High pressure/low temperature sintering of nanocrystalline alumina

Bulknanocrystalline α-Al₂O₃samples with a relative density >98% and a grain size < 50 nm have been produced by high pressure/low temperature sintering, using a toroidal-type high pressure apparatus. Nanocrystalline (n-) alumina powder with metastable γphase was used as the starting material. During sintering, the γphase transforms to αphase. The transformation temperature decreases from ~1075 °C at ambient pressure to about 460 °C at 8 GPa. Grain growth is limited by the low sintering temperature, and a multiplicity of nucleation events in the parent γ phase at very high pressure creates a nanoscale α grain size. The average grain size of the α-Al₂O₃ increases from 18 nm in the original powder to only about 49 nm in the sintered compact (98.2% dense). In addition, we found that high pressure could increase the nucleation rate while reducing the growth rate of the transformed α phase so that its grain size decreased with sintering pressure under the same sintering temperature and time. Due to its high surface area, n-Al₂O₃ powder readily absorbs chemical species from the environment. Alumina hydrates, formed by the reaction of Al₂O₃ with chemisorbed OH⁻ species during sintering, had a profound influence on sintering and phase transformation behaviors of n-Al₂O₃. To control grain size of the transformed α phase, it is essential to eliminate the hydrates before sintering.