Method for the preparation of an inorganic fiber containing silicon, carbon, boron and nitrogen

A method for the preparation of an inorganic fiber comprising the elements of silicon, carbon, boron and nitrogen which comprises the steps of:

• (a) heating a reaction mixture composed of an organosilicon compound having, in a molecule, at least one silicon-to-silicon linkage and at least one group denoted by R.sup.1, R.sup.1 being a hydrogen atom or a monovalent hydrocarbon group selected from the class consisting of methyl, ethyl, vinyl and phenyl groups, bonded to the silicon atom and an organoboron compound represented by the unit formula --BR.sup.2 --NR.sup.3 --, in which R.sup.2 is a monovalent group selected from the class consisting of monovalent hydrocarbon groups, trihydrocarbylsilyl-substituted alkyl groups of the formula --CH.sub.2).sub.n SiR.sup.4.sub.3, R.sup.4 being a monovalent hydrocarbon group and n being a positive integer, and substituted or unsubstituted amino groups of the formula --NR.sup.5.sub.2, R.sup.5 being a hydrogen atom or a monovalent hydrocarbon group, and R.sup.3 is a monovalent hydrocarbon group, in the molar ratio of silicon to boron in the range from 2:1 to 200:1 at a temperature in the range from 250.degree. to 500.degree. C. in an inert atmosphere to effect the thermal decomposition and polycondensation reaction of the reactants forming an organoborosilicon polymer having a number-average molecular weight in the range from 1,000 to 50,000;
• (b) spinning the organoborosilicon polymer into a fibrous form;
• (c) infusibilizing the thus formed fiber of the organoborosilicon polymer;
• and
• (d) calcining the infusibilized fiber of the organoborosilicon polymer at a temperature in the range from 900.degree. to 1800.degree. C. in an atmosphere of vacuum or an inert gas.