SYNTHESIS OF ULTRAFINE BETA''-ALUMINA POWDERS VIA FLAME SPRAY-PYROLYSIS OF POLYMERIC PRECURSORS

Flame spray pyrolysis of a polymeric precursor is used to prepare ultr afine powders that, when sintered, convert to essentially pure phase l ithium-doped sodium beta ''-alumina. The precursor 1.67Al10.67Li0.33[N (CH2CH2O)(3)](10.67)[OCH2CH2O]. x(HOCH2CH2OH) has been synthesized fro m stoichiometric amounts of metal hydroxides and triethanolamine (N(CH 2CH2OH)(3), TEA) in excess ethylene glycol, The precursor is dissolved in ethanol, and an atomized spray of the solution is combusted in a s pecially constructed flame spray apparatus. Combustion occurs at simil ar to 2000 degrees C, followed by immediate quenching. This procedure provides for a measure of kinetic control over the process. The result ing nanopowder particles are 50-150 nm in diameter and exhibit powder S-ray diffractometry patterns similar to beta ''-alumina. Heating the nanopowder at 30 degrees C/min to 1200 degrees C with a 1 h isotherm c onverts it to pure beta ''-alumina. In preliminary sintering studies, green powder compacts (similar to 65% theoretical density) sintered at 1600 degrees C for 12 min density to 3.0 +/- 0.1 g/cm(3) (similar to 92% theoretical density) with minimal loss of Na2O. This procedure off ers several processing and cost advantages over conventional beta ''-a lumina syntheses.