Elastocaloric cooling technology recently attracted significant attention as an environmental friendly alternative to vapor-compression technology. It is based on the elastocaloric effect, which ...occurs in superelastic shape memory alloys (SMAs) during stress-induced martensitic transformation. To date, several proof-of-concept devices (mostly based on tensile loading) have been developed, but limited fatigue life was shown to be one of the major issues. Compressive loading improves the fatigue life of such devices significantly, but in return buckling of SMA structure might occur. To overcome this challenge, it is crucial to understand the buckling phenomena of SMA structures, especially for thin-walled tubes that seem to be ideal candidates for application in elastocaloric cooling devices. Here, we experimentally investigated the effects of the diameter-to-thickness ratio (Dout/t) and slenderness (λ) on buckling stability of Ni-Ti tubes that were subjected to cyclic compressive loading. In total, 161 superelastic Ni-Ti tubes with outer diameter (Dout) ranging from 2 mm to 3 mm, Dout/t ranging from 5 to 25 and gauge lengths (Lg) ranging from 6 to 20 mm were tested. The loading procedure consisted of 3 parts: (I) 1 isothermal full-transformation loading cycle, (II) 50 training cycles, and (III) 20 adiabatic cycles to simulate loading conditions in elastocaloric device. We constructed experimental phase diagrams of buckling modes in λ - Dout/t space for constant Dout and in λ - Dout space for constant Dout/t ratio. Marked areas of functionally stable tubes in these phase diagrams give the design guidance for future developments of durable and efficient elastocaloric devices and other applications, e.g. actuators and dampers.
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•Experimental investigation of buckling stability of Ni-Ti tubes was performed.•A total of 37 different geometries with 7 different cross-sections were considered.•The tubes were subjected to cyclic loading in compression.•Defined phase diagrams serve as a guidance for the design of elastocaloric devices.•Functionally stable tubes for application in elastocaloric cooling are defined.
We report the occurrence of stress-induced burst-type martensitic transformation in a 〈116〉A oriented Cu70.5Al17.5Mn12 single crystal. The burst-type transformation is observed in the reverse ...transformation of unloading from 250 K to 370 K in tension, showing strong coupling with plastic deformation in the temperature range between 310 K and 370 K. Such coupling greatly exacerbates the stress hysteresis and energy dissipation in association with the stress-induced transformation, resulting in a significant enhancement in the mechanical damping and degradation in the elastocaloric response. In particular, the merit index for damping exhibits a remarkably high value of 0.903 at 370 K, demonstrating the huge potential of coupled burst-type transformation and plastic deformation in manipulating damping properties. Moreover, it is evidenced that the microstructural evolution of burst-type transformation greatly deviates from that of the thermoelastic transformation.
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Exploring novel materials with superior elastocaloric effect (eCE) is a central issue for the development of eCE refrigeration technique. In this work, we report a giant adiabatic temperature change ...ΔTad of −20.4 K in the 〈100〉-textured all-d-metal Ni50Mn31.75Ti18.25 bulk alloy. The ΔTad value, comparable to that of the sate-of-the-art Ni–Ti alloys, surpasses the existing conventional Ni–Mn-based alloys. Investigations show that the giant ΔTad is attributed to the large lattice volume change during martensitic transformation and the negligible negative contribution of magnetic entropy change owing to its weak magnetism. By performing X-ray diffraction measurements and ab-initio calculations, the weak magnetism is found to be intrinsically related to the B2 ordered structure in which the magnetic moments of the antiferromagnetically coupled Mn atoms located at 4a and 4b sites are cancelled out each other. Both experimental and theoretical studies confirm that Ni–Mn–Ti alloys possess the significantly enhanced mechanical properties compared with the conventional Ni–Mn-based alloys. Electronic structure investigations reveal that this could be ascribed to the replacement of d-d interaction for the strongly covalent p-d hybridization. Outstanding elastocaloric effect of the all-d-metal alloy opens a door for searching novel promising materials used as the elastocaloric refrigerants.
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•A giant adiabatic temperature change of −20.4 K is realized in the 〈100〉-textured Ni50Mn31.75Ti18.25 bulk alloy.•The giant elastocaloric effect is attributed to the large volume change and the negligible magnetic entropy change.•The weak magnetism of Ni–Mn–Ti alloys is ascribed to the intrinsically B2 ordered structure.•Ni–Mn–Ti alloys possess the significantly enhanced mechanical properties compared with the conventional Ni–Mn-based alloys.•Origin of the enhanced mechanical properties is the replacement of d-d interaction for strongly covalent p-d hybridization.
