SiC whiskers are produced on a commercial scale via the pyrolysis of rice hulls, which inherently have a high Si content. The growth mechanism for SiC whiskers produced by this method has been shown to be a variant of the VLS process,⁵⁰ thus illustrating that such whisker growth processes can be carried out on a small scale with economics⁵¹ that are acceptable for small speciality applications.

One of the most versatile methods for producing single-crystal fibers is the laser-heated pedestal growth (LHPG) technique.⁵² This is a variation on the float-zone pedestal growth process.⁵³ In the LHPG technique a source rod having the desired composition is heated by a laser focused onto one end of the rod, creating a molten zone from which the fiber is pulled (see Figure 3.18). The process does not employ a crucible, the position of the laser-heated zone being fixed in space while the source rod is translated into the laser beam at a rate necessary to conserve the melt volume. One of the principal advantages of this process is the ability of the laser to produce extremely sharp temperature gradients (˜1000°C/cm). The steep temperature gradient combined with the high surface-to-volume ratio of the fibers causes the fibers to cool very rapidly. This helps to suppress high-temperature, solid-state-phase transformations, which can be important for stabilizing metastable forms of the material for nonstructural applications. Another of the major advantages of this technique is the complete absence of hot furnace elements that can cause contamination of the fiber.

Processes emphasized earlier in this chapter have been found to be useful for the fabrication of high-performance fibers over a wide range of compositions. It is important to stress, however, that these processes represent only a select few chosen on the basis of demonstrated applicability. Other processes may also be useful in the development of new or improved fibers and for large-scale manufacturing. Fused composition processes such as in the traditional fabrication of glass filaments should not be forgotten. Commercial high-purity quartz yarn, used in dielectric composites, is made in France and the United States by drawing filaments from a melt fed by quartz precursor rods. Fusion and controlled spinning of some compositions, especially oxides, may be useful in the fabrication of some high-performance ceramic fibers. The "relic" process,² wherein porous fugitive organic fibers are impregnated with inorganic solutions followed by drying and pyrolyzing so as to form ceramic fibers, should be recalled. Fibers made by this process are not generally considered useful for reinforcement of composites, but determined efforts to control the microstructure of the resultant fibers should not be automatically dismissed as an impossible task. Metallic fibers may have high strengths and high modulus of elasticity values. Very small metal fibers with diameters of about 10 µm or less have been prepared by drawing heated metal wires encased in a sacrificial sheath (e.g., glass) that is removed. ¹ Metallic fibers, though handicapped in most cases by high-