磁场下钆基非晶/纳米晶材料制备及其磁制冷性能的研究

As an environment-friendly technology, magnetic refrigeration technology has many advantages, such as low noise, highly energy efficiency and minimization characteristics compared with traditional gas compression refrigeration technology. Rare earth based metallic glasses have many excellent properties, including tunable phase transition temperature, high electrical resistivity, smaller eddy current heating and better corrosion resistance which attracted wide attention in the research field of magnetocaloric effect (MCE). Due to these unique properties of disordered structure in amorphous materials, ferromagnetic metallic glasses have a wider magnetic transition zone which makes them excellent candidates for actual application as magnetic refrigeration materials. However, compared to crystalline counterparts, amorphous materials have lower Curie temperatures (Tc) and lower maximum magnetic entropy change (ΔSM) because they usually have second-order phase transition. Therefore, our design idea is to improve the magnetic coupling of magnetic atoms in monolithic amorphous materials, utilize the wider magnetic transition zone from the disordered structure of amorphous alloys and higher Tc and higher saturation magnetization Ms from the precipitated nanocrystalline in amorphous matrix. To realize the above-mentioned targets, an external magnetic field was innovatively introduced during the preparation of Gd-based materials. The specific procedure is to apply an external magnetic field 1) during the preparation process of amorphous alloy and subsequent relaxation annealing process in order to prepare amorphous materials having a strong anisotropy and local magnetic coupling, 2) during the process of preparation of amorphous/nanocrystalline composites and/or crystallization of Gd-based amorphous materials to obtain amorphous/nanocrystalline composite having a strong magnetic anisotropy. By introducing of an external magnetic field during the preparation Gd-based magentic refrigeration materials, the effect of magnetic field on magnetic coupling, local anisotropy, grain size of precipitated nanocrystallines and their distribution, and grain orientation on saturation magnetization and magnetocaloric effect was systematically investigated in order to fabricate magnetic refrigeration materials with excellent comprehensive properties, including wider magnetic transition zone, higher Tc, large ΔSM and high RC. The inner physical mechanism was also investigated and possible reasons were proposed to explain the experimental results as well.

非晶磁制冷材料由于其宽温区和高制冷能力RC，因而具有良好的应用前景。但是同晶体材料相比，非晶合金的居里温度Tc较低，磁熵变ΔSM不高。因此，本项目的设计思想是提高纯非晶中磁性原子之间的磁性耦合；在非晶中析出纳米晶材料利用非晶的无序结构带来的宽温区和析出的纳米晶的高Tc和高饱和磁化强度Ms。为实现以上目的，我们创新性地把外磁场引入材料制备过程，即在非晶及非晶纳米晶制备及退火过程中，施加外磁场来制备具有各向异性强局域磁性耦合及局域各向异性的影响的非晶，具有各向异性纳米晶析出的非晶/纳米晶磁制冷材料。通过制备过程中外磁场的引入，研究磁场对非晶中磁性原子之间磁性耦合及局域各向异性的影响，外磁场对析出纳米晶的尺寸、分布、取向的影响，并深入研究其对Gd基非晶饱和磁化强度、居里温度转变及磁制冷性能的影响，最终获得宽制冷温区、高Tc、大ΔSM和高RC的综合性能优良的磁制冷工质。

项目主要围绕Gd基非晶/纳米晶材料为主线，针对非晶/纳米晶磁制冷材料材料研究中的基础问题开展了系列研究工作。采用静磁场凝固的方法，成功制备了具有大磁熵变的Gd基非晶/纳米晶复合材料。采用静磁场凝固和退火的方法，成功制备了具有宽温度窗口和大磁熵变的哈斯勒合金，并对其机理进行了研究。采用弛豫退火和晶化退火的方法，成功制备了具有大磁熵变和高居里温度的非晶/纳米晶磁制冷材料，并对其内在机理进行了研究。这些非晶和晶体磁制冷材料的制备方法为高性能磁制冷材料的研究提供了重要前期工作基础。磁制冷材料的研究为开发出具有实用价值的新型磁制冷材料提供了理论依据和发展方向。