Synthesis of oxynitride powders via fluidized-bed ammonolysis .1. Large, porous, silica particles

Reaction of silica (SiO2) with triethanolamine (TEA, N(CH2CH2OH)(3)) and ethylene glycol (EG) under conditions (similar to 200 degrees C) where byproduct water is removed resulted in the formation of the neutral 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 degrees C, formed porous ceramic powders with high specific surface areas (>500 m(2)/g), These powders were nitrided via ammonolysis in a fluidized-bed reactor at temperatures of 700 degrees-1000 degrees C. The resulting nitrided powders were characterized by thermal and chemical analyses, 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 nitridation, 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, Under the nitridation conditions used, the extent of nitrogen incorporation correlated linearly with increases in material density, This linearity suggested that the change in density occurred primarily because of changes in coordination that occurred as trivalent nitrogen replaced divalent oxygen in the glass structure and nominally because of viscous flow, The linear density increase also suggested that pore trapping did not occur under these processing conditions, This work serves as a model for ongoing studies on the nitridation of high-surface-area ceramic powders produced by the rapid pyrolysis of mixed-metal TEA alkoxides.