Unveiling exceptional sinterability of ultrafine α-Al2O3 nanopowders

Scalable pressureless sintering of nanocrystalline alumina (Al2O3) ceramics is a challenging problem with great scientific and technological interest. This challenge was addressed in our recent works utilizing ultrafine α-Al2O3 nanopowders with exceptional sinterability combined with two-step sintering technique. Here the sintering mechanism and kinetic parameters (grain boundary diffusivity and its activation energy) were analyzed from constant heating-rate sintering experiments by three different sintering models and compared with existing sintering data in the literature. We found that the low-temperature sintering of 4.7 nm α-Al2O3 nanopowders can be well explained by conventional sintering mechanism via grain boundary diffusion, with reasonable activation energy of 4–5 eV that is smaller than that of coarse Al2O3 powders and enhanced diffusivity. However, unphysically small activation energy could be obtained if an inappropriate model was used. Lastly, successful two-step sintering was demonstrated under different heating rates. Our work illustrates that the exceptional sinterability of ultrafine α-Al2O3 nanopowders are most likely contributed by small size (short diffusion distance), large surface area (large sintering driving force) and good dispersity rather than new sintering mechanism, and highlights the importance of fast firing and the non-equilibrium nature for the low-temperature sintering of such nanopowders.