Large Isothermal Entropy Change Research Articles

Magnetostructural transformation and magnetocaloric response in Mn(Fe)NiSi(Al) alloys

An efficient magnetocaloric refrigerant must have certain characteristics such as a sharp transition near the desired working temperature, a large cyclic magnetocaloric response, and the use of raw materials with reduced costs and low environmental consequences. In this sense, this work focuses on a series of rare-earth- and Co-free Mn0.5Fe0.5NiSi1-xAlx alloys (x = 0.0525, 0.060, 0.0685) with promising magnetocaloric properties. The alloys were synthesized using combined arc melting and induction melting techniques, as this synthesis route provides improved control on the sample composition and homogeneity. We investigated how the heat treatment temperature and Al content affect the magnetostructural and magnetocaloric properties of the alloys. On the one hand, it is found that annealing at 1173 K for 7 days leads to a sharp magnetostructural transformation with no traces of impurities for the alloy with x = 0.0525. Under these conditions, a large isothermal entropy change of –11.5 J kg−1 K−1 for 1 T is obtained near room temperature, significantly improving the value of the as-cast sample. On the other hand, following this optimal heat treatment, the influence of Al content is studied: upon increasing the Al concentration the magnetostructural transformation shifts to lower temperatures, ranging from 320 K for x = 0.0525 to 220 K for x = 0.0685 (measured upon heating).

Effect of Fe on the microstructure and magnetic transition of Mn-Fe-P-Si Microwires

This work systematically investigated the effect of Fe content on the microstructure and magnetic transition behaviors of melt-extracting Mn-Fe-P-Si microwires. It was shown that the Fe content does not change the main phase of the samples, i.e. Fe2P compound, which decreased with the increasement of Fe content. The transition temperature (Ttran) decreased from 311 to 245.5 K while the thermal hysteresis (Thys) increased from 9.5 to 22.5 K increasing Fe from 0.90 to 1.05. The samples realized the first order magnetic transition at x = 1.00 and 1.05, correspondingly the samples demonstrated large magnetic hysteresis losses (Wy,) and isothermal magnetic entropy change (−∆Sisopeak) at 5 T, 59.1 J kg−1 and 14.5 J kg−1 K−1 for the sample x =1.00, 58.4 J kg−1 and 15.8 J kg−1 K−1 for the sample x = 1.05. In contrast, the samples at x = 0.90 and 0.95 indicated second order transition character, and the Wy and −∆Sisopeak were much smaller, i.e. 14.8 J kg−1 and 10.3 J kg−1 K−1, 9.7 J kg−1 and 10.7 J kg−1 K−1 respectively for the samples x = 0.90 and 0.95. The largest effective refrigerant capacity (RCE, 293.7 J kg−1, 5 T) is found at x = 0.90. The reduced hysteresis and maintained competitive isothermal magnetic entropy change make the melt-extracting Mn-Fe-P-Si microwires with optimized Fe content as a promising magnetocaloric material family applied in real magnetic refrigeration applications.

Colossal Barocaloric Effect near Ambient Temperature in 1-Dodecanol under a Low Pressure.

Solid-state refrigeration based on the barocaloric effect is an effective alternative to traditional vapor compression refrigeration. Here 1-dodecanol has been studied due to its large latent heat at a solid-liquid phase transition point around room temperature. The transition temperature will vary with the applied hydrostatic pressure, exhibiting with a sensitivity of 0.14 K MPa-1, which indicates its potential for refrigeration. A pressure of 40 MPa can result in a large isothermal entropy change of 520 J kg-1 K-1 (equivalent to that obtained in vapor compression refrigeration) at 297 K. A large adiabatic temperature change of >20 K in 1-dodecanol was acquired by direct measurement. A wide temperature window of ∼50 K (288-337 K) can be obtained in 1-dodecanol, which demonstrates broad application prospects. These discoveries offer promising prospects for barocaloric cooling and high-efficiency refrigeration technologies relying on solid-liquid phase transitions.

