$\hbox {Bi}_2\hbox {O}_{3}\hbox {-doped}$ $\hbox {SnO}_{{2}}$ varistors with low breakdown electric fields

Abstract

This work presents $\hbox {Bi}_{{2}}\hbox {O}_{\mathrm {3}}\hbox {-doped SnO}_{{2}}$ ceramic varistors prepared through conventional ceramic processing in the sintering temperature range of 1290–1320°C. The sample sintered at 1300°C exhibits a breakdown electric field as low as 11.6 V $\hbox {mm}^{\mathrm {-1}}$. Scanning electron microscopy images reveal that all the samples have a compact structure, and energy dispersive spectroscopy results for the sample sintered at 1300°C indicate that Bi distributes homogeneously along the grain boundaries and aggregates inhomogeneously on the grain surfaces. With increasing sintering temperature, the grain boundary barrier height remains nearly constant at 0.80 eV. In both the dielectric loss and electric modulus spectra of the sample sintered at 1300°C, obvious relaxations were observed and the activation energies obtained from the respective spectra were 0.33 and 0.15 eV, which are expected to be related to oxygen vacancies and interstitial ions, respectively. Complex impedance spectra are employed to develop a non-typical equivalent circuit model for the $\hbox {Bi}_{{2}}\hbox {O}_{\mathrm {3}}\hbox {-doped SnO}_{{2}}$ ceramic varistors at low voltage that yields an excellent fit to the data.