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DC flash sintering of both pure and 0.5 mol. % Bi2O3-doped ZnO at arelatively high activating field of 300 V/cm has been investigated. It isdemonstrated that even high-purity ZnO single crystals can "flash" at 870C. Incomparison, flash sintering occurs at a substantially lower onset temperatureof 550C in ZnO powder specimens, indicating the important roles of surfacesand/or grain boundaries. A model has been developed to forecast the thermalrunaway conditions and the predictions are in excellent agreements with theobserved onset flash temperatures, attesting that the flash starts as a thermalrunaway in at least these ZnO based systems. Interestingly, enhanced graingrowth is observed at the anode side of the pure ZnO specimens with an abruptchange in the grain sizes, indicating the occurrence ofelectric-potential-induced abnormal grain growth. With a large current density,the growth of aligned hexagonal single-crystalline rods toward the anodedirection is evident in the ZnO powder specimen. Moreover, Bi2O3 doping defersthe onset of flash sintering, which can be explained from the formation ofspace charges at grain boundaries, and it homogenizes the microstructure due toa liquid-phase sintering effect. The key scientific contributions of this studyinclude the development of a model to predict the thermal runaway conditionsthat are coincident with the observed onset flash sintering temperatures, theclarification of how flash starts in ZnO based specimens, and the observationand explanation of diversifying phenomena of sintering and microstructuraldevelopment under applied electric currents.

Thermal Runaway, Flash Sintering and Asymmetrical Microstructural Development of ZnO and ZnO-Bi2O3 under Direct Currents