Abstract
Electrocaloric (EC) materials are prominent candidates for new generations of scalable and green refrigeration devices. While most often the research on EC materials has been focused on achieving high magnitudes of the EC temperature change, little is known about electrical losses and self-heating effects, despite playing a critical role in the cooling performance of these materials. Here, we analyzed the behavior of a set of ceramic materials under EC-device-like electric-field-driving conditions. The EC temperature response was studied focusing on the contributions to the self-heating in three different compositions: relaxor Pb(Mg1/3Nb2/3)O3 and two different (undoped and Nb-doped) rhombohedral ferroelectric Pb(Zr,Ti)O3 compositions. The specific relaxor and ferroelectric nature of the analyzed materials enabled us to separate the different contributions, such as domain switching and electrical conductivity, to their EC responses. We show that besides having a large EC temperature change, low electrical losses, leading to reduced self-heating effects, are another key parameter to be considered in the engineering of materials for future EC cooling devices.
Highlights
The electrocaloric (EC) effect is the electric-field induced temperature change in dielectric materials.1,2 Until now, the research on EC materials was focused on finding a high EC effect under static conditions where a single EC field cycle is typically applied to the sample.3,4 The search has saturated, and the community is transcending to a second phase of the development focused on the implementation of promising materials into cooling prototypes and devices.5–8 Before a breakthrough in EC cooling, the behavior of EC materials under dynamic electric-field cycling conditions should be addressed
While the EC responses of the three samples were fully characterized in the temperature range between 28 and 65 ○C, in Fig. 1, the EC effect of PMN and PZT75/25Nb is shown for room temperature, while the response for PZT75/25 is shown for 65 ○C to illustrate the effect of the increased electrical conductivity
In contrast to PMN, the polarization and depolarization temperature peaks of PZT75/25-Nb [Fig. 1(b)] are not of the same magnitude, i.e., the adiabatic polarization temperature change is 99 mK, while that of the adiabatic scitation.org/journal/apm depolarization is 89 mK, 11% lower. We note that such asymmetry is in principle expected to lead to self-heating effects during continuous field cycling as the sample effectively heats more than it is cooled during each EC cycle
Summary
The electrocaloric (EC) effect is the electric-field induced temperature change in dielectric materials. Until now, the research on EC materials was focused on finding a high EC effect under static conditions where a single EC field cycle is typically applied to the sample. The search has saturated, and the community is transcending to a second phase of the development focused on the implementation of promising materials into cooling prototypes and devices. Before a breakthrough in EC cooling, the behavior of EC materials under dynamic electric-field cycling conditions should be addressed. Pb(ZrxTi1-x)O3 (PZT) and Pb(Mg1/3Nb2/3)O3 (PMN) are the most widely used and studied ferroelectric and relaxor materials, respectively.9–11 These two materials are natural choices for the purpose of self-heating investigations during EC cycling, considering their different nature, which in principle allows us to address the various mechanisms, such as domain switching and electrical conductivity, on the electric-field-induced EC temperature response.. Considering that in PZT75/25-Nb, the hysteresis is largely controlled by domain switching, while in undoped PZT75/25, the hysteresis may be influenced by the electrical conductivity, at low driving frequencies, it is expected that these two mechanisms will be reflected in the self-heating effects under unipolar EC-like field driving conditions This off-MPB composition was selected in order to prevent possible complications in the interpretation of the experimental data inherent to morphotropic PZT.. The results of this work show that the electrical losses are the key figure of merit in the selection of EC materials and should be more seriously considered in future studies
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