Process for preparing silicon carbide fibers

#This invention relates to a method for making silicon carbide fibers of high purity and high density, and to the fibers themselves.

#Silicon carbide is recognized as having excellent properties for high temperature applications. Silicon carbide articles have been made by sintering silicon carbide powder as taught in U.S. Pat. Nos. 4,144,207, 4,663,105, 4,693,988 and 4,702,869. However, formation of the desired shapes from particles can be difficult, and the use of silicon carbide particles does not lend itself to the manufacture of silicon carbide fibers.

#Seyferth and Wiseman, Am. Ceram. Soc., July 1984, pp. C132,133, report combining silicon carbide particles and polysilazane and sintering the mixture in a nitrogen atmosphere to form a bulk ceramic object. Under the conditions used, the polymer yields a substantial amount of silicon nitride. Fibers from the polymer are reported, but not from the combination of polymer and silicon carbide particles. U.K. Pat. No. 2,024,789 describes particular silanes from which shapes can be formed and sintered in an inert atmosphere to make silicon carbide shapes. Green fibers drawn from polymer are mentioned and the use of powdered silicon carbide as a filler is mentioned. However, there is no description of forming fibers from a powder/polymer mixture.

#TABLE 1 ______________________________________ Silicon carbide spin mix recipe. Component Amount Comment ______________________________________ OLOA 1200 (r) 2 g. Dissolve in 15 ml xylene SiC Powder 40 g. Add to above B.sub.4 C powder 0.4 g. Add to above Polycarbosilane 6.6 g. Dissolve in 10 ml xylene ("Nicalon" X9-6348) ______________________________________

#The dispersing aid (OLOA 1200) was dissolved in the solvent(xylene). The ceramic powders (commercially available alphasilicon carbide and boron carbide) were added and the resulting solution stirred until fluid. In another container, the polycarbosilane was dissolved in xylene. This solution was then added to the powder slurry and stirred.

#In order to insure the homogeneity of the dope, an excess of solvent over that needed for spinning was used during the mixing steps. In order to successfully spin, this solvent was removed to raise the viscosity of the dope. The excess solvent was evaporated by stirring in an open container on a hot plate. As the mix became more viscous, the stir bar was removed and stirring was continued by hand. Solvent was removed until the dope became quite viscous and dilatant at 60.degree. C. The mix was finally covered and cooled to room temperature. When cool, the leathery mixture was scooped out and put in a twin-piston spinning cell. Mixing was completed in four hours at 70.degree. C. while pumping between the two halves of the cell.