The electrocaloric effect (ECE) in dielectrics is characterized by the isothermal entropy change ΔS and adiabatic temperature change ΔT induced by changes of external electric fields. The Maxwell relation, which relates changes of polarization P with temperature T (pyroelectric coefficient) under a fixed electric field E to ΔS for finite intervals in E, provides a convenient way to deduce the ECE from polarization data P(T, E). Hence, this method, known as the indirect method, is widely used in ECE studies in ferroelectrics. Here, we first present the thermodynamic consideration for the Maxwell relation. We then use the indirect method and P(T, E) from bipolar and unipolar polarization curves to deduce the ECE in the normal ferroelectric phase of a P(VDF-TrFE) copolymer. The deduced ECE using the P(T, E) from bipolar polarization curves exhibits a giant negative ECE. In contrast, the directly measured ECE in the same polymer shows the weak and normal ECE. We discuss the constraints of the indirect method and its relation to the polarization–electric field curves measured in practical ferroelectric materials.

Polymer materials are actively used in dielectric capacitors, in particular for energy storage applications. An enhancement of the stored energy density can be achieved in composites of electroactive polymers and dielectric inorganic fillers with a high dielectric permittivity. In this article, we report on the energy storage characteristics of composites of relaxor terpolymer P(VDF-TrFE-CFE) and BaZr0.2Ti0.8O3 (BZT) nanoparticles. The choice of materials was dictated by their large dielectric permittivity in the vicinity of room temperature. Free-standing composite films, with BZT contents up to 5 vol.%, were prepared by solution casting. The dielectric properties of the composites were investigated over a wide range of frequencies and temperatures. It was shown that the addition of the BZT nanoparticles does not affect the relaxor behavior of the polymer matrix, but significantly increases the dielectric permittivity. The energy storage parameters were estimated from the analysis of the unipolar polarization hysteresis loops. The addition of the BZT filler resulted in the increasing discharge energy density. The best results were achieved for composites with 1.25–2.5 vol.% of BZT. In the range of electric fields to 150 MV/m, the obtained materials demonstrate a superior energy storage density compared to other P(VDF-TFE-CFE) based composites reported in the literature.

The temperature dependence of the dielectric permittivity and polarization hysteresis loops of P(VDF-TrFE-CFE) polymer films with different compositions are studied. Among them, the three compositions, 51.3/48.7/6.2, 59.8/40.2/7.3, and 70/30/8.1, are characterized for the first time. Relaxor behavior is confirmed for all studied samples. Increasing the CFE content results in lowering the freezing temperature and stabilizes the ergodic relaxor state. The observed double hysteresis loops are related to the field-induced transition to a ferroelectric state. The critical field corresponding to this transition varies with the composition and temperature; it becomes larger for temperatures far from the freezing temperature. The energy storage performance is evaluated from the analysis of unipolar polarization hysteresis loops. P(VDF-TrFE-CFE) 59.8/40.2/7.3 shows the largest energy density of about 5 J·cm−3 (at the field of 200 MV·m−1) and a charge–discharge efficiency of 63%, which iscomparable with the best literature data for the neat terpolymers.

A contactless technique for direct time-resolved measurements of the full dynamics of the adiabatic temperature change in electrocaloric materials is introduced. The infrared radiation emitted by the electrocaloric sample is sensitively detected with μs time resolution and mK temperature resolution. We present time-resolved measurements of the electrocaloric effect up to kHz frequencies of the driving electric field and down to small field strengths. The simultaneous recording of transients for applied electric field and induced polarization gives a comprehensive view of the correlation of electrocaloric and ferroelectric properties. The technique can further be applied to the continuous measurement of fatigue for [Formula: see text] electric field cycles.

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