Synthesis of Ultrafine β″‐Alumina Powders via Flame Spray Pyrolysis of Polymeric Precursors

Abstract

Flame spray pyrolysis of a polymeric precursor is used to prepare ultrafine powders that, when sintered, convert to essentially pure phase lithium‐doped sodium β″‐alumina. The precursor Na 1.67 Al 10.67 Li 0.33 [N(CH 2 CH 2 O) 3 ] 10.67 ‐[OCH 2 CH 2 O]·x(HOCH 2 CH 2 OH) has been synthesized from stoichiometric amounts of metal hydroxides and tri‐ethanolamine (N(CH 2 CH 2 OH) 3 , TEA) in excess ethylene glycol. The precursor is dissolved in ethanol, and an atom‐ized spray of the solution is combusted in a specially con‐structed flame spray apparatus. Combustion occurs at ∼2000°C, followed by immediate quenching. This proce‐dure provides for a measure of kinetic control over the process. The resulting nanopowder particles are 50–150 nm in diameter and exhibit powder X‐ray diffractometry pat‐terns similar to β″‐alumina. Heating the nanopowder at 30°C/min to 1200°C with a 1 hisotherm converts it to pure β″‐alumina. In preliminary sintering studies, green powder compacts (∼65% theoretical density) sintered at 1600°C for 12 min densify to 3.0 ± 0.1 g/cm 3 (∼92% theoretical density) with minimal loss of Na 2 O. This procedure offers several processing and cost advantages over conventional β″‐alumina syntheses.