Effects of Composition and Pre-precipitation on the Decomposition Kinetics and Mechanism of Spinel Solid Solutions in the System CuFeO₂-Fe₂O₃

Abstract

Kinetics and mechanism of the eutectoid decomposition of spinel phase CuFe₅O₈ into CuFeO₂ and Fe₂O₃ have been studied with special emphasis on the effects of Fe₂O₃-dissolution into and Fe₂O₃-precipitation from the spinet phase. Techniques employed are X-ray diffraction and optical microscopic analyses and Curie temperature measurement.
It is shown that the excess Fe₂O₃ over the stoichiometric composition checks the decomposition with different mechanisms depending on the temperature. Thus, Fe₂O₃-rich spinel separates CuFeO₂+2Fe₂O₃ at 850°C, whereas at 500°C the excess Fe₂O₃ separates simultaneously with CuFeO₂+2Fe₂O₃. Pre-precipitation of Fe₂O₃ from hypereutectoid spinet accelerated the rate of decomposition with a marked effect at 850°C. Kinetic analysis of the decomposition isotherms proposed that the effect of pre-precipitation should be attributed to the increased number of active sites for nucleation. The microscopic observations of the sintered bodies are in favor of this view.