Preparation And Magnetic Properties Of Nb,Ga Doped Mn-Fe-P-Si Magnetic Refrigeration Materials

The development of novel room-temperature magnetic refrigeration materials is one of the frontier topics in the research of magnetic functional materials and refrigeration technology.In this work,Mn-Fe-P-(Si,Ga)series compounds were prepared by rapid solidification strip throwing process and mechanical alloying.This is also the first systematic study on the partial and complete substitution of Ga for Si in this system,which including phase and structure,magnetism,microscopic characterization and magnetocaloric properties.In addition,the effects of Nb substitution for Mn on the crystal structure,thermal hysteresis,Curie temperature,magnetic and magnetocaloric properties of the system were also studied.The main research contents and results are as follows:1.Synthesis of Mn_0.55Fe_1.40P_0.70Si_0.30-xGa_x(x=0,0.03,0.05,0.08,0.10,0.12,0.30)by rapid solidification melt-spun process.The phase and structure,magnetic and microscopic properties of these series compounds were studied.Considering the low melting point of Ga and the volatility of P,we first prepared Fe-P pre-alloy by mechanical alloying,and then prepared this series of compounds by combining the advantages of accurate temperature control and fast solidification in rapid solidification technology.X-ray diffraction results show that with the gradual substitution of Ga,the crystal structure of the compound changes from Fe₂P-based hexagonal structure(03Si to(Fe,Mn)₃Ga.The magnetic measurement results show that the compound changes from ferromagnetic phase to paramagnetic phase,and its Curie temperature are higher than 400 K when x>0.05.The micro-morphology measurement results show that Ga and P are in two different phases,respectively.Comprehensive analysis shows that Ga does not successfully replace Si in the main phase(namely forming single-phase Mn-Fe-P-(Si,Ga)and Mn-Fe-P-Ga compounds)with the selected chemical composition and by using rapid solidification method.2.Synthesis of Mn_1.00Fe_0.90P_0.50Si_0.50-xGa_x(x=0,0.05,0.07,0.10)compounds by mechanical alloying process.Their crystal structure,magnetism,micro-morphology and magnetothermal properties were studied.In the process of sample preparation,Ga and Si powder were fully mixed/coated by water bath heating to solve the problems of low melting point of Ga and thus avoid clusters during ball milling.X-ray diffraction results show that the main phase of this series of compounds is Fe₂P hexagonal structure.With the increase of Ga content,the(Fe,Mn)₅Si₃secondary phase appears.The micro-morphology results show that the Ga concentration increases and enters the secondary phase with increasing of Ga content.Magnetic measurements results show that with the increase of GA content,the Curie temperature T_Cof this series of compounds decreases from 372 K with x=0 to 289 K with x=0.10.The thermal hysteresis increases with increasing of Ga content,the value increases from 14 K for x=0 to 38 K for x=0.10.Under the applied magnetic field change of 0-3 T,the maximum magnetic entropy of this series of compounds decreases with increasing of Ga content.When the introduction of GA,the Curie temperature,thermal hysteresis,magnetic moment and magnetic entropy change of the compound are greatly affected.3.Mn_1.20-xNb_xFe_0.70P_0.50Si_0.50(x=0,0.02,0.03,0.04)series of compounds was successfully synthesized.X-ray diffraction results show that the all the compounds crystalize in Fe₂P-based hexagonal structure,and behaviors in single-phase state.The substitution of Nb atoms for Mn ones has little influence on the crystal structure.The magnetic measurement results show that with the increase of Nb content,the Curie temperature of the compound gradually decreases,from 303 K with x=0to 288 K with x=0.03,and the value of thermal hysteresis show tiny fluctuation(?7 K).In addition,it is found that the maximum magnetic entropy increases with the introduction of Nb into the compounds.The micro-morphology results show that Nb atoms enter the main phase and are evenly distributed,though there are scattered Nb metal clusters in the sample.The comprehensive analysis shows that the substitution of Nb for Mn can adjust the Curie temperature of the compound.And due to the increase of magnetic moment and the faster ferro-paramagnetic phase change rate near the Curie temperature,the maximum magnetic entropy change of the compound increases about 20%.