Almost fully dense pure alumina could be prepared by fast firing (FF) at a high temperature (greater than 1689 °C) and a high heating rate (∼350 °C/s) with a whole sintering cycle in approximately 2 min without pressure. Dynamics of the ultrafast densification was explored by comparing the densifying characters of FF sintering and conventional sintering. Results show that overlapping of surface/grain boundary diffusion and lattice diffusion caused by the high heating rate, as well as rapid migration of nanoscale particles caused by the high heating rate and the sufficiently high sintering temperature, leads to the ultrafast densification, which differs from conventional time-cost material diffusion. The ultrafast densification dynamics is expected to be helpful for understanding or developing new fast sintering methods for ceramics.

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The densification process of sintered alumina is mainly controlled by surface, lattice, and interface diffusion, and many experimental researches show that heating rate can affect the transfer of matter. Thus, to further reveal the effect of heating rate on sintering mechanism of alumina nanoparticle, molecular dynamic simulations were performed at five different heating rates to examine the migration of atoms and evolution of microstructure in heating stage. Results show that the sintering process of heating is a typical thermal activation process. High displacement response temperature is caused by high heating rate, which results in the mechanism of atomic migration quickly changing from surface diffusion to overlapping of surface, interface, and lattice diffusion. Nonuniform microstructure and asymmetrical sintered neck forms due to unstable and nonuniform mass transfer. Sintered neck with small radius of curvature leads to high shrinkage rate and large driving force. The sintering mechanism is expected to be helpful for understanding or developing new fast sintering methods for ceramics.

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