Taking the temperature of phase transitions in cool materials

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.

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.

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.

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.

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.

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.

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.

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.