Giant Electrocaloric Effect in High-Energy Electron Irradiated P(VDF-TrFE) Copolymers
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Giant Electrocaloric Effect in High-Energy Electron Irradiated P(VDF-TrFE) Copolymers S. G. Lu1, X. Y. Li1,2, J. P. Cheng,1 L. Gorny1 and Q. M. Zhang1,2 1
Materials Research Institute, 2Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802, USA
ABSTRACT A direct calorimetry method was developed and used to measure the electrocaloric effect (ECE). A temperature change T of over 20 C and an entropy change S of over 95 J/(kgK) were procured at 33 C and 160 MV/m in the high-energy electron irradiated poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) 68/32 mol% copolymers, which were larger than those of terpolymer blends (T = 9 C, S=46 J/(kgK) at 180 MV/m and room temperature) and our earlier report on P(VDF-TrFE) 55/45 mol% normal ferroelectric copolymer (12 C and 55 J/(kgK) at 80 C). We observed that the value ((8.70.6)107 JmC-2 K-1) in the equation of S=1/2D2 derived from S - D2 relation for irradiated copolymers was larger than that of the terpolymer blends ((5.40.5)107 JmC-2K-1). It was also found that the irradiated copolymer showed a sharp depolarization peak at Td < Tm (maximum permittivity temperature), which is frequency independent, in the dielectric constant - temperature characteristics, a larger depolarization value at Td in the thermally stimulated depolarization current (TSDC) temperature relationship, and a larger volume strain/longitudinal strain ratio over terpolymer blends. The giant ECE in irradiated copolymer is regarded as due to the greater randomness present in the relaxor state. In irradiated copolymers, the long all-trans chains are broken by the high-energy electrons, which make the small sized all-trans sequences more easily reorient along the electric field, more remarkably affecting the permittivity, TSDC, and volume strain.
INTRODUCTION The electrocaloric effect (ECE) has been revived due to its potential applications as on-chip cooling devices and green refrigerators for daily life [1-5]. The ECE materials used in the early studies were mainly ferroelectric ceramics, crystals, and glass-ceramics [6-10]. One obvious pitfall of ferroelectric bulk materials is their low breakdown electric field, which leads to small ECEs observed, e.g. the largest temperature change observed is 2.5 C in (PbLa)(ZrSnTi)O3 ceramics [10]. With the advancement in thin film technologies, ferroelectric ceramic thin films have been paid much attention in order to exploit large ECE [1,4,11]. One typical result was reported in 2006 [1]. The temperature change was deduced to reach 12 C at 48 MV/m in Pb(Zr0.95Ti0.05)O3 thin film using a Maxwell relation. The shortcomings of this ECE material, however, are the smaller entropy change ~ 8 J/(kgK), higher phase transition temperature, ~ 222 C, and the presence of lead, which is not benign to the environment. In addition, in accordance with the phenomenological theory, the entropy change S=1/2D2, where (T-T0) is the phenomenological coefficient of D2 term in the phenomenological expansion,
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