Pseudo‐single crystal growth of the entropy‐stabilized compound CoTi2O5: Microstructural evolution and kinetics

Abstract

AbstractThe solid‐state transformation behavior of cobalt dititanate (CoTi2O5) has been studied at different temperatures. The starting structure consisted of a duplex (1:1 molar) mixture of CoTiO3 and TiO2 grains. A seed layer of CoTi2O5 grains was incorporated at the upper surface of the green, precursor powder samples. Cobalt dititanate is a member of the pseudobrookite family (Cmcm, orthorhombic); it is an entropy‐stabilized compound, hence is stable only at elevated temperatures (T > 1140°C). It was observed that transformation initiated adjacent to the seed layer. First, a narrow layer of CoTi2O5 forms at the diphase boundaries, creating a 3‐D network that propagates at a constant velocity through the structure. The velocity of the reaction front follows an Arrhenius dependence on temperature. Both the rates of nucleation and growth increase with increasing temperature. At 1300°C, the velocity was estimated to be in excess of 10 mm/h. The progression of the reaction front was modelled based on a discontinuous template growth mechanism, with enhanced diffusion along the boundary between the product and reactant phases. The activation energy derived for boundary diffusion was 475 ± 69 kJ/mol. Behind the reaction front, the microstructure consists of a continuous matrix of CoTi2O5, with isolated remnant grains of CoTiO3 and TiO2. Further transformation occurs more slowly by solid‐state diffusion through the CoTi2O5 phase. Studies of the crystallographic orientation of the CoTi2O5 phase showed that it was pseudo‐ single crystal, with a preferred growth direction of [100]. The system exhibits novel features, which have not been reported previously for ceramic systems.