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Synthesis, Separation and Electrical Properties of WO_{3-x} Nanopowders via Partial Pressure High Energy Ball-Milling

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Al Mohammad A.

Acta Physica Polonica A

2009

vol. 116

issue 2

240-244

Reduction processes of WO_{3} 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 WO_{3-x} nanopowder during reductions via partial pressure high energy ball-milling have been studied. During the carbon-reduction process the monoclinic WO_{3} structure transforms to nonstoichiometric Magneli phases W_{40}O_{118}, WO_{2.9} and finally to WO_{2} and W mixed phases. The Magneli WO_{3-x} phases exhibit specific fringe contrast imaging of well-ordered crystallographic shear planes. In comparison, the monoclinic WO_{3} structure transforms to hydrate WO_{3}·1/3H_{2}O, hexagonal WO_{3}, non-stoichiometric WO_{2.7} and finally to WO_{2} and W mixed phases during the hydrogen-reduction process. The inclusion of hydrogen atoms between the WO_{6} octahedral structure shifts the reduction steps to lower milling times. It demonstrates that the formation of hydrate WO_{3} 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.

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1 Acta Physica Polonica A

1 2009

1 Al Mohammad A.

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Synthesis, Separation and Electrical Properties of WO_{3-x} Nanopowders via Partial Pressure High Energy Ball-Milling