Cnitech: research and development of cycle stable magnetic refrigeration memory alloy

In today's energy materials, many material systems using solid-state phase change for energy conversion have some major problems, such as the lack of magnetoelastic effect and the scarcity of multiferrousness above room temperature, which lead to the space limitation of three energy forms of magnetic thermal mechanical interaction, so as to improve the conversion efficiency and enter the bottleneck stage It is the most effective way to overcome these energy conversion difficulties to develop new materials to avoid these intrinsic defects, develop new mechanisms to achieve multi energy intervention, and effectively improve conversion efficiency and functional stability Martensite formation, stress induced thermal effect and phase transformation stability of nimnin based Hasler alloy under magnetic field cycling (source: Ningbo Institute of materials) At present, the phase change materials of magnetoelastic strong coupling, including rare earth iron-based compounds and nickel based Hasler alloy, have the properties of room temperature multi iron with multi field controlled phase change, but the realization of function and the improvement of performance are at the expense of the reversibility of magnetic field, temperature and mechanical cycle phase change, which hinder the practical application of such materials In order to solve the problem of integration of function and structure, the laboratory of rare earth magnetic functional materials of Ningbo Institute of materials has been committed to obtain the balance key points of supporting magnetic and non-magnetic properties by means of microstructure control and advanced preparation and processing technology, such as the growth of [001] direction single crystal co50ni20ga30 alloy by high-pressure optical floating zone directional solidification method, which can be guaranteed in hundreds of mechanical cycles A stress-induced temperature change of 6K was maintained (scripta mater., 2017, 127, 1); a highly textured ni45mn36.5in13.5co5 alloy was grown by directional solidification with high temperature gradient liquid metal cooling, and 8K temperature change was obtained at 5% large strain (appl Phys Lett., 2017, 110, 021906); adding rare earth element TB or transition group element Cu into Ni Mn based heusler alloy, introducing the second phase of toughness appropriately, improved the fracture strength of the alloy, but the thermal effect was not significantly reduced (J alloys compd., 2017, 696, 538; scripta mater., 2017, 130, 278), and systematically studied the optimization conditions of hyperelastic dynamic parameters and refrigeration efficiency (SCI Reports, 2017, 7, 2084) Recently, the laboratory of rare earth magnetic functional materials of cnitech (Ningbo Institute of industrial technology, Chinese Academy of Sciences) cooperated with the University of technology of damstadt, Germany, to prepare a-Fe / La-Fe-Si biphasic magnetothermal materials, which can be processed into sheets with large specific surface area, and can still maintain the initial shape in the subsequent hydrogen absorption treatment Compared with the single-phase alloy, the thermal conductivity of the obtained biphasic hydrogen compounds increased from 2W / MK to 6W / MK at room temperature, and maintained good magnetothermal properties (adiabatic temperature change 5.5k at 1.9t) More importantly, the three-point bending strength of the two-phase alloy is 60 MPa, twice that of the polymer adhesive, and it can still maintain its initial shape after 105 magnetic field cycles The results preliminarily meet the requirements of high magnetocaloric, high thermal conductivity and high strength magnetic working medium, published in Acta mater (2017, 125, 506)   In addition, in the magnetic martensitic phase change materials, the laboratory researchers, in cooperation with Hong Kong University of science and technology, put forward a unified research strategy of phase change heat and cycle properties: the reversible phase change from cubic to monoclinic is characterized by large difference in structural symmetry before and after phase change, high latent heat of phase change and large deformation in superelastic region By designing lattice and adjusting the compatibility of microstructure, the latent heat is not sacrificed At the same time of deformation, the mechanical cycle characteristics and functional reversibility of this kind of materials are improved, and it is pushed to the application frontier of energy conversion materials A giant elastothermal ni50mn31.5in16cu2.5 magnetic shape memory alloy with adiabatic temperature variation up to 13 K was obtained by composition optimization The results of in-situ Laue X-ray diffraction and nonlinear geometric theory of martensite show that the length of the secondary axis of symmetry of twin martensite remains unchanged during reversible phase deformation Under the action of deformation tensor, the first type twin can form a compatible trigeminal boundary of the stress-free interface layer, and the second type twin can form a parallel boundary of the stress-free layer, thus ensuring the transformation boundary In the process of phase transformation, the material will automatically coordinate to form a special compatible microstructure This not only greatly reduces the original phase transformation lag, but also improves the material's functionality and reversibility Therefore, the transformation lag of the alloy reaches a low value of 3k, and the martensitic transformation remains highly stable before and after 105 magnetic field cycles This work provides theoretical support for the development of materials with both giant thermal effect and high phase change cycle stability The results are published in Acta mater (DOI: 10.1016 / j.actamat 2017.05.020) Paper link: http://www.sciencedirect.com/science/article/pii/s135964541730397x