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Silver nanoparticles were prepared by the method of mixing of two microemulsions having similar chemical composition but different reactants in their respective aqueous core. One microemulsion contains silver nitrate in the aqueous core and other contains sodium borohydride. The silver nanoparticles formed were characterized using UV-Visible absorption spectra and TEM micrographs. Effect of chain length of solvent and addition of Arlacel-20 to the AOT/Heptane, AOT/Decane and AOT/Dodecane microemulsion on particle size and absorption spectra of silver nanoparticles was studied. TEM photographs showed agglomerated and bigger particles in case of pure AOT/dodecane whereas addition of Arlacel-20 showed dispersed and smaller particles. Reaction kinetics was observed for silver nanoparticles using UV-visible spectrophotometer. Silver nanoparticles prepared using pure AOT surfactant showed plasmon band (416 nm) immediately after preparation whereas no absorption band of silver nanoparticles was observed for mixed surfactant microemulsion of AOT and Arlacel 20 for few hours indicating the reaction kinetics is slowed down upon addition of Arlacel-20. Growth rate of silver nanoparticles was plotted by monitoring absorption coefficient ratio (ε416/ε475) as a function of time. AOT/heptane system showed slower growth rate as compared to AOT/decane and AOT/dodecane and also larger particle size. Presence of Arlacel-20 significantly decreases the growth rate in all three alkanes and this observation can be explained using the concept of rigidity of surfactant film at the oil/water interface. It is proposed that higher the interfacial viscosity, slower is the coalescence rate of nanodroplets in the microemulsion system, and hence slower the growth rate of particles and smaller is the final particle size.
The basic building block of the ZnO varistor is the ZnO grain formed as a result of sintering. Nanosized ZnO particles are prepared by carrying out the reaction in the controlled size nanoreactors—the droplets of microemulsions. Chemical doping of the ZnO nanoparticles provides ZnO-based ceramic varistors displaying superior varistor properties. These varistors show a higher value of the nonlinear coefficient, lower leakage current, and higher critical electric field value as compared to those for conventional samples in their log E versus log J curve. The present work has also been aimed at studying the effect of processing variables such as sintering temperature and duration on the microstructure and grain growth of ZnO nanoparticles and ZnO-Bi2O3 ceramics. The activation energy calculated from this data is found to be 175 kJ/mol for pure ZnO. For Bi2O3-doped ZnO, the activation energy is found to decrease considerably (∼148 kJ/mol). All these advantages are due to greater structural homogeneity, smaller particle size, higher surface area, and higher density of the ZnO nanoparticles which are precursors for ZnO varistors, as compared to coarser particles for making varistors.
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