稀土-过渡金属间化合物纳米胶囊的制备及其室温磁制冷机理研究

The RxMy nanocapsules with graphite layers as shells and magnetic intermetallic compounds as cores will be prepared by the plasma arc-discharge technique. The compound cores are composed of lanthanide and transition metal elements and the magnetic cores have the characteristic that their Curie temperatures are higher than room temperature. We will systematically research the growth regularity of the nanocapsules with controlled phase composition and size distribution. The correlation between the magnetic properties and phase composition, doped elements of the nanocapsules will be studied in detailed. The principle that the Curie temperature and blocking temperature of the nanocapsules shift to room temperature through tuning the structure and size of the nanocapsules will be studied. In further, the nanocapsule system with wide refrigeration temperature range and highest magnetic entropy change at room temperature would be exploited, because we can utilized the characteristic that their magnetic phase transformation and superparamagnetic properties appear at room temperature at the same time. We also try to construct the semi-theoretical model to describe the correlation between the magnetic entropy change of the nanocapsules and their magnetic properties, structure, magnetic field and temperature. The active agent will be connected on the surface of nanocapsules to decrease their aggregation and magnetic interaction. In further, the nanocapsules will be prepared to the magnetofluid and their magnetocaloric efficiency will be measured on the refrigerating engine. The present proposal will not only supply a new way for exploiting more magnetic nanomaterials as the magnetocaloric materials at room temperature, but also supply more experimental datum and theoretical guide for understanding the physical mechanism of magnetic nanocapsules for achieving high magnetic entropy change at room temperature.

采用等离子体电弧共蒸发技术，制备居里温度高于室温的石墨包裹稀土-过渡金属间化合物RxMy纳米胶囊。系统研究不同制备条件对胶囊的物相与尺寸可控性的影响规律。深入研究胶囊的物相组成、元素掺杂等对其本征磁性质的调制规律。探索利用结构因素与尺寸大小调节胶囊的居里温度和冻结温度向室温移动的物理机制。利用胶囊磁相变与超顺磁现象同时出现在室温附近，得到室温附近具有宽制冷温区与最高磁熵变的最佳体系。建立磁性、结构、外场（温度与磁场）与胶囊室温磁熵变之间关系的半经验数学模型。在胶囊表面链接表面活性剂，减少胶囊间的团聚与相互作用。将胶囊制备成磁流体，在室温制冷样机上测试胶囊的磁制冷效率。本项目研究为开发新型室温磁制冷纳米材料提供了新的思路，为深入理解稀土-过渡金属间化合纳米胶囊在室温下具有高磁熵变的物理机制提供实验和理论指导。

为替代传统气体压缩制冷技术，减少压缩剂对大气环境的污染，一种新的磁制冷技术，由于它的节能、环保、无噪音、操作方便、可靠性高和工作周期长等优点，成为当前国内外研究的热点。具有高磁熵变(ΔSM)的磁性材料成为磁制冷技术能否应用的关键问题。本项目利用稀土-过渡金属化合物纳米胶囊的超顺磁特性，采用等离子体电弧技术制备了石墨或氧化物外壳包裹的RxMy磁性纳米胶囊，得到此类纳米胶囊的成相、晶体结构与尺寸控制规律。首先，发现RxMy的成相受到阳极合金靶材的成分与合金中元素蒸气压的联合控制，得到单相超顺磁纳米胶囊形成的最佳阳极合金成分范围是x=30-50, y=70-50。发现蒸发电流直接控制纳米胶囊的尺寸，找到了颗粒尺寸分布和平均粒径大小与蒸发电流呈指数关系，并由此得到最佳蒸发电流范围为80-100A。发现壳-核结构的形成，特别是石墨外壳的形成决定于碳原子在RxMy合金中溶解度，溶解度越大，所生成的石墨外壳越厚。其次，发现了RxMy纳米胶囊的磁矩密度直接决定于稀土离子在其晶格中的离子磁矩大小，找到了RxMy纳米胶囊具有最高磁矩密度的物相是RAl2系纳米胶囊，具有高磁熵变的最佳特征晶体结构是立方晶体结构，六方或四方晶体结构会诱导高磁晶各向异性，而降低其磁熵变。再次，得到了RxMy纳米胶囊居里温度随非磁性元素掺杂量增加而降低，随纳米胶囊尺寸变小而降低，冻结温度随纳米胶囊尺寸降低而降低。确定RxMy纳米胶囊磁熵变与外场的函数关系，即与温度成反比，与磁场成正比，从物理机制上得到本征磁性质，磁矩密度与磁晶各向异性常数是影响磁熵变的关键参数，即磁矩密度与磁熵变成正比，磁晶各向异性常数与磁熵变成反比，且颗粒越小磁熵变越大。根据Michel超顺磁理论，引入优化结合系数β，利用参数β得到超顺磁RxMy纳米胶囊的磁矩密度，磁晶各向异性，尺寸大小及分布的最优化结合，实现指导调节体系的参数，使体系达到磁熵变的最优化值的目的。本工作对理解化合物纳米胶囊的制备，基本磁性，磁制冷特性具有指导意义，为开发更多新型磁制冷材料奠定重要的基础。