Study On The Magnetic And Magnetocaloric Properties Of ?Mn,Fe?₂?P,Si,Ge,B? Compounds Prepared By Spark Plasma Sintering

Taking magnetic materials as refrigerant,magnetic refrigeration technology uses magnetocaloric?MCE?,which is an intrinsic property of magnetic materials,to achieve cooling purposes.Comparing with gas compression refrigeration,magnetic refrigeration technology shows advantages of high efficiency,energy saving,environmental protection and small machine volume due to the utilization of solid material with large MCE as working medium.MnFeP_1-x-x As_x compounds are potential magnetocaloric materials because of excellent magnetic properties,simple preparation process,low cost and giant MCE.However,the compounds contain highly toxic Arsenic.Therefore,non-toxic Si and Ge are usually used to replace As completely in?MnFe?₂?P,Si,Ge?compounds.The magnetic properties of the compounds can be optimized by adding B element.Normally,the?MnFe?₂?P,Si,Ge,B?compounds are prepared by ball milling-sintering method,rapid quenching method,melting-casting ingot method.These methods consume a lot of time and energy,and the resulting compounds exhibit low compactness.In this thesis,the?MnFe?₂?P,Si,Ge,B?compounds were prepared by spark plasma sintering?SPS?,which shows advantages of fast heating rate,short sintering time and low sintering temperature.The effects of sintering pressure,sintering temperature and sintering time on the microstructures and magnetic properties were studied in the compounds.Besides,the effects of adding boron element and adjusting the atomic ratio of Si to P on the magnetic properties and magnetocaloric effect were also investigated.The nominal composition Mn_1.15Fe_0.85P_0.65Si_0.13Ge_0.2B_0.02 has been selected as an object in the study.It is found that the Curie temperature decreases and the magnetic entropy change increases as the interring pressure improves.The value of T_C decreases from 287 K to 264 K and the maximum magnetic entropy increases from 7.9 to 14.6 J/kg K for a field change of 2T,when sintering pressure increases from 10 MPa to 50 MPa.The results of SEM show that increase of sintering pressure is beneficial to improving the density of the compounds.Considering the limited sintering pressure of the present SPS method,the effect of sintering temperature on the magnetic properties was studied with a maximum pressure of 50 MPa in the compounds.The results show that the magnetocaloric effect increases significantly when the sintering temperature is higher than 800?.However,when sintering temperature rises to 900?,the density of sample decreases due to the volatilization of P caused by excessive sintering.According to the analysis of magnetization data,the optimal sintering temperature is in the range of 830⁸70?.The effect of sintering time on the magnetic properties of the compound was studied with sintering temperature of 850?and sintering pressure of 50 MPa.For the sintering time of 5,10,15,20 and 30 minutes,the maximum-?_m values of the compound with a field change of 2T are 7.5,8.3,11.1,10.1 and 8.4 J/kg,the thermal hysteresis are 6,7,4,6 and8K,and the densities are 6.51,6.50,6.53,6.52 and 6.49 g/cm³,respectively.It is concluded that proper sintering time is helpful for increasing the crystal grain size and density.However,excessive prolongation of sintering time improves the volatilization of P,resulting in the decrease of both density and properties in the compounds.Therefore,the optimal sintering time range of 15²0 minutes has been obtained.The XRD patterns reveal that the Mn_1.15.15 Fe_0.85.85 P_0.65.65 Si_0.13.13 Ge_0.2.2 B_0.02 compounds prepared by SPS method exhibit Fe₂P-type hexagonal structure with the space group of6?28).The SEM measurements indicate that the sample contains a main phase with a small amount of secondary phase and holes.The influence of adding B element on magnetic properties of?MnFe?₂?P,Si,Ge,B?compounds has also been studied.For two series of compounds Mn_1.15Fe_0.85Si_0.16Ge_0.2P_0.64-xB_x?x=0.01,0.03,0.04,0.06,0.08,0.10,0.12,0.14,0.16?and Mn_1.15Fe_0.85Si_0.16Ge_0.2P_0.64B_y(y=0.01,0.02,0.03,0.04,0.06 and 0.08,the Curie temperature initially increases and then decreases with the increase of x and y.It can be explained that interstitial B atoms expand the a-b crystal plane which enhances ferromagnetic exchange interaction between Mn-Fe and weakens antiferromagnetic exchange interaction between Mn-Mn and Fe-Fe,leading to the increase of the Curie temperature.With further increase of B element content,partial B atoms may occupy 1b position of Si atoms,which changes the atomic ratio of Si to P.Finally,the Curie temperature decreases.In the last part,the effect of atomic ratio of Si to P on the magnetic properties in Mn_1.15Fe_0.85 Ge_0.2P_zSi_0.78-zB_0.02 compounds was also studied.The Curie temperatures are found to be 358,306,308 and 264 K,the corresponding thermal hysteresis values are 11.7,8.2,11.6,10.1K,and the maximum magnetic entropy changes are 4.4,8.7,6.9 and14.5J/kg K for a field change of 2T for z=0.50,0.55,0.60 and 0.65,respectively.In conclusion,the effects of spark plasma sintering parameters on the magnetic properties and microstructures of?MnFe?₂?P,Si,Ge,B?compounds were systematically studied,and the optimum technological parameters for material preparation were provided.It is expected to provide a room-temperature magnetic refrigeration material with large magnetocaloric effect,simple preparation method and low cost through adjusting the composition.