ON THE CHEMICAL-TRANSPORT OF TUNGSTEN USING HGBR2 - EXPERIMENTS AND THERMOCHEMICAL CALCULATIONS

Using HgBr2 as transport agent tungsten migrates in a temperature gradient from the region of higher temperature to the lower temperature (e.g. 1000 --> 900-degrees-C). The transport rates were measured for various transport agent concentrations (0.64 less-than-or-equal-to C(HgBr2) less-than-or-equal-to 11.74 mg/cm3; TBAR = 950-degrees-C) and for various mean transport temperatures (800 less-than-or-equal-to TBAR less-than-or-equal-to 1040-degrees-C). Under these conditions tungsten crystals were observed in the sink region. To observe the influence of tungsten dioxide (contamination of the tungsten powder) on the transport behaviour of tungsten, experiments with W/WO2 as starting materials were performed. According to model calculations the following endothermic reactions are important for the migration of tungsten: W(s) + 5/2HgBr2.g = WBr5,g + 5/2Hg(g) W(s) + 2HgBr2,g = WBr4,g + 2Hg(g) In the presence of H2O or WO2 other equilibria play a role, too. Using a special ''transport balance'' we observed a delay of deposition of tungsten (e.g. TBAR = 800-degrees-C; 15 h delay of deposition) with W and W/WO2 as starting materials. The heterogeneous and homogeneous equilibria will be discussed and an explanation for the non equilibrium transport behaviour of tungsten will be given.