Adiabatic Temperature Change ΔTad Research Articles

Improved elastocaloric effect of NiTi shape memory alloys strengthened by Ni4Ti3 nanoprecipitates

NiTi shape memory alloys (SMAs) exhibit extraordinary potential in the field of solid-state cooling due to their exceptional elastocaloric effect (eCE). However, the inherent trade-off between cooling efficiency and cooling capacity often makes it difficult to enhance both simultaneously, and the relatively poor cyclic stability poses a significant challenge to the application of such material as a solid-state refrigerant. A NiTi elastocaloric material containing uniformly distributed high-density Ni4Ti3 nanoprecipitates is obtained in this work, with a remarkable coefficient of performance of material (COPmat) as high as 52 and a satisfactory adiabatic temperature change (ΔTad) of 11 K, breaking through the limits of the eCE reported in existing SMAs. The precipitate-strengthened NiTi SMA fabricated in this work exhibits a low hysteresis and excellent cyclic stability, with the former attribute stemming from the strain-glass-like phase transformation and its reverse induced by the high-density Ni4Ti3 nanoprecipitates, and the latter resulting from the effective suppression of dislocation slip by the uniformly distributed high-density Ni4Ti3 nanoprecipitates. The precipitate-strengthened NiTi SMAs with a <001>{111} texture exhibit great eCE and simultaneously possess good strength and superelasticity controllability, which can maintain excellent cyclic stability even at a high stress level of 800 MPa. The precipitate-strengthened NiTi SMAs developed in this work not only hold significant advantages for solid-state refrigeration applications but also demonstrate extraordinary potential in high-precision control devices.

Enhanced Magnetocaloric Properties of the (MnNi)0.6Si0.62(FeCo)0.4Ge0.38 High-Entropy Alloy Obtained by Co Substitution.

In order to improve the magnetocaloric properties of MnNiSi-based alloys, a new type of high-entropy magnetocaloric alloy was constructed. In this work, Mn0.6Ni1-xSi0.62Fe0.4CoxGe0.38 (x = 0.4, 0.45, and 0.5) are found to exhibit magnetostructural first-order phase transitions from high-temperature Ni2In-type phases to low-temperature TiNiSi-type phases so that the alloys can achieve giant magnetocaloric effects. We investigate why chexagonal/ahexagonal (chexa/ahexa) gradually increases upon Co substitution, while phase transition temperature (Ttr) and isothermal magnetic entropy change (ΔSM) tend to gradually decrease. In particular, the x = 0.4 alloy with remarkable magnetocaloric properties is obtained by tuning Co/Ni, which shows a giant entropy change of 48.5 J∙kg-1K-1 at 309 K for 5 T and an adiabatic temperature change (ΔTad) of 8.6 K at 306.5 K. Moreover, the x = 0.55 HEA shows great hardness and compressive strength with values of 552 HV2 and 267 MPa, respectively, indicating that the mechanical properties undergo an effective enhancement. The large ΔSM and ΔTad may enable the MnNiSi-based HEAs to become a potential commercialized magnetocaloric material.

Frequency stabilization of adiabatic temperature change in Fe50Rh50 alloy in a cyclic magnetic field of 1.2 T

We present the results of direct measurements of the adiabatic temperature change (ΔTad) for the Fe50Rh50 alloy in a cyclic magnetic field (CMF) of 1.2 T. It is shown that increasing the frequency of the CMF from 1 to 30 Hz is accompanied by a shift of the position of temperature dependence ΔTad(T) maximum, Tmax, toward low temperatures. With an increase in the CMF frequency from 1 to 5 Hz, the ΔTmax value decreases by ∼12%. A further increase in frequency leads to stabilization of the effect. In the vicinity of the antiferromagnetic-ferromagnetic phase transition point TC = 370 K, ΔTad exhibits unconventional frequency behavior: while at T well above TC, the value of ΔTad monotonously decreases as frequency increases, at T = 370.4 K; an interval of frequency-independent ΔTad up to 10 Hz is observed, and at 368 K &amp;lt; T &amp;lt; TC, the maximum of ΔTad(f) dependence is found in the interval 1 &amp;lt; f &amp;lt; 10 Hz. Such behavior in the future can be applied in magnetic cooling technology due to large values of ΔTad and the frequency stability of the effect in alternating fields. The specific cooling power reaches giant values of ∼22 W/g at 20 Hz, which is comparable to the values under the same conditions for Gd −21.6 W/g. The unconventional behavior of ΔTad in the CMF is discussed in the context of the role of secondary phase localization, which leads to an enhanced internal local magnetic field and dynamic effects of ΔTad.

