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2 Bulletin of the Polish Academy of Sciences. Chemistry

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Preparation of silicon oxynitrides by the reaction of ammonia with high disperse silica

Truckenbrodt B., Breitbarth F. W., Vogelsberger W.

Bulletin of the Polish Academy of Sciences. Chemistry

2002

Vol. 50, No. 1

77-92

Nanosized silica is nitrided at different temperatures up to 1200° C by the reaction with gaseous ammonia. The nitridation degree of silica is determined as a function of time. After a nitridation time of 119 h and a temperature of 1200° C the mass fraction of nitrogen in the nitrided sample was found to be 35.4%. This corresponds to the stoi-chiometric formula SiO(18)Ni (1.21). It could be shown that the progressive nitridation of the samples is accompanied by a decrease of the specific surface area. The different degree of nitridation can be followed by IR spectroscopical investigations. Several absorption bands of silica and the nitrided species can be detected in dependence of nitridation time. EPR spectroscopical investigation of irradiated samples provide information about the bonding in the samples. Furthermore, a relation between the concentration of defect centers and the degree of nitridation is found. The process of nitridation is also determined by X-ray diffraction measurements. It was shown to be possible to separate the diffraction patterns of SiO(2)2, Si(2)ON(2) and Si(3)N(4). This separation could be established by NMR spectroscopy. Thus it was possible to follow the development of the mentioned three species in dependence of nitridation time. A phenomenological consecutive reaction scheme describes the time dependent appearance of the three species well.

EPR spectroscopic and rheological investigation during sol-gel transition in the systems TEOS/ethanol/water and sodium water-glass

Breyer T., Breitbarth F. W., Vogelsberger W.

Bulletin of the Polish Academy of Sciences. Chemistry

2000

Vol. 48, No. 4

325-336

Paramagnetic ions (V4+ , Mn2+ ) are used to detect mobility changes during the sol-gel transition of sols from sodium water glass as well as from TEOS hydrolysis. The viscosity of these samples is measured in the rotational and oscillating mode to determine the steady state viscosity and the storage and loss moduli. The overall kinetic rate constant for the formation of siloxane bonds has been determined in dependence on pH of the reaction mixture. In spite of the rapid increase of the viscosity during the solgel transition by several orders of magnitude, EPR spectra of solvated VO2+ and Mn2+ probe ions do not show significant changes of the mobility. These ions move freely in the space between the agglomerating sol particles until the gels are dried. Paramagnetic V(IV) species derived from reduction of VO(iPr)3, however, are adsorbed on the growing sol particles. They, therefore, show a powder-like EPR spectrum of immobilized ions even in the region of very low sol viscosity well before the sol-gel transition.