Nonoxide ceramic materials

Berzelius [1] first reported the formation of silicon carbide in 1810 and 1821, but it was later rediscovered during various electrochemical experiments, notably by Despretz [2], Schiitzenberger [3], and Moissan [4]. However, it was Acheson [5] who first realized the technical importance of silicon carbide as a hard material and, believing it to be a compound of carbon and corundum, he named the new substance carborundum . By 1891, Acheson had managed to prepare silicon carbide on a large scale such that, today, it has become by far the most widely used nonoxide ceramic material. 

Aluminum nitride is a nonoxide ceramic material. The GTE of AIN (4.5 ppm/°C) is substantially lower than that of alumina. Early attempts to use thick-film dielectric materials designed for alumina substrates on AIN were unsuccessful. Many of the glasses used in thick-film formulations designed for alumina substrates are chemically incompatible with AIN. Furthermore, the CTE differences between dielectrics designed for alumina and AIN substrates lead to cracking. Special thick-fihn dielectric formulations have been developed for use on AIN substrates. Table 8.12 shows properties of a typical thick-fihn dielectric on AIN substrates. " ... 

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. 

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

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. 

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 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. 

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. 

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 contrast with the oxide ceramics, the nonoxide ceramics are not thermodynamically stable in oxidizing environments, although their stability in other chemical media is excellent. Materials of silicon-containing compounds can nevertheless be used under oxidizing conditions up to ca. 1600°C, because a passivation layer of Si02 is formed at the surface which strongly hinders further oxidation. 

Silicon carbide is the only nonoxide ceramic product of major industrial importance, apart from carbon. The worldwide production of unworked SiC was 450 10- t/a in 1995, corresponding to 90% capacity utilization. The USA and Canada accounted for 85 10 t/a of this, Western Europe for 195 10 t/a, Japan and China for 110- 10- t/a and other regions for 90 10 t/a. The previously predicted strong growth in worldwide production has not materialized. The largest European production plants are in Norway and the Netherlands. 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

The term nanoparticles usually refers to particles with a size up to 100 nm [1]. Nanoparticles exhibit completely new or improved properties based on specific characteristics such as size, distribution, and morphology if compared with larger particles of their bulk material. Nanoparticles can be made of a wide range of materials, the most common being metal oxide ceramics, metals, silicates, and nonoxide ceramics. Even though other materials (e.g., polymer nanoparticles) exist, the former count for those used in most current applications [1]. 

The nonmetallic, mechanically resistant materials that are constituents of the ceramic materials can be subdivided into oxidic and nonoxidic materials. High melting points and hardnesses are the outstanding properties, as shown in Table 20.2. 

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. 

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