Pre-ceramic

Technological History. Archaeologists often divide the neolithic period, the latter part of what used to be called the Stone Age, into pre- and post ceramic, with reference to when ceramics came into production (88—92). Actually, there are occasions of pre-ceramic pyrotechnology, such as in the case of the fifth millenium BC mideastem plaster production (93). So far the eadiest occurrence of ceramics is in the 28th millenium BC in Eastern Europe, although here the technique was not used for the production of vessels but of figurines (94). 

To obtain information on the reason for NMR signal broadening and insight into the chemical nature of these pre-ceramic polymers, attempts were made to follow the reaction of the sym-tetrachlorodisilane and HMDZ by ySi NMR spggtroscopy. 

All of the steps preceding "Shape", step 10, Figure 1, are intended to make magnesium oxide and zirconia particles smaller and to mix them evenly. Figure 2 contains an illustration of what a classical "well mixed" pre-ceramic mixture might look like. Figures 3 and 4 are represenations of the crystal structures of magnesium oxide and zirconium (IV) oxide. 

Advanced Methods Towards Making Pre-Ceramic Powders... 

The following focuses on the structural characterization of the pre-ceramic polymers. 

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. 

Cone Calorimeter Data for Polymers and Pre-Ceramic Polymer-Polymer Blends... 

Hydrosilylation of 1 with trichlorosilane has been performed with Pt on charcoal (1% by weight) in quantitative yields (Scheme 1). The B-tris(trichlorosilylvinyl)borazine (2) was obtained with a high regioselectivity of proximately 80% trans hydrosUylation product [4], Pure 2 can be obtained by fiactional crystallization of the synthesized product fiom hexane. For further synthesis, both a- and P-hydrosilylation products can be used. No further hydrosilylation was observed in this case. In order to interconnect the single source precursor molecule 2 to a pre-ceramic polymer, methylamine was added to the solution of 2 in hexane, and a high viscosity, colorless oil was formed. By changing the reaction parameters (excess of CH3NH2, temperature), the viscosity of the polymer can be varied [5]. The obtained polymer (3P) is pure after evaporation of the solvent, which is checked by NMR. Other solvents like thf or toluene are also possible for the reaction, as well as for dissolution of the polymer. Furthermore, ethylamine leads to similar results in the formation of the polymer. 

The potential of silicon carbide pre-ceramic compounds was recognized especially after Yajima et al. had prepared silicon carbide-based ceramic fibers [1]. The development of tailor-made silicon carbide precursor molecules has to be seen in close relation to the tremendously fast growth of the field of... 

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. 

In order to overeome these difficulties, many hybrid processes have been studied for the fabrication of bulk ceramic components via polymer pyrolysis. Greil and coworkers proposed to reduce the shrinkage by the use of active fillers that can react during pyrolysis leading to a volume expansion [33], pre-ceramic polymers... 

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