Solid-state cooling based on elastocaloric effect (eCE), i.e., a temperature change coupled to an applied uniaxial stress in elastocaloric materials (eCMs), is an emerging refrigeration technology ...which has a great potential to replace the conventional vapor compression systems. The cyclic stability is vital during long term operation of cooling systems for real commercial applications. The multiple cycling under mechanical loading causes to produce structural and functional fatigue in eCMs. Recently, various feasible strategies, e.g., microalloying, toughening through texture, adjusting the compressive stress mode and grain refinement, etc., have been employed in shape memory alloys (SMAs) to enhance the working stability of eCMs. As structural–/functional fatigue is a crucial challenge for elastocaloric cooling that must be overcome to make the technology commercial, we summarize the state-of-the-art strategies to enhance the cyclic stability in numerous well-studied eCMs. The article elucidates the methodology of these approaches through tailoring the materials or composition, arresting the crack initiation via microstructural modifications and the influence of properties (i.e. ∆Tad) under the cyclic application of stresses. Finally, the current report provides a summary of directly measured adiabatic temperature change (∆Tad) for various eCE SMAs over multiple cycles.
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•The structural and functional fatigue behavior of elastocaloric cooling materials was reviewed.•Numerous up-to-date strategies enhancing the cyclic stability of elastocaloric cooling materials were summarized.•New strategies were proposed to meet the most stringent challenges of improving fatigue life.•The directly measured adiabatic temperature change was outlined for various well-studied elastocaloric cooling materials.
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Large-scale applications of elastocaloric cooling demand bulk materials showing both large adiabatic temperature change (ΔTad) and low-fatigue characteristics at room temperature. ...Using cold-rolling and aging treatment, we synthesize a bulk Ti49.2Ni40.8Cu10 polycrystalline alloy that has microstructural features of nanocrystallinity and epitaxially related Ti(Ni,Cu)2 nanoprecipitates. It exhibits a large ΔTad of 13.8 K and a coefficient of performance of 13 at room temperature. Moreover, the degradation of ΔTad is only 0.3 K after 450 tensile cycles. We attribute the favorable properties to the enhanced reversibility of martensitic transformation during stress cycling, aided by the internal epitaxy-generated stress at the interface between the Ti(Ni,Cu)2 nanoprecipitates and matrix, together with grain refinement. The results indicate that the alloy offers a good balance of multiple objectives, holding promise for solid-state refrigeration applications.
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.
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•A grain size dependent and thermo-mechanically coupled phase field model is newly proposed.•The effects of grain size, texture, gradient-nanograin and bimodal grain structures as well as geometric ...gradient on the elastocaloric effect of NiTi SMAs are revealed.•The COPmat of the optimal microstructure engineering scheme is up to 35.4, which is about 200% higher than that of uniform coarse-grained NiTi SMAs.•The microstructure engineering scheme with geometric gradient shows the lowest input stress of 195 MPa.
By newly introducing a grain boundary energy, a grain size (GS) dependent and thermo-mechanically coupled phase field model was proposed to investigate the elastocaloric effect (eCE) of NiTi shape memory alloys (SMAs), and discuss the effects of GS, texture, gradient-nanograin and bimodal grain structures as well as geometric gradient on the eCE through simulations. The simulated results show that with decreasing the GS, the gradual reduction of the degree of martensite transformation (MT) and the variation of transformation mode in the nano-polycrystalline NiTi SMA system lead to the decreasing absorption of transformation latent heat, and the gradual weakness and even disappearance of local instability during MT decreases the hysteresis, resulting in the increase of the coefficient of performance (COPmat); the deformation dependence of COPmat and the grain orientation dependence of transformation instability lead to a dependence of COPmat on texture, which can be enhanced with decreasing the GS. It is further found that the microstructure engineering (from the perspectives of GS, texture, gradient-nanograin and bimodal grain structures) can significantly improve the COPmat, and the introduction of geometric gradient can effectively reduce the MT start stress. Several microstructure and geometric schemes with considerable adiabatic temperature change (ΔTad) and high COPmat as well as relatively low MT start stress are reported, which can provide an important theoretical guidance for improving the eCE of NiTi SMAs and exploiting excellent SMA-based elastocaloric materials. The proposed phase field model and the characterization method for texture can be used as a reference to simulate the eCE and anisotropic MT of other SMAs.
