Low voltage varistors based on SrTiO3 ceramics

Varistors exhibiting nonlinear resistance at low voltages of 4.5 to 30 V mm−1 have been realised from SrTiO3 ceramics. These are processed from powders obtained from the gel-crystallite conversion technique. Doping with ≲ 1% Y3+ enhances the average grain size to over 50,um, when sintered around 1650 K in static air. On annealing the ceramics, in N2 + H2 atmospheres, they acquire a low resistivity of 0.4 to 3.7 Ω cm with positive temperature coefficient of resistance. They are painted with low melting point oxide mixtures of PbO + Bi2O3 + B2O3 and re-annealed. The energy dispersive X-ray results indicate selective melting reactions at the grain boundary layers with higher concentrations of the low melting point oxide constituents. The abnormally high dielectric constants, ελ = 104 to 105, point to the prevalence of GBL capacitance in these ceramics. Depending upon the conditions of the second annealing, the breakdown voltage could be adjusted from 0.2 to 1.5 V per grain boundary, without any change in the grain size. The nonlinearity coefficient a ranges from 6 to 15 and the barrier height from 0.15 to 0.3 eV This can be explained on the basis of variable pinning of traps at the interface and also the extent of trap filling.