Varistor behavior in a ternary system based on SnO2 doped with a hexavalent donor: SnO2-MnO2-WO3

Abstract

Pentavalent metals have been used as donors in tin dioxide, giving it varistor characteristics when associated with densifiers such as CoO or MnO2. However, attempts to obtain varistor behavior in SnO2-based ternary systems using hexavalent donors such as W6+ have been unsuccessful to date, leading to typical linear behavior. Here it is shown that one can achieve nonlinear behavior in this kind of system, given a suitable choice of densifier (MnO2). In order to get insights into the role of WO3 content on the potential barriers responsible for varistor properties, a combination of experimental and theoretical techniques was used. Measurements revealed two minor but relevant phases, Mn3O4, and MnWO4, whose presence was explained by theoretical calculations and whose amount was estimated as a function of WO3 content. This estimate revealed a correlation between Mn3O4 amount and varistor properties, interpreted with basis on Mn defects at SnO2/Mn3O4 interfaces. WO3 content was varied to optimize the varistor properties, resulting in nonlinear coefficient α≈ 6, breakdown electric field Eb> 10 kV/cm and leakage current Il≈ 200 μA. This finding is expected to stimulate further investigations on SnO2-based ternary systems containing hexavalent oxides, especially using other densifiers, such as CoO.