| Summary: | 博士 === 國立成功大學 === 資源工程學系碩博士班 === 101 === The effects of Ca2+, Zr4+, Mg2+, Y3+ and (Bi0.5Na0.5)TiO3 doping on the microstructure, crystal structure and dielectric properties in BaTiO3 were investigated. The XRD and Raman results confirm the crystalline phase of BCTZ and BCTZ with excess Ba2+ powders are both tetragonal phases and without any other second phases. The results also show that Ca2+ ions can be pushed from Ba-site to Ti-site via the addition of excess Ba2+. SEM micrographs indicate that a fine-grain microstructure of both BCTZ and BCTZ with excess Ba2+ can be obtained using a two-step sintering method. The average grain size of BCTZ was around 0.25 μm, and the average grain sizes of two BCTZ samples with extra addition of Ba2+ were 0.5 μm and 0.3 μm, respectively. The fine-grain samples were obtained by two-step sintering and had a higher Curie temperature and flatter temperature coefficient of capacitance (TCC) curves.
The single phase of BaTiO3-(Bi0.5Na0.5)TiO3 powders can be synthesized at 800 oC for 3 h. The tetragonality and dielectric constant decreased as the BNT concentration increased. The Curie temperature increased with the BNT concentration increased.The sintering temperature of BaTiO3-(Bi0.5Na0.5)TiO3 decreased when MgO concentration increased.The flattened TCC curve of BaTiO3-(Bi0.5Na0.5)TiO3 can be obtained when the MgO concentration increased.
The TEM micrographs show that, core-shell structure can be obtained in the BaTiO3 ceramics co-doped with MgO and Y2O3. In addition, the addition of MgO enhances sintering shrinkage and inhibits the grain growth of BaTiO3 ceramics. Moreover, the addition of 1.0 mol% Y2O3 can promote sintering shrinkage and suppress the grain growth of BaTiO3 ceramics. However, the sintering shrinkage is suppressed when the amount of Y2O3 is increased to 3.0 mol%. The diffusion depths of Mg2+ and Y3+ are obtained by high-resolution transmission electron microscopy. The results indicated that Y3+ dissolved in BaTiO3 lattice with 5~10 nm depths inside the grain whereas Mg2+ tended to stay at the grain surface rather than being incorporated into BaTiO3. It is considered that Mg2+ plays an important role as a shell maker in the formation of core-shell structure.
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