Silicon carbide grains

Fig. 3. 35. Isotopic variations of carbon and nitrogen in various silicon carbide grains, after Amari et at. (2001). Courtesy Sachiko Amari.

Besmehn A, Hoppe P (2003) A NanoSIMS study of Si- and Ca-Ti-isoptopic compositions of presolar silicon carbide grains from supemovae. Geochim Cosmochim Acta 67 4693-4703... 

Hoppe P, Amari S, Zinner E, Ireland T, Lewis RS (1994) Carbon, nitrogen, magnesium, silicon and titanium isotopic compositions of single interstellar silicon carbide grains from the Murchison carbonaceous chondrite. Astrophys J 430 870-890... 

Hoppe P, Ott U (1997) Mainstream silicon carbide grains from meteorites. In Astrophysical Implications of the Laboratory Study of Presolar Materials. Bematowicz TJ, Zinner E (eds) AlP, New York, p 27-59 Hsu W, Wasserburg GJ, Huss GR (2000) High time resolution by use of the Al chronometer in the multistage formation of CAL Earth Planet Sci Lett 182 15-29... 

Meyer BS (1994) The r-, s-,and p-processes in nucleosynthesis. Annu Rev Astronom Astrophys 32 153-190 Mostefaoui S, Lugmair GW, Hoppe P, El Goresy A (2003) Evidence for live iron-60 in Semarkona and Chervony Kut a nanosims study. Lunar Planet Sci XXXIV 1585 Murthy VR, Sandoval P (1965) Chromium isotopes in meteorites. J Geophys Res 70 4379-4382 Nicolussi GK, Davis AM, Pellin MJ, Lewis RS, Clayton RN, Amari S (1997) s-Process zirconium in presolar silicon carbide grains. Science 277 1281-1283... 

Nicolussi GK, Pellin MJ, Lewis RS, Davis AM, Amari S, Clayton RN (1998a) Molybdenum isotopic composition of individual presolar silicon carbide grains from the Murchison meteorite. Geochim Cosmochim Acta 62 1093-1104... 

Nicolussi GK, Pellin MJ, Lewis RS, Davis AM, Clayton RN, Amari S (1998b) Strontium isotopic composition in individual silicon carbide grains a record of s-process nucleosynthesis. Phys Rev Lett 81 3583-3586 Nicolussi GK, Pellin MJ, Lewis RS, Davis AM, Clayton RN, Amari S (1998c) Zirconium and molybdenum in individual circumstellar graphite grains new data on the nucleosynthesis of the heavy elements. Astrophys J 504 492-499... 

Theoretical modeling provides strong evidence that most presolar silicon carbide grains come from 1.5 to 3 M stars. As discussed in Chapter 3, stellar modeling of the evolution of the CNO isotopes in the envelopes of these stars makes clear predictions about the 12C/13C, 14N/15N, 170/160,180/160 ratios as a star evolves. For example, in the envelopes of low- to intermediate-mass stars of solar composition, the 12C/13C ratio drops to 40 (from a starting value of 89), and 14N/15N increases by a factor of six as carbon and nitrogen processed by... 

Carbon isotopic compositions of silicon carbide grains from the Murchison meteorite compared with the carbon isotopic compositions of carbon stars (low- to intermediate-mass AGB stars). The composition of carbon in the solar system is indicated by the vertical line. Note the similarity in the distributions of compositions in the two plots. These data indicate that the silicon carbide in the Orgueil meteorite came from a population of carbon stars very similar to that in the galaxy today. 

Silicon carbide grains provide a means to study the details of the s-process in low-mass stars. The structures of these grains accommodate a wide variety of minor and trace elements. The neutrons that drive the s-process come from two sources. One source is the reaction... 

Comparisons of the calculated compositions from models of stellar nucleosynthesis and the measured compositions of strontium, zirconium, molybdenum, ruthenium, and barium in the most common type of AGB silicon carbide grains SiC (Fig. 5.11) indicate that the 13C... 

Titanium isotopic data for mainstream silicon carbide grains versus 829Si. The correlation between excesses of minor titanium isotopes and minor silicon isotopes most likely reflects galactic chemical evolution. The offset of the S50Ti trend to pass above the solar composition probably reflects 5-process nucleosynthesis in the parent stars, which most strongly affects 50Ti. Data from Huss and Smith (2007) and references therein. 

The structure of presolar silicon carbide grains can provide information about the conditions of formation. Crystalline silicon carbide is known to form about 100 different polytypes, including cubic, hexagonal, and rhombohedral structures. Presolar silicon carbide exists in only two of these, a cubic (fi-SiC) polytype and a hexagonal (a-SiC) polytype (Daulton et al.,... 

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. 

Huss, G. R. and Smith, J. A. (2007) Titanium isotopes in isotopically characterized silicon carbide grains from the Orgueil Cl chondrite. Meteoritics and Planetary Science, 42, 1055-1075. 

Nicolussi, G. K., Pellin, M. J., Lewis, R. S. et al. (1998) Molybdenum isotopic compositions of individual presolar silicon carbide grains from the Murchison meteorite. Geochimica et Cosmochimica Acta, 62, 1093-1104. 

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. 

Fig. 6.4.7. Temperature distribution inside alundum and silicon carbide grains for different times of grain contact with the metal abraded.

Isotope ratios of Ca, Ti and Si on 37 presolar silicon carbide grains from supernova were examined using the NanoSlMS (Cameca). In aU grains of type X, a large enrichment in Si (up to five times Solar) was detected. This mass spectrometric finding implies that most X grains were formed from well mixed regions in supernova ejects. 

Silicon carbide grains were reduced from 100-200 mesh to 80 percent below 1 pm in a version of stirred bead mill, using 20-30 mesh silicon carbide as media (Hoyer, Report Investigations U.S. Bureau Mines 9097, 9 pp., 1987). 

Amari S., Hoppe P., Zinner E., and Lewis R. S. (1995a) Trace-element concentrations in single circumstellar silicon carbide grains from the Murchison meteorite. Meteoritics 30, 679-693. 

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