Method for formation of crystalline boron-doped silicon carbide and amorphous boron silicon oxycarbide fibers from polymer blends containing siloxane and boron

A method for preparing ceramic fibers comprising:

• i) preparing a blend comprising:
• a) 90 to 99.9 weight percent of a siloxane resin comprising units with a general formula R.sub.x R.sup.1.sub.y R.sup.2.sub.z SiO.sub.(4-x-y-z)/2, wherein each R is independently selected from unsaturated monovalent hydrocarbon groups, each R.sup.1 is independently selected from aryl groups of 6 to about 10 carbon atoms, and each R.sup.2 is independently selected from saturated monovalent hydrocarbon groups, x has a value of 1 or 2, y has a value of 0, 1, or 2, and z has a value of 0, 1, or 2 with the proviso that x+y+z=1, 2, or 3, and
• b) 0.1 to 10 weight percent of a boron containing polymer;
• ii) shaping the blend into green fibers;
• iii) curing the green fibers to cured fibers;
• and
• iv) pyrolyzing the cured fibers to ceramic fibers by heating to a temperature of 800 to 2,200.degree. C.