Patent Publication Number: US-2004052713-A1

Title: Refractory hard metals in powder form for use in the manufacture of electrodes

Description:
FIELD OF THE INVENTION  
       [0001] The present invention pertains to improvements in the field of electrodes for metal electrolysis. More particularly, the invention relates to refractory hard metals in powder form for use in the manufacture of such electrodes.  
       BACKGROUND ART  
       [0002] Aluminum is produced conventionally in a Hall-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.  
       [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] Extensive research has been carried out with refractory hard metals such as TiB 2 , as electrode materials. TiB 2  and other refractory hard metals are practically insoluble in aluminum, have a low electrical resistance and are wetted by aluminum. However, the shaping of TiB 2  and similar refractory hard metals is difficult because these materials have high melting temperatures and are highly covalent.  
       DISCLOSURE OF THE INVENTION  
       [0005] It is therefore an object of the present invention to overcome the above drawbacks and to provide a refractory hard metal in powder form suitable for the manufacture of electrodes by thermal deposition or powder metallurgy.  
       [0006] According to one aspect of the invention, there is provided a refractory hard metal in powder form comprising particles having an average particle size of 0.1 to 30 μm and each formed of an agglomerate of grains with each grain comprising a nanocrystal of a refractory hard metal of the formula:  
       A x B y X z    (I)  
       [0007] wherein A is a transition metal, B is a metal selected from the group consisting of zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, manganese, tungsten and cobalt, X is boron or carbon, x ranges from 0.1 to 3, y ranges from 0 to 3 and z ranges from 1 to 6.  
       [0008] The term “nanocrystal” as used herein refers to a crystal having a size of 100 nanometers or less.  
       [0009] The expression “thermal deposition” as used herein refers to a technique in which powder particles are injected in a torch and sprayed on a substrate. The particles acquire a high velocity and are partially or totally melted during the flight path. The coating is built by the solidification of the droplets on the substrate surface. Examples of such techniques include plasma spray, arc spray and high velocity oxy-fuel.  
       [0010] The expression “powder metallurgy” as used herein refers to a technique in which the bulk powders are transformed into preforms of a desired shape by compaction or shaping followed by a sintering step. Compaction refers to techniques where pressure is applied to the powder, as, for example, cold uniaxial pressing, cold isostatic pressing or hot isostatic pressing. Shaping refers to techniques executed without the application of external pressure such as powder filling or slurry casting.  
       [0011] The present invention also provides, in another aspect thereof, a process for producing a refractory hard metal in powder form as defined above. The process of the invention comprises the steps of:  
       [0012] a) providing a first reagent selected from the group consisting of transition metals and transition metal-containing compounds;  
       [0013] b) providing a second reagent selected from the group consisting of boron, boron-containing compounds, carbon and carbon-containing compounds;  
       [0014] c) providing an optional third reagent selected from the group consisting of zirconium, zirconium-containing compounds, hafnium, hafnium-containing compounds, vanadium, vanadium-containing compounds, niobium, niobium-containing compounds, tantalum, tantalum-containing compounds, chromium, chromium-containing compounds, molybdenum, molybdenum-containing compounds, manganese, manganese-containing compounds, tungsten, tungsten-containing compounds, cobalt and cobalt-containing compounds; and  
       [0015] d) subjecting the first, second and third reagents to high-energy ball milling to cause solid state reaction therebetween and formation of particles having an average particle size of 0.1 to 30 μm, each particle being formed of an agglomerate of grains with each grain comprising a nanocrystal of a refractory hard metal of formula (I) defined above.  
       [0016] The expression “high-energy ball milling” as used herein refers to a ball milling process capable of forming the aforesaid particles comprising nanocrystalline grains of the refractory hard metal of formula (I), within a period of time of about 40 hours. 
     
    
    
     DESCRIPTION OF DRAWING  
     [0017] In the accompanying drawing, the sole FIGURE shows the X-ray diffraction of the refractory hard metal in powder form obtained in Example 1. 
    
