SELECTIVE, LOW-TEMPERATURE SYNTHESIS OF NIOBIUM CARBIDE AND A MIXED (NIOBIUM TUNGSTEN) CARBIDE FROM METAL-OXIDE POLYACRYLONITRILE COMPOSITES BY CARBOTHERMAL REDUCTION

Composites of polyacrylonitrile (PAN) with the layered oxides (C6H13NH3)Nb3O8, (C8H17NH3)Nb3O8 and alpha-(C6H13NH3)NbWO6 undergo carbothermal reduction in an argon atmosphere at 1000-degrees-C to give the cubic carbides NbC(x) and (Nb, W)C(x) respectively. Reduction of the Nb3O8/PAN composites to NbC(x) proceeds via the formation of tetragonal NbO2, with no other intermediates being detected. Formation of NbC(x) begins at 800-degrees-C but is not complete until 1000-degrees-C. The resultant carbide appears in a highly porous form in admixture with approximately 50% wt/wt amorphous carbon. The carbide content, x, of cubic NbC, increases with heating time (at 1000-degrees-C) as expected. Values of x ranging from 0.69-0.95 have been observed. The cubic mixed carbide, (Nb, W)C(x), is formed similarly from the alpha-NbWO6 system via an alkylammonium form in the presence of PAN, although progressive separation into cubic NbC(x) and hexagonal WC(x) occurs at temperatures above 1000-degrees-C. The beta-NbWO6 system does not form a well-defined alkylammonium salt; instead a mixture of beta-HNbWO6 with PAN gives rise to a very poorly crystalline (Nb, W) carbide on reduction. In all cases, both a layered oxide and PAN are necessary to form the pure carbides at 1000-degrees-C. The oxide/PAN composites appear to be intimate physical mixtures rather than ordered layered nanocomposites.