| Non-toxic Gd5Si2Ge2 alloys with magnetic phase temperature close to room temperature and giant magnetocaloric effect(GMCE) are the promising candidates for refrigerant materials in recent years. However, their GMCE can only be obtained in high-cost superconducting magnetic fields(5-10 T), which brings out high costs in practical application. On the other hand, the low magnetic phase transformation temperature and magnetic hysteresis also impede the large-scale application of these alloys. Therefore, the improvement of magnetic phase transformation temperature and magnetocaloric effect and the reduction of magnetic hysteresis become currently hot topics of Gd5Si2Ge2 alloy to be investigatedIn this thesis, a systematic study was conducted on the magnetic phase transformation temperature, phase transformation property, magnetic hysteresis in phase transformation process, magnetocaloric effect and magnetic refrigeration capacity under a low magnetic field ranging from 0-1.5 T for the cast and heat-treated alloys with Zn substitution for Si and Ge simultaneously or individually. The mechanisms and roles of the enhancement of phase transformation temperature and magnetocaloric effect after adding Zn into Gd5Si2Ge2 alloys were also studied. The achievements of those studies provide an effective method and basic knowledge for obtaining the GMCE under low magnetic field. The main conclusions are as follows:1) The addition of trace amount of Zn element has a great impact on the phase formation in the solidification process of the alloy, benefiting the formation of Gd5Si2Ge2-type monoclinic structure and inhibiting the formation of Gd5Si4-type orthorhombic structure;The driving force for the first phase transformation is enhanced and the intensity of external magnetic field needed is reduced, and therefore, the GMCE is generated at a low magnetic field ranging from 0 to 1.5 T. For Gd5Si2-xGe2Znx and Gd5Si2-zGe2-zZn2z alloys, when x or 2z changed from 0 to 0.001, the maximum isothermal magnetic entropy change, magnetic phase transformation temperature, and magnetic refrigerant capacity are increased from 5.03 J/kg?K, 276 K, and 55.30 J/kg to 20.70 J/kg?K and 25.30 J/kg?K, 280 K and 284 K, and 96.14 J/kg and 101.00 J/kg, respectively, for a low magnetic field ranging from 0 to 1.5 T. These magnetocaloric properties are better than that of Gd5Si2Ge2 and GdSiGeGa cast alloys reported in literatures and the comprehensive properties are the best among the other Gd5Si2Ge2 alloys.2) The addition of trace Zn element enhances the interaction force between the 4f-4f electrons of Gd atoms, in which the interaction only happened through conduction electrons, and increases the spin-wave energy of Gd atoms, it in turn increases the magnetic moment of Gd atoms; the cell volume of monoclinic Gd5Si2Ge2 phase is reduced without destroying the crystal structure and the interaction energy between the Gd atoms is increased due to the similarity of the covalent radius of Zn, Si and Ge atoms, and therefore the magnetic phase transformation temperature is increased and the peak of ??SM? is widen. For Gd5Si2Ge2-yZny alloys, when y changed from 0 to 0.011, the magnetic phase transformation temperature and magnetic refrigerant capacity are increased from 276 K and 55.30 J/kg to 288 K and 196 J/kg, respectively, for a low magnetic field ranging from 0 to 1.5 T. These alloys show good comprehensive magnetocaloric properties.3) The magnetic anisotropy energy is reduced by adding appropriate amount of Zn element to Gd5Si2Ge2 alloy so that the movement resistance of the magnetic domain wall or the magnetic moment is reduced in the first phase transformation process. It makes the magnetic hystersis to reduce and the magnetic refrigerant capacity to enhance by decreasing the critical induced magnetic field.Whether Zn substitutes Si and Ge individually or simultaneously, the magnetic phase transformation temperature, the maximum isothermal magnetic entropy change, and the magnetic hystersis and cooling capacity of the alloys are improved for a low magnetic field ranging from 0 to 1.5 T, while the Zn substitution mode has an influence on the increment of respective characters.4) Heat treatment temperature significantly influences the magnetocaloric effect of GdSiGeZn alloys. When adding trace Zn to GdSiGe alloy, the heat treatment temperature may be effectively reduced and eutectoid reaction of Gd5Si2Ge2-type monoclinic structure (β-phase) at 773 K is prevented and therefore the stability ofβ-phase is increased; the Gd5Si4-type orthorhombic phase is inhibited and its volume fraction is decreased; the intrinsic performance ofβ-phase and its sensibility to the variation of temperature and magnetic field are improved, which lead to the enhancement of driving force for the first phase transformation. For Gd5Si2Ge2-yZny alloys with y of 0.011, heat treatment at 773 K is beneficial to increasing the magnetic phase transformation temperature, isothermal magnetic entropy change and magnetic refrigerant capacity, and lowering the magnetic hystersis in first phase transformation. When the heat treatment temperature is higher than 1413 K, the magnetic phase transformation temperature, isothermal magnetic entropy change and magnetic refrigerant capacity of the alloys decrease and the magnetic hystersis becomes worse due to the increase of structure defects and decrease of the driving force in the first phase transformation after fast cooling of the alloys.The alloys of GdSiGeZn with our own intellectual property rights are developed, of which the magnetic effect is enhanced about 4~5 times and the magnetic phase transformation temperature is increased about 4 K~6 K for a low magnetic field ranging from 0 to 1.5 T. The research results change, in some extent, the application philosophy that magnetic refrigeration technology should be relied on high applied magnetic field. It makes possible that magnetic refrigeration technology could be applied in lower magnetic field obtained by the cheap NdFeB magnets.
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