In situ phase equilibria determination of a manganese ferrite by electrical means
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Harry L. Tuller Crystal Physics and Optical Electronics Laboratory, Department ofMaterials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 3 November 1987; accepted 11 January 1988) Electrical conductivity and thermoelectric power have been measured for a series of MnZn ferrites as functions of the oxygen partial pressure under high-temperature equilibrium conditions. The isothermal variation of both properties was successfully correlated to the onset of phase transitions at characteristic POn 's. The ferrite 0.482MnO-0.518Fe2O3 was examined in some detail to locate the stability fields of the metallic alloy of iron and manganese, manganowustite, the spinel ferrite, and the hematitelike phase, and to extract the appropriate free-energy data. The results confirmed by x-ray diffraction are in satisfactory agreement with literature data.
I. INTRODUCTION Preparation of high-quality, high permeability ferrites requires control and optimization of a number of materials parameters, including bulk composition, purity, microstructure, divalent iron content or oxygen stoichiometry, and homogeneity.' Because these and other ceramics of technological interest, such as the negative temperature coefficient (NTC) thermistors2 and the new high-temperature superconductors,3 contain high concentrations of multivalent transition metals (e.g., Mn, Fe, Co, Cu) the stability limits of the desirable phases are often complex functions of temperature, Po^, and composition. In this article we present results of electrical conductivity and thermoelectric power measurements performed on a manganese ferrite at elevated temperatures and controlled atmosphere, and demonstrate the ability to map out the phase boundaries, in situ, of a relatively complex system. In an earlier report4 we presented preliminary conductivity and thermoelectric power data for a series of MnZn ferrites over an extensive range of POn 's under high-temperature equilibrium conditions. In that paper we conjectured that certain characteristic variations of the electrical properties with oxygen potential could be correlated to the onset of transitions. In this article we confirm that conjecture by x-ray diffractometry. Additional data obtained at other isotherms have enabled us further to establish more extensive phase stability diagrams. In particular, we illustrate the use of either conductivity or thermoelectric power for examining the phase equilibria in a multicomponent ferrite of nominal composition 0.482MnO-0.518 Fe 2 O 3 as an example. It may be noted that this work marks the first attempt to 552
J. Mater. Res. 3 (3), May/Jun 1988
http://journals.cambridge.org
apply thermoelectric power information to the study of phase equilibria. Phase stability of ferrites has been studied primarily by thermogravimetry.5"9 Electrical conductivity measurements have been used on a limited basis, to locate the phase boundary of spinel ferrites, e.g., by Volger10 and Tsuji et al.n In their work, however, oxygen partial pressu
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