Pre-ceramic material

The diversity in structure and properties found in metal-containing polymers has constantly aided the search for materials that respond to external stimuli. Metal-containing polymers can be found in countless applications performing as polyelectrolytes, photomasks and pre-ceramic materials, among others. As a consequence, active research in this field has constantly prompted for polymerization schemes that lead to well-defined structures, in order to understand the details associated with the structure-function relationships [1,2]. 

Even with this low amount of magnesium, the pre-ceramic PAA-derived material allowed the formation of Mg-PSZ ceramics with values approaching those of good Mg-PSZ ceramic materials. 

Instead of ceramic monoliths, high temperature Ni metal-based honeycomb carriers are available (Ref. 11, 12). In view of their overall weight and size advantages they are used as pre-catalysts and as retrofits. However on a price per volume basis, including canning, ceramic material currently is 2-3 times less expensive. 

Polymer pyrolysis to form advanced ceramics allows the production of highly covalent refractory components (fibers, films, membranes, foams, joints, monolithic bodies, ceramic matrix composites) that are difficult to fabricate via the traditional powder processing route [1-4]. Yajima was the first to demonstrate the feasibility of producing high-strength SiC-based fibers from pyrolysis of polycarbosilane [5]. In this process, a thermoplastic pre-ceramic polymer is first shaped into the desired form, cross-linked into a pre-ceramic network and finally converted into a ceramic material by a pyrolysis process in a controlled atmosphere (Fig. 1). A common feature of the polymer route is the formation of intermediates called amorphous covalent ceramics (ACC) [6]. These are formed after removal of the organic components and before crystallization that occurs at higher temperatures. 

Another advantage of this process is its high flexibility. Indeed, the composition, structure (amorphous or crystalline), microstructure, and properties of the ceramic material can be controlled and adjusted by varying many different parameters such as the composition and architecture of the pre-ceramic polymer, the amount and nature of the filler, the cross-linking step, and the pyrolysis parameters (atmosphere, heating rate, final temperature). Also, nonconventional heating systems such as laser or microwave heating or even athermal conversion processes like ion bombardment can be efficiently used. 

Since the mid 1990s, many research groups have concentrated on the development of fibers via the pyrolysis of appropriate pre-ceramic polymers [ 112]. In particular, the system Si-B-N-C has been of major interest due to the excellent high-temperature and oxidation stability of the resultant amorphous material [113]. In this case, the onset of crystallization may be as high as 1800 °C, while decomposition starts at 2000 °C in protective atmospheres. 

Hie need for a fine microstructure is usually encountered and justified in the context of the Griffith formula, which quantifies the stress a needed to propagate a pre-existing crack through a metal or ceramic material (Cottrell, 1975) ... 

Polysiloxanes have also been pyrolyzed to give ceramics and organic/ inorganic hybrid materials. " Ihe general topic of pre-ceramic polymers is discussed chapter 9. ... 

A second approach for the generation of structured pre-ceramic polymer-based materials that will be briefly addressed here is based on the self-assembly of organic-inorganic core-shell particles, also referred to as colloidal crystallization. The self-assembly of almost monodisperse colloidal micro- and nanoparticles is a feasible method for gaining access to ceramic functional materials for various applications, especially if the final materials feature an optical band gap [234-238]. In general, colloidal crystals can be prepared from their dispersions by various techniques of deposition or spin coating, which are depicted in Fig. 3 [239, 240]. 

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