Colossal barocaloric effect of plastic crystals imbedded in silicon frame near room temperature: Molecular dynamics simulation

Solid-state refrigeration technology has been attracting tremendous attention in recent decades. Plastic crystal pentaerythritol (PE) is a crucial barocaloric material in the solid-state refrigeration field due to its high entropy. However, its refrigeration temperature range and extremely low thermal conductivity are far from meeting the requirements of practical application. Here, we systematically investigate the barocaloric effect (BCE) of composite PE and silicon frame [consisting of silicon nanotube and silicene architectures (SNT-Sil)] and analyze the effects of different silicon models on the BCE performance based on molecular dynamics simulations and statistical analysis. A colossal BCE of PE/silicon frame composite is observed, and refrigeration temperature can be altered to the room temperature range by alloying neopentane (PA) into the PE matrix. It is found that the composite PE0.8PA0.2/SNT-Sil and PE0.9PA0.1/SNT-Sil demonstrate excellent comprehensive refrigeration performance near room temperature (300–320 K), with large isothermal entropy change ΔS (654–842 J kg−1 K−1), adiabatic temperature ΔT (34–47 K), and thermal conductivity κ (4.0–4.2 W m−1 K−1). The microscopic mechanism is discussed through pressure induced changes in bonding, structural, and vibrational properties. Importantly, the plastic crystal/silicon framework is easy to deform and requires smaller input work in the barocaloric refrigeration process compared to other nanomaterials such as carbon framework. This work provides important guidance on improving plastic crystals with colossal comprehensive refrigeration performance for practical applications.

Significantly enhanced reversibility and mechanical stability in grain-oriented MnNiGe-based smart materials

Materials that undergo a magnetostructural transition (MST) usually exhibit fascinating magnetoresponsive properties, making them an important class of smart materials. However, practical application of this class of smart materials has been hindered by structural degradation as well as large irreversibility of the MST during consecutive thermal and field cycles. Here we report a significant improvement of the reversibility and mechanical stability in grain-oriented MnNiGe-based alloys that were fabricated using a directional solidification method. The preferred grain orientation enables synergistic deformations between neighboring grains during the MST, leading to a substantial reduction in the transition-induced stress concentration. As a result, in situ and ex situ microscopic observations demonstrate a good mechanical stability of the textured alloys across the MST, in strong contrast to conventional MM'X (M, M’ = Mn, Fe, Co, Ni; X = Si, Ge) materials with randomly-oriented grains. The detailed transition stages of the MST have also been observed at the microscopic scale, and are reported for the first time in the MM'X family. Besides, a low thermal hysteresis (ΔThys) of ∼ 4 K was obtained in the textured alloys, which is the lowest ΔThys in the MM'X material family. Textured MnNiGe-based alloys show a large reversible isothermal entropy change (≥ 35.9 Jkg−1K−1) in a 5 T field change, which is the highest among typical magnetocaloric materials. Consequently, this work provides a promising strategy for enhancing the cyclic stability of materials with a MST, which may boost their practical applications in solid-state refrigerators, energy harvesters and high-precision actuators.

Electrocaloric effect in BaTiO3 multilayer capacitors with first-order phase transitions

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(Sc0.5Ta0.5)O3 multilayer ceramic capacitors (Nair et al 2019 Nature 575 468) 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 Science 370 125). 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.

Large reversible magnetocaloric effect across the first-order magneto-structural transition in (Mn0.45Fe0.55) (Ni0.6Co0.4) Si alloy

Magnetic materials with reversible magnetocaloric effect across their respective first-order magneto-structural transitions (MST) have vital significance for further advancement towards the ultimate goal of practical magnetic cooling applications. In this present study, the reversibility of the MST is investigated by analyzing the isofield curves under different minor and major thermal hysteresis loops and the isothermal curves under different magnetic field cycles. The reversibility of the magnetocaloric effect (MCE) depends upon the width of the nucleation energy barrier existing across the MST. Minor hysteresis loops are found to be minimizing the energy loss by preventing the nucleation of the martensite phase and can be easily returned to the austenite phase immediately while switching from cooling to heating. A large reversible isothermal magnetic entropy change ΔSM of −14.4 J kg−1 K−1 nearly at 322 K is observed for the (Mn0.45Fe0.55) (Ni0.6 Co0.4) Si alloy under a low magnetic field change of 3 T. The observed large reversible MCE, very low magnetic field-induced hysteresis loss, and large reversible RCeffe ∼ 155 J kg−1 will be beneficial for reducing the energy loss during several magnetic field cycles in practical cooling applications which makes this alloy promising for the development of efficient magnetic refrigerants.

