Realization of Large Low-Stress Elastocaloric Effect in TiZrNbAl Alloy

Seeking novel high-performance elastocaloric materials with low critical stress plays a crucial role in advancing the development of elastocaloric refrigeration technology. Here, as a first attempt, the elastocaloric effect of TiZrNbAl shape memory alloy at both room temperature and finite temperatures ranging from 245 K to 405 K, is studied systematically. Composition optimization shows that Ti-19Zr-14Nb-1Al (at.%), possessing excellent room-temperature superelasticity with a critical stress of around 100 MPa and a small stress hysteresis of around 70 MPa and outstanding fracture resistance with a compressive strain of 20% and stress of 1.7 GPa, demonstrates a substantial advantage as an elastocaloric refrigerant. At room temperature, a large adiabatic temperature change (ΔTad) of -6.7 K is detected, which is comparable to the highest value reported in the Ti-based alloys. A high elastocaloric cyclic stability, with almost no degradation of ΔTad after 4000 cycles, is observed. Furthermore, the sizeable elastocaloric effect can be steadily expanded from 255 K to 395 K with a temperature window of as large as 140 K. A maximum ΔTad of -7.9 K appears at 355 K. The present work demonstrates a promising potential of TiZrNbAl as a low critical stress and low hysteresis elastocaloric refrigerant.

Elastocaloric signatures of symmetric and antisymmetric strain-tuning of quadrupolar and magnetic phases in DyB2C2

The adiabatic elastocaloric effect measures the temperature change of a given system with strain and provides a thermodynamic probe of the entropic landscape in the temperature-strain space. Here, we demonstrate that the DC bias strain-dependence of AC elastocaloric effect allows decomposition of the latter into symmetric (rotation-symmetry-preserving) and antisymmetric (rotation-symmetry-breaking) strain channels, using a tetragonal [Formula: see text]-electron intermetallic DyB[Formula: see text]C[Formula: see text]-whose antiferroquadrupolar order breaks local fourfold rotational symmetries while globally remaining tetragonal-as a showcase example. We capture the strain evolution of its quadrupolar and magnetic phase transitions using both singularities in the elastocaloric coefficient and its jumps at the transitions, and the latter we show follows a modified Ehrenfest relation. We find that antisymmetric strain couples to the underlying order parameter in a biquadratic (linear-quadratic) manner in the antiferroquadrupolar (canted antiferromagnetic) phase, which are attributed to a preserved (broken) global tetragonal symmetry, respectively. The broken tetragonal symmetry in the magnetic phase is further evidenced by elastocaloric strain-hysteresis and optical birefringence. Additionally, within the staggered quadrupolar order, the observed elastocaloric response reflects a quadratic increase of entropy with antisymmetric strain, analogous to the role magnetic field plays for Ising antiferromagnetic orders by promoting pseudospin flips. Our results demonstrate AC elastocaloric effect as a compact and incisive thermodynamic probe into the coupling between electronic degrees of freedom and strain in free energy, which holds the potential for investigating and understanding the symmetry of a wide variety of ordered phases in broader classes of quantum materials.

Toughening of Ni-Mn-Based Polycrystalline Ferromagnetic Shape Memory Alloys

Solid-state refrigeration technology is expected to replace conventional gas compression refrigeration technology because it is environmentally friendly and highly efficient. Among various solid-state magnetocaloric materials, Ni-Mn-based ferromagnetic shape memory alloys (SMAs) have attracted widespread attention due to their multifunctional properties, such as their magnetocaloric effect, elastocaloric effect, barocaloric effect, magnetoresistance, magnetic field-induced strain, etc. Recently, a series of in-depth studies on the thermal effects of Ni-Mn-based magnetic SMAs have been carried out, and numerous research results have been obtained. It has been found that poor toughness and cyclic stability greatly limit the practical application of magnetic SMAs in solid-state refrigeration. In this review, the influences of element doping, microstructure design, and the size effect on the strength and toughness of Ni-Mn-based ferromagnetic SMAs and their underlying mechanisms are systematically summarized. The pros and cons of different methods in enhancing the toughness of Ni-Mn-based SMAs are compared, and the unresolved issues are analyzed. The main research directions of Ni-Mn-based ferromagnetic SMAs are proposed and discussed, which are of scientific and technological significance and could promote the application of Ni-Mn-based ferromagnetic SMAs in various fields.

