Activity coefficient of oxygen in copper-tellurium melts
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I.
INTRODUCTION
TUE thermodynamic behavior of oxygen dissolved in binary melts A-B containing a calcogenide B has been poorly investigated as compared with that in molten metals and alloys. Utilizing a modified coulometric titration technique, the activity coefficient of oxygen, 3'0, in T1-Te melts at 1073 and 1223 K ~ has been measured. The molar enthalpy change, AH~ and molar entropy change, AS~, for 1/2 O2(1 atm) ~ O(1 at. pct) as a function of solvent composition were determined. The 3'o values change abruptly around the composition T12Te as do the AH~ and AS~ values. Subsequently, the 3'o values in Cu-S melts at 1423 K, which are important in metallurgical processes, were measured. 2 The compositional dependence of the 3'o values is quite similar to that in T1-Te melts. These binary phase diagrams exhibit an immiscible region between metal and semiconductor-like phases. The activity coefficients of these solvents change abruptly around the composition T12Te or Cu2S. 3'4 The complexity of the 3'o vs composition curves appears to come from the solution properties of the solvents which deviate significantly from the ideal. In order to test this hypothesis, it was decided in the present study to measure the activity coefficient of oxygen, 3'0, in Cu-Te melts that possess physicochemical properties similar to T1-Te and Cu-S melts. 5'6 The measured 3'o values were compared with those predicted from several solution models proposed in the literature. 7-~~
II.
EXPERIMENTAL
The experimental technique used in the present study is the modified coulometric titration method previously proposed. H'12 The theoretical basis 12 and details of the experimental technique 13'14have been presented in earlier publications. A cylindrical electrochemical cell (O in Cu-Te melts/ZrO2(+CaO)/air, Pt) with an alumina stirring rod was constructed. A schematic diagram of the cell was shown in Reference 15, Figure 1. A ZrO2(+5 wt pct CaO) solid electrolyte tube (8 mm o.d., 5 mm i.d., and 300 mm long), SHINYA YAO, former name, SHINYA OTSUKA, Research Instructor, and ZENSAKU KOZUKA, Professor, are with the Department of Metallurgical Engineering, Faculty of Engineering, Osaka University, Yamada-oka 2-1, Suita-city, Osaka, 565, Japan. YOICHI KAKU, formerly Graduate Student, Osaka University, is now with the Nissan Motor Company Ltd., Zama-city, Kanagawa, 228, Japan. Manuscript submitted November 21, 1984.
METALLURGICALTRANSACTIONSB
closed at one end, was supplied by Nippon Kagaku Togyo Co. Ltd. The outside surface of the electrolyte was coated with platinum paste over a length of about 65 mm from its lower end. The copper rod used in this study was of 99.998 wt pct purity with the following impurities: Ag 4 ppm, Fe 4 ppm, Ni 3 ppm, Pb 2 ppm, S 4 ppm, Sn 1 ppm, and Zn 1 ppm. Tellurium used was of 99.999 wt pet purity. Preselected amounts of the copper and tellurium were sealed in an evacuated quartz glass tube. The sample was first melted at 1473 K and then quenched in water. Two starting material compositions of Cu2_xTe, i.e., 30.00 and
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