Taking the temperature of phase transitions in cool materials

09:05-09:30 Design of polar-dielectrics for electrocaloric cooling

Dr Qiming Zhang, Pennsylvania State University, USA

Abstract

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.

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09:45-10:15 Multicaloric propertires of P(VDF-TrFE-CTFE) terpolymer

Dr Gael Sebald, Université de Lyon, INSA-Lyon, LGEF, France

Abstract

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

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11:00-11:30 Electrocaloric effect of antiferroelectric thick films

Dr Qi Zhang, Cranfield University, UK

Abstract

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)La_x)(Zr_1-yTi_y)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.

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12:45-13:15 Electrocaloric and elastocaloric effects in soft materials

Professor Zdravko Kutnjak, Jozef Stefan Institute, Slovenia

Abstract

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.

13:30-14:00 Trajectories through parameter space of electrocaloric lead scandium tantalate

Sam Crossley, Stanford University, USA

Abstract

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

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14:45-15:30 Poster session

15:30-16:00 Energy efficient refrigeration near room temperature with transition metal based magnetic refrigerants

Dr Ekkes Brück, TU Delft, Netherlands

Abstract

With the advent of giant magnetocaloric effects (MCE) that occur in conjunction with magneto-elastic or magneto-structural phase transition of first order(FOT), room temperature applications became feasible. In this context the MnFe(P,X) system is of particular interest as it contains earth abundant ingredients that are not toxic. This material family derives from the Fe₂P compound, a prototypical example known since a long time to exhibit a sharp but weak FOT at 210 K (-63°C).

In this hexagonal system, the Fe atoms occupy two inequivalent atomic positions referred as 3f (in a tetrahedral environment of non-metallic atoms) and 3g (pyramidal). One intriguing aspect is the disappearance of the magnetic moments of iron atoms on the 3f sites when crossing T_C, whereas there is only a limited decrease on the 3g site. This observation has led to a cooperative description of the FOT linking the loss of long range magnetic order at T_C with the loss of local moments on 3f. This mechanism has recently been shown to be at the origin of the G-MCE observed in MnFe(P,Si).

The disappearance of the magnetic moments has been ascribed to a conversion from non-bonding 3f d electrons into a distribution with a pronounced hybridization with the surrounding Si/P atoms. Therefore, one can expect to adjust the properties of these compounds by substitutions on the non-metallic site. This solution has been used to optimize the properties of MnFe(P,Si) materials.

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16:15-16:45 (Magneto)caloric refrigeration: Is there light at the end of the tunnel?

Professor Vitalij Pecharsky, Ames Laboratory, USA

Abstract

Nearly twenty years old, the discovery of the giant magnetocaloric effect in Gd₅Si₂Ge₂ and other R₅T₄ compounds (R = rare earth metal and T is a Group 14 element) generated a broad interest in the magnetocaloric effect and magnetic refrigeration near room temperature in particular, and magneto-structural transitions in general.  Last decade has been also marked by rising interest in materials exhibiting strong electrocaloric, elastocaloric (a.k.a. thermoelastic), and barocaloric effects.  The rapidly increasing interest in calorics is related to the fact that residential and commercial cooling systems consume at least one out of every five kWh generated in the U.S., yet vapor-compression refrigeration asymptotically approaches its fundamental efficiency limit.  Whereas system-level studies predict lower environmental impact and higher efficiency for solid-state, caloric-based cooling compared to the vapor-compression cycle, successful market penetration of the energy-efficient caloric cooling technologies is impeded by lack of sustained, coordinated effort to provide the missing basic knowledge on both how to design the most effective solids, and how to control the processes in solids that change their state and temperature under the influence of external magnetic, electric, pressure, or stress fields.

A common feature observed in all materials that exhibit the giant magnetocaloric effect is the coupling of magnetic and elastic effects.  In addition to the interplay between magnetic and lattice entropies, both of which are intrinsic materials’ parameters that in principle can be modeled theoretically from first principles, extrinsic parameters such as microstructure and nanostructure, have been found to play a role in controlling  both the magnetostructural transition(s) and magnetocaloric effect.  Both the intrinsic and extrinsic parameters are, therefore, important in order to maximize magnetocaloric effect.  The role of different control parameters and the potential pathways towards materials exhibiting advanced magnetocaloric effect will be discussed.

