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

Abstract

27Al and 29Si solid-state NMR spectra and X-ray diffraction (XRD) patterns were obtained for α-SiAlON powders prepared by combustion synthesis, according to which the phase transformation and structure evolution of α-SiAlON were studied. It was found that in α-SiAlON 29Si chemical shift values (−48 < δ Si < −47) were close to those in β-Si 3N 4 and α-Si 3N 4, indicating that Si atoms kept SiN 4 coordination to a large extent in α-SiAlON despite the presence of O atoms. Dissimilarly, 27Al chemical shift values in α-SiAlON deviated clearly from that corresponding to AlN 4 coordination ( δ Al ∼ 112) and occurred in a range from δ Al 95.5 to 99.9, which should be assigned to tetrahedral AlO x N 4− x (0 ≤ x ≤ 4) coordination. The broadening effect of AlO x N 4− x peaks was noticed, which was suggested to induced by slight dispersion of α-SiAlON compositions. 27Al resonances assigned to AlO 4 or AlO 6 were also observed, revealing the existence of glassy phase in combustion products. By comparing the intensities of AlO 4 and AlO 6 resonance peaks, it was found that the composition and structure of the co-existing liquid assisting α-SiAlON formation were not constant but varied with temperature in combustion synthesis. In addition, with CaF 2 used as Ca-source α-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 α-SiAlON could be formed.