Investigation of local structures of silicon oxynitride glasses prepared from aerogels

Oxynitride glasses in Si-Al-O-N system were synthesized to investigate atomic arrangements in those glasses. Aerogels of silica (SiO 2 ) and silica-alumina (SiO 2 -Al 2 O 3 ) system were fabricated by drying wet gels in supercritical CO 2 condition. The SiO 2 gels were prepared from the silicon alkoxide with CH 3 groups and the SiO 2 -Al 2 O 3 gels were prepared from silicone and aluminum alkoxides. Ammonolysis were performed at T N = 750–1400 °C to synthesize oxynitride glasses. The nitrogen concentration in the resultant glasses increased with the increase in the ammonolysis temperatures and exceeded 34 eq.% by ammonolysis at 1300 °C. The specific surface area of these aerogels has been 1941 and 1159 m 2 g −1 , respectively. The glass structures were investigated by adopting 29 Si and 27 Al NMR measurements. In silicon oxynitride glasses, it was revealed that only one N atom occupies the nearest neighbor site around Si after ammonolysis at T N < 1200 °C, while two or more N atoms occupy the nearest neighbor site around Si after ammonolysis at T N > 1300 °C. In Si-Al-O-N glasses, the number of N atoms at around neighboring to Si atom varied with ammonolysis temperature but any traces of Al-N bonds were not found, indicating the bridging N in the form of Si-N-Al was absent in the glasses. Nitrogen contents of oxynitride glasses prepared from silica-based aerogels with CH 3 groups (ON-CH 3 ) and those doped with Al and Eu ions (ON-AE) increased with increase of ammonolysis temperature. The 29 Si NMR spectra differed between ON-CH 3 and ON-AE.