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09:00-09:30 Mastering hysteresis in magnetocaloric materials

Professor Oliver Gutfleisch, TU Darmstadt, Germany

Abstract

A MCE materials parameter´s library including the (cyclic) adiabatic temperature ΔTad, entropy ΔS, heat capacity cp and thermal conductivity λ, of large number of candidate materials is presented. This data base comprises the most relevant magnetic refrigerants LaFeSi-, Heusler- and Fe2P-type compounds but also manganites, FeRh and others. Primary magnetocaloric properties are investigated, intrinsic and extrinsic contributions to hysteresis are analysed and secondary engineering properties are studied. Most importantly their cyclic behaviours, their dynamic responses to various magnetic field rates and effects of fragmentation are compared and from there develop strategies how to fabricate efficient heat exchangers suitable for application. Finally, our magnetocaloric test bench which allows the assessment of the above materials under real working conditions – the latter being quite different from the elaborated measurements protocols commonly employed – is described.

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09:45-10:15 Magnetocaloric properties of ferromagnetic shape memory materials: from bulk to nano

Dr Franca Albertini, Institute of Materials for Electronics and Magnetism (CNR), Italy

Abstract

Ferromagnetic shape memory materials, introduced in 1996 have constantly shown new emerging properties exploitable in different fields of application, including solid state refrigeration. Giant effects can be driven by external fields, i.e. magnetic field, pressure and stress and by their combined application, enabling their multifunctional exploitation.

Two main physical properties underlie this challenging and rich phenomenology: a martensitic transformation (i.e. solid state diffusionless structural transition) and magnetically ordered states.

The wide tunability of crystal structure, magnetic interaction, critical temperatures by suitable changes of composition in the most representative class of materials, the Heusler compounds, is of great interest. With respect to the bulk materials, thin films offer the possibility to be integrated in micro/nanosystems and could be at the basis of new–concepts nanoactuators, energy harvesters and solid-state microrefrigerators.

In this talk the basic and functional properties of these compounds will be presented and their tailoring potential in bulk materials and thin films to improve their exploitation in solid state refrigeration and energy-related applications discussed.

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11:00-11:30 Characterisation of the magnetocaloric transition

Professor Lesley Cohen, Imperial College London, UK

Abstract

Materials exhibiting the magnetocaloric effect (MCE), in which a temperature change occurs in response to a change of magnetic field, have gained increasing interest for solid state magnetic cooling applications. Large MCE is often associated with coupled magnetostructural or magnetovolume effects, resulting in a first-order MC transition. Much interest has focused on materials with first-order transitions as they have large magnetic entropy and adiabatic temperature changes in response to magnetic field changes achievable using permanent magnets. First-order transitions are demarcated by a nucleation and growth process and consequently a region or period of coexistence of two phases which usually results in undesirable irreversibility, and magnetic and thermal hysteresis. When used in commercial-type cooling systems, cycling frequencies of at least a few Hz are imperative, with of course the desire for repeated cycling without degradation of the solid magnetocaloric coolant; all of these complicating factors associated with first-order transitions have to be considered. The study of time dependence and rate effects of the MCE transition is relevant to the potential for use as a practical magnetic refrigerant with reasonable operation frequencies. In this talk the use of microcalorimetry to determine the first order character of the transition will be discussed, and the use of Hall probe imaging and magnetometry to study the influence of geometry and thermal linkage on the evolution of the field driven magnetocaloric transition.

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11:45-12:15 Thermodynamics of multicaloric effects in multiferroic materials

Professor Antoni Planes, Universitat de Barcelona, Spain

Abstract

Ferroic and multiferoic materials thermally respond to externally driven changes of ferroic properties. Usually these changes are induced either by application or removal of the field thermodynamically conjugated to a given property. The corresponding isothermal change of entropy and adiabatic change of temperature are commonly used to quantify the caloric response of the studied material. From this perspective we provide a general thermodynamic framework to deal with multicaloric effects in multiferroic materials. This is applied to the study of both magnetostructural and magnetoelectric multiferroics. Landau models with appropriate interplay between the corresponding ferroic properties (order parameters) are proposed for metamagnetic shape-memory and ferrotoroidic materials, which respectively belong to each class of multiferroics. The multicaloric effect is obtained as a function of the two relevant applied fields in each class of multiferroics. It is further shown that multicaloric effects comprise the contributions from caloric effects associated with each ferroic property and the cross-contribution arising from the interplay between these ferroic properties. The obtained results will be compared with available experimental data.

