Temperature. Temperature influences the rate of all reactions occurring during reduction, hence the dynamic and partial pressure of the volatile [WO2(OH)2] which forms during reduction and which is responsible for the chemical vapor transport (CVT) of tungsten. Temperature and tungsten particle size are directly proportional while temperature and time required for final reduction are inversely proportional.
Oxide Feed. The tungsten mass flow determines the amount of H2O liberated during the entire reduction process. The higher the flow, the larger the grain size.
Tungsten Powder Layer Height. During reduction and accompanying water formation, the powder layer exerts a considerable diffusion resistance against water removal from the layer. The higher the layer, the greater the diffusion resistance and the more slowly the reaction water will be removed. The local humidity is higher at the bottom of growth conditions of the metal particles formed at a particular temperature. The layer height is directly proportional to powder grain size.
Porosity of the Powder Layer. The porosity of the powder layer, and thus its permeability, is determined by the macroporosity (intermediate space between the oxide particles) and by microporosity (porosity of the individual oxide particles). The higher the porosity of the powder layer, the better the material exchange H2O→H2 during reduction and the less the grains of the tungsten particles will grow, resulting in a smaller particle size.
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