Observation of Microstructure of Grain Boundaries of ZnO Varistors using Backscattered-electron

Abstract

The impurities segregated at the grain boundary of ZnO grains, such as Bi₂O₃, are important factor to obtain the nonlinear voltage-current (V-I ) characteristics of a Bi-added ZnO varistor. The deterioration of V-I characteristics progresses with voltage application. It has been reported that one of the reasons for this deterioration is the movement of oxide ions and interstitial Zn2+ ions across grain boundaries and around the neighborhood of grain boundaries. Thus, the mobility of ions and the pathway of the current formed by voltage application strongly correlate with the structure of grain boundary, such as the crystal orientation of ZnO grains, the phase of Bi₂O₃ at the grain boundary, and segregated grains. In the present studies, the structure of grain boundary for the ZnO varistor has been evaluated by composition images obtained from energy dispersive X-ray spectrometry (EDS) with scanning electron microscope (SEM) or with transmission electron microscope (TEM). However, the elemental mapping obtained from EDS with SEM is low resolution and the distribution of elements is obscure. Furthermore, the observation of varistor with TEM is difficult because the sample used for TEM needs precise processing. On the other hand, a composition image with high contrast of tone can be easily obtained using back-scattered electron (BSE) detector with SEM, although BSE detector is lack in the quantitative analysis. In this study, to clarify the microscopic distribution of impurities at the grain boundary with simple method, we observed the fractured surface of ZnO varistor using BSE detector with SEM. For the fractured surface, two types of fracture exist; grain boundary fracture and transcrystalline fracture. The microscopic distribution of impurities can be obtained by observing the surface of grain boundary fracture. The characteristics of ZnO varistor deeply correlate with the state of impurities at the grain boundary such as Bi₂O₃. To clarify the distribution of these impurities, the fractured surface of Bi-Mn-Co-Sb-added ZnO varistor was observed using SEM, EDS, and BSE detector. It was found that the deposit of Bi₂O₃ as additive had two types of shape on the surface of grain boundary fracture; spot-like and sheet-like, for Bi-Mn-Co-Sb-added ZnO varistor. With dissolving SiO2 in Bi₂O₃, the surface tension of Bi₂O₃ decreased and the sheet-like deposit of Bi₂O₃ increased, while the spot-like deposit of Bi₂O₃ decreased. Moreover, when the sample was annealed at 700°C, the surface free energy (surface tension) decreased because Zn²⁺ and Bi³⁺ were speculated to become the compound such as Bi_7.65 Zn_0.35 O_11.83 and move to the triple point or line.