Exploring microstructure and caloric effects in gas-atomized Ni–Mn–Sn–Co precursor for additive manufacturing

Recently, additive manufacturing (AM) of intrinsically brittle Ni–Mn–X alloys, in which obtaining high-quality powders is imperative, has received widespread attention. Here, a batch weight of about 34 kg Ni–Mn–Sn–Co powder was prepared using industrial gas atomization equipment. To explore the sinterability of the gas-atomized powder and provide a direct comparison in caloric effects for binder-based AM techniques, we sintered Ni–Mn–Sn–Co alloy powder samples at 1173–1273 K for 2–24 h by using a powder metallurgy process with air cooling. The sintered powder samples had a relative density of about 60.3–80.1 %, and 5M martensite and L21–ordered austenite were found to coexist at room temperature. For all sintered samples, martensitic transformation temperature width of about 11–17 K and hysteresis of about 21–23 K were obtained, and strong magneto-structural coupling was observed. The magnetocaloric and elastocaloric effects of the sample sintered at 1273 K for 2 h were examined. A magnetic entropy change of about 23.0 J kg−1·K−1 and an adiabatic temperature change (ΔTad) of −0.97 K were achieved for magnetic fields of 5.0 and 1.25 T, respectively. Furthermore, a ΔTad of −4.71 K was achieved upon unloading a compressive stress of 350 MPa. These competitive caloric effects in the gas-atomized powder lay the foundation for the AM of Ni–Mn–Sn–Co alloys with complex structures. This study provides a reference for the reliable manufacturing of composition-sensitive Ni–Mn–X Heusler alloys.

Impact of dimensionality on the magnetocaloric effect in two-dimensional magnets

Magnetocaloric materials, which exploit reversible temperature changes induced by magnetic field variations, are promising for advancing energy-efficient cooling technologies. The potential integration of two-dimensional materials into magnetocaloric systems represents an emerging opportunity to enhance the magnetocaloric cooling efficiency. In this study, we use atomistic spin dynamics simulations based on first-principles parameters to systematically evaluate how magnetocaloric properties transition from three-dimensional (3D) to two-dimensional (2D) ferromagnetic materials. We find that 2D features such as reduced Curie temperature, sharper magnetic transition, and higher magnetic susceptibility are beneficial for magnetocaloric applications, while the relatively higher lattice heat capacity in 2D can compromise achievable adiabatic temperature changes. We further propose GdSi2 as a promising 2D magnetocaloric material. Our calculation predicts that GdSi2 exhibits an isothermal entropy change ΔSM of 22.5 J kg−1 K−1 and an adiabatic temperature change ΔTad of 6.2 K, near the hydrogen liquefaction temperature (TC≈25 K). Our analysis offers valuable theoretical insights into the magnetocaloric effect in 2D ferromagnets and demonstrates that 2D ferromagnets hold promise for cooling and thermal management applications in compact and miniaturized nanodevices.

Enhancing elastocaloric performance: Ti-doped Co–V-Ga bulk polycrystalline alloys with minimal stress hysteresis

Currently, practical applications of shape memory alloy (SMA) solid elastocaloric refrigeration materials are limited by low elastocaloric effect (eCE), high applied stress, and large stress hysteresis (Δσhys). Here, we introduce a composition design strategy using Ti-doped Co–V-Ga SMAs to address these challenges. We combine first-principles calculations and experiments to verify this strategy. Our results show that the lattice distortion caused by the significant difference in atomic radius after Ti doping produces a solid solution strengthening effect inside the alloy, which significantly improves the superelasticity of the alloy, delays the plastic deformation of the austenite phase, and the volume fraction of martensitic transformation increases significantly. The newly developed Co51.7V31.3Ga16Ti1 bulk polycrystalline alloy exhibits a remarkable adiabatic temperature change (ΔTad) of −10 K at room temperature under a low stress of 400 MPa, a minimum Δσhys of 18 MPa, and a high coefficient of performance COPmat of 31.9. Importantly, the 100-cycle loading-unloading test at 350 MPa is done, and it reveals that the ΔTad for the Ti0 alloy was only 3.9 K, while it reached 6.2 K for the Ti1 alloy, marking a noteworthy 59 % performance enhancement. From the simplicity of the fabrication process to the comprehensive performance of the material, it exhibits a promising advantage for practical cooling applications This study presents a feasible design strategy for large-scale production of high-performance bulk polycrystalline alloys with high eCE, low stress, and Δσhys.

