This is an Accepted Manuscript for the Microscopy and Microanalysis 2020 Proceedings. This version may be subject to change during the production process.
A subset of isotopically “presolar” carbon
particles
extracted from the Murchison meteorite contain signatures expected
of particles formed in the atmosphere of red giant stars. Some of
these micron-size particles have spherical cores that show diffraction
rings from atom-thick graphene, possibly formed by solidification
of liquid carbon at low pressure. Electron phase-contrast transmission
electron microscopy (TEM) images suggest that these cores originate
from supercooled carbon droplets that formed graphene sheets on randomly
oriented 5-membered loops. In addition to presolar data, laboratory
synthesis in an “evaporating carbon oven” creates similar
core-rim structures by slow cooling of carbon vapor. In research studies,
it was shown that 5-membered loops are essential to the initiation
of carbon nanotubes on catalyst particles. In addition to offering
this experimental context, we present density functional theory (VASP)
computer simulations suggesting that 5-member loops are more likely
than 6-member loops in a solidifying carbon melt. These things suggest
that 5-member loops compete effectively as nucleation seeds for explaining
the faceted pentacones inferred from TEM images of presolar particle
cores. In that context, pent-first nucleation (along with the crowding
of growing sheets by nearby liquid atoms) may reduce the chances of
graphite layer formation and lead to unprecedented diffusion barrier
properties for this composite material.
Elemental carbon has important structural diversity,
ranging from
nanotubes through graphite to diamond. Transmission electron microscope
studies of micron-size core/rim carbon spheres suggest that unlayered-graphene
composite cores formed from (in some cases “pent-first”)
solidification of carbon-vapor droplets condensed in both stellar
atmosphere and laboratory settings, followed by gas-to-solid carbon
coating to form the graphite rims. In this work, we construct analytical
models for 2D reaction-limited nucleation and growth. We then generate
an analytical condensation and solidification model to compare with
presolar and lab-grown data on graphene sheet size and number density.
Unlike 3D metallic elemental liquids’ supercooling thresholds
of 30% of the melting temperature, our 2D analysis suggests containerless
supercooling thresholds for carbon droplets on the order of 50% of
the (inferred) melting temperature.
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