UCLA

Solid state phase transformations can be induced through mechanical, electrical, and thermal loading in ferroelectric materials that are compositionally close to morphotropic phase boundaries. Large changes in strain, polarization, compliance, permittivity, and coupling properties are typically observed across the phase transformation regions and are phenomena of interest for energy harvesting and transduction applications where increased coupling behavior is desired.

This work characterized and modeled solid state phase transformations in ferroelectric materials and assessed the potential of phase transforming materials for energy harvesting applications. Two types of phase transformations were studied. The first type was ferroelectric rhombohedral to ferroelectric orthorhombic observed in lead indium niobate lead magnesium niobate lead titanate (PIN-PMN-PT) and driven by deviatoric stress, temperature, and electric field. The second type of phase transformation is ferroelectric to antiferroelectric observed in lead zirconate titanate (PZT) and driven by pressure, temperature, and electric field. Experimental characterizations of the phase transformations were conducted in both PIN-PMN-PT and PZT in order to understand the thermodynamic characteristics of the phase transformations and map out the phase stability of both materials. The ferroelectric materials were characterized under combinations of stress, electric field, and temperature.

Material models of phase transforming materials were developed using a thermodynamic based variant switching technique and thermodynamic observations of the phase transformations. These models replicate the phase transformation behavior of PIN-PMN-PT and PZT under mechanical and electrical loading conditions. The switching model worked in conjunction with linear piezoelectric equations as ferroelectric/ferroelastic constitutive equations within a finite element framework that solved the mechanical and electrical field equations. This paves the way for future modeling work of devices that incorporate phase transforming ferroelectrics.

Studies on the energy harvesting capabilities of PIN-PMN-PT were conducted to gauge its potential as an energy harvesting material. Using the phase stability data collected in the characterization studies, an ideal energy harvesting cycle was designed and explored to ascertain the maximum energy harvesting density per cycle. The energy harvesting characteristics under non-ideal sinusoidal stress and constant electric load impedance were also investigated. Energy harvesting performance due to changes in loading frequency and electrical load impedance was reported.