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Abstract
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 (CaAl4O7, 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 883°C, amorphous powders with CA6 stoichiometry first crystallized into an intermediate structure without partitioning and then transformed into CA6 at 1175°C. 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, γ-Al2O3 and CA6 phases. The intermediate phase, identified as γ-Al2O3, was found to accommodate a significant concentration of Ca.
Acknowledgements
The authors thank Professor Nigel Browning and Dr Miaofang Chi for their help with EELS measurements. The work was initially supported by US Department of Energy through grant DE-FG02-01ER45883 and recently by 3 M Harry Heltzer Endowed Chair funds and an NSF international travel grant INT-0322622. The authors acknowledge use of facilities at the National Center for Electron Microscopy, Lawrence Berkeley Laboratory, which is supported by the US Department of Energy under Contract # DE-AC02-05CH11231. IA acknowledges funding from Sandia's Truman Fellowship LDRD; Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.