A,B-site Modification In Layer Structured Bismuth Oxide Materials(n=6,7)

Layer-structured Bi-containing Aurivillius oxides (Bi4Bin-3Fen-3Ti3O3n+3, n-BFTO are well known for their coexistence of ferromagnetism (FM) and ferroelectric ity (FE) at or above room temperatures, and therefore, have great potential applications in high-density storage, sensors, spintronic devices, etc.Unfortunately, the FM and FE properties of n-BFTO are not so satisfactory, which are suggested to be related to its layer number of n. In this work, several studies have been made to enhance the multiferroics of BFTO-n, including1). Impact of layer number (n) on the FM properties of Co-substituting n-BFTO;2) Effect of individual or co-substitution in A-site and B-site on the structure, ferromagnetic, ferroelectric, dielectric and photo luminescence properties of7-BFTO;3) Effect of A and B-sites co-substitution on the ferromagnetic, ferroelectric and leakage properties of6-BFTO oxide. And the main results are listed as follows:In the first chapter of thesis, the concept of multiferroic materials, the history and development of BFTO materials, methods for preparing and characterizing BFTO ceramics were introduced.In the second chapter, the effect of layer number n on the ferromagnetism of Bi4Bin-3Fen-3Coo.2Ti3O3n+3ceramics (n=4-7) were investigated. It is revealed that with the increase of n, the ferromagnetism was improved due to the increase of magnetic coupling ion pairs. Furthermore, odd-layer numbered compounds, which are monoclinic phase, are more beneficial to enhance the ferromagnetism than the evern-layer number ones with orthorhombic phase structure, because the more asymmetric crystal structure would increase the couping strength of Fe-O-Co.In the third chapter, we investigated the influence of partial substitution of Co for Fe on the lattice structure, ferroelectricity, ferromagnetism and dielectricity of seven-layered BigFe4-xCoxTi3O24(7-BFCT) ceramics. We found that thermodynamic process rather than tolerance factor is responsible for the maximum substituting amounts of cobalt ions. The maximum molar ratio of Co:Fe in such7-BFCT with stable seven-layered structure is0.43.6, and further increased Co3+ions, the seven-layered structure began to collapse and impurity phases appeared. For7-BFCT ceramics, the optimal ferromagnetism is obtained when the molar ratio of Co:Fe is0.4:3.6, and the compound with Co:Fe=0.3:3.3exhibits the largest ferroelectric polarization. The increased magnetic coupling pairs are responsible for the enhangced ferromagnetism, and the improved ferroelectricity could be attributed to the increased resistivity and the distortion of lattice structure induced by Co substitution.The chapter four is also focused on the seven layered BFTO, where the fluorescence, ferroelectric and ferromagnetic properties of Eu/Co codoped compounds were investigated. It is observed that the concentration quenching effect of Eu3+was suppressed partially in such layer-structured compounds. This phenomenon could be attributed to the unique layered structure of BFTO, which may modify the location of Eu3+and the distance between Eu3+ions. Besides, A-site doping with Eu3+also shows significant influence on the ferromagnetic and ferroelectric properties.In chapter five, we have reported some prophase work onpursuring the enhanced photoluminescence intensity in rare earth doped7-BFTO (BLnFTO) by using the surface plasmon of noble metal particles. The material of Ag/SiO2/BLnFTO with core/shell/shell structure was designed. But it was found that the Ag particles were easily oxidized or diffuse away from the material during heat treatment, which are unfavorable for the performance and life of the materials, thus they would not suitable to be acted as the surface plasmon center. At the end of this chapter, we also presented some subsequent experiment plans for pursuring the surface plasmon enhanced ptotoluminescence in rare earth doped7-BFTO.In the sixth chapter, the ferroelectric and ferromagnetic properties of La and Co co-doped Bi6-xLaxFe1.5Co1.5Ti3O21(6-BLFCT) were investigated. Both the magnetization and po larization reached the maximum at x=0.75, which could ascribed to the enhanced Fe3+-O-Co3+magnetic coupling and lattice distortion induced by La doping, respectively. Besides, it is also found that the most stable3+valence Bi3+and Fe3+, as well as the lowest conductivity also appear at x=0.75.Finally, we summarized the content of the whole thesis and forecasted the prospect of research directions in chapter seven.