1:5型钕钴基取向多晶合金的各向异性磁热效应研究

The rotation of anisotropic magnetic materials in a constant magnetic field is a new approach to magnetic cooling technology, compared with the traditional magnetic refrigeration. The rotating magnetocaloric effect (MCE) can be manipulated for simpler and more efficient refrigeration systems. It is necessary to exploit materials with large rotating MCE for the applications of this novel cooling technology. In this study, the regulation of spin reorientation transition and magnetocrystalline anisotropy in NdCo5-type polycrystalline alloys will be carried out to enhance the room temperature anisotropic MCE, through doping Si and/or Zr etc. By means of magnetic property measurements, first-principles calculations, X-ray and neutron powder diffraction, the effect of the composition on the spin reorientation temperature and the magnetocrystalline anisotropy constant will be investigated. Meanwhile, the relation between the anisotropic entropy change and magnetic field will be established. The occupation site of the atoms, the electronic, magnetic structure and magnetic domain will be systematically characterized. The regulation mechanism underlies spin reorientation transition, magnetocrystalline anisotropy and rotating MCE of the NdCo5-type alloys with doping elements will be unveiled based on the analysis of electronic structure and magnetic domain. The NdCo5-type alloys with giant rotating MCE in low magnetic fields will be obtained. Our investigation will be expected to provide experimental and theoretical support for the discovery of new anisotropic magnetocaloric materials in rare-earth-transition-metal intermetallics.

与传统的磁制冷相比，恒定磁场下旋转强各向异性材料的制冷方式有望提供简单、高效的制冷系统。开发巨旋转磁热效应材料是实现这一新型磁制冷技术的关键。本项目以具有自旋重取向的1:5型钕钴基多晶合金为研究对象，掺杂Si、Zr等元素，调控自旋重取向相变和磁晶各向异性，增强室温各向异性磁热效应。利用磁性测量、第一性原理计算和X射线/中子衍射等手段，重点研究材料自旋重取向相变温度和磁晶各向异性常数与合金成分的关联规律以及各向异性熵变与施加磁场的内在关系，并系统研究合金系的原子占位、电子结构、磁结构以及磁畴结构等微结构信息，从电子结构和磁畴结构层次揭示1:5型钕钴基合金自旋重取向相变、磁晶各向异性和旋转磁热效应的调控机理，获取低磁场驱动的巨旋转磁热效应钕钴基合金，为开发新型稀土—过渡族金属各向异性磁热材料提供实验基础和理论依据。

磁制冷是一种基于材料磁热效应的固态制冷方式。该制冷技术具有高效、环保、低噪音等特点，具有广泛应用前景。对于巨磁热效应材料，国内外以往研究主要集中于磁场诱导相变的材料体系。作为磁工质使用时，需要将其从磁场中移进移出才能获得制冷效果。据报道，NdCo5单晶在自旋重取向温度区域具有明显的各向异性磁热效应。因此，可以在恒定磁场中旋转单晶样品获得制冷效果，有望使制冷系统进一步简化。然而，NdCo5单晶制备困难、工作温度单一以及磁热效应不够大。本项目以NdCo5基合金为研究对象，结合实验和理论计算，系统研究了元素掺杂对RCo5-xMx (R=Nd, Pr; M=Si, Zr, Fe, Mn, C)合金微观组织和自旋重取向转变行为的影响，多角度阐明了掺杂原子对自旋重取向转变和旋转磁热效应的调控机理。（1）设计和搭建了磁场取向多晶制备装置，可简便易行地获取高度取向的NdCo5基多晶样品。（2）发现Si掺杂替换Co含量x到0.4，合金无杂相析出；合金的自旋重取向转变温度均大幅度提升，而旋转磁热效应几乎不降低。分析了Si原子占位与磁晶各向异性能、自旋重取向转变以及磁热效应的内在关联。（3）发现Fe掺杂替换Co含量x到1.0，合金均存在杂相，微观组织随Fe含量变化；适量Fe掺杂提升了合金自旋重取向转变温度，同时增强了磁热性能。从各向异性能变化角度讨论了旋转磁热效应的增强机理。（4）优化磁体烧结工艺，成功制备出高致密度、高热导率和高取向度的NdCo5基单相磁体。本项目的实施极大地拓宽了NdCo5基合金工作温度区间，并增强了其磁热性能，获得了一系列近室温旋转磁热效应材料。与所报道的同类材料相比，本项目开发的NdCo5-xFex合金系的磁热性能达到了最高水平。本项目的实施为探索新型旋转磁热效应材料提供了重要的实验数据和理论依据。