Preparation And Research Of Multiferroic Properties Of SrBi₂Nb₂O₉-based Ceramics

With the development of science and technology,the requirement for electronic devices in social production becomes more stringent and the demand for materials with multiple functions are growing.Multiferroic materials possess ferroelectric and magnetic orders simultaneously,which make multiferrous materials have more practical applications.Aurivillius compounds reduce the leakage current of perovskite-like layers in materials by separating them with high resistance?Bi₂O₂?²⁺layer.At the same time,when the center site of the oxygen octahedron is occupied by magnetic ions,the material shows good magnetism.In this work,SrBi₂Nb₂O₉ with m=2 was selected as the matrix material,which was modified by magnetic ions doping,reducing leakage current and fabrication of textured ceramics.The crystal structure,micro-morphology,dielectric properties and multiferrous properties of the ceramics were systematically studied.SrBi₂Nb_2-xFe_xO₉?x=0,0.1,0.2,0.3,0.4 and 1.0?and SrBi₂Nb_1.8-xFe_0.2Co_xO₉?x=0,0.1,0.2,0.3 and 0.4?ceramics were prepared by traditional solid-state reaction.With doping of Fe ion,the samples show a layered-perovskite structure.All ceramic samples show a typical plate-like grains.With the increase of doping content,T_c increases and permittivity decreases.There is obvious relaxation peak close to Curie peak.When Fe ions occupy central position of octahedron,the P_r and E_c decrease,and the minimum value is obtained when x=0.3.Then the P_r and E_c of the material increase,which is due to leakage current caused by acceptor doping of Fe ions.The magnetic properties of the material transform from diamagnetism to antiferromagnetism.Doping with Co ions,it was found that there was no second phase.Because the anisotropy during grain growth process was destroyed by the introduction of Co ions,the plate-like grains gradually transformed to the cubic.Increase of interfacial polarization and dielectric loss result from increase of oxygen vacancy concentration.The conductivity activation energies are 0.47 eV,0.37 eV,0.33 eV,0.31 eV and 0.30 eV by fitting resistivity.The hysteresis loops of materials become wider and leakage current increases.The double exchange interaction between different ions makes the material appear weak ferromagnetism.SrBi_2-xLa_xNb_1.6Fe_0.2Co_0.2O₉?x=0,0.15,0.30,0.45,0.60?ceramics,SrBi₂Nb_1.8Fe_0.1Co_0.1O₉ and SrBi₂Nb_1.8(Fe_0.1Co_0.1)_5/3O₉ ceramics were prepared by traditional solid-state reaction.With the increase of La ions,the structure changes from Orthorhombic to Tetragonal.Ceramics show a typical plate-like grains.When the x=0.3,the material obtains the maximum permittivity 107.1,the dielectric loss is below 0.05.In addition,the ferroelectric and magnetic properties have the best performance.La ions will replace Bi ions,reducing the oxygen vacancies in the material.Moreover,La ions will enhance the interaction between Fe-O-Co and improve the multiferrous properties.After non-stoichiometric doping,no second phase was found.The ceramics show typical plate-like grains.At 100 kHz,the permittivity of SBCFN1 and SBCFN2are 183.4 and 201.3,respectively.The dielectric loss is about 0.02.Curie temperature of ceramic is about 467 ^oC.The materials have higher resistivity and breakdown electric field strength,and their magnetic properties are further enhanced.SrBi₂Nb_1.8Fe_0.1Co_0.1O₉ powders with high aspect ratio were synthesized by molten salt method.The width of the plate-like particles was about 10?m and the thickness was below 100 nm.Then textured SrBi₂Nb_1.8Fe_0.1Co_0.1O₉ ceramics were prepared by cold isostatic pressing and sintering.XRD patterns show that the materials grow in?00l?direction.Upward,the palte-like grains are stacked layer by layer.In the direction perpendicular to the c-axis,it can be seen that all the grain sides are arranged in a certain direction.In the direction perpendicular to the c-axis,permittivity is 167.In the direction parallel to the c-axis,low dielectric loss,high resistance and high breakdown field strength are obtained.In addition,the orbits parallel to the c-axis are weakened by the interference between layers,which makes the material obtain higher magnetization in the direction parallel to the c-axis.