【作者】 陈远富；

【导师】 涂铭旌；

【作者基本信息】 四川大学 ， 材料学， 2001， 博士

【摘要】 在阅读了国内外大量文献的基础上，本文详细综述了磁致冷材料特别是近室温磁致冷材料的发展现状及趋势。针对近年来国际上取得突破性进展的GdSiGe系合金，首先以国产2N级和3N级金属Gd为原料，对Gd₅Si₂Ge₂合金的一级磁相变和巨磁热效应的重现性进行了研究；研究了非化学计量比GdSiGe合金的磁热性能；然后研究了国产2N级Gd为原料的低成本GxiSiC^系合金；研究了合金化对GdSiGe合金磁热性能的影响；采用同步辐射XPS对GdSiGe系合金费米面附近的电子能带结构进行了分析；最后建立了一级晶体磁相变的数学物理模型，并对其相变机制进行了探索。本研究达到了预期目标，取得了如下创新性研究成果和新的研究进展： 1．采用国产2N级和3N级金属Gd为原料，对Gd₅Si₂Ge₂合金的磁热性能进行了研究。研究发现：1)以国产2N级Gd原料时，Gd₅Si₂Ge₂合金未能重现一级磁相变和巨磁热效应；2)以国产3N级Gd为原料时，在2*10^-5乇的真空系统中，在高纯（5N）氩气的保护下熔炼出的Gd₅Si₂Ge₂合金，在一级磁相变及巨磁热效应两方面都基本上重现了AmesLab．采用4N高纯Gd原料的Gd₅Si₂Ge₂合金的研究结果。国产3N级Gd原料的成本较AmesLab．的4N级Gd原料降低50多倍。Cd₅Si₂Ge₂合金磁热性能重现的成功，对于原材料国产化、降低合金成本具有较大的工程意义。 2．首次研究了非化学计量比GdSiGe合金的磁热性能。研究发现：1)适量的非化学计量比（如Gd₅Si₂Ge₂合金）不仅重现了一级磁相变，而且增强了合金的磁热效应，且能使居里点得到一定提高；2)过量的非化学计量比(如Gd₅₂Si₂Ge₂合金)，则导致了合金一级磁相变的破坏以及磁热效应的大幅度降低。 四川 大学博士学位论文 3．以国产价格低廉的ZN级Gd为原料，通过调整臼：Ge比研究了低成本的Gd（SlyGel、h系合金。研究发现：l）当把a：Ge比从 2：2调整到 1．9：2．！、1．刀：228时，合金在居里点附近的等温磁化曲线从完全不表现出一级磁相变，过渡到不充分的一级磁相变，再到比较充分的一级磁相变；2)Gdsil．才抒地合金表现出比较充分的一级磁相变并获得了巨磁热效应。相对于3N级Gd原料而言，国产ZN级Gd的成本较 3N Gd再阐610倍左右。这对开发廉价、高性能的 Gdsthe系合金具有 ％重要的工程意义；同时对该系合牟的一级磁相变及巨磁热效应产生鹏的研究也具有理论和实践意义。 一，首次建立了ggff4＋描述一级晶体磁相变（一级磁相变）过程的数学物理模型一磁场诱发相变的滞后元模型并对该相变的机帘隧行了分析：1)合金被视为是由多种临界诱发磁场强度不同的滞后元组成；2)磁场诱发相变是逐步进行的，相变过程中铁磁相、JI3iffi＋g两相是共存的：3)在居里点以上，在磁场诱发相变中出现的铁磁相是亚稳态的，只有在磁场强度大于相应的临界相变磁场下才会存在，退磁后又发生可逆相变；4)预测 GdsiGe合金的一级磁相变属于一种新型的切变型相变，即磁场诱发马氏体相变；5)提出的切变相变机制能较好得解释杂质元素、替代元素及组织诚份均匀性、单相性等对一级磁相变进而对磁热效应的影响。 5．采用同步辐射XPS对GdsiGe k电子能带结构分析后，发现：l)合金化对GdsiGe系合金中Gd4f峰及附近低结合能端的复合能带峰、Ge－3d峰等在峰型、峰位、峰宽及相对强度等方面产生了明显的影响，并且这些影响随合金成份的变化出现了不同步变化，这表明GdSDe系合金随成份的变化，在磁相变、磁热性能等方面应该有较大的差异；2)鞭、氧陪因素使 Gd4与 Geod区间的髓峰半高宽减小、光滑性变差及出现小的锯齿峰，同时使oesd峰的相对强度减弱；这表明：杂质本身及因杂质而出现的窄带效应，明显影响了合金中电子的关联耦合，特别是对决定磁性能的4f电子的关联影响，从而对磁性能、磁相变可能产生重要影响，导致一级磁相变的破坏及磁热效应的大幅度下降。更多还原

