Next-generation electrocaloric and pyroelectric materials for solid-state electrothermal energy interconversion

Abstract

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...

Electrocaloric Cooling

“…small Joule heating. [2,5,6,8,26,51,53], the materials efficiency Φ mat , Eq. (22), and the figure of merit, Eq.…”

“…The latter is a vector field describing the electrical effect of free and bound charges in materials. Compared to VCR, the E plays the role of pressure and D plays the role of volume in vapour compression.More detailed descriptions can be found in a number of recent reviews of the EC effect [2,4,5] and its application in refrigerators [3,6,7], and a book on this topic [8]. …”

Amplitudon and phason modes of electrocaloric energy interconversion

“…Recently, there has been significant progress [1][2][3] in the development of polar (i.e., possessing spontaneous polarisation) dielectrics that display large EC temperature shifts ΔT under electric-field poling. These systems include a variety of ferroelectric ceramics, 4-6 polymers 7-10 and liquid crystals.…”

“…11 A few relaxor and antiferroelectric materials may exhibit negative EC effect, which is especially advantageous for solid-state refrigeration. 12 The best values of positive EC ΔT in modern nanoengineered materials range from 20 to 45 K, for electric-field sweeps ΔE ⩾ 500 kV/cm, [1][2][3][4][5][6]13,14 whereas negative EC ΔT remain below − 10 K for much smaller ΔE. [15][16][17][18] The magnitude of the EC ΔT is proportional to a logarithm of the number of possible polar states, or independent 'entropy channels' in the system.…”

“…[15][16][17][18] The magnitude of the EC ΔT is proportional to a logarithm of the number of possible polar states, or independent 'entropy channels' in the system. 3,19 Therefore, it is highly desirable to acquire precise understanding of physical phenomena underpinning the emergence of these states and develop practical methods for engineering additional entropy channels into dielectrics. Moreover, since entropic changes involving evolution of other ferroic order parameters can be cumulative with the EC effect, more advanced multicaloric materials concepts blending polar, magnetic and elastic energy-interconversion functionalities, are also being considered.…”