新型巨磁热体系一级相变过程滞后损耗机理和稳定性研究

Novel magnetocaloric materials, such as La(Fe,Si)13-based, Gd5(Si,Ge)4-based compounds, attracted lots of attention due to the giant magnetocaloric effect at high temperature even around room temperature. However, the notable hysteresis loss during first-order transition harms refrigeration efficiency. To investigate the machanism of hysteresis loss and obtain effective means of reducing hysteresis is an inevitable way to increase refrigeration efficiency, and enable practical magnetic refrigeration application of the novel magnetocaloric materials. In this project, we'll choose different magnetocaloric systems to extensively investigate intrinsic and extrinsic factors that affect hysteresis loss, and to study the relations between hysteresis loss and phase transition dynamics and the relations between activation energy barrier and electronic band structure. Through our effort we hope to find multi-ways to control phase transition and reduce hysteresis. On the other hand, we also plan to extensively study instability of different systems caused by different inducements, and the conresponding physics origin behind. Through our researches in this project, (1) we will overall understand the hysteresis loss behaviours and influencing factors; Through systematically adjusting phase boundaries, internal strain and other factors, we can get the intrinsic influence of electronic band structure on nucleation during phase transition, and eventually find the multi-means to reduce hysteresis loss; (2) Through studying critical particle size, critical pressure, critical temperature and other critical factors that cause instibalities in the systems with different particle size and different interstitial atom doping, such as C, H, N, we can overall understand the instability behaviours and the physics origin behind, and eventually guide practical applications.

新型巨磁热材料因呈现高温乃至室温区巨磁热效应而广受关注。然而其通常具有一级相变特征，呈现显著的滞后损耗影响制冷效率，研究滞后损耗机理找到降低滞后的有效手段是实现材料高温乃至室温磁制冷应用的必由之路。本项目将针对不同巨磁热体系深入研究影响滞后损耗的本征和非本征因素、研究滞后损耗与相变动力学的关系、激活能垒与能带结构的关系，找到调控相变、降低滞后损耗的多自由度调控方法。通过研究不同体系、不同诱因导致体系不稳定性的特点和分解后产物的不同，认识不稳定性出现的规律和物理诱因。通过本项研究（1）全面认识巨磁热一级相变体系滞后损耗的一般规律和影响因素；阐明相界、内应力、成核因素、能带结构等对滞后损耗影响途径和规律，找到降低滞后损耗的多途径调控方法；（2）澄清颗粒尺寸、间隙原子（C、H、N等）引入等诱导不稳定性出现的临界尺寸、临界压力、临界温度等因素及背后的物理根源，掌握不稳定性产生的规律，指导实际应用。

本项目针对不同巨磁热材料体系的一级相变特点，系统研究了影响磁共结构相变的因素，分析了滞后损耗来源和对磁热效应的影响。主要工作包括：..1、在六角MM’X体系获得了降低滞后损耗、增强制冷能力的有效手段。发现随着颗粒尺寸的减小，磁滞损耗减小；有效磁制冷能力随颗粒尺寸的减小先上升后下降。当颗粒尺寸在P2(20~40微米)时，有效制冷能力达到最大。本项工作揭示了残余应力、颗粒度调控的磁共结构相变特点、分析了滞后损耗的影响因素和根源，对于推动材料的应用具有重要实际意义。[Sci. Rep. 6,20993(2016)等]..2、利用应力干预手段成功实现了对MnCoGe基体系磁共结构相变过程、滞后损耗的调控。研究了相变动力学，发现残余应力的引入可使材料声学支声子软化，增加晶格的不稳定性，进而实现了宽温区内的巨大晶格负热膨胀。[J.Am.Chem.Soc. 137,1746(2015)等] ..3、利用MnCoGe基体系六角结构原子层间间距对应力高度敏感的特点，获得了静水压调控的巨大压热效应。0-3kbar压力产生的熵变达到52J/kgK, 超过现有绝大多数熵变材料在同等压力或 5 T 磁场驱动下的热效应。进一步地，与美国国家标准技术研究所(NIST)合作利用压力、磁场下的变温中子衍射手段揭示了相关机理。[Sci.Rep. 5,18027(2015)等] ..4、对比研究了化学压力、物理压力（静水压、薄膜应力）对不同磁共结构相变体系耦合和退耦合以及滞后损耗、磁热效应的影响规律。通过快淬技术引入、调控晶粒尺寸和应力分布，获得了具有小滞后损耗的Mn0.98CoGe磁共结构相变材料，有效提高了制冷能力。进一步地，研究了粘结La(Fe,Si)13基体系保持一级相变性质和巨磁热效应的根源；发现了几类具有低温区（2-30K）巨磁热效应的稀土过渡族化合物（如：RCuAl、RFeSi等）。[Sci.Rep. 5,14970 (2015)；Appl.Phys.Lett.104,062407 (2014)等] ..本项目共发表SCI论文29篇（包括：J. Am. Chem. Soc. 1 篇，Sci. Rep. 4 篇，Appl. Phys. Lett. 3 篇），发明专利 13 项（包括：新申请4项，获授权9项），国际、国内邀请报告 9 次。圆满完成项目研究计划，达到预期目标。