Focus on Caloric Materials and Devices

We consider well-known signatures of disorder in crystallographic and inelastic neutron scattering data. We show that these can arise from different types of disorder, corresponding to different values of the system entropy. Correlating the entropy of a material with its atomistic structure and dynamics is in general a difficult problem that requires correlating information between multiple experimental techniques including crystallography, spectroscopy, and calorimetry. These comments are illustrated with particular reference to barocalorics, but are relevant to a broad range of calorics and other disordered crystalline materials.

In recent years, elastocaloric cooling technology has been considered as one of the most promising alternatives to vapor compression technology. Given that elastocaloric technology is only in the early stages of development, a uniform method for evaluating the elastocaloric effect has not yet been established, and the thermodynamics of different elastocaloric cooling cycles have not yet been studied in detail. Therefore, the main goal of this work is to investigate these two important areas. Here, multiple thermodynamic cycles were studied, focusing on the parameters of the holding period of the cycle, which is essential for heat transfer between the elastocaloric material and the heat sink/source. The cycles were applied to commercially available superelastic thin-walled NiTi tubes under compressive loading and a thin NiTi wire under tensile loading. Isostress cycles with constant stress throughout the holding period, isostrain cycles with constant strain throughout the holding period and no-hold cycles (without a holding period) were studied across multiple stress/strain ranges. Based on the experimental results, a previously developed phenomenological model was applied to better understand and further evaluate the different cycles. The results revealed that the applied thermodynamic cycle significantly affects the thermomechanical response and thus the cooling/heating efficiency of the elastocaloric material. We show that by using isostress cycles and partial transformations, a Carnot-like thermodynamic cycle with improved heating/cooling efficiency can be generated. By applying the isostress cycles, an adiabatic temperature change of 30.2 K was measured, which is among the largest directly measured reproducible adiabatic temperature changes reported for any caloric material to date. Ultimately, this study intends to serve as a basis for establishing a uniform method for evaluating the elastocaloric effect in different materials that would allow for reliable and accurate one-to-one comparison of the reported results in the rapidly growing field of elastocalorics.

The magnetocaloric effect based Magnetic refrigeration (MR) was considered a novel energy-efficient and environmentally benign cooling method. However, the lack of suitable magnetic solids has slowed the development of its practical applications. We herein fabricated the RE₂NiTiO₆ (RE = Gd, Tb and Ho) double perovskite (DP) compounds and systematically determined their structural, magnetic and magnetocaloric properties by experimental determination and density functional theory calculations, in which the Gd₂NiTiO₆ was realized to exhibit promising cryogenic magnetocaloric performances. The results indicated that all the RE₂NiTiO₆ DP compounds crystallized in a distorted monoclinic structure with P2₁/n space group and underwent a second order type magnetic phase transition around 4.3, 4.5 and 3.9 K, for Gd₂NiTiO₆, Tb₂NiTiO₆ and Ho₂NiTiO₆, respectively. The magnetocaloric performances were checked by the parameters of maximum magnetic entropy change and relative cooling power, which are 31.28 J·kg⁻¹·K⁻¹ and 242.11 J·kg⁻¹ for Gd₂NiTiO₆, 13.08 J·kg⁻¹·K⁻¹ and 213.41 J·kg⁻¹ for Tb₂NiTiO₆, 11.98 J·kg⁻¹·K⁻¹ and 221.73 J·kg⁻¹ for Ho₂NiTiO₆ under the magnetic field change of 0–50 kOe, respectively. Evidently, the Gd₂NiTiO₆ compound exhibit promising magnetocaloric performances and therefore is of potential for practical cryogenic MR applications.

Effective and, at the same time, efficient active magnetic regenerator (AMR) performance requires balanced geometry and operating conditions. Here the influence of regenerator shape, magnetocaloric material size, operating frequency, and utilization on the performance of gadolinium packed-particle bed AMRs is demonstrated experimentally. Various metrics are applied to assess effectiveness and efficiency. Observed temperature spans and cooling powers across a wide range of operating conditions are used to evaluate system performance and estimate exergetic cooling power and exergetic power quotient. A new metric combining exergetic cooling power and pump power provides an estimate of the maximum achievable second law efficiency. Five regenerator geometries with equal volumes and the aspect ratio from 1.0 to 3.8, and four different ranges of Gd spherical particles between 182 and 354 µm, are investigated. Improvements in system performance are demonstrated by a boost in specific cooling power of gadolinium from 0.85 to 1.16 W g⁻¹ and maximum temperature span from 8.9 to 15.1 K. The optimum exergetic cooling power is observed for 1.37 utilization and 3 Hz operating frequency, exergetic power quotient exhibits a maximum at the same utilization but at 2 Hz frequency, while the highest efficiency is recorded at 1 Hz and utilization of 0.5, demonstrating that multiple performance metrics must be balanced to achieve regenerator design meeting all performance targets.

