基于晶粒间协同作用调控MnFe(P,Si)系磁制冷材料机械稳定性及磁热效应的研究

Magnetic materials, exhibiting a first-order magnetic transition (FOMT), are key components for room-temperature magnetic refrigeration applications due to their giant (large) magnetocaloric effect (MCE). However, the common drawback of mechanical instability, as well as the remaining gap between the current and the desired magnetocaloric performance, hinders their application in real magnetic refrigerators. Here we are proposing a new way to tailor the mechanical stability and the MCE for MnFe(P,Si)-type magnetocaloric materials. The main idea of our proposal is to tune the mechanical stability and the MCE taking advantage of the intergranular synergistic effect. We will investigate the nano- and micro-scale structural evolution during the magnetoelastic transition by means of structural analysis and finite-element simulations. Particular attention will be paid to the distribution of internal stress and its relation with the relative orientation of neighboring grains, which will shed light on the fracture mechanism for the MnFe(P,Si)-type magnetocaloric materials. Besides that, we will try to reveal the influence of intergranular stress coupling on the magnetoelastic transition and the MCE. Finally, by tailoring the orientation of grains, the intergranular synergistic effect will be triggered during the magnetoelastic transition, which will lead to the enhancement of mechanical stability and the MCE for the MnFe(P,Si)-type polycrystalline materials. Our work will offer new insight into the magneto-structural-stress coupling in the FOMT materials and paves the way for their applications in real magnetic refrigerators.

一级磁相变材料由于表现出巨（大）磁热效应，有望应用到室温磁制冷领域，成为颠覆传统蒸气压缩制冷的关键材料。本项目针对一级磁相变材料机械稳定性差的共性问题和增强磁热效应的实际需求，以MnFe(P,Si)系材料为研究对象，提出调控该体系机械稳定性和磁热效应的新思路，其核心思想在于通过调控晶粒取向来优化晶粒间的应力耦合，促使相邻晶粒在磁弹相变时发生协同作用。具体研究内容包括：通过结构表征和有限元模拟，系统研究该体系材料在微纳尺度上的结构演变特性，建立相变诱导内应力的分布规律以及与晶粒取向之间的关联性，进而揭示其断裂机制。同时，阐明相变诱导内应力对磁热效应的影响机理。据此，通过调控晶粒取向，优化晶粒间应力耦合，使得相邻晶粒在磁弹相变时发生协同作用，从而达到增强机械稳定性和磁热效应的目的。该项目的研究成果对于丰富一级磁相变材料中磁-结构-应力耦合的物理内涵，推动其在室温磁制冷领域的实用化有重要意义。

一级磁相变材料由于表现出巨磁热效应，有望应用到室温磁制冷领域，成为颠覆传统蒸气压缩制冷的关键材料。本研究针对一级磁相变材料机械稳定性差的共性问题和增强磁热效应的实际需求，重点开展了四项研究内容：（1）MnFe(P,Si)合金磁弹相变过程多尺度结构演变规律与断裂机制；（2）相变诱导内应力对磁弹相变特性和磁热效应的影响机理；（3）晶粒择优取向的磁制冷材料制备及其机械、磁热性能；（4）高热导和机械稳定性的磁制冷复合材料制备及其性能。通过本项目的相关研究，揭示了相变诱导内应力的起源及其对一级磁相变特性的影响规律，提出了调控相变诱导内应力的有效手段。进而通过调控晶粒取向，获得了晶粒择优取向的磁相变材料，优化了晶粒间的应力耦合作用，使得相邻晶粒在相变时发生协同作用，降低了弹性应变能和相变能垒，提高了相变可逆性，相邻晶粒的协同变形还有效避免了在晶界上造成应力集中，从而达到了同时提高磁制冷材料机械稳定性和磁热效应的目的。此外，在(Mn,Fe)2(P,Si)合金中引入热导率高且延展性好的增强相Cu，制备出了(Mn,Fe)2(P,Si)/Cu复合磁制冷材料，增强相Cu呈连续的三维网格状分布，两相界面平滑、致密，极大地提高了 (Mn,Fe)2(P,Si)材料的热导率和机械稳定性。本研究对于丰富一级磁相变材料中磁-结构-应力耦合的物理内涵，推动其在室温磁制冷领域的实用化有重要意义。