Evolving corundum nanoparticles at room temperature

Metals and ceramics in nanometric forms attract much attention due to their unique properties, yet their stability is often questioned. Nevertheless, the shape and size instabilities are thought to be suppressed at ambient conditions, especially for refractory materials with high melting points. Contrary to this notion, here we report gradual coarsening of sub-10 nm corundum ( α-Al ₂O ₃ ) nanoparticles during room-temperature storage, indicating active surface diffusion at a surprisingly low temperature (13% of the absolute melting point). Prolific surface diffusion is confirmed by in-situ transmission electron microscopy mechanical deformation, performed at room temperature and demonstrating remarkable high-strain-rate Coble pseudoelasticity. Anomalously small activation energy and growth stagnation are found in high-temperature coarsening experiments, suggesting additional chemical driving forces beyond purely physical capillarity. We expect similar phenomena may also exist in other ceramic materials, which is an important factor to consider for quality control, storage, and applications of ultrafine nanomaterials.