SYNTHESIS OF BAAL2SI2O8 FROM SOLID BA-AL-AL2O3-SIO2 PRECURSORS - II, TEM ANALYSES OF PHASE EVOLUTION

Conventional transmission electron microscopy (TEM), coupled with ener gy-dispersive X-ray (EDX) analysis, has been used to examine the inter mediate phases produced during the transformation of Ba-Al-alpha-Al2O3 -SiO2 (cristobalite) precursors into celsian, BaAl2Si2O8, Analyses wer e conducted on samples that had been exposed to pure, flowing oxygen a t peak temperatures of 300 degrees C for 24 h, 650 degrees C for 72 h, or 900 degrees C for 24 h, Particles of Al2O3 retained after the 300- 900 degrees C heat treatments were coated with a layer of BaAl2O4. An amorphous, alumina-poor Ba-Al-Si-O (G1) phase was observed in contact with residual SiO2 particles after the 650 degrees C heat treatment, A lthough not a starting component of the precursor, elemental silicon w as also detected after the 300-900 degrees C heat treatments. Silicon particles produced during the 300 degrees C heat treatment were surrou nded by an amorphous Ba-Al-Si-O (G2) phase that contained more alumina than the G1 phase. Pine-grained barium orthosilicate (Ba2SiO4) and sa nbornite (BaSi2O5) were observed after the 650 degrees C treatment, al ong with thin, plate-shaped grains of BaAl2Si2O8, The latter two phase s were surrounded by an amorphous Ba-Al-Si-O (G3) phase that possessed less alumina than the G2 phase. Smaller platelets of BaAl2Si2O8 were also detected after the 300 degrees C treatment. After annealing at a peak temperature of 1650 degrees C, BaAl2Si2O8 was the only silicate c ompound detected by TEM, Novel reaction paths to BaAl2Si2O8, which are consistent with the present TEM observations and prior XRD and SEM/ED X analyses (Part I), are discussed.