Showing papers on "Silicon carbide published in 1975"

Continuous silicon carbide fiber of high tensile strength

Abstract: Continuous silicon carbide fiber of high-tensile strength (about 350 Kg/mm2) was synthesized by means of the heat-treatment of organosilicon polymer obtained from dodecamethylcyclohexasilane. X-ray analysis and high voltage electron microscopic observations revealed that the obtained continuous fiber is an ultrafine grain structure of β-silicon carbide.

Hot-pressing behaviour of silicon carbide powders with additions of aluminium oxide

Abstract: The hot-pressing behaviour of different silicon carbide powders (average particle sizes ranging from ∼ 0.5 to 9 μm) with aluminium oxide additions ranging from 0.01 to 0.15 volume fractions was investigated. Using powders with an average particle size < 3 μm, densities ≥ 99% theoretical could be achieved at 1950° C (1 h) with 28 MN m−2 for volume fractions of AI2O3 $$\bar > $$ 0.02. A liquid phase forms at high temperatures which dissolves the silicon carbide particles to promote densification by a solution-reprecipitation mechanism.

Structural analysis in continuous silicon carbide fiber of high tensile strength

Abstract: The starting material of a continuous silicon carbide fiber with very high tensile strength is a polycarbosilane. It begins to decompose from 300°C in vacuum and is gradually converted into a β-silicon carbide fiber by the heat-treatment at the temperature above 800°C.

Sintering of Silicon Carbide

Abstract: The absence of densification during sintering of pure SiC is the result of its high grain boundary to surface energy ratio. Whenever this ratio exceeds a certain critical value, a solid will fail to densify without external pressure as there is not enough energy available to extend the grain boundaries.

Oxidation of 6H‐α Silicon Carbide Platelets

Abstract: Oxidation on the 2 crystal faces of SiC platelets was studied. The oxidation of the C face followed a simple parabolic law and was oxide-diffusion controlled, whereas that of the opposite crystal face exhibited a linear growth rate which was surface-reaction controlled. Previous investigators, who studied the oxidation of Sic by measuring weight change, found that the oxidation rate follows a simple parabolic law. At high temperatures and long reaction times, there is an increase in the oxidation rate which has previously been attributed to a change in the state of the oxide. The present investigation, however, indicates that this change occurs as the linear oxidation rate becomes dominant.

Materials having non-linear electrical resistance characteristics

Abstract: A polymer composition containing silicon carbide of a critical maximum average particle size can be processed into heat-shrinkable articles possessing nonlinear electrical resistance characteristics. An embodiment of the invention provides a heat-shrinkable article comprising a laminate of a layer of elastomer, including silicon carbide particles, and a layer of a heat-shrinkable polymeric material. The elastomeric layer may be a blend of a rubbery polymer and a low density polyethylene having dispersed therein particulate silicon carbide of particle size finer than about 300 grit.

Gauge Length and Surface Damage Effects on the Strength Distributions of Silicon Carbide and Sapphire Filaments

Abstract: The assumption of the constancy of the Weibull distribution parameters with variations in gauge length was tested for SiC-on-carbon and sapphire filaments. Surface damage was superimposed on the ga...

Silicon carbide electrolyte retaining matrix for fuel cells

Abstract: In a fuel cell utilizing an acid electrolyte, such as H 3 PO 4 , the electrolyte retaining matrix is made from silicon carbide. The silicon carbide has been found to be virtually inert to H 3 PO 4 at fuel cell operating temperatures and provides all the other necessary and desirable matrix properties. This matrix is expected to have a life of at least 40,000 hours under normal fuel cell operating conditions.

Sintered silicon carbide ceramic body

Abstract: Pressureless sintering of silicon carbide to produce ceramic bodies having 75% and greater theoretical densities, can be accomplished by firing shaped bodies, containing finely divided silicon carbide, boron source such as boron carbide, carbon source such as phenolic resin and a temporary binder, at a sintering temperature of from about 1900° C. to about 2500° C.

