FORMATION OF DENSE SILICON CARBIDE BY MOLTEN SILICON INFILTRATION OF CARBON WITH TUNED STRUCTURE

摘要

Carbon preforms are being developed from cellulose-based precursors for the formation of fully dense SiC monoliths by a liquid phase Si infiltration technique. The carbon structural characteristics such as bulk density, microporosity and solid reactivity are paramount in determining the effectiveness of the SiC formation process. To adjust the carbon structural properties thus allowing successful liquid Si infiltration and SiC formation, a precursor composite of cellulose and phenolic resin was created. For the carbonization process, researchers adopted a hybrid procedure of low-temperature oxidative curing in air atmosphere followed by high-temperature carbonization in argon to obtain carbons with desired bulk densities and macroporosities. In the SiC formation process, carbon disks were embedded in fine Si powder inside graphite crucibles. These crucibles were then placed in a high-temperature graphite vacuum furnace at a temperature of 1800°C and pressures of about 0.5 Torr for various time lengths. After studying the samples produced by this process, researchers discovered that SiC disks with apparent densities of greater than 91 percent of that of pure SiC can be achieved with just one infiltration cycle from a precursor composite of 6:4 mass ratio of cellulose to phenolic resin. The successful SiC conversion this material exhibits is attributable to the high reactivity of the cellulose-derived phase and the structural rigidity and low reactivity of the phenolic phase combined with a narrow pore size distribution above 1 μm. The reaction mechanism of this process is believed to be dissolution of carbon by molten Si followed by the nucleation and growth of SiC microcrystallites.