Electrocaloric effects in multilayer capacitors for cooling applications
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Electrocaloric effects Electrocaloric (EC) effects1–8 are reversible thermal changes that arise in electrically polarizable materials due to changes in the electric field, ΔE. An example of how EC effects can be exploited in cooling cycles for heat pumps is shown in Figure 1.9,10 EC effects are normally parameterized as adiabatic changes in temperature, ΔT, or isothermal changes in entropy, ΔS, which can be expressed1 using equilibrium thermodynamics as: ∆T = −∫
Ef Ei
E T ∂P dE and ∆S = ∫E c ∂T E i
f
∂P dE , ∂T
(1)
E
where P is the electrical polarization, c is the specific heat capacity, Ei is the initial applied field, and Ef is the final applied field. EC effects are expected to be large near ferroelectric phase transitions,1 where the electrical polarization varies strongly with temperature. For several decades, EC effects in ferroelectric oxides and polymers have been investigated1–8 using thinned bulk samples and films of various thicknesses. The EC effects tend to be small in thinned bulk materials, as the resulting samples are too thick to support very large electric fields without breakdown.1,5 In contrast, thin films can support large electric fields and therefore show larger EC effects. EC thin films, however, can only pump a small amount of heat, and they are typically attached to substrates that constitute thermal anchors.
Multilayer capacitors (MLCs) based on EC materials do not suffer from any of the above problems, because they comprise many electrode films.11 Moreover, the metallic electrodes can be exploited to help transfer heat between the thermally insulating EC layers11 and the rest of the thermal pathway in a given cooling device. Here, we describe MLCs based on good EC materials, and discuss their use and potential in EC cooling devices.
MLCs A capacitor is an electronic device that stores electrical charge using two closely spaced metallic plates separated by an electrically polarizable medium (or free space). Over the last three decades, trends toward miniaturization, high performance, and low power consumption have dominated the ceramic capacitor industry, motivated to a large extent by the rise of mobile phones and compact computers. A MLC comprises many electrically polarizable layers that lie electrically in parallel due to interdigitated inner electrodes (Figure 2). The large resulting capacitance between the two terminals (outer electrodes) permits a large amount of charge to be stored in a small volume. MLCs based on micron-thick layers of high-permittivity ceramic oxides are mass-produced worldwide via the following fabrication process, which is used to produce billions of units
Xavier Moya, Department of Materials Science, University of Cambridge, UK; [email protected] Emmanuel Defay, Luxembourg Institute of Science and Technology, Luxembourg; [email protected] Neil D. Mathur, Department of Materials Science, University of Cambridge, UK; [email protected] Sakyo Hirose, Murata Manufacturing Co., Ltd., Japan; [email protected] doi:10.1557/mr
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