Study On The Structure And Magnetocaloric Effect Of Transition Group Fe-mn Series Alloys

The traditional refrigeration technology is characterized by high energy consumption and pollution of the environment,so the refrigeration technology urgently needs innovation.Among the many emerging refrigeration technologies,magnetic refrigeration technology has become the most promising alternative traditional refrigeration technology due to its advantages of high energy conversion efficiency,environmental friendliness,and low noise.Magnetic refrigeration technology mainly includes magnetic refrigeration materials,magnetic refrigeration prototypes and related theories.The comprehensive performance of magnetic refrigeration materials determines the level of refrigeration efficiency,so the problem that needs to be urgently solved at present is to find magnetic materials with low cost,no toxicity,large refrigeration interval and high refrigeration efficiency.Among the many magnetic refrigeration materials,the transition metal alloy Fe-Mn series alloys have attracted much attention due to their low raw material prices,large magnetic moments,and stable performance.The literature shows that the magnetic physical characteristics of Fe-Mn series alloys are varied,and the magnetocaloric properties of the material can be optimized by means of related composition design,element doping,microstructure control.Under this background,this thesis takes Fe-Mn series alloys as the research object,and enhances the magnetocaloric effect as the research goal,and explores the secondary phase change material Fe-Mn-Al alloy and first-order phase change material Fe-Mn-Ni-Si-Al alloy by means of composition optimization and element doping.The influence rules and physical mechanism of the composition on its crystal structure,magnetocaloric effect and corrosion resistance were studied.Firstly,the compositions of Fe_15-xMn₄₀Al_45+x?x = 0,1,2,3?and Fe₁₅Mn_40-yAl_45+y?y = 1,2,3?were designed,and the effect of Fe/Mn atoms on structure and magnetocaloric properties were studied in detail.The results show that all samples are Cs Cl type body-centered cubic structure.For Fe₁₅Mn_40-yAl_45+y alloy,with the increase of Mn content,the lattice constant increases,and the Curie temperature also increases from 288K to 330K.As for Fe_15-xMn₄₀Al_45+xalloy,with the change of Fe content,the lattice constant and Curie temperature did not change significantly.Under the change of 5T magnetic field,the maximum magnetic entropy of the material is 3.09 J/?kg?K??x=1?.Although the peak values of the magnetic entropy change are relatively small,the values of the cooling capacity RC are large,reaching 179.1 J/?kg?K?.Without thermal hysteresis and magnetic hysteresis,the Fe-Mn-Al alloy is a typical secondary phase change material,and also has good corrosion resistance.Secondly,for the problem of small values of the magnetic entropy change of Fe-Mn-Al alloys,this thesis explores the Fe_0.6Mn_0.4Ni Si_1-xAl_x?x=0,0.02,0.03,0.04,0.05,0.06,0.08,0.12?alloys.The experimental results show that with the increase of Al content,the alloys undergo a transition from Ti Ni Si-type orthorgonal structure to Ni₂In-type hexagonal structure at room temperature,and its structural phase transition temperature and Curie temperature also gradually decrease.At the same time,the temperature can also drive the structural transition.The upper and lower limits of the Curie temperature window are 318K and 136K.When the martensitic transformation temperature is within this temperature range,a magnetic and structural coupling phase transition occurs,resulting in a large magnetic entropy change.The magnetic and structural phase diagram of alloys shows that when the composition is x=0.02to 0.08,the alloy is a first-order phase change material with magnetic and structural coupling,and the other components are second-order phase change materials.Under the change of 5T magnetic field,the maximum magnetic entropy change reach 52.9 J/?kg?K??x=0.04?,the thermal hysteresis is 51K with the magnetic hysteresis is zero,and the RC value also reaches126.5 J/kg.The alloy is a special first-order phase change material with a large thermal hysteresis but no magnetic hysteresis and the corrosion resistance of the alloy is also good.In summary,this thesis optimized the magnetocaloric properties of Fe-Mn series alloys by means of composition design,element doping,microstructure control,etc.By adjusting the temperature window of the structural phase change and the magnetic phase change,the coupling between them is achieved,and a large magnetocaloric effect is obtained,which lays the foundation for the development of magnetic refrigeration.