Effects of MnO2 on phase composition, microstructure, and varistor properties of ZnO-SrCO3-Cr2O3-Co3O4-based ceramics

Bi-free ZnO-SrCO 3 -Cr 2 O 3 -Co 3 O 4 based varistor ceramics with the composition (97.4- x ) mol% ZnO-2 mol% SrCO 3 -0.1 mol% Cr 2 O 3 -0.5 mol% Co 3 O 4 - x mol% MnO 2 ( x  = 0–0.7) were prepared by a conventional solid-state reaction method. The effects of MnO 2 on the microstructure, grain-boundary barriers and nonlinear electrical properties were systematically investigated. MnO 2 addition induced a non-monotonic change in ZnO grain size, altered the morphology and distribution of the Sr rich intergranular phases, and promoted the formation of crystalline Sr 2 Mn 2 O 5 at x  ≥ 0.5 mol%. The sample with x  = 0.5 mol% exhibited the best initial nonlinear electrical characteristics among the compositions investigated, with E 1 mA = 614 ± 18 V/mm, α = 33.2 ± 1.7, J L = 3.66 ± 0.37 μA/cm 2 , and ϕ b = 0.866 ± 0.022 eV. Further increasing the MnO 2 content increased the Sr 2 Mn 2 O 5 phase fraction and refined the ZnO grains but reduced the nonlinear coefficient, indicating that the electrical response depended on the combined effects of phase composition, Sr-rich intergranular morphology, grain size, and grain-boundary-related electrical characteristics rather than on the amount of Sr 2 Mn 2 O 5 alone. These results demonstrate that appropriate MnO 2 addition can effectively regulate the microstructure and grain boundary related electrical response of Sr-Cr-based varistor ceramics.