Magnetocaloric properties of a wire composite of La (Fe,Mn,Si)13-H alloy powders in a Gd cylindrical tube

The enhancement of magnetocaloric effect (MCE) and the extension of effective magnetic refrigeration temperature region near room temperature are critical requirements for highly efficient and environment friendly cooling technologies. The large isothermal entropy change of the first-order phase transition material and broader temperature range of the second-order phase transition material are combined into the composite form of La(Fe,Mn,Si)13-H and Gd with their transition temperatures close to room temperature. In this work, wire composites were fabricated using the La (Fe,Mn,Si)13-H powders filled inside the Gd tube. The magnetocaloric properties of the wire composite are convoluted from individual constituents. The dual transitions temperature were observed at 287 K and 292 K. The isothermal entropy change response of the wire composite can be obtained by using the simulation with the combined weight of the Gd tube and La (Fe,Mn,Si)13-H powders. The maximum isothermal entropy change (−ΔSiso) at μoH= 5 T calculated from isothermal magnetization measurements are 8.7 for Gd tube, 16.9 for La(Fe,Mn,Si)13-H powders, and 10.2 J/kg•K for a wire composite. The magnetocaloric properties including the order of phase transition of the wire composite are close to second-order type due to the higher weight fraction of Gd tube.

Large magnetocaloric effect near room-temperature by tuning the magneto-structural transition in (Mn, Fe)(Ni, Fe)Si alloy

The observation of near room-temperature coupling of both magnetic and structural transitions in Fe substituted Mn-Ni-Si alloy leads to an interesting magneto-structural transition (MST) phenomenon which intensifies the required magnetocaloric parameters for magnetic cooling applications. The crystal structure analysis reveals that temperature as well as Fe substitution work as the driving forces for deforming the orthorhombic unit cell and to stabilize the hexagonal phase. The correlation between the unit cell parameters of both phases confirms that a large change in unit cell volume (∼2.9%) is mainly responsible for the structural transition. The simultaneous substitution of Fe across Ni and Mn sites helps to tune the MST temperature. Both magnetization and calorimetric studies confirm the existence of first-order type structural transition across MST. The [Mn0.64 Ni0.64 Fe0.72 Si] alloy exhibits a large isothermal magnetic entropy change (ΔS M ) of −19.2 0.4 J kg−1 K−1 accompanied by a large value of effective refrigerant capacity (RC effe ) of 181.8 3.7 J kg−1 due to a magnetic field change of 0–30 kOe. The estimated total entropy change associated with the complete structural transition from the calorimetric studies sets an upper limit for the entropy change of the system. The low cost and non-toxic nature of constituent elements, composition-dependent tunability of the transition temperature, and near room-temperature large magnetocaloric parameters make this alloy potentially suitable to be used as a refrigerant in solid-state magnetic cooling technology.

High Energy Storage Performance and Large Electrocaloric Response in Bi0.5Na0.5TiO3-Ba(Zr0.2Ti0.8)O3 Thin Films.

