Microstructural kinetics of phase transformations during electrochemical reduction of titanium dioxide in molten calcium chloride

An investigation is made of the microstructural kinetics of a reaction sequence that occurs during electrochemical reduction of cathodically polarised samples of porous TiO2 in molten CaCl2. Initial reactions, when the TiO2 both reduces to Ti2O3 via higher-order sub-oxides and reacts with the electrolyte to form CaTiO3 around the surfaces of the reduced oxide particles, have no obvious kinetic barriers other than transport limitations. However, further reduction of the Ti2O3 to TiO requires a reconstructive transformation that limits the rate of reduction. In the following chemical reaction of TiO with CaTiO3 to CaTi2O4, limited nucleation combines with rapid, anisotropic growth to reconstruct dramatically the microstructure into large CaTi2O4 needles. These are then reduced and decomposed to small TiO particles at a moving transformation front. An observation-based reaction mechanism is proposed to account for this. Possible atomic transport mechanisms are also discussed.