Linear resistance limiting the current of intergranular barriers in varistor ceramics

Abstract

Method for measuring the linear resistivity ρlin, which affects the volt-ampere characteristic (VAC) nonlinearity of varistor materials in a high current region is proposed. For ZnO based varistor ceramics the dependence of ρlin on amplitude density Jmax of current pulse is studied in the range 5 A cm-2 – 4600 A cm-2. At Jmax < 200 A cm-2 a sharp decrease of ρlin with increasing Jmax takes place. This behavior indicates a non-uniform distribution of high current over the cross section of ceramics sample. At Jmax > 1000 A cm-2 ρlin does not depend on Jmax and has a value of 0.49 – 0.50 Ohm cm corresponding to the volume resistivity ρg of ZnO grains. The decrease in nonlinearity of VAC at high currents is due to the formation of conductive paths of electric current between the electrodes of a sample. Along such paths, all intergranular double Schottky barriers are in a reversible electrical breakdown mode, probably due to impact ionization. The current in these paths is limited by small linear volume resistance of the grains. A spread of grain sizes is the reason for existence of current paths with different numbers of contacts between grains and therefore with different breakdown voltages. At voltage increases, the sequential formation of parallel-connected conductive paths occurs. This explains the inhomogeneous distribution of high current over the cross section of a sample at Jmax < 200 A cm-2. At Jmax > 1000 A cm-2 almost all current paths in a sample are switched on and the value of ρlin limiting the barriers current is close to the volume resistivity ρg of grains.