 2007-01-04 | Very Small Particle Company Pty Ltd | Method of making metal oxides |
| CN100507091C (en) * | 2005-11-10 | 2009-07-01 | 东北大学 | Metal-base composite material inert anode for aluminium electrolysis and preparation method thereof |
| US20100183816A1 (en) * | 2006-02-17 | 2010-07-22 | Steward Advanced Materials, Inc. | Low velocity oxygen-fueled flame spray method and apparatus for making ferrite material products and products produced thereby |
| US9119906B2 (en) * | 2008-09-24 | 2015-09-01 | Integran Technologies, Inc. | In-vivo biodegradable medical implant |
| US20100085685A1 (en) * | 2008-10-06 | 2010-04-08 | Avx Corporation | Capacitor Anode Formed From a Powder Containing Coarse Agglomerates and Fine Agglomerates |
| JP2010245299A (en) | 2009-04-06 | 2010-10-28 | Three M Innovative Properties Co | Composite thermoelectric material and method of manufacturing the same |
| CA2850856C (en) * | 2011-10-20 | 2019-09-17 | Institut National De La Recherche Scientifique | Inert anodes for aluminum electrolysis and method of production thereof |
| WO2013137857A2 (en) | 2012-03-12 | 2013-09-19 | The Massachusetts Institute Of Technology | Stable binary nanocrystalline alloys and methods of identifying same |
| KR102372737B1 (en) | 2013-03-14 | 2022-03-10 | 메사추세츠 인스티튜트 오브 테크놀로지 | Sintered nanocrystalline alloys |
| JP6291370B2 (en) * | 2014-07-02 | 2018-03-14 | 株式会社東芝 | Strain detection element, pressure sensor, microphone, blood pressure sensor, and touch panel |
| WO2017105570A2 (en) | 2015-09-17 | 2017-06-22 | Massachusetts Institute Of Technology | Nanocrystalline alloy penetrators |
| SE539594C2 (en) | 2015-11-06 | 2017-10-17 | Winloc Ag | A set of profile members in combination with a key plug, a method to manufacture such a key plug and a combination also including an associated key |
| CN108863368A (en) * | 2018-07-12 | 2018-11-23 | 百色皓海碳素有限公司 | The production technology of fluting prebaked anode |
| WO2021188674A1 (en) * | 2020-03-18 | 2021-09-23 | Xtalic Corporation | Nanostructured palladium-based alloys and related methods |
| CN111974986A (en) * | 2020-08-06 | 2020-11-24 | 东莞材料基因高等理工研究院 | Aluminum metal composite powder and laser additive prepared from same |
| CN113087515B (en) * | 2021-03-26 | 2022-08-02 | 重庆上甲电子股份有限公司 | Manganese zinc ferrite with high saturation magnetic induction intensity, wide temperature range and low magnetic core loss, preparation method thereof and application of potassium tetrafluoroaluminate |
| CN117004256A (en) * | 2023-04-07 | 2023-11-07 | 中铝郑州有色金属研究院有限公司 | Metal electrode for aluminum electrolysis and coating composition and preparation method thereof |
| TWI846464B (en) * | 2023-05-12 | 2024-06-21 | 富宸材料國際股份有限公司 | Composite inert electrode, electrolyzer comprising the same and method of forming the same |
| TWI849901B (en) * | 2023-05-12 | 2024-07-21 | 富宸材料國際股份有限公司 | Composite inert electrode, electrolyzer comprising the same and method of forming the same |
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| RU2003130746A (en) | 2005-02-10 |
| JP2004531644A (en) | 2004-10-14 |
| WO2002075023A3 (en) | 2003-07-17 |
| WO2002075023A2 (en) | 2002-09-26 |
| NO20034198L (en) | 2003-11-13 |
| CN1498287A (en) | 2004-05-19 |
| US20040045402A1 (en) | 2004-03-11 |
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