Enhancing the proportion of active grain boundaries in ZnO varistor via ZnO@Bi2O3@SiO2 core-shell-shell structured nanopowders

Abstract

Active grain boundaries form the basis for the non-linear characteristics exhibited by ZnO varistors. To enhance the proportion of active grain boundaries in ZnO varistors, a method using ZnO@Bi₂O₃@SiO₂ core-shell-shell structured nanopowder as the sintering feedstock was proposed.

Using ZnO@Bi₂O₃@SiO₂ varistor samples (sintered from ZnO@Bi₂O₃@SiO₂ core-shell-shell structured nanopowder) as the experimental group, and ZnO@Bi₂O₃-SiO₂ varistor samples (sintered from ZnO@Bi₂O₃ core-shell structured nanopowder doped with SiO₂) and ZnO-Bi₂O₃-SiO₂ varistor samples (sintered from mixed powder) as control groups. The inhibition effect of the dense SiO₂ shell on Bi₂O₃ volatilisation, the improvement of Bi distribution by the core-shell-shell structure, and the influence of this structure on the formation of double Schottky barriers within varistors were systematically investigated using TEM-EDS, F-SEM, XRD, TEM-BSE, and AFM-KPFM. In addition, the enhancement in the proportion of active grain boundaries in the ZnO@Bi₂O₃@SiO₂ sample as compared with the mixed sample was estimated based on the work function difference ΔΦ between the active grain boundary and its adjacent grains, the potential gradient E_1mA, and the total number of grain boundaries per unit height N. The results indicate that, compared with mixed samples, using ZnO@Bi₂O₃@SiO₂ nanopowder as sintering feedstock can effectively increase the proportion of active grain boundaries. When Zn²⁺: Bi³⁺:Si⁴⁺ = 1:2.5 mol%:7.5 mol%, the proportion of active grain boundaries increases by approximately 77.2%. This is attributed to the close and thorough contact between SiO₂ and Bi₂O₃, which promotes the transformation of the liquid phase into a Si-containing phase. Furthermore, the protective effect of the dense SiO₂ shell prevents Bi₂O₃ from being exposed to air, thereby suppressing the volatilisation of Bi₂O₃ and improving the distribution of Bi₂O₃.