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

Monolithic Al2O3 and Al2O3/SiC nanocomposite powders were prepared by sol–gel processing. The process involved the precipitation of Al(NO3)3·9H2O with NH4OH in excess water to form boehmite (AlOOH). XRD indicates that the subsequent thermal reaction proceeds by the phase transformation sequence AlOOH, γ-, δ-, θ-, to α-Al2O3. The 27Al NMR spectra indicate a gradual increase in the proportion of Al in the tetrahedral sites of the γ-, δ- and θ-Al2O3 formed at increasing calcination temperatures. Complete transformation to octahedral Al (α-Al2O3) is marked by the abrupt disappearance of tetrahedral Al. Al2O3/SiC nanocomposite powders were prepared by adding α-SiC powder to the boehmite precursor at the precipitation stage. Upon heating, the 29Si NMR spectra of the Al2O3/SiC powders reveal α-SiC, Al2O3·xSiO2 and SiO2 phases. Stable α-Al2O3 and α-Al2O3/SiC nanocomposite powders are formed at 1200 and 1300 °C, respectively. It appears likely that the presence of SiC modifies the thermal behaviour of the Al2O3 in the nanocomposites by stabilising the Al2O3 phases with concomitant oxidation of SiO2.