Caloric Materials Research Articles

Kinetic origin of hysteresis and the strongly enhanced reversible barocaloric effect by regulating the atomic coordination environment

Hysteresis is an inherent property of first-order transition materials that poses challenges for solid-state refrigeration applications. Extensive research has been conducted, but the intrinsic origins of hysteresis remain poorly understood. Here, we report a study of the kinetic origin of hysteresis and the enhanced barocaloric effect (BCE) in MnCoGe-based alloys with ~2% nonmagnetic In atoms. First-principles calculations demonstrate that substituting In atoms at Ge sites rather than Co sites results in a lower energy barrier, indicating a narrower hysteresis for the former. Combining neutron powder diffraction (NPD) with magnetic and calorimetric measurements completely verified the theoretical prediction. Electron local function (ELF) calculations further reveal the atomic coordination origin of regulated hysteresis due to weaker Co–Ge bonds when In atoms replace Ge, which is opposite to Co sites. Moreover, we experimentally investigate the BCE and find that although MnCo(Ge0.98In0.02) has a lower barocaloric entropy change ΔSP than does Mn(Co0.98In0.02)Ge, the reversible ΔSrev of the former is advantageous owing to a smaller hysteresis. The maximum ΔSrev of MnCo(Ge0.98In0.02) is 1.7 times greater than that of Mn(Co0.98In0.02)Ge. These results reveal the atomic-scale mechanism regulating hysteresis and provide insights into tailoring the functional properties of novel caloric refrigeration materials.

Contactless thermal diffusivity characterization of second order magnetocaloric materials under magnetic field using modulated photo-thermal radiometry method with very low power probe beam

The thermal diffusivity of magnetocaloric materials has a transition point at a given temperature that depends on the intensity of the applied magnetic field. Consequently, a fine temperature resolution on the material sample is needed to obtain an accurate determination of the thermal diffusivity variation with temperature. The coupling between the external and the internal fields has to be carefully mastered since the pertinent operating condition is fixed by the actual internal field which is not directly measurable and may be heavily affected by any element in contact with the sample. Therefore, contactless methods such as Photo-Thermal Radiometry (PTR) are privileged. The latter is based on a radiative excitation of the front face of a thin sample and the detection of the thermal effect on the opposite face. However, the powerful radiative source may significantly increase the sample temperature which is not suitable for caloric materials. In this work, a low power modulated PTR method is proposed to characterize second order magnetocaloric materials under magnetic field. It was compared to high energy thermo-flash PTR and validated on common materials such as steel and stainless steel, and then applied to gadolinium which is the reference magnetocaloric material for magnetic refrigeration and heat pumping study. The thermal diffusivity of gadolinium samples is measured in the 285.1 K to 305.1 K temperature range, including the magnetic transition temperature without and under an external magnetic flux density of 0.5 T in the 13 mm air gap of the permanent magnet magnetic circuit. The low power probe beam ensures a temperature stability with a negligible sample temperature fluctuation less than 0.05 K on the incident sample surface and less than 0.03 K on the measurement surface. The experimental results without magnetic field align with those using other methods including the magnetic transition temperatures determination. This low-power optical method proved its efficiency to characterize highly temperature dependent materials such as magnetocaloric materials sensitive to magnetic field. The data obtained partly fills the lack of information in the literature on excited gadolinium.

General approach for efficient prediction of refrigeration performance in caloric materials

General approach for efficient prediction of refrigeration performance in caloric materials

Introduction of novel method of cyclic self-heating for the experimental quantification of the efficiency of caloric materials shown for LaFe11,4Mn0,35Si1,26Hx

