Non-Ohmic conduction in tin dioxide based varistor ceramics

Abstract

A phenomenological model of non-Ohmic conduction in a ceramic varistor is suggested and it is applied to explain current–voltage characteristics of SnO 2–Bi 2O 3–Co 3O 4–Nb 2O 5–Cr 2O 3 varistor obtained at different temperature. It is assumed that electrical conduction in SnO 2 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 = σ 0 E exp( αE). This equation can be used for the approximation of j( E) characteristics of varistors instead of frequently used empirical expression j = BE β . The nonlinearity factor α or the normalized nonlinearity coefficient β E = β/ E 1 ≅ α can be used instead of traditional nonlinearity coefficient β = ( E/ j)(d j/d E) (electric field E 1 is calculated at current density 10 −3 A cm −2). Varistors in a SnO 2–Bi 2O 3–Co 3O 4–Nb 2O 5–Cr 2O 3 system have the highest normalized nonlinearity coefficient β E among SnO 2 based varistors known up to now.