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Kinetics, morphology and thermodynamics of the solid-liquid transition of non-metals. Progress report, March 1, 1979-February 28, 1980

Description: Some previous work on Internal Centrifugal Zone Growth was documented. New calculations have been made to show that for large rf skin depths, the temperature of the sample depends in a systematic way on only three dimensionless parameters; these characterize the rf power level, the surface heat transfer coefficient, and the ambient temperature. Critical values are given for the ambient temperature below which curves of sample temperature versus RF power level are S-shaped. Based on this improvement in understanding, our previous numerical results, valid for arbitrary skin depths, are being prepared for publication. Work continues toward the measurement of the solid-liquid surface tensions of non-metallic materials via the grain-boundary groove technique. Degassed samples of GeO/sub 2/ have been obtained, but the necessary temperature caused damage to the quartz tube in our present apparatus, necessitating a new design. While the new apparatus is under construction, sodium sulfate will be used as a prototype material to enable work on the optical system. Use of an astronomical telescope in conjunction with the optical viewpoint leads to poor image quality so we are considering the substitution of a microscope with a large working distance. Previous difficulties with numerical calculation of the temperature profiles in the system have been alleviated by using finer grid sizes for the finite difference scheme. Further effort has been expended to form the basis of new work on the application of Onsager's theory of reciprocity to transport phenomena in solids.
Date: August 1, 1979
Creator: Sekerka, R.F.
Partner: UNT Libraries Government Documents Department

Dispersion aspects of silicon carbide gelcasting

Description: The principal objective of this research was to increase the solid loading of silicon carbide (SiC) powder, in an appropriate liquid medium, to a level that is useful for gelcasting technology. A number of factors that determine the maximum concentration of silicon carbide that can be incorporated into a pourable ceramic suspension have been identified. The pH of the system is the most critical processing parameter. Its proper adjustment (pH 11 to 13) allows SiC concentrations exceeding 50%, based on volume, to be routinely achieved without the use of additional dispersing agents. The particle size of SiC was also found to affect the maximum, attainable concentration. The surface area of the powder and the presence of free carbon in the powder, though not significantly influencing the suspension properties, determine the concentration of initiator required to induce polymerization and gelation. SiC specimens have been gelcast for powders in the size range of 0.8 to 8.5 {mu}m; the powders employed contain either {approximately} 0 or 19% carbon by weight. Finally, the generation of bubbles, typically encountered by the use of ammonia to adjust pH has been circumvented by the use of tetramethylammonium hydroxide.
Date: September 1, 1991
Creator: Bleier, A.
Partner: UNT Libraries Government Documents Department

rf plasma synthesis of ultrafine, ultrapure silicon carbide powder

Description: Ultrafine, ultrapure silicon carbide powder has been produced by reaction of silane and methane in a high temperature rf plasma. Preliminary studies include the effect of gas composition and of powder (plasma temperature) on the stoichiometry of the powder. The carbon-to-silicon ratio of the powder was varied from 1.0 to 1.9 by changing the process conditions. The powder has a BET surface area of 101 m/sup 2//g, which is equivalent to a particle diameter of 18.5 nm. A particle size in the range of 10 to 20 nm was measured by transmission electron microscopy. X-ray diffraction results indicate a domain size of 7.5 nm and a crystal structure of beta (cubic) silicon carbide. Spectrographic analysis shows that metallic impurities are lower than high quality grade commercial powder. Because of the high surface area, the powder must be stored and processed in an inert atmosphere to prevent severe contamination with oxygen.
Date: January 1, 1983
Creator: Hollabaugh, C.M.; Hull, D.E.; Newkirk, L.R. & Petrovic, J.J.
Partner: UNT Libraries Government Documents Department