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Use of metastable equilibria for determination of Gibbs energy of solids

Published online by Cambridge University Press:  31 January 2011

K. T. Jacob  and
S. Srikanth
K. T. Jacob
Affiliation:
Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India
S. Srikanth
Affiliation:
Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India
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Abstract

Attempts are made to measure activities of both components of a binary alloy (A–B) at 650 K using a solid-state galvanic cell incorporating a new composite solid electrolyte. Since the ionic conductivity of the composite solid electrolyte is three orders of magnitude higher than that of pure CaF2, the cell can be operated at lower temperatures. The alloy phase is equilibrated in separate experiments with flourides of each component and fluorine potential is measured. The mixture of the alloy (A–B) and the fluoride of the more reactive component (BF2) is stable, while (A–B) + AF2 mixture is metastable, Factors governing the possible use of metastable equilibria have been elucidated in this study. In the Co–Ni system, where the difference in Gibbs energies of formation of the fluorides is 21.4 kJ/mol, emf of the cell with metastable phases at the electrode is constant for periods ranging from 90 to 160 ks depending on alloy composition. Subsequently, the emf decreases because of the onset of the displacement reaction. In the Ni–Mn system, measurement of the activity of Ni using metastable equilibria is not fully successful at 650 K because of the large driving force for the displacement reaction (208.8 kJ/mol). Critical factors in the application of metastable equilibria are the driving force for displacement reaction and diffusion coefficients in both the alloy and fluoride solid solution.

Type
Articles
Information
Journal of Materials Research , Volume 3 , Issue 4 , August 1988 , pp. 687 - 693
Copyright
Copyright © Materials Research Society 1988

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References

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