With regard to the global energy crisis and environmental pollution, ferroelectric thin films with unique polarization behavior have garnered considerable attention for energy storage and electrocaloric refrigeration. Herein, a series of (1 - x)Bi0.5Na0.5TiO3-xBa(Zr0.2Ti0.8)O3 (x = 0.3-0.9; (1 - x)BNT-xBZT) films were fabricated on Pt(111)/Ti/SiO2/Si substrates. Incorporating BZT can tune the polarization behavior and phase transition temperature of BNT. A high recoverable energy density ≈ 82 J cm-3 and optimized efficiency ≈ 81% were realized for the (1 - x)BNT-xBZT thin film with x = 0.7. The thin film exhibits excellent stability in energy storage performance, a wide working frequency range (0.5-20 kHz), a broad operating temperature window (20-200 °C), and reduplicative switching cycles (107 cycles). In addition, the 0.5BNT-0.5BZT film exhibits a desirable electrocaloric effect with a large adiabatic temperature change (ΔT ≈ -22.9 K) and isothermal entropy change (ΔS ≈ 33.4 J K-1 kg-1) near room temperature under a moderate applied electric field of 2319 kV cm-1. These remarkable performances signify that the (1 - x)BNT-xBZT system is a promising multifunctional electronic material for energy storage and solid-state cooling applications.

Gadolinium oxyorthogermanate Gd2GeO5: An efficient solid refrigerant material for magnetic cryocoolers

We report here the magnetic interactions, magnetocaloric effect and thermodynamic parameters of an efficient magnetic refrigerant material, Gd2GeO5, through magnetization and heat capacity measurements, as well as a mean-field and scaling analysis approach. An isotropic antiferromagnetic exchange coupling with energy scale εex = JexS2 ≈ 2.6 K is estimated from analysing the magnetization dependences, in agreement with the couplings derived from the density functional theory (DFT). The Gd2GeO5 shows an exceptionally large isothermal magnetic entropy change of 0.36 J K−1 cm−3 (50.3 J K−1 kg−1) and relative cooling power of 4.34 J cm−3 (611.3 J kg−1) for a field change of 8.9 T, with the highest adiabatic temperature change being 22.2 K, which is quite impressive for use in helium-free magnetic cryocoolers.

Large magnetocaloric effect and magnetoresistance in Fe-Co doped Ni50-(FeCo) Mn37Ti13 all-d-metal Heusler alloys

The effect of substituting FeCo in Ni site maintaining their same ratio on structural, magnetocaloric, and magneto-transport properties of all- d -metal Ni 50- x (FeCo) x Mn 37 Ti 13 ( x = 16, 18, and 20) Heusler alloys are investigated. These alloys undergo a first-order magnetostructural transition (MST) between low temperature weak magnetic martensite (monoclinic or orthorhombic structure) to a high temperature ferromagnetic (FM) austenite (cubic structure) phase around room temperature. The MST temperature is observed to shift towards the lower temperature with increasing (FeCo) x content. These alloys with x = 16, 18, and 20 exhibit isothermal magnetic entropy change (∆S M ) as large as about ~ 13.8 , ~ 12.7 and ~ 11.8 JkgK −1 across their MST associated with an effective refrigerant capacity (RC eff ) of about 119.9 , ~ 126.2 and ~ 194.4 J/kg respectively due to an application of 5 T magnetic field. It is interesting to note that ∆S M remains almost identical over a large temperature region with (FeCo) x content from 16 to 20 at%. In addition, the maximum values of large magnetoresistance (MR)~ − 26.1 % for x = 16, − 25.4 % for x = 18, and − 24.3 % for x = 20 are achieved at the field of 5 T. These materials with large magnetocaloric response in an extended temperature window around room temperature as well as large negative MR can be considered as potential candidates for multifunctional application. • We report the co-doping effect on magnetocaloric effect and magnetoresistance in all- d -metal Heusler alloys. • The magnetostructural transition (MST) shifts towards lower temperature with doping contents. • We observe large isothermal entropy change ( ∆ S M ) , and large magnetoresistance near room temperature.

Driving Barocaloric Effects in a Molecular Spin-Crossover Complex at Low Pressures.

