The carbonization mechanism in polyacrylonitrile (PAN) based carbon nanofibers is studied using ReaxFF molecular dynamics simulations. Simulations are performed at two carbonization temperatures, 2500 and 2800 K, and also at two densities, 1.6 and 2.1 g/cm(3), that are relevant to the experimental carbonization conditions. The results are analyzed by examining the evolution of species with time, including carbon-only ring structures and gaseous species. Formation mechanisms are proposed for species like N(2), H(2), NH(3), and HCN and five-, six-, and seven-membered carbon-only rings, along with polycyclic structures. Interestingly, the formation of five-membered rings follows N(2) formation and usually occurs as a precursor to six-membered rings. Elimination mechanisms for the gaseous molecules are found that are in agreement with previously proposed mechanisms; however, alternative mechanisms are also proposed.

Excited-state geometries and electronic spectra of butadiene, acrolein, and glyoxal have been investigated by the symmetry adapted cluster configuration interaction (SAC-CI) method in their s-trans conformation. Valence and Rydberg states below the ionization threshold have been precisely calculated with sufficiently flexible basis sets. Vertical and adiabatic excitation energies were well reproduced and the detailed assignments were given taking account of the second moments. The deviations of the vertical excitation energies from the experiment were less than 0.3 eV for all cases. The SAC-CI geometry optimization has been applied to some valence and Rydberg excited states of these molecules in the planar structure. The optimized ground- and excited-state geometries agree well with the available experimental values; deviations lie within 0.03 A and 0.7 degrees for the bond lengths and angles, respectively. The force acting on the nuclei caused by the excitations has been discussed in detail by calculating the SAC-CI electron density difference between the ground and excited states; the geometry relaxation was well interpreted with the electrostatic force theory. In Rydberg excitations, geometry changes were also noticed. Doubly excited states (so-called 2 (1)A(g) states) were investigated by the SAC-CI general-R method considering up to quadruple excitations. The characteristic geometrical changes and large energetic relaxations were predicted for these states.

Abstract. Time dependent variation perturbation calculations have been performed for estimating the transition energies, oscillator strengths and transition probability values for a few dipole allowed states of compressed hydrogen atom confined in a weakly coupled plasma. The compression is obtained by embedding the atom at the centre of an impenetrable spherical box. The dipole polarizability of the atom is evaluated at each confinement radius with respect to different plasma screening parameters. The effect of pressure due to spatial confinement on the dipole polarizability and other atomic properties is analyzed. Results obtained are useful for the diagnostic determination of astrophysical and laboratory plasmas and for the calculation of collision rate coefficients needed for computing opacity of stellar envelopes -a quantity of importance in the context of stellar structure and pulsations.

Carbon nanotubes are being widely studied as a reinforcing element in high-performance composites and fibers at high volume fractions. However, problems with nanotube processing, alignment, and non-optimal stress transfer between the nanotubes and surrounding matrix have so far prevented full utilization of their superb mechanical properties in composites. Here, we present an alternative use of carbon nanotubes, at a very small concentration, as a templating agent for the formation of graphitic structure in fibers. Continuous carbon nanofibers (CNF) were manufactured by electrospinning from polyacrylonitrile (PAN) with 1.2% of double wall nanotubes (DWNT). Nanofibers were oxidized and carbonized at temperatures from 600 °C to 1850 °C. Structural analyses revealed significant improvements in graphitic structure and crystal orientation in the templated CNFs, with the largest improvements observed at lower carbonization temperatures. In situ pull-out experiments showed good interfacial bonding between the DWNT bundles and the surrounding templated carbon matrix. Molecular Dynamics (MD) simulations of templated carbonization confirmed oriented graphitic growth and provided insight into mechanisms of carbonization initiation. The obtained results indicate that global templating of the graphitic structure in fine CNFs can be achieved at very small concentrations of well-dispersed DWNTs. The outcomes reveal a simple and inexpensive route to manufacture continuous CNFs with improved structure and properties for a variety of mechanical and functional applications. The demonstrated improvement of graphitic order at low carbonization temperatures in the absence of stretch shows potential as a promising new manufacturing technology for next generation carbon fibers.