Elastocaloric Waste/Natural Rubber Materials with Various Crosslink Densities

The characterization of the mechanical behavior of elastocaloric materials is essential to identify their viability in heating/cooling devices. Natural rubber (NR) is a promising elastocaloric (eC) polymer as it requires low external stress to induce a wide temperature span, ΔT. Nonetheless, solutions are needed to further improve DT, especially when targeting cooling applications. To this aim, we designed NR-based materials and optimized the specimen thickness, the density of their chemical crosslinks, and the quantity of ground tire rubber (GTR) used as reinforcing fillers. The eC properties under a single and cyclic loading conditions of the resulting vulcanized rubber composites were investigated via the measure of the heat exchange at the specimen surface using infrared thermography. The highest eC performance was found with the specimen geometry with the lowest thickness (0.6 mm) and a GTR content of 30 wt.%. The maximum temperature span under single interrupted cycle and multiple continuous cycles were equal to 12 °C and 4 °C, respectively. These results were assumed to be related to more homogeneous curing in these materials and to a higher crosslink density and GTR content which both act as nucleating elements for the strain-induced crystallization at the origin of the eC effect. This investigation would be of interest for the design of eC rubber-based composites in eco-friendly heating/cooling devices.

High-performance cooling and heat pumping based on fatigue-resistant elastocaloric effect in compression

In recent years, elastocaloric cooling has shown great potential as an alternative to vapor-compression refrigeration. However, there is still no existing elastocaloric device that offers fatigue-resistant operation and yet high cooling/heat-pumping performance. Here, we introduce a new design of an elastocaloric regenerator based on compression-loaded Ni-Ti tubes, referred to as a shell-and-tube-like elastocaloric regenerator. Our regenerator design, which can operate in both cooling and heat-pumping modes, enables durable operation and record performance with a maximum temperature span of 31.3 K in heat-pumping mode or maximum heating/cooling powers of more than 60 W, equivalent to 4,400 W/kg of the elastocaloric material (at temperature span of 10 K). In terms of both maximum performance metrics, these results surpass all previously developed caloric (magnetocaloric, electrocaloric, and elastocaloric) devices and demonstrate the enormous potential of compression-loaded elastocaloric regenerators to be used in elastocaloric devices for a wide range of cooling and heat-pumping applications.

Elastocaloric Properties of Polycrystalline Samples of NiMnGaCu Ferromagnetic Shape Memory Alloy under Compression: Effect of Improvement of Thermoelastic Martensitic Transformation

Shape memory alloys (SMAs) and ferromagnetic shape memory alloys (FeSMAs) have recently attracted interest for solid state refrigeration applications. Among NiMnGa-based quaternary systems, NiMnGaCu exhibits an interesting giant magnetocaloric effect thanks to the overlapping of the temperatures related to the magnetic transition and the thermoelastic martensitic transformation (TMT); in particular, for compositions with Cu content of approximately 6 at%. In the present work, we investigated the improvement effect of TMT on the total entropy change (ΔS) in the elastocaloric performances of polycrystalline Ni₅₀Mn_18.5Cu_6.5Ga₂₅ at% alloy samples, just above room temperature. We report an extensive calorimetric and thermomechanical characterization to explore correlations between microstructural properties induced by the selected thermal treatment and elastocaloric response, aiming at providing the basis to develop more efficient materials based on this quaternary system. Both ΔT and ΔS values obtained from mechanical curves at different temperatures and strain recovery tests under fixed load vs. T were considered. Maximum values of ΔS = 55.9 J/KgK and ΔT = 4.5 K were attained with, respectively, a stress of 65 MPa and strain of 4%. The evaluation of the coefficient of performance (COP) was carried out from a cyclic test.

