UC Berkeley

Advances in materials displaying strong electrothermal phenomena can enable next-generation devices that besides powering themselves utilizing ambient waste heat can also cool down electronics that, in times to come, will dissipate nearly all the input energy into heat. Pyroelectrics is one such class of materials that exhibits a complex interplay of electrical polarization and heat that has the potential for waste-heat energy conversion and solid-state cooling. While pyroelectricity and its thermodynamic converse, the electrocaloric effect have been known for decades, there remains a significant gap in the fundamental understanding of how to control and exploit this electrothermal phenomena, including significant differences between theoretical and experimental observations, under utilization of advances in the design and synthesis of complex-oxide ferroelectric thin films, and inadequate characterization techniques (particularly for thin films). The research presented here, therefore, aims to bridge this gap by employing a comprehensive approach to the design, synthesis, and characterization of complex oxide thin films and explore routes to enhance electrothermal responses. First, new insights into the nature of growth of these complex ferroelectric materials will the developed. This will include understanding the role of epitaxial strain in exerting control over the film morphology and the ferroic domain structure. Next, a direct technique for measuring both pyroelectric and electrocaloric effect in epitaxial ferroelectric thin films will be demonstrated. Combined, this will facilitate a thorough investigation of how domain structures effect the electrothermal and electromechanical susceptibility in thin films of the canonical ferroelectric PbZr$_{1-x}$Ti$_{x}$O$_{3}$. Next, the potential of thin-film ferroelectrics in high-power waste-heat harvesting will be explored in the relaxor ferroelectric 0.68Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$-0.32PbTiO$_{3}$ using solid-state thermodynamic cycles that mimic traditional gas-phase thermodynamic cycles. Using field- and temperature-dependent pyroelectric measurements, the role of polarization rotation and field-induced polarization in mediating the large pyroelectric effect will be explained. Finally, moving beyond the prototypical pervoskite ferroelectrics like PbZr$_{1-x}$Ti$_{x}$O$_{3}$ and (1-x)Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$-xPbTiO$_{3}$, electrothermal response in new lead-free and CMOS-compatible ferroelectric HfO$_{2}$ will be explored.