Thin-film electrocaloric and pyroelectric electrothermal interconversion energy sources have recently emerged as viable means for primary and auxiliary solid-state cooling and power generation. Two significant advances have facilitated this development: (1) the formation of high-quality polymeric and ceramic thin films with figures of merit that project system-level performance as a large percentage of Carnot efficiency and (2) the ability of these newer materials to support larger electric fields, thereby permitting operation at higher voltages.This makes the power electronic architectures more favorable for thermal to electric interconversion. Current research targets to adequately address commercial device needs include reduction of parasitic losses, increases in mechanical robustness, and the ability to form nearly free-standing elements with thicknesses in the range of 1-10 m. This article describes the current state-ofthe-art materials, thermodynamic cycles, and device losses and points toward potential lines of research that would lead to substantially better figures of merit for electrothermal interconversion.Keywords: Material type-polymer (143), Material type-ceramic (121), Functionality-energy generation (189), , Material form-film (231), Transport-ferroelectricity (288)
Fundamentals of ferroelectrics materials: Pyroelectrics and ElectrocaloricsIt has long been known that, when heated, materials such as the borosilicate tourmaline have the ability to attract objects such as pieces of feather, pollen, and cloth. This is due to the appearance of a surface charge in response to a temperature change. The history of this phenomenon, which is known as the pyroelectric effect (PE), was charted by Lang, 1 from its first description by Dielectrics whose structures possess both a unique axis of symmetry and lack a center of symmetry (i.e., that are "polar") display a spontaneous polarization (PS) and will exhibit a PE due to temperature-induced changes in PS.
MRS BulletinThese variations in PS result in uncompensated charge appearing along surfaces that have a component normal to the polar axis, generating a net voltage across the dielectric. If the surfaces are provided with electrodes that are connected through an external circuit, this surface charge can cause a current to flow, potentially resulting in useful work. In the absence of an applied electric field or applied stress, the pyroelectric coefficient p(T) is defined as the rate of change of spontaneous polarization with temperature such that p(T) = dPS/dT. If electrodes are applied to the major faces perpendicular to the polar axis, as illustrated in Figure 1a, and the temperature is changed at a rate dT/dt, then the short-circuitThe converse of the pyroelectric effect is called the electrocaloric effect (ECE; see Figure 1b). Here, an electric field applied to a polar dielectric causes a change in temperature in the material. Conceptually, the ECE is somewhat harder to grasp than the PE, but it is analogous to the changes in temperature and en...