Morphological, structural and physical characterization of commercially available low voltage ZnO-based varistors

The set of ZnO-based varistors from different manufacturers was examined before and after aging with 8/20 µs current lightning strokes. Aging resulted in an increase in leakage current of up to 25%. Powder X-ray diffraction (pXRD) analysis allowed identifiation except dominant ZnO oxide also δ-Bi₂O₃ phases at grain boundaries, as well as additional phases such as pyrochlore (Zn₂Bi₃Sb₃O₁₄) and spinel (Zn₇Sb₂O₁₂), indicating recrystallization of the samples after aging and an increase in microstrains (ε). Scanning electron microscopy (SEM) confirmed recrystallization caused by Joule heating and showed grain sintering inhomogeneities due to improper manufacturing processes. Electron paramagnetic resonance (EPR) studies indicated that aging caused an increase in the concentration of Mn²⁺ and Co²⁺, which can be explained by a partial conversion of Mn³⁺ and Co³⁺ ions to their detectable states (Mn²⁺ and Co²⁺). Dielectric spectroscopy demonstrated up to 40% changes in permittivity and distinct relaxation mechanisms between pristine and aged ceramics. The results highlight the necessity of employing comprehensive material studies in the assessment of commercial low voltage varistor quality.