| Fe2P-based materials can be used as candidates for permanent magnet materials and magnetocaloric materials due to their multifunctional properties such as magnetocrystalline anisotropy and magnetocaloric effect,and have good application prospects in the fields of industry,energy,and national defense.Binary Fe2P alloy crystallizes in the Fe2P hexagonal structure(space group:P-62m)and exhibits a large magnetocrystalline anisotropy,with the easy magnetization axis corresponding to the c-axis,resulting in a large magnetocrystalline anisotropy constant(at 4.2 K,K1?2.3 MJm-3),once became a potential candidate for permanent magnet materials.However,its Curie temperature is too low(TC?214 K)and cannot be directly used in practical applications.In order to combine high Curie temperature and large magnetocrystalline anisotropy,in this work,Fe1.93-xCoxP1-ySiyseries quaternary compounds were prepared by solid-phase sintering method.X-ray diffractometer,Vibrating sample magnetometer,Scanning electron microscope and other instruments were used to characterize their crystal structure,magnetic properties and microstructures.The main research content of the thesis can be divided into the following three parts:1.In this chapter,the crystal structures and different phases of Fe1.93-xCoxP1-ySiycompounds were systematically studied,where 0?y?0.5,0?x?1.1.The results shows that the lattice parameters and crystal structures of this series of compounds evolve with the content of Co and Si elements.Based on Full Prof refined results of the obtained XRD data,we systematically studied the variation of the crystal structure with the chemical compositions,and established the phase diagram of the Fe-Co-P-Si quaternary compounds.The stability range of Fe2P hexagonal structure can be effectively extend by controlling the proportions of transition metals Fe and Co,and non-metallic P and Si.In Fe1.93-xCoxP1-ySiycompounds,with higher Si and lower Co contents,a distortion of the crystal structure occurs,resulting in the appearance of a body-centered orthorhombic structure(BCO structure).At high Co content,when the Fe2P unit cell volume reaches a critical volume of about 102.4(?)~3,the crystal structure transforms into a Co2P-type orthorhombic structure.Compared with the ternary Fe2(P,Si)and(Fe,Co)2P compounds,the simultaneous substitutions of Co for Fe and Si for P in the quaternary compound increase the stability range of the Fe2P-type hexagonal structure,which is positive for the realization of their application for permanent magnet.2.In this chapter,the magnetic properties and magnetocrystalline anisotropy of Fe1.93-xCoxP1-ySiyseries compounds in the range of 0?y?0.5 and 0?x?1.1 were studied.In the Fe1.93-xCoxP1-ySiyseries of quaternary compounds,the effect of Si substitutions for P and Co substitutions for Fe on their Curie temperature and magnetocrystalline anisotropy was systematically studied,and the effect of the simultaneous changes of the two was obtained.The results show that this series of compounds are all ferromagnetic at room temperature,but their Curie temperatures do not show a linear change with the element substitutions of Si and Co,rather shows a complex behavior.The results show that the changes of their Curie temperatures are not only related to the c/a ratio.Change of crystal structure(Fe2P-type hexagonal transform to Co2P orthorhombic),band filling and different magnetic moments due to Co substitutions may play a role.When the Si content is low,the TCincreases with the increase of Co content.When the Si content is 0.2,the change of TCdue to Co content is not obvious.When the Si content is higher,the TCshows a decreasing trend with the increase of the Co content.The polycrystalline Fe1.93-xCoxP1-ySiymaterials showing a clear preferential orientation along c-axis were prepared by powder orientation in an external magnetic field.It was found that the simultaneous substitutions of Si and Co elements lead to a rapid weakening of the magnetocrystalline anisotropy until the easy magnetization c-axis turns to the easy magnetization a-b plane.This results fill the gap in the study of magnetocrystalline anisotropy of Fe1.93-xCoxP1-ySiyseries quaternary compounds.3.In this chapter,the(Fe,Co)2(P,Si)nanoparticles were prepared,and their permanent magnetic properties were studied by changing the ball-milling time and doping content.By estimating the particle size of(Fe,Co)2(P,Si)nanoparticles prepared by dry ball milling using ball milling time of 1 h,5 h,8 h and 16 h,we found that the ball milling time within 8 h results in a significant reduction in grain size,decreased to about 20 nm.But the reduction of the particle size is smaller with further increase of ball milling time.Therefore,selecting the ball milling time of8 h,we studied the influence of simultaneous substitutions of Si for P and Co for Fe on the permanent magnet performance.The results show that the substitution of Si element effectively increases the coercivity and the maximum magnetic energy product,but the substitution of Co element reduces the above properties.Currently,the coercivity of0?0.08 T was obtained at room temperature in the(Fe,Co)2(P,Si)quaternary compound with Fe2P hexagonal structure,which clearly demonstrate the potential of this series of materials for permanent magnet.The theoretical value of the maximum magnetic energy product of this series compounds can reach about 130 k Jm-3,while the experimental value is only about 8%of this value.Considering the research of(Fe,Co)2(P,Si)nanoparticles remains at their early stage,we believe that there is still a large room for improvement in their permanent magnet properties such as coercivity and maximum magnetic energy product by optimizing the synthesis conditions and magnet preparation conditions. |
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