Solid-state cooling based on the caloric effect of phase transformation materials has attracted considerable interest with the increased demand for energy-efficient and environmentally friendly ...cooling technologies. Here, we have systematically studied the microstructural evolution, martensitic transformation (MT) behaviors, and elastocaloric effect (eCE) of directionally solidified Ni35.5Co14.5Mn35Ti15 all-d-metal Heusler metamagnetic shape memory alloys. Electron backscatter diffraction (EBSD) analysis revealed the coexistence of Heusler-type 〈001〉-oriented dendritic crystal and Ti-poor interdendritic phase for as-solidified alloys. Upon homogenization annealing, the sample presents a columnar-like morphology with 〈105〉 textured five-layer-modulated (5 M) martensite along the growth direction. Besides, the multi-modulated martensite with 7 M and 8 M structures was identified by transmission electron microscopy (TEM). Using a unique technique of in situ digital image correlation (DIC) with the combination of infrared thermography, MT and eCE behaviors were studied. Ni35.5Co14.5Mn35Ti15 alloy yielded a large adiabatic temperature change (ΔTad) of 11.5 K with a low critical stress (σcr) of 38 MPa and a moderate stress hysteresis (Δσhy) of 54 MPa when subjected to uniaxial stress, resulting in the largest value of |ΔTad/σcr| = 0.31 K MPa−1. This improved eCE is attributed to the enhanced compatibility in the oriented polycrystalline alloy and the high mobility of the low-energy twin boundary in the multi-modulated martensite.
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•A machine learning workflow was developed to evaluate elastocaloric property.•The adiabatic temperature change of ternary NiMn-based SMAs was predicted.•The volume change of NiMnTi SMA was ...calculated to verify elastocaloric property.
Shape memory alloys (SMAs) have the potential to improve the efficiency of solid-state refrigeration technology through coupled excitation of multiple thermal effects. Aiming to achieve high elastocaloric NiMn-based SMAs, this paper utilized machine learning to predict the adiabatic temperature change and first-principle calculations to elucidate the mechanism. Based on the optimal XGB Regressor model, the Ni50Mn33Ti17 SMA through directional solidification is predicted to have the highest adiabatic temperature change of 10 K (test temperature = 298 K, applied stress = 300 MPa). In addition, the volume change ratio after martensitic transformation reaches 2.375 % with first-principles calculations, which is expected to provide sufficient entropy and thus obtain an excellent elastocaloric effect. This study provides an available pathway to design and optimize the elastocaloric property of NiMn-based SMAs.
Ti–Ni based shape memory alloys are promising elastocaloric materials for solid-state refrigeration. In this paper, we made a comprehensive study on elastocaloric effect of a nanocrystalline ...Ti–44Ni–5Cu–1Al (at%) alloy exhibiting successive B2–B19–B19′ transformation. The maximum adiabatic temperature decrease by stress removal from 600 MPa is ΔTadi∼25 K with a small temperature distribution of ∼0.5 K. The value of ΔTadi is consistent with that calculated from strain-temperature relation. The effective working temperature window is ∼55 K, resulting in a high refrigeration capacity of RC = 4.2 kJ/kg. Material coefficient of performance reaches COP ∼9.6 when an Otto cycle is considered. The middle eigenvalue of the transformation matrix is λ2 ∼ 0.99, implying high lattice compatibility between the parent and martensite phases. These properties are related to the diffuse nature of successive B2–B19–B19’ martensitic transformation of this alloy.
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Cyclability of elastocaloric effect is of crucial importance for practical applications of elastocaloric refrigeration which is a promising alternative to the conventional cooling technology based on ...vapor compression. The well-known Ni-Mn-based magnetic shape memory alloys exhibit fascinating multicaloric effects (including elastocaloric, magnetocaloric and barocaloric effects), but they are intrinsically brittle because of weak grain boundary cohesion, which results in poor cyclic stability of elastocaloric effect. Here we demonstrate that microalloying with boron is very effective in enhancing the mechanical properties and cyclic stability of elastocaloric effect in Ni-Mn-In intermetallic magnetic shape memory alloys. The elastocaloric effect of the boron-free Ni51.5Mn33In15.5 alloy degrades rapidly after only ∼20 cycles; in contrast, that of the boron-doped (Ni51.5Mn33In15.5)99.7B0.3 alloy remains stable with almost no degradation for more than 150 cycles. The enhancement of mechanical properties and cyclic stability of elastocaloric effect is mainly attributed to the increase of gain boundary cohesion and grain refinement resulting from microalloying with boron. Furthermore, by virtue of its enhanced mechanical properties, a high adiabatic temperature change up to 6.6 K (under 550 MPa), a large stress-induced entropy change of 20.0 J kg−1 K−1 (under 300 MPa) and a high coefficient of performance of 18 were successfully achieved in the boron-doped (Ni51.5Mn33In15.5)99.7B0.3 alloy showing a working temperature just around room temperature. These advantages make this alloy promising for room-temperature elastocaloric refrigeration. This study is instructive for designing high-performance elastocaloric materials for solid-state mechanocaloric cooling applications.