Caloric cooling is an attractive family of technologies owing to their environmental friendliness and potential for higher efficiency than present refrigeration systems. Cooling devices based on the electrocaloric (EC) effect specifically have the added benefit of being easily miniaturized, enabling applications in electronic thermal management, wearables and localized cooling. A challenge in prior compact EC cooling devices has been the need for a separate actuation mechanism to cyclically contact the EC material with hot and cold interfaces. Here, we propose a self-actuated EC polymer heat pump, exploiting recent discoveries of giant EC and electromechanical responses under low electric fields in P(VDF-TrFE-CFE-FA) (VDF: vinylidene fluoride, TrFE: trifluoroethylene, CFE: chlorofluoroethylene, FA: fluorinated alkynes) relaxor tetrapolymers. We show that the transverse electroactuation of P(VDF-TrFE-CFE-FA) relaxor tetrapolymer films can be tailored over a broad range, from strong actuation to weak actuation, without affecting the high EC response. Using this principle, a unimorph actuator was constructed from two EC tetrapolymer layers with large differences in electroactuation. This device autonomously achieves a large displacement between the heating and cooling cycles of the EC films, which could be used to switch thermal contact between hot and cold interfaces. This concept could thus enable highly efficient and compact EC heat pumps.

Cu-based superelastic shape memory alloys are promising for low-stress elastocaloric cooling. We have synthesized bulk alloys of 68Cu–16Al–16Zn under different conditions in order to promote its grain growth and enhance its elastocaloric properties. High-temperature x-ray diffraction of untreated 68Cu–16Al–16Zn alloy showed that the phase boundary between the α + β mixed phases and the high temperature phase (β phase) was between 973 K and 1023 K. Based on this result, the 68Cu–16Al–16Zn alloy was heated and cooled in a furnace repeatedly between 773 K and 1173 K. The maximum grain size after heat treatment of the ingot rolled to 67% reached 11.1 mm. The latent heat of the martensitic transformation after grain growth was 6.3 J g⁻¹, which is higher than the previously reported value for the compound. The stress–strain curve of 68Cu–16Al–16Zn rolled to 67% rolling with cyclical heat treatments showed a maximum stress of 106 MPa at 4.5% strain, with adiabatic temperature change of 5.9 K in heating during stress loading and 5.6 K in cooling in stress removal. Furthermore, no fatigue in the stress–strain behavior was observed up to at least 60 000 mechanical cycles at 2% strain.

Multiferroic materials with strong coupling between different degrees of freedom are prone to exhibit giant multicaloric effects resulting from the application or removal of diverse external fields. These materials exhibit a synergic response to the combined action of two fields when the monocaloric effects are both conventional (or both inverse), while a non-synergic response occurs when one of the monocaloric effects is conventional and the other is inverse. In all cases, the multicaloric properties (isothermal entropy and adiabatic temperature changes) do not result from the simple addition of the corresponding monocaloric quantities because there is a contribution from the interplay between degrees of freedom (cross-coupling term). In this paper, we analyse in detail the contribution of the cross-coupling term to the multicaloric entropy values obtained for both synergic and non-synergic multicaloric materials. We first introduce basic thermodynamic concepts accounting for the multicaloric effects, and next the contribution from the cross-coupling term is illustrated via several model examples. We finally analyse the realistic situation for two prototype materials with synergic and non-synergic multicaloric effects.

