Interfacial Coupling Boosts Giant Electrocaloric Effects in Relaxor Polymer Nanocomposites: In Situ Characterization and Phase‐Field Simulation

Abstract

efficient and environment friendly refrigeration systems. [4] Among all the EC materials, ferroelectrics are the most promising candidate owing to their large spontaneous polarization that can be aligned by the applied electric field. The large isothermal entropy change (ΔS) induced by switching the dipoles during the polarization/depolarization process gives rise to a substantial change in the isothermal heat (Q) and adiabatic temperature (ΔT). Maximal ECE could be achieved in the vicinity of the critical Curie temperature (T c ) where the ferroelectric-paraelectric (FE-PE) phase transition induces the largest polar entropy change (ΔS). [5] For instance, by using the Maxwell relation, a giant ECE of ΔT = 12 °C was observed in Pb(Zr 0.95 Ti 0.05 ) O 3 thin films at T c of 222 °C [6] ; the same temperature change of ΔT = 12 °C was also obtained at T c of 80 °C in the organic poly(vinylidene fluoride-tri-fluoroethylene) [P(VDF-TrFE)] ferroelectric films. [7] Apparently, the T c of normal ferroelectrics are much too high to make them viable candidates for room-temperature EC cooling devices. [8] To address this issue, element doping has been adopted to transform normal ferroelectrics into relaxors in order to lower their T c to room temperature as well as to expand the narrow temperature windows of their FE-PE phase transitions, hence making them operational in a broader temperature range. [9] For instance, BaTiO 3 could be transformed from normal ferroelectrics to relaxors by doping Zr ions in B-sites. [10] As an analog to ferroelectric ceramics, ferroelectric polymers P(VDF-TrFE) can also be converted into a relaxor by copolymerization with a bulky monomer such as CFE (chlorofluoromethylene) or CTFE (chlorotrifluoroethylene). [7,11] The ternary polymers exhibit phase transitions within a large temperature span near room temperature, which is beneficial to achieving the desirable temperature windows for the ECE. Relaxor ferroelectrics also feature polar nanoregionsand lower energy barriers for dipole switching, which gives rise to large maximal electric polarizations (P s ), small remnant polarization (P r ), and low coercive electric field (E c ). [7,[12][13][14] All these features indicate that a large dipolar entropy change can be induced at a lower electric field and hence is beneficial to achieving a large EC strength that is defined as ΔT/E. [8,15] A high EC strength is much desired for EC materials to lower working voltages for The electrocaloric effect (ECE) refers to reversible thermal changes of a polarizable material upon the application or removal of electric fields. Without a compressor or cooling agents, all-solid-state electrocaloric (EC) refrigeration systems are environmentally benign, highly compact, and of very high energy efficiency. Relaxor ferroelectric ceramics and polymers are promising candidates as EC materials. Here, synergistic efforts are made by composing relaxor Ba(Zr 0.21 Ti 0.79 )O 3 nanofibers with P(VDF-TrFE-CFE) to make relaxorrelaxor-type polymer nanocomposites. The ECEs of the ...

Extraordinarily Large Electrocaloric Strength of Metal‐Free Perovskites

“…Assuming a constant heat capacity of 1000 J K −1 kg −1 for MDABCO at the temperature of interest (300-500 K), the obtained EC temperature changes ΔT EC (Figure 2c) are 16 K for ΔE of 2 MV m −1 and 19 K for ΔE = 10 MV m −1 . Figure 3a,b show a comparison of isothermal entropy change ΔS EC and adiabatic temperature change ΔT EC between MDABCO and a number of existing materials [16,[18][19][20][21][22][23][24][25][26][27][28][29] known to possess giant or colossal EC effects. For example, the recently developed polymer nanocomposites [19] made up of relaxor Ba(Zr 0.21 Ti 0.79 )O 3 nanofibers and P(VDF-TrFE-CFE) matrix have Figure 1.…”

“…Figure 3a,b show a comparison of isothermal entropy change ΔS EC and adiabatic temperature change ΔT EC between MDABCO and a number of existing materials [16,[18][19][20][21][22][23][24][25][26][27][28][29] known to possess giant or colossal EC effects. For example, the recently developed polymer nanocomposites [19] made up of relaxor Ba(Zr 0.21 Ti 0.79 )O 3 nanofibers and P(VDF-TrFE-CFE) matrix have Figure 1. a) The thermodynamic free energy density difference ΔG = G(T,σ ij = 0, P i ) − G(T,σ ij = 0, P i = 0) as function of temperature calculated by assuming the ferroelectric phase tetragonal, orthorhombic, and rhombohedral symmetries, respectively and b) the spontaneous polarization versus temperature obtained from the developed thermodynamic potential.…”

“…Figure a,b show a comparison of isothermal entropy change Δ S EC and adiabatic temperature change Δ T EC between MDABCO and a number of existing materials known to possess giant or colossal EC effects. For example, the recently developed polymer nanocomposites made up of relaxor Ba(Zr 0.21 Ti 0.79 )O 3 nanofibers and P(VDF‐TrFE‐CFE) matrix have the highest Δ S EC ≈ 180 J K −1 kg −1 , but measured under a very high Δ E of 200 MV m −1 .…”

Self‐Actuating Electrocaloric Cooling Fibers

“…[ 7 ] The maximum adiabatic Δ T could potentially be much higher for P(VDF‐TrFE‐CFE) and its composites loaded with barium zirconate titanate nanofibers. [ 5,12 ]…”

Analytical description of the size effect on pyroelectric and electrocaloric properties of ferroelectric nanoparticles

“…Finite element modeling (FEM) has been performed to find the solution of a coupled equations system (1)-(4) for BaTiO 3 nanoparticles placed in a polymer matrix, since such nanocomposites have been intensively studied for energy storage, PEE, and ECE application [71,72,73]. We have chosen BaTiO 3 because it is a classical proper ferroelectric with the relatively high spontaneous polarization at room temperature, relatively low FEPT temperature and well-known material parameters.…”