Nonoxide preceramic polymers

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. 

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. 

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. 

This chemical method has therefore opened new avenues for the synthesis of nonoxide ceramics with enhanced properties. The area of preceramic polymer chemistry is now about 20 years old. In this time a number of preceramic polymer systems have been developed. These systems include polymer precursors for Si3N4, SiC, AIN, BN, B4C, TiBa, and TiN. In this discussion, silicon-based nonoxide ceramics have been generally excluded they are dealt with more extensively in a related chapter by Professor Okamura. 

The purpose of this chapter is to provide an overview of the chemistry, processing and application of boron-containing preceramic polymers in the BN system. The nonoxide precursor route, also called the Polymer Derived Ceramics (PDCs) route, represents a chemical approach based on the use of air- and/or moisture-sensitive (molecular or polymeric) precursors by means of standard Schlenk techniques and vacuum/argon lines. This precursor route allows the chemistry (e.g., elemental composition, compositional homogeneity and atomic architecture) of molecular precursors to be controlled and tailored in order to provide the ensuing preceramic polymers... 

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,... 

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