Adiabatic Temperature Change ΔTad Research Articles

Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications

AbstractMagnetic refrigeration relies on a substantial entropy change in a magnetocaloric material when a magnetic field is applied. Such entropy changes are present at first‐order magnetostructural transitions around a specific temperature at which the applied magnetic field induces a magnetostructural phase transition and causes a conventional or inverse magnetocaloric effect (MCE). First‐order magnetostructural transitions show large effects, but involve transitional hysteresis, which is a loss source that hinders the reversibility of the adiabatic temperature change ΔTad. However, reversibility is required for the efficient operation of the heat pump. Thus, it is the mastering of that hysteresis that is the key challenge to advance magnetocaloric materials. We review the origin of the large MCE and of the hysteresis in the most promising first‐order magnetocaloric materials such as Ni–Mn‐based Heusler alloys, FeRh, La(FeSi)13‐based compounds, Mn3GaC antiperovskites, and Fe2P compounds. We discuss the microscopic contributions of the entropy change, the magnetic interactions, the effect of hysteresis on the reversible MCE, and the size‐ and time‐dependence of the MCE at magnetostructural transitions.

Direct measurement of the magnetocaloric effect in MnZnSb intermetalic compound

The equiatomic intermetallic alloy MnZnSb with a tetragonal Cu2Sb-type crystal structure (space group P4/nmm) was melted in the resistance furnace in the evacuated quartz ampoule. The adiabatic temperature change (ΔTad) and the isothermal variation of the magnetic entropy (ΔSM) of the MnZnSb compound near to the magnetic phase transition were studied. The ΔTad in a magnetic field up to 1.25 T was studied by the direct method. It was found that temperature dependencies of both ΔTad and ΔSM show a sharp peak near room temperature with a maximum at Curie temperature TC = 317 K. It was shown that there is no temperature hysteresis of the ΔTad in MnZnSb, and the maximum of ΔTad when the compound is heated or cooled is detected at the same temperature. The estimated value of ΔTad in a field of 14 T is 4.5 K. It was shown that near the Curie temperature, the field dependence of the maximum of magnetic entropy change is adequately described by the thermodynamic Landau theory for magnetic second-order phase transitions.

Specific heat in magnetic field and magnetocaloric effects of α-R2S3 (R = Tb, Dy) single crystals

The magnetocaloric effects (MCE) of α-Tb2S3 and α-Dy2S3 single crystals exhibiting successive antiferromagnetic (AFM) transitions have been investigated by analyzing specific heat measured in magnetic field. The temperature dependence of specific heat in the vicinity of the successive transitions shows obvious distinction depending on the orientations of the applied magnetic field for both α-Tb2S3 and α-Dy2S3 that having orthorhombic crystal structures. When the magnetic field is increased, the specific heat is as follows: For α-Tb2S3 in H‖b, the peak around TN2 shifts to lower temperature but the other one peak around TN1 barely moves; In H⊥b, the peak around TN2 has no shift almost within 3 T but suddenly moves to lower temperature in 4 T and the other one peak around TN1 shifts to lower temperature in specific heat versus temperature. In the case of α-Dy2S3, the two peaks around TN2 and TN1 shift to lower temperatures in H‖b but move to higher temperatures when the magnetic field is increased up to 5 T by H⊥b in spite of antiferromagnetic transitions. Therefore, the maximum value and corresponding temperature of both isothermal magnetic entropy change (ΔSm) and adiabatic temperature change (ΔTad) in the magnetic field H⊥b are extremely different in low temperature range from that in the field of H‖b. The results propone that the MCE of α-Tb2S3 and α-Dy2S3 could be controlled at low temperature by the magnitude and orientation of magnetic field. It also indicates that the refrigerating capacity and thermal absorption capacity will be controlled by changing magnitude and orientation of magnetic field on the α-Tb2S3 and α-Dy2S3 single crystals.

