Analogs for Unlayered-Graphene Droplet-Formation in Stellar Atmospheres

P Fraundorf (1, 2) , Melanie Lipp (1) , Taylor Savage (1)

1 University of Missouri [St. Louis]
2 WUSTL - Washington University in Saint Louis

The unlayered spherical graphene cores, found in a subset of micron-sized presolar graphite onions extracted from primitive meteorites, show graphene but not graphite inter-sheet spacings, sheet " coherence widths " of about 4nm, and (in electron-phase-contrast images) evidence of edge-on obtuse-angle graphene-sheet junctions. The literature on solidification of metallic liquids predicts supercooling by as much as 30% below the melt-temperature, suggesting that at low pressures carbon vapor will predictably condense first as a liquid. Density measurements, possible TEM evidence of faceted-pentacones, and preliminary studies of 5 and 6 atom loop formation in a solidifying melt, all suggest that these unlayered graphene cores (which bear isotopic signatures of formation in asymptotic-giant-branch star atmospheres after third dredge-up) may have resulted from dendritic solidification of " slow-cooled " carbon droplets. Preliminary notes on a strategy for slow-cooling carbon vapor-condensation in the laboratory (with help from an evaporating oven) are also provided.

Mots clés: Graphene synthesis; liquid carbon; transmission electron microscope; presolar grains; cool stellar atmospheres

Domaines: Science des matériaux [cond-mat.mtrl-sci]; Astrophysique stellaire et solaire [astro-ph.SR]; Micro et nanotechnologies/Microélectronique

P Fraundorf, Melanie Lipp, Taylor Savage. Analogs for Unlayered-Graphene Droplet-Formation in Stellar Atmospheres. Microscopy and Microanalysis (M&M 2016), Jul 2016, Columbus, OH, United States. pp.1816-1817. ⟨hal-01356394⟩