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Flash sintering is a novel technique of ultrafast densification. There is a continuous debate over the mechanisms that are responsible for such fast sintering. In this work, we have assessed the role of Joule heating and thermal runaway on densification using a combined experimental and modeling approach. First, flash sintering (FS) experiments have been carried out for three different oxides that have very different electrical and thermal conductivities, namely, 3-and 8-mol% yttria-stabilized zirconia and titania (TiO 2 ). Then, we modeled the densification during FS, for identical experimental conditions, using a novel combined finite element modeling and master sintering curve approach. The results obtained through experiment and modeling have been compared. Finally, our results show that ultrafast densification observed during FS cannot be replicated through Joule heating and thermal runaway alone at low current density (<100 mA/mm 2 ), suggesting that a contribution from other mechanisms is likely essential to explain the observed ultrafast densification. However, Joule heating and thermal runaway can account for the ultrafast densification at higher than 100 mA/mm 2 current density.

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