Electrocaloric (EC) materials, presenting large adiabatic temperature change or isothermal entropy change under the application (or removal) of electric fields, offer an efficient alternative to caloric heat pumps for replacing hazardous gases used in traditional vapor-compression systems. Recently, a large EC temperature change of 5.5 K have been reported in Pb(Sc_0.5Ta_0.5)O₃ multilayer ceramic capacitors (Nair et al 2019 Nature575 468) thanks to its strong first-order phase transition and a temperature span of 13 K has been reported in a prototype based on these capacitors (Torelló et al 2020 Science370 125). However, the toxicity of lead forces researchers to find eco-friendly materials exhibiting competitive EC performances. Here, we study the EC effect in lead-free BaTiO₃ multilayer capacitors using an infrared camera. Unlike commercial BaTiO₃ capacitors, we prepared our samples without sacrifying the first-order phase transition in BaTiO₃ while a low amount of 0.2 mol% Mn was added as an acceptor dopant to improve electrical resistivity. Their EC adiabatic temperature variations show two peaks versus temperature, which match BaTiO₃ two first-order phase transitions, as observed by differential scanning calorimetry. We measured a temperature drop of ∼0.9 K over a temperature range of 70 K under 170 kV cm⁻¹, starting at 30 °C near the tetragonal-to-orthorhombic phase transition. Under the same electric field, a maximum temperature change of 2.4 K was recorded at 126 °C, at BaTiO₃'s Curie temperature. Our findings suggest that further optimized BaTiO₃ capacitors could offer a path for designing lead-free caloric cooling prototypes.

The magnetically frustrated manganese nitride antiperovskite family displays significant changes of entropy under changes in hydrostatic pressure near a first-order antiferromagnetic to paramagnetic phase transition that can be useful for the emerging field of solid-state barocaloric cooling. In previous studies, the transition hysteresis has significantly reduced the reversible barocaloric effects (BCE). Here we show that the transition hysteresis can be tailored through quaternary alloying in the Mn₃Cu$_{1-x}$Sn$_{x}$N system. We find the magnitude of hysteresis is minimised when Cu and Sn are equiatomic (x = 0.5) reaching values far less than previously found for Mn₃AN ($A =$ Pd, Ni, Ga, Zn), whilst retaining entropy changes of the same order of magnitude. These results demonstrate that reversible BCE are achievable for p < 100 MPa in the Mn₃(A, B)N family and suggest routes to modify the transition properties in compounds of the same family.

Rare-earth-free magnetostructural MnNiSi-based solid solutions are considered as promising candidates for solid-state cooling applications. In this paper, we use density functional theory calculations to study the energetics, variations in atomic displacements and bond length, and magnetic properties of high-entropic, intermetallic MnNi-X (X = Si_0.2Ge_0.2Sn_0.2Al_0.2Ga_0.2) magnet in both the low-symmetry Pnma and high-symmetry $P6_3/mmc$ structures, where we confine the large configurational entropy to the non-magnetic X-site of the compound. Our calculations reveal that the high-entropic chemical substitution of Si_0.2Ge_0.2Sn_0.2Al_0.2Ga_0.2 in the X-site carry fingerprints that favor a reduction in magnetostructural transition temperature with minimal impact of total magnetization. These results motivate a promising path of high-entropic X-site substitutions to tune the magnetostructural properties of MnNiSi-based solid solutions.

Caloric cooling enlisting solid-state refrigerants is potentially a promising eco-friendly alternative to conventional cooling based on vapor compression. The most common refrigerant materials for elastocaloric cooling to date are Ni-Ti based superelastic shape memory alloys. Here, we have explored tuning the operation temperature range of Ni_50.8Ti_49.2 for elastocaloric cooling. In particular, we have studied the effect of thermal treatments (a.k.a. aging) on the transformation temperature, superelasticity, and elastocaloric effects of Ni_50.8Ti_49.2 shape memory alloy tubes. The isothermal compressive test revealed that the residual strain of thermally-treated Ni-Ti tubes at room temperature approaches zero as aging time is increased. Short-time aging treatment at 400 °C resulted in good superelasticity and elastocaloric cooling performance with a large tunable austenite finish (A_f) temperature range of 24.7 °C, as determined from the A_f temperature of the samples that were aged 5–120 min. The main reason of the property change is the formation of a different amount of Ni₄Ti₃ precipitates in the NiTi matrix. Our findings show that it is possible to tailor the A_f temperature range for development of cascade elastocaloric cooling systems by thermally treating a starting single composition Ni-Ti alloy.

