Silicon carbide condensation

The second interesting feature of this isotope is that minuscule grains of silicon carbide extracted from meteorites have been found to be very rich in calcium-44, as mentioned earlier. They have been identified with presolar grains that condensed in the ejecta of supernovas during their first few years of expansion. Could it be that supernovas have been throwing sand in our eyes Data gathered by the ISO (Infrared Space Observatory), yet another experiment with strong participation by the French CEA, clearly demonstrates that new dust condensed inside the Cas A remnant very soon after explosion of the supernova that caused it. °... 

Stephens, J. R., 1980. Visible and ultraviolet (800-130 nm) extinction of vapor-condensed silicate, carbon, and silicon carbide smokes and the interstellar extinction curve, Astrophys. J., 23H, 450-461. 

Circumstellar condensates carry information about the environments in which they formed and about the environments through which they passed on the way to being incorporated into meteorites and IDPs. To date, studies to extract this information have been done primarily on presolar silicon carbide and graphite (Bematowicz et al., 2006). 

Silicon carbide grains are known to contain subgrains of titanium carbide. Equilibrium thermodynamics predicts that titanium carbide will condense before silicon carbide (Fig. 5.13). The titanium carbide grains were apparently accreted by the growing silicon carbide grains and were enclosed as the silicon carbide grains continued to grow. 

This reaction is endothermic and additional energy must be provided to sustain it, usually by induction heating, or by adding silicon carbide grain which chlorinates exothermically. The product gases are cooled below 200°C to condense and collect the zirconium—hafnium tetrachloride as a powder. The offgas stream then is refrigerated to obtain by-product silicon tetrachloride liquid. 

Two principal mechanisms that may be responsible for mass loss from red giants are considered shock wave-driven winds and radiatively (dust)-dr iven winds. Effect of the periodic shocks accompanying nonlinear oscillations of red giants is most prominent in the outer layers of the stellar atmosphere where shocks are able not only to expel gas but also increase gas density so that some molecular components become supersaturated. In 0-rich stars the most abundant condensible species are silicon monoxide and iron, whereas in C-rich stars these are carbon, silicon carbide and iron. 

In C-rich stars almost all atoms of oxygen are blocked in CO molecules, whereas the large fraction of carbon is tied in molecules C H and CgHg. The rest of carbon exists in the form of free atoms and molecules of silicon carbide SIC. Both carbon and silicon carbide are the most abundant condensible species in C-rich stars. The presence of SiC circumstellar grains is compelling due to the ii.3 im spectral feature identified firstly by Treffers and Cohen (1974). 

There are several reports of the condensation of bis-silanes of the type H3Si-X-SiH3 (primary silanes) and H2RSi-X-SiR H2 (secondary silanes) principally with the aim of producing polymers that would be precursors to silicon carbide ceramic material. The polymers would, in principle,... 

Gas phase condensation dc- and rf-magnetron sputtering, 11 deposition of palladium on silicon carbide support, 12... 

Other more complex polymers have been employed as silicon carbide precursors. For instance, the mixture of methylchlorodisilanes obtained as a by-product in the direct synthesis of Me2SiCl2 can be redistributed with catalysts to give a polycyclic, partially cross-Uuked polymer with the approximate composition shown in equation (44). Pyrolysis of this precursor produces silicon carbide in good yield. Partially cross-linked polymers made by condensing vinylmethyldichlorosilane and other methylchlorosilanes with sodium also are efficient precursors for silicon carbide. 

Clayton D. D., Arnett W. D., Kane J., and Meyer B. S. (1997) Type X silicon carbide presolar grains type la supernova condensates Artropliyi. J. 486, 824-834. 

Silicon carbide whiskers are generally manufaclured by a metal-catalyzed process, in which carbon and silicon are condensed from the gas phase. Another jtroccss utilizes rice husks decomposed at 500°C. At temperatures of 1800°C the carbon/Si02-mixture formed is converted into a-SiC. Potassium titanate whiskers are synthesized from potassium molybdate, potassium carbonate and litanium dioxide at temperatures above 1200°C. 

A silicon carbide rod, held in place by two electrode connector clamps, is fitted into a Liebig condenser. The electrode clamps are sealed into Pyrex caps, which fit over the groimd-glass male joints of the condenser. 

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