Fraction Crystalline from Electron Powder Patterns of Unlayered Graphene in Solidified Carbon Rain

Abstract

This is an Accepted Manuscript for the Microscopy and Microanalysis 2020 Proceedings. This version may be subject to change during the production process.

Five-Membered Loop-Initiated Nucleation of Unlayered Graphene Sheets in a Carbon Melt, with Diffusion Barrier Possibilities

“…Table 2 compares the loops resulting from VASP calculations with the number of graphene sheets per carbon atom condensed in the atmosphere of red giant stars and in laboratory environments. 15 As indicated in the table, diffraction profiles from lab-grown cores are best modeled with a mixture of carbon atoms, 12% of which are in graphene sheets with a mass-weighted coherence width of 12.6 Å, i.e., in clusters on the order of 60 atoms in size, plus diffuse scattering from the remaining carbon atoms. Diffraction profiles from presolar cores instead best fit a mixture of carbon atoms, 40% of which are in graphene sheets with a mass-weighted coherence width of 40 Å, i.e., in clusters on the order of 600 atoms in size.…”

“…The context for, and general conclusions of, our HRTEM observations have been previously described for the astrophysics and microscopy communities. Previously reported high-frequency tails on the graphite ( hk 0) peaks with no sign of graphite’s usually dominant (002) layering periodicity in core diffraction patterns indicated the presence of unlayered graphene sheets in an otherwise disordered carbon matrix.…”

“…Similar core/rim onions have also been synthesized in the laboratory in the interior of an “evaporating carbon oven” . Using Debye scattering factor analysis of electron powder diffraction patterns, we have characterized average sheet size, fraction crystalline, and, by inference, the effective number density of sheets in both presolar and laboratory cores . The key take-home there is that in the laboratory vs presolar cores, the inferred sheet size (60 vs 600 atoms) and fraction crystalline (12 vs 40%) were much smaller, but the number of clusters per cm 3 was larger (18 compared to 6 times 10 19 per cm 3 ).…”

“…High-resolution transmission electron microscopy (HRTEM) suggests that many of the graphene sheets were nucleated on pentagonal loops, resulting in a carbon composite made of randomly oriented (and unlayered) graphene sheets in a disordered carbon matrix. Similar core-rim structures in the diameter range 100–400 nm have now been synthesized in the lab by slow cooling of carbon vapor created by evaporation from a resistively heated carbon rod. − …”

“…Elemental carbon is best known as either graphite (which consists of three atoms bonded together) or diamond (which consists of four bonds per atom). These solid-state materials are capable of being transformed into a liquid phase at high pressure and high temperature in planetary interiors, but in the absence of pressures above 100 atm, such elemental carbon sublimates to a gas (at around 3900 K) before forming a liquid. , However, rapidly solidified spheres of supercooled liquid carbon have been reported, e.g., after laser ablation experiments in the lab, showing that supercooled droplets of liquid carbon can be created at low pressure by cooling carbon vapor. − …”