Densification and Grain Growth of Al ₂ O ₃ Nanoceramics During Pressureless Sintering

α ‐ Al 2 O 3 nanopowders with mean particle sizes of 10, 15, 48, and 80 nm synthesized by the doped α‐Al 2 O 3 seed polyacrylamide gel method were used to sinter bulk Al 2 O 3 nanoceramics. The relative density of the Al 2 O 3 nanoceramics increases with increasing compaction pressure on the green compacts and decreasing mean particle size of the starting α‐Al 2 O 3 nanopowders. The densification and fast grain growth of the Al 2 O 3 nanoceramics occur in different temperature ranges. The Al 2 O 3 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.