Quantum chemical molecular dynamics (QM/MD) simulations using periodic boundary conditions show that hot giant fullerene (GF) cages can both eject and capture C2 molecules dependent on the concentration of noncage carbons in the simulated system, and that the cage size can therefore both increase and decrease under high temperature conditions. The reaction mechanisms for C2 elimination and incorporation involve sp3 carbon defects and polygonal rings larger than hexagons, and are thus closely related to previously described mechanisms (Murry, R. L.; Strout, D. L.; Odom, G. K.; Scuseria, G. E. Nature 1993, 366, 665). The atoms constituting the cage are gradually replaced by the two processes, suggesting that a fullerene cage during high-temperature synthesis is a dissipative structure in the sense of Ilya Prigogine’s theory of self-organization in nonequilibrium systems. Explicit inclusion of Lennard-Jones-type helium or argon noble gas atoms is found to increase the GF shrinking rate. Large GFs shrink at a greater rate than small GFs. The simulations suggest that in an idealized, closed system the fullerene cage size may grow to a dynamic equilibrium value that depends on initial cage size, temperature, pressure, and overall carbon concentration, whereas in an open system cage shrinking prevails when noncage carbon density decreases as a function of time.

Using density-functional tight-binding (DFTB)-based quantum chemical molecular dynamics at 2500 and 3000 K, we have performed simulations of benzene combustion by gradually reducing the hydrogen to carbon (H/C) ratio. The accuracy of DFTB for these simulations was found to be on the order of 7-9 kcal/mol when compared to higher-level B3LYP and G3-like quantum chemical methods in extensive benchmark calculations. Ninety direct-dynamics trajectories were run for up to 225 ps simulation time, during which hydrocarbon cluster size, curvature, and C(x)H(y) composition, carbon hybridization type, and ring count statistics were recorded. Giant fullerene cage formation was observed only after hydrogen was completely eliminated from the reaction mixture, with yields of around 50% at 2500 K and 42% at 3000 K. Cage sizes are mostly in the range from 152 to 202 carbon atoms, with the distribution shifting toward larger cages at lower temperature. In contrast to previous simulations of dynamics fullerene assembly from ensembles of C(2) molecules, we find that the resulting cages show smaller number of attached carbon chains (antenna) surviving until cage closure. Again, no direct formation pathway for C(60) from smaller fragments was observed. Our results challenge the idealized picture of "ordered" growth of PAHs along a route involving only maximally condensed and fully hydrogenated graphene platelets, and favor instead fleeting open-chains with ring structures attached, featuring a large number of hydrogen defects, pentagons, and other nonhexagon ring species.

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Functional groups and their associated charges are responsible for the binding and release of molecules from the surfaces of particles in nanodiamond colloids. In this work, we describe a combined set of experimental and computational techniques that are used to characterize these functional groups quantitatively. The surfaces of the particles examined during this study are amphoteric, as one would expect for surfaces made of carbon, with high concentrations of phenols, pyrones, and sulfonic acid groups; the average 50-nm-diameter nanodiamond aggregate has approximately 22000 phenols, 7000 pyrones, and 9000 sulfonic acids. The aggregates also have at least 2000 fixed positive charges, stabilized within pyrones and/or chromenes. No evidence for a significant concentration of carboxylic acid groups was found, although some are probably present. Hydroxyl and epoxide groups are present on some areas of the surfaces. The surfaces are graphitized, so the presence of phenols and pyrones is not surprising because such groups are common on graphitic surfaces. The sulfonic acid is due to the sulfuric acid treatment used to remove amorphous carbon and graphite during particle cleaning. The fixed charges are also due to the cleaning procedure that includes the use of KMnO 4 with the sulfuric acid.Based on titration and zeta potential experiments, elemental and particle size analyses, and modeling using semiempirical quantum mechanics, a model is proposed for the types and concentrations of surface groups. The modeling shows how functional groups form during the bead milling and cleaning used in the preparation of the colloid. It also shows that the pK a associated with the phenols and pyrones that are formed (pK a = 7.6−10.0) is consistent with that predicted using titration experiments (pK a ≥ 7.3). The positive surface potential means that the latter pK a value is significantly larger than a Henderson− Hasselbalch-based estimate. The model is shown to be useful in explaining a number of recent experiments in which nanodiamonds were used to bind and release therapeutic drug and polymer molecules.