Enhancing Electrocaloric Effects in “Freestanding” Epitaxial Thin Films of Ferroelectric Relaxors

Ferroelectric materials are attractive for electrocaloric studies due to the pronounced evolution of the polarization and lattice with applied electric fields. Relaxor ferroelectrics are particularly noteworthy for their ability to exhibit significant polarization changes over a broad temperature range, making them potentially ideal candidates for electrocaloric applications driven across wide temperature windows. For example, (1-x)PbMg_1/3Nb_2/3O₃-(x)PbTiO₃ (PMN-xPT) is already known for its large pyroelectric coefficient which, in turn, hints at its potential as a promising material for electrocaloric effects. In general, however, the complex chemistry of relaxors such as PMN-xPT means that making high-quality thin-film versions of these materials can be a challenge. As a result, there have been inconsistent reports regarding the performance of PMN-xPT films, largely due to difficulties in growing low-defect, uniform films. To gain deeper insight into the real properties, their physical origins, the mechanisms behind any enhanced electrocaloric effects, and, eventually, to achieve a pathway to maximal electrocaloric responses, it is crucial to synthesize high-quality, uniform versions of these materials and fabricate them into the geometries required to produce the biggest effects.
Despite extensive efforts to produce high-quality PMN-xPT thin films, many studies have reported the formation of defect phases (e.g., pyrochlores), due to the material’s complex chemistry. Here, epitaxial PMN-0.32PT films have been successfully synthesized without detectable defect phases, achieving full-width half-maximum (FWHM) values of <0.03 in X-ray rocking curve studies, and confirming their excellent crystalline quality. Although epitaxial thin films offer a means to study high-quality materials, the inherent constraints imposed by the substrate can limit the magnitude of the electrocaloric response. To address this, the current work also explores the use of approaches to produce “freestanding” films, which have been released from the substrate by introduction of a selectively etchable LaMnO₃ buffer layer between the film and the substrate. These freestanding films retained their structural quality. In turn, both substrate-supported and freestanding films were subsequently fabricated into circular capacitors and microfabricated electrothermal test structures. Using these devices, the dielectric and piezoelectric responses were measured, both of which were enhanced in the freestanding films. The dielectric permittivity (at 10 kHz) for the substrate-supported films was ~2600 and that of the freestanding films was ~3600 and the electromechanical strain (at 800 kV cm^-1) was 0.37% for the substrate-supported films and that of the freestanding films was 1.07%. The polarization was measured as a function of electric field and temperature (from 300 to 460 K), and the electrocaloric effect was indirectly calculated from these measurements using Maxwell relations and, in turn, these data are compared to direct measurements of the electrocaloric temperature change. Notably, the freestanding films exhibit a temperature change of ~6 K – more than twice that observed in the substrate-supported films (~2.6 K). The significant enhancement can be attributed the greater freedom of dipole moment rotation in the freestanding films, which facilitates lager entropic changes and, by extension, temperature evolution. Additional details of the mechanism underlying these effects will be discussed. All told, our ability to produce high-quality, freestanding PMN-xPT films opens new avenues for exploring the fundamental physics of electrocaloric effects in this material system. Furthermore, it offers the potential to integrate these freestanding films with a variety of substrates, including flexible or thermally insulating supports, to further optimize and exploit the electrocaloric properties of PMN-xPT for advanced thermal management and energy-conversion applications.