MATERIAL SYSTEM ENGINEERING FOR ADVANCED ELECTROCALORIC COOLING TECHNOLOGY

Open Access
Author:
Qian, Xiaoshi
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 30, 2015
Committee Members:
  • Qiming Zhang, Dissertation Advisor
  • Qiming Zhang, Committee Chair
  • Iam Choon Khoo, Committee Member
  • Zhiwen Liu, Committee Member
  • Srinivas A Tadigadapa, Committee Member
  • Md Amanul Haque, Special Member
Keywords:
  • Electrocaloric effect
  • Cooling devices
  • Dielectrics
  • Ferroelectrics
Abstract:
Electrocaloric effect refers to the entropy change and/or temperature change in dielectrics caused by the electric field induced polarization change. Recent discovery of giant ECE provides an opportunity to realize highly efficient cooling devices for a broad range of applications ranging from household appliances to industrial applications, from large-scale building thermal management to micro-scale cooling devices. The advances of electrocaloric (EC) based cooling device prototypes suggest that highly efficient cooling devices with compact size are achievable, which could lead to revolution in next generation refrigeration technology. This dissertation focuses on both EC based materials and cooling devices with their recent advances that address practical issues. Based on better understandings in designing an EC device, several EC material systems are studied and improved to promote the performances of EC based cooling devices. In principle, applying an electric field to a dielectric would cause change of dipolar ordering states and thus a change of dipolar entropy. Giant ECE observed in ferroelectrics near ferroelectric-paraelectric (FE-PE) transition temperature is owing to the large dipolar orientation change, between random-oriented dipolar states in paraelectric phase and spontaneous-ordered dipolar states in ferroelectric phases, which is induced by external electric fields. Besides pursuing large ECE, studies on EC cooling devices indicated that EC materials are required to possess wide operational temperature window, in which large ECE can be maintained for efficient operations. Although giant ECE was first predicted in ferroelectric polymers, where the large effect exhibits near FE-PE phase transition, the narrow operation temperature window poses obstacles for these normal ferroelectrics to be conveniently perform in wide range of applications. In this dissertation, we demonstrated that the normal ferroelectric polymers can be converted to relaxor ferroelectric polymers which possess both giant ECE (27 Kelvin temperature drop) and much wider operating temperature window (over 50 kelvin covering RT) by proper defect modification which delicately tailors ferroelectrics in meso-, micro- and molecular scales. In addition, in order to be practical, EC device requires EC material can be driven at low electric fields upon achieve the large ECE. It is demonstrated in this dissertation that by facially modifying materials structure in meso-, micro- and molecular scale, low-field ECE can be greatly improved. Large ECE, induced by low electric fields and existing in wide temperature window, is a major improvement in EC materials for practical applications. Besides EC polymers, this thesis also investigated EC ceramics. Due to several unique opportunities offered by the EC ceramics, Ba(ZrxTi1-x)O3 (BZT), that is studied. (i) This class of EC ceramics offers a possibility to explore the invariant critical point (ICP), which maximizes the number of coexistent phase and provides a nearly vanishing energy barrier for switching among different phases. As demonstrated in this thesis, the BZT bulk ceramics at x~ 0.2 exhibits a large adiabatic temperature drop Tc=4.5 K, a large isothermal entropy change S = 8 Jkg-1K-1, a large EC coefficient (|Tc/E| = 0.52×10-6 KmV-1 and S/E=0.93×10-6 Jmkg-1K-1V-1) over a wide operating temperature range Tspan>30K. (ii) The thermal conductivity of EC ceramics is in general, much higher than that of EC polymers, and consequently they will allow EC cooling configurations which are not accessible by the EC polymers. Moreover, in the same device configuration, the high thermal conductivity of EC ceramics (> 5 W/mK, compared with EC polymer, ~ 0.25 W/mK) allows higher operation frequency and therefore a higher cooling power. (iii) Well-established fabrication processes of multilayer ceramic capacitor (MLCC) provide a foundation for the EC ceramic toward mass production. In this thesis, BZT thick film double layers have been fabricated and large ECE has been directly measured. EC induced temperature drop (T) around 6.3 °C and entropy change (S) of 11.0 Jkg−1K−1 are observed under an electric field of E=14.6 MV/m at 40 °C was observed in BZT thick film double layers. The result encourages further investigations on ECE in MLCC for practical applications. This thesis also explores ECE in dielectric fluids and asks the question of whether a high ECE is possible in dielectric fluids. Compared with solid state EC materials, dielectric fluids that possess large ECE would provide unique routes to design a cooling device. Fluids are widely used in industrial communities for passive cooling and thermal management, for which it is also known as coolants or heat exchange fluids. If there is a dielectric fluid that can provide active cooling by responding to external electric field, such fluid can thus provide functionalities of both active refrigerant and heat exchange media, opening grand opportunities to design much simpler cooling devices. In this dissertation, we demonstrated that ECE indeed exists in a class of fluid, liquid crystals, which possess large dielectric anisotropy and several first-ordered phase transitions near RT, i.e. smectic, nematic and isotropic phases. Large ECE is demonstrated in a widely studied liquid crystal, 5CB, near its nematic-isotropic (N-I) transition temperature that is near RT. An isothermal entropy change of more than 24 Jkg-1K-1 and an adiabatic temperature change of 5.2 K was observed near 39 ºC, which is slightly above the N-I transition temperature. The studies suggest that great potential of refrigeration designing lies in small molecules with functional group of large dipole moment and liquid or liquid crystal phases. Previous reported ECE generates cooling and heating cyclically in response to an electric pulse (normal or negative ECE). This thesis asked the question that whether the cooling and heating signals can be unbalanced and even generate only cooling under and electric pulse. Since ECE is dipolar entropy in responses to electric field induced polarization change, there is no fundamental reason for ECE to always generate balanced cooling and heating signals. In the last chapter, an anomalous ECE (A-ECE) that exhibit only cooling but without subsequent heating, under an small electric pulse, is demonstrated, which may provide instant cooling with high reliability for applications such as the thermal management for on-chip hot-spots. The A-ECE is observed in internal-biased normal ferroelectric/relaxor ferroelectric composites. When such composites are de-poled, the dipolar-ordering in the internal-biased composites becomes random, and stay random when electric field is removed owing to the existence of randomness induced by relaxor ferroelectric participant. The observation of A-ECE is consistent with detailed polarization study. Upon de-poling such a polymer blend, the remnant polarization reduced to zero and stayed at zero-state. Therefore, the instant cooling (S=10 J/kgK) without subsequent heating is realized. The observation paves a way to produce anomalous, large EC effect through engineering inhomogeneous dipolar interactions, and thus could lead to many device applications. These devices would absorb heat from the surrounding area without subsequent heat releasing to the surrounding area.