PREPARATION OF MOLYBDENUM AND TUNGSTEN CARBIDES FROM SOLUTION DERIVEDPRECURSORS

Synthesis principles have been developed for the preparation of soluti on derived precursors that can be selectively converted to desired ref ractory materials, as powders, fibres or thick films. The precursors a re prepared from aqueous or ethylene glycol solutions containing a pyr olysable organic compound (carbonaceous gel or saccharose) and a solub le molybdenum or tungsten compound (peroxo acid or ethylene glycolate) . The concentrated solutions exhibit rheological properties that allow fibre drawing and production of thick films. The sequence of the high temperature reactions - pyrolysis, reduction, carburization - was inv estigated by thermal gravimetric analysis (TGA), X-ray analysis and ni trogen adsorption. Because of the high homogeneity of the reactants, c arbon and transition metal suboxides in the pyrolysed precursors, the observed carburization temperatures (800-1200 degrees C) are lower tha n those used industrially. Most of the intermediate and final products - carbon/oxide composites, Mo(C,O), Mo2C, W2C, and WC - are materials of a high Brunauer-Emmett-Teller (BET) surface area (50-200m(2) g(-1) ). (C) 1998 Chapman & Hall.