A Brilliant Magnetic Refrigerant Operating Near Liquid Helium Temperature: Enhanced Magnetocaloric Effect in Ferromagnetic EuTi0.75Al0.125Zr0.125O3

AbstractRare earth‐based perovskites have become an attractive research interest in the field of cryogenic magnetic refrigerants due to their unique advantages in practical applications. The remarkable magnetocaloric effect (MCE) renders EuTiO3 a potential magnetic refrigerant in the liquid helium temperature range. More impressively, the tunability between antiferromagnetism (AFM) and ferromagnetism (FM) provides the feasibility of tailoring the magnetism and enhancing the magnetocaloric performance. In this study, the magnetism of EuTi0.75Al0.125Zr0.125O3 is investigated in depth through first‐principles calculations and experimental methods. Both theoretical calculations and experimental results reveal that it exhibits significant ferromagnetism due to the AFM‐FM magnetic transition promoted by the co‐substitution of Al and Zr. Lattice expansion and altered electronic interactions are responsible for the FM behavior, which leads to a significant enhancement of the MCE. With the field change of 0−1 T, the peak values of magnetic entropy change (−ΔSM), refrigerating capacity (RC), and adiabatic temperature change (ΔTad) reach 18.9 J kg−1 K−1, 77.7 J kg−1, and 7.4 K, respectively. More surprisingly, the values of maximum magnetic entropy change () and maximum adiabatic temperature change for EuTi0.75Al0.125Zr0.125O3 reach 11.4 J kg−1 K−1 and 3.7 K under the field change of 0−0.5 T, respectively. The remarkable magnetocaloric performance proves it to be a brilliant magnetic refrigerant operating near liquid helium temperature.

Low-fatigue large elastocaloric effect in NiTi shape memory alloy enabled by two-step transition

NiTi shape memory alloys are promising elastocaloric materials owing to their substantial adiabatic temperature change (ΔTad). However, the simultaneous attainment of large ΔTad and low fatigue poses challenges due to the significant hysteresis and severe functional fatigue associated with the autocatalytic avalanche-like martensitic transformation. This study demonstrates a continuous two-step transition in Ni50.8Ti49.2 (at.%), showcasing cyclic-stable superelasticity with large recoverable strain (5.9 %), substantial ΔTad (19.1 K) and high coefficient of performance. In-situ loading analysis indicates a stress-induced continuous transition from the B2 to R and subsequently to B19′. Nanoscale lattice analysis exposes heterogeneous strain network, harboring metastable R phase preceding the B19′ phase. By exploiting differences in critical stress and transformation potential between R and B19′ phases, this study demonstrates the possibilities to synergistically integrate functional performances of different martensitic phases in NiTi into a sequential and continuous two-step transition to provide controlled strain release with unprecedented properties.

Exploring magnetocaloric effect of coordination polymer based on Mn(II) and Nb(IV) by relaxation calorimetry

Magnetocaloric properties of {MnII2(imH)2(H2O)4 [NbIV(CN)8]·4H2O}n molecular magnet are reported. The coordination polymer crystallizes in the monoclinic P21/c space group and displays a magnetic second-order phase transition at Tc ≈ 24.1 K. The relaxation calorimetry measurements are analyzed in order to determine the temperature dependence of the magnetic entropy change ΔSM as well as the adiabatic temperature change ΔTad for an array of applied field changes ranging from 0 to 9 T. For μ0ΔH = 5 T the entropy change ΔSM attains the maximum value of 6.55 J K−1 mol−1 (9.5 J K−1 kg−1) at 25.5 K, while the corresponding maximum value of ΔTad amounts to 2.02 K at 25.1 K. The critical behavior of the isothermal entropy change is discussed. In particular, the analysis of the field dependence of ΔSM in terms of the power law implies that the studied compound belongs to the mean-field universality class.

