Microstructure silicon carbide

Daulton, T. L., Bematowicz, T. J., Lewis, R. S. et al. (2003) Polytype distribution in circumstellar silicon carbide Microstructural characterization by transmission electron microscopy. Geochimica et Cosmochimica Acta, 67, 4743-4767. 

Sintered silicon carbide retains its strength at elevated temperatures and shows excellent time-dependent properties such as creep and slow crack growth resistance. Reaction-bonded SiC, because of the presence of free silicon in its microstructure, exhibits slightly inferior elevated temperature properties as compared to sintered silicon carbide. Table 2 (11,43) and Table 3 (44) show selected mechanical properties of silicon carbide at room and elevated temperatures. 

Chemical and phase purity are not always desirable. For example, H- and N-doped silicon carbide films behave as high temperature semiconductors, while silicon carbonitride glasses offer properties akin to glassy carbon with room temperature conductivities of 103 2 cm-118. Additional reasons for targeting materials that are not chemically or phase pure stem from the desire to control microstructural properties. 

In some instances, subtle changes in the precursor architecture can change the composition and microstructure of the final pyrolysis product. For example, pyrolysis of —[MeHSiNH] — leads to amorphous, silicon carbide nitride (SiCN) solid solutions at >1000°C (see SiCN section). At ca 1500 °C, these material transform to SisN SiC nanocomposites, of interest because they undergo superplastic deformation20. In contrast, chemically identical but isostructural — [F SiNMe] — transforms to Si3N4/carbon nanocomposites on heating, as discussed in more detail below21. 

Suganuma, K., Sasaki G., Fujita, T. etal., Mechanical properties and microstructures of machinable silicon carbide, J. Mater. Sci., 1993, 28(5) 1175. 

Wang, C.M., Mitomo, M., and Emoto, H., Microstructure of liquid phase sintered superplastic silicon carbide ceramics , J. Mater. Res. 1997, 12, 3266-70. 

Turan, S., (1995), Microstructural characterisation of silicon nitride-silicon carbide particulate composites , Ph.D. thesis, University of Cambridge. 

Microstructures of the two siliconized silicon carbide materials, (a) SiC-C and (b) SiC-S, observed with reflected light under an optical microscope, showing different reflective indexes between Si (light) and SiC (dark). 

Less-conventional processing techniques are also used to make ceramic matrix composites. Siliconized silicon carbide, for example, is made by liquid infiltration.16,17 A compact of SiC particles is formed and then presintered, or reaction bonded. Liquid silicon is then infiltrated into the structure. Many different microstructures of siliconized silicon carbide can be made in this manner. The volume fraction of SiC particles can be as high as 90vol.%. Bimodal structures have also been made by this technique. These materials are used for radiant heaters and heat exchangers.17,19... 

Fig. 4.4 Examples of the microstructure of siliconized silicon carbide, (a) Carborundum KX01 contains approximately 33 vol.% Si, (b) COORS SCRB 210 contains approximately 18 vol.% Si.

J. R. Porter and A. Chokshi, Creep Performance of Silicon Carbide Whisker-Reinforced Alumina in Ceramic Microstructures 86 The Role of Interfaces, eds. J. Pask and A. Evans, Plenum Press, New York, NY, 1987, p. 919. 

E. R. Fuller, Jr., R. F. Krause, Jr., J. Kelly, R. N. Kacker, E. S. Lagergren, P. S. Wang, J. Barta, P. F. Jahn, T. Y. Tien, and L. Wang, Microstructure, Mechanical Properties, and Machining Performance of Silicon Carbide Whisker-Reinforced Alumina, J. Research NIST, in press. 

The multifilament fiber (10-20 xm diameter) as commercially produced consists of a mixture of /3-SiC, free carbon and SiOj. The properties of this fiber are summarized in Table 6.5. The properties of Nicalon start to degrade at temperatures above about 600°C because of the thermodynamic instability of composition and microstructure. A ceramic grade of Nicalon, called Hi Nicalon, having low oxygen content is also available Yet another version of a multifilament silicon carbide fiber is Tyranno, produced by Ube Industries, Japan. This is made by pyrolysis of poly (titano carbosilanes) and contains between 1.5 and 4wt% titanium. 

The synthesis of nanophase ceramics is one of these concepts, it allows micro-porous ceramic materials with ceramic grains in the nanometer range to be obtained. Research in the field of nanophase materials is very active. A number of results on the control of microstructure and temperature stability of metal oxide ceramics can be applied to membrane preparation. Works carried out on non-oxide ceramics such as silicon carbide, silicon oxinitride or aluminum nitride should be regarded in order to extend the domain of available membrane materials. 

Microstructure of Chemical Vapour Deposition SiC Figure 6.8 shows the X-ray diffraction (XRD) patterns of CVD SiC deposited in a temperature range of 1000 to 1300°C. Detailed analyses of the X-ray results indicate that the deposits are pure silicon carbide mainly composed of //-SiC (cubic 3C crystal structure) with a small amount of er-SiC (hexagonal 4H crystal structure). It is clear from the figure that the diffraction angles of 35.6°, 41.3°, 60.1°, 72.1° and 75.5° correspond to //-SiC and the diffraction angle of 33.7° corresponds to er-SiC. As the deposition temperatures decrease, the deposits become poorly crystallised because the diffraction peaks become broader or its intensity shown in Y axis become lower. At the deposition temperature of 1000°C, the deposits are in a quasi-amorphous state. 

Key words a-silicon carbide, liquid phase, sintering, mechanical properties, microstructure... 

MICROSTRUCTURE OF LIQUID PHASE SINTERED SILICON CARBIDE CERAMICS WITH HIGH FRACTURE TOUGHNESS ... 

Microstructure of Liquid Phase Sintered Silicon Carbide Ceramics... 

Sung-Gu Lee and Young-Wook Kim, Relationship between Microstructure and Fracture Toughness of Toughened Silicon Carbide Ceramics, / Am. Ceram. Soc., 84, 1347-53 (2001). 

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