Investigation On Magnetic Refrigerant Capacity Of Ni-Mn-Ga Alloy Microwires At Room Temperature

In this thesis,Ni-Mn-Ga microwires were prepared by melt-extraction method.The microstructure,martensitic transformation,thermal hysteresis,magnetic hysteresis,magnetic properties and magnetocaloric properties of the microwires after annealing under different conditions?including different temperatures,different time durations,ordering or high temperatures?were investigated.The measures and mechanisms for the reduction of the hysteresis were studied.Then the mixing rules obtained by the comparison of theoretical mixing and practical mixing were used for theoretical prediction.The study has shown that the microstructure and transformation temperatures of three types of prepared microwires are completely different.When the annealing temperature T?1223K,with the increase of temperature,the grain grew in the microwires and the transformation hysteresis decreased gradually.However,the composition,microstructure and transformation temperature barely changed.When T>1223K,bamboo grains were formed in the microwires,and the composition changed greatly.As the temperature rises,the room temperature tissue changed from 7M to 5M.Also,the transformation temperature,transformation interval and transformation hysteresis all increased while the magnetic hysteresis was always less than 2 J/kg in contrast.With the prolongation of annealing time,the grain grew in the microwires;and in the room temperature tissue,the 5M increased but the 7M decreased.However,the composition and transformation temperature did not change significantly.The transformation hysteresis increased in both stages and the magnetic hysteresis was less than 0.5 J/kg.After annealing for 6h,the demarcation temperature T_dem of reverse transformation was between 325K and 331K.When T<T_dem,7M?5M transformation occurred.When T>T_dem,7M?A and 5M?A transformation occurred.Compared with the chemical ordering annealing,the composition changed slightly and the room temperature tissue did not change after the high temperature annealing.The transformation interval and the transformation hysteresis were larger,except for the case where the microwires were in a coupled state.Under different annealing conditions,the law of M-H curves was consistent.Increasing temperature and extending time were beneficial to the improvement of magnetic properties.Under 50 kOe,the maximum magnetic entropy changes?35?S_max of NMG3-1223-2,NMG3-1223-6 and NMG3-1223-10 microwires were-4.06 J/kg?K,-2.86J/kg?K,-4.48 J/kg?K,respectively.Their working temperature interval?T_FWHMWHM were 5K,15K,9.5K,resulting in the effective refrigerant capacity RC_eff were 15.32 J/kg,33.64 J/kg,29.09 J/kg,respectively.In addition,the?35?S_max of NMG3-1273-2 and NMG3-1323-2microwires were-5.32 J/kg?K and-4.36 J/kg?K,respectively.Since the?35?S_m of the first-order martensitic transformation?FOMT?and the second-order magnetic transformation?SOMT?was continuous,the?T_FWHM was greatly broaden,up to 70K and 71K,leading to a significant improvement in RC_eff,which was 277.31 J/kg and 253.92 J/kg,respectively.The transformation temperature and the Curie temperature T_c of the mixed microwires NMG-MIX-1 and NMG-MIX-2 were basically the same.Compared with the NMG-MIX-2,the transformation interval and transformation hysteresis of NMG-MIX-1were slightly lower,while the magnetization was higher.Under 50 kOe,the RC values of NMG-MIX-1 microwires at FOMT and SOMT were 72.08 J/kg and 214.64 J/kg,respectively,while the RC values of NMG-MIX-2 microwires at FOMT and SOMT were61.25 J/kg and 219.54 J/kg,respectively.In general,the magnetic entropy change of the mixed microwires conformed to the mass mixing law,but the value of practical testing is slightly different from the value of theoretical calculation.Under 50 kOe,the theoretically optimal mixing ratios of NMG-MIX-3,NMG-MIX-4 and NMG-MIX-5 microwires were5:5,9:1 and 5:9:6.Their RC values were 38.02 J/kg,36.91 J/kg and 36.45J/kg,respectively,which were all higher than any other components.