The influence of area/volume ratio on microstructure and non-Ohmic properties of SnO₂-based varistor ceramic blocks

This work deals with the electrical properties of SnO₂-based varistor systems with different area–volume (A/V) ratio of the green compact. The influence of A/V ratio specially on microstructural homogeneity and different diameter-thickness (D/t) ratio of sintered compact mainly as a requisite to the existence of non-Ohmic properties is evaluated. The results evidence that, contrary of what is generally observed for ZnO-based varistor system, in the SnO₂-based system, the A/V ratio of the green compacts does not influence the non-Ohmic properties, i.e. the homogeneity of the microstructure and the composition after sintering is conserved independently of the A/V ratio employed in the green compacts. Such independence was specifically observed by performing non-Ohmic measurement after cutting the sintered blocks in different slices and observing that the current–voltage curve of the slices are very similar for different A/V ratio of the green compacts before sintering. The observed behavior has its origin on the fact that A/V ratio does not affect the microstructure development during sintering due to the minimal CoO losses by vaporization for SnO₂-based system, i.e. the stability of the dopant oxides is high when compared with that used in commercial ZnO · Bi₂O₃-based varistors in which, for instance, Bi₂O₃ volatilizes critically. On the other hand, it was found a critical value of A/V = 5.0 cm⁻¹ for the ceramic blocks to effectively served as varistor, below this ratio the ceramics were highly resistive because of a high number of effective barriers ∼85% which is higher than normally found for ZnO-based systems (35%).