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

Mo migrates under the influence of HgBr2 in a temperature gradient (e.g. 1000 --> 900-degrees-C). Besides elementary Mo we observed in some experiments the occurence of MoBr2 and MoO2 (from oxygen containing impurities) respectively. The transport behaviour (deposition sequence; deposition rates of various phases) has been enlightened by continous measurement of the mass change during the transport experiments using a special ''transport balance''. Thus obtained deposition rates m(Mo) for molybdenum reached in the temperature region 800 less-than-or-equal-to TBAR less-than-or-equal-to 1040-degrees-C a maximum at TBAR = 980-degrees-C independend from the starting material (Mo or Mo/MoO2 mixtures). For variable densities D of transport agent at a constant temperature (TBAR = 950-degrees-C) increasing values for m(Mo) were observed (m(Mo) = 23 mg/h, D(max) = 8.61 mg HgBr2/cm3). Thermochemical calculation give strong evidence for the migration of Mo via the endothermal reaction Mo(s) + 2HgBr2,g = MoBr4,g + 2Hg(g). The experimental deposition rates are about half as large than the calculated values. Good agreement between calculations and experiments were obtained only assuming the presense of oxygen in the starting materials.