Preparation Of A-site Doped Bi₆Fe_1.4Ni_0.6Ti₃O₁₈ Polycrystalline Material

Layered bismuth Arrivillius structure oxide is a kind of single-phase material periodically arranged along c direction by(Bi2O2)2+ and perovskite-like structure[An-1BnO3n+1]2-(n:perovskite-like layer number)units.The single-phase multi-iron Aurivillius compound with atomic scale(0.1nm)superlattice composed of the widely studied magnetoelectric material BiFeO3 and ferroelectric material Bi4Ti3O12 is generally expressed as Bi4Bin-3Ti3Fen-3O3n+3(BFTO).Recently,due to the emergence of its potential ME coupling,in this case,the magnetization of the electric field and/or the ferroelectric polarization of the magnetic field can be realized,enabling them to be in the fields of sensors,memories,spintronics and spintronics.Bi6Fe2Ti3O18(n=5)is a typical Aurivillius compound with two(Bi2O2)2+ layers sandwiched by(Bi4Fe2Ti3O16)2-5-layer perovskite units along the c-axis.The multiferroic material BFTO with a layered perovskite structure exhibits its unique ferroelectric and ferromagnetic properties because the Ti and Fe ions in the center of the oxygen octahedron in the perovskite-like layer form a strong bond with the surrounding O ions.The interaction thus deviates from the center of the oxygen octahedron.In this paper,Bi6Fe1.4Ni0.6Ti3O18(BFNT)with five layers of perovskite structure was used as the matrix material.The effects of different doping elements,oxygen vacancy concentration and sol-gel preparation methods on the microstructure and multi-iron and dielectric properties of the materials were investigated.The first chapter introduces the research background of multi-iron materials,including the related concepts and research directions of multi-iron materials and the research status and progress of Bi5FeTi3O15(n=4)and Bi6Fe2Ti3O18(n=5)multi-iron materials.the preparation methods,characterization methods and principles of polycrystalline perovskite materials are also introduced.The second chapter of the thesis mainly introduces Bi5.75R0.25Fe1.4Ni0.6Ti3O18(R=La,Sm,Nd,Eu)ceramic samples prepared by sol-gel method,by doping the A site with different rare earth elements(La,Sm,Nd,Eu),studied its effect on the microstructure and multi-iron properties of single-phase multi-iron materials.It is found that the multi-iron properties of the materials are improved to some extent after the magnetic elements containing the rare earth elements La,Sm,Nd and Eu areinserted into the ferroelectric matrix material due to the different ionic radii of the four ions.Through research and comparison,it is found that a greater degree of distortion caused by doping of elements with smaller A-site ionic radius may be the cause of magnetic lift.The third chapter of the thesis mainly introduces Bi6-xNdxFe1.4Ni0.6Ti3O18(x=0.1,0.2,0.25,0.3)ceramic samples prepared by sol-gel method.The effects of different Nd doping amounts on the microstructure and properties of the materials were studied.The results show that the grain size decreases with the increase of Nd doping amount.When the doping amount of the A site was 0.20,the sample obtained a maximum saturation magnetization(2Ms)of 4.132 emu/g.At an electric field of 190 kV/cm,the sample obtained a maximum residual polarization strength(2Pr)of 19.7?C/cm2 when the doping amount of the A site was 0.25.And the measurement results of Raman and XPS show that a small amount of Nd doping can enhance the stability of Bi ions and help to improve the ferroelectric properties of the samples.The fourth chapter introduces a self-designed rapid preparation of powder device with a modified sol-gel method.The device can improve the preparation cycle,the uneven ion distribution and the insufficient combustion when the sample is prepared by the sol-gel method.The sample can be pre-sintered under different protective atmospheres by the device to increase the strength and continuity of the gel fibers.Alternatively,the gel fiber can be sintered under an oxygen atmosphere to solve the problem of insufficient combustion.The fifth chapter of the thesis is a summary of the content of the full-text research,and a prospect for the research work that can be carried out in the next step.