Hysteresis and the associated production of dissipative heat during first order phase transitions are often major contributors to thermodynamic losses in caloric heat pumps. The figure of merit (FOM), defined as the ratio of adiabatic temperature change and the thermal hysteresis of the caloric material, quantifies these losses, and can also be used to calculate the maximum potential efficiency of a caloric material in a thermodynamic cycle. This paper presents a novel and simple method to determine the heat loss and thus the FOM can be determined from self-heating of the caloric material during repeated field cycling. As this method mainly requires temperature readings and the ability to cycle the caloric material in a field, most test setups that directly measure the adiabatic temperature change should already be able to perform dissipative heat measurements with this technique. With the presented method, we were able to determine the efficiency of a commercial LaFeSiMnH-sample with a high degree of accuracy. A maximum FOM of 37±1 was determined for the selected LaFeSiMnH-sample. In an ideal cascaded magneto caloric system, this corresponds to a system efficiency of 90%, with an ideal heat regeneration this could theoretically even be increased to 97%.

Numerical and thermal analysis of a caloric refrigeration device operating near room temperature

The application of external stimuli such as the magnetic and electric field in magnetocaloric and electrocaloric materials, and stress and pressure in elastocaloric and barocaloric materials give rise to a new generation of a refrigeration technology based on caloric materials which are considered an emerging alternative to classical refrigeration. Active caloric regenerator (ACR) made in parallel plates is studied under a large number of materials with Comsol multiphysics for a 2D numerical model. In this work, we compare various types of caloric materials, in terms of their thermodynamic properties, working mechanisms, and potential applications as solid refrigerant on caloric refrigeration devices. For this purpose, the energy equation, Navier-Stocks equation, and continuity equation are considered to study the heat transfer phenomena in refrigerator. The water was used as a carrier fluid to transport the thermal energy from the solid refrigerants to heat exchanger. This study is performed at velocity 0.06 m/s and the frequency 2 Hz at room temperature. Among them, Gadolinium show the best results in term temperature span, coefficient of performance, and the cooling power, higher than every other caloric materials, conferring to magnetocaloric cooling globally the most promising system. Our analysis provides insights into the selection and optimization of caloric materials for caloric refrigeration, which can contribute to the development of sustainable energy systems.

Caloric effects in liquid crystal-based soft materials

With the increased environmental awareness, the search for environmentally friendlier heat-management techniques has been the topic of many scientific studies. The caloric materials with large caloric effects, such as the electrocaloric (EC) and elastocaloric (eC) effects, have increased interest due to their potential to realize new solid-state refrigeration devices. Recently, caloric properties of soft materials, such as liquid crystals (LCs) and LC elastomers (LCEs), are getting more in the focus of caloric materials investigations, stimulated by large caloric effects observed in these materials. Here, an overview of recent direct measurements of large caloric effects in smectic LC 14CB and main-chain LCEs is given. Specifically, high-resolution thermometric measurements revealed a large EC response in 14CB LC exceeding 8 K. Such a large effect was obtained at a relatively moderate electric field of 30 kV cm−1 compared to solid EC materials. We demonstrate that such a small field can induce the isotropic to smectic A phase transition in 14CB, releasing or absorbing relatively large latent heat that enhances the EC response. Furthermore, it is demonstrated that in main-chain LCEs, the character of the nematic to isotropic transition can be tuned from the supercritical towards the first-order regime by decreasing the crosslinkers’ density. Such tuning results in a sharper phase transition and latent heat that enhance the eC response, exceeding 2 K and with the eC responsivity of 24 K MPa−1, about three orders of magnitude larger than the average eC responsivity found in the best shape memory alloys. Significant caloric effects in soft LC-based materials, observed at much smaller fields than in solid caloric materials, demonstrate their ability to play an important role as new cooling elements, thermal diodes, and caloric-active regeneration material in new heat-management devices.

