Adiabatic Temperature Change ΔTad Research Articles

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.

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 0-1.28GPa and 0-0.74GPa 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 15.89K/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, 7.5K/GPa, 0.211%/GPa and 12.2K/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 185.52J/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 1.28GPa. 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.

Effect of aspect ratio on the elastocaloric effect and its cyclic stability of nanocrystalline NiTi shape memory alloy

NiTi shape memory alloy (SMA) has been suggested as an environmentally friendly solid-state refrigerant that is expected to replace the traditional vapor compression technology. However, its various refrigeration indicators still need to be further explored and enhanced. Nanocrystalline NiTi SMA has significant advantages in terms of elastocaloric effect (eCE) and cyclic stability compared with traditional coarse-grained NiTi SMA. In this work, the influence of aspect ratio (L/W, where L and W are the gauge length and width of the samples, respectively) on the eCE and its cyclic stability of nanocrystalline NiTi SMA was investigated experimentally. The in-situ cyclic tensile tests were performed to observe the axial strain field and temperature field on the surface of the specimens with different aspect ratios. Some microscopic observations were also carried out to clarify the internal mechanism. The experimental results show that as the aspect ratio decreases, the adiabatic temperature change ΔTad (elasocaloric cooling capacity) of the nanocrystalline NiTi SMA gradually decreases, while the coefficient of the performance of material (COPmat) increases, and the cyclic stability of eCE was enhanced. However, the dependence of eCE and its cyclic stability on the aspect ratio is gradually weakened when L/W > 4. The eCE and its cyclic stability of nanocrystalline NiTi SMA are not related to the dislocation slipping but rather attributed to the cyclically accumulated residual martensite. This work can provide valuable reference for designing efficient and reliable nanocrystalline NiTi SMA based solid-state refrigeration devices.

Observation of superior magnetocaloric performance in multicomponent (Ce0.71Pr0.07Nd0.22)2Fe17-xAlx (x = 0.6, 0.8) compounds

Magnetocaloric effect (MCE) in multicomponent (Ce0.71Pr0.07Nd0.22)2Fe17-xAlx (x = 0.6, 0.8) alloys was studied using the heat capacity and magnetization data. Both the samples crystallize in a rhombohedral Th2Zn17-type structure. The maximum values of magnetic entropy change (-ΔSM) without hysteresis loss are found to be 1.19 Jkg−1K−1 and 1.23 Jkg−1K−1 with relative cooling power (RCP) values of 52.3 Jkg−1 and 66.4 Jkg−1 under an applied field change of 0–1 T for x = 0.6 and 0.8, respectively. Meanwhile, a wide working temperature range (44/54 K) and a large adiabatic temperature change (ΔTad) of 5.6/5.8 K were also achieved for the present compounds.

Novel Class of Additive Manufactured NiTi-Based Hierarchically Graded Chiral Structure with Low-force Compressive Actuation for Elastocaloric Heat Pumps

Elastocaloric refrigeration is the most promising green solid-state refrigeration technology to replace conventional vapor compression refrigeration. The development direction of the elastocaloric component that acts as a key part of the elastocaloric refrigeration system contains a large elastocaloric effect, low stress hysteresis, high heat exchange performance, and small driving loads. The first two indices can be realized by material modification; however, the last two are more dependent on a novel porous structure design. However, the conventional porous structure is confronted with some critical challenges, including inhomogeneous stress, a significant hysteresis area, and deformation instability under the alternating cyclic loading. In this study, a NiTi-based elastocaloric structure model with chirality feature and gradient design as innovative elements was presented, bio-inspired by the structure of the plant tendrils. A quantitative optimization for the NiTi-based elastocaloric structure was performed using the finite element analysis (FEA) method. Strain and martensite volume fraction (MVF) fields during the loading and unloading processes were predicted and evaluated. The simulated results indicated that increasing the thickness gradient g1 of the strip or decreasing the diameter gradient g2 of the structure was beneficial to achieving more homogeneous strain and martensite distribution, simultaneously with higher energy storage efficiency and specific surface area. In addition, these NiTi-based chiral structures with different structural parameters were fabricated by laser powder bed fusion (LPBF). At the optimized structure parameters of g1 = 2 and g2 = 1.11, the LPBF-fabricated NiTi-based chiral structure could achieve an adiabatic temperature change ΔTad of 2.3 K, driving force of as low as 149.11 N, and |ΔTad/F| of as high as 15.42 K/kN at a recoverable compressive strain of 10%.