【Abstract】 In this thesis, magnetic refrigerants, especially room temperature magnetic refrigerants, are comprehensively reviewed. Firstly, native 2N and 3N Gd are used to try to repeat the magnetocaloric properties of GdsSi2Ge2 alloy reported by scientists of Ames Lab (AL). Secondly, the magnetocaloric properties of non-stoichiometric GdSiGe alloys are studied and the cheaper GdSiGe alloys are made. Thirdly, the effect of alloying on the magnetocaloric effect of GdSiGe alloy is investigated. Then, the structure of electronic energy band for GdSiGe alloys is studied by Synchrotron Radiation X-Ray Photoemission Spectroscopy (XPS). Finally, the mathematic and physical model for FOCMPT is presented to describe the phase hysteresis. On the basis of the model, the mechanism of the magnetocaloric effect (MCE) and magnetic phase transition has been analyzed. The main creative results and new progresses in this thesis are presented as follows:1. The result shows that the GdsSi2Gc2 alloy using 2N Gd does not demonstrate first order crystallographic-magnetic phase transition (FOCMPT) and giant magnetocaloric effect (GMCE). However, when 3N Gd as the starting material, the expected FOCMPT and GMCE of GdjSi2Ge2 alloy appear and is basically the same with the results using 4N high pure Gd in AL. The cost of native 3N Gd drops to 1/57 compared with that of AL 4N grade high pure Gd, which is important in engineering.2. For the first time, the magnetocaloric properties of non-stoichiometric GdSiGe alloys are studied. The results show that slight non-stoichiometric GdSiGe alloy, such as Gd5.jSi2Ge2, can not only posses the FOCMPT, but also enhance the magnetocaloric effect (MCE) and raise the Curie temperature. However, excessive non-stoichiometric GdSiGe alloy, such as Gds.2Si2Ge2, can not only destroy the FOCMPT, but also decrease the MCE seriously.3. By mean of adjusting the Si:Ge ratio, native 2N Gd has been used to develop cheaper GdSiGe alloys. When the Si:Ge ratio is 2:2, the alloy using 2N Gd material does not show FOCMPT. However, when the Si:Ge ratio changing from 1.9:2.1 to 1.72:2.28, the GdSiGe alloy demonstrates partial and almost complete FOCMPT. The GdjSii -266228 using 2N? Gd material gains GMCE. However, the cosf of 2N Gd drops to 1/10 compared with that of 3N Gd, which has both engineering significanceto develop cheap and high property GdSiGe alloys, and practical and theoretical significance to explore the mechanism of FOCMPT and GMCE.4. A mathematical and physical model, hysteresis element model of magnetic induced phase transition, is presented for the first time and the phase transition mechanism is discussed. The main ideas of the model are as follows: 1) The GdSiGe alloy is thought as composed of many hysteresis elements with various critical induce magnetic field; 2) The magnetic induced phase transition occurs step by step and the paramagnetic phase and ferromagnetic phase coexist during the phase transition process; 3) The ferromagnetic phase induced by magnetic field is metastable and can only exist when the magnetic field is higher than the critical magnetic field of the reverse phase transition. The nature of FOCMPT induced by magnetic field can be predicted as a new type shear movement phase transition, magnetic-induced martensitic transformation. With the help of shear movement mechanism, some facts can be explained about the effect of the impurities, the substitute elements and the homogeneity of constitution and phase on magnetic phase transition and MCE.5. Synchrotron Radiation X-Ray Photoemission Spectroscopy is used to study the electronic energy band/gap structure of GdSiGe alloys. The results show as follows: 1) There are some obvious changes on me shape, position, FWHW and intensity of Gd-4f peak, Ge-3d peak and the complex peak nearby the Gd-4f because of alloying effect, but these changes are not synchronous with the change of the alloy constituent, which suggests that the magnetic phase transition and magnetocaloric property of GdSiGe alloys should be great dif更多还原