Varistors in the new system ZnO-Bi₂O₃-SiO₂ were prepared through conventional ceramic processing route. The effect of sintering temperature and time (0·5 h to 2 h between 1000° and 1250°C) on the microstructure and current/voltage characteristics of the varistors of the new system were investigated and the results were compared with those of ZnO-Bi₂O₃ system varistors prepared. An increase in nonlinear coefficient (α) value was observed in the SiO₂ added varistors. The microstructure and the phase of the varistors were examined by means of SEM and XRD. The Zn₂SiO₄ spinel phase was found to be present in the intergranular region. The grain growth exponent was determined to be 2·5±0·2 and the activation energy for the ZnO grain growth was estimated to be 251±11 kJ/mol. These values were compared with those estimated for ZnO-Bi₂O₃ system varistors.

Carbon nanostructures (CNS) are often grown using oxide nanoparticles as catalyst in chemical vapour deposition and these oxides are not expected to survive as such during growth. In the present study, the catalysts of cobalt- and nickel oxide-based nanoparticles of sizes varying over a range have been reduced at 575$^{\circ}$C under environment resulting from the introduction of C$_2$H$_2$ $+$ NH$_3$ during growth of CNS as well as under introductionof C$_2$H$_2$ and NH$_3$ separately. The structure of the reduced nanoparticles is often different from the equilibrium structure of the bulk. Nanoparticles of cobalt oxide with and without doping by copper oxide are reduced to cobalt alloy or cobalt nanoparticles having fcc structure, but the rate of reduction is relatively less in NH$_3$ environment. However, reduced nickel oxide nanoparticles with and without doping shows a mix of fcc and hcp phases. The presence of hydrogen and nitrogen in the environment appears to discourage the formation of hcp nickel as inferred from the results in NH$_3$ environment. Cobalt carbide forms when the 10 wt.% or less doped cobalt oxide is reduced in C$_2$H$_2$ $+$ NH$_3$ environment. At higher level of doping of 20 wt.%, separate metallic phase of copper appears andcarbide formation gets suppressed.