U.U. Gomes

Synthesis of tungsten carbide through gas–solid reaction at low temperatures

Abstract Conventionally, WC powders are produced heating a mixture of W powder and carbon black. This method is based on solid state reaction controlled by diffusion and phase transformations. Thus high temperatures (>1400°C) and long times are necessary. This paper presents a method able to synthesize WC at low temperatures (∼850°C) in 2 h. It is based on a gas–solid reaction between a W source and a C-containing gas. The gas phase is a mixture of H2 and CH4. Ammonium paratungstate and tungsten blue oxide are used as the tungsten source. The higher reactivity of the precursor material and the better contact between the gas–solid reactants explain the lower temperatures and higher reaction rates of this method compared to the conventional one. Very fine WC crystallites (

A low temperature synthesized NbC as grain growth inhibitor for WC–Co composites

Abstract Niobium carbide can be used to inhibit WC grain growth in hardmetal. The performance of a NbC powder produced at low temperature by solid–gas reaction (an experimental powder) as WC grain growth inhibitor is compared with that of a commercial NbC powder. It is verified that NbC effectively inhibits heterogeneous WC coarsening. This results in an increase in hardness. The commercial and experimental NbC powders exhibit a comparable performance in inhibiting the WC grain coarsening, in spite of a significant difference in particle size and shape. The commercial NbC powder is very fine while the experimental one is coarse and porous, but its crystallites are finer than those of the commercial product. The milling procedure used to prepare the alloys is able to reduce the particle size of the experimental NbC, and thus guarantee a dispersion of the particles with a quality level comparable to that found for the alloy prepared with the commercial NbC.

The thermal decomposition of monohydrated ammonium oxotris (oxalate) niobate

The thermal decomposition in argon of the monohydrated ammonium tris(oxalate)niobate, (NH4)3NbO(C2O4)3.H 2O, has been studied by TG/DTG, DTA, DSC, IR absorption and XRD. The material decomposes stepwise by the release of CO, CO2, NH3 and water. The atomic structure of the complex initially becomes amorphous and then crystallizes twice in the form of orthorhombic and monoclinic niobium pentoxide.