Thermodynamic properties of liquid tin-tellurium alloys at high temperature
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1 INTRODUCTION M O L T E N Sn, Te alloys were previously investigated by calorimetric measurements, 1~ but to our knowledge, only Predel et al 7have published values of the activities at 1281 K. In a previous study of binary alloys exhibiting strong interactions, s we have reported some results on the thermodynamic measurements of liquid tin, tellurium alloys (TexSn~_x) over the range of temperature 673 to 913 K and with compositions from 0.756 to 0.978. Subsequently we have examined further tellurium rich alloys by emf measurements on galvanic cells over the temperature range 673 to 1123 K. In addition, measurements have been made by solution calorimetry and differential thermal analysis (DTA) in order to check the emf results. The results obtained are the subject of the present paper. 2 EXPERIMENTAL 2.1 Electromotive Force in Galvanic Cells
2.1.1 Principle and Experimental Techniques. The emf measurements made use of the following galvanic cell Sn(l)/Sn2+ /Sn, Te(l) The thermodynamic properties of tin are related to the measured emf's E by the relations:
AGs.=RTlnas.=-2EF;
ASs.= 2F(0~)
", XSa
_ ?EJq Arts. = - 2F ~0--]~T-] ~so' J. RAKOTOMAVO,who is preparing his thesis, and C. PETOT, Professor, are at the Universityof Paris Nor& M.-C. BARON is an Engineer at CNRS, Laboratoire d'Etudes des Proprirtrs Mecaniques et Thermodynamiquesdes Mat~riaux, Universit6de Paris Nord, 93430 Villetaneuse, France. Manuscript submitted October 2, 1980.
where AGs,, AS-s., AHs, and as, are respectively the partial molar Gibbs energy, entropy, enthalpy and the activity of tin referred to pure liquid standard state; F is the Faraday constant. Techniques for these measurements are similar to those described in a previous investigation on Au, Sn alloys. 9 The galvanic cell is shown schematically in Fig. 1. It contains three pure Sn reference electrodes and five different concentrations of (TexSn ~_x) alloys. The tin and tellurium used to make the alloys were both 99.999 mass pct purity. The liquid electrolyte was the eutectic mixture of KC1 and LiC1 containing about 2 mass pct of SnC12. The electrical connections were made using tungsten wires protected by alumina tubes. Temperature was measured with a chromel-alumel thermocouple, protected by an alumina sheath placed in the center of the cell. After transfer of the electrolyte into the alumina crucible containing the thermocouple and alloys, the cell was evacuated with a mechanical vacuum pump, filled with purified argon at atmospheric pressure and then sealed. The alloys were homogenized "in situ" by heating for a sufficiently long time at a temperature above the melting point. The emf values were measured at constant temperature using a Tacussel electronic millivoltmeter, Aries 10000, the impedance of which is equal to 1013Ohms. At every operational temperature the stability of emf was checked over a period of 30 to 45 min. The emf measurements were made at various temperatures when the specimens were heated from 673 to 1123 K and repeated when cooling over the same range of tempe
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