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 TiO 2 in molten CaCl 2. Initial reactions, when the TiO 2 both reduces to Ti 2O 3 via higher-order sub-oxides and reacts with the electrolyte to form CaTiO 3 around the surfaces of the reduced oxide particles, have no obvious kinetic barriers other than transport limitations. However, further reduction of the Ti 2O 3 to TiO requires a reconstructive transformation that limits the rate of reduction. In the following chemical reaction of TiO with CaTiO 3 to CaTi 2O 4, limited nucleation combines with rapid, anisotropic growth to reconstruct dramatically the microstructure into large CaTi 2O 4 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.