The High-Temperature Oxidation of Hot-Pressed Silicon Carbide

Abstract: For the past several years, we have been studying the oxidation behavior of a number of silicon-based materials, the aim being to define the rate-determining mechanism of oxidation. These materials have a bright future in high-temperature energy systems such as MHD generators, rocket engines, re-entry vehicles, and advanced air-breathing propulsion systems. Desirable properties include retained strength at elevated temperatures, chemical inertness, thermal stability, and high strength-to-weight ratio.

Process for forming high density silicon carbide

Abstract: High density sintered silicon carbide articles are formed by first forming a green blank or billet out of powdered silicon carbide, heat treating the billet to cause it to lightly sinter and become partially densified, shaping the billet to the desired final dimensions and configuration, and fully sintering and densifying the partially sintered shape by heat alone or by heat in the presence of silicon. The product of the process possesses high mechanical strength and may be very fine grained.

Filamentary silicon carbide crystals by VLS growth in molten iron

Abstract: A method of growing silicon carbide whiskers from a gaseous phase by means of a vapor-liquid-solid mechanism on a substrate using iron in a finely divided state as a solvent for the silicon carbide.

On the chemical erosion of a carbon first wall proposed for Tokamak devices for controlled nuclear fusion

Abstract: Preliminary results on the chemical attack of several carbon materials by hydrogen plasma are presented and compared with the literature data on the reaction of hydrogen atoms with carbon in an afterglow. The chemical erosion of silicon carbide and of pyrolytical graphite should not be dangerous for the ignition of fusion assuming that the reaction probability of hydrogen atoms with the particular material would not be increased due to radiation damage of the surface.

Polycrystalline silicon carbide with increased conductivity

Abstract: Polycrystalline silicon carbide with increased electrical conductivity at room temperature is produced by shaping a particulate mixture of β-silicon carbide, boron additive, beryllium carbide and a carbonaceous additive into a green body and sintering the body producing a sintered body having a density of at least about 85% of the theoretical density of silicon carbide.

Hot‐Pressing of Silicon Carbide with 1% Boron Carbide Addition

Abstract: The densification behavior of pressure sintered cubic silicon carbide containing 1 wt% boron carbide was studied as a function of temperature (1750° to 1950°C). Specimens of theoretical density were obtained at 1950° with a pressure of 3000 psi. Experimental results showed that densification proceeded by a plastic flow mechanism. Interpretation of the data in terms of Murray's equation yields an activation energy of (11S ± 18) kcal/mol. At 1850C and above, tabular grains of 6H and 2H SiC were observed in a matrix of fine grains of 3C SiC.

Graphite fiber/metal composites

Abstract: A novel graphite fiber/metal composite material in which the graphite fibers have an adherent coating of silicon oxide and silicon carbide. The coating protects the graphite surface from attack by carbide forming matrix metals such as aluminum, titanium, magnesium and nickel. In a preferred embodiment of the invention the coating is formed by an intermediate temperature vapor deposition technique involving the reduction of silicon tetrachloride in the presence of hydrogen and an oxygen containing gas.

Elevated Temperature Strength of Silicon Carbide-on-Carbon Filaments

Abstract: HE CONTINUOUS SILICON carbide (SiC) filament currently being considered T for reinforcement in high temperature metal matrices [1] is manufactured by chemical vapor deposition (CVD) from reactant gases onto a heated tungsten substrate [2]. Recently, investigators at AVCO Systems Division’ reported the development of a silicon carbide filament manufactured by CVD on a carbon monofilament [2]. It has been shown previously that the reaction of the tungsten core with the SiC coincides with the loss of strength at high temperature [3]. This investigation of the high temperature strength of the SiC-on-carbon filaments was conducted to determine if the absence of this strength degradation mechanism results in higher strengths at the temperatures where the SiC-W reaction becomes appreciable. A model TM-SM Instron testing machine was used to tensile test the filaments employing the hypodermic needle/pivot bearing grips described elsewhere [4]. The thermal treatments were performed in the Instron with a 10&dquo; split tube furnace having a 2&dquo; hot zone. The 12&dquo; long filaments were tested at a strain rate of 0.033 min-l. The SiC-on-carbon filaments supplied by Avco consisted of a 0.001&dquo; diameter monofilament carbon substrate coated with ==0.0001&dquo; layer of pyrolytic graphite upon which the SiC was deposited to form an overall diameter of 0.004&dquo;. In the final stages of deposition excess carbon is added to form a carbon rich layer which is more abrasion resistant than stoichiometric SiC as witnessed by the fact that the strength of the pristine filaments is not degraded by self-abrasion. Scanning electron microscopic examination of fractures surfaces of the filaments revealed that fracture always seems to initiate at the carbon/SiC interface or in the carbon core as illustrated in Figure 1. 1 AVCO Systems Division, Lowell Industrial Park, Lowell, MA