Persistent values of magnetocaloric effect in the multicomponent Laves phase compounds with varied composition

In this work we investigate the magnetocaloric effect (MCE) in the multicomponent RCo2-type compounds with a Laves phase structure varied by substitutions within both the rare earth (R) and Co sublattices. Complex substitutions increase dramatically the Curie temperatures at which the maximum MCE is observed. The Tbx(Dy0.5Ho0.5)1-xCo2 (0 ≤ x ≤ 0.5) compounds have transitions of the first order at the Curie temperature (TC) and exhibit a large magnetocaloric effect (with the adiabatic temperature change ΔTad as a measure of the effect) decreasing linearly from ΔTad = 3.6 K to 2.2 K (at μ0ΔH = 1.8 T) as x varies between 0 and 0.5. At the increasing Tb concentrations x above 0.5, the transition at TC in Tbx(Dy0.5Ho0.5)1-xCo2 changes order from first to the second while MCE remains constant ∼2.2 K. Volume magnetostriction is of the order of 1000 ppm in Tbx(Dy0.5Ho0.5)1-xCo2 in the vicinity of TC. Sets of compounds Tbx(Dy0.5Ho0.5)1-xCo1.75T0.25 (T = Fe, Al) having equal MCE values, appreciable magnetostrictive strains and continuously increased TC are found.

Influence of particle size on the mechanical properties and magnetocaloric effect of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn composites

The particle size dependence of the mechanical properties and the magnetocaloric effect (MCE) in La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn composites were studied. The compressive strength (σbc) was in the range of 180–200 MPa for composites with particle sizes less than 180 μm, which is much higher than the compressive strength of larger size powders (136 MPa). When the particles were larger than 45 μm, the observed maximum magnetic entropy change (−ΔSM)max of 7.66–7.99 J/(kg⋅K) shows that surface/interface anisotropy effects have a negligible impact on MCE. The adiabatic temperature change (ΔTad) increased from 1.74 K@1.4 T, for particles in the size range of 0–45 μm, to 1.91 K@1.4 T for particles in the size range of 45–100 μm. The ΔTad was in the range of ∼2.0 K@1.4 T when the particle size increased from 100 to 250 μm. Magnetic hysteresis in these second-order phase transition alloys showed negligible change in the particle size range of 0–250 μm. These results are useful of La(Fe,Si)13-based compounds for magnetocaloric applications.

Specific heat and the influence of hydrostatic pressure on the phase transitions in Ni50Mn35In14.25B0.75

The magnetic properties, magnetocaloric effects, and martensitic transition in Ni50Mn35In14.25B0.75 have been investigated using specific heat and magnetic measurements as a function of hydrostatic pressure. A shift in the martensitic transition temperature (TM) toward room temperature (by 35 K to higher temperature) was observed with the application of pressure (P = 0.54 GPa). The isothermal entropy changes obtained from magnetization and heat capacity measurements were found to be in good agreement. A large inverse adiabatic temperature change (ΔTad) of 2.7 K was determined from heat capacity data at TM for a magnetic field change of 1.8 T. The density of states and the Debye temperature were found to be 4.9 states/eV/f.u. and 322 K, respectively. The magnetic entropy analysis agrees fairly well with the theoretical maximal magnetic entropy, R ln(2J + 1).

Simultaneously achieved large reversible elastocaloric and magnetocaloric effects and their coupling in a magnetic shape memory alloy

Large reversible caloric effects covering a broad temperature region are essential for high-efficiency and environment-friendly solid-state caloric refrigeration that can potentially replace the traditional vapor-compression-based cooling technology. Here, we report the simultaneously achieved large reversible magnetocaloric and elastocaloric effects in a Ni43Co6Mn40Sn11 magnetic shape memory alloy. A reversible near-room-temperature magnetic entropy change ΔSm of as high as 19.3 J kg−1 K−1 under 5 T was experimentally obtained and the corresponding adiabatic temperature change ΔTad was estimated to be 7.7 K. Meanwhile, a large reversible elastocaloric effect with a directly measured ΔTad up to 7.1 K was attained. The elastocaloric effect exhibits high cyclic stability with no apparent degradation during 380 cycles of loading and unloading. Furthermore, we propose and demonstrate the utilization of the multicaloric approach under the coupled uniaxial stress and magnetic field to enlarge the refrigeration temperature region of reversible caloric effects. By combining the reversible magnetocaloric and elastocaloric effects and the reversible multicaloric effect under the coupling of uniaxial stress and magnetic field in the hysteresis region, large reversible caloric effects covering a broad temperature region from 257 K to 383 K can be obtained. This study may pave the way for designing advanced caloric materials with cyclically stable and reversible large caloric effects and wide refrigeration temperature region for solid-state refrigeration.

