Applications of carbon-carbon composite (4)

Key properties of CC composite, such as strength retention at temperatures approaching 5000F, low density, dimensional stability, and complex shape fabricability, make CC an appropriate material for a variety of evolving applications. Substitution of CC limiters because of good structural properties and toughness allows a thin shell design and passive cooling by thermal radiation. The limiters consist of 24 U-shaped segments in a 5- to 6-ft-diameter toroidal arrangement with the base of the U-shaped segments facing inward. The CC limiter is based on a 0-90°, 2D CC composite using staple fiber PAN yarn as a knit cloth. The width of the U-shaped segment is 21cm and the leg height is approximately 6cm with a wall thickness of 1cm. The CC limiters must function at approximately 3990F with short-duration spikes of 5000F to 6000F. Carbon-carbon is being considered for containers is which nuclear wastes are stored because of the high temperature that might be generated. Good thermal stability also makes CC attractive for use on laser shields to protect space-based satellite systems from the heat of high-powered laser beams in a space defense scenario. In a completely different environment, the compatibility of carbon with body tissues makes CC an interesting bone replacement in areas such as the hip instead of the currently used stainless steel.

Carbon-carbon is used in fuel cells in a commercial application that is related to electric power generation. The basic fuel cell system consists of a fuel processor that converts raw material fuel into hydrogen-rich gas, fuel fells that directly convert chemical energy into electrical energy, and a power conditioner to convert the fuel cell dc current into ac current. In practice, many individual fuel cells are connected in series to form fuel cell stacks; the power generator consists of many modules of these stacks. Currently, the most developed system is based on a phosphoric acid electrolyte with CC electrodes and other structural components. The cells operate at 400F and generate from 200kw to 11Mw of electrical energy. The CC functions well because of its relatively high thermal and electrical conductivity and its resistance to the fuel cell environment.

Carbon-carbon use in glass container forming machines as an asbestos replacement for hot-end glass contact applications illustrates its potential commercial growth. The CC material showed wear characteristics from 100 to 300 times greater than asbestos for these applications, and because it does not get wet by molten glass and does not require external cooling or frequent replacement, it is a cost-effective replacement for asbestos.

Additional commercial applications of K-Karb CC include vanes for rotary vane compressors and vacuum pumps, in which the CC replaces other composite and graphite parts to increase service life; nuts, bolts, and fittings to assemble major graphite elements in vacuum furnaces and increase working life over formerly used amorphic graphite parts; and flat and cylindrical heating elements for hot isostaic presses to increase work life up to 10 times over amorphic graphite, provide consistent resistance numbers to minimize power source adjustments, and allow increased working temperatures to 4000F. Other commercial applications include sintering trays for carbonizing and carbiding furnaces in which toughness of the CC tolerates rough shop usage better than brittle graphites; clutch disks for racing and other high-performance cars to provide high-temperature stability; and planar bearings in which temperatures are too high for Tefloon or similar materials and in which graphite suffers from catastrophic failures.

The applications and ultimate markets for CC materials are continually developing for both military and commercial use. New domestic and foreign suppliers have contributed to the acceptance of this unique and versatile material through the use of improved high-modulus and high-strength carbon fibers, the development of high-char organic polymers, improved pitch matrix precursors, new rapid processing CVI technology, a better understanding of fiber-matrix interface phenomenology, and ever-increasing production capabilites. The applications discussed here indicate the variety of roles that CC can fulfill; it is hoped that they will lead to additional future uses.



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