I. Microstructure engineering schemes: polycrystalline systems with different gradient-nanograin structures and bimodal grain structure; II. Simulated microstructure evolution; III. Improved elastocaloric effect Display omitted
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•Decreasing the specific driving force of NiTi sheet to 5.64 N·g − 1 by bending.•Elastocaloric air cooler with single-row bent NiTi sheets and an air duct is built.•Cooler outputs an ...air flow of 5.5 K lower than room temperature.•Concept of lift air duct design for the elastocaloric air cooler.•Great potential to develop a 1 kW elastocaloric air cooler.
Elastocaloric cooling has emerged as a promising alternative to traditional vapor-compression refrigeration, and miniaturization is essential for commercialization of this cooling technology. Here, a compact NiTi elastocaloric air cooler with low force bending actuation is presented. The phase transformation of bent NiTi sheets (with a maximum strain of 7.20% on the surface of tension side) is induced at a specific driving force of 5.64 N·g−1, which is at least two orders of magnitude lower than those of tensive and compressive modes. The cooler with single-row bent NiTi sheets and air duct is studied, achieving an output air flow of 5.5 K temperature reduction in the cooling cycle and a maximum specific cooling power of 0.137 W·g−1 in experiments. The cooler with multi-row NiTi sheets produces a maximum cooling power of 11.5 W at the current stage. These results show a development potential for the compact and miniature elastocaloric cooling device.
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•Elastocaloric material-polymer composite structures can be actuated by bending.•Active materials are entirely under either tension or compression in the composite.•Forces required to ...actuate NiTi prototype are reduced by more than 50 %.
The elastocaloric effect underpins a promising solid-state heat pumping technology that, when adopted for commercial and residential applications, can revolutionize the cooling and heating industry due to low environmental impact and substantial energy savings. Known operational demonstration devices based on the elastocaloric effect suffer from low endurance of materials and, in most experimental systems, from large footprints due to bulky actuators required to provide sufficient forces and displacements. We demonstrate a new approach which has the potential to enable a more effective exploitation of the elastocaloric effect by reducing the forces required for actuation. Thin strips of NiTi were incorporated into composite structures with base polymer, such that bending the structures results in either exclusively compression or exclusively tension applied to the elastocaloric strips. The structures allow compression of thin elastocaloric strips without buckling, realize more than 50 % reduction in required forces for a given strain compared with axial loading, and open up a wide range of possibilities for compact, efficient elastocaloric devices.
The NiTi SMAs were successfully in-situ synthesized by LDED additive manufacturing, and the microstructural evolution and elastocaloric effect of the NiTi SMAs along parallel and perpendicular to the ...build direction (BD) were investigated. The NiTi SMAs at room temperature consisted of B2 austenite, B19’ martensite and the precipitation phase Ti2Ni, which was diffusely distributed within the B2 grains and at grain boundaries. Grain morphology that crosses the molten pool boundary and elongates along the thermal gradient is found in the plane parallel to the BD direction. NiTi SMAs loaded perpendicular to the BD direction have less energy loss and fewer stress mechanical training times to obtain fully recoverable stress-strain behavior, which shows better stress-strain response behavior than that loaded parallel to the BD direction. The maximum strain and adiabatic temperature changes follow a monotonically increasing trend from the maximum stress when the compressive stress was applied along parallel and perpendicular to the BD direction and rapidly removed. When the 900 MPa stress was removed, the adiabatic temperature changes reached −5.2 K and −5.9 K, respectively. The degradation of the elastocaloric effect of loading parallel to the BD direction reaches 15% after 100 stress-strain cycles (800 MPa). In contrast, the degradation of loading perpendicular to the BD direction is negligible. This work provides a new idea to realize the preparation of NiTi SMAs and to study the elastocaloric effect of NiTi SMAs.
