| In this dissertation, the theoretical research on the mechanism of the magnetic phase transition for the room temperature magnetic refrigeration -GdSiGe alloys and their nano-calculations are presented. Firstly, a large number of researches on the room temperature magnetic refrigerants, especially GdSiGe alloys with the giant magnetocaloric effect, are analyzed extensively. Then the deep-going study on the room temperature magnetic refrigeration -GdSiGe alloys is carried on from the three different aspects: the microscopical theory, the phenomenological theory, and the nano-theory. On the microscopical theory, by applying the important Ising model to the GdSiGe alloys, the magnetic ordering temperatures of various dimensional lattices are calculated unitedly, and the two different kinds of the correlative interactions in the Ising magnetic lattices are compared exactly. On the phenomenological theory, by applying the Landau-Devonshire theory to the magnetic phase transition of the GdSiGe alloys, a phenomenological description for the magnetic phase transition of these alloys is presented reasonably, and the main experiment results of these alloys are described correctly. And on the nano-theory, by applying the nano-calculation to the GdSiGe alloys, the maximum values of the magnetic entropy changes for the nano-microcrystal system in certain conditions are calculated attentively. Therefore, this work obtains some new creative results and progresses, and reaches expect goals.1.Applying successfully the Ising model to the GdSiGe alloys, this work calculates unitedly the magnetic transition temperatures of the various dimensional Ising lattices for the first time, and evaluates qualitatively the correlative interaction parameters between the magnetic moments in the GdSiGe alloys. Therefore the necessary foundation is laid for the research on the microscope mechanism for the room temperature magnetic refrigeration.2.It is initiative to compare exactly the two kinds of correlations in the Ising magnetic lattices, such as the nearest neighbor versus next-nearest neighbor correlations in the normal Ising lattice, the inter-chain versus intra-chain correlations in the two-chain Ising lattice. Consequently the correlative interactions, including their extent in different temperature regionsand the related values for the different kinds of correlations, are analyzed in detail. Therefore the reliable theoretical bases are presented to study microscopically the phase transition mechanism of the room temperature magnetic refrigeration.3.For the first time the Landau-Devonshire theory is applied to the magnetic transition for the GdSiGe alloys, and the critical external field and the critical temperature corresponding respectively to the thermal hysteresis and the magnetic hysteresis are obtained, and the related magnetic phase diagram is plotted. It is a foundation of the phenomenological theory of the magnetic phase transition for the GdSiGe alloys, and acquires a broader perspective and application of the Landau theory.4. With the help of the Gd sublattice the phenomenological theory of the magnetic phase transition for the GdSiGe alloys is established for the first time. The main experiment results are described correctly, and up to now more reasonable magnetizations of the GdSiGe alloys are obtained theoretically. Therefore the transition essence of the GdSiGe alloys can be comprehended deeply.5. Applying the nano-calculation for materials to the room temperature magnetic refrigeration, this work treats the nano-magnetic-material as a cluster system with a changeable interaction for the first time, and connects this changeable correlation with the nano-technology-parameters. The theoretical description to the magnetic entropy change of the nano-microcrystal system is given out, and the maximum magnetic entropy changes for the nano-microcrystal system in certain conditions are obtained. In the mean time, it is initiative to introduce the temperature reliability...
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