Non-Ohmic conduction in tin dioxide based varistor ceramics

A phenomenological model of non-Ohmic conduction in a ceramic varistor is suggested and it is applied to explain current–voltage characteristics of SnO2–Bi2O3–Co3O4–Nb2O5–Cr2O3 varistor obtained at different temperature. It is assumed that electrical conduction in SnO2 varistor is controlled by the grain-boundary potential barriers and the barrier height is decreased with electric field. The reasonable agreement between the calculations and the experimental results is found. The assumed decrease of the barrier height (the activation energy of electrical conduction) with electric field is observed experimentally. This model gives a relationship between the current density j and the average electric field E in a form j = σ0E exp(αE). This equation can be used for the approximation of j(E) characteristics of varistors instead of frequently used empirical expression j = BEβ. The nonlinearity factor α or the normalized nonlinearity coefficient βE = β/E1 ≅ α can be used instead of traditional nonlinearity coefficient β = (E/j)(dj/dE) (electric field E1 is calculated at current density 10−3 A cm−2). Varistors in a SnO2–Bi2O3–Co3O4–Nb2O5–Cr2O3 system have the highest normalized nonlinearity coefficient βE among SnO2 based varistors known up to now.