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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. [ABSTRACT FROM AUTHOR]

The topologically protected vortex–antivortex (V–AV) domain structure in ferroelectric hexagonal manganites has been highly concerned recently, but its stability against intrinsic defects remains to be understood, given the claim that a topological structure would be robust against defects and other perturbations. In fact, it is also known that the V–AV structure is sensitive to the sample quality, and such a well-developed structure is hardly observed in thin films and defective single crystals. In this work, we investigate the influence of anti-trimer point defects on the stability of the V–AV domain structure by employing the phase-field simulation based on the Landau–Devonshire phenomenological theory. It is revealed that the characteristic V–AV structure essentially relies on the anti-trimer point defects under consideration. These defects lower the trimerization transition temperature on one hand and produce pinning effect on the vortex cores/walls on the other hand. However, the V–AV structure does remain robust if the anti-trimer magnitude of these defects is relatively weak but will be eventually destroyed if the anti-trimer magnitude is strong. [ABSTRACT FROM AUTHOR]

In this work, we have predicted and classified the temperature-misfit strain phase diagrams of (001)-oriented K1−xNaxNbO3 (KNN, 0 ≤ x ≤ 0.5) thin films using three classical machine learning algorithms: k-nearest neighbors, support vector machine, and deep neural networks, which have a very excellent prediction accuracy rate of about 99%. Furthermore, various physical properties including ferroelectric, dielectric, piezoelectric, and electrocaloric properties have been calculated and studied based on the phenomenological Landau–Devonshire theory. The calculated results show that the dielectric constant ɛ33, piezoelectric coefficient d33, and isothermal entropy change ΔS of the KNN thin films can be enhanced at the orthorhombic–rhombohedral phase boundary. This work will provide theoretical guidance for experimental studies of KNN thin films. [ABSTRACT FROM AUTHOR]

Electrically and thermally induced transitions between ferroelectric and relaxor states are of great importance for Bi1/2Na1/2TiO3 (BNT)-based materials because of their close relevance to electromechanical properties. In this study, the electric field-temperature (E-T) phase diagrams of Fe doped Bi1/2(Na0.8K0.2)1/2TiO3 (BNKT) ceramics are constructed via an experiment and theory combined approach. A novel phenomenological model based on the Landau-Devonshire theory and the Preisach model is proposed to describe the electric field induced phase transitions. Using this model, an approximate free-energy landscape is obtained by fitting the experimental double polarization-electric field loop, and then the electric field representing ferroelectric and relaxor two-phase equilibrium is calculated from the free-energy landscape for the construction of an E-T phase diagram. The constructed E-T phase diagrams meet basic thermodynamics requirements, for example, the Clausius-Clapeyron relation, and, therefore, overcome some of the shortcomings of previously reported E-T phase diagrams for BNT-based ceramics. The relationship between the E-T phase diagram and electromechanical properties is also established. From the E-T phase diagrams, it is predicted that Fe doping could lower the threshold field of triggering giant strains of BNKT ceramics at a high temperature. This prediction is successfully verified by experimental measurement of the electric field induced strain. At the optimized temperature for strain property, the threshold field of triggering giant strain is estimated to be about 2.6 kV/mm for a 3.0% Fe doped sample, significantly lower than 3.5 kV/mm for the undoped sample. This shows that the E-T phase diagram can provide valuable guidance for the improvement of electromechanical properties of BNT-based ceramics. [ABSTRACT FROM AUTHOR]