Structure And Magnetocaloric Effect Of Mn-based Room Temperature Magnetic Refrigrant Materials

With the increasingly serious environmental problems and energy crisis,people have a strong demand for new energy saving and environmental protection refrigeration technology.Among the many new technologies,magnetic refrigeration technology,with its unique advantages to meet the needs of social development,has become one of the most promising alternative to traditional refrigeration technology.The refrigeration technology based on the magnetocaloric effect?MCE?has many advantages,such as high energy conversion efficiency,environmental friendliness and low noise and so on.The core of the magnetic refrigeration technology is to find a materials with low cost,no poison,continuous adjustable working range and large magnetic entropy,which has became an important research topic.In recent years,Mn based materials with martensitic transformation got lots of attentions due to their large magnetic entropy changes.In this thesis,the composition,phase stability,structure and MCE of MnNiSi based compounds have been systematically studied.Firstly,the structure,phase transition,magnetic properties and MCE of Mn_0.4Fe_0.6NiSi_1-xGa_x alloy were studied.The results show that with the increase of Ga content,the crystal structure at room temperature is gradually changed from orthorhombic to hexagonal structure,implying that the phase transition temperature movies successively decreased.The formed Curie temperature window?CTW?as wide as 275 K,incluing the room temperature.The two samples of x=0.04 and 0.06 in the CTW possess a maximum magnetic entropy change(-?S_max)of 13.6J/kgK and7.8J/kgK,respectively,under the magnetic field change of 3T.Their refrigerant capacity?RC?were calculated to be 110 J/kg and 136 J/kg.Secondly,in order to optimize the performance of the system,the Mn_0.6Fe_0.4NiSi_1-xGa_x and Mn_0.8Fe_0.2NiSi_1-xGa_x series were also studied.The change of crystal structure and phase transition temperature is the same as that of the Mn_0.4Fe_0.6NiSi_1-xGa_x alloys.However,the CTW in Mn_0.6Fe_0.4NiSi_1-xGa_x and Mn_0.8Fe_0.2NiSi_1-xGa_x samples is much wider,reaching to 332 and 320 K,respectively.Therefore,the whole system can show a magnetostructural transition in a temperature range from 128 to 534 K,which is 406 K wide.It is concluded that,reducing the content of Fe can enhance the Curie temperature?T_c?of the hexagonal phase,and also increase the magnetization significantly.The M_s of the Mn_0.8Fe_0.2NiSi_1-xGa_x alloy is14 emu/g higher than the Mn_0.6Fe_0.4NiSi_1-xGa_x alloy.The increase of magnetization directly lead to the enhancement of the MCT.The values of?S_max in field change of3T for the Mn_0.6Fe_0.4NiSi_1-xGa_x alloys are all above-20J/kgK.And the largest?S_maxax and RC values were obtained in the Mn_0.8Fe_0.2NiSi_0.84Ga_0.16 alloy,which is-48.2J/kgK and 197 J/kg,respectively,under a magnetic field change of 3T.At last,the effect of Co and Ge doping on the phase transition and magnetic properties of MnNiSi system has been investigated.With the introducing of main group element,not just Co,the phase transition of?MnNiSi?_1-x?Co₂Ge?_x system was successfully decreased to 250 K,and a CTW of 221K was obtained.The-?S_max of the x=0.31 and 0.3 samples are 15.87 J/kgK and 9.1 J/kgK,respectively.