Phase-field simulation and machine learning of low-field magneto-elastocaloric effect in a multiferroic composite

Achieving appreciable elastocaloric effect under low external field is critical for solid-state cooling technology. Here, a non-isothermal Phase-Field Model (PFM) coupling martensitic transformation with mechanics, heat transfer and magnetostrictive behavior is proposed to simulate Magneto-elastoCaloric Effect (M-eCE) that is induced by magnetic field in a multiferroic composite (e.g., Magnetostrictive-Shape Memory Alloys (MEA-SMA) composite). In the PFM, a nonlinear constitutive hyperbolic tangent model is utilized to model the macroscopic magnetostrictive behavior of MEA, and the heat transfer coupled with phase transformation is employed to calculate the adiabatic temperature change (ΔTad) during M-eC cooling cycles. The influences of magnetic field, geometrical dimension, and ambient temperature on ΔTad are comprehensively investigated. Machine Learning (ML) is further conducted on the database from PFM simulations to accelerate the prediction and design of MEA-SMA composite with an improved ΔTad. It is found that a large ΔTad of 10–14 K and a wide working temperature window of 30 K can be achieved under ultra-low magnetic field of 0.15–0.38 T by optimizing the composite’s geometrical dimension. The present work combining PFM and ML for evaluating M-eCE provides a theoretical framework for the optimization of M-eC cooling devices, and is also potentially extended to other multicaloric effects (e.g., electro-elastocaloric effect).

Sublimed fine-grained dysprosium: Significant magnetocaloric effect

As is known, rare-earth metals (REMs) are promising magnetocaloric materials. The magnitude of the magnetocaloric effect (MCE) of REMs significantly depends on their purity. This paper presents results of studies of the magnetic and magnetocaloric properties of sublimed dysprosium, prepared in the course of the present study, with an emphasis on its impurity and structure perfection. The comprehensive analysis of the chemical composition of sublimed dysprosium, which was performed for the first time by atom probe tomography, showed that the metal corresponds to high-purity rare-earth metals (3 N+). The MCE effect was studied using direct measurements of the adiabatic temperature change (ΔTad) in pulsed (up to 50 T) and steady (up to 14 T) magnetic fields. The studies of the MCE of polycrystalline sublimed Dy by direct method showed that the high ΔTad value for sublimed Dy are comparable with those obtained for single-crystal Dy in magnetic fields up to 5 T. The vacuum sublimation, which is more economical and technologically advanced in contrast to single crystal growing, can be used to create magnetocaloric REM-based materials with high MCE values.

Pressure dependence of phase transformation, thermal expansion and barocaloric property in a polycrystalline Ni54Mn23Ga23 alloy

An experimental investigation was conducted to understand the influence of hydrostatic pressure on phase transition, thermal expansion and barocaloric property of a polycrystalline Ni54Mn23Ga23 alloy. Two pressure ranges of 0–1.28 GPa and 0–0.74 GPa were selected for thermal expansion and barocaloric property measurements due to the different test limits of instrumentals. The pressure dependence of fourteen parameters was studied systematically. It was found that the sample experiences a martensitic transformation (MT) near room temperature and the response of MT peak temperature (TM) to pressure is about 15.89 K/GPa. The phase transition strain (λtr), transformation width (ΔTtr) relative volume change (|ΔV/V|) and maximum adiabatic temperature change (ΔTad)max increase linearly with pressure and their shifting rates are about 0.071 %/GPa, 7.5 K/GPa, 0.211 %/GPa and 12.2 K/GPa, respectively. Simultaneously, applying hydrostatic pressure can also enhance the barocaloric property of the sample, leading to a large increase in the absolute value of the maximum entropy change (|ΔS|max), the refrigerating capacity (RC) and their reversible values. On cooling, the sensitivities of the reversible RC and its proportion to pressure are around 185.52 J/kg GPa and 16.7 %/GPa, respectively. Finally, the average linear expansion coefficient (αave) of both martensite and austenite phases keeps unchanged while in the MT region it decreases from -99.84×10–6/K to -86.04×10–6/K with increasing pressure from 0 to 1.28 GPa. These findings indicate that applying hydrostatic pressure can effectively adjust the phase transformation, thermal expansion and barocaloric property of Ni-Mn-Ga alloys, which are meaningful for their application in solid-state refrigeration.

