Densification During Stage I: The Incubation Period in Flash Sintering

ABSTRACT Flash experiments can be carried out by applying an electric field while holding the furnace at an elevated temperature. The full onset of flash, signaled by a sudden increase in conductivity, requires an incubation time, also called Stage I. There is evidence that the conductivity at first rises gradually before the sudden increase, called Stage II, when sintering occurs concomitantly with the increase in current. Here we study this incubation period. We report measurements of densification rate and sintering during this period in rutile TiO 2 . The densification rates are several times higher than predicted from Arrhenius extrapolation of data from conventional sintering. These results point toward flash onset behaving like a nucleation and growth phenomenon. Some concepts emerging from recent work on defect generation during flash form the basis for our suggestions. For example, we show that the oxygen vacancy generation or annihilation rate induced by current flow in Stage I was approximately four orders of magnitude higher than expected from thermally generated Frenkel oxygen vacancies. The results further confirm that Joule heating alone cannot account for the observed behavior. Instead, the applied electric field and elevated temperature act synergistically to trigger defect avalanches, significantly enhancing diffusion and enabling ultrafast densification. Comparisons with 3YSZ highlight material‐specific differences arising from distinct conduction mechanisms.