Thermophysical Properties of Perovskite Ferroelectrics and their Applications in Energy Harvesting

The thermophysical properties of perovskite ferroelectrics were investigated by high accuracy determinations of the heat capacity (CP), thermal expansion (ΔL/L) and thermal conductivity (k). The ferroelectric phenomena that govern thermophysical behavior near phase transitions in these materials were investigated and compared, and were analyzed using the thermodynamic Ginzburg-Landau phenomenological theory. For the normal ferroelectric BaTiO3, the temperature dependence of the coefficients of the Landau free energy were experimentally determined from heat capacity and dielectric data and these data were used to validate predictions of low crystallographic anisotropy of polarization at inter-ferroelectric transitions. For the solid solution Pb(Zr1-xTix)O3 (PZT) the tricritical points along the Curie line were experimentally determined and compared with predictions of a low order approximation of the Landau free energy taking into account the composition dependence of the expansion terms. Lastly, for the relaxor ferroelectric solid solutions Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), a method was developed to separate the reversible and root mean square (RMS) contributions to the polarization from heat capacity data. The relaxor ferroelectric materials exhibited thermophysical properties near phase transitions that differed qualitatively from their normal ferroelectric counterparts. The anomalous behavior for the relaxor materials was associated with built-in disorder arising from random orientation of the polarization. Quantitative comparisons of phase transition energetics for the different ferroelectric perovskites studied revealed very low transition enthalpies for the relaxor materials that were linked to very weak crystallographic anisotropy of polarization and an associated easy rotation of polarization at the inter-ferroelectric transitions between phases at the morphotropic boundary. The easy reorientation of polarization near inter-ferroelectric transitions observed for relaxor ferroelectrics was exploited in mechanical energy harvesting. A phase transforming piezocrystal energy harvesting device was designed, built and tested under cycling stress loading conditions as functions of electrical resistive load and drive frequency. The efficiency, power and energy density were determined and found to be six times greater than those in the linear single phase region. The performance characteristics of phase transforming and resonant mode energy harvesting devices are compared and contrasted