Electrocaloric Effect in Relaxor Ferroelectric Materials

Open Access
Author:
Li, Xinyu
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 11, 2013
Committee Members:
  • Qiming Zhang, Dissertation Advisor
  • Zhiwen Liu, Committee Member
  • Noel Christopher Giebink, Committee Member
  • Christopher Rahn, Committee Member
Keywords:
  • electrocaloric effect
  • relaxor ferroelectric
  • polymer blends
Abstract:
Electrocaloric Effect (ECE) refers to the entropy change and/or temperature change in dielectric materials due to electric field induced change of dipole states. ECE may provide an effective means of realizing cooling devices for a broad range of applications in both household appliances as well as industrial facilities. Refrigeration based on ECE has the potential of reaching higher efficiency compared with vapor-compression cycle systems. This dissertation focuses on ECE in relaxor ferroelectric polymers, which exhibit large electrocaloric response over a broad temperature range. Various relaxor ferroelectric polymers have been examined in this work, through theoretical considerations, structural analysis and electrical measurements, to explore general rules to develop material systems with giant ECE. First, the background and principle of ECE, along with prototypical materials with notable electrocaloric responses are introduced. It is shown that ECE can be generated in various materials including ceramics (bulk and thin film), polymers, polymer composites, polymer/nanoparticle composites, dielectric fluids and ionic crystals. Compared with other types of materials, poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE))-based relaxor ferroelectric polymers exhibit superior properties in many ways. ECE cooling device has been discussed to derive critical parameters and requirements for designing elctrocaloric materials. The electrocaloric performance of relaxor ferroelectric polymers can be predicted and understood using theoretical guidelines such as thermodynamic analysis, Landau-Devonshire phenomenological theory, coexisting phases rule near invariant critical points. Based on these considerations, many efforts have been made to tailor material systems in order to generate large ECE response. Specially, defect modifications, such as high energy electron irradiation and copolymerization with monomer with larger size, provide a few facile ways to increase random states and thus improve material properties. To characterize ECE, two different methods, i.e.,indirect measurement and direct measurement are utilized. It should be noted that indirect method would result in a large deviation when used to deduce ECE in relaxor ferroelectrics, due to the non-ergodic nature of relaxors. High energy electron irradiated copolymers are then discussed. The impact of high energy electron irradiation, which reduces dipole correlations and thus increases ECE response of P(VDF-TrFE) by introducing defects into the copolymers, is studied in different ferroelectric copolymers. As irradiation dose increases, P(VDF-TrFE) 65/35 mol% copolymer gradually transforms from normal ferroelectric into relaxor ferroelectric material, with dielectric constant peak and ECE response peak shifting from high temperature to room temperature. It is also demonstrated that high energy electron irradiation alleviates hysteresis loss while increases the number of disorder states, therefore, irradiated P(VDF-TrFE) 65/35 mol% copolymer with 20 Mrads shows a large entropy change as high as 130 JKg-1K-1 and large temperature change of 28 °C. While For P(VDF-TrFE) with high VDF content (>75 mol%), the high energy electron irradiation fails to complete the conversion of these copolymers form normal ferroelectrics to relaxor ferroelectrics. Similarly, for poly(vinylidene chlorotrifluoroethylene) (P(VDF-CTFE)) and poly(vinylidene hexafluoropropylene) (P(VDF-HFP)), the impact of irradiation on electrocaloric effect is also feeble. Another typical relaxor ferroelectric polymer, the poly(vinylidene fluoridetrifluoroethylene- chlorofluoroethylene) (P(VDF-TrFE-CFE)) tempolymer is elaborated in the following chapter. The influence of uniaxial stretching on electrocaloric and other properties of P(VDF-TrFE-CFE) relaxor terpolymer has been investigated to probe how material preparation process may affect its behaviors. Although dielectric constants are almost identical in the nonstretched and stretched samples, it has been found that ECE response varies upon uniaxial stretching. Data reveal that the relaxor terpolymer maintains a high ECE over a broad temperature range, which is in sharp contrast to what observed in the normal ferroelectric polymer where ECE exhibits a sharp peak at ferroelectric paraelectric phase transition temperature. Around 30 °C, both films shows an adiabatic temperature change DT of 15 °C under 150 MVm-1. Besides, the directly measured electrocaloric effect and the indirectly measured results P(VDF-TrFE-CFE) terpolymer are compared. The results show that the directly measured ΔT in the relaxor terpolymer is much larger than that indirectly deduced from Maxwell relation. The large difference between the directly measured ΔT and that deduced indicates that the Maxwell relation, which is derived for ergodic systems, is not suitable for deducing ECE in the relaxor ferroelectric polymers, which are nonergodic (polar-glass) material systems. Moveover, P(VDF-TrFE-CFE)-based composites, e.g., P(VDF-TrFE-CFE)/P(VDF-TrFE) and P(VDF-TrFE-CFE)/ZrO2, are analyzed. It is shown that the electroactive properties, especially ECE, of PVDF-based ferroelectric polymers can be tailored by blending. When P(VDF-TrFE) copolymer content is low in the terpolymer/copolymer composite (<15 wt%), interfacial coupling between the relaxor terpolymer and the nano-phase copolymer increases the crystallinity of the composite, resulting in an enhanced relaxor polarization response and a significant increase in the electrocaloric effect. At high copolymer content, the blends exhibit mixed structures of the two components and ECE gradually decreases as copolymer becomes more dominant. A larger electrocaloric effect is also observed in relaxor ferroelectric terpolymer (P(VDF-TrFE-CFE))/ZrO2 nanocomposites with modest ZrO2 content. The interface effect between the polymer matrix and nano-fillers increases the polarization response and provides additional electrocaloric entropy changes. Furthermore, the internal bias electric field generated in poled blends of P(VDF-TrFE-CFE)/P(VDF-TrFE) is favorable to obtaining higher polarization and enhanced ECE. Besides keeping the effort to develop materials with giant ECE response, future works also need to focus on increasing the thermal conductivity of electrocaloric materials and reducing the driving electric field required for generating large ECE.