基于中子衍射实验的稀土基多相变体系磁热效应的研究

Materials with large magnetocaloric effect (MCE) hold good prospects on magnetic refrigeration for they can release or absorb heat under magnetic field. How to simultaneously improve magnetic entropy change, adiabatic temperature change, refrigerant temperature span and refrigerant capacity for given MCE materials is one of the challenges in the research area of MCE materials and this project aims to solve this problem. This project takes multi-phase-transition system R-M-T (R=rare earth, M=Ni, Ga, Co, T=Cu, Al, Ge, Si) as research object, takes neutron diffraction as main research method, takes strengthening synergy effect of phase transitions and exploring related physical mechanism as kernel research content, takes simultaneously improving the key MCE parameters as research goal. Complex magnetic structure and physical mechanism of complex order-order transition will be studied by neutron diffraction method. By regulating order-order transition temperature and order-disorder transition temperature discriminatively and purposefully, make the magnetic order reach critical state in the temperature range different transitions overlapping, simultaneously improve the key MCE parameters including magnetic entropy change, adiabatic temperature change, refrigerant temperature span and refrigerant capacity, build and enhance the synergy effect of phase transitions in the multi-phase-transition system. The critical magnetic state in the temperature range different transitions overlapping will be detected at different temperatures and different fields by neutron powder diffraction experiment. The evolution process of magnetic structure driven by magnetic field will be clarified, the essence of synergy effect of phase transitions will be analyzed and the physical mechanism based on which the key MCE parameters can be improved simultaneously will be explored.

磁热效应材料在磁场调控下可以放热或吸热，因而在磁制冷领域具有重要的应用前景。如何同步提高磁熵变、绝热温变、制冷温跨和制冷能力几个参数是面临的挑战之一，本项目将努力解决这一难题。本项目以多相变R-M-T (R=稀土，M=Ni, Ga, Co, T=Cu, Al, Ge, Si)体系为研究对象，以中子衍射为主要研究手段，以强化相变协同作用和探究相关物理机理为核心研究内容，以同步提高几个关键参数为研究目标。借助中子衍射手段研究复杂磁结构和复杂有序-有序相变的物理机理；通过定向调控有序-有序相变与有序-无序相变的温度，使相变交叠温区的磁有序达到临界状态，获得磁熵变、绝热温变、制冷温跨、制冷能力等参数的同步提升，从而在多相变体系构建并强化相变协同效应；借助中子衍射手段对相变交叠温区的临界磁有序态进行变温和变场探测，明确磁场驱动下磁结构的演变过程，分析相变协同作用的本质以及磁热参数同步提高的物理机理。

制冷在生活、生产中具有广泛的应用，基于磁热效应的磁制冷技术在未来的应用中被寄予厚望。低温制冷领域面临的一些问题，比如常用的低温资源液氮、液氦的制备和储存，未来清洁能源氢能的液化和储存以及航空航天中的助燃剂液氧的液化和储存等，都有望通过磁制冷技术来解决。低温磁制冷领域的核心课题之一是低温磁制冷材料的研发。本项目以多相变R-M-T (R=稀土，M=Ni, Ga, Co, T=Cu, Al, Ge, Si)体系为研究对象，以中子衍射为主要研究手段，以强化相变协同作用和探究相关物理机理为核心研究内容，以同步提高几个关键参数为研究目标。通过本项目的实施，发现了以重稀土基R-Co-Si、R-Cu-Si、R-Ni-Al、R-Ni-Si为代表的一系列具有大磁热效应的低温磁制冷材料；又基于自旋调节对Tm-Y-Ga、Ho-Tm-Ni等体系的磁热效应进行了优化提高；还基于中子衍射、洛伦兹透射电镜等先进技术手段，对Cr-Te、Ni-As体系的反常热膨胀行为进行了深入研究并厘清了其中的负热膨胀机理，对Tm-Ho-Ga、Er-Tm-Al、TbGa等体系的磁结构、磁相变进行了系统分析；另外又发现了Tm-Ni体系在液氦温区的巨低场磁热效应，申请了专利。在上述成果中，TmCoSi化合物表现出优异的磁热性能，在0-1 T的磁场变化下其绝热温变峰值高达6.2 K，该数值是目前合金类低温磁制冷材料报道的最大值。该项目的实施为未来探索、设计高性能新型磁性功能材料奠定了重要的材料和物理基础。