Nonoxide ceramics composites

Pyrolysis of organosilicon polymers in nonoxidizing atmospheres provides a route to nonoxide ceramic compositions. 

Preceramic polymer precursors (45,68) can be used to make ceramic composites from polymer ceramic mixtures that transform to the desired material when heated. Preceramic polymers have been used to produce oxide ceramics and are of considerable interest in nonoxide ceramic powder processing. Low ceramic yields and incomplete burnout currently limit the use of preceramic polymers in ceramics processing. 

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. 

Many nonoxide ceramics form gaseous reaction products when oxidized. For example, when an alumina/silicon carbide composite is exposed to an oxidizing environment, SiC will oxidize, forming carbon monoxide via the following reaction ... 

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. 

A wide range of methods have been used to synthesize ceramic powders, with different compositions. Although most of these methods are applicable to transparent ceramic power synthesis, some of them have not been employed in the fabrication of transparent ceramics. Solid-state reaction is still the key method, while wet-chemical routes are increasingly used, whereas gas-phase reactions are only limited to nonoxide ceramics. For large scale applications, cost-effectiveness, environmental friendliness and less energy consumption, are all very important considerations, when selecting synthetic methods. 

Upon cooling, these liquid phases remain as glassy phases or as secondary crystalline phases in the sintered materials consequently, these hquid-phase-sintered ceramics are actually composite materials consisting of a matrix of grains and dispersed secondary phases. The thermal conductivity of these composite materials will depend on the amount, distribution state and thermal conductivity of each constituent phase in the structure. The effects of secondary phases on the thermal conductivity ofliquid-phase-sintered nonoxide ceramics are discussed in the following subsection. 

Ceramic composites often consist of mixtures of oxide and nonoxide ceramics. Although oxides generally exhibit good oxidation resistance, most nonoxides do... 

Engineering ceramics can be classified into three—oxides, nonoxides, and composites. Examples for oxides are alumina and zirconia. Carbides, borides, nitrides, and silicides come under nonoxides. Particulate-reinforced oxides and nonoxides are examples for composites. Oxide ceramics are characterized by oxidation resistance, chemical inertness, electrical insulation. 

This group can be composed of a combination of oxide ceramics-nonoxide ceramics oxide-oxide ceramics nonoxide-nonoxide ceramics ceramics-polymers, and so on—and an almost infinite number of combinations are possible. The reinforcements can be granular, platy, whiskers, and so on. When it is a combination of ceramics and polymers, the objective is to improve the toughness of the ceramics, which otherwise is brittle. When we combine two ceramics, the intention is to improve the hardness, so that the combination becomes more suited to a particular application. This is a somewhat new area of development, and compositions can also include metals in particulate or matrix forms. 

Table 2. Nonoxide-Based Ceramic-Matrix Composites...

The excellent high-temperature properties of the ceramic materials strongly depend on the molecular structure and composition of the polymeric precursors. This chapter reviews the fundamentals of synthetic approaches to silicon-based nonoxide preceramic polymers and briefly discusses their processing. 

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