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

Abstract

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^1,2,3,4. The nanoparticles produced by these methods consist mainly of the undesired δ phase with some γ- and θ-Al₂O₃. Nano-t-aluminas should provide access to dense nano/submicrometre-grained α-Al₂O₃ shapes offering significant advantages over micrometre-grained shapes^{5,6,7,8,9,10,11}. Unfortunately, polymorphism coupled with the high activation energy for nucleating α-Al₂O₃ greatly impedes efforts to process dense α-Al₂O₃ with controlled grain sizes, especially for submicrometre materials. Typically α-Al₂O₃ nucleation within t-aluminas is sporadic rather than uniform, leading to exaggerated grain growth and vermicular microstructures without full densification⁵. Thus, production of quantities of nano-α-Al₂O₃ from multiple nano-t-aluminas for seeding or direct processing of α-Al₂O₃ monoliths could greatly change how α-Al₂O₃ components are processed. We report here that liquid-feed flame spray pyrolysis³ of nano-t-aluminas converts them to dispersible 30–80 nm α-Al₂O₃ powders (50–85% phase transformed). Surprisingly, the powder surfaces are fully dehydrated. These powders pressureless sinter to more than 99.5% dense α-Al₂O₃ with final grain sizes ≤500 nm without sintering aids.