Vitreous bonded cubic boron nitride abrasive articles

Abstract: A vitreous bonded cubic boron nitride grinding wheel is provided which optionally contains silicon carbide or other abrasive having a coefficient of thermal expansion substantially the same as the coefficient of thermal expansion of cubic boron nitride, and wherein the vitreous bond, having a coefficient of thermal expansion substantially the same as the coefficient of thermal expansion of cubic boron nitride, is substantially non-reactive with cubic boron nitride and the optional abrasive.

Process for fabricating silicon carbide articles

Abstract: Silicon carbide articles are formed by wet milling a mixture of silicon carbide having an average particle size less than 10 microns, and colloidal graphite; eliminating powder agglomerates larger than 325 mesh (U.S. Standard Sieve Series); drying the milled powder and pressing it to the desired shape; and finally, firing the preform at approximately 2000° C. in the presence of silicon, converting the graphite to silicon carbide.

Thermal stability of silicon carbide fibres

Abstract: THE potential of high strength silicon carbide fibres for use in composite materials1 depends on their thermal stability; specifically on the ability to retain their strength after exposure to the temperatures of composite fabrication and subsequent service. Aveston2 has reported a marked reduction in the strength of such fibres after they have been exposed to high temperatures. This has not been observed in single crystal and bulk polycrystalline silicon carbide which exhibits no degradation of mechanical strength with temperature3,4. We have studied the influence of high temperature on the tensile fracture strengths of pyrolytically deposited silicon carbide fibres from several sources. We found that their strengths after exposure were considerably higher than may have been expected from the previously published data.

Alumina-silicon carbide refractories and their method of manufacture

Abstract: Alumina-silicon carbide refractory characterized by comprising predominantly an inter growth texture. The alumina-silicon carbide is manufactured by adding fine aluminum powder to a body of silica-alumina type refractory compound, mixing, molding and drying the resultant mixture to obtain a green body and finally firing the green body in an atmosphere containing predominantly a carbon oxide gas.

Method of forming a silicon carbide article

Abstract: A method of forming a silicon carbide article is disclosd. Selected weight percentages of silicon carbide particles, graphite particles, if desired, and a thermosetting binder are mixed together and molded into an article by molding techniques which operate on the basis that the thermosetting binder forms a continuous medium about all the particles supported therein. The molded article is heated in the absence of oxygen and the thermosetting binder breaks down to form a low density, vitreous carbon phase. The article is heated in the absence of oxygen to a selected temperature at which the article is maintained for a period of time in a gaseous environment consisting of nitrogen preferably with a small amount of hydrogen therein. The hydrogen-nitrogen treatment is effective to cleanse the article and insure that there is an adequate pore structure through the article for a later siliciding operation to be carried out thereon. The article is silicided at an elevated temperature by penetration of the article through its pore structure with a reactable form of silicon.

Effects of annealing on profiles of aluminum implanted in silicon carbide

Abstract: The effects of isothermal annealing on the profiles of heavy (4×1016 Al+/cm2 at 60 keV) implantations of Al in SiC were investigated by means of the 27Al(p,γ)28Si nuclear resonance technique. Annealing at 1400 °C for 15 min resulted in the apparent outdiffusion of about 30% of the implanted Al, an accumulation of Al at the surface, and a residual peak at the depth of the as−implanted profile. Subsequent annealing showed continued apparent outdiffusion with the release of Al from trapping sites as the rate−limiting process.