镍锰镓多晶纤维磁致冷能力优化设计

Magnetic entropy change, working temperature interval and magnetic hysteresis loss of magnetic refrigerant material determines its magnetic refrigerant capacity. In view of the narrow working temperature interval and large magnetic hysteresis loss of the Ni-Mn-based alloys, magnetic refrigerant capacity optimization was studied in polycrystalline Ni-Mn-Ga microwires in the present project. Two methods of compounding microwires with different transition temperatures and introducing partially coupled magnetic-structural transition into the microwires were designed to enlarge the working temperature interval. Combined with the decreased hysteresis loss attributing to the enhanced grain free surface of the one dimensional microwires, improvement of magnetic refrigerant capacity in Ni-Mn-Ga alloy was expected. Based on the compositional dependence of the transformation temperatures in Ni-Mn-Ga alloys and the increased specific surface area of the microwire, different kinds of heat treatments were applied to adjust the transformation temperatures and first order transition range of the microwires. The effect of heat treatments on the transformation temperatures was investigated and several partially coupled states were expected. Effect of grain size and interfaces on the hysteresis of the microwires was studied. Effectively reduced thermal hysteresis (< 3K) and average magnetic hysteresis (< 3 J/kg) were expected after certain heat treatments, and the micro-mechanism of the reduction was studied. Magnetocaloric effect of the compounds and the partially coupled microwires was investigated. Working temperature interval > 50 K and refrigerant capacity > 300J/kg were expected under 50 kOe, and the inner mechanism was addressed. The novel design idea of the present project provides technical methods and guidance for the magnetocaloric effect optimization of similar small size refrigerant materials.

磁致冷材料的制冷能力由其磁熵变、制冷工作温度区间和磁滞后综合决定，针对镍锰基合金制冷温度区间窄、磁滞后大的问题，本项目以镍锰镓多晶纤维为研究对象，通过设计纤维复合化和引入磁-结构部分耦合相变两种方法宽化制冷工作温度区间，并结合一维纤维材料界面可动性强滞后小的特性，优化其综合磁热性能。基于镍锰镓相变温度依赖成分以及纤维比表面积大的特征，采用热处理调节纤维相变温度和一级相变宽度，研究热处理工艺对相变温度的影响规律，获得磁-结构相变部分耦合状态的纤维。研究晶粒尺寸和界面等对纤维滞后的影响，获得有效降低相变热滞后（< 3K）和磁滞后（平均值 < 3J/kg）的热处理方法，并阐明其微观机制。研究复合化状态和磁-结构部分耦合状态纤维的磁热性能，最终实现50 kOe下磁致冷温度工作区间> 50K且制冷能力> 300J/kg，并阐明内在机理。本项目的设计思想可为类似小尺寸材料的磁热性能优化提供技术途径和导向。

磁制冷技术具有高效、绿色环保、结构简单、噪音小等优点，有望替代传统气体压缩制冷技术应用于日常生活中。然而，目前在制冷材料机械稳定性、制冷工作温度区间、滞后等方面，仍有很多重要的基础性和关键性的问题没有得到解决。本项目利用小尺寸材料比表面积大的特征研究了热处理对镍锰镓纤维成分、马氏体结构和相变的影响规律，获得了有效降低滞后的方法及其微观机制，镍锰镓纤维热滞后和磁滞后分别可达1.1K和0.08J/kg。通过复合化的方法宽化镍锰镓纤维磁制冷工作温度区间，获得了优化复合配比并提高综合磁热性能的理论预测方法，为优化类似小尺寸材料的磁热性能提供了技术途径。实现了热处理对磁-结构相变耦合程度的调控，获得了磁-结构相变不耦合、部分耦合和完全耦合状态的镍锰镓纤维，揭示了磁熵变和制冷工作温度区间随磁-结构相变耦合程度的全流程变化规律和内在机理，最终获得了迄今为止镍锰基合金中50kOe下最大的净磁制冷能力值255J/kg和最大的制冷工作温度区间~71K，实现了在单种材料中提高磁制冷能力的目的，并且镍锰镓纤维净磁制冷能力随磁场呈现线性变化特征，60kOe下可超过300J/kg。在以上研究的基础上，拓展高比表面积的一维纤维形态到零维颗粒、二维带材、三维多孔泡沫等材料形态，并拓展材料体系至反磁热材料体系Ni-Mn-X(X=In，Sn，Sb)及其四元、五元合金，大大降低了该类型材料磁致结构相变过程的滞后，改善了材料的韧性，并显著提高了材料的磁制冷能力。此外，还将研究扩展到一维纤维材料的弹热性能，实现了在同一纤维中对晶粒形态的调控，揭示了晶粒形态对记忆合金纤维弹热性能的影响及其内在机制。本项目的完成可为类似多维多尺度材料的磁/弹热性能优化提供理论与实验基础。