| Abstract Scope |
Nanocrystalline ceramics with <100 nm grain sizes and superior properties are of great interest. Much has been discussed about ultrafine grain sizes, but little is known about ultra-uniformity, defined as grain size distribution narrower than predicted by the classical theory of Hillert. Here we provide a generalized growth theory unifying the textbook knowledge of Ostwald ripening and normal grain growth. For curvature driven grain growth, we find a steady-state size distribution that is analytically solvable for growth exponent n>1 and it narrows with increasing n. Experimental validation of this prediction is found in porous alumina ceramics at various intermedium and final stage sintering, which sheds light on the hidden role of porosity to self-homogenize the microstructure. When coupled with low-temperature two-step sintering that freezes grain coarsening in the final stage sintering, it creates dense alumina nanoceramics with 34 nm average grain size and an extremely uniform microstructure below 1150 degreeC. |