Carbon filaments, silicon carbides

According to the data in Table 25.5 and to Eq. (25.6) the compressive strength of filaments of refractory materials such as carbon and silicon carbide have compressive strengths about 10 times as large as those of organic fibres. This would seem to be a serious restriction to the use of organic polymers such as aramids in their application in composites. For most of the applications this restriction is of minor importance, however, since long before ac max is reached, instability in the construction will occur. The resistance of a column or a panel under pressure is proportional to the product of a load coefficient and a material efficiency criterion ... 

Ceramic abrasives and machine tools are refractory as they glow orange-hot on the job. In advanced, high-strength filament-reinforced composites, there is at least one refractory component, such as carbon or silicon carbide. The legs of a high-temperature thermocouple are also made of these two materials. Nuclear reactors use graphite, uranium oxide, and carbide. Military armor is made of refractory materials. 

A continuous CVD fiber coating process is being explored for the preparation of a low cost, high strength, thermally stable silicon carbide fiber tow. By depositing a 5 pm thick layer of stoichiometric SiC onto each filament of a carbon fiber tow it is possible to prepare fibers that are 89 vol.% SiC which have twice the tensile strength of the commonly employed Nicalon fiber. In addition, the CVD fiber has superior resistance to creep. An economic analysis indicates that the fibers could be produced for 50/lb compared to 300/lb for Nicalon. 

Structures have been produced that utilize all of the above materials and even some other binders. Found in this group of materials are carbon fibers infiltrated and held together with pyrolytic carbon, silicon carbide, glassy carbon, FIFE, methyl methacrylate, epoxies, and petroleum pitches as well as combinations thereof. The structures may contain randomly oriented chopped fibers or long filaments oriented in random, 2, 3 and n dimensions. 

Silicon CarbidG Fibers. Silicon carbide (SiC) filaments are produced by a CVD technique. The y3-SiC is obtained by the reaction of silane and hydrogen gases with the carbon filament being the substrate for deposition. The SiC fibers have mechanical and physical properties equal to those of boron, and can be used at higher temperatures than the present boron fiber when available in production quantities. CVD SiC fibers are primarily used for reinforcing metal and ceramic matrices. Alternatively, SiC fibers can be made from a polycarbosilane precursor which is meltspun at 350°C. The final form is obtained by pyrolyzing the fiber at 1200°C in an inert environment. 

SILICON CARBIDE FIBERS. There are two forms of SiC fibers, neither of which is available commercially. One consists of a pyrolytic deposit (CVD) of SiC on an electrically conductive, usually carbon, continuous filament. Fiber diameter is about 140 fim. This technology has been used to make filaments with both graded and layered structures, including surface layers of carbon which provide a toughness-enhancing parting layer in composites having a brittle matrix (sdicon nitride, for example). 

Electrothermal atom cells have changed radically since their inception in the late 1950s. The majority of electrothermal devices have been based on graphite tubes that are heated electrically (resistively) from either end. Modifications such as the West Rod Atomizer (a carbon filament) were also devised but were later abandoned. Tubes and filaments made from highly refractory metals such as tungsten and tantalum have also been made, but they tend to become brittle and distorted after extended use and have poor resistance to some acids. Their use continues, however, in some laboratories that need to determine carbide-forming elements. For example, silicon reacts with the graphite tube to form silicon carbide, which is both very refractory and very stable. The silicon is therefore not atomized and is lost analytically. Use of a metal vaporizer prevents this. 

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