Design of polar-dielectrics for electrocaloric cooling
Dr Qiming Zhang, Pennsylvania State University, USA
The direct and efficient coupling between the electric signals and the elastic, thermal, optical and magnetic signals in ferroelectrics makes them attractive for exploring a broad range of cross-coupling phenomena which have great promise for new device technologies. This talk will present the recent advances at Penn State in developing electrocaloric materials which may provide alternative cooling technology to the century old vapor compression cycle (VCC) based cooling. Electrocaloric effect (ECE), which is the temperature and/or entropy change of dielectric materials caused by the electric field induced polarization change, is attractive to realize efficient cooling devices. Recently, we demonstrated that large ECE can be achieved in several classes of ferroelectric materials with tailored nano- and meso-structures. Experimental results on the ECE in the relaxor ferroelectric polymers and general theoretical considerations for achieving large ECE will be presented. This talk will also discuss considerations on and present recent works in using nanocomposites to further enhancing the ECE beyond the pure relaxor polymers, on the giant ECE in a class of dielectric liquid, and in ferroelectric ceramics near the invariant critical point. The works related to developing the EC cooling devices, making use of the newly developed large ECE in ferroelectric materials and featuring high cooling power density and high efficiency, will also be presented.
Multicaloric propertires of P(VDF-TrFE-CTFE) terpolymer
Dr Gael Sebald, Université de Lyon, INSA-Lyon, LGEF, France
In the framework of caloric materials, electrocaloric effect on the one hand and elastocaloric effect on the other hand may be used for cooling systems. Electrocaloric effect refers to the electric field driven entropy change whereas elastocaloric effect refer to strain-driven entropy change. In some materials, such as P(VDF-TrFe-CTFE) and P(VDF-TrFe-CFE) terpolymer, both effects may exist. In this presentation, the characterization of the electrocaloric effect of terpolymer is presented. It is based on two complementary direct characterizations. In a first experiment, the isothermal heat exchange is measured upon the application of electric field steps using modified Differential Scanning Calorimetry equipment. In a second experiment, the adiabatic temperature change is measured upon fast electric field variation using a thermal imaging camera. It is shown that both characterizations are consistent. In case of elastocaloric effect, the characterization technique is first presented and the results obtained on stretched terpolymer are presented and compared to other elastocaloric materials such as natural rubber. The entropy variation in case of electrocaloric effect is believed to be related to the polar phase whereas elastocaloric effect is better related to the amorphous phase entropy. Based on thermodynamics considerations, theoretical conditions for the additivity of both effects are finally presented and discussed
Electrocaloric effect of antiferroelectric thick films
Dr Qi Zhang, Cranfield University, UK
When a sufficiently high external electric field is applied to an antiferroelectric (AFE) material, a ferroelectric state can be induced in AFE, and this phase transition is often accompanied by larger strains and polarization changes, and normally occurs at far lower temperature than its Curie temperature. Therefore, Pb-based AFE materials have attracted increasing attention for the potential applications in cooling devices through electrocaloric effect near room temperature. Thick (1-100 μm) films possess large breakdown strength that bulk materials lack and relatively large heat-sinking capacity compared with thin films.
In this work, antiferroelectric (AFE) thick films (1 μm) of (Pb(1-3x/2)Lax)(Zr1-yTiy)O3 with x = 0.08 -0.14 and their compositionally graded multilayer thick films were deposited on LaNiO3/Si(100) substrates by using a sol-gel method. A large reversible adiabatic temperature change of ΔT = 25 ºC was presented in the PLZT thick film with x=0.08 at 127 ºC at 990 kV/cm and also a large reversible adiabatic ΔT (=28 ºC) was presented in the compositionally graded thick films at room temperature at 900 kV/cm. The refrigerant capacities (ΔT x ΔS) of the compositional graded thick films and single composition PLZT thick film with x = 0.08 show comparable values with the best thin films’ values. These properties of the thick AFE films indicate that the thick films have strong potential application in cooling devices.
Electrocaloric and elastocaloric effects in soft materials
Professor Zdravko Kutnjak, Jozef Stefan Institute, Slovenia
Materials with large caloric effect have the promise of realizing solid state refrigeration which is more efficient and environmentally friendly compared to current techniques. A review of recent direct measurements of the large electrocaloric effect in liquid crystalline materials and large elastocaloric effect in liquid crystal elastomers will be given. In liquid crystalline materials and mixtures of liquid crystals with functionalized nanoparticles the electrocaloric effect exceeding 8 K was found in the vicinity of the isotropic to smectic phase transition. Direct measurements indicate that the elastocaloric response of similar magnitude can be found in main-chain liquid crystalline elastomers. Both soft materials can play significant role as active cooling elements and parts of thermal diodes or regeneration material in development of new cooling devices.
Trajectories through parameter space of electrocaloric lead scandium tantalate
Sam Crossley, Stanford University, USA
Measurements of electrocaloric (EC) effects can be challenging in various ways. I will review a variety of strategies that have been developed to meet these challenges over the past decade, and present fresh measurement protocols using lead scandium tantalate. Notably, I will show how to deduce EC temperature change from adiabatic contours on maps of total entropy. Separately, I will show how equivalent results may be obtained from electrical measurements in which thermodynamic conditions are varied via the measurement timescale