Study On Processing And Properties Of Bismuth Layered Structure SrBi₄Ti₄O₁₅ Piezoelectric Ceramics

With prominent ferroelectric properties such as negligible polarization fatigue, high Curie temperature and low leakage current, bismuth layered structure materials(BLSF) are proper to apply at high temperature and high frequency conditions and have attractive application in ferroelectric memory fields. As the materials have a relative inferior piezoelectric properties, their applications were limited. Heating treatment (heat-pressing, hot-forging), partly-order reaction (TGG) and proper doped processing are some potential methods to resolve the problem. In this paper, synthesis parameters and sintering processing of SrBi4Ti4O15(m=4),SrBi2Nb2O9(m=2) were investigated to study their relationship between structure and ferroelectrics properties. Texturing and doping methods were employed to improved properties of SrBi4Ti4O15 ceramics.Compared with conventional solid state (CS) synthesis method, molten salt synthesis (MSS) method could produce plate-like SBT powder at lower synthesized temperature that resulted less volatilization of Bi2O3 in high temperature. Due to abundant dopant in grain boundary restraining grain growth, grain size of La-doped SBT decreased with the increasing of dopant amount. However, microstructure of Zr-doped SBT grain remained plate-like structure.Layer structure of doped SBT was almost not destroyed by incorporating doping ion. Because of the Bi2O3 volatilization and defects resulting from doping, SrTiO3 was appeared in some doped SBT with the doping amount increasing.To investigate structure distortion of doped SBT, quantum chemistry method was employed to calculate total energy and covalent bonding strength. La3+ ion could enter the (Bi2O2) layer firstly which was very difficult to substitute, and then replace the Bi site of pseudo peroskite layer when the doping amount was increasing. Due to much more distortion of crystal lattice for La-doped SBT, some dopant would exist at grain boundary when doping amount increased. The total energy was lowest when B-site substitution ion that we chose all replaced Ti (3) site and Sr2+ ion was at (6 )site. It was probably because that Ti (3) O6 octahedron near Sr (6) could be so weak that Nb5+ and Ti4+ ions could easily substitute into.Temperature dependence of the Raman spectra for SBT(40℃-590℃ )was tested and influence of different doping ion on structure was studied in this paper. The Raman modes at 200~400cm-1 could be assigned as the Eg modes, attributed to the internal mode of TiO6 octahedra originating from the Ti-0 bonds. The Raman modes at 470~490cm"1 could assigned as Ti-0 bonds torsion of TiO6 octahedra. The Raman modes at 314cm"1 and 464cm-1 were influenced by tilting and distortion of TiO6 octahedra. The mode at 870cm-1 had A1g modes' characteristics, attributed to symmetrical stretching of TiO6 octahedra. Temperature and different ion doping had little influence on 870cm"1 vibrating peak. B-site doping had great effects on Ti-0 bonds of TiO6 octahedra that resulted the internal mode of TiO6 octahedra broadened. With complex doping mechanism, the Raman spectra of A-site doping SBT changed a lot with doping amount increasing.Doping is efficiency means to improve properties of SBT. Nb-doped SBT was donor doping that could enhance the polar voltage to exhibit its high piezoelectric performance. D33 of Nb-doped SBT increased with doping amount increasing, maximum was 17pC/N. Properties of Mg-doped SBT improved not so distinct as Nb doping, maximum value was 14pC/N. Because of La3+ substituting the Bi3+ site of pseudo peroskite layer, oxygen vacancies were restrained. On the other hand, La3+ doping made less lattice distortion. The two aspects competed with each other, resulting piezoelectric constant increased firstly, and then decreased later. The maximum value was about 10pC/N. Piezoelectric properties of Zr-doped SBT were fluctuated with maximum value 11pC/N.With little influence on structure of SBT, Nb-doped SBT had sharp permittivity peak at 520℃. Compared with B site doping, A-site doping had lattice structure so great distortion t...