Influence of MCO3 (M=Ca, Sr, Ba)-doping on the non-ohmic properties of the ceramic varistor system SnO2-Co3O4-Cr2O3-Nb2O5

In this study, the influence of the addition of calcium, strontium and barium carbonates on the microstructure and the electrical properties of the SnO2-Co3O4-Cr2O3-Nb2O5 ceramic varistor system was investigated. The powder mixtures were previously studied by thermogravimetric analysis and differential scanning calorimetry showing the decomposition of carbonates into CaO, SrO, and BaO below 1000 degrees C. Then, green tablets were obtained and sintered at 1350 degrees C for 1 h under an ambient atmosphere. Later characterization by X-ray diffraction demonstrates a systematic increase on the lattice parameters as CaCO3 SrCO3, and BaCO3 were added. Scanning Electron Microscopy reveals a decreasing tendency for the average grain size. The electrical performance was evaluated in terms of electrical breakdown field (E-b) and non-linear coefficient (alpha). Although all the samples exhibited non-ohmic electrical behavior, the SrCO3-doped ceramic showed the highest non-linear coefficient (alpha = 10) compared to those doped with CaCO3 or BaCO3. A defect model is proposed to explain the way that Ca2+, Sr2+, or Ba2+ incorporates into SnO2 lattice.