Magnetothermal properties including magnetocaloric performance of the ternary rhombohedral Laves phase of Pr2Rh3Ge

The article presents the study of the magnetocaloric effect (MCE) for the rhombohedral Laves phase of Pr2Rh3Ge, which shows a magnetic order below TC=8.5 K. We have established that the compound exhibits a continuous second-order type of transition which was demonstrated and confirmed by several different techniques, mainly by analyzing universal curves of normalized entropy change as a function of scaled temperature. The observed MCE, in our opinion, is a consequence of an indirect exchange coupling between the magnetic sublattices of the rare earth ions, which, however, does not exclude the potential contribution of sublattice-containing transition metals. In this paper, the procedure to evaluate the MCE from magnetization and specific heat data is described. As a result, important parameters such as the isothermal magnetic entropy change (ΔSM), adiabatic temperature change (ΔTad), relative cooling power (RCP), and the temperature averaged entropy change (TEC) were determined. The highest values of −ΔSM, ΔTad, and RCP for a field change (Δμ0H) of 5 T at TC are 5.96 J/kgK, 3.87 K, and 72.62 J/kg, respectively. These results obtained for Pr2Rh3Ge seem to be, however, low compared to the values obtained for the rhombohedral Laves phases, belonging to the group of ternary germanides RE2Rh3Ge containing heavy rare earth metals (RE = Gd, Tb, Ho, and Er). Nevertheless, we believe that the results presented in this work extend and complement the current knowledge on the magnetocaloric properties of this family of materials.

Colossal barocaloric effect of the spin-crossover compound {Fe(pz)2(BH3CN)2} near room temperature

As one of the most likely alternatives to traditional vapor compression refrigeration technology, solid refrigeration technology based on the barocaloric effect (BCE) has attracted extensive attention in recent years. Spin-crossover (SCO) compounds are considered suitable for working at low driving pressures due to high-pressure sensitivity and small hysteresis width. However, the entropy change (ΔSSCO) of the SCO compound is smaller than that of other excellent barocaloric materials (plastic crystals and two-dimensional perovskites). Here, we report the BCE of the SCO compound {Fe(pz)2(BH3CN)2} (pz = pyrazine) with a smaller molar mass and a third source of entropy change besides electron and vibrational entropy changes. Compound {Fe(pz)2(BH3CN)2} exhibits high pressure sensitivity (dT1/2dP= 20.2 K kbar−1) as well as entropy change (ΔSSCO= 202 J kg−1 K−1). The maximum values of reversible isothermal entropy change (ΔSit,rev,max) and adiabatic temperature change (ΔTad,rev,max) at 1 kbar are only 103 J kg−1 K−1 and ∼0 K, respectively, due to the hysteresis behavior. However, at sufficiently high driving pressures, ΔSit,rev,max exceeds 200 J kg−1 K−1, and ΔTad,rev,max can reach ∼47 K, which exceeds all SCO compounds reported in BCE studies and is comparable to some plastic-crystalline and two-dimensional perovskite barocaloric materials. The excellent BCE of the SCO compound {Fe(pz)2(BH3CN)2} is mainly due to its small molar mass, which makes the unit mass compound exhibit higher ΔSSCO, while the introduction of the third source of entropy change—the reorientation entropy change (ΔSreo), only plays a small role. This is expected to promote the practical application of SCO compounds as barocaloric refrigerants.

Magnetocaloric effect in Gd1-xCexNi (x = 0–0.6): A cost-effective approach to tuning Curie temperature from 70 K to 30 K

Magnetic refrigeration is a promising alternative to traditional gas compression refrigeration technologies, as it offers significantly higher energy efficiency. One factor hindering the widespread implementation of magnetic refrigeration is the dependence on expensive and critical rare earth elements needed to produce a strong magnetocaloric effect. Inside this work we explore substituting a non-critical rare earth element, Ce, into an established magnetocaloric material of choice, GdNi. Structural characteristics are investigated via x-ray powder diffraction, and the magnetocaloric effect is evaluated indirectly from heat capacity measurements as well as calculated using the mean field method. Our results show that small substitutions of Ce into GdNi results in the dilution of the magnetocaloric effect while maintaining relative cost effectiveness for small substitutions of Ce. As Ce content is increased, the cost effectiveness of the adiabatic temperature change (ΔTad) increases as high as 20% for (Gd0.4Ce0.6)Ni while the magnetic entropy change (-ΔSm) shows minimal improvement in cost effectiveness, falling from 3.93 J/kg*K in GdNi to 1.75 J/kg*K in (Gd0.4Ce0.6)Ni at 1 T applied field. This demonstrates that using Ce substitutions can be a powerful tool in both tuning Curie temperature and mitigating the reliance on critical elements as we look to develop towards a more sustainable future.