In most cases, substitution studies that aim to optimize magnetic properties are performed at the magnetic atomic site. However, in the case of MnB, magnetic substitutions at the Mn site significantly decrease the once promising magnetocaloric and magnetic properties. This study employs computationally directed search to optimize the magnetocaloric properties of MnB where partial substitutions of boron atoms (Mn₅₀B_50−xSi_x and Mn₅₀B_50−xGe_x where x = 3.125, 6.25, and 12.5) reveal new compounds with a greater magnetocaloric effect than pure MnB at the same Curie temperature. These new compounds were obtained by arc melting the pure elements and further characterized. The computationally driven screening process is based on density functional theory calculations that do not require large databases of known compounds. This work demonstrates that using simple computational screening procedures to search for new magnetocaloric materials with improved properties can be done quickly, cost-effectively, and while maintaining reliability.

Magnetocaloric hydrogen liquefaction could be a 'game-changer' for liquid hydrogen industry. Although heavy rare-earth based magnetocaloric materials show strong magnetocaloric effects in the temperature range required by hydrogen liquefaction (77–20 K), the high resource criticality of the heavy rare-earth elements is a major obstacle for upscaling this emerging liquefaction technology. In contrast, the higher abundances of the light rare-earth elements make their alloys highly appealing for magnetocaloric hydrogen liquefaction. Via a mean-field approach, it is demonstrated that tuning the Curie temperature (T_C) of an idealized light rare-earth based magnetocaloric material towards lower cryogenic temperatures leads to larger maximum magnetic and adiabatic temperature changes (ΔS_T and ΔT_ad). Especially in the vicinity of the condensation point of hydrogen (20 K), ΔS_T and ΔT_ad of the optimized light rare-earth based material are predicted to show significantly large values. Following the mean-field approach and taking the chemical and physical similarities of the light rare-earth elements into consideration, a method of designing light rare-earth intermetallic compounds for hydrogen liquefaction is used: tuning T_C of a rare-earth alloy to approach 20 K by mixing light rare-earth elements with different de Gennes factors. By mixing Nd and Pr in Laves phase (Nd, Pr)Al₂, and Pr and Ce in Laves phase (Pr, Ce)Al₂, a fully light rare-earth intermetallic series with large magnetocaloric effects covering the temperature range required by hydrogen liquefaction is developed, demonstrating a competitive maximum effect compared to the heavy rare-earth compound DyAl₂.

$\mathrm{La}(\mathrm{Fe}_{x}\mathrm{Si}_{1-x})_{13}$ and derived quaternary compounds are well-known for their giant, tunable, magneto- and barocaloric responses around a first-order paramagnetic-ferromagnetic transition near room temperature with low hysteresis. Remarkably, such a transition shows a large spontaneous volume change together with itinerant electron metamagnetic features. While magnetovolume effects are well-established mechanisms driving first-order transitions, purely electronic sources have a long, subtle history and remain poorly understood. Here we apply a disordered local moment picture to quantify electronic and magnetoelastic effects at finite temperature in $\mathrm{La}(\mathrm{Fe}_{x}\mathrm{Si}_{1-x})_{13}$ from first-principles. We obtain results in very good agreement with experiment and demonstrate that the magnetoelastic coupling, rather than purely electronic mechanisms, drives the first-order character and causes at the same time a huge electronic entropy contribution to the caloric response.

Magnetic refrigeration is a highly active field of research. The recent studies in materials and methods for hydrogen liquefaction and innovative techniques based on multicaloric materials have significantly expanded the scope of the field. For this reason, the proper characterization of materials is now more crucial than ever. This makes it necessary to determine the magnetocaloric and other physical properties under various stimuli such as magnetic fields and mechanical loads. In this work, we present an overview of the characterization techniques established at the Dresden High Magnetic Field Laboratory in recent years, which specializes in using pulsed magnetic fields. The short duration of magnetic-field pulses, lasting only some ten milliseconds, simplifies the process of ensuring adiabatic conditions for the determination of temperature changes, $\Delta T_{\mathrm{ad}}$. The possibility to measure in the temperature range from 10 to 400 K allows us to study magnetocaloric materials for both room-temperature applications and gas liquefaction. With magnetic-field strengths of up to 50 T, almost every first-order material can be transformed completely. The high field-change rates allow us to observe dynamic effects of phase transitions driven by nucleation and growth as well. We discuss the experimental challenges and advantages of the investigation method using pulsed magnetic fields. We summarize examples for some of the most important material classes including Gd, Laves phases, La–Fe–Si, Mn–Fe–P–Si, Heusler alloys and Fe–Rh. Further, we present the recent developments in simultaneous measurements of temperature change, strain, and magnetization, and introduce a technique to characterize multicaloric materials under applied magnetic field and uniaxial load. We conclude by demonstrating how the use of pulsed fields opens the door to new magnetic-refrigeration principles based on multicalorics and the 'exploiting-hysteresis' approach.

