大磁卡效应化合物的居里温度研究

The magnetic refrigeration technology is considered as one of the promising new cooling technique by building technologies office of U.S. Department of Energy in 2014 because it is more efficient and environmentally friendly than conventional refrigeration systems. Recently, Astronautics Corporation of America has constructed a large-scale rotary magnetic refrigerator of 2 kW cooling power over a temperature span of 12 K with Electrical Coefficient Of Performance (COPe) > 2. More interesting, the rotary magnetic refrigerator has used the enhancement effect of six layers LaFeSiH compounds with their Curies temperatures (Tc) to be different only in turn 2.5 degrees. It means that highly uniform Tc in everywhere of each layer LaFeSiH is required to obtained large cooling power. Also it is reported LaFeSiH experiences degradation in its magnetocaloric properties and split in its Tc when held close to its Curie temperature. Therefore the uniformity and stability of Curie temperature must be paid attention for the practical application of LaFeSiH compounds. This project aims to find out the major factors affecting the uniformity and stability of Curie temperature as well as appropriate way to control Tc. Specifically, the effect of composition segregation on Tc, and the diffusion behavior of the Tc sensitive elements in peritectoid reaction. Establish the dynamic diffusion model of the sensitive elements in solid phase transformation. It would be interesting, effects of doping elements on the hydrogen atoms distribution in the material, the effects of temperature and pressure in the hydrogen gas - solid reaction on the hydrogen atoms diffusion into interstitial positions, the change of the microstructure and split of Curie temperature in the temperature cycle near its Tc, the hydrogen atoms move in the interstitial positions when the coexistence of the ferromagnetic phase and paramagnetic phase.

美国宇航公司利用6种居里温度(Tc)依次仅相差2.5度的LaFeSiH的叠加效果，使磁制冷样机得到2005W大输出制冷量。证实了磁制冷技术可以实用化。也表明大制冷输出量要求每一种LaFeSiH化合物的Tc处处相同,高度均匀。另外,发现制冷机运行时还会有Tc不稳定而劈裂分解。所以磁制冷材料Tc的稳定性和一致性对磁制冷是否能实用很重要。项目研究制备过程中成分偏析引起Tc分布不均匀; 影响Tc敏感元素在包析反应中均匀扩散的关键因素; 建立Tc敏感元素在固态相变中动态扩散模型。研究固气反应条件和添加元素对氢向LaFeSiH材料的间隙位置扩散的影响、一级相变材料在Tc附近经历变温循坏时微观结构渐变和Tc劈裂规律、在铁磁和顺磁相同时存在时H原子在化合物中的扩散。弄清影响Tc一致性和稳定性的关键因素, 揭示LaFeSiH化合物的Tc和微观组织结构的关联,找出精确控制LaFeSiH化合物Tc的方法。

选择了调节居里温度最有效的添加元素Co替代铁后的LaFe13-x-yCoxSiyC0.15合金来研究合金中居里温度Tc的一致性。发现大尺寸块体合金的Tc不均匀，合金中Co和Si元素有偏析。分析是凝固速度缓慢的缘故。Tc不均匀的块体LaFe13-x-yCoxSiyC0.15磁熵变化最大值△S偏低。计算表明，偏低的△S是由Tc不均匀引起。结果对用添加元素来调节La(Fe,Si)13合金Tc的材料具有重要的参考价值。研究了薄带状La0.7Ce0.3Fe11.6-xMnxSiy的Tc和包析反应退火气氛的关联。发现在低于一个大气压的氩气中退火，合金中的La, Ce, Si,Mn会产生挥发，造成Tc随退火时间增加而明显提高。测试了饱和氢化后的La0.7Ce0.3Fe11.6-xMnxSiyHmax氢化物放置在Tc附近的温度下两年后Tc的变化。La0.7Ce0.3Fe11.6-xMnxSiyyHmax的Tc随放置时间的增加而降低，但是潜热升高。这些改变和La0.7Ce0.3Fe11.6-xMnxSiyyHmax中的Mn含量没有明显的关联。而Mn元素对La0.7Ce0.3Fe11.6-xMnxSiy合金的Tc调节作用在饱和氢化后不如氢化前明显。用中子衍射首次从实验上证明了La0.7Ce0.3Fe11.28Mn0.22Si1.5Hmax 合金中Mn原子替代Fe原子后处于96i (feii) 位置。该结果弄清楚了Mn替代铁后在La(Fe,Mn,Si)13合金的NaZn13结构中所处的位置。制备了公斤级的La0.7Ce0.3Fe11.6-xMnxSi1.45Hmax氢化物,在1T的低磁场下得到了15J/kg.K的巨磁熵变值。La0.7Ce0.3Fe11.6-xMnxSiyHmax氢化物是目前公认最有实用价值的磁致冷材料。本项目研究结果对实际应用非常有用。最后从相形成角度统一研究了各种元素替代铁后合金的Tc和磁性的改变。发现Cr和Ni 元素在1:13相中固溶度很低，只有微量的Cr和Ni元素可以进入 1:13 相，它们导致一级磁相变变为二级相变，△S急剧降低。合金元素 Nb在材料 LaFe11.0Nb0.5Si1.5 中形成了Fe2Nb，未进入1:13相，但降低了 1:13相中Fe含量，虽然材料依然为一级磁相变，Tc有变化。以上替代元素规律适用于镧铁硅合金中铁的任何替代元素。