Gibbs energies of formation of rare earth oxysulfides
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I.
INTRODUCTION
THE use of rare earth elements at high temperatures in the treatment of iron and steel for purposes such as deoxidation, desulfurization, control of inclusion morphology in steel, and graphite morphology in cast iron necessitates the evaluation of the thermodynamic properties of rare earth compounds. Investigations in this field are, however, limited and often accompanied by large uncertainties in the thermodynamic properties. The present investigation is part of a larger study on thermodynamic properties of rare earth compounds. Fruehan ~ determined the Gibbs energy of formation of CezO:S by equilibrating CeO2 with CO-CO2-SO2 gas mixtures in the temperature range 1070 to 1750 K. For fixed CO/CO~ ratios, the SO2 concentration at which the sample was completely converted to Ce202S was determined. The equilibrium constant for the reaction, 2 CeOz + 4 CO + SO2--* Ce2OzS + 4 CO2
[1]
was calculated and combined with auxiliary thermodynamic data for CeO2, CO, CO2, and SO2 in the literature to obtain the standard free energy of formation of Ce202S. The value was a sensitive function of the fourth power of the CO/CO2 ratio. The value obtained was less negative by 13 kJ than the value of -1418 (• kJ mol -~ estimated by Gschneidner et al. 2 at 1100 K. Dwivedi and Kay 3 used a galvanic cell technique with calcia stabilized zirconia as the solid electrolyte to measure the standard free energies of formation of Ce202S and Y202S. The sulfur potential corresponding to the equilibrium between the rare earth oxide and oxysulfide was fixed by the Cu/Cu2S equilibrium. The experiments were performed in a limited temperature range, from 1040 to 1085 K. The Gibbs energies of formation reported by Dwivedi and Kay 3 are - 1423 (+--12.5) kJ mol -j for Ce202S at 1072 K and -1482.4 (--_12.5)kJ mol -J for YzO2S at 1068 K. The corresponding values from Gschneidner et al. 2 R. AKILA, Graduate Student, Department of Metallurgy and Materials Research Laboratory, K. T. JACOB, Professor, Department of Metallurgy, and A. K. SHUKLA, Assistant Professor, Solid State and Structural Chemistry Unit, are with Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted July 1, 1986.
METALLURGICALTRANSACTIONS B
are -1426 ( • mol -~ and -1494 ( • kJ mol -~, respectively. Other studies include the free energy determination of La202S by Kumar and Kay 4 and those of Fukatsu et al. 5 for the oxysulfides of Ce, Nd, Gd, and Y. Kumar and Kay 4 used an air-Pt reference electrode with a (CaO)ZrO2 solid electrolyte tube. A C u / C u z S mixture was employed to fix the sulfur pressure over the two phase oxide/oxysulfide electrode. The study was performed in the temperature range 1080 to 1350 K, with an accuracy of 1.6 kJ in the free energy change associated with the conversion of the oxide to the oxysulfide. Fukatsu et al. 5 used an S-O gas mixture at a total pressure of one atmosphere to fix the sulfur pressures of the oxide/oxysulfide equilibria. The equilibrium oxygen potential was measured with a (CaO)ZrO2 electrolyte having an
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