Silicon-alloyed carbon fibers

Keywords Polyaromatic Mesophase / Silicon-Alloyed Carbon Fibers / Co-pyrolysis / Monomeric and Polymeric Precursors... 

J. Dautel, W. Schwarz, "Precursors for Silicon-Alloyed Carbon Fibers , in Organosilicon Chemistry III - From Molecules to Materials (Eds. N. Auner, J. Weis), Wiley-VCH, Weinheim, 1998, p. 632. 

The polyaromatic mesophase (PA-MP) is a nematic, discotic, chemotropic liquid crystal. Owing to its high density (about 1.5 gcm ), its high carbon yield of about 90 %, and its thermoplasticity, it is unique as a precursor of structure carbons. An important application is the manufacture of high modulus (HM) and ultra-high modulus (UHM) carbon fibers [1]. By alloying with silicon, physical and chemical properties of the materials, such as strength, hardness and oxidation resistance, can be improved. These modified carbons were available by chemical vapor deposition (CVD) processes only up to now. The preparation by liquid phase pyrolysis is novel, economic, and thus opens a completely new field of applications. 

A melt infiltrated ceramic matrix composite (MI-CMC) is a continuous fiber composite whose matrices are formed by a reaction of carbon with molten silicon (or silicon alloy) infiltration into a porous SiC- and/or C-containing preform [53,54]. There are two general approaches to the fabrication of MI-CMCs, generally referred to as the slurry casting (start with SiC fiber) and the prepreg (start with SiC fiber with a coating) Ml approaches (Fig. 12.9(b)). 

Germanium is less common than carbon or silicon and less well known than tin or lead. Yet it is used extensively in all fiber optics, and it is used in some semiconductors too. It used to play a major part in the electronics industry as a rectifier because it could transmit electricity in one direction but not in the reverse direction. It was the element that was first used to make a transistor and was used in the first commercial transistor radios. But today it is less common to find it because it has been replaced by the cheap and prevalent silicon. It s still used in specialty devices such as wide-angle camera lenses, detectors, and some alloys. 

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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