Abstract
The electrocaloric effect represents an adiabatic temperature change or isothermal entropy change upon the application or removal of an external electric field. However, the effect of the repetitive bipolar/unipolar electric field on the electrocaloric effect in ferroelectrics is not well understood. In this work, the electrocaloric effect after both bipolar and unipolar electrical cycling in Pb0.92La0.08Zr0.65Ti0.35O3 relaxor thin films has been indirectly evaluated using the Maxwell relationship. It was found that at room temperature, the electrocaloric temperature change ΔT under higher electric fields decreases gradually from a positive value to a negative one with the increase in the bipolar/unipolar fatigue cycle number. Intriguingly, around the Curie temperature, the positive ΔT of the film after bipolar/unipolar cycling was found to decrease under low or intermediate electric fields, while it remains almost unchanged under high electric fields. This behavior is attributed to the pinning effect of defects caused by phase decomposition during both bipolar and unipolar fatigue. Note that the increase in polarization by temperature induced domain depinning close to room temperature after the fatigue process is manifested by using the Maxwell relation indirectly. Our work further reveals the origin of both the electrocaloric effect and ferroelectric electrical fatigue and is important for the applications of practical electrocaloric refrigerators.
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