Synthesis of BaAl ₂ Si ₂ O ₈ from Solid Ba‐Al‐Al ₂ O ₃ ‐SiO ₂ Precursors: II, TEM Analyses of Phase Evolution

Abstract

Conventional transmission electron microscopy (TEM), coupled with energy‐dispersive X‐ray (EDX) analysis, has been used to examine the intermediate phases produced during the transformation of Ba‐Al‐alpha‐Al 2 O 3 ‐SiO 2 (cristobalite) precursors into celsian, BaAl 2 Si 2 O 8 . Analyses were conducted on samples that had been exposed to pure, flowing oxygen at peak temperatures of 300°C for 24 h, 650°C for 72 h, or 900°C for 24 h. Particles of Al 2 O 3 retained after the 300‐900°C heat treatments were coated with a layer of BaAl 2 O 4 . An amorphous, alumina‐poor Ba‐Al‐Si‐O (G1) phase was observed in contact with residual SiO 2 particles after the 650°C heat treatment. Although not a starting component of the precursor, elemental silicon was also detected after the 300‐900°C heat treatments. Silicon particles produced during the 300°C heat treatment were surrounded by an amorphous Ba‐Al‐Si‐O (G2) phase that contained more alumina than the G1 phase. Fine‐grained barium orthosilicate (Ba 2 SiO 4 ) and sanbornite (BaSi 2 O 5 ) were observed after the 650°C treatment, along with thin, plate‐shaped grains of BaAl 2 Si 2 O 8 . The latter two phases were surrounded by an amorphous Ba‐Al‐Si‐O (G3) phase that possessed less alumina than the G2 phase. Smaller platelets of BaAl 2 Si 2 O 8 were also detected after the 300°C treatment. After annealing at a peak temperature of 1650°C, BaAl 2 Si 2 O 8 was the only silicate compound detected by TEM. Novel reaction paths to BaAl 2 Si 2 O 8 , which are consistent with the present TEM observations and prior XRD and SEM/EDX analyses (Part I), are discussed.