Chemical vapor synthesis (CVS) and characterization of tungsten carbide-cobalt nanocomposite powder

The purpose of this research was to synthesize nanosized tungsten carbide and tungsten carbide-cobalt composite powders by chemical vapor synthesis (CVS). The CVS process was carried out by the reduction and carburization of volatile reactant precursors by gaseous reducing and carburizing sources in a tubular reactor and a plasma reactor.;The reduction and carburization of the volatilized tungsten hexachloride together with cobalt chloride in some runs, by a hydrogen-methane mixture in the tubular reactor produced nanosized WC and WC-Co composite powder, which sometimes contained small amounts of W2C and W phases. The products obtained from the plasma reactor with tungsten hexachloride or ammonium paratungstate (APT) as the W precursor were nanosized WC1-x, which sometimes contained small amounts of W2C and/or WC phase. When cobalt oxide (Co3O4) was fed together with APT, WC1-x -Co composite powder that contained small amounts of W2C and W phases was synthesized. The particle sizes of product obtained from the tubular reactor and the plasma reactor were less than 30 nm and 20 nm, respectively. The effects of reaction temperature, CH4 to WCl 6 molar ratio, CoCl2 content, residence time, precursors feeding rate, and H2 to CH4 molar ratio on the product composition and particle size were investigated. In addition, the effects of plasma torch power, the flow rate of plasma gas, molar ratio of reactant gases, and the addition of secondary plasma gas (H2) in the plasma reactor system on the product composition and particle size were also examined.;The synthesized powders were subjected to a hydrogen heat treatment to fully carburize W2C, W and/or WC1-x phases to the WC phase as well as to remove excess carbon in the product. Finally, nanosized WC and WC-Co composite powders of particle sizes less than 100 nm were obtained from this process.;Experimental runs in the tubular reactor system for the synthesis of WC from WCl6 with CH4 and H2 were simulated by the use of computational fluid dynamics (CFD). In this work, the mechanisms of WC particle formation were further studied, the nucleation (kn) and growth (kg) rate constants were obtained, and an apparent activation energy was determined.