Fundamental and Novel Applications of Electrocaloric Effect

Abstract

The discovery of materials with a giant electrocaloric (EC) coefficient in the last decade triggered the research community to explore the materials with high pyroelectric and electrocaloric coefficient that revived the research on the development of EC-based solid-state cooling devices, as alternatives of conventional vapor compression (VC)-based technology, which was abandoned few decades back due to less pyroelectric and electrocaloric coefficient of then known materials. Electrocaloric materials (ECMs) offer a great potential for solid-state cooling applications and currently the research on the subject is being carried out in two different domains: the development of new ECMs and design of EC-based cooling device. For the former, the characterization of the EC effect is the key and several ECMs have been studied and reported for last 15 years where primarily two methods are used for their characterization: direct and indirect methods. A challenge, which is associated with the characterization of the ECMs, is that huge variations in the coefficients of the electrocaloric effect (ECE) have been observed during characterization by these two methods. For the latter, one of the problems in the development of EC-based solid-state cooling device is the involvement of moving parts – either active components or cooling/ heating fluids are moved physically to transfer heat from the source (cold end) to the sink (hot end). This approach reduces the efficiency of EC-based coolers/ heat pumps and adds complexities limiting their exploitation in small scale applications. In this research, the above mentioned issues with EC cooling technologies have been addressed. Indirect method that is based on the thermodynamic relations and direct method that is the direct measurement of temperature change in dielectric materials upon application/removal of electric field should give the same results in characterization coefficients for linear dielectric materials. However, most ECMs are nonlinear in nature that causes the difference in the EC coefficient obtained using direct and indirect methods. Moreover, the mechanical condition has a strong influence on the EC coefficient, but during the measurements, either perfect constant stress or constant strain condition cannot be achieved. Additionally, adiabatic process is needed, but it is difficult to achieve, especially when dealing with the thin films where substrates act as thermal anchors, and complete adiabatic conditions are not achievable. More importantly, giant EC coefficient has been obtained in relaxor ferroelectrics, but the results from direct and indirect methods show a huge difference. By fundamental physics, the relaxor ferroelectrics should not have the EC effect. The fundamental factors behind these phenomena are discussed using phenomenological theory by considering nonlinear effects, such as the electric field and temperature dependence of the permittivity of the dielectric material. New relationships are introduced. Based on these, a new EC-like phenomenon is studied that is independent of crystal structure and becomes dominant at the higher electric fields that are involved in the EC characterization. On the device side, to address the issue of moving parts, a multilayer system of EC and non-EC bodies is devised that is capable to achieve the directional heat flow without the involvement of any moving parts in the system. In other words, all the bodies remain in thermal contact throughout the heat conduction process in the system and no moving parts are involved. Two EC layers have been sandwiched between source (SO) and sink (SI) and the heat is pumped from source to sink in a complete silent operation. A thermal cycle (precisely an electric field cycle) is applied on the EC layers alternatively in such a way that it creates a temperature gradient to achieve a directional heat flow in a system of bodies (SI/ECs/SO), that are otherwise in thermal equilibrium. Most of the ECMs being anisotropic may be simplified to one dimensional case. So, the problem of one-dimensional (1D) transient heat conduction within a multilayer system of four connecting bodies (SI/ECs/SO), in which two finite bodies that are EC, and two semi-infinite bodies that are non-EC, has been solved analytically. The temperature of EC-bodies can be instantaneously changed by external electric field to establish the initial temperature profile. Then, the temperature distribution in the bodies as a function of time/space (1D) and the heat flux through the interfaces as a function of time have been determined analytically. Each of these analytical solutions includes five infinite summation series. It is proved that each of these series is convergent, and the sum of each infinite series can be approximated and calculated using the first N terms of the series. The formula for calculating the value of N is provided. This idea of multilayer system of EC/non-EC bodies has been employed to achieve a directional heat flow from source to sink with complete silent operation, i.e., without involvement of any physical movements of the components, in an EC-based heat pump. For a sustainable operation, a specially designed thermal cycle comprising of three steps is applied on EC bodies, where in first step heat is pumped from source to sink and in the subsequent steps, EC bodies are recovered for the next cycle. In these two steps, the net heat flow between SO and SI through the interfaces cancels each other so the net heat transfer is the heat that was transferred in the first step. The analytical solution has been implemented numerically for several scenarios depending on the thermal properties of EC and non-EC materials and has been found that a huge thermal mismatch between ECMs and SI/SO (i.e. low value of contacting coefficient), offers ideal conditions for the continuous operation of heat pump. This novel approach can be a viable solution in thermal management of high power density electronics and for portable medical applications. Moreover, most of the typical ECE-based devices reported in the literature are either prototypes or numerical simulations, so the performance of the device is mostly based on the experimental results. The optimization of the devices requires that there must be some formulation so that the EC devices can be designed for high efficiency and cooling power. During absorption of heat (depolarization of ECM), the typical ECM is coupled with source on one side and there is air/ or other material on the other side. On the other hand, during polarization, the ECM is coupled with sink on one side and with air/other material on the other side to reject the heat. This scenario has been solved analytically, and the transient solution for generalized initial conditions, and for bodies with different thermal properties has been given that is much versatile and flexible to be used in the thermal analysis of most of the ECE-based devices for the determination of relaxation time, temperature profiles and heat fluxes.