Cascaded pyroelectric conversion: Optimizing the ferroelectric phase transition and electrical losses

Abstract

The dramatic increase in the pyroelectric coefficient around the ferroelectric–paraelectric phase transition suggests high potential energy conversion efficiencies, but the reality is more complicated when thermal and electrical losses are considered. The performance of prototype mono-domain lead titanate thin films is simulated around phase transition using a phenomenological modeling approach. Thermodynamic properties are calculated using a modified Landau–Devonshire potential that is a function of temperature, applied electric field, and uniaxial tensile stress for bulk films. Significant performance enhancement near the ferroelectric–paraelectric phase transition is observed. However, increases in both the specific heat and the dielectric constant reduced the anticipated improvement. Critically, electrical losses during charging and discharging processes within the energy conversion cycle are included and calculated using the dielectric dissipation factor. Cascaded pyroelectric conversion cycles are considered where heat for each subsequent stage is provided by the previous stage, allowing for the segmentation of large temperature changes into multiple cycles. The implementation of a multi-stage or cascade approach could greatly increase the cycle efficiency over a large temperature range while utilizing lower, more realistic, electric field strengths. We show how each stage could be optimized through a secondary multi-caloric effect where the ferroelectric–paraelectric transition temperature is shifted using an applied biaxial stress. Loss tangents as low as 5% are shown to negate the benefits of cascading for high number of stages (n > 10). Using a stress-tuned optimum phase transition temperature for cascade stages provides roughly a 100% increase in thermal efficiency vs the unoptimized material for low electric field cycles.