Reduction kinetics and microstructures of Al³⁺-containing cobalt ferrites

Abstract

Dense, polycrystalline CoFe₂O₄, CoAl_0.02Fe_1.98O₄, and CoAl_0.1Fe_1.9O₄ were reduced in flowing H₂/0.01% H₂O at temperatures between 500 and 800° C. The reactions proceeded in a topochemical fashion. The rate of advance of the reaction interface was determined by direct measurement and by thermogravimetric analysis. An anomalous decrease in the reduction kinetics was observed for CoFe₂O₄ around 650° C and for CoAl_0.02Fe_1.98O₄ at 750° C. This reaction rate anomaly could be attributed to the appearance of a wüstite-type subscale. The effect of the substitutional Al³⁺ ions was to decrease the interfacial reaction rates. In the lower temperature range, the reaction was dominated by the interface reaction. With increasing temperatures, the importance of the gas transport resistance through the porous metal product scale increased. The microstructure of the scales was examined extensively. Pronounced grain-boundary attack was observed at lower temperatures leading to the formation of a distributed reaction interface. At higher temperatures, the reaction interface was better defined. The pore structure of the scales was examined after polishing and sputter etching. While changes in the pore morphology were observed, they were not correlated with the anomalous reaction rate effects.