Texture and grain alignment in piezoelectric bismuth titanate ceramics

The factors controlling the development of an aligned microstructure in bismuth titanate (Bi4Ti3O12) ceramics were investigated. Bismuth titanate is a candidate for use in high-temperature piezoelectric devices as a result of its high Curie temperature (675 0 C). Grain oriented bismuth titanate was fabricated by using hot forging technique, which makes use of the deformation and sliding of grains at high temperatures and by seeding with aligned, plate-like grains, which preferentially grow to yield a highly textured ceramic. Although hot forging and tape casting are widely used for texturing of oxide ceramics the role of various parameters such as initial density, applied pressure and temperature during hot forging and the size and concentration of the plate-like particles for the seeding technique in achieving highly oriented microstructures have received little attention. One aim of this research was to understand the role played by these parameters in achieving the desired microstructure. The addition of seeds had a significant improvement in terms of final microstructure in comparison to unseeded Bi4Ti3O12. Slight compositional variations in Bi4Ti3O12 had a dramatic effect on microstructure evolution. The effect of slight excess of Bi2O3 and TiO2 on the microstructure development was examined. The textured microstructures were characterized by using SEM and XRD. To show the considerable anisotropy exhibited by the textured ceramic, dielectric measurements were performed. The hot forged Bi4Ti3O12 had dielectric constants almost similar to the single crystal values indicating almost complete grain orientation. Bi4Ti3O12 fabricated using the seeding technique had a low degree of texture as compared to the hot forged ceramic, but the anisotropy in dielectric properties was significant as compared to randomly oriented samples. Undoped Bi4Ti3O12 is extremely lossy especially in the textured direction, which makes poling of this material difficult --Abstract, page iii