On the spinel formation in Co1 − xO/Co2TiO4 composites via reactive sintering, exsolution and oxidation

The formation mechanism and microstructural development of the spinel phases in the Co1 − xO/Co2TiO4 composites upon reactive sintering the Co1 − xO and TiO2 powders (9:1 molar ratio) at 1450 °C and during subsequent cooling in air were studied by X-ray diffraction and analytical electron microscopy. The Co2TiO4 spinel occurred as inter- and intragranular particles in the matrix of Ti-doped Co1 − xO grains with a rock salt-type structure during reactive sintering. The submicron sized Co2TiO4 particles were able to detach from grain boundaries in order to reach an energetically favorable parallel orientation with respect to the host Co1 − xO grains via a Brownian-type rotation/coalescence process. Upon cooling in air, secondary Co2TiO4 nanoparticles were precipitated and the Ti-doped Co1 − xO host was partially oxidized as Co3 − δO4 spinel by rapid diffusion along the {1 1 1} and {1 0 0}-decorated interphase interface and the free surface of the composites.