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With environmental issues and energy crisis becoming increasingly severe, new solid-state refrigeration technologies have attracted the attention of researchers from various countries with their green and efficient refrigeration potential. Compared to traditional gas-compression refrigeration, the use of solid-state phase-change materials, such as magneto-thermo-mechanical energy conversion, is environmentally friendly and energy-efficient. However, there are two major challenges in the current development of solid phase change materials: First, poor machinability and low thermal conductivity; Second, when using a single external field excitation, the phase transition of the non-holonomic path will lead to limited latent heat of phase change. Therefore, multiphase composites have attracted much attention due to their high thermal conductivity and good mechanical properties. In addition, the enhancement of phase-change thermal effect through multi-field coupling is a powerful means to promote the development of low-energy solid-state refrigeration technology. When the low magnetic field or low stress cannot fully drive the phase change, the action of the original drive field is supplemented by the bias stress or magnetic field excitation, which further promotes the phase change to occur, resulting in a large thermal effect.
Among the solid phase change materials found so far, Nd-Fe-Si alloy is considered to be one of the most promising magnetic refrigeration materials with its great magnetic entropy change, low cost, and adjustable Curie temperature. In the laboratory of rare earth magnetic functional materials of Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, LaFeCoSi/α-Fe dual phase composites were prepared by introducing the endogenous second phase on the basis of the positive ratio components for room temperature yttrium iron silicon materials. The results of the study indicate that when the Fe content is properly increased, the change of matrix composition improves the thermal conductivity of the material, and the endogenous phase plays a role in maintaining good magnetocaloric performance and improving mechanical properties (Applied Physics Letters, 2015, 107, 152403; Patent 201510078240.8). In addition, a hybrid hot press forming technique of hydride and low melting point metal has also been proposed. The composite material manufacturing method significantly shortens the preparation cycle. By controlling the hot-pressing temperature, LaFeSiH/Sn composites with continuously variable phase transition temperatures between 1-17°C, thermal conductivity of 7 W/mK, and compressive strength of 400 MPa were obtained. The overall performance exceeds the commonly used Polymer bonded magnetocaloric complex (Scripta Materialia, 2016, 120, 58; patent 201510975703.0).
Compared with neodymium iron silicon, magnetic martensitic phase change nickel iron neodymium alloy has better mechanical properties and is a highly ductile material. Under the drive of uniaxial stress, phase change occurs accompanied by the release and absorption of latent heat, showing high latent heat and low hysteresis. Recently, Ningbo Institute of Materials and Shanghai University co-cultivation student Li Yang et al. prepared a [420]-oriented Ni54Fe19Ga27 single crystal by optical floating zone method, and obtained a reversible adiabatic temperature of 7.5K under a low critical stress of 30 MPa. The change reveals the nonlinear intrinsic correlation between the non-steady-state kinetic deformation conditions and the thermal effects of phase change, and proposes an effective way to reduce the stress lag by optimizing the deformation conditions, and provides a theoretical basis for solving engineering problems such as cooling efficiency and fatigue life (Scientific Reports, 2016, 6, 25500). In addition, it has been found that super-elastic stress can be greatly reduced by means of mechanical training (Scripta Materialia, 2016, 114, 1), which is very advantageous for the development of low-stress, large-temperature, elastic-thermal materials.
This study was funded by the National Natural Science Foundation of China (51531008) and the General Program (51371184), the Natural Science Foundation of Zhejiang Province (Y40111DA07), the Outstanding Youth Project (LR14E010001), and the General Program (LY16E010002).
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