On the efficiency of caloric materials in direct comparison with exergetic grades of compressors

Efficiency improvements in heat pump can drastically reduce global energy demand. Caloric heat pumps are currently being investigated as a potentially more efficient alternative to vapor compression systems. Caloric heat pumps are driven by solid-state materials that exhibit a significant change in temperature when a field is applied, such as a magnetic or an electric field as well as mechanical stress. For most caloric materials, the phase transition results in a certain amount of power dissipation, which drastically impacts the efficiency of a caloric cooling system. The impact on the efficiency can be expressed by a figure of merit (FOM), which can directly be deduced from material properties. This FOM has been derived for 36 different magneto-, elasto-, electro and barocaloric material classes based on literature data. It is found that the best materials can theoretically attain second law efficiencies of over 90%. The FOM is analogous to the isentropic efficiency of idealized compressors of vapor compression systems. The isentropic efficiency can thus be directly linked to the theoretically achievable efficiency of a compressor-based refrigeration system for a given refrigerant. In this work a theoretical comparison is made between efficiency of caloric heat pumps and vapor compression systems based on the material losses for the caloric heat pump and the efficiency of the compressor for vapor compression systems. The effect of heat regeneration is considered in both cases. In vapor compression systems, the effect of the working fluid on the efficiency is also studied.

High-performance polymer-based regenerative elastocaloric cooler

Alternative methods of refrigeration are critical for performance optimization and environ- mental sustainability. Prototype systems using caloric materials have shown the advantages of a high coefficient of performance (COP) and low global warming potential. Elastocaloric elastomers can meet ongoing needs, but their implementation in functional heat pumps or coolers remains challenging due to their requirement for a large deformation and low thermal conductivity. Moreover, a scalable design is required to achieve high cooling power. As a leap forward for the use of polymers in larger scale caloric devices, we developed experimental elastocaloric polymer coolers with two different geometries using natural rubber tubes. We confirmed that a suitable geometry partially compensated for the low thermal conductivity and led to performances comparable to those of other caloric devices. Several operating conditions were tested to determine the optimal temperature span, cooling power, and COP for both device geometries. Under a specific operating condition, one of the prototypes exhibited a maximum temperature span > 8 K, a maximum COP of 6, and an output cooling power of 1.5 W; this power outperforms other prototypes based on polymers or ceramics. The parallelization of rubber tubes will facilitate realistic elastocaloric polymer coolers on a larger scale.

Thermal properties and phase transition behaviors of possible caloric materials Bi0.95Ln0.05NiO3

Thermal properties and phase transition behaviors of possible caloric materials Bi0.95Ln0.05NiO3 (Ln = La, Nd, Sm, Eu, Gd, Dy), which show intersite charge transfer between Bi and Ni ions, were investigated. Although...

Order-disorder phase transition and molecular dynamics in the hybrid perovskite [(CH3)3NH][Mn(N3)3

We present a temperature-dependent Raman scattering study of a [(CH3)3NH][Mn(N3)3] hybrid organic–inorganic azide-perovskite, in which we have analysed in detail the wavenumber and full width at half-maximum (FWHM) of lattice modes and internal modes of the NC3 skeleton, N3– and CH3 molecular groups. In general, the modes exhibited unusual behaviour during the phase transitions, including discontinuity in the phonon wavenumber, bandwidth, and unconventional shifts upon temperature variation. Spectral features on heating reveal the absence of significant distortions in the NC3 skeleton and a relatively restricted order–disorder process of the TrMA+ cations. On the other hand, linewidth anomalies of the δNC3 and νasNC3 modes have been attributed to the molecular dynamics of encapsulated cations. The unconventional blue shift of the symmetric stretching modes of azide ligands indicates the weakening of the intermolecular interactions between the TrMA+ cations and azido-bridges, and the strengthening of the intramolecular bonds. Additionally, we have used differential scanning calorimetry to confirm the subtle monoclinic to monoclinic (P21/c → C2/c) phase transition at around 330 K; and the phase transition to trigonal structure (R3¯m) above 359 K, whose associated entropy variation turns to be |ΔS| ∼ 22.3 J·kg−1 K−1 and displays a barocaloric (BC) tunability |δTt/δP| ∼ 3.17 K kbar−1, according to our estimations using the Clausius-Clapeyron method. Although the obtained values of entropy change and BC tunability are very close to those reported on formate-perovskites and other important caloric materials, those parameters are much lower than the giant entropy change of ∼80 Jkg−1 K−1 and large BC tunability ∼12 K kbar−1 observed for the analogue azide-perovskite [(CH3)4N][Mn(N3)]3 (TMAMnN3). Very interestingly, our combined study shed light to understand such different behaviour, as they reveal that the hydrogen bonds created between the TrMA+ cations and the framework prevent an extensive order–disorder process that is needed to obtain large entropy changes and large BC coefficients as it occurs in the case of related azide-perovskites with no H-bonds between the A cations (for example TMA) and the framework.

