Researches On The Micro Structure And Magnetic Properties Of LaFeSi Series Magnetic Refrigerating Materials

In this paper,the effects of transition elements Cr,Co,Ni and Nb on the microstructure,crystal structure and magnetic properties of La(Fe.Si)13 alloy were studied.It was found that the Curie temperature of the large-sized Co containing material LaFe13-x-yCoxSiyC0.15 was inhomogenerous.and the Curie temperature decreased along the radial direction from edge to sub-edge region.Due to the redistribution of the Co and Si elements in micro solidification zone between solid and liquid during cooling,the Curie temperature of the material was not uniform.It was found that the non-uniform Curie temperature reduced the maximum value of magnetic entropy change.The doping elements Ni and Cr mainly entered into LaFeSi phase and ?-Fe phase in the as-cast material,respectively.By annealing with a peritectoid action.the Cr and Ni entered into 1:13 phase,which resulted in the disappearance of the first-order transition characteristics of the material and a sharp decrease in the magnetic entropy change.The Nb formed a stable Fe2Nb phase and none Nb entered into the 1:13 phase after annealing.Therefore,the doping Nb had a little effect on the microstructure and magnetic properties of the LaFe11.0Nb0.5Si1.5 alloy that maintained the first order phase transition with large magnetic entropy change about 18J/kgK under 0-3T.The critical contents for Ni and Cr resulting in the change of the nature of the magnetic phase transition for the LaFe11.52-xMxSi1.48(M=Cr/Ni)alloys from first to second order were 0.08 and 0.25,respectively.The nature of the magnetic phase transition for the material changed with the increase of doping Cr/Ni,neither the microsuture nor the crystal structure of the material changed.Due to the increase of the content of Cr/Ni element in the I:13 phase,the nature of the magnetic phase transition of the material changed.The magnetic properties of' the materials near the critical content of Cr and Ni were further studed.It was found that the effect of Ni on the nature of the magnetic phase transition.Curie temperature and magnetic entropy change was greater than that of Cr element.The critical behavior of the LaFe11.7-xCoxSi1.3C0.15(x=0.7,0.8 and 0.9)alloys near the Curie temperature was investigated based on the modified Arrott plots(MAP)and the Oesterreicher-Parker method(OP method).The relatively reliable critical exponents(?,y,and ?)were obtained.The critical exponents obtained by the OP method was in good agreement w ith the prediction of the mean field model,which means that the magnetism of LaFe11.7-xCoxSi1.3 3C0.15 alloys is governed by long-range interaction.In addition,the relative slope(RS)method was used to study the accuracy of the two methods.It was found that the the OP method and the MAP method were more suitable for LaFe10.9Co0.8Si1.3Co.15 and LaFe10.8Co0.9Si1.3C0.15.respectively.The Curie temperature and the temperature window of the negative thermal expansion of the LaFe11.52-xCrxSi1.48 alloy can be adjusted by changing the content of Cr element.At the temperature range of 150-220K,the thermal expansion cofficients of LaFe11.52-xCrxSi1.48 and LaFe11.52-xCrxSi1.48 are-53.46×10-6K-1 and-31.30×10-6K-1,respectively.