Effect Of Carbon Doping On The Structure,Phase Transition And Magnetocaloric Properties In（MnFe）₂PSi Compounds

Magnetic refrigeration technology,due to its many advantages such as energy saving,environmental protection and small size,has attracted much attention as a promising alternative to conventional gas-compression cooling for large-scale application.The performance of the magnetocaloric effect material as the core of the magnetic refrigeration system is related to the refrigeration efficiency of the actual application and has become the most critical indicator.MnFePSi first-order magnetic transition（FOMT）material is regarded as one of the most promising room temperature magnetic refrigeration materials due to its low cost,giant magnetocaloric effect（GMCE）and no toxic elements.Although the MnFePSi material has giant magnetocaloric effect,there is still much room for improvement in the entropy change at a low magnetic field.In addition,though the Curie temperature can be tuned in a wide range by changing the composition,the entropy change is also reduced,and this series of alloys are often accompanied by large thermal hysteresis due to structural phase transition,which is not conducive to practical applications.In this paper,a non-stoichiometric（MnFe）_1.95PSi was used as the research object.Non-metallic small atoms C are introduced into the structure to tune the phase transition and magnetic properties and the effects of C doping on the structure、phase transition and magnetocaloric properties were investigated.Firstly,Mn_1.15Fe_0.80P_0.50Si_0.50（x=0,0.01,0.03,0.05）compounds with less impurity phase were prepared by arcmelting followed by twice sintering process.The experiment results showed that all samples crystallize into hexagonal Fe₂P-type structure（space group of P-62m）.Some carbon atoms enter Fe₂P-type phase and occupy the interstitial site with enlarging the unit cell volume,but a part of carbon atoms form some carbon-like phase like amorphous carbon or microcrystalline graphite,leading that the true concentration of carbon atoms entering the Fe₂P crystal structure is different from nominal carbon content.As a whole,T_c can be increased from 289.2 K（x=0）to 321.4 K（x=0.01）andΔT_hys is reduced by about 30%.The samples undergo a field-induced magnetic transition at a lower magnetic field due to adding carbon atoms,soΔS_M can be increased under 1 T and 2 T and GMCE is also well preserved at higher field（5 T）.Next,Mn_1.15Fe_0.80P_0.50-xSi_0.50（x=0,0.01,0.03,0.05）compounds were prepared with minor impurity phase by arcmelting followed by twice sintering process.The experiment results showed tha all samples crystallize into hexagonal Fe₂P-type structure（space group of P-62m）.Except for a small part of the carbon atoms entering the（Mn,Fe）₃Si impurity phase,the other carbon atoms first enter the Fe₂P phase structure and occupy the interstitial site to increase the unit cell volume.And with increasing carbon content,excess carbon atoms tend to form some carbon-like phase like amorphous carbon or microcrystalline graphite.After carbon doping,T_c can be increased significantly andΔT_hys is reduced by nearly 50%.T_c is 289.2 K,346.3 K,346.7K,and 347.2 K andΔT_hys is 31.5 K,16.3 K,17.7 K,and 15.8 K,for x=0,0.01,0.03,and 0.05,respectively.The samples undergo a field-induced magnetic transition at a lower magnetic field due to adding carbon atoms,soΔS_M of x=0.01 can be increased and large magnetic entropy change of x=0.03 and 0.05 can be preserved under 1 T and 2 T.But GMCE under 5 T of carbon-containing samples decrease due to the reduction of latent heat from structural transition.Finally,this study explicates the effects of C doping on MnFePSi from the theoretical perspective.Considering size effects of the dopants,the preference of C for interstitial occupation can be proved from the enlarged volume of carbon-containing samples.The interstitial occupation of C leads to the lattice expanding along a axis and the larger magnetic moments on the Fe atoms（3f）,which enhances the ferromagnetic exchange interaction and finally improves the total saturated magnetic moment and magnetic entropy change.Since the strong positive correlation betweenΔT_hysΔS_maxand|Δa/a|²,enlarged saturation magnetic moment tends to reduce theΔT_hys with limited|Δa/a|² nearby the phase transition.The results of large magnetocaloric effect under low magnrtic field,tuned T_c and lowΔT_hysys indicate that C doping in（MnFe）₂PSi compounds will be an useful tool to adjust the performance of MCE materials.