Lead-free (Ba,Ca)(Zr,Ti)O 3 Based Electrocaloric Devices: Challenges and Perspectives
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Lead-free (Ba,Ca)(Zr,Ti)O3 Based Electrocaloric Devices: Challenges and Perspectives Gunnar Suchaneck and Gerald Gerlach Solid State Electronics Laboratory, TU Dresden, 01062 Dresden, Germany ABSTRACT In this work, we analyze the requirement to (Ba,Ca)(Zr,Ti)O3 thin films for applications in electrocaloric devices. We demonstrate that large temperature changes are realized mostly independent of the used material by applying sufficient electric fields. Ferroelectrics exhibiting a diffuse phase transition are beneficial for electrocaloric applications, but they change the range of operational temperatures. INTRODUCTION The electrocaloric (EC) effect was derived by Sir William Thompson for pyroelectrics from thermodynamic consideration in 1878:”If the preceding explanation of pyroelectricity be true, it must follow that a pyroelectric crystal moved about in an electric field will experience cooling effects or heating effects … in virtue of the wholly latent electric polarity of a seemingly neutral pyroelectric crystal (that is to say, a crystal at the surface of which there is an electrification neutralizing for external space the force due to its internal electric polarity), the same cooling and heating effects will be produced by moving it in an electric field, as similar motions would produce in a similar crystal which, by having been heated in hot water, dried at the high temperature, and cooled, is in a state of pyroelectric excitement” [1]. The first experimental investigation of the EC effect in Rochelle salt and BaTiO3 date back to 1930 [2] and 1952 [3], respectively. However, the EC effect values reported since that time (at maximum 2.5 K in Pb0.99Nb0.02(Zr0.75Sn0.20Ti0.05)O3 ceramics [4]) were too small for practical use. Nevertheless, a first EC refrigerator was operated at the Moscow Power Institute in the 1980s [5]. This device was based on 0.3 mm thick plates of PbSc0.5Ta0.5O3 (PST) with a gap of 0.05 mm between the plates for pumping a heat-transfer medium. Media such as helium and pentane were cycled at 0.3–0.4 Hz. The maximum EC temperature change amounted to 1.3 K (at an applied field of 2.6 V/µm) and 5 K (at 6 V/µm) for a gaseous and liquid heat-transfer medium, respectively. In the latter case, an efficiency of 45% was estimated. The interest in EC cooling was revitalized in 2006 based on a study of PbZr0.95Ti0.05O3 thin films with a thickness of 350 nm enabling the application of high electric fields [6]. An adiabatic temperature change of 12 K was deduced from the integrated pyroelectric effect for a field change from 77.6 to 29.5 V/µm near the ferroelectric Curie temperature (TC) of 222°C. Recently, an EC temperature change of 7.1 K at an applied field of 80 V/µm was reported for a stack of 63 BaTiO3 multilayer thick films (thickness about 3 µm) with inner Ni electrodes fabricated by tape casting [7]. Commercially available multilayer capacitors (MLCs) with a BaTiO3-based Y5V formulation comprising 200 ceramic layers of 6.5 µm thickness exhibit an EC temperature change of 0.5 K at 30 V/µm [8]. A
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