Excellent elastocaloric and mechanical performance in Mn-doped Co–V-Ga as-cast polycrystalline alloy with simple preparation

With the rapid development of solid-state refrigeration technology, elastocaloric materials face higher requirements, such as a higher elastocaloric effect (eCE), better mechanical properties, and simpler and faster large-scale preparation. In this work, we propose a composition design strategy for Mn-doped Co–V-Ga shape memory alloys, which can enhance both the eCE and the mechanical properties. We combine first-principles calculations and experiments to demonstrate this strategy. Our results reveal that the lattice distortion caused by the difference in atomic radii produces a solid-solution-strengthening effect within the alloy, impeding dislocation motion and significantly improving the mechanical properties. Through composition optimization, the low-cost Co51.7V31.3Ga14.75Mn2.25 as-cast polycrystalline alloy exhibits a large adiabatic temperature change (ΔTad) of −11.2 K at room temperature, while its maximum compressive strength and strain can reach up to 1859 MPa and 26% at room temperature, respectively. Importantly, the alloy exhibits excellent cycle stability, and its ΔTad does not decay after 100 loading and unloading cycles, indicating that it has the potential for practical application. Further comparison with the annealed sample shows that the secondary defects deteriorate the ΔTad of the alloy. Our research provides an effective strategy for the simple, rapid and large-scale production of as-cast eCE alloys.

Giant irreversibility of the inverse magnetocaloric effect in the Ni47Mn40Sn12.5Cu0.5 Heusler alloy

Direct studies of the adiabatic temperature change (ΔTad) in the Ni47Mn40Sn12.5Cu0.5 Heusler alloy in steady magnetic fields up to 8 T by the extraction method and in pulsed magnetic fields up to 50 T were carried out in this paper. The alloy Ni47Mn40Sn12.5Cu0.5 demonstrates a magnetostructural phase transition (MSPT) of the first order in the 254–283 K temperature range as well as a second order phase transition near the Curie temperature TC = 313 K. An inverse magnetocaloric effect (MCE) was found in the region of the MSPT, and it reaches the maximum value ΔTad = −12 K in 20 T at the initial temperature T0 = 275 K. The irreversible part of the MCE reached ΔTir = −10 K when the field is completely removed. We consider the dynamics of the MCE in the vicinity of the MSPT and discuss the mechanisms that cause the giant irreversibility of the MCE as well as the possibilities of its application in hybrid cooling systems.

Fatigue-resistant elastocaloric effect in hypoeutectic TiNi58 alloy with heterogeneous microstructure

Shape memory alloys with high fatigue life and large adiabatic temperature change (ΔTad) are critical for elastocaloric effect in practical solid-state refrigeration applications. Here, we report on the discovery of a hypoeutectic TiNi58 alloy by heterogeneous microstructure design to perform ΔTad of 6.1 K after 2 million fatigue loading-unloading cycles. The fatigue-resistant elastocaloric effect is achieved by a highly reversible quasi-linear superelasticity with strength of 2.2 GPa and modulus of 44.9 GPa. Such stable mechanical behavior is attributed to the prepared unique heterogeneous microstructure, as characterized by multi-scale microscopic analysis. The heterogeneous microstructure is consisted of dendrite regions with Ti2Ni3 precipitates and the surrounding eutectic structure with lath TiNi3 phase. The lath intermetallic TiNi3 phase surrounding dendrite regions works as skeletal reinforcement, providing extra mechanical fatigue stability and giving rise to high strength. The cubic structured Ti2Ni3 precipitates produce local lattice strain due to lattice mismatch within the surrounding B2 matrix with near equiatomic composition. Detailed phase-field simulation based on the local strain field distribution in B2 matrix reveals a continuous transition behavior of martensite under external loads distinct from conventional stress-induced violent autocatalytic martensitic transformation, which is the origin of controlled strain release in quasi-linear superelasticity with ultrahigh fatigue stability. This work demonstrates the potential of hypoeutectic TiNi58 alloy for long-term elastocaloric service and provides guidance to design new elastocaloric materials with high fatigue resistance in bulk mass through simple controlled casting process.

