Ablation of Spacecraft Thermal Protection Systems Using Multiscale Simulations (NASA ESI 2015)

Phenolic resin-based thermal protection systems are commonly used to protect re-entry spacecrafts from atmospheric heating. We study the ablation of such thermal protection systems (TPS) using multiscale simulations. A crosslinked phenolic formaldehyde resin is modeled and its pyrolysis kinetics are determined using ReaxFF-based molecular dynamics (MD) simulations. The activation energy  and rate constant  of thermal decomposition are obtained. We use the MD derived kinetics in a higher order mesoscale model to determine the effective surface recession rate of phenolic resin as a function of temperature. To extend the analysis to realistic length- and time-scales we propose a continuum level formulation informed by the MD and mesoscale data. We validate the model by direct comparison with previous arc jet and wind tunnel experiments and use it to characterize the Avcoat TPS. The temperature profile within Avcoat TPS, the thickness of resulting char layer as well as the pyrolysis gas blowing rates are calculated for atmospheric re-entry from low-earth orbit.

Patterning of Graphene by Hydrogen Plasma Treatment

Scalable and precise nanopatterning of graphene is an essential step for graphene-based device fabrication. Hydrogen-plasma reactions have been shown to narrow graphene only from the edges, or to selectively produce circular or hexagonal holes in the basal plane of graphene, but the underlying plasma-graphene chemistry is unknown. Here, we study the hydrogen-plasma etching of monolayer graphene supported on SiO2 substrates across the range of plasma ion energies using scale-bridging molecular dynamics (MD) simulations based on reactive force-field potential. Our results uncover distinct etching mechanisms, operative within narrow ion energy windows, which explain the differing plasma-graphene reactions observed experimentally.  Understanding the complex plasma-graphene chemistry opens up a means for controlled patterning of graphene nanostructures.


Solubility of Carbon in Ni and Cu for Controlled CVD Growth of Graphene

Chemical vapor deposition (CVD) is now a widely accepted, low-cost, and scalable method of growing macroscale graphene sheets. The CVD process starts with the decomposition of methane into active C species on catalytic metal substrates which are usually late transition metals (Ni, Pd, Pt) and coinage metals (Cu, Ag, Au). Using first principle calculations, we study the surface-to-bulk diffusion of C atoms in Ni(111) and Cu(111) substrates, and compare the barrier energies associated with the diffusion of an isolated C atom versus multiple interacting C atoms. We find that the preferential Ni-C bonding over C–C bonding induces a repulsive interaction between C atoms located at diagonal octahedral voids in Ni substrates. This C–C interaction accelerates C atom diffusion in Ni with a reduced barrier energy of ∼1 eV, compared to ∼1.4-1.6 eV for the diffusion of isolated C atoms. The diffusion barrier energy of isolated C atoms in Cu is lower than in Ni. However, bulk diffusion of interacting C atoms in Cu is not possible due to the preferential C–C bonding over C–Cu bonding, which results in C–C dimer pair formation near the surface. The dramatically different C–C interaction effects within the different substrates explain the contrasting growth mechanisms of graphene on Ni(111) and Cu(111) during chemical vapor deposition.