Cu-Al-Mn alloys display martensitic transformation over a wide range of temperatures. In addition to low cost, this alloy is known for its low transformation stress with reasonable latent heat favoring elastocaloric applications. However, the ductility of Cu-Al-Mn can be limited owing to ordering and intergranular fracture. Through rapid solidification by melt spinning, we show that Cu-Al-Mn ribbon can be made highly ductile (greater than 8% tensile strain in the as-spun state and 10% tensile strain after heat treatment). The ductility of the melt-spun ribbon is related to the suppression of L2₁ ordering that is characterized through magnetic property measurement. Heat treatment of the ribbon promotes bamboo grain formation, and the latent heat is increased to 6.4 J g⁻¹. Under tensile conditions, we show that the ribbon exhibited about 4 °C temperature change (4.4 °C on heating and 4.2 °C on cooling from 6.3% strain).

The emergent evolution of first-order phase transitions from magneto-structural to magneto-elastic and magnetocaloric effect (MCE) have been investigated by X-ray diffraction, differential scanning calorimetry and magnetization measurements. Applying the isostructural alloying principle, the martensitic transition temperature (T_M) increases effectively and the Curie temperatures of the two phases increase slightly by substituting the Si content (x). With an appropriate amount of Fe and Si content, an emergent first-order antiferromagnetic–ferromagnetic magnetoelastic transition with thermal hysteresis in the martensitic state occurs for MnCo_0.7Fe_0.3Ge_1–xSi_x (x = 0.15–0.40) alloys, which results from the decrease in the nearest-neighbor Mn–Mn distance. Moreover, the values of magnetic entropy change (ΔS_M), refrigeration capacity (RC) and temperature-averaged entropy change (TEC, 10 K) with ΔH = 50 kOe reach −12.2 J kg⁻¹ K⁻¹, 112.8 J kg⁻¹ and 11.4 J kg⁻¹ K⁻¹ for MnCo_0.7Fe_0.3Ge_0.8Si_0.2 undergoing the ferromagnetic magneto-structural transition in the Curie temperature window. The results facilitate the magnetocaloric/magnetoelastic performance and tunability of multiple phase states in a wider temperature range.

The electrocaloric (EC) effect is the adiabatic temperature change of a material in a varying external electric field, which is promising for novel cooling devices. While the fundamental understanding of the caloric response of defect-free materials is well developed, there are important gaps in the knowledge about the reversibility and time-stability of the response. In particular, it is not settled how the time-dependent elements of microstructure that are always present in real materials act on the field-induced temperature changes. Ab initio based molecular dynamics simulations allow us to isolate and understand the effects arising from domain walls (DWs) and defect dipoles and to study their interplay. We show that DWs in cycling fields do not improve the response in either the ferroelectric (FE) phase or at the FE phase transition, but may result in irreversible heat losses. The presence of defect dipoles may be beneficial for the EC response for proper field protocols, and interestingly this benefit is not too sensitive to the defect configuration.

The quest for better performance from magnetocaloric devices has led to the development of thermal control devices, such as thermal switches, thermal diodes, and thermal capacitors. These devices are capable of controlling the intensity and direction of the heat flowing between the magnetocaloric material and the heat source or heat sink, and therefore have the potential to simultaneously improve the power density and energy efficiency of magnetocaloric systems. We have developed a new type of thermal control device, i.e., a silicon mechanical thermal switch capacitor ( TSC). In this paper we first review recently developed thermal switches based on micro-electromechanical systems and present the operation and structure of our new TSC. Then, the results of the parametric experimental study on the thermal contact resistance, as one of the most important parameters affecting the thermal performance of the device, are presented. These experimental data were later used in a numerical model for a magnetocaloric device with a thermal switch-capacitor. The results of the study show that for a single embodiment, a maximum cooling power density of 970 W m⁻² (510 W kg_mcm⁻¹) could be achieved for a zero-temperature span and an operating frequency of 5 Hz. However, a larger temperature span could be achieved by cascading multiple magnetocaloric elements with TSCs. We have shown that the compact TSC can be used in caloric devices, even with small temperature variations, and can be used in a variety of practical applications requiring thermal regulation.

