Effect of Grain Size on the Electrocaloric Properties of Polycrystalline Ferroelectrics

The physical properties, including the electrocaloric effect (ECE), of polycrystalline ferroelectrics are highly dependent on grain size in the materials. Using a phase-field model based on the time-dependent Ginzburg-Landau equation, the electrocaloric properties of ferroelectric ceramics with different grain sizes are investigated. The ferroelectric hysteresis loops under different temperatures are calculated and the adiabatic temperature changes (ATCs) are obtained through the indirect method based on the Maxwell relation. Among the investigated models with different grain sizes, the model with larger grain size possesses a larger value of ATC as well as a higher temperature at which the maximum ATC exhibits. In contrast, for the model with smaller grain size, the value of ATC declines and the temperature related to the maximum ATC shifts to a lower temperature. The significant influence of grain sizes on the ECE of polycrystalline ferroelectrics is due to the different type of domain patterns formed inside the grains. Compared to the vortex domains formed in the model with smaller grain size, the 90\ifmmode^\circ\else\textdegree\fi{} domains and single domains shown in larger grain size contribute to a more pronounced change of temperature-dependent polarization, which results in the larger ECE. In addition, the shifting of corresponding temperature to a lower one in smaller grain sizes is due to the size-effect-induced decrease of transition temperature. This work suggests a degree of freedom to understand the grain-size-dependent ECE properties of polycrystalline ferroelectrics from the perspective of domain structure and its evolution with temperature changing, which can be further studied to improve the performance of solid-state cooling devices.