Giant Elastocaloric Effect in Ni-Mn-Ga-Based Alloys Boosted by a Large Lattice Volume Change upon the Martensitic Transformation

High-performance elastocaloric materials are highly sought in developing energy-efficient and environmentally friendly solid-state elastocaloric refrigeration. Here, we present an effective strategy to achieve a giant elastocaloric response by enlarging the lattice volume change ΔV/V₀ upon the martensitic transformation. Using the Ni₅₀Mn₅₀ binary alloy as the prototype, a large transformation entropy change ΔS_tr can be tailored in the vicinity of room temperature by simultaneously doping Cu and Ga. Especially, the |ΔS_tr| values in the ⟨001⟩_A-textured Ni₃₀Cu₂₀Mn_39.5Ga_10.5 and Ni₃₀Cu₂₀Mn₃₉Ga₁₁ alloys prepared by directional solidification can be as large as 47.5 and 46.7 Jkg^-1 K^-1, respectively, due to the significant ΔV/V₀ values, i.e., 1.81 and 1.82%, respectively. Such enhanced ΔS_tr values thus yield giant ΔT_ad values of up to -23.5 and -19.3 K on removing the compressive stress in these two alloys, being much higher than those in Heusler-type alloys reported previously. Moreover, owing to the relatively low driving stress endowed by the highly textured microstructure, the specific adiabatic temperature change (|ΔT_ad/Δσ_max|) in the present work can be as large as 77.2 K/GPa. This work is expected to provide new routes in designing high-performance elastocaloric materials with the combination of a giant elastocaloric response and low driving stress.

Elastocaloric signature of nematic fluctuations

The elastocaloric effect (ECE) relates changes in entropy to changes in strain experienced by a material. As such, ECE measurements can provide valuable information about the entropy landscape proximate to strain-tuned phase transitions. For ordered states that break only point symmetries, bilinear coupling of the order parameter with strain implies that the ECE can also provide a window on fluctuations above the critical temperature and hence, in principle, can also provide a thermodynamic measure of the associated susceptibility. To demonstrate this, we use the ECE to sensitively reveal the presence of nematic fluctuations in the archetypal Fe-based superconductor Ba([Formula: see text])₂[Formula: see text] By performing these measurements simultaneously with elastoresistivity in a multimodal fashion, we are able to make a direct and unambiguous comparison of these closely related thermodynamic and transport properties, both of which are sensitive to nematic fluctuations. As a result, we have uncovered an unanticipated doping dependence of the nemato-elastic coupling and of the magnitude of the scattering of low-energy quasi-particles by nematic fluctuations-while the former weakens, the latter increases dramatically with increasing doping.

Optimizing the Caloric Properties of Cu-Doped Ni-Mn-Ga Alloys

With the purpose to optimize the functional properties of Heusler alloys for their use in solid-state refrigeration, the characteristics of the martensitic and magnetic transitions undergone by Ni₅₀Mn_25−xGa₂₅Cu_x (x = 3–11) alloys have been studied. The results reveal that, for a Cu content of x = 5.5–7.5, a magnetostructural transition between paramagnetic austenite and ferromagnetic martensite takes place. In such a case, magnetic field and stress act in the same sense, lowering the critical combined fields to induce the transformation; moreover, magnetocaloric and elastocaloric effects are both direct, suggesting the use of combined fields to improve the overall refrigeration capacity of the alloy. Within this range of compositions, the measured transformation entropy is increased owing to the magnetic contribution to entropy, showing a maximum at composition x = 6, in which the magnetization jump at the transformation is the largest of the set. At the same time, the temperature hysteresis of the transformation displays a minimum at x = 6, attributed to the optimal lattice compatibility between austenite and martensite. We show that, among this system, the optimal caloric performance is found for the x = 6 composition, which displays high isothermal entropy changes (−36 J·kg⁻¹·K⁻¹ under 5 T and −8.5 J·kg⁻¹·K⁻¹ under 50 MPa), suitable working temperature (300 K), and low thermal hysteresis (3 K).

