The application of electron microscopy methods in the study of inversion boundaries in ZnO-based varistors

Inversion boundaries in ZnO-based varistor ceramics were studied by various electron microscopy techniques. Scanning electron microscopy observations disclose a flat internal boundary in every ZnO grain. Etched cross-sections reveal the inversion of crystal domains over the fault plane. Microdiffraction experiments using a conventional transmission electron microscope proved that the faults are inversion boundaries, lying in basal planes of the hexagonal ZnO structure. The local structure was assessed by high-resolution transmission electron microscopy and chemistry of the fault was interrogated by energy dispersive spectroscopy. Regardless the type of the dopant (Sb, Sn, Ti, In, Fe, Ga, etc.) inversion boundaries possess a local cubic structure comprising a mixed close-packed cationic layer of octahedral coordination.