Discontinuous oxide fibers

Discontinuous Oxide Fibers 1.2.1. Melt-Spim Aluminosilicate Fibers... 

Aerospace applications of ceramic matrix composites to date have been limited. Perhaps the most significant are the aircraft engine flaps used on a French fighter. There are two types. Both use silicon carbide matrices. One is reinforced with carbon fibers, and the other with a multifllament silicon carbide fiber. Another application is a missile diverter thruster made of carbon fiber-reinforced silicon carbide. Again, the process used to make this part is CVI. The Space Shuttle Orbiter thermal protection system (TPS) makes extensive use of tiles composed of a three-dimensional network of discontinuous oxide fibers with silicate surface layers. While there is no continuous matrix for most of the tile, the surface region is a form of CMC. In a sense, this can be considered to be a type of functionally graded material. 

Metal-Matrix Composites. A metal-matrix composite (MMC) is comprised of a metal ahoy, less than 50% by volume that is reinforced by one or more constituents with a significantly higher elastic modulus. Reinforcement materials include carbides, oxides, graphite, borides, intermetahics or even polymeric products. These materials can be used in the form of whiskers, continuous or discontinuous fibers, or particles. Matrices can be made from metal ahoys of Mg, Al, Ti, Cu, Ni or Fe. In addition, intermetahic compounds such as titanium and nickel aluminides, Ti Al and Ni Al, respectively, are also used as a matrix material (58,59). P/M MMC can be formed by a variety of full-density hot consolidation processes, including hot pressing, hot isostatic pressing, extmsion, or forging. 

A more defective form of sp carbon fibers, carbon nanotubules (CNTbs), grown by pyrolytic deposition of carbon into anodized aluminum oxide (AAO) nanochannels, are also used as nanoelectrode arrays for electrochemical sensors. Small graphitic crystallites are deposited on the inner surface of the nanochannels. Since the crystallites do not extend very long (less than micrometers) and are structurally discontinued, the resistance is orders of magnitude higher than CNFs... 

The temperature resistance of carbon fiber reinforced ceramics is limited by the oxidation of carbon fiber at temperatures above 500°C and it is necessary to provide some form of protective coating. However, the attributes of increased toughness, strength and modulus are worthwhile considerations. Continuous fiber gives improved tensile properties, but may be more difficult to impregnate with the matrix material than discontinuous fiber. 

To improve the mechanical properties of activated carbon fiber, a dual viscose and PAN based carbon filament has been developed [14], with the core made of PAN based fiber and an external layer of viscose based fiber, which can readily be activated with CO2. The central core comprises of about 50 discontinuous PAN filaments held together by a continuous filament of PVOH and concentrically wrapped by discontinuous viscose filaments using a friction spinning process. The dual filament is carbonized in an inert atmosphere at 900°C and activated by CO2 at 900°C. Another way of achieving a coating of cellulose would be to apply a solution of cellulose in a mix of water and 4-methylmorpholine-A-oxide. 

Ceramic matrix composites (CMCs) can be thought of as an improved form of carbon matrix composite in which the carbon matrix is replaced with ceramics that are stronger and much more resistant to oxidation. CMCs employ a variety of reinforcements including continuous fibers, discontinuous fibers, whiskers, and particles. Continuous fibers provide the best properties. There are many different types of CMCs, and they are at various stages of development. 

The superalloys, as well as alloys of aluminum, magnesium, titanium, and copper, are used as matrix materials. The reinforcement may be in the form of particulates, both continuous and discontinuous fibers, and whiskers concentrations normally range between 10 and 60 vol%. Continuous-fiber materials include carbon, silicon carbide, boron, aluminum oxide, and the refractory metals. However, discontinuous reinforcements consist primarily of silicon carbide whiskers, chopped fibers of aluminum oxide and carbon, or particulates of silicon carbide and aluminum oxide. In a sense, the cermets (Section 16.2) fall within this MMC scheme. Table 16.9 presents the properties of several common metal-matrix, continuous and aligned fiber-reinforced composites. 

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