Crystallization of CaAl<inf>4</inf>O<inf>7</inf> and CaAl <inf>12</inf>O<inf>19</inf> powders

Calcium is always present in alumina systems as an unintentional (or intentional) dopant, and yet the fundamental effect of its incorporation into the aluminas is not well understood, and is further complicated by the presence of Si. The synthesis of powders of two calcium aluminate phases (CaAl 4O7, which is also known as CaO 2Al2O3 or CA2, and CaAl12O19, which is also known as CaO 6Al2O3 or CA6) has been investigated using low-temperature chemical-processing techniques. The crystallization of these powders from the amorphous precursor has been examined using various characterization techniques. The precursors for the powders were prepared by mixing stoichiometric proportions of the nitrate salts into a 5 wt% aqueous solution of poly(vinyl alcohol). Conversion of the amorphous precursors to crystalline powders and the subsequent phase transitions were monitored using differential thermal analysis (DTA), thermogravimetric analysis (TGA) and powder X-ray diffractometry (XRD). While powders with CA2 stoichiometry crystallized directly at 883C, amorphous powders with CA6 stoichiometry first crystallized into an intermediate structure without partitioning and then transformed into CA6 at 1175C. Fully and partially crystallized powders were analyzed using transmission electron microscopy and electron energy-loss spectroscopy (EELS). Measured near-edge structures (Al-L2,3, Ca-L2,3 and O-K) are presented for the CA2, -Al2O3 and CA6 phases. The intermediate phase, identified as -Al2O3, was found to accommodate a significant concentration of Ca.