Twinning and charge compensation in Nb2O5-doped SnO2-CoO ceramics exhibiting promising varistor characteristics

We investigated the effects of dual doping of SnO2 varistor ceramics with 1 mol% CoO and different amounts of Nb2O5 (0.1-2 mol%) on the formation of twin boundaries, microstructure development and electrical properties. Nb2O5 addition shifts densification to higher temperatures (up to 1430 °C), producing microstructures composed of twinned SnO2 grains. Already 0.1 mol% Nb2O5 triggers a three-fold increase in growth rate via the diffusion induced grain boundary mobility (DIGM). At 0.5 mol% of Nb2O5 chemical equilibrium is achieved and SnO2 grains undergo normal grain growth. Electron back-scatter diffraction (EBSD) has shown that the prevailing type of twins is {101}. Cyclic twins are common. High-angle annular dark-filed scanning transmission electron microscopy (HAADF-STEM) image analysis revealed non-uniform segregation of Nb along the twin boundaries, indicating that they are not directly triggered by Nb2O5, but are a result of yet unexplained sequence of topotaxial replacement reactions. Energy dispersive spectroscopy (EDS) has shown that by dual doping of SnO2 with CoO and Nb2O5 the amount of Co dissolved in SnO2 grains is always ~4x lower compared to the amount of incorporated Nb and propose the following mechanism of tin out-diffusion: 6SnIVSn(IV)×⇋SnIISn(IV)''+CoIISn(IV)''+4NbVSn(IV)⋅. Optimal electrical properties were achieved at 1 mol% Nb2O5 addition displaying high nonlinearity (α=50), moderate break-down voltage (571±12 V/mm) and low leakage current (IL = 4.2 µA). The addition of 2 mol% of Nb2O5 has an inhibiting effect on densification and SnO2 grain growth, resulting in a collapse of nonlinearity and increase of leakage current.