Magnetocaloric Effect Of Mn_1.25Fe_0.65P_0.5-xSi_0.5+x Bonded Composites

Nowadays,the research and development of magnetocaloric materials for novel room-temperature magnetic refrigeration have been approaching maturity.Among all the magnetocaloric materials,Mn-Fe-based compounds show high efficiency and are energy-saving,environment-friendly,and moreover,it is easy preparation. In this paper,Mn_1.25Fe_0.65P_0.5-xSi_0.5+x（x=0.02,0.04,0.06,0.08）series compounds were prepared by mechanical alloying and solid reaction.Besides the magnetocaloric effects,the powderization treatment,metal-bonding effects were also investigated.Chapter 1 gives a brief introduction to the background of magnetic refrigeration,magnetocaloric effects,principle of magnetic refrigeration,research on the existing magnetocaloric materials and the thermodynamic basis of magnetocaloric effect.Chapter 2 introduces the equipment that involved during the experiment and data analyzing principle.Chapter 3 reports the magnetic and crystal properties of the Mn_1.25Fe_0.65P_0.5-x Si_0.5+x（x=0.02,0.04,0.06,0.08）compounds.It is found that the whole series of compounds show a first-order phase transition（FOMT）.With increasing Si content,the Curie temperature tends to increase gradually,while the thermal hysteresis is kept constant and the value is about 5 K.The highest Curie temperature（300 K）is achieved for the compound with x=0.58.The largest magnetic entropy change is achieved for the compound with x=0.52,the value is about 2.83（J/kg·K）for a field change of 1.5 T.All the compounds crystallize in Fe₂P-type hexagonal structure with a space group ofPˉ62m.In Chapter 4,The Sn-bonding effects on the structure and magnetocaloric effects of Mn_1.25Fe_0.65P_0.5-xSi_0.5+x（x=0.02,0.04,0.06,0.08）compounds were discussed.It is found that besides the reflection peaks of main phase（Fe₂P-type hexagonal structure）,the reflection peaks of Sn is also observed in XRD pattern.The lattice parameters a,c and the cell volume V are hardly affected.The studies on the magnetocaloric effect of the sample show that the magnetic entropy curves have been broadened by the Sn-bonding.However,it shows the side effect on the sharpness of magnetic phase transition and the magnitude of magnetocaloric effect.In Chapter 5,the effects of metal Zn-bonding on the structure and magnetocaloric effect of Mn_1.25Fe_0.65P_0.5-xSi_0.5+x（x=0.02,0.04,0.06,0.08）series were studied.It is found that besides the reflection peaks of main phase（Fe₂P-type hexagonal structure）,the reflection peaks of Zn is also observed in XRD pattern.The lattice parameters a,c and the cell volume V are hardly affected.The studies on the magnetocaloric effect of the sample show that the magnetic entropy curves have been broadened by the Zn-bonding.However,the sharpness of magnetic phase transition and the magnitude of magnetocaloric effect are slightly influenced by Zn-bonding.The full-width at half maximum（FWHM）of magnetic entropy change curves are broadened.