Bulk PbMg_0.5W_0.5O₃ (PMW) is an antiferroelectric in which an electric field of 12 V μm⁻¹ is sufficient to initiate a nominally reversible transition to a dipole-aligned (ferroelectric) phase if operating just below the Néel temperature T_N, near room temperature (Li et al 2021 Adv. Funct. Mater.31 2101176). Here we describe multilayer capacitors (MLCs) of PMW that permit 27 V µm⁻¹ to be applied without breakdown. Below T_N, nominally reversible driving of the partial (full) antiferroelectric–ferroelectric (AF–FE) transition over a wide (narrow) range of temperatures yields large inverse electrocaloric (EC) effects that peak at ΔT_j ∼ –2.6 K when applying 25 V μm⁻¹ at 293 K (ΔT_j denotes directly measured temperature jumps). Above T_N, nominally reversible driving of the partial (full) paraelectric–ferroelectric (PE–FE) transition yields large conventional EC effects that peak at ΔT_j ∼ +5.2 K when applying 25 V μm⁻¹ at 302 K. This good EC performance near room temperature implies that MLCs of PMW could be exploited in prototype EC coolers.

Solid-state cooling/heating technology based on the elastocaloric effect is one of the promising alternatives to vapor compression systems. Large elastocaloric temperature modulation is often generated through the non-linear strain-induced structural transition by applying large strain and/or stress to ferroelastic materials. Recently, an unconventional approach to expand the application possibilities of the elastocaloric effect was demonstrated by processing elastocaloric materials into kirigami structures, which was inspired by the art of paper cutting. Using this approach, only a small stretch of processed conventional plastics can locally provide more efficient performance of elastocaloric temperature modulation than that of ferroelastic materials. To further improve such a unique functionality, it is necessary to find plastic or polymeric materials showing large elastocaloric effects in the linear elastic response regime that can be driven by a MPa-order weak stress application, where the non-linear structural transition is irrelevant. In this work, by means of a recently developed measurement technique for the elastocaloric effect based on the lock-in thermography, we found that shape memory polymers (SMPs) show prominent performance for elastocaloric temperature modulation that is larger than conventional plastics. SMPs enable the control of crystallinity by changing the cross-linking agents, melting temperature by changing the degree of polymerization, and orientation of the polymer chain segment by the shape memory effect. By utilizing the unique properties of SMPs, we manipulated their elastocaloric performance. The experimental results reported here will highlight the potential of smart polymers for flexible and durable elastocaloric applications.

The magnetocaloric effect (MCE) in La_1−zR_z(Fe_0.89−xMn_xSi_0.11)₁₃H_ymax (R = Ce and Pr) is verified in view of correlation between alloying recipes such as selection of doping elements and fundamental physics that governs MCE. The Ce-doped specimen with z = 0.3 & x = 0.017 exhibits a peaky isothermal entropy change ΔS_M profile with a maximum value of 20 J kg⁻¹ K under a field change of 0.8 T at the Curie temperature of 285 K. In contrast, the enlarged field dependence of the Curie temperature and diminished hysteresis results in the adiabatic temperature change ΔT_ad of 2.7 K under a field change of 0.8 T at the Curie temperature of 289 K for the Pr-doped specimen.

In an all-solid-state electrocaloric arrangement, an absolute temperature change which exceeds twice the electrocaloric adiabatic temperature change is locally realized, using just the distributed thermal capacitances and resistances and spatio-temporal distributed electric field control. First, simulations demonstrate surface temperature changes up to four times (400%) the electrocaloric adiabatic temperature change for several implementations of all-solid state distributed element configurations. Then, experimentally, an all-solid-state assembly is built from commercial electrocaloric capacitors with two independently-controlled parts, and the measured surface temperature change was 223% of the adiabatic electrocaloric temperature change, which clearly exceeds twice the adiabatic temperature change and verifies the practical feasibility of the approach. This allows a significant increase of the maximum temperature difference per stage in cascaded and thermal switch-based electrocaloric heat pumps, which was previously limited by the adiabatic electrocaloric temperature change (100%) under no-load conditions. Distributed thermal element simulations provide insight in the spatio-temporal temperatures within the all-solid-state electrocaloric element. Since only the distributed thermal capacitance and resistance is used to boost the temperature change, the maximum absolute temperature change occurs only in parts of the all-solid-state element, for example close to the surfaces. A trade-off of the approach is that the required electrocaloric capacitance increases more than the gained boost of the absolute temperature change, reducing the power density and electrical efficiency in heat pump systems. Nevertheless, the proposed approach enables to simplify electrocaloric heat pumps or to increasing the achievable temperature span, and might also improve other electrocaloric applications.