Magnetocaloric properties of Ni49.9Mn19.6Cu5.7Ga24.8 single crystal processed by Bridgman method with stationary crucible

Bridgman method with stationary crucible was used to grow NiMnCuGa single crystal. Microstructure observations, as well as XRD pattern, revealed a presence of martensitic structure at room temperature. Crystal structure was determined as a non-modulated tetragonal martensite. The phase transition occurred at 296 K and 290 K on heating and cooling, respectively. Magnetic entropy change (ΔSM) reached a peak value of −9.86 J/kgK at the field of 1520 kA/m. Refrigerant capacity (RC), calculated at FWHM of the ΔSM peak, reached a value of 28.5 J/kg. Directly measured adiabatic temperature change (ΔTad) spread over 51 K range and its maximum value reached 0.23 K at 1520 kA/m field.

A large barocaloric effect and its reversible behavior with an enhanced relative volume change for Ni42.3Co7.9Mn38.8Sn11 Heusler alloy

Both the barocaloric effect (BCE) and its reversible behavior during martensitic transformation (MT) have been experimentally investigated in Ni42.3Co7.9Mn38.8Sn11 quaternary Heusler alloy. The experimental results show that the studied alloy experiences the MT from the ferromagnetic L21-type cubic structure to the paramagnetic-like three-layered (3M) modulated monoclinic structure, resulting in a value of ∼1.1% for the relative volume change ε between two phases. Due to appearance of an enhanced ε, the migration rate of martensitic transformation with respect to the hydrostatic pressure reaches ∼4.7 K/kbar, while a high transition entropy change (∼28 J/kg K) is still maintained. Based on an indirectly estimated method, the calculated maximum value for isothermal entropy change ΔST achieves ∼ -23 J/kg K with the pressure change of 6.2 kbar, which yields the values of ∼10 K for adiabatic temperature change ΔTad and ∼786 J/kg for relative cooling power (RCP), performing a large BCE. According to the change of phase fraction at selected hydrostatic pressures, a detailed schematic diagram has been established to evaluate the reversibility of this effect. With applying an external cyclic pressure of 6.2 kbar, the maximum repeatable values calculated for both ΔST and ΔTad are decreased to ∼ -15 J/kg K and ∼5 K, respectively. This is attributed to an insufficient driving force, which can only partially transform this material, leading to a reversible MT appeared in the mixed state.

Double magnetocaloric peak feature observed in quaternary Ni-Mn-In based Heusler alloys

We present the results of the studies of magnetic and magnetocaloric (MCE) properties of quaternary Heusler alloys Ni48.5Mn35In15Co1.5 and Ni50Mn35In13.5Al1.5 in temperature range between 80 K and 400 K. While doping initial ternary Ni-Mn-In alloy with 1.5 at.% Al does not change TCA as well as MS, doping with 1.5 at.% Co significantly increases both TCA and MS. The direct magnetocaloric measurements around martensitic transition and austenite Curie temperature reveal the competition between the contributions of different signs. The latter is related to the vicinity of transition temperatures of austenite and martensite, resulting in mixed state, where both phases undergo magnetic order-disorder transformation in the same temperature region. Additionally temperature dependencies of adiabatic temperature change ΔTad and magnetic entropy ΔSM of the system exhibit additional peak feature which we attribute to intermartensitic transition occurring in these alloys.

Kinetic effects in the magnetic and magnetocaloric properties of metamagnetic Ni50Mn35In14.25B0.75

Ni50Mn35In14.25B0.75 is a typical representative of the family of metamagnetic In-based Heusler alloys which shows large magnetocaloric effects generated from first order magnetostructural and ferromagnetic-paramagnetic transitions. For this alloy, a study of the adiabatic temperature change ΔTad by a direct method and through thermomagnetic measurements in magnetic fields up to 14 T has been performed. It has been shown that the rate of heating/cooling can affect the magnitude and sign of ΔTad. The application of magnetic fields larger than 12 T (on cooling) results in the “kinetic arrest” of the ferromagnetic austenitic phase. The effects of the temperature and magnetic history, and kinetic effects, on ΔTad of these Ni-Mn-In-based Heusler alloys in high magnetic fields are discussed.

The study of entropy change and magnetocaloric response in magnetic nanoparticles via heat capacity measurements

In our work, we have investigated the magnetocaloric effect (MCE) of core@shell Co@Au nanoparticles by heat capacity Cp measurement in temperature range 1.9–55 K under external magnetic field. After the subtraction of lattice heat capacity contribution, the maximum of isothermal magnetic entropy change ΔSm = 3.11 J K−1 kg−1 was obtained at field change from 0 to 9 T. Adiabatic temperature change ΔTad = 3.31 K, refrigerant capacity power RCP = 88.9 J kg−1 and efficiency of adiabatic temperature change η = 0.37 K/T at the same external magnetic field change were established for the investigated system. The comparison of the results to the independent magnetic measurements data revealed the accordance of fundamental magnetocaloric characteristics obtained by both methods. Thus, this work is also the first experimental confirmation of relation between magnetocaloric response of nanoparticles and the low temperature magnetic transition from superparamagnetic to blocking/freezing state.

