Study On The Microstructure And Magnetoclaoric Properties Of Gd₅(Si_xGe_1-x)₄ Alloys

This thesis presents a study of microstructure and related magnetocaloric properties of Gd₅(Si_xGe_1-x)₄ alloys, where x near 0.5. The discovery of the giant magnetic entropy changes in the series of Gd₅(Si_xGe_1-x)₄ intermetallic compounds made room temperature magnetic refrigeration technology and this alloy system attractive subjects for research and development.An overview on the comparisons of traditional refrigeration techniques with magnetic refrigeration, the advances of room temperature magnetic refrigeration materials, and the comparisons of some parameters, such as isothermal magnetic entropy changes, adiabatic temperature changes, and relative cooling power, of the candidates for magnetic refrigeration is presented. The thermodynamics and calculation methods of MCE are also presented. After that, the importance of developing of room temperature techniques and the motivation of this study is point out in Chapter 1.From point of view of materials engineering, the relationships among the composition, structure, and properties of Gd₅(Si_xGe_1-x)₄ should be established for future applications. In Chapter 2, the microstructure of series Gd₅(Si_xGe_1-x)₄ alloys, with x=1, 0.7, 0.5, 0.35, 0, are investigated and the results show that the column grains and line features in them are the two distinct characteristics in the arc-melted Gd₅(Si_xGe_1-x)₄ alloys and there may be some 5 : 3 Gd₅(Si,Ge)₃-type or 1: 1 Gd(Si,Ge)-type structure phase come along with 5 : 4 matrix.So far, the differences of structure of Gd₅(Si_xGe_1-x)₄, where x near 0.5, prepared by high purity Gd and low purity Gd metals have not made very clear. Therefore, themagnetocaloric effect materials Gd5(SixGei.x)4 with x=0.475 and 0.43 based on 99.94wt% and 99.2wt% purity Gd have been synthesized and their phase components have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) in Chapter 3. Room temperature XRD studies have demonstrated that GdsSi[.9Ge2.i with high purity Gd is Gd5Si2Ge2-type single-phase; whereas Gd5Si1.72Ge2.28 with high purity Gd, whose actual chemistry is shifted into the 5 '. 4 + 1 '. 1 field, has GdsSi2Ge2-type and GdGe-type phases. Both Gd5Sii.gGe2.i and Gd5Si1.72Ge2.2s with low purity Gd consist of Gd5Si2Ge2-type, Gd5Si3-type, and GdGe-type multi-phase. Low temperature XRD studies have confirmed the existence of GdsSi3-type phase in the alloys prepared using low purity Gd. EDS studies have verified the presence of Gd5Si2Ge2-type and GdGe-type phases in the compounds containing low purity Gd and in Gd5Si1.72Ge2.28 with high purity Gd. Moreover, GdSi2-x-type phase might exist in the two as-cast alloys prepared using low purity Gd and Gd5Si1.72Ge2.28 with high purity Gd according to the EDS measurements. Further surveying on the non-stoichiometric alloys indicate that it is difficult to obtain single-phase Gds(SixGei-x)4 materials, where x is near 0.5, if low purity Gd is used.The effects of processes on the structure and magnetic entropy changes of GdsSi2Ge2 alloy are researched in Chapter 4. The magnetic entropy changes of Gd5Si2Ge2 prepared by high purity Gd are -15.91/kg"1^1 and -lSJlkg^K'^O-ST) for arc-melted and 1200°C/lh heat treated GdsSi2Ge2 respectively. The reason of rising of MCE after heat treatment can be attributed to the ascension of the content of GdsSi2Ge2-type structure. The annealing and in situ high temperature powder XRD confirmed that the essence of phase transformation near 300 °C is from the room temperature monoclinic Gd5Si2Ge2 structure to the orthorhombic GdsSL* structure. Stress may induce the (3-y transformation and the transformation temperature can be advanced to about 130°C. The magnetic entropy change of 1200°C/lh heat treated Gd5Si2Ge2 prepared by low purity Gd is -17.4 Jkg'K'^O-ST) and reason of the GMCE is the arising of large amount of Gd5Si2Ge2-type phase. The melt-spinning process verified that the effect of solidification velocity on the structurealloy, that is to say, there is some GdsSLrtype phase arising in the alloy with the rising of velocity. When the 50m/s velocity is adopted, there is a metastable-stable transformation in the DSC curve of Gd5Si2Ge2.The fine structure of Gd5Si2Ge2.2 alloy prepared using high purity Gd and GdsSi2Ge2 prepared using low purity Gd are investigated in Chapter 5. We can observed large amount of stack faulting in Gd5Si2Ge2-type phase. There is some beam-like orthorhombic GdsSL^-type phase depositing on the GdsSi2Ge2 matrix for the alloy prepared using high purity Gd. The line feature in the SEM micrographs is not twinning or secondary phase, which is confirmed by the SADs and it is faulting aggregations where impurities concentrate. There are some dispersive rings present in the SADs of the arc-melted Gd5Si2Ge2 which indicated the arising of amorphous structure in the alloy.