An optical in-situ study of a reacting spinel crystal

Results are reported of high-temperature optical investigations of heterogeneous and homogeneous reactions of cobalt aluminate spinel, CoAl₂O₄. The formation reaction of cobalt aluminate spinel from its quasi-binary components CoO and Al₂O₃ is studied using differently oriented single crystalline Al₂O₃. The experiments give access to the early stages of the reaction, where a linear rate law is observed. The orientation dependent kinetics are discussed in relation with the microscopic mechanism of the spinel formation and the role of the phase boundary control of the reaction kinetics. Jump-type relaxation experiments are used to study (i) the relaxation of the cation distribution and (ii) the kinetics of the redox behaviour of CoAl₂O₄ at high temperatures under in-situ conditions.