Barocaloric effects─thermal changes in a material induced by applied hydrostatic pressure─offer promise for creating solid-state refrigerants as alternatives to conventional volatile refrigerants. To enable efficient and scalable barocaloric cooling, materials that undergo high-entropy, reversible phase transitions in the solid state in response to a small change in pressure are needed. Here, we report that pressure-induced spin-crossover (SCO) transitions in the molecular iron(II) complex Fe[HB(tz)3]2 (HB(tz)3- = bis[hydrotris(1,2,4-triazol-1-yl)borate]) drive giant and reversible barocaloric effects at easily accessible pressures. Specifically, high-pressure calorimetry and powder X-ray diffraction studies reveal that pressure shifts as low as 10 bar reversibly induce nonzero isothermal entropy changes, and a pressure shift of 150 bar reversibly induces a large isothermal entropy change (>90 J kg-1 K-1) and adiabatic temperature change (>2 K). Moreover, we demonstrate that the thermodynamics of the SCO transition can be fine-tuned through systematic deuteration of the tris(triazolyl)borate ligand. These results provide new insights into pressure-induced SCO transitions and further establish SCO materials as promising barocaloric materials.

Understanding of the giant magnetic entropy change around the co-occurrence point of martensitic and magnetic transitions in Ni-Mn-In Heusler alloy

Magnetic refrigeration technique based on magnetocaloric effect has attracted increasing interest due to the high energy-converting efficiency and environmental friendliness. From application aspect, a large cooling capacity with a wide working temperature span, i.e., a combination of large isothermal entropy change (|ΔS|) and large adiabatic temperature (|ΔTad|) is desired, yet these two indices are hardly achieved. In this work, we report a combination of both large |ΔS| (40 J kg−1K−1) and large |ΔTad| (15 K) under 5 T around room temperature in Ni-Mn-In Heusler alloys. This excellent combination is found to occurred near the co-occurrence point of martensitic and ferromagnetic transitions. Based on the systematic microstructure and Landau theory analysis, the magnetocaloric properties can be understood by the free energy barrier and the temperature range of field induced magnetic transition results when approaching the co-occurrence point. Our study may provide a promising strategy on designing novel magnetocaloric materials with both large cooling capacity and wide working temperature span through approaching the co-occurrence point of martensitic and magnetic transitions.

Tuning the giant magnetocaloric effect in MnCoGe alloy with external pressure

We report the effect of hydrostatic pressure on the magnetocaloric effect across the magneto-structural transition in an off-stoichiometric Mn34.5Co33.1Ge32.4 alloy. A very large isothermal entropy change of 38.95 J kg−1 K−1 is observed at nearly 303 K for a magnetic field excursion of 7 T at ambient pressure. The temperature of the peak entropy change and, thus, the working temperature range of the alloy drastically decrease by about 45 K with the application of a moderate hydrostatic pressure of 297 MPa. This shift is explained by correlating the results of x-ray diffraction measurements performed under high pressure for studying the structural aspect of the transition. Our results highlight the potential of the Mn–Co–Ge alloy system for multi-caloric applications in which the temperature change can be achieved by more than one external stimulus. The present work may also lead to future experiments, which study the response of multi-calorics driven by many external stimuli within the same experimental environment.

Giant room temperature electrocaloric effect in a layered hybrid perovskite ferroelectric: [(CH3)2CHCH2NH3]2PbCl4

Electrocaloric effect driven by electric fields displays great potential in realizing highly efficient solid-state refrigeration. Nevertheless, most known electrocaloric materials exhibit relatively poor cooling performance near room temperature, which hinders their further applications. The emerging family of hybrid perovskite ferroelectrics, which exhibits superior structural diversity, large heat exchange and broad property tenability, offers an ideal platform. Herein, we report an exceptionally large electrocaloric effect near room temperature in a designed hybrid perovskite ferroelectric [(CH3)2CHCH2NH3]2PbCl4, which exhibits a sharp first-order phase transition at 302 K, superior spontaneous polarization (>4.8 μC/cm2) and relatively small coercive field (<15 kV/cm). Strikingly, a large isothermal entropy change ΔS of 25.64 J/kg/K and adiabatic temperature change ΔT of 11.06 K under a small electric field ΔE of 29.7 kV/cm at room temperature are achieved, with giant electrocaloric strengths of isothermal ΔS/ΔE of 0.86 J·cm/kg/K/kV and adiabatic ΔT/ΔE of 370 mK·cm/kV, which is larger than those of traditional ferroelectrics. This work presents a general approach to the design of hybrid perovskite ferroelectrics, as well as provides a family of candidate materials with potentially prominent electrocaloric performance for room temperature solid-state refrigeration.

