Bi0.5+x(Na0.78K0.22)0.5-3xTiO3 ceramics with an excess Bi3+ and a deficiency of Na+ and K+ were synthesized by a conventional solid state reaction method. The structure and morphology ofBi0.5+x(Na0.78K0.22)0.5-3xTiO3 ceramics were characterized by X-ray diffraction and field emission scanning electron microscopy. The electric polarization and mechanical strain induced by external electric field, and the temperature dependence of dielectric constant were investigated. These results demonstrated that anergodic relax or phase can be induced by controls of the mole ratio of Bi3+, Na+ and K+. A phase boundary between non-ergodic and ergodic relaxor phases can be observed at ambient temperature. Theergodic relaxor phase can be transferred to the ferroelectric phase by application of the electric field. The stability of the induced ferroelectric phases strongly depends on the mole ratio of Bi3+, Na+ and K+. The maximum strain of 0.31% was observed in Bi0.51(Na0.78K0.22)0.47TiO3 ceramics sintered at 1,150℃ for 2 h.

In this study, we have investigated the effects of Mn and Co co-doping on defects, J-E curves and grain boundary characteristics of ZnO-Bi2O3 (ZB) varistor. Admittance spectra and dielectric functions show two bulk defects of Zn ·· (0.17∼0.18 eV) and V· (0.30∼0.33 eV). From J-E characteristics the nonlinear coefficient (α) and resistivity (ρgb) of pre-breakdown region decreased as 30 to 24 and 5.1 to 0.08 GΩcm with sintering temperature, respectively. The double Schottky barrier of grain boundaries in ZB(MCo) (ZnO-Bi2O3-Mn3O4-Co3O4) could be electrochemically single type. However, its thermal stability was slightly disturbed by ambient oxygen because the apparent activation energy of grain boundaries was changed from 0.64 eV at lower temperature to 1.06 eV at higher temperature. It was revealed that a co-doping of Mn and Co in ZB reduced the heterogeneity of the barrier in grain boundaries and stabilized the barrier against an ambient temperature (α-factor= 0.136).

In this study, we have investigated the effects of Co doping on I-V curves, bulk trap levels and grain boundary characteristics of ZnO-Bi2O3 (ZB) varistor. From I-V characteristics the nonlinear coefficient (a) and the grain boundary resistivity (ρgb) decreased as 32→22 and 18.4→0.6×10(9) Ωcm with sintering temperature (900∼1,300℃), respectively. Admittance spectra and dielectric functions show two bulk traps of zinc interstitial, Zn(i)·· (0.16∼0.18 eV) and oxygen vacancy, Vo· (0.28∼0.33 eV). The barrier of grain boundaries in ZBCo (ZnO-Bi2O3-Co3O4) could be electrochemically single type. However, its thermal stability was slightly disturbed by ambient oxygen because the apparent activation energy of grain boundaries was changed from 0.93 eV at the 460∼580 K to 1.13 eV at the 620∼700 K. It is revealed that Co dopant in ZB reduced the heterogeneity of the barrier in grain boundaries and stabilized the barrier against the ambient temperature.

In this study, we have investigated the effects of Mn dopant on the bulk trap levels and grain boundary characteristics of Bi2O3-based ZnO (ZB) varistor using admittance spectroscopy and dielectric functions (such as Z*, Y*, M*, ε*, and tanδ). Admittance spectra and dielectric functions show two bulk traps of Zn(i) (0.20 eV) and Vo (0.29~0.33 eV) in ZnO-Bi2O3-Mn3O4 (ZBM). The barrier of grain boundaries in ZBM could be electrochemically single type. However, its thermal stability was slightly disturbed by ambient oxygen because the apparent activation energy of grain boundaries was changed from 0.79 eV at lower temperature to 1.08 eV at higher temperature. The grain boundary capacitance Cgb was decreased slightly with temperature as 1.3~1.8 nF but resistance Rgb decreased exponentially. The relaxation time distribution can result from the heterogeneity of the barriers constituting the varistor. It is revealed that Mn dopant in ZB reduced the heterogeneity of the barrier in grain boundaries and stabilized the barrier against the ambient temperature.