Energy-Efficient Elastocaloric Cooling by Flexibly and Reversibly Transferring Interface in Magnetic Shape-Memory Alloys

Elastocaloric cooling is currently under extensive study owing to its great potential to replace the conventional vapor-compression technique. In this work, by employing multiscale characterization approaches, including in situ neutron diffraction in a loading frame, in situ transmission electron microscopy observation at different temperatures, in situ synchrotron X-ray Laue microdiffraction, and high-resolution infrared thermal imaging, we have investigated the thermal and stress-induced martensitic transformation, the stability of superelastic behavior and the associated elastocaloric effect for a Heusler-type Ni_50.0Fe_19.0Ga_27.1Co_3.9 single crystal. On the basis of transformation from cubic austenite into monoclinic martensite with a flexibly and reversibly transferring interface, this unique single crystal exhibits a giant elastocaloric effect of 11 K and ultralow fatigue behavior during above 12 000 mechanical cycles. The numerical simulation shows that the Ni_50.0Fe_19.0Ga_27.1Co_3.9 alloy offers 18% energy saving potential and 70% cooling capacity enhancement potential compared to the conventional shape-memory nitinol alloy in a single-stage elastocaloric cooling system, making it a great candidate for energy-efficient air conditioner applications.

Elastocaloric Effect in Carbon Nanotubes and Graphene

Carbon nanotubes are famous for their many extraordinary properties. We use a thermodynamical approach, experimental data from the literature, and atomistic simulations to reveal one more remarkable property of the carbon nanotubes that has so far been overlooked. Namely, we predict the existence of very large elastocaloric effect that can reach up to 30 K under moderate loads. Potentially even larger values could be achieved under extreme loads, putting carbon nanotubes in the forefront of caloric materials. Other remarkable features of the elastocaloric effect in carbon nanotubes include linearity of elastocaloric temperature change in applied force (compressive or stretching), very weak dependence on the temperature, and an absence of hysteresis. Such features are extremely desirable for practical applications in cooling devices. Moreover, a similarly large elastocaloric effect is predicted for the graphene. The prediction of a large elastocaloric effect in carbon nanotubes and graphene sets forward an unconventional strategy of targeting materials with moderate caloric responses but the ability to withstand very large loads.

Elastocaloric effect in CuAlZn and CuAlMn shape memory alloys under compression

This paper reports the elastocaloric effect of two Cu-based shape memory alloys: Cu68Al16Zn16 (CuAlZn) and Cu73Al15Mn12 (CuAlMn), under compression at ambient temperature. The compression tests were conducted at two different rates to approach isothermal and adiabatic conditions. Upon unloading at a strain rate of 0.1 s(-1) (adiabatic condition) from 4% strain, the highest adiabatic temperature changes (ΔTad) of 4.0 K for CuAlZn and 3.9 K for CuAlMn were obtained. The maximum stress and hysteresis at each strain were compared. The stress at the maximum recoverable strain of 4.0% for CuAlMn was 120 MPa, which is 70% smaller than that of CuAlZn. A smaller hysteresis for the CuAlMn alloy was also obtained, about 70% less compared with the CuAlZn alloy. The latent heat, determined by differential scanning calorimetry, was 4.3 J g(-1) for the CuAlZn alloy and 5.0 J g(-1) for the CuAlMn alloy. Potential coefficients of performance (COPmat) for these two alloys were calculated based on their physical properties of measured latent heat and hysteresis, and a COPmat of approximately 13.3 for CuAlMn was obtained.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.

(Magneto)caloric refrigeration: is there light at the end of the tunnel?

Caloric cooling and heat pumping rely on reversible thermal effects triggered in solids by magnetic, electric or stress fields. In the recent past, there have been several successful demonstrations of using first-order phase transition materials in laboratory cooling devices based on both the giant magnetocaloric and elastocaloric effects. All such materials exhibit non-equilibrium behaviours when driven through phase transformations by corresponding fields. Common wisdom is that non-equilibrium states should be avoided; yet, as we show using a model material exhibiting a giant magnetocaloric effect, non-equilibrium phase-separated states offer a unique opportunity to achieve uncommonly large caloric effects by very small perturbations of the driving field(s).This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.

Giant room-temperature elastocaloric effect in ferroelectric ultrathin films

Environmentally friendly ultrathin BaTiO3 capacitors can exhibit a giant stress-induced elastocaloric effect without hysteresis loss or Joule heating. By combining this novel elastocaloric effect with the intrinsic electrocaloric effect, an ideal refrigeration cycle with high performance (temperature change over 10 K with a wide working-temperature window of 60 K) at room temperature is proposed for future cooling applications.