Pyrolytic ceramization

Scheme 31 Upper panel-. Complex formation between acetylenes and cobalt carbonyls. Lower panel Formation of polymer complexes 81 and 82 via metal complexation and transformation of the complexes into soft ferromagnetic materials 83 and 84 by pyrolytic ceramization...

Figure 10 Fe 2p photoelectron spectra of ceramics (curve A) 2(12A) and (curve B) 3(10A) prepared by pyrolytic ceramizations of h3fperbranched (curve A) poly[l,l ferrocenylene(methyl)silyne] [1(1)] at 1200°C and (curve B) poly[l,l -ferrocenylene(vmyl)silyne] [1(V)] at 1000°C imder argon.

One potential solution to these problems, suggested some 20 years ago by Chantrell and Popper (1), involves the use of inorganic or organo-metallic polymers as precursors to the desired ceramic material. The concept (2) centers on the use of a tractable (soluble, meltable or malleable) inorganic precursor polymer that can be shaped at low temperature (as one shapes organic polymers) into a coating, a fiber or as a matrix (binder) for a ceramic powder. Once the final shape is obtained, the precursor polymer can be pyrolytically transformed into the desired ceramic material. With careful control of the pyrolysis conditions, the final piece will have the appropriate physical and/or electronic properties. 

The cyclotrisilazane (R = Me) produced in reaction (14) is recycled at 650°C [by reaction with MeNHo) the reverse of reaction (14)] to increase the yield of processible polymer. Physicochemical characterization of this material shows it to have a softening point at 190°C and a C Si ratio of 1 1.18. Filaments 5-18 pm in diameter can be spun at 315°C. The precursor fiber is then rendered infusible by exposure to air and transformed into a ceramic fiber by heating to 1200°C under N2- The ceramic yield is on the order of 54% although, the composition of the resulting amorphous product is not reported. The approach used by Verbeek is quite similar to that employed by Yajima et al. (13) in the pyrolytic preparation of polycarbosilane and its transformation into SiC fibers. 

Pyrolytic Carbon Coating Artificial Heart Valves Pyrolytic Carbon Coatings Spinal Surgery Bloactive Glass-Ceramic HA... 

Not only must precursor fibers be self-supporting as extruded, they must also remain intact (e.g. not melt or creep) during pyrolytic transformation to ceramic fibers. Thus, precursor fibers (especially melt spun fibers) must retain some chemical reactivity so that the fibers can be rendered infusible before or during pyrolysis. Infusibility is commonly obtained through reactions that provide extensive crosslinking. These include free radical, condensation, oxidatively or thermally induced molecular rearrangements. 

This criterion, which is product rather than precursor-property driven, is critical to the design and synthesis of new precursors. The need for high ceramic yields arises because of the excessive volume changes associated with pyrolytic conversion to ceramic materials. Scheme 1 illustrates these changes for a SiC precursor with an 80% ceramic yield of phase pure SiC (3.2 gml-1). Most precursors densities are close to 1 gml-1, whereas most Si ceramic densities range from 2.5 to 3.5 gml-1. 

Figure 9.1 Definitions of the various species used as starting materials and condensation products during pyrolytic conversion to ceramics.

The pyrolytic formation of carbon fiber provides an introduction to the conditions and temperature programming that is required for the formation of ceramics that contain silicon, boron, aluminum, and other elements. 

It has become clear that the carbon-rich hb-PYs are readily curable (from ca. 150 °C), thermally stable (up to ca. 550 °C), and pyrolytically carboniz-able (yield up to 80% at 900 °C). Furthermore, their triple bonds are easily metallizable by the complexations with cobalt carbonyls. Since the polymer complexes contain a large number of metal atoms, we tried to utilize them as precursors for fabrication of metalloceramics. The pyrolyses of the polyyne-cobalt complexes at 1000 °C for 1 h under nitrogen furnished ceramic products 83 and 84 in 50%-65% yields (cf., Scheme 31). All the ceramics were magnetizable and could be readily attracted to a bar magnet. 

Synthesis of Some Organosilicon Polymers and Their Pyrolytic Conversion to Ceramics... 

This chapter gives an introduction to the preceramic polymer route to ceramic materials and focuses on the reasons why this new approach was needed and on the chemical considerations important in its implementation, with examples from research on organosilicon polymers. Novel polysilazanes have been prepared by the dehydro-cyclodimerization reaction, a new method for polymerizing suitably substituted cyclooligosilazanes. The living polymer intermediate in this reaction has been used to convert Si-H-containing organosilicon polymers that are not suitable for pyrolytic conversion to ceramics into useful preceramic polymers. 

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