Exaggerated Anisotropic Grain Growth in Hexagonal Barium Titanate Ceramics

BaTiO3 ceramic sintered in a reducing atmosphere in the presence of a TiO2-rich eutectic exhibits exaggerated anisotropic grain growth. The growth mechanism principally involves the dissolution of cubic matrix grains, a mass transport through the liquid phase and precipitation of a hexagonal polymorph. Under these conditions hexagonal grains grow anisotropically with the highest growth rate in the direction of the prismatic planes. The kinetics of the process can be described by the Avrami equation with an exponent of 2.5. The promoted direction of anisotropic growth is correlated with the growth of parallel (111) twins in cubic BaTiO3. The presence of Ti3+ ions triggers the formation of the Ti2O9 coordination groups which considerably lowers the formation energy of the hexagonal stacking, and hence the cubic–hexagonal transition temperature. Three-dimensional Monte Carlo simulations were performed to match the experimental microstructures and to demonstrate that the surface energy anisotropy is the important impetus for anisotropic grain growth.