Reactive single-step hot-pressing and magnetocaloric performance of polycrystalline Fe2Al1.15−xB2GexGax (x = 0, 0.05) MAB phases

Reactive single-step hot-pressing at 1473 K and 35 MPa for 4 h produces dense, bulk, near single-phase, low-cost, and low-criticality Fe2Al1.15B2 and Fe2Al1.1B2Ge0.05Ga0.05 MAB samples, showing second-order magnetic phase transition with favorable magnetocaloric properties around room temperature. The magnetic as well as the magnetocaloric properties can be tailored upon Ge and Ga doping, leading to an increase in the Curie temperature TC and the spontaneous magnetization mS. The maximum isothermal entropy change ΔsT,max of hot-pressed Fe2Al1.15B2 in magnetic field changes of 2 and 5 T amounts to 2.5 and 5 J(kgK)−1 at 287.5 K and increases by Ge and Ga addition to 3.1 and 6.2 J(kgK)−1 at 306.5 K, respectively. The directly measured maximum adiabatic temperature change ΔTad,max is improved by composition modification from 0.9 to 1.1 K in magnetic field changes of 1.93 T. Overall, we demonstrate that hot-pressing provides a much faster, more scalable, and processing costs reducing alternative compared to conventional synthesis routes to produce heat exchangers for magnetic cooling devices. Therefore, our criticality assessment shows that hot-pressed Fe-based MAB phases provide a promising compromise of material and processing costs, criticality, and magnetocaloric performance, demonstrating the potential for low-cost and low-criticality magnetocaloric applications around room temperature.

Magnetothermal Properties and Magnetocaloric Effect in the 3d Ferromagnetic Elements: Fe, Co, and Ni

We present a calculation of magnetothermal properties and magnetocaloric effect (MCE) for the ferromagnetic elements: Fe, Co, and Ni. In particular, we calculated the temperature and field dependences of magnetization, heat capacity, entropy, isothermal entropy change Delta {S}_{m}, adiabatic temperature change ΔTad, and the two figures-of-merit: the relative cooling powers RCP(S) and RCP(T). We have used the mean-field theory in calculating the magnetization, magnetic heat capacity, and magnetic entropy. The lattice and electronic contributions to the total heat capacity and entropy were calculated using standard relations to subsequently calculate Delta {T}_{ad}. Those contributions depend on the Debye temperature ϴD and the coefficient of the electronic heat capacity γe respectively. The Maxwell relation is used to calculate Delta {S}_{m} and Delta {T}_{ad}. As an example of our results, the maximum Delta {S}_{m} for the three elements, in 6 T, is between 0.17 to 0.36 J/mol K and the maximum Delta {T}_{ad} mathrm{} is between 0.46 to 1.5 K/T for the same field change. The relative cooling power RCP(S) is in the 15–36 J/mol range for the three elements in a 6 T field. Also, the relative cooling power, RCP (T), is in the 162–1044 {K}^{2} range for the same field. For Fe and Co the RCP (T) per Tesla values, i.e., 139 and 174 {K}^{2}/T respectively are comparable to that of Gd and other Gd-based magnetocaloric materials. The behavior of the magnetization, magnetic heat capacity, and magnetic entropy shows that the phase transition in these three elements is of the second order. The universal curve and Arrott plots further support this conclusion.

Magnetocaloric properties of La0.9Pr0.1Fe11.2Co0.7Si1.1 compound through direct measurements under cyclic magnetic fields up to 30 Hz

Direct measurements of the adiabatic temperature change (ΔTad) in cyclic magnetic fields with a frequency of up to 30 Hz in the La0.9Pr0.1Fe11.2Co0.7Si1.1 alloy were carried out. For this alloy, the magnetocaloric effect (MCE) under magnetic field change of 1.8 T is 2.8 K and the temperature hysteresis does not exceed 0.5 K. Under magnetic field change of 0.62 T with frequencies of up to 20 Hz, the MCE shows a weak frequency dependence (less than 5%), while under magnetic field change of 1.2 T the frequency dependence is clear pronounced. Long-term exposure to a low frequency cyclic magnetic field does not result in a change in the MCE value, while at frequencies more than 10 Hz noticeable degradation of the MCE is observed. The results of the study show that the upper limit of the operation of magnetic refrigerators can reach several tens of Hz.

