Lignin-silica hybrids as precursors for silicon carbide

Lignin, prepared by digesting cedar with acetic acid, and tetraethoxysilane have been allowed for a sol-gel reaction in tetrahydrofuran using H2SO4 as catalyst to yield lignin-SiO2 hybrids in the bulk gel form. The solid-stat e C-13 nuclear magnetic resonance spectra of the hybrids and products forme d solely from the lignin under the acidic conditions revealed that the lign in underwent crosslinking during the sol-gel reaction. The degree of crossl inking increased with an increasing amount of H2SO4. The powdered hybrids h ave been heated at 1500 degrees C in Ar for carbothermal reduction, resulti ng in the formation of SiC powders. The lignin-to-tetraethoxysilane mixing ratio of the starting solutions varied free carbon content in the SiC powde rs. In addition, the amount of carbonaceous residue formed from the Lignin upon heating depended on the degree of crosslinking of the lignin. Thus, to adjust the amounts of both the lignin and H2SO4 was necessary for producin g the hybrids suitable for precursors for SiC powders with high purity. Cri tical adjustment of the amounts led to the formation of SiC powders with a free carbon content of 0.57 wt %, implying that the lignin is a beneficial carbon source for the production of SiC powders by the hybrid route. (C) 19 99 John Wiley & Sons, Inc.