Origin of the Large Entropy Change in the Molecular Caloric and Ferroelectric Ammonium Sulfate (Adv. Funct. Mater. 45/2022)

Molecular Caloric In article number 2207717, Phillips and co-workers show that the large entropy change in the caloric material ammonium sulfate arises from librations of the ammonium ions in flat-bottomed, “bathtub-shaped” potential wells rather than an order-disorder phase transition. Engineering similar energy landscapes may be a promising route to designing new caloric molecular materials.

Long-Term Stable Elastocaloric Effect in a Heusler-Type Co51V33Ga16 Polycrystalline Alloy

Cyclic stability is of utmost importance for the long-term work of caloric materials in solid-state refrigeration. Heusler-type shape memory alloys show a large elastocaloric effect under low driving stress, but their intrinsic brittleness always leads to poor cyclic stability. Here, we demonstrate the simultaneously achieved large elastocaloric effect and long-term cyclic stability in a Heusler-type Co51V33Ga16 polycrystalline alloy. Over a wide temperature range of 290–340 K, large adiabatic temperature change (ΔTad) values of −6.0 to −10.9 K can be obtained upon fast unloading. Moreover, by synthetically considering the influences of deformation parameters on the ΔTad values, stress hysteresis, energy dissipation, and temporary residual strain to balance cooling capacity and cyclability, large ΔTad values higher than 6.0 K are maintained for more than 50 000 superelastic cycles performed under the compressive strain of 3.5% and strain rate of 1.1 × 10–2 s–1, showing a very low degradation rate of 2 × 10–5 K per cycle. Such cyclic stability of elastocaloric effect well outperforms those reported in Heusler-type alloys so far.

Magnetocaloric response with significant mechanical efficiency in frustrated intermetallic compound Pr2Co0.86Si2.88

Magnetically frustrated materials are considered as a promising unconventional members of caloric materials. Here, the magnetocaloric properties of the frustrated Pr2Co0.86Si2.88 have been investigated and discussed with the aid of density functional theory (DFT) calculations. The material exhibits a magnetic entropy change (−ΔSM) of 13.1 J/kg-K for ΔH = 70 kOe around low-temperature transition TL ∼ 4 K associated with the antiferromagnetic coupling between localized Pr-4f and itinerant Co-3d moments. Despite the absence of any long-range magnetic order, the obtained − ΔSM is one of the highest among the known Pr-based good magnetocaloric materials at cryogenic temperature range. It also exhibits a relative cooling power (RCP) of ∼ 201 J/kg and an adiabatic temperature change of 6.3 K around TL at 70 kOe. Moreover, the compound also exhibits a high mechanical efficiency and moderate electrical efficiency, being beneficial for possible technological applications.

Magnetocaloric effect in ScGdTbDyHo high-entropy alloy: Impact of synthesis route

