Structures and chemistry of tungsten powder from doped and undoped tungsten blue oxide

Abstract

Blue tungsten oxide, both without dopant and doped with aluminium, silicon and potassium was reduced to tungsten by heating at 750 and 900° C in flowing hydrogen. The morphology, chemistry, and crystal structure were determined by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis, X-ray photoelectron spectroscopy and Auger electron spectroscopy. Undoped oxide reduced at a lower rate than did the doped oxide. The potassium, silicon and aluminium appeared to be distributed non-uniformly on the surface of the doped oxide particles. Nucleation of tungsten crystals began earlier in the doped oxides leading to a bimodal distribution of crystal sizes in the doped material. There were pockets of potassium inside the tungsten crystals from the doped blue oxide. This suggests that the tungsten crystals nucleated earlier and at regions of high potassium concentration on the surface of the doped blue oxide. Tungsten was then transported from the surrounding oxide to the nucleus to grow the crystals trapping the potassium inside the crystal. Nucleation of the tungsten crystals occurred more homogeneously on the surface of the oxide that was low in potassium and, of course, on the undoped oxide particles, leading to the smaller size of the tungsten crystals from the undoped oxide.