Abstract
The reduction mechanism of the composite powders mixed with <TEX>$WO_3$</TEX> and CuO has been studied by using thermogravimetry (TG), X-ray diffraction, and microstructure analyses. The composite powders were made by simple Turbula mixing, spray drying, and ball-milling in a stainless steel jar with the ball to powder ratio of 32 to 1 at 80 rpm for 1 h without process controlling agents. It is observed that all the oxide composite powders are converted to W-coated Cu composite powder after reducing treatment under hydrogen atmosphere. For the formation mechanism of W-coated Cu composite powder, the sequential reduction steps are proposed as follows: CuO contained in the ball-milled composite powder is initially reduced to Cu at the temperature range from 20<TEX>$0^{\circ}C$</TEX> to 30<TEX>$0^{\circ}C$</TEX>. Then, <TEX>$WO_3$</TEX> powder is reduced to W <TEX>$O_2$</TEX> via W <TEX>$O_{2.9}$</TEX> and W <TEX>$O_{2.72}$</TEX> at higher temperature region. Finally, the gaseous phase of <TEX>$WO_3(OH)_2$</TEX> formed by reaction of <TEX>$WO_2$</TEX> with water vapour migrates to previously reduced Cu and deposits on it as W reduced by hydrogen. The proposed mechanism has been proved through the model experiment which was performed by using Cu plate and <TEX>$WO_3$</TEX> powder.
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