Nonoxide ceramics

The production of technical or engineering ceramics, as compared to earlier traditional ceramic production, is a much more demanding and complex procedure. High-purity materials and precise methods of production must be employed to ensure that the desired properties of these advanced materials are achieved in the final product. 

Over the years, several processing routes have been used to produce fully dense transparent polycrystalline AlON ceramics. Initial work was reported by McCauley et al. [279] in 1979, using reactive sintering of AI2O3-AIN mixtures. Then the reactive sintering technique was widely used by other researchers [280]. 

A reactive SPS method has been developed to prepare AlON transparent ceramics [291]. AI2O3 and AIN powder mixtures were used as precursors. Reactive SPS was conducted at temperatures between 1400 °C and 1650 °C for 15-45 min at 40 MPa 

The single-phase AlON powder used in this experiment was obtained at relatively low temperature by the solid-state reaction method with nanosized AIN powder and nanosized alumina powder [292]. The obtained powder was ground by ball milling and doped with Y2O3. Then, it was shaped into pellets. Transparent ceramics sintering was carried out at 2153 K (1880 °C) for 5, 10, and 20 h. Obtained 

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Y. Murata and R. Smoak in S. Somiya and S. Saito, eds., Proc. Int. Sjmp. of Factors in Densification and Sintering of Oxide and Nonoxide Ceramics, Gakujutsu Biinken Fukyu-Kai, Tokyo, 1979, p. 382. 

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. 

Mechanistic studies are particularly needed for the hydrolysis and polymerization reactions that occur in sol-gel processing. Currently, little is known about these reactions, even in simple systems. A short list of needs includes such rudimentary data as the kinetics of hydrolysis and polymerization of single alkoxide sol-gel systems and identification of the species present at various stages of gel polymerization. A study of the kinetics of hydrolysis and polymerization of double alkoxide sol-gel systems might lead to the production of more homogeneous ceramics by sol-gel routes. Another major area for exploration is the chemistry of sol-gel systems that might lead to nonoxide ceramics. 

Table 11.4 lists a number of nonoxide ceramics that have been produced from the... 

Nonoxide Ceramics Produced from the Pyrolysis of Polymeric Materials... 

There are a number of other nonoxygen or nonoxide ceramics including phosphonitric chlorides (PN backbone), boron nitriles (BN), aluminum nitriles (AIN), titanocarbosilanes (Si-Ti-C backbone), and silazanes (Si-C-N backbones). 

When the rate of the chemical reaction occurring at the surface is the rate-limiting step, the principles we have described to this point apply. The reaction rate can have any order, and the gas reacts with the ceramic substrate to produce products. Although our discussion to this point has focused on oxide ceramics, there are a number of nonoxide ceramics, such as carbides, nitrides, or borides, that are of importance and that undergo common decomposition reactions in the presence of oxygen. These ceramics are particularly susceptible to corrosion since they are often used at elevated temperatures in oxidizing and/or corrosive enviromnents. For example, metal nitrides can be oxidized to form oxides ... 

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,... 

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. 

FIGURE 9.3 Sdiematic of force balance for Young s equation with example of water wetting on an oxide ceramic and nonwetting on a nonoxide ceramic (i.e., i aitudes and nitrides). 

Glassman, I., Davis, K. A., and Brezinsky, K., A gas-phase combustion synthesis process for nonoxide ceramics. TWenty-fourth Symposium (International) on Combustion. The Combustion Institute, 1877 (1992). 

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

Clarke, D. R. (1979a). High-resolution techniques and application to nonoxide ceramics. J. Amer. Ceramic Soc., 62, 236-46. 

Preparative organosilicon chemistry offers manifold possibilities for the synthesis of precursors for nonoxide ceramics (Scheme 18.1). The focus has been on the synthesis of polymers such as polysilanes A, polysilazanes B, polycarbosilanes... 

Silicon carbide Manufacture and applications in the nonoxide ceramics sector, see Section 5.5.5.4. 

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