CVD silicon carbide

CVD silicon carbide fibers are a recent development with prom-ising potential which may take over some of the applications of CVD boron fibers or other refractory fibers, providing that the production cost can be reduced. 

Figure 19.2. Cross section of a CVD silicon-carbide fiber, conrtesy of Textron Specialty Materials.

Figure 19.3. Tensile strength of CVD silicon-carbide fiber as a function of temperature.

Goodale, D. B., Reagan, P. Miskolczy, G., Lieb, D. and Huffman, F. N. "Characteristics of CVD Silicon Carbide Thermionic Converters", 16th Intersociety Energy Conversion Engineering Conference, Atlanta, GA, 1981. 

Giannuzzi, L.A., C.A. Lewinsohn, C.E. Bakis, and R.E. Tressler. 1997. High temperature creep of CVD silicon carbide fibers, submitted to the Journal of the American Ceramic Society. 

The key component is the hot-shell-emitter (HS-EM) structure, which operates between the emitter and collector temperatures. This trilayer composite stmcture (tungsten-graphite-silicon carbide) is fabricated by CVD. First, the graphite is machined to the desired configuration. Next, the inside of the graphite is coated with CVD tungsten to form the emitter and its electrical lead. Finally, the outside is coated with CVD silicon carbide to protect the emitter from the combustion atmosphere. 

Schiroky, G. H., Price, R. J. and Sheehan, J. E., Oxidation Charcteristics of CVD Silicon Carbide and Silicon Nitride, GA-A18696, General Atomics, La Jolla, CA (1986). 

Specific grinding energy (top) and area percent grinding-induced surface fracture (bottom) vs. the grain depth-of-cut to the -4/3 power for CVD silicon carbide. Tomita, Y. and Eda, H., Development of new bonding materials for fixed abrasive of grinding stone instead of free abrasives processing. Bull JSPE (in Japanese), 61, 10 (1995) 1428 Lawn, B.R. and Swain, M.V., Microfracture beneath point indentations in brittle solids, J Mater Sci, 10 (1975) 113. 

Tiny VLS nanowhiskers can be obtained by chemical vapor deposition with less than 1/100th the diameter of ordinary VLS-CVD silicon carbide whiskers by proper selection of the metal catalyst and process conditions. Indeed, these "cobweb" [2] or nanowhiskers have been made with diameters of <20 nm. To the naked eye these whiskers look like a blue cloud created by light scattering similar to that which causes the sky to appear blue. 

DJ Larkin, PG Neudeck, JA Powell, LG Matus. Site-competition epitaxy for controlled doping of CVD silicon carbide. Institute of Physics Conference Series No. 137. London IDP, 1994, p 51. HM Hobgood, RG Glass, G Augustine, RH Hopkins, J Jenny, M Skowronski, WC Mitchel, M Roth. Semi-insulating 6H-SiC grown by physical vapor transport. Appl Phys Lett 66 1364, 1995. GL Pearson, J Bardeen. Electrical properties of pure silicon and sdicon aloys containing boron and phosphorus. Phys Rev 75 865, 1949. 

Synthesis of chemical vapor deposited (CVD) silicon carbide (SiC) thick plates... 

Shiroky G H, Price R J, Sheehan J E, Oxidation characteristics of CVD silicon carbide and sihcon nitride , Rept No. GA-18696, GATechnologies, San Diego, CA, 1986. 

Also noted is the rapid expansion of a number of materials produced by CVD, which include copper, tungsten, diamond, silicon carbide, silicon nitride, titanium nitride, and others. The coverage of the chemistry and deposition techniques of these materials has been greatly expanded. 

Fluidized-bed CVD was developed in the late 1950s for a specific application the coating of nuclear-fuel particles for high temperature gas-cooled reactors. PI The particles are uranium-thorium carbide coated with pyrolytic carbon and silicon carbide for the purpose of containing the products of nuclear fission. The carbon is obtained from the decomposition of propane (C3H8) or propylene... 

Carbides produced by CVD include the refractory-metal carbides and two important non-metallic carbides boron carbide and silicon carbide. The refractory-metal carbides consist of those of the nine transition elements of Groups IVa, Va, and Via and the 4th, 5th, and 6th Periods as shown below in Table 9.1. 

Silicon carbide (SiC) is a major industrial material with a considerable number of applications. CVD plays a significant role in its development and production, SiC is a covalent carbide with two phases a and [3. The phase of major interest here is pSiC, which has a cubic zinc blend structure. It is the one reported here. 

CVD is a maj or process in the production of thin films of all three categories of electronic materials semiconductors, conductors, and insulators. In this chapter, the role of CVD in the fabrication of semiconductors is reviewed. The CVD production of insulators, conductors, and diffusion barriers is reviewed in the following chapter. The major semiconductor materials in production or development are silicon, germanium, ni-V and II-VI compounds, silicon carbide, and diamond. 

Diamond, however, is not the universal semiconductor panacea it is an indirect bandgap semiconductor and does not lase. In addition, present semiconductor materials, such as silicon and gallium arsenide, are solidly entrenched with a well-established technology, and competing with them will not be an easy task. CVD diamond will also compete with silicon carbide, which has also an excellent potential as a high-performance semiconductor material and is considerably easier and cheaper to produce. 

CVD is used in the industrial production of inorganic structural fibers such as boron and silicon carbide. Boron fibers are, in... 

Thistable(and Table 19.2below) shows that the major competitor to CVD SiC is carbon as both fibers have similar properties and are in the same cost bracket. Another competitor is boron but it is expensive and may eventually be replaced by silicon carbide. 

Silicon carbide CVD Sol-gel High strength High modulus High cost High-temp composites... 

Two fibers are presently produced by CVD on a commercial scale boron and silicon carbide. The production of these two fibers requires a monofilament starter core capable of being heated resistively such as a tungsten or graphite fiber. I l The deposition apparatus is shown schematically in Fig. 19.1. 

Ellison, A., Silicon Carbide Growth by High Temperature CVD Techniques, Ph.D. thesis, Linkoping University, 1999. 

A review article on the CVD processes used to form SiC and Si3N4 by one of the pioneers in this area, Erich Fitzer [Fitzer, E., and D. Hegen, Chemical vapor deposition of silicon carbide and silicon nitride—Chemistry s contribution to modem silicon ceramics, Angew. Chem. Int. Ed. Engl, 18, 295 (1979)], describes the reaction kinetics of the gas-phase formation of these two technical ceramics in various reactor arrangements (hot wall, cold... 

Silicon 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 Ot-SiC fiber approach offers the potential of high performance and extreme temperature stability at a relatively low cost. A comparison of the manufacturing methods and properties of various SiC fibers is presented in Table 4 (121,122). 

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