Nano-α-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(1-4). The nanoparticles produced by these methods consist mainly of the undesired delta phase with some gamma- and theta-Al2O3. Nano-t-aluminas should provide access to dense nano/submicrometre-grained alpha-Al2O3 shapes offering significant advantages over micrometre-grained shapes(5-11). Unfortunately, polymorphism coupled with the high activation energy for nucleating alpha-Al2O3 greatly impedes efforts to process dense alpha-Al2O3 with controlled grain sizes, especially for submicrometre materials. Typically alpha-Al2O3 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-Al2O3 from multiple nano-t-aluminas for seeding or direct processing of alpha-Al2O3 monoliths could greatly change how alpha-Al2O3 components are processed. We report here that liquid-feed flame spray pyrolysis(3) of nano-t-aluminas converts them to dispersible 30-80 nm alpha-Al2O3 powders (50-85% phase transformed). Surprisingly, the powder surfaces are fully dehydrated. These powders pressureless sinter to more than 99.5% dense alpha-Al2O3 with final grain sizes <= 500nm without sintering