Study On Phase Transition And Magnetocaloric Effect In Ni-Mn-based Heusler Alloys

The magnetic refrigeration technology is based on the magnetocaloric effects,which is defined as the reversible temperature change of a magnetic material upon the application or removal from magnetic field in an adiabatic process.Magnetic refrigeration technology has been mentioned to apply in the next generation of new refrigeration technology,because of its environmental friendly,low noise,low power consumption,small size,high efficiency and with the possibility about replacement of gas compression refrigeration.The magnetocaloric-effect materials involve first order magnetic transition?FOMT?and second-order magnetic transition?SOMT?.FOMT materials have bigger entropy changes,but also bigger thermal hysteresis which can reduce cooling efficiency during the cooling cycle.In contrast,SOMT materials show a lower but fully reversible entropy changes.This thesis focuses on the second-order phase transition in Ni-Mn-based Heusler alloys due to their low cost,simple fabrication process and excellent magnetocaloric properties.Thus it is of great significance for application to investigate the effects of composition adjustment to phase transtion and magnetic properties in details.Firstly,Ni₅₀Mn_50-xSn_x?x=14,15,16,17?ribbons were prepared by melt spinning,then the effects of Mn/Sn ratio on phase transition,phase structure and magnetic properties were investigated.The experiment results showed that the inverse martensitic transition was restrained with increasing Sn contents.The martensitic-austenitic phase transition temperature T_M-A of ribbons decreased from 254K?x=14?to 172K.When x=15 and 16,the inverse martensitic transition disappeared.With increasing Sn contents,the Curie temperature of austenitic phaseincreased slowly.At room temperature,all samples crystalized in the cubic L2₁ structure and impurity was not found in all samples.Under external magnetic field change of 5T,the maximal isothermal magnetic entropy changes were 4.0,3.8,3.1 and 3.0 J kg^-1 K^-1 and the relative cooling power were 320.4,300.2,226.3 and 207.7 J kg^-1 for Ni₅₀Mn_50-xSn_x?x=0,1,2,3?ribbons.Secondly,Ni_50-xCu_xMn₃₆Sn₁₄?x=1,2,3?ignots were prepared by arc melting and Ni_50-xCu_xMn₃₆Sn₁₄?x=0,1,2,3?ribbons were prepared by melt spinning,then the effects of copper dopping on phase transition,phase structure and magnetic properties were investigated.Furthermore,the effects of heat treatment temperature for Ni₅₀Mn₃₆Sn₁₄ ribbons were investigated.The experiment results showed that the inverse martensitic transition was restrained with increasing Cu contents.The T_M-A of ribbons decreased from 250K?x=0?to 123K?x=3?.With increasing Cu contents,thenearly remained constant.At room temperature,all ribbons crystalized in the cubic L2₁ structure and impurity was not found in all ribbons.While some impurities were found in ignots.Taking Ni₄₉CuMn₃₆Sn₁₄ ignot and ribbon for example,the maximal isothermal magnetic entropy change increased from 3.8 J kg^-1 K^-1 to 3.9J kg^-1 K^-1 by 2.6%and the relative cooling power increased from 243.2 J kg^-1 to 366.6 J kg^-1 at the external magnetic field of 5T.For Ni₅₀Mn₃₆Sn₁₄ ribbon,the magnetocaloric effect was best in the heat treatment temperature of 1173K.Thirdly,(Ni_43-x3-x Co_xMn₇)Mn₃₉Sn₁₁?x=0,1,2,3?ribbons were prepared by melt spinning,then the effects of cobalt dopping on phase transition,phase structure and magnetic properties were investigated.Furthermore,the effects of wheel speed for(Ni₄₂CoMn₇)Mn₃₉Sn₁₁ ribbons were investigated.The experiment results showed that that the inverse martensitic transition was restrained with increasing Co contents.The T_M-A of ribbons decreased from 205K?x=0?to171K?x=2?.When x=3,the inverse martensitic transition disappeared.With increasing Co contents,thewas linearly increased from 279K?x=0?to 354K?x=3?.At room temperature,all ribbons crystalized in the cubic L2₁ structure and impurity was not found in all ribbons.Under external magnetic field change of 5T,the maximal isothermal magnetic entropy changes were 3.1,3.3,3.1 and 2.8 J kg^-1 K^-1 and the relative cooling power were 285.2,389.4,406.1and 456.4 J kg^-1 for(Ni_43-xCo_x Mn₇)Mn₃₉Sn₁₁?x=0,1,2,3?ribbons.For(Ni₄₂CoMn₇)Mn₃₉Sn₁₁ribbon,the magnetocaloric effect was best in the wheel speed of 20m/s.Finally,all Ni-Mn-based Heusler alloys show second order phase transition in the vicinity of the Curie temperature when analysed with Arrott plots and universal curves theories.