CHEMICAL-VAPOR TRANSPORT OF TUNGSTEN DIOXIDE USING HGBR2 AS TRANSPORTAGENT - EXPERIMENTS AND THERMOCHEMICAL CALCULATIONS

Tungsten dioxide migrates under the influence of HgBr2 in a temperatur e gradient from the region of higher temperature to the lower temperat ure. The transport behaviour (deposition rate and mass loss of dioxide by solution in the gas phase) has been determined by continuous measu rement of mass change during the transport experiments using a special ''transport balance''. Deposition rates for WO2 reached a maximum (m( WO2) almost-equal-to 100 mg/h) in the temperature region 600 less-than -or-equal-to TBAR less-than-or-equal-to 1000-degrees-C at TBAR almost- equal-to 750-degrees-C (C(HgBr2) = 0.2 mmol/20 cm3). Thermochemical ca lculations are in very good agreement with the experimental results at higher mean transport temperature (TBAR greater-than-or-equal-to 750- degrees-C). The heterogeneous and homogeneous equilibria that are resp onsible for chemical vapour transport of tungsten dioxide based on the thermochemical calculations will be discussed. An explanation for the non-equilibrium transport behaviour of WO2 at TBAR less-than-or-equal -to 725-degrees-C will be given.