Flash sintering activated by bulk phase and grain boundary complexion transformations

Abstract A naturally-occurring coupled thermal and electric runaway, resulted from an Arrhenius temperature-dependent specimen conductivity, can trigger flash sintering in many ceramics. This study reveals another possibility to activate flash sintering: a bulk phase transformation or a grain boundary (phase-like) complexion transition can cause an abrupt rise in the specimen conductivity to jump start flash sintering (prior to the occurrence of a natural thermal runaway). In undoped and Al2O3-doped ZnO, the flash sintering is activated by natural thermal runways that can be quantitively predicted from an Arrhenius extrapolation of low-temperature specimen conductivity. In contrast, a bulk eutectic reaction and the associated formation of premelting-like intergranular films (IGFs) in Bi2O3-doped ZnO can lead to a nonlinear rise in the specimen conductivity (above the Arrhenius extrapolation) to trigger flash sintering prior to the occurrence of the predicted natural thermal runaway. Yet, a natural thermal runaway can still take place in Bi2O3-doped ZnO before the occurrence of the interfacial and bulk transformation if the initial electric field is increased to a sufficiently high level. All five cases can be fully explained in a consistent framework so that this set of experiments systematically validate our theory of flash initiation. This work uncovers the roles of the bulk phase and interfacial (phase-like) complexion transformations in initiating flash sintering, thereby suggesting a new direction to understand and tailor the flash sintering process. An observation of ultra-fast field-induced migration of aliovalent cations during the flash sintering of Al2O3-doped ZnO is also reported.