MAGIC-ANGLE-SPINNING NUCLEAR-MAGNETIC-RESONANCE CHARACTERIZATION OF NANOMETRIC SI C/N POWDERS FORMED BY LASER SPRAY-PYROLYSIS/

Ultrafine amorphous Si/C/N powders were obtained via ultrasonic inject ion of aerosol droplets into the beam of a high-power CO2 laser using hexamethyldisilasane as precursor and an ammoniac argon mixture as car rier gas. The evolution of the structure after heat treatment under ni trogen (1000-1600 degrees C) was studied by Si-29 magic angle spinning -nuclear magnetic resonance. At low temperature (1000 degrees C), the spectra show an unstructured massif which can be associated with an am orphous structure. At higher temperature the massif is split into broa d and sharper structured lines. The broad line is attributed to silico n carbide, and the sharper structured line to well-crystallised silico n nitride. The Si-29 spin-lattice relaxation times are different in th ese two environments, The presence of a large number of paramagnetic c entres detected by electron paramagnetic resonance spectroscopy could be associated with one of these environments.