Pyrolysis of Organosilicon Gels to Silicon Carbide

Abstract

Sol-gel precursors to silicon carbide have been prepared using trifunctional chloro and alkoxysilanes which contain both the silicon and carbon necessary for SiC formation. Crosslinked gels having the ideal formula [RSiO_{1 5}].]_n have been synthesized by a hydrolysis/condensation scheme for a series of saturated and unsaturated R groups. The starting gels have been characterized by a variety of elemental analysis, spectroscopic and physical measurements including IR. XRD. TGA.. surface area and pore volume. A particularly powerful method for characterizing these gels is the combination of ¹³C and ²⁹ Si solid state NMR which can provide information about the degree of crosslinking as well as residual hydroxy/alkoxy content.

The controlled pyrolysis of these gels has been used to prepare silicon carbide-containing ceramic products with surface areas in excess of 600m²/gm. The pyrolysis products are best described as a partially crystalline, partially amorphous mixture of β-SiC, silica and carbon. The effect of carbon chain length and the degree of unsaturation in the R group on the composition and surface area of the product has been determined. The origin of the high surface area of the pyrolysis products has been identified and its implications on potential uses of these materials is discussed.