Synthesis, Separation and Electrical Properties of WO3-x Nanopowders via Partial Pressure High Energy Ball-Milling

Reduction processes of WO3 nanopowder either with carbon or with hydrogen were observed using X-ray powder diffraction and transmission electron microscope. The phase transformations, separation, grain size and electrical conductivity of WO3-x nanopowder during reductions via partial pressure high energy ball-milling have been studied. During the carbon-reduction process the monoclinic WO3 structure transforms to nonstoichiometric Magneli phases W40O118, WO2.9 and finally to WO2 and W mixed phases. The Magneli WO3-x phases exhibit specific fringe contrast imaging of well-ordered crystallographic shear planes. In comparison, the monoclinic WO3 structure transforms to hydrate WO3.1/3H(2)O, hexagonal WO3, non-stoichiometric WO2.7 and finally to WO2 and W mixed phases during the hydrogen-reduction process. The inclusion of hydrogen atoms between the WO6 octahedral structure shifts the reduction steps to lower milling times. It demonstrates that. the formation of hydrate WO3 phases enhances the amenability of the system to reduction. The activation energy for conduction was deduced from the Arrhenius equation and was found to depend on oxygen partial pressure or presence of the hydrogen atoms. The defect band model was used for interpretation of these behaviors. It supposes that the surface oxygen vacancies introduce donor levels in the gap of semiconductor, so free electrons are produced by reduction.