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.

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.

The influence of neutron and gamma irradiation on the low- and high-field dielectric and electrocaloric (EC) properties of Mn-doped 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃ (PMN–10PT) ceramic is studied. Upon exposure to neutron fluences of up to 10¹⁷ cm⁻² and gamma-ray doses of up to 1200 kGy the Mn-doped PMN–10PT exhibits a lower saturated polarization, increased internal bias field and reduced EC temperature change. In comparison, the respective properties of the undoped PMN–10PT remain almost unchanged upon exposure to neutrons and gamma rays. In Mn-doped PMN–10PT, the acceptor-oxygen vacancy defect complexes, introduced via doping, contribute to the lowering of the threshold radiation dose that the material survives without noticeable changes in properties. Radiation-induced degradation of the EC response of Mn-doped PMN–10PT can be partially healed by annealing at 450 °C. The study provides guidance for designing EC ceramic materials for solid-state cooling applications in environments of high ionizing radiation, such as the medical field or space technologies.

Scanning thermal microscopy (SThM) is emerging as a powerful atomic force microscope based platform for mapping dynamic temperature distributions on the nanoscale. To date, however, spatial imaging of temperature changes in electrocaloric (EC) materials using this technique has been very limited. We build on the prior works of Kar-Narayan et al (2013 Appl. Phys. Lett.102 032903) and Shan et al (2020 Nano Energy67 104203) to show that SThM can be used to spatially map EC temperature changes on microscopic length scales, here demonstrated in a commercially obtained multilayer ceramic capacitor. In our approach, the EC response is measured at discrete locations with point-to-point separation as small as 125 nm, allowing for reconstruction of spatial maps of heating and cooling, as well as their temporal evolution. This technique offers a means to investigate EC responses at sub-micron length scales, which cannot easily be accessed by the more commonly used infrared thermal imaging approaches.

The nature of the phase transition has been studied in MnNi_1−xCo_xGe_0.97Al_0.03 (x= 0.20–0.50) through magnetization, differential scanning calorimetry and x-ray diffraction measurements; and the associated reversibility in the magnetocaloric effect has been examined. A small amount of Al substitution for Ge can lower the structural phase transition temperature, resulting in a coupled first-order magnetostructural transition (MST) from a ferromagnetic orthorhombic to a paramagnetic hexagonal phase in MnNi_1−xCo_xGe_0.97Al_0.03. Interestingly, a composition-dependent triple point (TP) has been detected in the studied system, where the first-order MST is split into an additional phase boundary at higher temperature with a second-order transition character. The critical-field-value of the field-induced MST decreases with increasing Co concentration and disappears at the TP (x= 0.37) resembling most field-sensitive MST among the studied compositions. An increase of the hexagonal lattice parameter a_hex near the TP indicates a lattice softening associated with an enhancement of the vibrational amplitude in the Ni/Co site. The lattice softening leads to a larger field-induced structural entropy change (structural entropy change≫ magnetic entropy change, for this class of materials) with the application of a lower field, which results in a larger reversibility of the low-field entropy change (|ΔS_rev| = 6.9 J kg⁻¹ K for Δμ₀H = 2 T) at the TP.

This paper presents a novel approach to characterizing the relevant mechanical, thermal and caloric properties of elastocalorics material in a single testing device. Usually, tensile experiments are performed to determine the rate- and process-depending stress/strain behavior of nickel-titanium-based shape memory alloys and potentially other elastocaloric materials made from metallic alloys. These tests are relevant for, e.g., characterization of hysteresis properties and subsequent calculation of mechanical work input. In addition, simultaneous observation with an infrared camera is useful to understand temperature evolution and maximum temperature changes achievable during the loading/unloading process. Characterization of the caloric properties of the materials determines latent heats and, together with the mechanical work, also the material coefficient of performance. It is typically carried out via differential scanning calorimetry (DSC), which is performed in a separate device and requires a second experiment with different types of samples. Furthermore, DSC measurements do not reflect the way mechanically induced phase transformations trigger the release and absorption of latent heats as it is the case for elastocalorics. In order to provide a more consistent understanding of the relevant elastocaloric material properties, we here present a novel method that (a) allows for a systematic determination of load-dependent latent heats and (b) introduces a comprehensive testing setup and suitable testing routine to determine the mechanical, thermal and caloric parameters in the same experimental device and with the same sample, thus greatly simplifying the overall procedure.

