Pyroelectric Energy Conversion with Relaxor Ferroelectric Thin Films

The need for efficient energy utilization is driving research into ways to harvest ubiquitous waste heat. Here, we explore pyroelectric energy conversion (PEC) from low-grade thermal sources that exploits strong field- and temperature-induced polarization susceptibilities in the relaxor ferroelectric 0.68Pb(Mg_1/3Nb_2/3)O₃–0.32PbTiO₃ (PMN-PT). Relaxors have attracted considerable attention due to their intriguing dielectric and piezoelectric properties including high reversible strains and high-temperature operation, but there has been only minimal study of their electrothermal effects. Here, we develop a comprehensive picture of the relationship between epitaxial strain, structure, properties, and local polar order in relaxor thin films. We will discuss how high-quality, coherently-strained films of PMN-PT provide new understanding of structure and properties¹ which opens doors for new applications. In particular, we explore how the electric-field-driven enhancement of the pyroelectric response (−550 μC m⁻² K⁻¹) and suppression of the dielectric response (by 72%) yield substantial figures of merit for PEC. Field- and temperature-dependent pyroelectric measurements highlight the role of polarization rotation and field-induced polarization in mediating these effects. Solid-state, thin-film devices that convert low-grade heat into electrical energy are demonstrated using pyroelectric Ericsson cycles, and optimized to yield maximum energy density, power density, and efficiency of 1.06 J cm⁻³, 526 W cm⁻³ and 19% of Carnot, respectively; the highest values reported to date and equivalent to a thermoelectric with an effective ZT ≈ 1.16 for a temperature change of 10 K.² We will also explore the potential for electrocaloric effects and routes to enhance the energy conversion potential of materials.
1. J. Kim, et al. under review (2018).
2. S. Pandya, et al. Nature Mater. 17, 432 (2018).