Effect Of Mischmetal And Strontium Doping On Phase Structure And Magnetic Properties Of NaZn₁₃ Alloys

La-Fe-Si alloys have been considered as one of the most applicable magnetic refrigerants near room temperature due to their low cost and without toxic elements.It has been reported that the magnetocaloric effect of La-Fe-Si alloy can be enhanced by using other rare earth elements instead of La element.In this thesis,La element was partially replaced by mischmetal in non-stoichiometric alloys and the phase structure and magnetocaloric properties were studied.In addition,the phase structure and magnetocaloric properties of La_1-xSr_xFe_11.5Si_1.5nominal alloys were researched.Three series of alloys with nominal compositions of La_0.5+xMM_0.5+x(Fe_1-ySi_y)₁₃and La_1-xSr_xFe_11.5Si_1.5were prepared by arc melting and subsequent high temperature heat treatment.X-ray diffractometer was used to detect the crystal structure of the alloys,and Jade software was used to qualitatively analyze the XRD patterns.The microstructure of the alloys was determined by scanning electron microscopy（SEM）and the content of elements in each phase was determined by energy dispersive spectrometer（EDS）.The magnetocaloric properties of the alloys were characterized by measuring M-T and M-H curves using vibrating sample magnetometer（VSM）.The results showed that the content of Si has significant effect on the phase formation and magnetic properties of the non-stoichiometric alloys.The quantity of 1:13 phase is low in the alloys with Si content of 1.4.The microstructure uniformity is poor due to the enrichment of excess rare earth for x>0.1.The peritectic reaction occurred sufficiently as the Si increased to 1.8,and the 1:13 phase content still maintained more than 90 wt.%for x=0.2.The finite solid solution of Ce,Pr and Nd of the mischmetal in 1:13 phase weakened the interaction strength of Fe-Fe atoms,resulting in the rapid decrease of the Curie temperature with the increase of x value.The Curie temperature kept almost unchanged for x>0.2.The non-stoichiometric alloys showed obvious characteristics of the first-order phase transformation,and large isothermal magnetic entropy change.The maximum isothermal magnetic entropy changes of some alloys were found to be about 20 J/kg K under the change of external magnetic field of 0-2 T.It revealed that the magnetocaloric properties of the alloys can be enhanced by adding about total amount of 1.4 excessive rare earth.The element Sr and its compounds were not detected in the XRD and EDS spectra in La_1-xSr_xFe_11.5Si_1.5nominal alloy.It was concluded that Sr element did not entry in the La-Fe-Si solid solution and form 1:13 phase due to its low melting point.The results of EDS spectrums showed that with the increase of x in the nominal composition,Si content in the main phase increased,the Curie temperature improved,the characteristics of first-order phase transformation weakened and the isothermal magnetic entropy change reduced.When x=0.2,the maximum isothermal magnetic entropy of the material at 210 K under 0–2 T magnetic field is 4.6 J/kg K.The present study provided some guidance for the development of practical magnetic refrigerants near room temperature.