The reduction of cobalt doped ammonium paratungstate to nanostructured W-Co powder

Cobalt doped ammonium paratungstate (Co-doped APT) precursors for the fabrication of cemented carbides were synthesized by co-precipitation, where W and Co are mixed on the atomic scale. The effect of cobalt content on the reduction of W-Co precursor was studied. W-Co precursors with different ratios of tungsten to cobalt were prepared and calcined at 220 degreesC for 5 h in air, followed by a reduction in hydrogen gas at 600 and 650 degreesC for different times. The resulting powders were characterized with respect to phase constituent, specific surface area, particle size, density, and morphology of the reduced powders. It has been found that the addition of cobalt results in a higher reduction rate toward the metallic phases. However, the particle size of the reduced powders increases with increasing cobalt content, while its relative density decreases. The specific surface area of reduced powders at 650 degreesC increases for short reduction times, due to reduction of oxides to metal powder, and decreases for long reduction time due to agglomeration of metal powder. Cobalt plays the role as a catalyst during the reduction processing. On the other hand, it promotes the agglomeration of small particles. However, the completely reduced powders were well nanostructured with an average particle size between 20-50 nm. (C) 2002 Elsevier Science Ltd. All rights reserved.