Polymer derived ceramics route

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. 

Table 4-1 summarizes the processes used to prepare commercially available polymer-derived ceramic fibers, as well as fibers being actively developed. Although the fiber compositions vary widely, they are all prepared by a process route similar to the one shown in Figure 4-1. Advantages inherent to the polymer approach include (Lipowitz, 1997a) ... 

The open porosity of the PAlCs-derived ceramic samples shows a weak dependence from the amount of aluminum and falls in the range f 8-13%. It increases up to w20% for the PCS-derived component. Such low values of open porosity have been already reported in the literature for similar polymer-derived ceramics [56] and indicate that this processing route is suitable for the fabrication of dense (up to 92% of theoretical density) covalent ceramics at low temperature. The porosity values measured for the ceramic products are comparable with the starting pre-ceramic samples. Indeed, results of the mercury infiltration experiments indicate that the pyrolysis process leads to the removal of the finer pore fraction (below lOnm) leaving almost unchanged the amount and the size of the coarser pores. 

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... 

Gao, Y., Mera, G., Nguyen, H., Morita, K., Kleebe, H. J., Riedel, R. (2012). Processing route dramatically influencing the nanostructure of carbon-rich SiCN and SiBCN polymer-derived ceramics. Part 1 Low temperature thermal transformation. Journal of the European Ceramic Society, 32(9), 1857-1866. doi 10.1016/j. jeurceramsoc.2011.09.012. 

Borazine and its derivatives are also possible educts to synthesize precursors for Si-B-N-C ceramics. Sneddon and co-workers prepared Si-B-N-C preceramic precursors via the thermal dehydrocoupling of polysilazanes and borazines [7]. A further synthesis route is the hydroboration of borazines. The work group of Sneddon found that definite transition metal reagents catalyze hydroboration reactions with olefins and alkynes to give 5-substituted borazines [8]. Recently, Jeon et al. reported the synthesis of polymer-derived Si-B-N-C ceramics even by uncatalyzed hydroboration reactions from borazines and dimethyldivinylsilane [9]. 

The production of boron containing Si-C-N-ceramics provides an efficient system for materials with high thermal stability. Previously, the preparation of quaternary systems such as Si-B-C-N was realized by blending boron-containing compounds with polysilazanes This processing route leads to an inhomogenous elemental distribution in the finally received ceramic. In the last few years several investigations of ceramics derived from different polymers have been performed [1-3]. Therefore, the stoichiometry and basic structure units determine the properties of the final non-oxide ceramic materials [4-5]. 

The following discussion covers the chemical synthesis of ceramics derived from organometallic polymers BN, AIN, TiN, TiC, and TiBa. It should be emphasized here that some of the syntheses involve starting materials, monomers, and intermediates, as well as polymers, that are oxidatively unstable and/or susceptible to hydrolysis. These syntheses therefore generally require inert atmospheres and the extensive use of vacuum (Schlenk type) line or dry-box techniques. This makes it obvious that collaborations between synthetic chemists and materials and ceramic scientists and engineers is important. Here we outline a selected number of synthetic routes to preceramic polymers. 

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