A-SiC

Fig. 5. Radioactivity after shutdown per watt of thermal power for A, a Hquid-metal fast breeder reactor, and for a D—T fusion reactor made of various stmctural materials B, HT-9 ferritic steel C, V-15Cr-5Ti vanadium—chromium—titanium alloy and D, siUcon carbide, SiC, showing the million-fold advantage of SiC over steel a day after shutdown. The radioactivity level after shutdown is also given for E, a SiC fusion reactor using the neutron reduced...

Sihcon carbide whiskers typically have diameters of a few micrometers and lengths up to 5 cm. They may be composed of either P SiC or a-SiC, the latter in one or more polytypes, and occur mosdy as hair- or ribbonlike crystals. Despite many attempts to produce SiC whiskers on a large scale at low cost, they have never acquired a wide importance. SiC whiskers have been reviewed (111—120). 

Sihcon carbide is also a prime candidate material for high temperature fibers (qv). These fibers are produced by three main approaches polymer pyrolysis, chemical vapor deposition (CVD), and sintering. Whereas fiber from the former two approaches are already available as commercial products, the sintered SiC fiber is still under development. Because of its relatively simple process, the sintered a-SiC fiber approach offers the potential of high performance and extreme temperature stabiUty at a relatively low cost. A comparison of the manufacturing methods and properties of various SiC fibers is presented in Table 4 (121,122). 

Fig. 3. Load—deflection curve for a SiC—C—SiC composite in four-point bending. Note the extreme change in behavior fora composite fabricated with a 0.17-p.m carbon layer between the SiC fiber and the SiC matrix as compared with a composite with no interfacial layer (28).

Fig. 10. Crack pinning by a SiC fiber in a glass matrix, photographed using an optical microscope and Nomarski contrast. Fiber ties perpendicular to plane of micrograph lines represent crack position at fixed intervals of time, crack mnning left to right.

A SiC buffer layer was grown on a silicon wafer at 1150-1300°C from one to 45 minutes using C3Hg and H2 as reactant gases. The thickness of the film increased gradually by diffusion of Si into the deposit until a thickness controlled by temperature and silicon etching was reached. 

Spectroscopic ellipsometry has also been applied in the characterization of compositionally graded a-SiC H alloys [353], where the flow ratio z —... 

Absorption in the p-layer can be reduced by using an a-SiC H alloy with a bandgap of about 2 eV [584]. Carbon profiling within the p-layer further improves the window properties [585]. An intentionally graded p-i interface (buffer layer) 10 nm in thickness enhances the spectral response in the blue [125, 494, 586], which can be attributed to a reduced interface recombination. 

Other stacking sequences than these are also possible, for example AaBpAaCy... or statistical sequences without periodic order. More than 70 stacking varieties are known for silicon carbide, and together they are called a-SiC. Structures that can be considered as stacking variants are called polytypes. We deal with them further in the context of closest-sphere packings (Chapter 14). 

Pt on Vulcan XC-72 with 50% PTFE the electrolyte 100% H3P04 immobilized in a SiC matrix. 

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