Magnetic energy conversion systems based on magnetocaloric effect promise to be an efficient and eco-friendly alternative to widespread gas cooling systems. Progress in this field initiates an active search for high-performance magnetic refrigerants with suitable characteristics. Among various caloric materials, rare-earth multi-principal element alloys are of particular interest to achieve these purposes. Since structure and properties in these materials strongly depend on synthesis conditions and fabrication prehistory, these issues need to be considered for each multi-element systems. This study is aimed to address structure formation, magnetic and magnetocaloric properties in ScGdTbDyHo high-entropy alloy fabricated under different conditions. We analyze the liquid and solid phases in this system using the ab initio molecular dynamics simulations and find that the alloy has a strong tendency to form a single-phase solid solution. Within the experimental study, as-cast, rapidly quenched, thermally annealed and severely cold-deformed alloy samples have been examined. We have found that all the fabricated specimens are single-phase hexagonal close-packed solid solutions characterized by different density of structural defects. The magnetic measurements reveal a complex magnetic structure in these materials. The Neel point in the studied alloys varies in the range of 139–144 K. A field-induced metamagnetic transition from antiferromagnetic to ferromagnetic state is observed in the alloy samples at all temperatures below the Neel point. In the magnetically ordered state, all the materials demonstrate large coercive force, reaching 4 kOe. Despite of the magnetic hysteresis, the alloys demonstrate the largest values of relative cooling power (920–984 J/kg at 5 T magnetic field) among similar rare-earth high-entropy systems reported so far. Analysis of the experimental results obtained for the alloy samples fabricated under different synthesis conditions allows us to conclude that there is a correlation between structural defectiveness and refrigerant capacity in this rare-earth system.

Elastocaloric-like effect in strain-mediated Mn3SnC/PZT magnetoelectric hetero-composite for solid-state refrigeration

Concerning the part of global warming caused by conventional refrigerant gasses, past decades of research on solid-state cooling technologies based on caloric effects present an attractive alternative with zero-greenhouse gas emission and higher operating efficiency. However, the search for materials with large caloric effects near room temperature has become a challenge in modern material physics. Mn-based antiperovskite compounds are favorable caloric materials which generate a reversible enthalpy change by applying an external field. We report a novel approach of piezo-enhanced elastocaloric-like effect on Mn3SnC for solid-state refrigeration application. In the as-designed Mn3SnC/PZT magnetoelectric hetero-composite, the reversible caloric effect of Mn3SnC was regulated by the electric field-induced strain in the PZT piezoelectricity layer without any external magnetic field. In addition, we proposed an experimental setup for the direct adiabatic temperature change measurement of the caloric effect. The adiabatic temperature change is as high as ∼ 0.57 K at 280 K under an electric field of 0.8 kV/cm, which is approximately 2 factors larger than that of the magnetocaloric effect value of Mn3SnC under 3 T. By adopting the first-principles theory, we estimated the entropy change is approximately 2.6 J Kg−1 K−1 at 280 K which is close to the previous reports. This work demonstrates a novel approach to effectively enhance reversible caloric effects in first-order phase transition materials via electric fields.

Room‐Temperature Colossal Elastocaloric Effects in Three‐Dimensional Graphene Architectures: An Atomistic Study

AbstractSolid‐state cooling exploits the thermal response of caloric materials when subjected to external physical fields and represents a promising alternative to conventional refrigeration technologies. However, existing caloric materials are often limited by relatively small caloric response and hysteresis issues rooted in first‐order structural transitions. Here, colossal and reversible elastocaloric effects near room temperature in superelastic graphene architectures are predicted by thermodynamic analysis and atomistic calculations. The estimated adiabatic temperature change can reach a value of 155 K under a compressive stress of 0.7 GPa in a wide temperature range of around 300 K, yielding outstanding elastocaloric strength and efficiency. More unique is that both cooling and warming can be realized in the materials by applying tensile and compressive strains to host conventional and inverse elastocaloric effects, respectively, with almost no fatigue behavior and hysteresis effect. Such unprecedented elastocaloric performance results from a strain‐induced large change in configurational entropy in the graphene architectures. These effects are potentially extendable to other superelastic nanomaterials and hence suggest new material settings for developing high‐performance solid‐state refrigerants.