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•NiTi shape memory alloys can realize solid-state cooling technology.•Microstructure of NiTi SMAs prepared by LDED in different directions was analyzed.•Elastocaloric of NiTi SMAs along different directions were studied.•Cyclic stability of elastocaloric effect along different directions was compared.
•The cast Ni-Mn-Sn-Fe samples are obtained by liquid cooling solidification.•These alloys show prominent eCE and excellent superelasticity.•The dendrites of as-cast alloys improve mechanical ...properties.•The underlying mechanism of enhanced mechanical property is researched.
The Ni-Mn-Sn ferromagnetic shape memory alloys exhibit excellent both magnetic field-induced magnetocaloric effect and uniaxial stress-induced elastocaloric effect. However, the intrinsic brittleness and narrow refrigeration temperature regions hinder their practical applications in elastocaloric and magnetocaloric as well other fields. In this work, the as-cast Ni44-xFexMn46Sn10 (x = 1, 2, 3) alloys are prepared directly by cooling solidification. The as-cast alloys improve the mechanical properties and broaden the martensitic transformation temperature region due to the presence of dendrites. The dendrites prevent cracking formation and propagation along the grain boundaries, and the addition of Fe-atoms enriched in the interdendritic region further optimizes the mechanical performance. In addition, the as-cast Ni41Fe3Mn46Sn10 alloy exhibits a large adiabatic temperature variation up to −10.3 K at a moderate stress of 350 MPa. Accordingly, direct cooling solidification of as-cast alloys could be a novel and feasible method for studying elastocaloric cooling technology.
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•Effect of intermetallics on the elastocaloric effect (eCE) of NiTi is studied.•Microcomposites consisting of NiTi and intermetallic phases are designed by phase-field ...modeling.•Highest hitherto reported coefficient of performance (COP) of 67.13 is achieved.•Higher intermetallic volume fraction ensures higher COP but lower temperature change.•Finer aspect ratio of the intermetallic phase induces higher eCE.
The non-transforming intermetallic Ni3Ti phase generated in NiTi matrix by additive manufacturing was previously reported to create elastocaloric composites with a great coefficient of performance (COP) between 11 and 22 Hou et al., Science 366 (6469) (2019) 1116–1121. In this work, we use a fully thermomechanical coupled phase-field model to design microarchitectures considering the effects of all the possible non-transforming intermetallics (Ni4Ti3, Ni3Ti, and Ti2Ni) in NiTi. Our simulations show possibilities of increasing the COP by guiding the type, shape and volume fraction of intermetallics, which are controllable by processing parameters. With 50% intermetallic fraction arranged in strips of 500 nm width perpendicular to the loading direction, the Ti2Ni intermetallic induces higher COP (67.13) than Ni3Ti (16.18) and Ni4Ti3 (14.29), all surpassing that of the bulk NiTi without intermetallics (12.92). Additionally, the COP increases to 79.94 for 65% volume fraction of Ti2Ni and decreases to 56.31 for 35% Ti2Ni content. Even nontrivial designs with 50% of circular or square transforming NiTi domains display high COP of 40.06 and 29.22, respectively. A high COP is achievable by introducing intermetallics having high modulus (for low input energy), thermal conductivity (for temperature change) and heat capacity (for the output energy).
In the present study, the reversible adiabatic temperature change (ΔTad) of shape memory alloys was shown to be proportional to the mechanical work released by the reverse martensitic transformation ...during unloading (ΔWu). As there exists a considerable amount of data (ΔWu) on superelastic stress-strain measurements in shape memory alloys, the proposed relationship allows us to predict ΔTad without caloric measurements. The estimated ΔTad from the ΔWu of different Ti-Ni alloys shows good linear relationship with the directly measured values. Moreover, following such a design criterion and by tuning of composition and thermo-mechanical treatment, a group of Ti-Ni binary shape memory alloys with directly measured ΔTad larger than 35 K under tension were achieved. The large ΔTad and ΔWu can be ascribed to the grain refinement and the heterogeneous internal stress fields after the thermo-mechanical treatment, which have enhanced the critical stress of superelasticity and recoverability of martensitic transformation during unloading.
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