Solid-state NMR and XRD study of α-SiAlON powders prepared by combustion synthesis

Al-27 and Si-29 solid-state NMR spectra and X-ray diffraction (XRD) patterns were obtained for alpha-SiA1ON powders prepared by combustion synthesis, according to which the phase transformation and structure evolution of ot-SiAlON were studied. It was found that in alpha-SiA1ON Si-29 chemical shift values (-48 < delta(Si) < -47) were close to those in beta-Si3N4 and alpha-Si3N4, indicating that Si atoms kept SiN4 coordination to a large extent in alpha-SiA1ON despite the presence of 0 atoms. Dissimilarly, Al-27 chemical shift values in alpha-SiA1ON deviated clearly from that corresponding to AlN4 coordination (delta(Al) similar to 112) and occurred in a range from 3Al 95.5 to 99.9, which should be assigned to tetrahedral AIQN4-, (0 < x < 4) coordination. The broadening effect of AlOxN4-x, peaks was noticed, which was suggested to induced by slight dispersion of alpha-SiAlON compositions. 27 Al resonances assigned to AlO4 or AlO6 were also observed, revealing the existence of glassy phase in combustion products. By comparing the intensities of AlO4 and AlO6 resonance peaks, it was found that the composition and structure of the co-existing liquid assisting alpha-SiAlON formation were not constant but varied with temperature in combustion synthesis. In addition, with CaF, used as Ca-source alpha-SiAlON with low oxygen contents (n < m/2) were prepared, and it was found that there was a low threshold of oxygen content only beyond which alpha-SiAlON could be formed. (c) 2006 Elsevier B.V. All rights reserved.