Nonoxide ceramics boron carbide

Boron-containing nonoxide amorphous or crystalline advanced ceramics, including boron nitride (BN), boron carbide (B4C), boron carbonitride (B/C/N), and boron silicon carbonitride Si/B/C/N, can be prepared via the preceramic polymers route called the polymer-derived ceramics (PDCs) route, using convenient thermal and chemical processes. Because the preparation of BN has been the most in demand and widespread boron-based material during the past two decades, this chapter provides an overview of the conversion of boron- and nitrogen-containing polymers into advanced BN materials. 

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. 

Included in the term nonoxide ceramics are all non-electrically conducting materials in the boron-carbon-silicon-aluminum system. The industrially most important representatives, apart from carbon (see Section 5.7.4), are silicon carbide (SiC), silicon nitride (Si3N4), boron carbide (B4C), boron nitride (BN) and aluminum nitride (AIN). 

In nonoxide ceramics, nitrogen (N) or carbon (C) takes the place of oxygen in combination with silicon or boron. Specific substances are boron nitride (BN), boron carbide (B4C), the silicon borides (SiB4 and SiBg), silicon nitride (SisN4), and silicon carbide (SiC). All of these compounds possess strong, short covalent bonds. They are hard and strong, but brittle. Table 22.5 lists the enthalpies of the chemical bonds in these compounds. 

Compare oxide ceramics such as alumina (AI2O3) and magnesia (MgO), which have significant ionic character with covalently bonded nonoxide ceramics such as silicon carbide (SiC) and boron carbide (B4C see Problems 19 and 20) with respect to thermodynamic stability at ordinary conditions. 

Nonoxide ceramics, such as silicon carbides, silicon nitrides, and boron nitrides, have unique mechanical and functional characteristics. Silicon carbides with high thermal conductivity, high thermal stability, excellent mechanical strength, and chemical inertness are especially considered as effective catalyst supports. 

Silicon carbide (SiC) is the most widely used nonoxide ceramic. Its major application is in abrasives because of its hardness (surpassed only by diamond, cubic boron nitride, and boron carbide). Silicon carbide does not occur in nature and therefore must be synthesized. It occurs in two crystalline forms the cubic P phase, which is formed in the range 1400-1800°C, and the hexagonal a phase, formed at >2000°C. 

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. 

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