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 α-(C₆H₁₃NH₃)NbWO₆ undergo carbothermal reduction in an argon atmosphere at 1000 °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 °C but is not complete until 1000 °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_x increases with heating time (at 1000 °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 α-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 °C. The β-NbWO₆ system does not form a well-defined alkylammonium salt; instead a mixture of β-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 °C. The oxide/PAN composites appear to be intimate physical mixtures rather than ordered layered nanocomposites.