Oxynitride ceramics via ammonolysis of high surface area oxides produced from triethanolamine metal-alkoxides.

Materials chemistry was applied toward processing oxynitride ceramic powders by studying the nitridation of high surface area oxide powders by a gas/solid reaction with ammonia (NH$\sb3),$ termed ammonolysis. High surface area oxide powders were produced via pyrolysis of triethanolamine (TEA) chelated-metal alkoxides using two different techniques: rapid precursor pyrolysis and flame spray pyrolysis (FSP). Processable precursors were synthesized by reacting TEA directly with amorphous metal oxides and hydroxides in excess ethylene glycol ($\sim$200$\sp\circ$C). The resulting precursors are soluble in common alcohols, and show prolonged stability against hydrolysis in ambient air. Amorphous metal hydroxides were synthesized for use as starting materials by precipitating acid solutions with ammonium carbonate. Porous powders were produced by a rapid, solution pyrolysis that favors formation of a highly porous structure. Two silicate systems were studied: silica (SiO$\sb2)$ and cordierite $\rm(MgO{\cdot}2Al\sb2O\sb3{\cdot}5SiO\sb2).$ The highly porous silica powders had specific surface areas (SSAs) exceeding 500 m$\sp2$/g, as a result of micro- and meso-porosity. The cordierite powders had SSAs up to 470 m$\sp2$/g with the largest contribution coming from 2.5-20 nm meso-pores. Processing windows for nitriding highly porous silicates were determined by experiment with fluidized bed reactions over the temperature range of 700-1000$\sp\circ$C. Nitrogen contents for the SiON system approached 21 wt.% N, and up to 12.7 wt.% N incorporation for MgSiAlON. High SSAs remain in the powders after processing to give highly porous, amorphous, oxynitride powders. Flame spray pyrolysis of precursor solutions in alcohol was also used to produce high SSA powders. However, FSP produces solid, dense, ultra-fine particles (UFPs), where the high SSA comes solely from external surfaces. The first system studied was spinel $\rm(MgO{\cdot}Al\sb2O\sb3).$ FSP of titanatrane precursors produced anatase/rutile UFPs with specific surface areas exceeding 40 m$\sp2$/g. Nitridation of these titania (TiO$\sb2)$ UFPs by fixed-bed ammonolysis was done over the range of 550$\sp\circ$ to 1200$\sp\circ$C, resulting in ultra-fine TiN.Ph.D.Applied SciencesMaterials scienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/129942/2/9711924.pd