Studies On Preparation And Properties Of BiFeO₃ Based Multiferroic Ceramics With Composition At Morphotropic Phase Boundary

Multiferroic materials have wide application prospects in the fields of magnetic sensors,transducers and memory storages due to the coupling effect among various internal ferroic orders.BiFeO₃is the only multiferroic material which exhibits both strong ferroelectricity and weak ferromagnetism at room temperature.It has high ferroelectric Curie temperature（T_C¹103 K）and antiferromagnetic Néel temperature（T_N⁶43 K）,so that it becomes one of the hotspots in the field of material science and condensed matter physics.In this thesis,several BiFeO₃based multiferroic ceramics were selected as the material candidates,the follow works have been carried out:Firstly,Bi_1-xSm_xFe_0.95Mn_0.05O₃（x=0⁰.2）ceramics have been synthesized by introducing Sm and Mn elements into BiFeO₃ceramic.The substitution of 5 mol%Mn for Fe can significantly refine the grains and inhibit valence fluctuation of Fe³⁺,and thus enhancing the insulation properties of ceramics.The substitution of Sm（x≤0.12）can promote grain coarsening and inhibit the appearance of holes and improve the density of ceramics,and thus significantly optimizing the insulation and ferroelectricity.Meanwhile,appropriate amount of Sm can induce the phase transition from R3c phase to Pnma phase and inhibit the spiral spin magnetic structure to enhance the magnetic properties.However,excessive Sm（x≥0.16）not only damages the magnetic structure seriously,but also leads to a large number of polarized R3c phase transforming to antipolar/nonpolar Pnma phase,which degrades the ferroelectric and magnetic properties of ceramics.When x=0.12,the ceramics are at morphotropic phase boundary（MPB）and exhibit excellent multiferroic properties,the remnant magnetization and polarization of the ceramics are 0.055 emu/g and 9.52μC/cm²,respectively.When x=0.16,the electric field can reverse the transformation from R3c phase to Pnma phase caused by the increase of Sm amount in the ceramics.But,the macroscopic magnetoelectric coupling coefficient cannot be detected in all the investigated Bi_1-xSm_xFe_0.95Mn_0.05O₃（x=0⁰.2）ceramics.Then,（1-x）（0.8BiFeO₃-0.2BaTiO₃）-xK_0.5Na_0.5NbO₃（x=0⁰.5）ceramics have been prepared by introducing K_0.5Na_0.5NbO₃into BiFeO₃-BaTiO₃ceramic matrix.With the increase of K_0.5Na_0.5NbO₃content,the crystal structure of ceramics transforms from biphase consisting of R3c phase and P4mm phase to single P4mm phase,and the average grain size increases initially and then decreases.The insulation,ferroelectricity and dielectric properties of ceramics are enhanced with the increase of K_0.5Na_0.5NbO₃ content,while the magnetism of ceramics initially increases and then decreases due to the suppression and excessive destruction of the spiral magnetic structure.When x=0.3,the ceramics are at MPB and exhibit low leakage current density（J=2.19×10^-77 A/cm²）,high remnant polarization（P_r=8.6μC/cm²）and excellent remnant magnetization（M_r=2.09 emu/g）.Smilarly,the macroscopic magnetoelectric coupling coefficient cannot be detected either in all the investigated（1-x）（0.8BiFeO₃-0.2BaTiO₃）-xK_0.5Na_0.5NbO₃（x=0⁰.5）solid solutions.Finally,0.6BiFeO₃-0.4Bi_0.5K_0.5TiO₃ ceramics were chosen as candidates to observe the room temperature magnetoelectric coupling effect.MnO₂ was introduced to optimize the multiferroic properties of ceramics and enhance the magnetoelectric coupling.0.6BiFeO₃-0.4BaTiO₃+x mol%MnO₂（x=0¹0）ceramics were prepared.The appropriate amount of MnO₂（x≤2.5）can promote grain growth and inhibit various defects,and thus reduce leakage current density and optimize ferroelectric,dielectric and insulation properties.Excessive MnO₂（x≥5）hinders grain growth and produces a large number of defects,and thus increases leakage conductance and deteriorates ferroelectric,dielectric and insulation properties of ceramics.When x=2.5,the ceramics exhibit the most excellent multiferroic properties with low leakage current（J=1.83×10^-88 A/cm²）,general remnant magnetization（M_r=0.0067 emu/g）,high remnant polarization（P_r=36.2μC/cm²）,observable remenant magnetization（M_r=0.0067 emu/g）and maximum magnetoelectric coupling coefficient(α_ME=42.56 mV cm^-11 Oe^-1).