Synthesis of novel core-shell nanocomposites for fabricating high breakdown voltage ZnO varistors

Abstract

A novel varistor-composite synthesis for the fabrication of high breakdown voltage ZnO varistors has been successfully developed. It involves coating ZnO nanoparticles with SiO₂ through the hydrolysis of tetraethyl orthosilicate (TEOS) and the subsequent coating of additives on the core of the SiO₂-coated ZnO particles, producing a double core-shell structure. The effect of the amount of TEOS used on the morphology of the synthesized composites and the microstructure, as well as the electrical property of the sintered samples from the double core-shell composites are fully studied. Such materials, with the appropriate amount of TEOS added (Si⁴⁺/Zn²⁺ = 0.15 moles) enables the superior electrical performance of ZnO varistors: high breakdown voltage (V_B = 1123 V mm⁻¹), excellent nonlinear coefficient (α = 42.5) and low leakage current (I_L = 8.5 μA), compared with current commercial varistors and the sample from single core-shell composites. The high performance is attributed to the smaller ZnO grain size and perfect additive homogeneity. The SiO₂-coating provides a better way to control the varistor microstructure through the formation of a nearly circular distribution of blocky Zn₂SiO₄ around the ZnO grains.