Thermal Expansion and Pyroelectricity in Lead Titanate Zirconate and Barium Titanate

Abstract

Curves of thermal expansion parallel and perpendicular to the polar axis are shown for poled barium titanate and lead titanate zirconate ceramics. Anistropy is remarkably higher in the latter due to greater alignment by 90° switching during poling. The anisotropy disappears after heating through the Curie point, and is also virtually eliminated within a given temperature range below the Curie point after exposure to a temperature only slightly above this range. The contraction in the polar direction upon first heating is due largely to a decrease in the alignment of domains originally reoriented by 90° during poling. The reversible expansion on repeated heating is due to a combination of domain anisotropy and a reversible increase in nonpolar alignment during heating. The pyroelectric effect is also greatly reduced in subsequent heating, although piezoelectric constants are reduced only slightly. The pyroelectric effect is found to be largely primary in both barium titanate and lead titanate zirconate ceramics. The increase in nonpolar 90° domain alignment on second heating of the lead titanate zirconate causes the primary and secondary pyroelectricity to be of opposite sign near room temperature.