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Elastocaloric cooling attracts broad interest and rapidly growing attention due to its potential for high efficiency and low environmental impact. While it is common knowledge that triggering ...reversible entropy and temperature changes with stress applied in compression prevents rapid failures of materials, realizing this regime in elastocaloric systems is highly challenging because nearly all geometries suited for efficient heat transfer are prone to buckling even under modest loads. This work describes a concept of a novel composite, where an active NiTi layer is embedded into a polymer support structure such that the elastocaloric material is entirely in compression when the assembly is subjected to bending. The active layer achieves 8.1 K temperature change at 2.5% compressive strain without buckling. After 10,000 cycles at 2% compressive strain, the composite maintains mechanical integrity without degradation of the elastocaloric effect. The results demonstrate that NiTi and, potentially, other elastocalorically active materials in geometries previously thought impossible can be successfully implemented in regenerative cooling systems operating in compression.
•A rotary prototype for air conditioning based on elastocaloric effect is introduced.•A 2D rotative numerical model has been used to design the prototype.•A map of the energy performances under ...variable operating conditions is presented.•28.5 K and 5400 W kg-1 are the achievable peaks of temperature span and cooling power.•6.22 is the maximum coefficient of performance weighted as 60% of II law efficiency.
The urgent needing to address global warming and the rapid depletion of fossil fuel reserves has led to a demand for immediate research in sustainable and clean energy technologies. Elastocaloric cooling is a promising proposal for clean refrigeration because of the zero global warming potential of the shape memory alloys, which are solid-state materials showing elastocaloric effect. The latter manifests when the shape memory alloys are stressed through a mechanical loading, transforming from the austenite phase toward the martensite one and releasing heat, dually transforming from the martensite phase into the austenite phase and absorbing heat. The updated literature accounts for 15 elastocaloric cooling devices, but none is close to commercialization. The efforts are devoted to making this decisive step by implementing new efficient devices.
This paper analyses the energy performances of an elastocaloric rotary prototype employing binary NiTi wires through the first rotary bidimensional numerical model based on the finite element method to attain the device's potential cooling and heating capacities. In this paper, the energy performances of an elastocaloric rotary prototype employing binary NiTi wires are analyzed through a 2D numerical model based on the finite element method to attain the device's potential cooling and heating capacities. The model reproduces the thermo-fluid-dynamic behaviour of an experimental rotary device for air conditioning; meanwhile, the secondary fluid in the device is air. The accuracy of the rotary model easily allows to optimize the operating parameters of the elastocaloric prototype under construction.
Results in terms of outlet air temperature, cooling power and coefficient of performance are presented for different air velocities inside the air channel and different rotation frequencies of the device. A performance map has been obtained by exploring the device's behaviour in the cooling mode under variable working conditions to identify the optimal configuration. A maximum COP of 6.22 (corresponding to a second law analysis efficiency of 60%) was evaluated under an airflow speed of 6 m s-1 and a frequency of 0.3 Hz, corresponding to ϕ∗=0.44. 28.5 K and 5400 W kg-1 are the reached peaks of temperature span and cooling power.
Structural fatigue is the major obstacle that prevents practical applications of the elastocaloric effect (eCE) in cooling or heat-pumping devices. Here, the eCE and fatigue behaviour of Ni-Ti plates ...are systematically investigated in order to define the fatigue strain limit and the associated eCE. Initially, the eCE was evaluated by measuring adiabatic temperature changes at different strain amplitudes and different mean strains along the loading and unloading transformation plateaus. By comparing the eCE with and without pre-strain conditions, the advantages of cycling an elastocaloric material at the mean strain around the middle of the transformation plateau were demonstrated. In the second part of this work, we evaluated the fatigue life at the mean strain of 2.25% at the loading plateau and at the unloading plateau after initial pre-straining up to 6% and 10%, respectively. It is shown that on polished samples, durable operation of 105 cycles can be reached with a strain amplitude of 0.50% at the loading plateau, which corresponds to adiabatic temperature changes of approximately 5 K. At the unloading plateau (after initial pre-strain of 10%), durable operation was reached at a strain amplitude of 1.00%, corresponding to adiabatic temperature changes of approximately 8 K. The functional fatigue was analysed after the cycling and it is shown that once the sample has been stabilized there is no further degradation of the eCE, even after 105 cycles. These results present guidelines for the design and operation of efficient and durable elastocaloric devices in the future.
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