| 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...
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