基于低温热压烧结与晶界扩散的LaFeSi基磁制冷复合材料的制备及性能调控

Based on the fact that La(Fe,Si)13-based compounds are brittle and hard to process. In this project, NaZn13 type LaFeSi-based compounds with a first-order phase transition characteristic and RE-Co (RE=La, Ce and Pr, etc.) low melting point alloy powders are used as raw materials. LaFeSi-based composite materials for magnetic refrigeration with large magnetic entropy change, high thermal conductivity, high mechanical strength, low (no) magnetic/thermal hysteresis effect will be prepared by low-temperature hot pressing sintering technology combined with grain boundary regulation and grain boundary diffusion techniques. We will explore the mechanism of Low-temperature hot pressing sintering for preparing LaFeSi-based composite materials for magnetic refrigeration. And the thermal conductivity, mechanical properties, magnetic properties (Curie temperature, magnetic entropy change), thermal/magnetic hysteresis of the composites correlated with the process parameters of hot pressing sintering and diffusion annealing, particle size and the content of low melting point alloy will also be clarified. Meanwhile, the influence of grain boundary regulation and grain boundary diffusion on the magnetic properties, thermal hysteresis and magnetic hysteresis of LaFeSi-based composite materials for magnetic refrigerant will be revealed, which will provide practical basis and theoretical guidance for the controllable preparation of high-performance LaFeSi -based composite materials for magnetic refrigeration. It is of great significance to not only solve the problem of commercial application of high-performance LaFeSi-based magnetic refrigeration materials because of their intrinsic brittleness, but also promote the development of room temperature magnetic refrigeration materials and technology.

基于La(Fe,Si)13基化合物性脆、难加工现实，本项目拟以具有一级相变特征的NaZn13型LaFeSi基化合物及RE-Co(RE=La、Ce和Pr等)低熔点合金粉末为原料，利用低温热压烧结并结合晶界调控与晶界扩散技术制备大磁熵变、高热导率、高机械强度、低(无)磁/热滞效应的LaFeSi基复合磁制冷材料；探索LaFeSi基磁制冷复合材料的低温热压烧结机理，阐明复合材料的热导率、机械性能、磁性(居里温度、磁熵变)、热/磁滞与热压烧结及扩散退火工艺参数、化合物颗粒大小以及低熔点合金含量的相互关系；揭示晶界调控与晶界扩散对LaFeSi基复合磁制冷材料的磁性、热/磁滞的影响规律及作用机制，为高性能LaFeSi基磁制冷复合材料的可控制备提供实践依据和理论指导。这不仅有利于解决高性能LaFeSi基磁制冷材料因其本征脆性而限制其商业化应用的问题，而且对促进室温磁制冷材料及技术的发展具有重要意义。

磁制冷技术具有高效、环保、节能等优点而有望应用于室温制冷领域。在众多的磁制冷材料体系中，NaZn13型La(Fe,Si)13基合金具有磁熵变大、居里温度可控、无毒、价格便宜的优点而备受关注。但具有巨磁热效应的三元LaFeSi合金存在居里温度远低于室温，成相困难，以及由于磁体积效应易破碎等问题，严重限制了其在近室温领域的商业化应用。本项目以具有一级相变特征的NaZn13型LaFeSi基化合物及RE-Co(RE=La、Ce和Pr等)低熔点合金粉末为原料，利用低温热压烧结并结合晶界调控与晶界扩散技术制备出了大磁熵变、高热导率、高机械强度、低(无)磁/热滞效应的LaFeSi基复合磁制冷材料。例如，在高于La70Co30合金熔点温度热压制备的LaFe11.6Si1.4/La-Co复合材料，并分别在1323 K扩散热处理。结果表明，熔融态的La70Co30合金在热压过程中起到粘结剂的作用，不仅能有效填充到复合材料的内部空隙，而且能有效降低大尺寸主相颗粒的破碎程度甚至不破碎。随着La70Co30粘结剂的增加，热压复合材料的力学性能亦显著提高。经过1323 K扩散热处理后，来自La-Co粘结剂的Co元素扩散进入LaFe11.6Si1.4主相内部。在扩散热处理过程中，La-Co晶界相具有促进1:13型磁热相重新形成的作用。扩散热处理后，LaFe11.6Si1.4/16wt.%La70Co30复合材料的居里温度由~199 K提升至~238 K，在0-2 T外磁场下，最大磁熵变由5.62 J/kg‧K提升至9.17 J/kg‧K。复合材料的力学性能在扩散热处理后亦进一步的提高。这为低温热压制备NaZn13型LaFeSi基复合磁制冷材料并采用高温扩散退火处理对其性能进行调控提供了新思路。本项目探索了LaFeSi基磁制冷复合材料的低温热压烧结机理，阐明了复合材料的热导率、机械性能、磁性(居里温度、磁熵变)与热压烧结及扩散退火工艺参数、化合物颗粒大小以及低熔点合金含量的相互关系；揭示了晶界调控与晶界扩散对LaFeSi基复合磁制冷材料的磁性、热/磁滞的影响规律及作用机制，为高性能LaFeSi基磁制冷复合材料的可控制备提供实践依据和理论指导。这不仅有利于解决高性能LaFeSi基磁制冷材料因其本征脆性而限制其商业化应用的问题，而且对促进室温磁制冷材料及技术的发展具有重要意义。