Probing the densification mechanisms during flash sintering of ZnO

Using ZnO as a model system, the densification mechanisms of flash sintering are investigated. Controlled experiments via limiting the maximum current or the effective ramp rate suggest that both the maximum specimen temperature and the high heating rate (on the order of 200 C/s) are essential for the rapid densification during the flash sintering. Moreover, benchmarking rapid thermal annealing (RTA) experiments, which were conducted to mimic the heating profiles in the flash sintering, achieved similar densification and grain growth rates with comparable heating profiles, attesting that the ultrafast densification is mainly enabled/determined by the T(t) profile. The combination of these experiments suggest that, at least for ZnO, the rapid heating profile is a key factor for the observed rapid densification in flash sintering, while various electric field/current effects could also exist. A clear and consistent correlation between the grain sizes and relative densities is also evident for specimens made by both flash sintering and RTA with different conditions, suggesting the same conventional grain growth mechanism in both cases under the current experimental conditions. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.