Hydrated Tungsten Oxide
In the process of hydrogen reduction of tungsten oxide, the vapor pressure of tungsten oxide increases obviously when there is water vapor at high temperature.
This increase in vapor pressure is due to the reaction of various tungsten oxides with water vapor to form a volatile hydrated tungsten oxide. In the W - O - H system, the hydrated tungsten oxide is much more volatile than other volatile tungsten compounds. Most scholars at home and abroad believe that it is the volatile hydrated tungsten oxide that causes the chemical vapor transfer (CVT) process of tungsten. Moreover, the CVT process runs through the whole stage of the reduction of tungsten oxide and hydrogen. It has a decisive influence on the physical and chemical properties of tungsten powder.
In the CVT process, tungsten and its oxides are contacted with water vapor to produce hydrated tungsten oxide , which is volatilized to the gas phase and undergoes a homogeneous reduction reaction with H2. The reduced product is deposited on the nucleated metal tungsten grains. The reduced product is deposited on the nucleated metal tungsten grains so that the tungsten powder particles grow.
The reaction rate of CVT process is fast, and the morphology of reduction products will change significantly compared with that of raw materials. The actual partial pressure of hydrous tungsten oxide WO2(OH)2 during hydrogen reduction is related to the reduction temperature and the actual humidity of the atmosphere. With the progress of reduction process, the partial pressure (humidity) of oxygen decreases continuously, and at the end of the reduction process, the partial pressure of hydrated tungsten oxide WO2(OH)2 is getting lower and lower.
In addition, hydrous tungsten oxide also occurs during the equilibrium annealing of hydrogen reduction, which causes small particles of oxide to evaporate and large particles grow, which eventually makes the annealed oxide markedly coarser.
To sum up, the key to prepare ultrafine tungsten powder is to reduce the formation of volatile hydrated tungsten oxide WO2(OH)2, and to inhibit the process of chemical vapor migration, under certain temperature conditions, the equilibrium partial pressure of WO2(OH)2 depends mainly on hydrogen humidity, and the greater the humidity is, the greater the equilibrium partial pressure of WO2(OH)2. Therefore, the key to the preparation of ultrafine tungsten powder is to rapidly remove water vapor from the reaction atmosphere.
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