Abstract
•ZnO nanoparticles were used to make varistor with the superior properties and lower breakdown voltage.•The strong solid-state reaction during sintering that attributed to the high surface area of the 20nm ZnO nanoparticles.•The annealing process at different ambient had a significant effect on the grain crystallinity.•The optical band gap increased with increase in the in-plane compressive stress.•Most optical properties were attributed to the defects introduced into the sample.
ZnO–Bi2O3–Mn2O3 varistors based on ZnO micro- and nanoparticle powders were prepared via conventional ceramic processing and the effects of ZnO particle size and different annealing conditions on their properties were investigated. The strong solid-state reaction during sintering may be attributed to the high surface area of the 20nm ZnO nanoparticles. The annealing process improves grain crystallinity, as shown in the decrease in intrinsic compressive stress based on the X-ray diffraction lattice constant and full-width at half-maximum (FWHM) data. The reduced particle size significantly influences the electrical properties, showing a sharp drop in the breakdown voltage which has been found to strongly depend on grain size and potential barrier formation. The observed shift in the energy band gap from a higher value to a lower one derives from the conversion from compressive to tensile stresses with annealing. The Raman spectra indicate the sizes of particles and dopants to have significant effects on the E2(high) phonon mode and ZnO crystal structures.