Nonoxide ceramic fibers

FIGURE 1-6 Single filament of boron nitride-coated Nippon Carbon Nicalon non-oxide ceramic fiber. Nonoxide fibers discussed in this report include polycrystalline SiC fibers and multiphase (amorphous or crystalline) fibers consisting of B,C,N,Ti, or Si. Current manufacturers include Bayer, Dow Corning, Nippon Carbon, Textron, Tonen, and Ube. Source Dow Coming Corporation. 

The last quarter of the twentieth century saw tremendous advances in the processing of continuous, fine diameter ceramic fibers. Figure 6.4 provides a summary of some of the important synthetic ceramic fibers that are available commercially. We have included in Fig. 6.4 two elemental fibers, carbon and boron, while we have excluded the amorphous, silica-based glasses. Two main categories of synthetic ceramic fibers are oxide and nonoxides. A prime example of oxide fibers is alumina while that of nonoxide fibers is silicon carbide. An important subclass of oxide fibers are silica-based glass fibers and we devote a separate chapter to them because of their commercial importance (see chapter 7). There are also some borderline ceramic fibers such as the elemental boron and carbon fibers. Boron fiber is described in this chapter while carbon fiber is described separately, because of its commercial importance, in Chapter 8. 

Ceramic fibers of the nonoxide variety such as silioon carbide, silicon oxycarbide such as Nicalon, silicon nitride, boron carbide, etc. have become very important because of their attractive combination of high stiffiiess, high strength and low density. We give brief description of some important nonoxide fibers. 

High temperature stability of these nonoxide fibers in air is another critical problem. Thermal stability of ceramic fibers derived from polymeric precursors is of special concern mainly because, as mentioned above, they frequently have undesirable phases present in them. Polycarbosilane-derived SiC fibers, such as Nicalon or Tyranno, involve a thermal oxidation curing process as described above and can contain as much as 10 wt % oxygen (Okamura and Seguchi, 1992). Such fibers decompose at temperatures above 1200 C in a nitrogen or argon atmosphere with SiO and CO gas evolution ... 

FIGURE 12.14 (a) Plot of the wave number and bandwidth (full width at half height) of the C-C raman peaks for different SiC fibers after various thermal treatment in air or in nonoxidizing atmospheres, (b) Comparison between ultimate tensile strength and sp peak FWHH as a function of thermal treatment for SiC Hi Nicalon fibers. (Adapted from Colomban, P., Raman microscopy and imaging of ceramic fibers in CMCs and MMCs, Ceramic Trans., 103, 517, 2000. With permission.)... 

Fabrication routes in which a solution of metal compounds is converted into a solid body are sometimes referred to as liquid precursor methods. The sol-gel process has attracted considerable interest since the mid-1970s and forms the most important liquid precursor route for the production of simple or complex oxides. The pyrolysis of suitable polymers to produce ceramics (mainly nonoxides such as SiC and Si3N4) has attracted a fair degree of interest in the past 20 years. It is an important route for the production of SiC fibers. 

Nonoxide fibers, such as carbides, nitrides, and carbons, are produced by high temperature chemical processes that often result in fiber lengths shorter than those of oxide fibers. Mechanical properties such as high elastic modulus and tensile strength of these materials make them excellent as reinforcements for plastics, glass, metals, and ceramics. Because these products oxidize at high temperatures, they are primarily suited for use in vacuum or inert atmospheres, but may also be used for relatively short exposures in oxidizing atmospheres above 1000°C. 

In the last 10 years, significant advances in fibrous monolithic ceramics have been achieved. A variety of materials in the form of either oxide or nonoxide ceramic for cell and cell boundary have been investigated [1], As a result of these efforts, FMs are now commercially available from the ACR company [28], These FMs are fabricated by a coextrusion process. In addition, the green fiber composite can then be wound, woven, or braided into the shape of the desired component. The applications of these FMs involve solid hot gas containment tubes, rocket nozzles, body armor plates, and so forth. Such commercialization of FMs itself proves that these ceramic composites are the most promising structural components at elevated temperatures. 

SiC is an excellent nonoxide ceramic with high-temperature stability and suitable mechanical properties. Since silicon-containing polymers are generally used for preparing nonoxide ceramics, various polymeric precursors with different structures have been designed. Preceramic polycarbosilane (PCS), used for preparing commercial Nicalon fiber,... 

Carbon fiber reinforced ceramic composites also find some important applications. Carbon is an excellent high temperature material when used in an inert or nonoxidizing atmosphere. In carbon fiber reinforced ceramics, the matrix may be carbon or some other glass or ceramic. Unlike other nonoxide ceramics, carbon powder is nonsinterable. Thus, the carbon matrix is generally obtained from pitch or phenolic resins. Heat treatment decomposes the pitch or phenolic to carbon. Many pores are formed during this conversion from a hydrocarbon to carbon. Thus, a dense and strong pore-free carbon/carbon composite is not easy to fabricate. 

The purpose of this chapter is to provide an overview of ceramic materials used for photonic crystals, their synthesis, and macroscopic structures and architectures. Particularly close attention is given to the fabrication of silica colloidal crystals, since these forms are the most commonly studied. Initial efforts into devices are discussed, as are newer ceramic photonic crystal structures, including an overview of work in photonic crystal optical fibers. For completeness, nonoxide and organic photonic crystals also are included briefly. 

Fiber preparation via the polymer precursor route provides many desirable properties for use in continuous-fiber ceramic matrix composites intended for high-temperature uses in oxidative and nonoxidative environments [65]. These fibers, especially those having low electrical conductivity and good dielectric properties, are being investigated for use in radiation-transparent structures, such as radomes [66]. 

The history of ceramics is as old as civilization, and our use of ceramics is a measure of the technological progress of a civilization. Ceramics have important effects on human history and human civilization. Earlier transitional ceramics, several thousand years ago, were made by clay minerals such as kaolinite. Modem ceramics are classified as advanced and fine ceramics. Both include three distinct material categories oxides such as alumina and zirconia, nonoxides such as carbide, boride, nitride, and silicide, as well as composite materials such as particulate reinforced and fiber reinforced combinations of oxides and nonoxides. These advanced ceramics, made by modem chemical compounds, can be used in the fields of mechanics, metallurgy, chemistry, medicine, optical, thermal, magnetic, electrical and electronics industries, because of the suitable chemical and physical properties. In particular, photoelectron and microelectronics devices, which are the basis of the modern information era, are fabricated by diferent kinds of optical and electronic ceramics. In other words, optical and electronic ceramics are the base materials of the modern information era. 

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