Next-generation electrocaloric and pyroelectric materials for solid-state electrothermal energy interconversion
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undamentals of ferroelectrics materials: Pyroelectrics and electrocalorics It 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 Theophrastus in the fourth century BC, through studies by various well-known scientists (including Sir David Brewster, Lord William Thomson Kelvin, and Pierre and Jacques Curie), to its widespread use in infrared sensing and thermal imaging.2,3 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. These 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 of dT/dt, then the short-circuit pyroelectric current ip is ip = Ap(T )
dT . dt
(1)
The converse of the PE 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 entropy that occur when a gas is compressed or in a rubber band when it is stretched. The entropy and corresponding temperature changes are due to the relative movement of the
S. Pamir Alpay, Department of Materials Science and Engineering, University of Connecticut, Storrs, CT, USA; [email protected] Joseph Mantese, United Technologies Research Center, East Hartford, CT, USA; [email protected] Susan Trolier-McKinstry, The Pennsylvania State University, University Park, PA, USA; [email protected] Qiming Zhang, The Pennsylvania State University, University Park, PA, USA; [email protected] Roger W. Whatmore, Department of Materials, Faculty of Engineering, Imperial College London, UK; [email protected] DOI: 10.1557/mrs.2014.256
© 2014 Materials Research Society
MRS BULLETIN • VOLUME 39 • DECEMBER 2014 • www.mrs.org/bulletin
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NEXT-GENERATION ELECTROCALORIC AND PYROELECTRIC MATERIALS
Figure 1. (a) Pyroelectric effect: A change in temperature results in a variation in the polarization that generates a pyroelectric cur
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