Relationship Between Microstructure And Properties Of Typical Oxide Ceramics

Based on the advanced multi-functional scanning electron microscope platform,the microstructural characteristics such as composition distribution,grain growth,second phase distribution,defects,and the microstructural evolution process of Bi-doped Sr_0.8Ba_0.2TiO₃ ceramics and ZnO-Bi₂O₃-Sb₂O₃ ceramics sintered at low temperature were systematically investigated.The innovation point lied in that we firstly investigated the point defects characteristics by cathodoluminescence?CL?spectroscopy in the core-rim structure of Sr_0.8Ba_0.2TiO₃ ceramics and related the defects'distribution with the core-rim formation process during liquid phase sintering.In addition,the study of microstructures and its effects on properties of ZnO-Bi₂O₃-Sb₂O₃ ceramics indicated that the addition of Sb₂O₃ induced the formation of inversion boundaries?IBs?which could influence the grain growth.The varistor voltage could be changed by adjusting the grain sizes which were affected by the amount of Sb₂O₃.The doping elements mainly dissolved into the second phases and no segregation was found at the grain boundaries.The intrinsic point defects determined the nonlinearity of the varistor.The main contents were listed as follows:?1?A novel core?rim structure,core with double layered rim,was observed in Bi-doped Sr_0.8Ba_0.2TiO₃ ceramics.There was a sharp interface between Core and Rim',but not for Rim'and Rim".Energy dispersive spectra revealed not only a small amount of Ba,Bi in the core,but also a significantly different Ba and Bi solution levels in the Rim.The change tendency of Bi was consistent with Ba but contrary to that of Sr.Ti was evenly distributed throughout the grain.The Core?rim structure was formed from solution?reprecipitation process.The undissolved SrTiO₃ particles remained as cores.The larger diffusivities of Ba²⁺and Bi³⁺in the sintering liquid and constitutional supercooling led to their segregation at Core?Rim'interface.The diffusivities of Ba²⁺and Bi³⁺decreased with the volatilization of LiF,resulting in the dispersion distribution of Ba and Bi in the rim??.?2?Point defects characteristics of the Bi-doped Sr_0.8Ba_0.2TiO₃ ceramics were investigated by Cathodeluminescence?CL?spectroscopy.Three CL peaks at 2.9eV,2.4eV and 2.0eV named Peak?,?,?,respectively,were found.The excited electron trapped in the doubly ionized oxygen vacancy recombined with a free hole in the valence band and gave off Peak?.Peak?was induced by the recombination of a free electron with a hole trapped at a strontium vacancy.It was also associated with an optical transition from d(t_2g)conduction band levels to new Ti 3d-e_g state introduced by oxygen vacancy inside the TiO₆ octahedral.Because of Schottky defects were generated in pairs,the core area exhibited enhanced luminescence intensities of peak I and peak III.The charge transfer between Ti⁴⁺and O^2-in the TiO₆ octahedron gave off peak?and the intensity of it depended on electron concentration.It might also correlate with the new energy state introduced by Bi³⁺.Higher Bi content in the Rim'led to the higher peak intensity.?3?The microstructural investigation on the ZnO-Bi₂O₃-Sb₂O₃ ceramics showed that Bi₂O₃ and Sb₂O₃ could significantly reduce the sintering temperature and the ceramics could be sintered and densified at a temperature of 950°C.In addition to ZnO and Bi₂O₃,spinel phase and pyrochlore phase were formed.The grain size varied with Sb₂O₃ doping amount and reached the maximum mean value in x=0.20 sample,while the change tendency of varistor voltage was contrary to that of grain size.Only a small amount of Co was dissolved into the grains.The Co and Mn solutions were detected in the spinel and pyrochlore phases.Due to the excessive Sb₂O₃ doping in x=0.24 sample,the amount of the pyrochlore phase increased resulting in a significant increase in the leakage current value.?4?There was no obvious segregation of doping elements at the grain boundaries of ZnO-Bi₂O₃-Sb₂O₃ ceramics,which were observed by transmission electron microscopy.CL spectra revealed four peaks of 380 nm?Peak1?,525 nm?Peak2?,680nm?Peak3?and 890 nm?Peak4?.Peak1 was directly recombined by the electrons in the conduction band and holes in the valence band.Peak2 was related to zinc vacancies,and Peak3 was related to solid solution of Co element.The intrinsic point defect determined the grain boundary Schottky barrier and thus presented the nonlinear characteristics.