  Nanocrystalline metals and ceramics with <100 nm grain sizes and superior
properties (e.g., mechanical strength, hardness, fracture toughness and stored
dielectric energy) are of great interest. Much has been discussed about
achieving nano grains, but little is known about maintaining grain-size
uniformity that is critical for material reliability. An especially intriguing
question is whether it is possible to achieve a size distribution narrower than
what Hillert[1] theoretically predicted for normal grain growth, a possibility
suggested, for growth with a higher growth exponent, by the generalized
mean-field theory[2] of Lifshitz, Slyozov, Wagner (LSW)[3,4] and Hillert but
never realized in practice. We demonstrate that this can be achieved in bulk
materials with an appropriately designed two-step sintering route that (a)
takes advantage of the large growth exponent in the intermediate sintering
stage to form a most uniform microstructure despite porosity remaining, and (b)
freezes the grain growth thereon while continuing densification to reach full
density. The resultant dense bulk Al2O3 ceramic has an average grain size of 34
nm and a much narrower size distribution than Hillert's prediction. Bulk Al2O3
with a grain-size distribution narrower than the particle-size distribution of
starting powders was also demonstrated using this strategy, as were highly
uniform bulk engineering metals and ceramics of either high purity and high
melting points (Mo and W-Re) or highly complex compositions (core-shell BaTiO3
and 0.87BaTiO3-0.13Bi(Zn2/3(Nb0.85Ta0.15)1/3)O3).
