The refractory metal borides, carbides and nitrides have already been proposed for use as components of electrolytic aluminum production cells, particularly those components which in use may normally be contacted with molten aluminum and/or cryolite, such as an electrowinning cathode or cathode current feeder. Borides and in particular titanium diboride are often preferred on account of their excellent wettability by molten aluminum and good electronic conductivity. However, many problems have been encountered in producing these materials with a sufficient resistance to attack by molten aluminum and by the conventional alumina-containing cryolite based melts used in aluminum electrowinning cells.
Considering for the moment the production of borides, this may be accomplished by the reduction of a metal oxide or some other metal compound, such as by using carbon, boron or boron carbide. Regarding the use in the reduction of compounds other than metal oxide, it has been disclosed in U.S. Pat. No. 2,973,247 that the titanium component can be in the form of a titanic acid hydrolysate, sometimes known as basic titanium sulfate or basic titanium chloride. More recently, it has been disclosed in U.S. Pat. No. 3,379,647 that solutions of ingredients are desirable. For example, finely divided titanium dioxide can be combined with sucrose and boric acid in an ethylene glycol medium to form an intimate mixture for subsequent heating.
Continuing in the context of the preparation of diborides, it has been shown that the heating of some diborides under vacuum and at substantially elevated temperature will lead to boron loss. It has nevertheless been often taught to prepare diboride with either stoichiometric amounts, or slight excesses of boride reactant. Thus, in the above mentioned U.S. Pat. No. 2,973,247 it has been taught that typically 10% excess of boric acid can be expected to be sufficient for use in the carbothermic reduction of titanium hydrate with an oxidic compound of boron. U.S. Pat. No. 3,379,647 has disclosed that the appropriate quantities of reactants can be calculated quite closely from known equations for the production of the desired product.
On the other hand, some teachings away from stoichiometry have been suggested. In British Pat. No. 1,004,585 it is disclosed that boric oxide may be tolerated in an excess up to 50 percent by weight when considered for reaction with titanium dioxide. It is further disclosed therein that whereas low production temperature may be desirable, as a practical matter more elevated temperatures are actually employed.
However, the outcome of all of the foregoing taken with the conventional sintering approach of these patents has been found to provide products, which when formed into articles by pressing and sintering, are nevertheless prone to severe grain boundary corrosion when exposed, for example, to molten aluminum.
It is to be understood that there are other, well recognized methods of preparing the desired final product, e g., fused salt electrolysis, reductions by other metals, or the vapor phase reaction of the metal halide. However, such reaction methods are not the subject of the present invention.