Formation of a Volatile Tungsten Oxide Hydrate

Timgsten oxides and tungsten metal exhibit an apparent increase in vj or pressure at elevated temperatures in the presence of water vapor [3.27-3.31]. This increase is due to a reaction between the respective oxide or the metal and the Avater vapor, to form a volatile tungsten oxide hydrate. The reactions can be described by the following equations  

The presence of the monomere, volatile oxide hydrate [WO3 H2O resp. W02(OH)2] was proven by mass spectroscopy [3.28], and thermodynamic data are available for all of the above phase equilibria [3.10,3.29]. Based on these free energy data as well as on those of the solid oxides [3.23], the equilibrium partial pressure of the volatile compound can be calculated as a function of humidity. The result of such a calculation is shown in Fig. 3.2 for a temperature of 1000 °C [3.32]. In addition, the equilibrium pressures of the other volatile tungsten compoimds are also presented. From these calculations it is evident, that the oxide hydrate is by far the most volatile tungsten compound in the W-O-H system. 

The oxide hydrate is responsible for the chemical vapor transport of tungsten, which occurs throughout the reduction sequence, a d which decisively codetermines the physical and chemical properties of the metal powder. Its actual partial pressure during reduction will depend on both the temperature and the prevailing humidity. It will be lower toward the end of reduction, due to the steady decrease in the oxygen partial pressure (humidity) of the system as the reduction proceeds. 

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