Materials, physics and systems for multicaloric cooling

Calls to minimize greenhouse gas emissions and demands for higher energy efficiency continue to drive research into alternative cooling and refrigeration technologies. The caloric effect is the reversible change in temperature and entropic states of a solid material subjected to one or more fields and can be exploited to achieve cooling. The field of caloric cooling has undergone a series of transformations over the past 50 years, bolstered by the advent of new materials and devices, and these developments have contributed to the emergence of multicalorics in the past decade. Multicaloric materials display one or more types of ferroic order that can give rise to multiple field-induced phase transitions that can enhance various aspects of caloric effects. These materials could open up new avenues for extracting heat and spearhead hitherto unknown technological applications. In this Review, we survey the emerging field of multicaloric cooling and explore state-of-the-art caloric materials and systems (devices) that are responsive to multiple fields. We present our vision of the future applications of multicaloric and caloric cooling and examine key factors that govern the overall system efficiency of the cooling devices.

On the Thermal Capacity of Solids.

The term thermal capacity appears to suggest a storable thermal quantity. However, this claim is not redeemed when thermal capacity is projected onto “heat”, which, like all energy forms, exits only in transit and is not a part of internal energy. The storable thermal quantity is entropy, and entropy capacity is a well-defined physical coefficient which has the advantage of being a susceptibility. The inverse of the entropy capacity relates the response of the system (change of temperature) to a stimulus (change of entropy) such as the fluid level responses to a change in amount of fluid contained in a vessel. Frequently, entropy capacity has been used implicitly, which is clarified in examples of the low-temperature analysis of phononic and electronic contributions to the thermal capacity of solids. Generally, entropy capacity is used in the estimation of the entropy of a solid. Implicitly, the thermoelectric figure of merit refers to entropy capacity. The advantage of the explicit use of entropy capacity comes with a descriptive fundamental understanding of the thermal behaviour of solids, which is made clear by the examples of the Debye model of phonons in solids, the latest thermochemical modelling of carbon allotropes (diamond and graphite) and not least caloric materials. An electrocaloric cycle of barium titanate close to its paraelectric–ferroelectric phase transition is analysed by means of entropy capacity. Entropy capacity is a key to intuitively understanding thermal processes.

Harmonic analysis of temperature profiles of active caloric regenerators

The design of a thermal regenerator is initially carried out by considering the fundamental influencing variables. For novel solid-state cooling systems using active caloric regenerators, the non-linearity of the coupled phenomena of material properties, heat transfer, and hydraulic flow can complicate the interpretation of experimental and simulated results. Based on the boundary conditions of a sinusoidal magnetic field and fluid flow, we elucidate the operation of active regenerators by deriving easy-to-manage analytical expressions for the temperature transients of the caloric materials and heat transfer fluid. An internal temperature measurement system with an estimated uncertainty of ±0.3 K for packed bed regenerators has been developed for validation. The derived expressions have acceptable accuracy relative to the experimental and numerical results for temperature profiles in both magnitude and sensitivity, where average and maximum errors are ∼10% and ∼15%, respectively. Useful figures of merit are post-calculated using the derived temperature profiles. We found that the average temperature profiles are linear for passive regenerators and nonlinear for active regenerators, and their transients are nonlinear functions of the configuration and operating parameters.

Atomic-scale insights into the colossal barocaloric effects of neopentyl glycol plastic crystals

Neopentyl glycol has become an important candidate material for solid-state refrigeration in the future because of its environmental protection, high energy efficiency, high stability, and economy. However, the complete micro-dynamic mechanism remains to be established, which restricts its further applications. In this work, we investigate one representative material-plastic crystal neopentyl glycol (NPG) by means of large-scale molecular dynamics simulation. It is found that NPG exhibits colossal barocaloric effects (CBCEs) with high isothermal entropy changes and potentially large adiabatic temperature changes, which closely relates to the reversible order disorder change in NPG's molecular orientation, in which the non-bond interaction between molecules plays a key role. Further analysis of orientational dynamics and hydrogen bond energy during phase transition along with pressure dependent thermal conductivity sheds light on the underlying microscopic mechanism. Our work reveals the molecular mechanism of CBCEs in NPG as a prototypical plastic crystal, providing valuable insight into achieving practical caloric materials in future cooling technology.