New molecular precursors from the reaction of hydrazine and aluminum alkoxide for the synthesis of powders in the Al-O-N system

A novel low-temperature sol-gel-based process has been developed for the synthesis of amorphous and crystalline powders in the Al-O-N system. The process involves reacting aluminum tri-sec-butoxide with anhydrous hydrazine using a solvent mixture of acetonitrile (ACN) and chloroform (at ≈60 °C) to result in solid precursors. The as-prepared precursors have been pyrolyzed in nitrogen and in anhydrous ammonia to yield amorphous and crystalline oxynitride or nitride powders. The powders have been characterized using X-ray diffraction (XRD), fourier transform infrared (FTIR), and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy techniques. Results of the structural spectroscopic analyses conducted on the pyrolyzed powders obtained at 800 °C indicate the formation of an amorphous or nanocrystalline aluminum oxynitride containing Al bonded to O in buta-, penta-, and hexacoordination. Evidence is also presented suggesting the influence of hydrazine in facilitating replacement of the butoxy groups during heat treatment and creating molecular species containing Al-N linkages in the solid precursor. Continued heat treatment of these precursors in NH₃ at 1300 °C leads to the formation of crystalline AlN. The role of hydrazine in the formation of the nitride was also verified by conducting the reaction using controlled amounts of deionized (DI) water. The addition of water competes with the nitridation ability of hydrazine leading to the formation of either oxides or oxynitrides of aluminum. The approaches thus developed demonstrate the potential and flexibility of alkoxides for synthesizing precursors that lead to either single phase of AlN or molecularly mixed composites of Al₂O₃, AlON, and AlN, in the Al-O-N system.