Study of the preparation of bulk powder tungsten carbides by temperature programmed reaction with CH4+H-2 mixtures

The synthesis of bulk tungsten carbides by carburization of W metal or of WO3 with mixtures of CH4 in hydrogen at various pressures has been studied in temperature programmed experiments. The resulting solids have been characterized by elemental analysis, X-ray diffraction, XPS analysis, and specific surface area measurements. The carburization occurs in two distinct steps: W2C is formed in the first step taking place at about 650 degrees C at atmospheric pressure with a 20% CH4-H-2 mixture, while the formation of WC occurs only at higher temperatures. During carburization some free carbon is deposited, the importance of which is very much dependent on CH4 partial pressure and on the temperature of carburization. It has also been shown that direct carburization of WO3 by CH4-H-2 does not take place, but that the carburization occurs via the reduction of WO3 to W metal. The rate of reduction of WO3 and that of carburization of W metal are very much dependent on, respectively, hydrogen partial pressure and CH4 partial pressure. The extent of reduction of WO3 into W metal required for carburization which takes place also depends on CH4 partial pressure, indicating a competition between carburization of W metal at the surface and diffusion of W metal into the bulk of the solid. (C) 1996 Academic Press, Inc.