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 Co_1 − xO/Co₂TiO₄ composites upon reactive sintering the Co_1 − 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 Co_1 − 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 Co_1 − xO grains via a Brownian-type rotation/coalescence process. Upon cooling in air, secondary Co₂TiO₄ nanoparticles were precipitated and the Ti-doped Co_1 − xO host was partially oxidized as Co_3 − δO₄ spinel by rapid diffusion along the {1 1 1} and {1 0 0}-decorated interphase interface and the free surface of the composites.