Varistors based on n-BaTiO3 ceramics

Stable and highly reproducible voltage-limiting characteristics have been observed at room temperature for polycrystalline ceramics prepared from donor-doped BaTiO3 solid solutions containing isovalent lattice substitute ions that lower the Curie point Tc. When the ambient temperature Ta is decreased such that Ta < Tc, the same ceramics show current-limiting behaviour. The leakage current, the breakdown voltage and the non-linear coefficient (α = 30−50) could be varied with grain-boundary layer (GBL) modifiers and postsintering annealing. The magnitude of the abnormally high dielectric constant (epsilon (Porson)r greater than, approximately 105) indicates the prevalence of GBL capacitance in these ceramics. Analyses of the current-voltage relations show that GBL conduction at Ta < Tc corresponds to tunnelling across asymmetric barriers formed under steady state Joule heating. At Ta > Tc, trap-related conduction gives way to tunnelling across symmetric barriers as the field strength increases.