Fibrous silicon carbide

Uptake ean similarly be increased by exogenous sources of AOS. Brief exposure to whole cigarette smoke, which is a highly concentrated source of AOS and other radicals, and then to a mineral dust suspension produces considerably greater uptake of asbestos, titanium dioxide, fibrous silicon carbide, and talc (51 -53 Fig. 2). The increase in uptake is proportional to the dose of cigarette smoke, and the smoke effect can be inhibited by catalase, superoxide dismutase (which destroys superoxide anion), or deferoxamine (see Fig. 2). The effects of low levels of ozone are similar (Fig. 3), but differ from cigarette smoke in that superoxide dismutase is not protective (126). Smoke also fails to enhance the uptake of nonfibrous silicon carbide or iron oxide (hematite 53) the latter observation is particnlarly interesting because it was recently shown (127) that hematite does not catalyze the formation of AOS. This observation emphasizes the idea that redox-active surface iron, as opposed to compositional iron, is cracial to AOS formation and particle nptake. 

Fibrous glass Poly(sulfur nitride) Poly(boron nitride) Silicon carbide... 

Baskaran, S., and Halloran, J.W. (1993), Fibrous monolithic ceramics II, Flexural strength and fracture behavior of the silicon carbide/graphite system , J. Am. Ceram. Soc., 76(9) 2217-2224. 

One can classify fibers in a variety of ways. For example, one may divide the whole field of fibers into apparel and nonapparel fibers, i.e. based upon the final use of fibrous material. The apparel fibers include synthetic fibers such as nylon, polyester, spandex, and natural fibers such as cotton, jute, sisal, ramie, silk, etc. Nonapparel fibers include aramid, polyethylene, steel, copper, carbon, glass, silicon carbide, and alumina. These nonapparel fibers are used for making cords and ropes, geotextiles, and structural applications such as fiber reinforcements... 

Of the two most commonly used preform materials, one, alumina, is chemically inert during DMO while the other, silicon carbide, is reactive. We first examine the growth of a DMO composite into an alumina particulate, followed by that into SiC preforms, and finally the DMO infiltration of a suitably coated fibrous preform. Coatings of calcium sulfate with calcium silicate may be used to ensure that oxidative growth ceases at the external boundary of the preform, thereby ensuring near-net-shape fabrication. The mechanism by which these barriers poison the oxidation without impeding the flow of oxygen is not clear at this time. 

Conclusive experimental studies on the effects of respirable fibrous SiC dusts (whiskers) are not yet available [255]. In the United States ASTM has published recommendations for safe handling procedures related to all ceramic fibers, including silicon carbide whiskers [256]. 

Granulomatous pulmonary foci with lymphoid fol-Hcles, round folHcular groups of histiocytes, and a patchy infiltration of eosinophils and much well advanced fibrous tissue had developed in a 33-year-old male who had worked as a precision grinder, grinding a hard metal compound (stellite) with a silicone carbide wheel and without any exhaust ventilation (Joseph 1968). The chemical analysis of the lung biopsy showed only titanium, but no tungsten or cobalt. 

The development of ceramic materials for armor since 1970 has been extensive, In addition to alumina and titanium diboride, the most widely used ceramic materials are silicon carbide, boron carbide, and aluminum nitride, as monolithic plates and shapes, which are bonded to a fibrous laminate of fiberglass or Kevlar . A typical impact sequence is shown in Fig. 16.2. On impact, the ceramic plate fractures the projectile core and absorbs a major part of the kinetic energy. The backing material absorbs the residual energy. 

Anhydrous fibrous silicas formed in connection with high temperature metallurgical operations were noted as long ago as 1852 by Schnabel and 1859 by Rose. Soft, silky fibers of more than 98% SiO, were classed as aphanitic (invisible) silica, and also known as lussatite. Around 1910. in the mouths of electric furnaces making silicon carbide, a soft spongy gray deposit called elephant s ear" was identified as microfibrous amorphous silica (67). It is likely that all of these were silica W. 

Silicon carbide (SiC) whiskers are extremely anisotropic, short fibrous crystals and grow from SiC particles along the [111] plane through catalysis. At present, SiC whiskers are usually prepared by vapor reaction and solid methods, and the latter is more economical and more suitable for industrial production. Figure 3.1 shows the microstructures of SiC whiskers. Table 3.1 shows the chemical and physical properties of SiC whiskers. 

Silicon carbide (hexagonal) [Moissanitej lower limits for fibrous SIC SIC 409-21-2 10 mg/m ... 

Materials that are classified as fibers are either polycrystalline or amorphous and have small diameters fibrous materials are generally either polymers or ceramics (e.g., the polymer aramids, glass, carbon, boron, aluminum oxide, and silicon carbide). Table 16.4 also presents some data on a few materials that are used in fiber form. 

---