    
     MODES FOR CARRYING OUT THE INVENTION  
     [0018] Typical examples of refractory hard metals of the formula (I) include TiB 1.8 , TiB 2 , TiB 2.2 , TiC, Ti 0.5 Zr 0.5 B 2 , Ti 0.9 Zr 0.1 B 2 , Ti 0.5 Hf 0.5 B 2  and Zr 0.8 V 0.2 B 2 . TiB 2  is preferred.  
     [0019] Examples of suitable transition metals which may be used as the aforesaid first reagent include titanium, chromium, zirconium and vanadium. Titanium is preferred. It is also possible to use a titanium-containing compound such as TiH 2 , TiAl 3 , TiB and TiCl 2 .  
     [0020] Examples of suitable boron-containing compounds which may be used as the aforesaid second reagent include AlB 2 , AlB 12 , BH 3 , BN, VB, H 2 BO 3  and Na 2 B 4 O 7 . It is also possible to use tetraboron carbide (B 4 C) as either a boron-containing compound or a carbon-containing compound.  
     [0021] Examples of suitable compounds which may be used as the aforesaid third reagent include HfB 2 , VB 2 , NbB 2 , TaB 2 , CrB 2 , MoB 2 , MnB 2 , Mo 2 B 5 , W 2 B 5 , CoB, ZrC, TaC, WC and HfC.  
     [0022] According to a preferred embodiment, step (d) of the process according to the invention is carried out in a vibratory ball mill operated at a frequency of 8 to 25 Hz, preferably about 17 Hz. It is also possible to conduct step (d) in a rotary ball mill operated at a speed of 150 to 1500 r.p.m., preferably about 1000 r.p.m.  
     [0023] According to another preferred embodiment, step (d) is carried out under an inert gas atmosphere such as a gas atmosphere comprising argon or helium, or under a reactive gas atmosphere such as a gas atmosphere comprising hydrogen, ammonia or a hydrocarbon, in order to saturate dangling bonds and thereby prevent oxidation of the refractory hard metal. An atmosphere of argon, helium or hydrogen is preferred. It is also possible to coat the particles with a protective film or to admix a sacrificial element such as Mg or Ca with the reagents. In addition, a sintering aid such as Y 2 O 3  can be added during step (d).  
     [0024] In the particular case of TiB 2  or TiC wherein titanium and boron or carbon are present in stoichiometric quantities, these two compounds can be used as starting material. Thus, they can be directly subjected to high-energy ball milling to cause formation of particles having an average particle size of 0.1 to 30 μm, each particle being formed of an agglomerate of grains with each grain comprising a nanocrystal of TiB 2  or TiC.  
     [0025] The high-energy ball milling described above enables one to obtain refractory hard metals in powder form having either non-stoichiometric or stoichiometric compositions.  
     [0026] The refractory hard metals in powder form according to the invention are suitable for use in the manufacture of electrodes by thermal deposition or powder metallurgy. Due to the properties of refractory hard metals, the emission of toxic and greenhouse effect gases during metal electrolysis is lowered and the lifetime of the electrodes is increased, thus lowering maintenance costs. A lower and constant inter-electrode distance is also possible, thereby decreasing the electrolyte ohmic drop.  
     [0027] The following non-limiting examples illustrate the invention.  
     EXAMPLE 1  
     [0028] A TiB 2  powder was produced by ball milling 3.45 g of titanium and 1.55 g of boron in a hardened steel crucible with a ball-to-powder mass ratio of 4.5:1 using a SPEX 8000 (trademark) vibratory ball mill operated at a frequency of about 17 Hz. The operation was performed under a controlled argon atmosphere to prevent oxidization. The crucible was closed and sealed with a rubber O-ring. After 5 hours of high-energy ball milling, a TiB 2  structure was formed, as shown on the X-ray diffraction pattern in the accompanying drawing. The structure of TiB 2  is hexagonal with the space group P6/mmm (191). The particle size varied between 1 and 5 μm and the crystallite size, measured by X-ray diffraction, was about 30 nm.  
     EXAMPLE 2  
     [0029] A TiB 2  powder was produced according to the same procedure as described in Example 1 and under the same operating conditions, with the exception that the ball milling was carried out for 20 hours instead of 5 hours. The resulting powder was similar to that obtained in Example 1. The crystallite size, however, was lower (about 16 nm).  
     EXAMPLE 3  
     [0030] A TiC powder was produced according to the same procedure as described in Example 1 and under the same operating conditions, with the exception that titanium and graphite were milled.  
     EXAMPLE 4  
     [0031] A TiB 2  powder was produced by ball milling titanium diboride under the same operating conditions as in Example 1, with the exception that the ball milling was carried out for 20 hours instead of 5 hours. The starting structure was maintained, but the crystallite size decreased to 15 nm.  
     EXAMPLE 5  
     [0032] A TiB 1.8  powder was according to the same procedure as described in Example 1 and under the same operating conditions, with the exception that 3.6 g of titanium and 1.4 g of boron were milled.  
     EXAMPLE 6  
     [0033] A TiB 2.2  powder was according to the same procedure as described in Example 1 and under the same operating conditions, with the exception that 3.4 g of titanium and 1.7 g of boron were milled.  
     EXAMPLE 7  
     [0034] A Ti 0.5 Zr 0.5 B 2  powder was according to the same procedure as described in Example 1 and under the same operating conditions, with the exception that 1.9 g of titanium, 3.1 g of zirconium diboride and 0.8 g of boron were milled.  
     EXAMPLE 8  
     [0035] A Ti 0.9 Zr 0.1 B 2  powder was according to the same procedure as described in Example 1 and under the same operating conditions, with the exception that 2.9 g of titanium, 0.6 g of zirconium and 1.5 g of boron were milled.  
     EXAMPLE 9  
     [0036] A Ti 0.5 Hf 0.5 B 2  powder was according to the same procedure as described in Example 1 and under the same operating conditions, with the exception that 0.9 g of titanium, 3.3 g of hafnium and 0.8 g of boron were milled.  
     EXAMPLE 10  
     [0037] A Zr 0.8 V 0.2 B 2  powder was according to the same procedure as described in Example 1 and under the same operating conditions, with the exception that 3.5 g of zirconium, 0.5 g of vanadium and 1.0 g of boron were milled.