Traditional ceramic sintering takes hours. The powder compact sits in a furnace at high temperature while atoms diffuse across grain boundaries, slowly eliminating pores and consolidating the material. The process is well understood: diffusion is slow, so densification is slow.
Ultrafast sintering compresses this to seconds. Flash sintering, laser sintering, plasma sintering, and induction methods all achieve full densification in timeframes that conventional diffusion theory says are too short. The flash event in particular initiates through a coupled thermal and electrical runaway — the ceramic becomes electrically conductive at high temperature, which generates Joule heating, which raises the temperature further, which increases conductivity, which accelerates the heating. The runaway is the mechanism.
What remains incompletely understood is why this works at all. The heating rates are so extreme that conventional diffusion models predict insufficient time for mass transport to close the pores. Something other than equilibrium solid-state diffusion must be operating — possibly liquid-phase formation at grain boundaries from local overheating, possibly enhanced vacancy concentrations from the electrical current, possibly plastic deformation from thermal shock stresses. The industry is adopting the technique faster than the science can explain it.
The gap between the technology and its theory is itself informative. Traditional sintering was understood before it was optimized. Ultrafast sintering is being optimized before it is understood. The sequence usually runs: understand, then control. Here it runs: discover a runaway that works, then try to explain why. The explanation trails the application by years.