Effects Of Al Doping On The Magnetocaloric And Magnetoresistance Properties Of Ni-Co-Mn-Sb Alloys

Heusler alloy has a promising prospect in practical application and has received an extensive attention used in magnetic refrigeration and magnetic sensors due to its large magnetocaloric and magnetoresistance effects.Among Heusler alloys,Ni₅₀Mn_25+xX_25-x(X=In,Sn,Sb)is unique and has a phase transition in both crystallographic and magnetic structure from paramagnetic martensitic phase to ferromagnetic austenite phase simultaneously within a certain range of concentration and temperature.Besides,the Ni₅₀Mn_25+xX_25-x(X=In,Sn,Sb)Heusler alloy with a positive deviation from the stoichiometry has many distinctive physical properties,such as shape memory effect,thermoelastic effect,magnetoelastic,magnetoresistance and magnetocaloric effects and so on,which results in Ni₅₀Mn_25+xX_x(X=In,Sn,Sb)Heusler alloys becomes one focus of the researches in magnetic and magnetic materials at current time.In this thesis,the effects of different concentration of Al doping on the martensitic transformation,magnetocaloric and magnetoresistance effects were studied for the Ni₄₆Co₄Mn₃₈Sb₁₂ Heusler alloy.The main research contents and experimental results were summarize as follows:1?Ni₄₆Co₄Mn₃₈Sb_12-xAl_x(x=0,0.5,1.0,1.5,2.0)serial alloys with Al doping were prepared by high vacuum arc melting method with a post annealing treatment.The composition,fracture morphology,phase structure and transition temperature were characterized by XRD,SEM,EPMA and DSC for Ni₄₆Co₄Mn₃₈Sb_12-xAl_x series.2?The experimental studies on Ni₄₆Co₄Mn₃₈Sb_12-xAl_x(x=0,0.5,1.0,1.5,2.0) alloys show that the phase structure remains almost the same while the transition characteristic temperature gradually increases after doping Al.When the doping of Al is 0,0.5 and 1.0,the SEM results show that there is no significant difference on the fracture.EMPA shows that the composition is uniform,but the interval of phase transition temperature decreases gradually.When the Al dopant is 1.5 and 2.0,the SEM results show that there is a significant difference in the fracture,and the bean-like grains appear.The EMPA results show that the Al enrichment phenomenon occurs,and the phase transition temperature interval increases greatly.3?The magnetocaloric and magnetoresistance effects were studied for Ni₄₆Co₄Mn₃₈Sb_12-xAl_x(x=0,0.5,1.0,1.5,2.0)alloys doped with different concentration of Al.Compared with the Ni₄₆Co₄Mn₃₈Sb₁₂ matrix alloy,the magnetocaloric and magnetoresistance effects are improved to different degrees for Ni₄₆Co₄Mn₃₈Sb_12-xAl_xwhen the doping amount of Al is small at 0.5 and 1.0.Under the magnetic field of 30 k Oe,the martensitic transformation temperature respectively decreases from 8.5 K for undoped Ni₄₆Co₄Mn₃₈Sb₁₂ to 5 K for Ni₄₆Co₄Mn₃₈Sb_11.5Al_0.5 and 4 K for Ni₄₆Co₄Mn₃₈Sb₁₁Al₁ after doping,the magnetic entropy increases from 9.3 J/kg K to 17.80 J/kg K and 13.50 J/kg K,and the corresponding cooling capacity increased from 42.2 J/kg to 72.4 J/kg and 56.3 J/kg,respectively.In addition,the alloy samples exhibit a negative magnetoresistance(MR)effect.Under the constant magnetic field of 30 k Oe,the MR value is increased from–17.1%for the undoped Ni₄₆Co₄Mn₃₈Sb₁₂ alloy to-32.7%for the Ni₄₆Co₄Mn₃₈Sb_11.5Al_0.5 alloy and-20.7%for Ni₄₆Co₄Mn₃₈Sb₁₁Al₁after doping,respectively.When the atomic percentage of doped Al is 1.5 and 2.0,the magnetocaloric effect and MR effect are reduced to different degrees compared with the matrix Ni₄₆Co₄Mn₃₈Sb₁₂ alloy.The experimental results show that a small amount of Al doping can enhance the martensitic transformation temperature and decrease the martensite transformation interval temperature,improve the refrigeration capacity and negative MR effect for the Ni₄₆Co₄Mn₃₈Sb₁₂ Heulser alloy.