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13:30-14:00 Thermoelastic cooling: mechanism, materials and systems

Professor Ichiro Takeuchi, University of Maryland, USA

Abstract

We are actively developing prototypes of thermoelastic (elastocaloric) cooling systems using mechanisms based on compression of shape memory alloys. In the latest design, we are compressing bundles of NiTi tubes while heat exchange fluid flows through the tubes. A key component is a heat recovery system implemented in order to maximize the efficiency of the heat exchange. We have observed cooling T of 4.5 K at 70 W, as directly measured in cooled water. We have demonstrated that when properly loaded, shape memory alloy tubes can survive up to at least ~ 0.3 million cycles without any degradation in cooling capacity. We will also discuss our efforts in developing alternative shape memory alloys.

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14:15-14:45 TiNi-based thin films for elastocaloric applications

Professor Eckhard Quandt, Kiel University, Germany

Abstract

Caloric materials have the potential to serve as an environmentally friendly and more efficient alternative substitute in conventional vapor compression cooling. The principle of ferroic cooling is based on a solid state phase transformation initiated by an external field, in the case of elastocalorics by an external stress field. Combined with thin film processes this technology enables the development of small scale cooling devices required for mobile applications. Up to now, the major obstacle for the implementation of elastocaloric materials in cooling devices are the functional degradation and structural failure of the material. To investigate the underlying microstructural mechanisms TEM and synchrotron analyses of NiTiCu- based thin films are conducted in the pristine state and after superelastic cycling. A strong difference of superelastic degradation for Ti-rich compositions compared to near equiatomic compositions is found. While near equiatomic compositions already degrade severely during the first cycles, Ti-rich compositions are functionally stable for 107 full superelastic cycles (1). Using stress dependent in situ synchrotron investigations the change of lattice constants of B2 phase and stress induced B19 phase during the superelastic transformation can be quantified. This measurement enables the compatibility calculation of austenite and martensite phases which is known to have a strong influence on the superelastic hysteresis and the thermally induced transformation stability. The microstructural influences of grain size, precipitates and crystallographic compatibility on the functional degradation of NiTiCu-based thin films will be discussed.

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15:30-16:00 Large elastic strain and elastocaloric effect in an Fe-Pd alloy exhibiting a weak first-order martensitic transformation

Professor Tomoyuki Kakeshita, Osaka University, Japan

Abstract

A disordered Fe-31.2Pd (at%) alloy exhibits a weak first-order martensitic transformation from a cubic structure to a tetragonal structure near 230 K. This transformation is associated with significant softening of an elastic constant (C’), and the softening is probably due to band Jahn-Teller effect of the alloy. In this study, we have examined mechanical properties and phase diagram under compressive stress in the [001] direction by using a single crystal. The alloy shows an elastic-like behavior when the stress is applied above the transformation temperature. That is, the hysteresis between stress applying and removing processes is very small, and the residual strain is negligibly small. The stress-induced strain depends strongly on test temperature, and a large elastic-like strain of more than 6% appears when the stress is applied in the vicinity of the transformation temperature. Above the yield point, mechanical twinning with {111} type twinning plane is introduced in the specimen. The phase boundary between the parent and the martensite phases in the stress-temperature phase diagram seems to have a critical point (40MPa, 280K), above which the first order nature of the transformation completely disappears. In addition, because of significant temperature dependence of elastic-like strain, the alloy shows a high elastocaloric effect in a wide temperature range (175K<T<335K) for both the parent and the martensite phases. The refrigeration capacity calculated in this temperature range is 5MJ/m³.

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16:15-17:00 Mechanocaloric effects in ferroic materials

Professor Lluis Mañosa, Universitat de Barcelona, Spain

Abstract

Mechanocaloric effects refer to  isothermal entropy and adiabatic temperature changes of a body when subjected to an external stress. Depending on wether the stress is uniaxial or hydrostatic the effect is known as elastocaloric or barocaloric. Commonly these changes are tiny,  but they can become very pronounced when a solid is in the vicinity of a ferroic phase transition which involves a structural change. Actually, a variety of materials undergoing purely structural and coupled (magneto-structural and electro-structural) transitions have already been reported to exhibit giant elastocaloric and barocaloric effects.  

The talk will cover  some general aspects  on the thermodynamics of mechanocaloric effects and how thermodynamic quantities can be extracted from different experimetal protocols. A critical comparative analysis of entropy and temperature changes reported for diverse giant magnetocaloric materials will also be presented.  

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