Effect of temperature treatment on the properties of ZnO nanoparticle-Bi2O3-Mn2O3 varistor ceramics

In the current study, 20 nm zinc oxide nanoparticles were used to make high-density ZnO discs doped with Bi₂O₃ and Mn₂O₃ via the conventional ceramic processing method. Different sintering temperatures were found to have significant impacts on the ZnO discs, especially on enhancing grain growth even at a low sintering temperature of only 980°C. The strong solid-state reaction during sintering may be attributed to the high surface area of the 20 nm ZnO nanoparticles that promoted a strong surface reaction even at low sintering temperatures. Moreover, the sintering process also improved the grain crystallinity, as shown in the lowering of the intrinsic compressive stress based on the X-ray diffraction lattice constant and full-wave half-maximum data. The sintering temperatures also significantly influenced the electrical properties of the doped ZnO discs with a marked drop in the breakdown voltage from 335 V (sample at 980°C) to 130 V (sample at 1220°C). The resistivity also experienced a dramatic drop from 303.5 kΩ.cm (sample at 980°C) to 180.7 kΩ.cm (sample at 1220°C). The observed shift in the energy band-gap from a higher to a lower value may be attributed to the conversion of compressive stress to tensile stress with increasing sintering temperature. Therefore, the sintering process can be used as a new technique for controlling the breakdown voltage of doped ZnO discs made from ZnO nanoparticles with improved structural and optical properties.