Sol–gel processing and phase characterization of Al2O3 and Al2O3/SiC nanocomposite powders

Monolithic Al{sub 2}O{sub 3} and Al{sub 2}O{sub 3}/SiC nanocomposite powders were prepared by sol-gel processing. The process involved the precipitation of Al(NO{sub 3}){sub 3}.9H{sub 2}O with NH{sub 4}OH in excess water to form boehmite (AlOOH). XRD indicates that the subsequent thermal reaction proceeds by the phase transformation sequence AlOOH, {gamma}-, {delta}-, {theta}-, to {alpha}-Al{sub 2}O{sub 3}. The {sup 27}Al NMR spectra indicate a gradual increase in the proportion of Al in the tetrahedral sites of the {gamma}-, {delta}- and {theta}-Al{sub 2}O{sub 3} formed at increasing calcination temperatures. Complete transformation to octahedral Al ({alpha}-Al{sub 2}O{sub 3}) is marked by the abrupt disappearance of tetrahedral Al. Al{sub 2}O{sub 3}/SiC nanocomposite powders were prepared by adding {alpha}-SiC powder to the boehmite precursor at the precipitation stage. Upon heating, the {sup 29}Si NMR spectra of the Al{sub 2}O{sub 3}/SiC powders reveal {alpha}-SiC, Al{sub 2}O{sub 3}.xSiO{sub 2} and SiO{sub 2} phases. Stable {alpha}-Al{sub 2}O{sub 3} and {alpha}-Al{sub 2}O{sub 3}/SiC nanocomposite powders are formed at 1200 and 1300 deg. C, respectively. It appears likely that the presence of SiC modifies the thermal behaviour of the Al{sub 2}O{sub 3} in the nanocomposites by stabilising the Al{sub 2}O{sub 3} phases with concomitant oxidation of SiO{sub 2}.