Estimating Joule heating and ionic conductivity during flash sintering of 8YSZ

Sintering is a crucial processing step used in a wide variety of fields. Flash sintering is a recent technique to quickly densify conductive ceramics in the presence of an electric field. Full densification can occur in seconds and at low temperatures. However, the mechanisms responsible for such extreme densification rates remain a matter of debate. We provide evidence that Joule heating sets off a chain of events resulting in flash sintering of 8% yttria-doped zirconia (YSZ). We show that prior to flash onset, Joule heating raises sample temperature relative to the thermal equilibrium established with the furnace. Flash sintering occurs once the sample temperature is high enough to cause densification. We derive an empirical model to estimate the conductivity of 8YSZ during flash sintering. We show that the model is equally valid under constant heating rate and in isothermal conditions, regardless of applied electric field strength.