Alpha silicon carbide

Table 3. Steady-State Creep for Alpha-Silicon Carbides...

Resistivity measurements of doped, alpha-silicon carbide single crystals from —195 to 725°C showed a negative coefficient of resistivity below room temperature, which gradually changed to positive above room temperature (45). The temperature at which the changeover occurred increased as the ionization of the donor impurity increased. This is believed to be caused by a change in conduction mechanism. 

Radiation Effects. Alpha silicon carbide exhibits a small degree of anisotropy in radiation-induced expansions along the optical axis and perpendicular to it (58). When diodes of silicon carbide were compared with silicon diodes in exposure to irradiation with fast neutrons (59), an increase in forward resistance was noted only at a flux about 10 times that at which the increase occurs in a silicon diode. In general, it appears that silicon carbide, having the more tighdy bound lattice, is less damaged by radiation than silicon. 

Lasday, S. B. Alpha Silicon Carbide Properties Advantageous for Automotive Water Pump Seal Faces Produced at New Facility in W. Germany, Ind. Heat. [35-39] (1990). 

Storm, R.S., Boecker, W.D.G., McMurtry, C.H., and Srinivasan, M. (1985) Sintered alpha silicon carbide ceramics for high temperature structural application -Status review and recent developments, NASA Technical Reports, ASME PAPER 85-1GT-127. 

L. A. Giannuzzi, K. L. Rugg and R. E. Tressler, High temperature strength and creep behavior of Carborundum alpha silicon carbide fibers, Ceram. Trans., 38,655-666 (1993). 

Polymorphism and polytypism. Silicon carbide has two polymorphs. At temperatures above 2000°C alpha silicon carbide (a-SiC), with a hexagonal crystal structure, is the more stable polymorph with iridescent and twinned crystals with a metallic luster. At temperatures lower than 2000°C, beta silicon carbide ((3-SiC) exhibits a face-centered cubic (fee) crystal structure. 

A basic requirement for a valid experiment with the CNB specimen is stable crack extension. Unstable crack extension, which may occur in test pieces with poorly prepared notches, typically causes the test piece to be overloaded and leads to overestimates of fracture toughness. Generally, notch width between 0.25 and 0.30 mm at any point on the specimen surface is available from commercial machine shops. Larger notch thickness is acceptable provided that stable crack extension occurs. The offset between the notch planes is particularly critical. For alpha silicon carbide, notch widths of 0.225 to 0.250 nun with an offset less than 0.030 mm were consistently stable. For sapphire, notch widths of 0.300 to 0.325 mm with notch offsets of 0.075 to 0.110 mm were consistently unstable. However, notch widths of 0.250 to 0.300 nun with offsets of 0.025 to 0.075 nun were stable. Wider notches may allow for more total offset. 

Figure 6. SCF, CNB and SEPB fracture toughness test results for specimens from one lot of alpha silicon carbide [37], SCF fracture toughness values overestimate the correct value if the amount removed, at X nh, is insufficient to remove lateral cracks that interfere with the main precrack.

Silicon carbide exists in a cubic form, beta silicon carbide, and in several noncubic forms lumped under the term alpha silicon carbide. 

Thus friction of alpha silicon carbide appears to be tribochemically controlled and very dependent on the reactive gases present in the surrounding en-... 

J. M. Martin, T. Le Mogne and M. N. Gardos, Friction of Alpha Silicon Carbide under Oxygen Partial Pressure High Resolution Analysis of Interface Films, Proc. Japan Int. Tribology Conf., Nagoya, Japan. 1990. p. 1407. 

Storm, R.S., Net Shape Fabrication of Alpha Silicon Carbide Turbine Components. ASME Publication 82-GT-216 (19te). 

Rottenholber, P., Langer, M., Storm, R.S. and Frechette, F., Design Fabrication and Testing of an Experimental Alpha Silicon Carbide Turbocharger Rotor. Society of Automotive Engineers USA Publication No. 810523 (1981). 

Oxidation—In general, the metals-silver, vanadium, copper, molybdenum, platinum, palladium, cobalt, nickel, manganese, tin, and lead-in either oxide or metallic form, mounted on suitable supports, are used in the oxidation of hydrocarbons. For complete oxidation, active supports such as gamma aluminas have been used. For selective oxidation, alpha alumina or silicon carbide are employed. Sometimes bi-metals, tin-vanadium, iron-molybdenum, and vanadium-molybdenum are mounted on the carrier. 

Cross-sections of the embedded layers were also investigated by SEM. The substrates were cut using a diamond cutting blade about 2-3 mm from the end and mounted into a mixture of cold mounting epoxy resin and an epoxy system (1 1) for metallographic preparation. Cross-sections of the samples were polished with silicon carbide (SiC) grinding papers (180, 500, 1200,2400,4000) and with 1.0 and 0.3 pm of alpha alumina suspension. 

Silicon carbide is produced most often by the Acheson process, discovered in 1891 by Edward Goodrich Acheson. Later in 1891 he formed Carborundum Corporation to manufacture Sic abrasives. In this process current is passed through a mixture of carbon and sand causing the reaction SiOj -H 3C — SiC + 2CO. When carried out at 2600°C or higher the result is alpha the beta form can be achieved at 1500 to 1600°C. ... 

SILICON CARBIDE PLATELETS. Single crystals of alpha-phase hexagonal crystal structure. Four size ranges currently are produced —100, H-200 mesh (100-300 /rm in diameter, 5-15 /rm thick) —200, H-325 mesh (50-150 /rm in diameter, 1-10 /rm thick) —325 mesh (5-70 /rm in diameter, 0.5-5 /rm thick) and 00 mesh (3—30 /rm in diameter, 0.5-3 /rm thick). The finest size... 

Nesquehonite Calcium fluosilicate Beta silicon carbide Sodium sulfide Sodium fluoride Magnesium carbonate chloride hydroxide hydrate Magnesium pyrophosphate Anorthoclase Alpha cristobcdite Sodium hydroxide Calcium aluminum oxide sulfate... 

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