Determination of oxygen in W-C-Co nanopowders

Abstract

The oxygen content of W-C-Co nanopowders produced by plasma reduction of WO3, followed by low-temperature carburization in hydrogen has been determined by carrier gas hot extraction. The oxygen in adsorbed water, carbon-oxygen complexes weakly bound to the surface, and surface oxides (WO x and CoO x ) has been determined separately. Freshly prepared, passivated WC and WC + 8% Co powders with a specific surface area of 6–11 m2/g were found to contain 0.03–0.07 μg/cm2 of oxygen, not counting adsorbed water. Strongly bound oxygen in the form of surface oxides accounts for at least 80% of the total oxygen. These oxygen contents are equivalent to surface coverages from 1.2 to 2.5 oxygen monolayers (monolayer density of 1015 at/cm2). The water content varies from 0.15 to 0.3%, which corresponds to a water film no thicker than a monolayer. All of the water is physisorbed. The products of the plasma reduction of WO3 have a complex phase composition (W2C, WC1 − x , W, α-WC) and a specific surface area from 21 to 24 m2/g. In spite of the high content of the readily oxidizable phases W and W2C, the plasma-synthesized mixtures have submonolayer surface coverages with oxygen. They are protected from air oxidation by thin (one to three monolayers) pyrolytic carbon films, while the small amount of oxygen present originates from unreacted particles. In dry air, the powders oxidize insignificantly. At 100% humidity, stoichiometric WC powders are the most stable, while WC + 8% Co shows the lowest stability. The oxidation rate of W-C powders is proportional to the overall content of W and W2C.