Evolution of small silicon carbide to 3C or 2H SiC nanoparticles from meteorites: A Theoretical Study
Duan, Xiaofeng (AFRL DSRC / HITS TEAM SAIC)
Advanced Materials Development
Small silicon carbide clusters and grains are important in astrochemical processes. Si2C2 cluster is a dominant small Si-C molecule observed in C-rich ABG stellar clouds. Large amounts of particulate SiC is observed in these clouds some of which has been isolated as pre-solar 3C- or 2H- SiC nanoparticles from meteorites. Using our Stochastic Potential Surface and Car-Parinello Simulated Annealing Simulation method, we found two Si2C2 low-temperature dimerization mechanisms both of which lead to the most stable Si4C4 cluster through different transition state and intermediate structures. One mechanism of Si2C2 polymerization proceeds at low temperature, without significant thermal activation. In pursuit of an explanation for the formation of 3C-SiC, the other two reactions of formation of Si4C4H4 and C4Si4H8 in the work. The first reaction is initiated from reaction of acetylene with disilicon carbide while the later one is originated from reaction of acetylene plus disilyne. Assuming dipole-driven self-assembly of the C2Si2H4 molecules, DFT calculations predict a barrierless, enthalpically-driven formation of C4Si4H8. We employed B3LYP and other functionals with cc-pVTZ basis sets to perform DFT calculations to optimize geometries on the ground and transition states. We obtained the potential surface for the two reaction paths by integrating the intrinsic reaction coordinate starting from the TS structures connecting intermediates or products. All the calculations were carried out on HPCs in AFRL DSRC utilizing Gaussian and GAMESS computational chemistry applications installed on the systems.