Synthesis, Separation and Electrical Properties of WO_3-xNanopowders via Partial Pressure High Energy Ball-Milling

Abstract

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 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 WO3·1/3H2O, 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.

Nanocrystalline nickel dispersed with nano-size WO3 particles synthesized by electrodeposition

“…The mechanism of phase transition from monoclinic to tetragonal structures is not clear yet, but there are two possibilities. One possible , and finally to WO 2 and W [24]. Therefore, it may be possible that, during rapid potential drops, particles are deoxidized to more complex stoichiometric or nonstoichiometric compounds near WO 3 , which induces disequilibrium phase transition.…”

Effect of solution concentration on physicochemical and gas sensing properties of sprayed WO3 thin films

“…The films showed n-type semiconducting behavior which is confirmed by the thermoelectric power measurements (not included). Activation energy of the films was calculated using formula [35] and shown in table (1).…”

Evaluation of the WO_x Film Properties for ReRAM Application

“…10). From the Arrhenius plot, the calculated activation energy (Ea) of tungsten oxidization is 0.73 eV, close to but smaller than 0.82 eV for the monoclinic WO 3 [7]. With the evidences from the XPS [3] and XRD results, the WO X layer contains not only WO 3 but also tungsten of other oxidized states, which contribute the reduction of the calculated Ea.…”