Structural, electronic, magnetic, and magnetocaloric properties in metallic antiperovskite compound Mn3GaC

The structural, electronic, magnetic, and magnetocaloric properties of the metallic antiperovskite compound Mn3GaC were investigated using several theoretical methods such as: First principle calculations, Monte Carlo simulations and mean field theory. The metallic antiperovskite compound Mn3GaC exhibits a second-order ferromagnetic-paramagnetic phase transition around TC=249K. Using the first principle calculations, the magnetic moment and the exchange coupling interactions values are 1.37μB and J1=35.78meV,J2=40.16meV, respectively. The total magnetization, the susceptibility and the specific heat of this compound are calculated. The critical temperature obtained is in good agreement with the experimental results. Obviously, the large MCE with no hysteresis loss is obtained around TC. The maximum values of the magnetic entropy change (ΔSmag), adiabatic temperature change (ΔTad) and the relative cooling power (RCP) are 13.41J/kg.K, 15.96K, 748J/kg respectively, under applied an external magnetic field of h=5.0T.

Tunable magnetocaloric properties of Gd based alloys via Zn and Cd additions

Magnetocaloric effect by employing direct measurements of the adiabatic temperature changes (ΔTad) with a low magnetic field change (ΔH) up to 1.2T are investigated, for the Gd based alloys with Zn and Cd substituted for Gd individually or synchronously. A comparative study of the adiabatic temperature change (ΔTad) and magnetic entropy change (ΔSM) near room temperature has been reported in three different alloys of Gd1−xZnx, Gd1−yCdy and Gd0.95(Zn1−zCdz)0.05. Effects of Zn and Cd content on magnetocaloric effect and the Curie temperature (Tc) are studied in different alloys. A direct correlation between phase structure and ΔTad, ΔSM, Tc was discussed in three different types alloys of Gd1−xZnx, Gd1−yCdy, and Gd0.95(Zn1−zCdz)0.05. Mechanism of Zn improving the ΔTad and Cd increasing the Tc is revealed. It is expected that magnetic refrigeration near room temperature has a potential for superseding the traditional gas refrigeration.

Magnetocaloric, thermal, and magnetotransport properties of Ni50Mn35In13.9B1.1 Heusler alloy

The structural, thermal, magnetic, magnetocaloric, and magnetotransport properties of the Heusler alloy Ni50Mn35In13.9B1.1 were studied using room temperature X-ray diffraction, differential scanning calorimetry (DSC), field dependent magnetization, and electrical resistivity measurements. Ni50Mn35In13.9B1.1 exhibits inverse and direct magnetocaloric effects in the vicinity of its magnetostructural transition (MST) and at its Curie temperature (TC), respectively. A maximum magnetic entropy change of 16J/kgK was observed at 295K for a magnetic field change of 5T. The values of the refrigeration capacity (RC) at the MST and at TC were found to be 140J/kg and 230J/kg, respectively, at μ0ΔH=5T. The values of the latent heat (L=10.8J/g) and total entropy change (ΔST=31J/kgK) have been determined through DSC measurements. The magnetoresistance associated with the martensitic transformation was found to be 56% for a magnetic field change of 5T at room temperature. The maximum value of the adiabatic temperature change (ΔTad) was found to be 2.2K/T as estimated from experimental data. The relevant parameters affecting the value of magnetocaloric effects are discussed.

Mixed rare earth oxides derived from monazite sand as an inexpensive precursor material for room temperature magnetic refrigeration applications

An inexpensive perovskite type (REMIX)0.67Sr0.33MnO3 compound is synthesized via solid state method using mixed rare earth oxide precursor derived from monazite sand. Rietveld refinement of X-ray powder diffraction patterns confirms the compound is a mixture of rare earth manganites which is the major phase and CeO2 as the secondary phase. The compound shows a second order ferromagnetic to paramagnetic transition near room temperature and exhibits a magnetic entropy change (-ΔSM) of 3.28Jkg−1K−1 with a relative cooling power (RCP) of 120Jkg−1 and an adiabatic temperature change (ΔTad) of 2.11K at 310K under 50kOe magnetic field. The Debye temperature of the compound is found to be 543K and room temperature thermal conductivity is 4.26Wm−1K−1. The temperature variation of electrical resistivity shows a metal to insulator transition around 180K, which gets shifted towards higher temperature upon the application of magnetic field. The compound shows a large value of −ΔSM and this work makes an endeavor to develop low–cost materials for the magnetic refrigeration applications near the room temperature.

