Nano-alpha-Al2O3 by liquid-feed flame spray pyrolysis

Nanometre-sized particles of transition (t)-aluminas are important for the fabrication of high-quality alumina ceramics. Multiple tons are produced each year using a variety of gas-phase processes. The nanoparticles produced by these methods consist mainly of the undesired delta phase with some gamma- and theta-Al(2)O(3). Nano-t-aluminas should provide access to dense nano/submicrometre-grained alpha-Al(2)O(3) shapes offering significant advantages over micrometre-grained shapes. Unfortunately, polymorphism coupled with the high activation energy for nucleating alpha-Al(2)O(3) greatly impedes efforts to process dense alpha-Al(2)O(3) with controlled grain sizes, especially for submicrometre materials. Typically alpha-Al(2)O(3) nucleation within t-aluminas is sporadic rather than uniform, leading to exaggerated grain growth and vermicular microstructures without full densification (5). Thus, production of quantities of nano-alpha-Al(2)O(3) from multiple nano-t-aluminas for seeding or direct processing of alpha-Al(2)O(3) monoliths could greatly change how alpha-Al(2)O(3) components are processed. We report here that liquid-feed flame spray pyrolysis of nano-t-aluminas converts them to dispersible 30-80 nm alpha-Al(2)O(3) powders (50-85% phase transformed). Surprisingly, the powder surfaces are fully dehydrated. These powders pressureless sinter to more than 99.5% dense alpha-Al(2)O(3) with final grain sizes < or =500 nm without sintering aids.