磁-结构耦合材料的熵变大小及组成研究

Novel giant magnetocaloric materials are almost magnetostructural coupling materials. The magnetic entropy change contains multi-parts, such as magnetic and structural entropies. In this project, the study objects are magnetostructural coupling compounds in Tb-Dy-Fe, Sm-Dy-Fe, R-Co (R is rare earth) magnetostrictive systems, and magnetic shape memory compounds in Ni-Mn-Ga system. By using isothermal magnetization, the temperature dependence of specific heat, Yong’s modulus under different magnetic fields, stress-strain curves under different magnetic fields, to investigate the phase transition and the companying entropy induced by the magnetic field, stress, and magnetic field together with stress. We try to reveal each component part of the entropy changes: magnetic, structural, and magnetostructural coupling entropies depending on compound type, the order and type of phase transition, even the sign of magnetostriction, the difference and relationship of phase transition and the companying entropy change induced by single field with multi-field, and the contribution of the magnetostructure coupling to the phase transition and the entropy change. It is expected that a way to increase entropy change by increasing one of subparts of the entropy and to decrease induced field by using multi-field to intrigue will be found. The experimental and theoretical basement will be supported in this project to make magnetostructural coupling materials be promising materials for fridge application.

新型巨磁致冷材料基本都是磁-结构耦合材料，它们的熵变中包含磁熵和结构熵多个组成部分。本项目以磁-结构耦合Tb-Dy-Fe、Sm-Fe、R-Co (R为稀土)等磁致伸缩体系化合物，以及磁形状记忆合金Ni-Mn-Ga化合物为研究对象，通过测量等温磁化、磁场下比热温度曲线、不同磁场下杨氏模量温度曲线、不同磁场下拉伸(压缩)曲线等方法，研究磁场诱发相变、应力场诱发相变、磁场和应力场共同作用诱发相变的相变规律及其伴随的熵变大小，揭示熵变的各组成部分：磁有序熵、结构熵、磁-结构耦合熵大小和占比与化合物体系、相变级别、相变性质以及磁致伸缩符号等的关系。多场作用与单场诱发相变的熵变的区别与内在联系，明确磁-结构耦合对相变及熵变的贡献。探求增大结构熵或耦合熵以增大磁-结构耦合材料的相变总熵以及多场作用降低诱发场的途径。为磁致冷材料的商业应用提供实验支持和理论依据。

固态致冷具有高效率，环境友好等优点，是最有希望取代传统气体压缩致冷的致冷方式，其中磁致冷技术路线成熟，已用于超低温和大型冷库等特殊条件的致冷，压热和弹热致冷是新兴的，最有研究发展前途的固态致冷方式，本项目是研究这几种固态致冷的致冷工质。.在以新戊二醇（NPG）为代表的一系列塑晶材料里，发现了最高达687 J kg-1K-1的熵变，比传统固态制冷材料高一个数量级，并揭示了塑晶材料出现庞压卡效应的深层次物理机制。从本质来源角度确认了庞压卡效应的发现。有机材料压卡效应所需驱动压力小、成本低廉，具有明显的应用价值。同时，将塑晶引入固态相变制冷材料研究领域，将极大地丰富固态相变制冷研究的材料体系，为发现和设计性能更加优异的材料提供了可能。发现著名的Ti-Ni形状记忆合金在低于奥氏体转变结束温度Af下仍具有大的弹热效应，且在略低于Af处弹热熵变有最大值，未发生塑性变形时，Maxwell关系和Clausius-Clapeyron关系均适用于计算弹热熵变，两种方法得到的熵变相差不大，发生塑性变形时，由Maxwell和Clausius-Clapeyron关系得到的弹热熵变相差较大。温度处于超弹性温区时，马氏体相变的临界应力与温度基本呈线性关系，弹热熵变和绝热温度变化随温度变化都较小。研究生长了Fe3Sn2, BaZnBi2, Cr5Te8, ErAl2Ge2，ErNi3Al9等多种新型金属单晶，发现有些单晶的熵变比多晶大25%以上，ErAl2Ge2在20 kOe下即具有大的旋转磁熵变，ErNi3Al9在50 kOe下旋转磁熵变大于它自身的熵变，这两种化合物可用于结构简单的旋转磁致冷机的致冷工质。