29Si AND 13C MAGIC ANGLE SAMPLE SPINNING NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY OF CERAMIC FIBERS PREPARED BY PYROLYSIS OF ORGANOSILICON POLYMERS

Silicon oxycarbide (Si-C-O) and oxycarbonitride (Si-N-C-O) fibers, prepared by pyrolysis of organosilicon polymers, have been characterized by 29Si and 13C magic angle spinning(MAS) NMR. The Si-C-O fibers are partially amorphous and the Si-N-C-O fibers are totally amorphous. A single-pulse FT-NMR technique using ≥ 60s pulse delay with phase cycling and with multiple signal acquisitions (>200) was used. A spinning rate > 4 KHz is required to remove spinning sideband interference. Boron nitride rotors were used for sample spinning in 13C NMR experiments. 29Si chemical shifts of the five possible tetrahedral species about silicon (SiC4, SiC3O, SiC2O2, SiCO3, SiO4) in silicon oxycarbides (NILCALON™ fibers) can be partially resolved at 6.3 Tesla (29Si resonance frequency of 53.8 MHz). These Si-C-O compositions were previously shown to contain β-SiC nanocrystallites, 1 to 3 nm in size, and a carbon nanophase distributed throughout the continuous amorphous phase. Chemical shift assignments, relative to external TMS, are based on analogous organosilicon compounds and on the various silicon carbide polytypes.