Sintering and Characterization of Nanophase Zinc Oxide

Nanocrystalline, single‐phase undoped ZnO was sintered to 95%–98% of theoretical density at 650°–700°C, using pressureless isothermal sintering. The density increased very rapidly at 500°–600°C, remained constant with sintering temperature until ∼900°C, and then decreased slightly. The estimated activation energy for densification at 600°–700°C (275 kJ/mol) was comparable to grain‐growth activation energies previously reported for microcrystalline ZnO but much greater than the grain‐growth activation energy measured in the present work. A bimodal microstructure, consisting of nanocrystalline grains within larger ensembles (“supergrains”), was observed, and both modes grew as the sintering temperature increased. The grain‐growth activation energy for the nanocrystalline grains was extremely low, ∼20 kJ/mol. The activation energy for the growth of the supergrains depended strongly on temperature but was ∼54 kJ/mol at >500°C. The important mechanisms probably are rearrangement of the nanoparticle grains, with simultaneous surface and boundary diffusion, and vapor transport above 900°C.