SYNTHESIS OF OXYNITRIDE POWDERS VIA FLUIDIZED-BED AMMONOLYSIS .1. LARGE, POROUS, SILICA PARTICLES

Reaction of silica (SiO2) with triethanolamine (TEA, N(CH2CH2OH)(3)) a nd ethylene glycol (EG) under conditions (similar to 200 degrees C) wh ere byproduct water is removed resulted in the formation of the neutra l silatrane glycolate complex, N(CH2CH2O)(3)SiOCH2CH2OH (or TEASiOCH(2 )CH(2)OH) in essentially quantitative yield, Solutions of this neutral precursor in EG, when rapidly pyrolyzed and then oxidized at 500 degr ees C, formed porous ceramic powders with high specific surface areas (>500 m(2)/g), These powders were nitrided via ammonolysis in a fluidi zed-bed reactor at temperatures of 700 degrees-1000 degrees C. The res ulting nitrided powders were characterized by thermal and chemical ana lyses, diffuse reflectance infrared spectroscopy, gas sorption, and X- ray photoelectron spectroscopy, The apparent activation energy for the nitridation process was determined to be 54 kJ/mol, Following nitrida tion, the powders were amorphous and had nitrogen contents as high as 21 wt% with retained surface areas >300 m(2)/g at 1000 degrees C, Unde r the nitridation conditions used, the extent of nitrogen incorporatio n correlated linearly with increases in material density, This lineari ty suggested that the change in density occurred primarily because of changes in coordination that occurred as trivalent nitrogen replaced d ivalent oxygen in the glass structure and nominally because of viscous flow, The linear density increase also suggested that pore trapping d id not occur under these processing conditions, This work serves as a model for ongoing studies on the nitridation of high-surface-area cera mic powders produced by the rapid pyrolysis of mixed-metal TEA alkoxid es.