Thermodynamic study of the NiO-MgO System in the temperature range 1073 to 1473 K by a galvanic cell technique
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I.
INTRODUCTION
THE thermodynamic properties of binary isostructural solid solutions are of interest, because they form a starting point for the understanding of more complex oxide systems frequently encountered in the field of ceramics as well as in process metallurgy. The Department of Theoretical Metallurgy is currently involved in a study of the kinetics of interdissolution of oxides by an electromotive force (EMF) method,tU and the system NiO-MgO has been studied by this technique. The interpretation of the EMF data to arrive at the diffusion coefficients necessitates an understanding of the activity-composition relationship in this system. As the available thermodynamic data in the literature show a wide scatter, the present work was undertaken to examine the thermodynamics of the system NiO-MgO by the solid-electrolyte galvanic cell method. A study of this system is also quite relevant in developing techniques of joining nickel metal to magnesia using a NiOMgO "composite filler having a stepwise controlled concentration gradient. "I2~ A thermodynamic study of oxide solid solutions is interesting even from a fundamental viewpoint. Oxide solid solutions of a rock salt structure involving oxides like NiO, CoO, and MgO present a simple structural arrangement and, hence, suitable for understanding the coupling between thermodynamic properties and structure. Earlier work I31 has indicated that the thermodynamic properties in such systems could be affected by factors like next-nearest neighbor interactions, cationic sizes, and crystal-field stabilization. It is imperative that accurate and reliable thermodynamic data are generated in order to establish proper correlations with structural properties. The present work aims at measuring the thermodynamic activities in this system by the solid-electrolyte galvanic cell method using ZrO2-CaO electrolytes. The application of this method to thermodynamic studies of oxide systems has been described in an earlier review. HI A. JAKOBSSON, Graduate Student, DU SICHEN, Research Associate, and S. SEETHARAMAN, Professor, are with the Department of Theoretical Metallurgy, Royal Institute of Technology, S-100 44 Stockholm, Sweden. Manuscript submitted February 22, 1993. METALLURGICAL TRANSACTIONS B
II.
PREVIOUS WORK
The system NiO-MgO shows complete miscibility both in a liquid as well as in a solid state, tSj Lattice parameter measurements by X-ray diffraction carried out by a number of previous workers t6-91 show that the system obeys Vegard's law. Extensive measurements of the thermodynamic properties of this system have been carried out by a variety of experimental techniques like gasequilibration, galvanic cell measurements involving ZrO2-CaO or CaF 2 solid electrolytes, as well as hightemperature calorimetry. A list of all the thermodynamic studies in this system along with the results obtained are summarized in Table I. III.
EXPERIMENTAL
A. Principle The galvanic cell method involving ZrO2-CaO solid electrolyte was the major experimental technique employed in the pre
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