Plastic crystals have emerged as benchmark barocaloric (BC) materials for potential solid-state cooling and heating applications due to huge isothermal entropy changes and adiabatic temperature changes driven by pressure. In this work we investigate the BC response of the neopentane derivative 2-methyl-2-nitro-1-propanol (NO₂C(CH₃)₂CH₂OH) in a wide temperature range using x-ray diffraction, dilatometry and pressure-dependent differential thermal analysis. Near the ordered-to-plastic transition, we find colossal BC effects of $\simeq$400 J K⁻¹ kg⁻¹ and $\simeq$5 K upon pressure changes of 100 MPa. Although reversible effects at the transition are obtained only from higher pressure changes due to hysteretic effects, we do obtain fully reversible BC effects from any pressure change in individual phases, that become giant at moderate pressures due to very large thermal expansion, especially in the plastic phase. From our measurements, we also determine the crystal structure of the low-temperature phase and estimate the contribution of the configurational disorder and the volume change to the total transition entropy change.

We report on the design and characterization of a demonstrator device for miniature-scale elastocaloric (eC) cooling using a series of natural rubber (NR) foil specimens of 9 × 26.5 mm² lateral size and thicknesses in the range of 290–900 μm. NR has the potential to meet the various challenges associated with eC cooling, as it exhibits a large adiabatic temperature change in the order of 20 K and high fatigue resistance under dynamic load, while loading forces are low. Owing to the large surface-to-volume ratio of rubber-based foils, heat transfer to heat sink and source elements is accomplished by mechanical contact enabling compact designs. Two actuators are implemented to control the performance in loading direction independent from the performance of mechanical contacting. The study of operation parameters is complemented by lumped-element modeling to understand the cycle frequency-dependent dynamics of heat transfer and resulting cooling capacity. The single-stage device operates in the strain range of 300%–700% and exhibits a temperature span up to 4.1 K, while the specific cooling power reaches 1.1 Wg⁻¹ and the absolute cooling power 123 mW. The performance metrics show a pronounced dependence on foil thickness and heat transfer coefficient indicating a path toward future device optimization.

Climate change and the increasing demand for energy globally have motivated the search for a more sustainable heat-pumping technology. Magnetic refrigeration stands as one of the most promising alternative technologies for clean and efficient heat pumps of the future. The rotating magnetocaloric effect (RMCE) has previously been studied in materials with magnetocrystalline anisotropy due to its potential to improve devices by requiring only a single magnetic field region, but these materials are fragile and costly to obtain, making them inviable for applications. It has been shown that by exploiting the demagnetizing effect, an RMCE is, in fact, attainable in any polycrystalline magnetocaloric sample with an asymmetric shape, without requiring magnetocrystalline anisotropy. Using gadolinium as a case study, we provide a theoretical framework for computing the demagnetizing field-based RMCE and present thorough experimental verification for different magnetic field intensities and a wide temperature range. Direct measurements of the RMCE in gadolinium reveal that a significant adiabatic temperature difference (1.2 K) and refrigerant capacity (7.44 J kg⁻¹) can be attained within low magnetic field amplitudes (0.4 T). Utilizing lower magnetic field intensities in a magnetocaloric heat pump can significantly diminish the need for permanent magnet materials, thus reducing the overall device cost, size, and weight, ultimately enhancing the feasibility of mass-producing such devices.

The elastocaloric effect denotes the ability of a material to release or absorb heat when the material is stretched and released respectively. This effect may be used to design an alternative cooling device. This work focuses on the development of a cooling device using natural rubber (NR) as the elastocaloric material. It consists of a solid–solid heat exchange between a cyclically stretched elastocaloric material and two exchangers, respectively put in contact with the elastocaloric material when it is stretched or released. An experimental device was designed and tested in order to assess the temperature span and cooling power (PC) achievable by NR based single stage device. The effect of the thickness of the NR is also discussed. It is shown that it was possible to transfer nearly 60% of the heat absorption potential of the NR from the cold heat exchanger. From the measurements, the highest PC was found to be 390 mW (430 W kg⁻¹) for a 600 µm thick sample, and 305 mW (540 W kg⁻¹) for a 400 µm thick sample. The temperature span was found to be similar for both materials, ranging 1.5 °C–1.9 °C.