Magnetocaloric Effect in R6Fe23: R = Dy, Ho, Er, and Tm

We present a mean field study on the R6Fe23 system, where R = Dy, Ho, Er, and Tm, to calculate the magnetization, magnetic heat capacity, and the magnetocaloric effect (MCE) (isothermal entropy change (ΔSm) and the adiabatic temperature change (ΔTad)) for different field changes up to 5 T and at temperatures ranging from 0 to 600 K. The maximum ΔSm, using the trapezoidal method, for the R6Fe23 system is in the range 4.9–9.8 J/K mol, and the maximum ΔTad is in the range 9.56–15.17 K for a field change ΔH = 5 T. The largest ΔSm and largest ΔTad are found for Tm6Fe23 to be 9.8 J/K mol and 15.17 K at Curie temperature Tc = 489 K, for ΔH = 5 T. The relative cooling power RCP(S) is in the range 148–560 J/mol for ΔH = 5 T, which is comparable to that of bench-mark materials, e.g., Gd. Also, the RCP based on the adiabatic temperature change, RCP(T) is in the range 449–1092 K2 for ΔH = 5 T, which is comparable also to that of bench-mark materials, e.g., Gd. We investigated the type of phase transition in the light of universal curves, Arrott plots, and the behavior of the magnetic moment, magnetic heat capacity, and MCE (ΔSm, ΔTad), which confirm that the type of phase transition at Tc of this system is second-order phase transition (SOPT). A calculation of some critical exponents adds more evidence that the MFT is fairly suitable to handle the aforementioned properties in the studied systems.

Ultrahigh cyclic stability and giant elastocaloric effect in directionally solidified (Ni50Mn28Fe2.5Ti19.5)99.4B0.6 alloy

Cyclic stability of elastocaloric materials is critically important for practical applications in solid-state refrigeration. Ni-Mn-based Heusler-type alloys exhibit a large elastocaloric response to low driving stress. However, these alloys can only be worked for short-term cyclic operation due to their intrinsic brittleness. Here, we demonstrate the achievement of giant elastocaloric effect (eCE) and long-term fatigue performance in a directional solidification (Ni50Mn28Fe2.5Ti19.5)99.4B0.6 alloy. At 303 K, the maximum compressive strength and strain of the alloy can be as high as 2734 MPa and 20.5%, respectively. During fast loading and unloading with the stress of 435 MPa and the strain of 8%, giant adiabatic temperature change (ΔTad) of 25.1 K and -24.5 K can be obtained, respectively. In particular, a large ΔTad of about 5.8 K is generated during cycling at 360 MPa stress and 3.5% strain, and can be maintained for more than 20,000 cycles.

Improvement of mechanical properties and elastocaloric effect in Ag doped Ni-Mn-In magnetic shape memory alloys

Mechanical properties and elastocaloric effect are the most important parameters of Ni-Mn-In shape memory alloys in the refrigeration field. This paper mainly studies the influence of Ag doping and different preparation methods on the mechanical properties and elastocaloric effect of the alloys. The doping of Ag element not only increases the martensitic transformation temperatures and reduces the critical phase transformation stress, but also improves the mechanical properties and elastocaloric effect of the alloys. The adiabatic temperature change (ΔTad) of Ni50Mn34In15.5Ag0.5 prepared by arc melting was − 7 K at 4% strain, and the alloy fractured after 69 cycles with 3% strain. Furthermore, Ni50Mn34In15.5Ag0.5 powders prepared by gas-atomization were used by SPS sintered to obtain 10 µm grains alloys. This preparation method enables the fracture stress and fracture strain to reach 1625.8 MPa and 17.1%, respectively. At the same time it remains stable after 300 cycles with 6.5% strain, indicating the significant improvement of mechanical properties and cyclic stability in Ni-Mn-In metamagnetic shape memory alloy.