PYROLYSIS CHEMISTRY OF POLY(ORGANOSILAZANES) TO SILICON CERAMICS

The thermal evolution of a commercial and an in-house-prepared organosilazane polymer to silicon nitride and silicon carbide was examined systematically for temperatures up to 1400-degrees-C. Characterization methods employed include thermal gravimetric analysis, X-ray powder diffraction, infrared spectroscopy, BET nitrogen surface area, solution H-1 and Si-29 NMR spectroscopy, cryoscopic and viscosimetric molecular weight determinations, and elemental analysis. In addition, the volatile gaseous pyrolysis byproducts were analyzed by gas-phase infrared spectroscopy and mass spectrometry. The commercial polymer, which shows enhanced solubility due to its lower fraction of higher molecular weight material and limited cross-linking, has a decreased pyrolytic ceramic yield due primarily to loss of volatile silazane species between 300 and 600-degrees-C. This decreased yield can be offset by the addition of small quantities of a trichlorosilane prior to pyrolysis. The major gaseous byproduct from both polymers is methane which is evolved at temperatures greater than 450-degrees-C. Under nitrogen and forming gas pyrolysis atmospheres there is evidence for some methylene bridge forming reactions. These phases are not found when an ammonia atmosphere is used in the pyrolyses.