A process for the preparation of substantially polycrystalline silicon carbide fibers are provided. The fibers may be fabricated to have a small diameter and are thermally stable at high temperature. The process is carried out by initially forming fibers from a preceramic polymeric precursor comprising phenyl-containing polyorganosiloxane resins. The fibers are then infusibilized to render them nonmelting followed by a pyrolysis step in which the fibers are heated to a temperature in excess of 1600.degree. C. in a nonoxidizing atmosphere to form substantially polycrystalline silicon carbide fibers. The substantially polycrystalline silicon carbide fibers which are formed have at least 75% crystallinity and have a density of at least about 2.9 gm/cm.sup.3. The polymeric precursor or the fibers contain, or have incorporated therein, at least about 0.2 % by weight boron. This incorporation of boron may be accomplished either prior to or during formation of the fibers or during at least one of the infusibilizing or pyrolyzing steps of the process.

This method for forming a measuring diaphragm which measures the volume of a passing gas with high accuracy and has a prolonged service life because the diaphragm moves completely in the same way to both sides of the measuring vessel in which it is fixed. The measuring diaphragm when produced in quantity has an undulating structure of high uniformity and variation in quality of the diaphragm is small. The method of preparation of the measuring diaphragm includes providing each of a pair of molds with a flat part at the center and providing a multiplicity of grooves and ribs at the periphery parallel to an edge of the central part; placing the grooves and ribs in such a way that the grooves of one mold fit the ribs of the other mold, such that the depth of a groove on a mold is larger than the height of a rib on the other mold which fits the groove; pressing an elastomer composite sheet material between the molds, such that one side of the pressed elastomer composite sheet material is pressed to the surface of the ribs of the mold and fixed there with tension and the other side of the elastomer composite sheet material is out in contact with the surface of the grooves of the mold; and then heating the elastomer composite sheet material to crosslink the elastomers composite sheet and to form a desired shape.

This invention relates to the preparation of thermally stable, substantially polycrystalline silicon carbide ceramic fibers derived from a polycarbosilane resin. The unexpected thermal stability of these fibers is achieved by the incorporation of boron prior to ceramification.

High tensile carbon fibers are provided with a high yield process in which, after oxidation of a precursor, the fibers are first precarbonized in an inert atmosphere to to about 600.degree. C. while imparting 5-10% stretch. In precarbonizing, the fibers are intially heated in a sweeping manner with substantial volumes of hot inert gas which is extracted along with products of decomposition before the fibers are cooled to a low, non-reactive exit temperature. The arrangement minimizes redeposition of tars on the fibers and stretches the fibers in a range in which substantial off-gassing occurs. Thereafter the fibers are finally carbonized at a higher temperature with a different tension being applied, to provide a more reliable less sensitive process that enables oxidation to be effected more rapidly.

This invention relates to the preparation of thermally stable, substantially polycrystalline silicon carbide ceramic fibers derived from a polycarbosilane resin. The unexpected thermal stability of these fibers is achieved by the incorporation of boron prior to ceramification.