Silicon oxycarbide

Zank, G. A. Preceramic Polymer-Derived Silicon Oxycarbides. In Silicon-Containing Polymers. The Science and Technology of Their Synthesis and Applications Jones, R. G., Ando, W., Chojnowski, J., Eds. Kluwer Dordrecht, 2000 pp 697-726. 

As observed by D. Johnson and J. Stiegler, "Polymer-precursor routes lor fabricating ceramics offer one potential means or producing reliable, cost-effective ceramics. Pyrolysis of polymeric metalloorganic compounds can be used to produce a wide variety of ceramic materials." Silicon carbide and silicon oxycarbide fibers have been produced and sol gel methods have been used In prepare line oxide ceramic powders, such as spherical alumina, as well as porous and fully dense monolithic forms. 

Although a number of Si02 precursors are used industrially, most are used for CVD of silica insulating layers in electronic device manufacture1. As such, they will not be discussed here. Many of the silicon oxycarbide precursors discussed above can be oxidized to Si02 and thus can be considered precursors. However, this is simply a processing approach. The chemistry remains the same as described above. Thus, further discussion is not warranted. 

Combining aspects of carbon and silicon chemistry while at the same time expanding the tool box of the periodic table, recently, a first report on the templated synthesis of mesoporous silicon oxycarbide (SiOC) and silicon carbonitride (SiCN) as analogs of the well-known mesoporous silica materials discussed in many chapters of the book has appeared (Fig. 25.5),58 opening an even wider horizon for the exploration of SiC-related nanomaterials in the fields covered in this book. 

Ceramic fibers of the nonoxide variety such as silioon carbide, silicon oxycarbide such as Nicalon, silicon nitride, boron carbide, etc. have become very important because of their attractive combination of high stiffiiess, high strength and low density. We give brief description of some important nonoxide fibers. 

Figure 16.22 shows that silica (Si02) has a tetrahedral elementary unit. To reduce the k value of silica, some oxygen atoms are replaced with F, C, or CH3. The addition of CH3 not only introduces less polar bonds but also creates additional free volume. Therefore, materials such as silicon oxycarbides (SiOCH) are constitutively porous [61]. 

Figure 4.39. spectrum of commercial Nicalon SGN silicon oxycarbide fibre, showing peaks corresponding to a range of oxycarbide units. From Hatfield and Carduner (1989), by permission of... 

Martfnez-Crespiera, S., lonescu, E., Schlosser, M. et al. 2011. Fabrication of silicon oxycarbide-based microcomponents via photohthographic and soft lithography approaches. Sensors and Actuators... 

In another attempt to obtain diamond from SiC, no evidence for diamond growth was found by means of XRD and SEM after hydrothermal hydrolysis of P SiC powder at 140 MPa and 800°C [57]. Formation of quartz, a second acicular phase, graphite and maybe nanosized diamond crystals was reported. TEM analysis of whiskers of the second acicular phase revealed lattice spacing of 0.9 nm, a <7-value of 0.56nm and a composition of SiOi 6Co.2- A very sharp band at 1330cm was found in a Raman spectrum and assigned to the silicon oxycarbide, SiOi 6C0.2, rather than to diamond. 

C. Above this temperature the slight decrease of E is probably related to the viscous deformation of the amorphous silicon oxycarbide phase that shows a glass transition temperature of 1300°C [76]. 

This work reports the development of a polymeric/sol-gel route for the deposition of silicon carbide and silicon oxycarbide thin films for applications such as heat-, corrosion-, and wear-resistant coatings, coatings on fibers for controlling the interaction with the matrix in ceramic matrix composites, or films in electronic and optoelectronic devices. This method, in which the pre-ceramic films are converted to a ceramic coating either by a conventional high temperature annealing or by ion irradiation, is alternative to conventional methods such as chemical or physical vapor deposition (CVD, PVD), molecular beam epitaxy, sputtering, plasma spray, or laser ablation, which are not always practical or cost efficient. 

It is thus clear that the processing atmosphere strongly affects the resulting composition and microstructure of both SiOC and SiC coatings, and that vacuum is the only environment which allows the synthesis of both silicon carbide and silicon oxycarbide thin films [51],... 

G. D. Soraru, Q. Liu, L. V. Interrante, and T. Apple, The role of precursor molecular structure on the microstructure and high temperature stability of silicon oxycarbide glasses derived from methylen-bridged polycarbosilanes, Chem. Mater. 1998, 10, 4047-4054. 

P. Colombo, T. E. Paulson, and C. G. Pantano, Atmosphere effects in the processing of silicon carbide and silicon oxycarbide thin films and coatings, J. Sol-Gel. Sci. Technol. 1994,2,601-604. 

P. Colombo and M. Modesti, Silicon oxycarbide ceramic foams from a preceramic polymer, J. Am. Ceram. Soc. 1999, 82, 573-578. 

But CMCs will be commercially successful only when they are produced cost-effectively. Polymer-derived ceramic (PDC) technology is one of the most promising low cost fabrication methods for ceramic matrix composites, particularly for large, complex shapes. In PDC technology, a silicon-based polymer (siloxane, carbosilane, silazane, etc) with fiber or particle reinforcement is shaped and cured in the polymer condition and then pyrolyzed in a controlled atmosphere to form a stable silicon-based ceramic, such as silicon carbide, sihcon nitride, silicon oxycarbide, or silicon oxynitride. 

This chapter will describe the processing and properties of an oxide fiber reinforced ceramic matrix composite with a silicon oxycarbide matrix based on a PDC technology, introduced by AlliedSignal (now Honeywell International) under the trademark of Blackglas ceramic. The oxide fiber in this CMC system is the Nextel 312 fiber (3M, Inc.) that has been treated to form a boron nitride surface coating. The information that follows was primarily developed from Low Cost Ceramic Matrix Composites (LC ) program funded by DARPA from 1991-1997. 

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