Ultrafine Tungsten Powder Preparation -- Hydrogen Reduction Method (2)
Hydrogen reduction is the main method for preparing ultrafine tungsten powder.
It is a rather complicated physical and chemical process, including chemical transformation, phase change process, solid phase diffusion and chemical vapor transfer.
In W-O-H there are 6 solidification phase, which are: tungsten trioxide (WO3), beta tungsten oxide (WO2.9, W20O58, blue tungsten) gamma tungsten oxide (WO2.72, W18O49, Purple Tungsten) two tungsten oxide (WO2), -W alpha and beta -W. Therefore, the process of hydrogen reduction reaction of tungsten oxide is complex, and different reduction processes will occur with the change of temperature and humidity: In a high temperature and a humid reducing atmosphere, the reduction proceeds in distinct stages ( WO3 → WO2.9 → WO2.72 → WO2→ W ); In low temperature and dry hydrogen, it is possible to reduce tungsten oxide to tungsten powder directly from high valence tungsten oxide ( WO2.9 → W，WO2.72 → W ), the process is roughly: tungsten oxide → hydrogen reduction → screening → batch → tungsten powder.
The reaction pathway of tungsten oxide reduction process is different, and the rate of reduction reaction is different, which leads to different particle size of tungsten powder after reduction. Some scholars have shown that tungsten powder obtained by WO2.9 → WO2 → W ( or WO2.72 → WO2 → W ) is different from tungsten powder obtained by WO2.9 → W ( or WO2.72 → W ).
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