Fabrication of Transparent γ‐Al2O3 from Nanosize Particles

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Abstract
The compaction and heat‐treatment behavior of nanosize γ‐Al2O3 powder (average diameter = 20 nm) was studied. A diamond anvil high‐pressure cell was used to compact the powder at pressures up to 3 GPa, both in air at room temperature and under liquid nitrogen, followed by pressureless heat treatment at 800°C. For all conditions studied, the fabricated compacts were optically transparent. X‐ray diffraction confirmed retention of the γ‐phase. The compacts were also characterized before and after heat treatment by microhardness measurements and by transmission electron microscopy. For both ambient and cryogenic compaction, sample hardness increased with pressure, and heat treatment resulted in about a 50% increase in hardness independent of the initial green‐state value. Samples compacted in LN2 were significantly harder (up to 9.6 GPa) than those compacted in air. TEM examination revealed a random‐dense‐packed particle structure and interconnected porosity; interstitial void dimensions, however, were always less than the average particle diameter (20 nm). Observed effects on the increase in hardness could not be explained by microstructural changes normally attributed to increased compaction pressure or heat treatment, most notably densification. Alternative explanations are proposed.