Gd induced modifications in the magnetocaloric properties of dysprosium manganites

We report structural, electronic, magnetic, and magnetocaloric studies of polycrystalline DyMnO3 and its A-site Gd substituents, Dy1−xGdxMnO3, where 0 ≤ x ≤ 1. Detailed comparison of various magnetocaloric parameters shows that Dy0.5Gd0.5MnO3 has the best values with a large isothermal magnetic entropy change, −∆Smmax~ 15.35 J/kg K, adiabatic temperature change, ∆Tadmax~ 6.15 K and relative cooling power, RCP ~ 307 J/kg, suggesting that rare earth mixing plays a vital role in enhancing the magnetocaloric properties. In addition, absence of magnetic and thermal hysteresis also makes it one of the best candidates for magnetic refrigeration among all known potential candidates for cryogenic temperature applications.

On the structural and thermo-magnetic study of the magnetocaloric Heusler alloy Ni2Mn1-x Cu x Ga0.8Al0.2

Full Heusler alloys present martensitic transition and shape memory effect related phenomena and several technological applications can be envisaged. One promising area is the magnetocaloric effect (MCE) as the magnetic and structural transitions combine to produce a large isothermal entropy and adiabatic temperature change useful for heating and cooling applications. In this work, we study a Ni–(Mn, Cu)–(Ga, Al) Heusler alloy family which has a giant MCE when the chemical composition is fine-tuned to bring the temperature of the second-order magnetic transition close the first-order structural one. Our results show that, for a certain range of copper concentration, the samples show interesting physical properties captured by calorimetric, microscopy imaging, and magnetization measurements, leading to a high MCE with minimized hysteresis.

Elastocaloric effect in bamboo-grained Cu71.1Al17.2Mn11.7 microwires

Bamboo-grained Cu71.1Al17.2Mn11.7 microwires with diameter ∼150 μm were created by Taylor-Ulitovsky method and subsequent annealing. The grain architecture dependent superelasticity (SE) and elastocaloric effects (eCE) were confirmed. The bamboo-grained microwires exhibited low stress hysteresis loss during SE due to the reduced constraints caused by grain boundary, which is favorable for the mobility of martensite-austenite interface. Consequently, a large isothermal entropy change ∼21 J/kg K with a wide temperature range 90 K was achieved. The produced adiabatic temperature change (ΔTad) reached −11.9 K under a maximum stress 400 MPa. A stable ΔTad of −5.6 K showing little degradation in 200 eCE cycles was found with applied stress 325 MPa. This work reveals a fact that the bamboo-grained Cu–Al–Mn microwire may act as a desirable material for miniature solid-state refrigeration devices.

Printing (Mn,Fe)2(P,Si) magnetocaloric alloys for magnetic refrigeration applications

The fabrication of brittle magnetocaloric materials into heat exchangers is currently challenging for conventional manufacturing techniques. In the present study, we demonstrated that selective laser melting (SLM), an emerging 3D printing technique, offers a new and promising solution to fabricate the (Mn,Fe)2(P,Si) magnetocaloric alloys for magnetic refrigeration applications. The hexagonal (Mn,Fe)2(P,Si) phase, which shows a first-order magnetic transition, is successfully formed in the SLM-printed samples with a very small amount of impurity phase (less than 2% in volume fraction). The SLM processing parameters (e.g., the laser power and scanning speed) have a strong influence on the microstructure, magnetic phase transition and magnetocaloric properties of the printed samples. The microscopic mechanism underlying the changes in the microstructure and phase transition temperature has also been discussed. Besides that, a large isothermal entropy change, more than three times higher than that of the benchmark Gd material in a field change of 1 and 2 T, has been obtained in the SLM-printed samples.