Densification and grain growth of Al2O3 nanoceramics during pressureless sintering

alpha-Al2O3 nanopowders with mean particle sizes of 10, 15, 48, and 80 nm synthesized by the doped alpha-Al2O3 seed polyacrylamide gel method were used to sinter bulk Al2O3 nanoceramics. The relative density of the Al2O3 nanoceramics increases with increasing compaction pressure on the green compacts and decreasing mean particle size of the starting alpha-Al2O3 nanopowders. The densification and fast grain growth of the Al2O3 nanoceramics occur in different temperature ranges. The Al2O3 nanoceramics with an average grain size of 70 nm and a relative density of 95% were obtained by a two-step sintering method. The densification and the suppression of the grain growth are achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. The densification was realized by the slower grain-boundary diffusion without promoting grain growth in second-step sintering.