宽温域复合驱动制冷材料高压区熔凝固过程控制

NiMn-based Heusler magnetocaloric materials have attracted much attention during the past two decades, as they are environmental-friendly while at low costs. However, limited operating temperature window restraints its application. Available studies concentrated on the magnetic-field-induced martensitic transformation, while the elasto cooling effect has not been sufficiently explored. Recent investigations revealed that both elasto and magneto cooling could be coupled together to enhance the refrigeration efficiency for elasto cooling with a flexible driving mode. Considering strong anisotropy demonstrated in both magneto and elasto-driven martensitic transformation, the key innovative concept of the present proposal is to integrate the elasto cooling into the magnetically-driven refrigeration system, using high-quality controllable unidirectional Ni-Mn-Sn crystals prepared in a newly customer-designed infrared optical zone-melting furnace filled with high-pressure atmosphere. There are several research objectives. Firstly,the single crystal growth behavior will be identified, especially the solutal partition on the front of solid-liquid interface and the competitive mechanism of crystal orientation. Secondly, the fundamental transformation mechanisms governing the elasto cooling during the reverse stress-induced transformation will be addressed from atomistic and microstructural aspects. Finally, functional fatigue, which produces strong influence on the stability of cyclically cooling, will be investigated, and the effects of texture orientation and strain rate will be clarified to optimize the maximum heat effects while keeping the highest functional stability. This research will develop a new scientific concept of magneto-elasto cooling based on Heusler NiMn-based materials, and more importantly, enriching the solidification theory in single crystal growth to support future research in unidirectional intermetallic compounds containing high manganese or rare-earth elements.

低成本Heusler型NiMn基磁制冷材料不含稀土和有毒元素，具有广阔的应用前景，但制冷工作温区狭窄成为制约其发展的瓶颈。近期发现磁制冷与弹性制冷可以高效耦合并存，但目前研究主要集中在磁制冷，对弹性制冷及二者耦合并存效应关注甚少。申请者在高压浮区技术基础上发展出高压区熔技术，据此提出通过控制凝固过程，基于Ni-Mn-Sn制备出高品质定向材料，利用材料的各向异性，突破现有单纯磁制冷概念，发展一种宽温域磁弹性复合驱动制冷材料。研究内容包括：1）高压区熔定向凝固过程中溶质分凝行为及晶体生长取向的竞争选择机制；2）弹性制冷微观结构影响机制；3）制冷性能表征及功能性疲劳效应。通过本课题，揭示高压凝固过程-材料微观组织结构-制冷性能-功能性疲劳效应之间的内在关联，制定出性能调控优化原则，为开发新型复合制冷材料提供理论依据，同时发展普适于高熔点易氧化强挥发型材料的高压区熔定向凝固理论。

本课题针对富锰或稀土类高活性金属磁制冷材料，采用改进型高压区熔定向凝固过程控制装置完成了高品质定向材料制备，完成了凝固过程溶质分配及晶体生长择优取向行为、微观组织结构演化及磁性能表征与解析。得到以下主要结论：1、采用改进型高压区熔法可以实现高品质高活性磁制冷定向材料制备，操作过程简单且具有较好的重复性；而采用光学浮区法无法完成高活性磁制冷定向材料制备，晶体生长过程中熔区表面极易氧化而形成严重氧化皮，导致生长过程无法正常进行。2、采用改进型高压区熔法制备出的定向材料沿轴向依次为等轴晶区、再结晶区、定向生长过渡区及稳定生长区。其中在7mm/h拉速条件下制备出的Ni-Mn-Sn定向材料，平行和垂直于晶体生长方向施加磁场，磁学性能未表现出显著各向异性；而在14mm/h拉速条件下获得的Mn-Ni-In定向材料，在30kOe平行磁场方向材料的磁制冷能力达到232J/kg，垂直磁场方向磁制冷能力为247J/kg，后者磁制冷能力提升得益于一阶马氏体相变的磁滞损耗显著降低。3、采用改进型高压区熔法制备出La-Fe-Si复相共生定向材料，这种组织花样为金属功能材料强韧化提供了新思路，据此利用快速液态离心成型技术成功制备出复相共生La-Fe-Si薄板材，其力学强度性能与高温热处理时间之间的关系符合经典JMAK方程，高温固态相变属于一维扩散生长模式；4、基于第一性原理密度泛函理论提出的晶体结构重构法较好应用于B、C等微量轻元素合金化La-Fe-Si占位机制解析。