Xuechen Huang

Ab initio study of doping mechanisms in BaTiO3-BiYO3

A density functional plane-wave pseudopotential method is used to study the doping mechanisms of impurity defects(BiBa, YTi) in BaTiO3-BiYO3. Single BiBa and YTi impurities have little structure distortion. Bi forms ionic bond with nearby O atom in single Bi impurity, Y formed [YO6] octahedral in single Y impurity. However, in the co-doped BiBa and YTi structure, Bi formed three valence bonds with nearby O atom, which causes the large structure distortion. The doped ion makes the mobile of Ti4+ difficult and loss local ferroelectricity, which will broaden the dielectric constant temperature curve and increase the temperature stability of BaTiO3 ceramic matrix.

High-Temperature Dielectrics in BNT-BT-Based Solid Solution

Bi_0.5Na_0.5TiO₃-BaTiO₃ (BNT-BT)-based ternary solid solutions were investigated for high-temperature capacitor applications. Through a comprehensive investigation of the (1 - x)(0.92Bi_0.5Na_0.5TiO_3-0.08BaTiO₃)-x NaNbO₃ [(1 - x)(BNT-BT)- xNN, x = 0-0.45] system, 0.85(BNT-BT)-0.15NN was selected as the parent matrix due to its relatively high permittivity (>1800) and favorable energy-storage density (0.56 J/cm³ at 7 kV/mm). The effect of bismuth substitution on the dielectric properties of the matrix was further characterized. The introduction of bismuth greatly broadened the operational temperature range of 0.85(BNT-BT)-0.15Na_1-3yBi_yNbO₃ ceramics to over 327 °C for a ±15% tolerance. The dc resistivities were of the order of 10⁸-10¹¹ Ω · m magnitude from room temperature to 300 °C. An activation energy of 1.1-1.2 eV in 200-350 °C was obtained from dc resistivity data, suggesting that the conduction process in this temperature range may be associated with oxygen vacancy migration. Furthermore, the energy-storage properties were largely improved by the addition of bismuth. When the substitution of Bi over Na achieved was up to 7%, the energy-storage density and efficiency reached 0.62 J/cm³ and 88% at 7 kV/mm, respectively. These results confirm that a BNT-BT-based solid solution is a promising candidate for lead-free high-temperature capacitor applications.

Structure and dielectric properties of Bi 2 O 3 doped BaTiO 3 -(Na 1/4 Bi 3/4 )(Mg 1/4 Ti 3/4 )O 3 lead-free ceramics

Bi₂O₃ doped BaTiO₃-(Na_1/4Bi_3/4)(Mg_1/4Ti_3/4)O₃ (BT-NBMT) polycrystalline ceramics were prepared by solid state reaction method. The effects of Bi₂O₃ additive on the microstructure and dielectric properties of BT-NBMT ceramics were investigated. The Bi₂O₃ effectively lowered the sintering temperature and improved the bulk density of BT-NBMT ceramics. The low-temperature stability first improved and then deteriorated with increasing Bi₂O₃ concentration. The core-shell microstructure undertook responsibility for the good dielectric temperature stability. The dielectric ceramic samples with good permittivity and low dielectric loss were obtained at a moderated sintering temperature. The results also suggest that the developed BT-NBMT ceramics may serve as a promising candidate for multilayer ceramic capacitors.