Magnetocaloric effect in textured polycrystalline Ho2GdAl2

Textured Ho2GdAl2 alloy exhibits highly anisotropic magnetic properties due to the coexistence of preferred crystallographic orientation and magnetocrystalline anisotropy (MCA). The variation in MCA energy (Ea) induced by the rotation of textured Ho2GdAl2 from a perpendicular to parallel direction under constant field results in a rotating magnetocaloric effect (MCE), which is larger than the conventional MCE. Moreover, the peaks of rotating magnetic entropy change (ΔSR) and rotating adiabatic temperature change (ΔTad,R) broaden asymmetrically towards low temperatures due to the different variation of MCA below and above the Curie temperature TC, thus leading to a table-like rotating MCE over the liquefaction temperature of natural gas.

Reversibility of minor hysteresis loops in magnetocaloric Heusler alloys

The unavoidable existence of thermal hysteresis in magnetocaloric materials with a first-order phase transition is one of the central problems limiting their implementation in cooling devices. Using minor loops, however, allows achieving significant cyclic effects even in materials with relatively large hysteresis. Here, we compare thermometric measurements of the adiabatic temperature change ΔTad and calorimetric measurements of the isothermal entropy change ΔST when moving in minor hysteresis loops driven by magnetic fields. Under cycling in 2 T, the Ni-Mn-In-Co Heusler material provides a reversible magnetocaloric effect of ΔSTrev= 10.5 J kg–1 K−1 and ΔTadrev= 3.0 K. Even though the thermodynamic conditions and time scales are very different in adiabatic and isothermal minor loops, it turns out that after a suitable scaling, a self-consistent reversibility region in the entropy diagram is found. This region is larger than expected from basic thermodynamic considerations based on isofield measurements alone, which opens new opportunities in application.

Microstructural and magnetic properties of Mn-Fe-P-Si (Fe2 P-type) magnetocaloric compounds

Fe2 P-based magnetocaloric compounds are an important and industrially relevant material class for magnetic refrigeration, yet their microstructure and its influence on the magnetic properties is hardly discussed in literature. We prepared Mn-Fe-P-Si-based samples using a powder metallurgical process and analyzed their microstructural and thermomagnetic properties. XRD, SEM, EDX and EBSD analysis reveal a phosphorous depleted cubic secondary phase in many samples with distinct microstructural properties giving an insight into the phase formation process. A porous morphology was found, hindering the direct application of the materials a magnetocaloric heat exchanger in bulk-like structures. The “virgin” effect could be in-situ observed for the first time on a macroscopic scale using temperature-dependent optical microscopy. Thermomagnetic measurements reveal a difference in transition temperature Tt in comparison to literature values which is attributed to a processing induced deviation from the nominal composition. The isothermal entropy change ΔST and adiabatic temperature change ΔTad were studied as well as their cyclic behavior. The effect of secondary phases is discussed and the importance of the metal/non-metal (M/NM)-ratio is shown. The article presents a road map for the preparation of Mn-Fe-Si-P-based alloys with highest quality.

Giant field-induced adiabatic temperature changes in In-based off-stoichiometric Heusler alloys

Direct measurements of the adiabatic temperature change (ΔTAD) of Ni50Mn35In14.5B0.5 have been done using an adiabatic magnetocalorimeter in a temperature range of 250–350 K, and with magnetic field changes up to ΔH = 1.8 T. The initial susceptibility in the low magnetic field region drastically increases with temperature starting at about 300 K. Magnetocaloric effects parameters, adiabatic temperature changes, and magnetic entropy changes were found to be a linear function of H2/3 in the vicinity of the second order transitions (SOT), whereas the first order transitions do not obey the H2/3 law due to the discontinuity of the transition. The relative cooling power based on the adiabatic temperature change for a magnetic field change of 1.8 T has been estimated. Maximum values of ΔTAD = −2.6 K and 1.7 K were observed at the magnetostructural transition (MST) and SOT for ΔH = 1.8 T, respectively. The observed ΔTAD at the MST exceeds the ΔTAD for Ni50Mn35In14X with X = In, Al, and Ge by more than 20% and is larger than the Gd based Heusler alloys.