Patent Publication Number: US-2004045402-A1

Title: Inert electrode material in nanocrystalline powder form

Description:
TECHNICAL FIELD  
       [0001] 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  
       [0002] Aluminum is produced conventionally in a Hall-Héroult reduction cell by the electrolysis of alumina dissolved in molten cryolite (Na 3 AlF 6 ) at temperatures of up to about 950° C. A Hall-Héroult 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.  
       [0003] During electrolysis in Hall-Héroult cells, the carbon anode is consumed leading to the evolution of greenhouse gases such as CO and CO 2 . The anode has to be periodically changed and the erosion of the material modifies the anode-cathode distance, which increases the voltage due to the electrolyte resistance. On the cathode side, the carbon blocks are subjected to erosion and electrolyte penetration. A sodium intercalation in the graphitic structure occurs, which causes swelling and deformation of the cathode carbon blocks. The increase of voltage between the electrodes adversely affects the energy efficiency of the process.  
       [0004] 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 U.S. Pat. No. 6,162,334, ceramics such as proposed in U.S. Pat. Nos. 3,960,678 and 4,399,008, and cermets such as proposed in U.S. Pat. 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-Héroult 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).  
       [0005] U.S. Pat. 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 NiO and Fe 2 O 3 , 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.  
       [0006] 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.  
       [0007] 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.  
       [0008] 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  
       [0009] 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.  
       [0010] 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 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.  
       [0011] 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 μm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a single phase ceramic material.  
       [0012] 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 μm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a metal.  
       [0013] 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 μm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of an alloy.  
       [0014] 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 μm) powder mixtures for a same densification level. Thus, the overall cost of the sintering process is considerably decreased.  
       [0015] 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.  
       [0016] 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.  
       [0017] 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:  
       [0018] 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;  
       [0019] 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 the metal;  
       [0020] c) mixing the first and second powders to form a powder mixture; and  
       [0021] 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 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 nanocrystal of the metal.  
       [0022] 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:  
       [0023] 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;  
       [0024] 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 the metals;  
       [0025] c) mixing the first and second powders to form a powder mixture; and  
       [0026] 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 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.  
       [0027] 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 μm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a single phase ceramic material.  
       [0028] 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 μm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of the metal.  
       [0029] 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 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 μm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of an alloy of the metals.  
       [0030] 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  
       [0031] 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.  
       [0032] 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 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.  
       [0033] 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.  
       [0034] 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.  
       [0035] According to a preferred embodiment of the process of the invention, 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 100 to 2000 r.p.m., preferably about 1200 r.p.m.  
       [0036] 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.  
       [0037] 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.  
       [0038] 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.  
       [0039] 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.  
       [0040] The following non-limiting examples illustrate the invention. 
     
    
    
     EXAMPLE 1  
     [0041] A NiFe 2 O 4  spinel powder was produced by ball milling 51.7 wt. % NiO and 48.3 wt. % Fe 2 O 3  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 NiFe 2 O 4  spinel with excess NiO was formed. The particle size varied between 0.1 and 5 μm and the crystallite size, measured by X-ray diffraction, was about 30 nm.  
     [0042] 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 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 μm and the crystallite size, measured by X-ray diffraction, was about 40 nm.  
     [0043] 80 wt. % of the NiFe 2 O 4  spinel 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 NiFe 2 O 4  spinel and nanocrystals of the Cu—Ag alloy was obtained. The particle size varied between 5 and 10 μm. 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  
     [0044] A NiFe 2 O 4  ferrite powder was produced by ball milling 51.7 wt. % of NiO and 48.3 wt. % Fe 2 O 3  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 NiFe 2 O 4  spinel was produced with an excess of nanocrystalline NiO. The particle size varied between 0.1 and 5 μm.  
     [0045] 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 μm and the crystallite size, measured by X-ray diffraction, was about 20 nm.  
     [0046] 81.3 wt. % of the above NiFe 2 O 4  spinel powder, 16.6 wt. % of the above Cu—Ag alloy powder and 2 wt. % of CAPLUBE G (trademark) 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 10:1 using a SIMOLOYER rotary ball mill operated at 800 r.p.m. After 15 minutes of high-energy ball milling, a nanocrystalline powder comprising particles each formed of an agglomerate of grains comprising nanocrystals of the NiFe 2 O 4  spinel and nanocrystals of the Cu—Ag alloy was obtained. The particle size varied between 5 and 10 μm. 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.  
     EXAMPLE 3  
     [0047] A NiFe 2 O 4  ferrite powder was produced by ball milling 51.7 wt. % of NiO and 48.3 wt.% Fe 2 O 3  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 NiFe 2 O 4  spinel was produced with an excess of nanocrystalline NiO. The particle size varied between 0.1 and 5 μm.  
     [0048] 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 μm and the crystallite size, measured by X-ray diffraction, was about 20 nm.  
     [0049] 81.3 wt. % of the above NiFe 2 O 4  spinel 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 NiFe 2 O 4  spinel and nanocrystals of the Cu—Ag alloy was obtained. The particle size varied between 5 and 10 μm. This nanocrystalline powder was then uniaxially 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.  
     EXAMPLE 4  
     [0050] A coarse-grained ZnO powder (99.9% pure) having an average grain size of 1 μm and a specific surface area of 3 m 2 /g was used as starting material. 0.008 wt. % Al 2 O 3  and 2 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 15 hours of high-energy ball milling, a nanocrystalline ZnO powder having a particle size between 1 and 5 μm and an average grain size smaller than 100 nm was obtained. The specific surface area of the nanocrystalline grains was 40 m 2 /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  
     [0051] 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. 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 μm 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.