Magnetocaloric properties of TbCrO3 and TmCrO3 and their comparison with those of the other RCrO3 systems (R = Gd, Dy, Ho, and Er)

Magnetocaloric properties of TbCrO3 and TmCrO3 are reported and compared with those of the previously reported rare-earth chromites RCrO3 (R = Gd, Dy, Ho, and Er) and other perovskite-type oxides. The samples of TbCrO3 and TmCrO3 in this work were synthesized using a citrate gel combustion technique, and their magnetic properties were investigated and compared with those reported previously on RCrO3 (R = Gd, Dy, Ho, and Er). The Cr3+–Cr3+ ordering temperatures were found to strongly depend on the ionic radii of the rare-earth. By fitting the dc magnetization data with modified Curie–Weiss law including the Dzyaloshinsky–Moriya antisymmetric exchange interaction (D) and the symmetric exchange constant Je, spin canting angles (α) were obtained. In general, α was found to increase with the decreasing ionic radii of R3+ in RCrO3. The magnetocaloric properties investigated included the magnetic entropy change (−ΔS) for a given change in magnetic field (ΔH), the corresponding adiabatic temperature change (ΔTad), and their relative variations (ΔTad/ΔH) and (−ΔS/ΔH). It is observed that for RCrO3, (−ΔS) measured in the vicinity of the ordering temperature of R3+–R3+, varies almost as G2/3 where G is the de Gennes factor. Among RCrO3, GdCrO3 shows the largest value of (−ΔS/ΔH), because of its largest G factor and its magnitudes of (ΔTad/ΔH) and (−ΔS/ΔH) compare well with the reported values for the perovskites GdFeO3 and EuTiO3. These comparisons presented here provide useful information on the potential use of these materials in magneto-refrigeration technology.

A comparison between different materials with elastocaloric effect for a rotary cooling prototype

Solid-state refrigeration based on the caloric effects has recently received widespread consideration because of its high efficiency and environmental protection. Compared with other solid-state refrigeration technologies, the elastocaloric effect-based one has great application potential. The elastoCaloric Effect (eCE) is defined as the reversible adiabatic temperature change (ΔTad) and the reversible isothermal entropy change (ΔST) in Shape Memory Alloys (SMA), following a variation in the mechanical field applied. To date, the elastocaloric materials, considered as benchmark, since offering the best compromise between thermal and mechanical performances are represented by the NiTi binary alloys.In this paper, a comparison among elastocaloric materials was carried out to identify the best one to be used in an experimental device for air conditioning based on the elastocaloric effect. The device is a rotary that ensures continuous flows of hot or cold air and it lodges the elastocaloric material in the form of 600 wires (diameter 0.5 mm and length 300 mm) for a total mass of 230 g. The model can reproduce the thermofluido-dynamic behavior of the air flowing in the device exchanging heat with the wires of elastocaloric material. For a correct dimensioning of the prototype, a two-dimensional model was built to simulate the real rotation of the device. Before building the prototype, an energy performance analysis was conducted for various elastocaloric materials to identify the most promising one. The materials used for the comparison are: Ni50.8Ti49.2, Ni55.9Ti44.1, Ni45Ti47.2Cu5V2.75, (Ni50Mn31.5Ti18.5)99.8B0.2 and PbTiO3.The innovations contained in this article are: i) the design of a device for environmental conditioning that can have competitive performance respect to traditional systems; ii) the presentation of a 2D model able to simulate the real rotation of the device; iii) the construction of an energy performance map for various elastocaloric materials to identify the most promising one.The numerical test campaign has been carried out at a fixed cycle frequency (0.5 Hz) varying the air velocity in the range 3.0–20.0 m s−1. The results reveal very encouraging energy performances, attributing the prototype as suitable for the macro-scale application: (Ni50Mn31.5Ti18.5)99.8B0.2 and Ni45Ti47.2Cu5V2.75 ensure the achievement of a greater than 30 K temperature span, with a peak of 38.6 K for the former material while air flows at 3 m s−1. Both the materials allow the overcoming of the 1KW thresholds for cooling power, even if (Ni50Mn31.5Ti18.5)99.8B0.2 carries the device up to 6.66 as Coefficient Of Performance (COP) at 12 m s−1, corresponding to 86% as a second-law efficiency.