Search

We present a combined experimental and computational study of the elastic behavior of a series of highly anisotropic pyrocarbons, with crystallite sizes L a in the 2–10 nm range, under a -axis compressive load. The materials include a rough laminar and a regenerative laminar pyrocarbon, as-prepared by chemical vapor deposition and after various heat treatments up to 2600 °C, for which a -axis nanoindentation experiments have been performed, showing a significant decrease in the indentation modulus and hardness with increasing L a (or heat treatment temperature). To rationalize this behavior, molecular dynamics simulations of the uniaxial compression of accurate atomistic models of the materials as well as pristine graphite were performed, unraveling significant out-of-plane deformations in the models with increasing compressive strain, leading to elastic softening. More precisely, significant kinks were observed around extended screw dislocation-like defects in the most disordered pyrocarbon at rather large strain levels (∼ 3%). Conversely, graphite rather shows the formation of extended buckles, starting at very low strain values. Finite element modelling shows that such kinking/buckling transitions should take place in a large area under the indenter tip within usual nanoindentation conditions. Both finite element calculation and analytical approximation of the indentation modulus predict the correct trend of decreasing modulus with increasing L a when applied with the elastic tensors computed after the buckling/kinking transitions, certainly proving the importance of the latter in the observed experimental indentation moduli. [ABSTRACT FROM AUTHOR]

We present a mechanistic model of carbon nucleation and growth from a fluid at elevated temperature (T) and pressure conditions, typical of those found in the shallow Earth's lithosphere. Our model uses a replica exchange reactive molecular dynamics framework in which molecular configurations are swapped between adjacent T replica at regular intervals according to underlying statistical mechanics. This framework allows predicting complex molecular structures and thermodynamics while remaining computationally efficient. Here we simulate the reactivity of an unstable mixture of CO 2 and CH 4 at 1000 K and 1 GPa. We find that the path to thermodynamic equilibrium is initially entropy-driven, producing a diversity of short-lived species, including various alcohols with intermediate carbon oxidation states. Cyclic and polycyclic radicals that are sometimes resonance-stabilized form next and set the stage for carbon nucleation. The carbon exsolution process releases abundant water, is exothermic and starts with the nucleation of a large aggregate of hydrogenated graphene flakes from covalently bonded polycyclic units. The carbon backbone of this nucleus subsequently grows into a hydrogen-depleted fullerene-like structure, before evolving toward a partially bilayered graphene layer. Overall, our results show that the mechanism of graphitic C formation is certainly not bimolecular, and that it may involve a combination of key condensation and radical chain reactions. This will help understand the isotopic, and reactive characteristics of carbon-bearing fluids during their upward transit through the Earth's mantle and crust. Moreover, the mechanistic insights outlined here present intriguing similarities with the process of soot and interstellar dust formation, which suggests that the widespread distribution of abiotic polyaromatic and graphitic material on Earth and beyond may reflect the prevalence of a fundamental chemical pathway. [ABSTRACT FROM AUTHOR]

Dense anisotropic carbons are praised materials for thermostructural applications, yet, so far, a detailed structure–property relationship for these materials is still lacking, especially for the pyrocarbon (pyC) matrices in carbon/carbon composites. Here we compute the full elastic tensors of 210 recently introduced nanoscale models of anisotropic carbons (Polewczyk et al., 2023), covering domain sizes (L c and L a) and orientation angles (OA, as a measure of nanotexture) relevant to most as-prepared and moderately heat-treated pyC matrices: L c ∈ [1.5:8 nm]; L a ∈ [2:5.5 nm]; and OA ∈ [25:110°]. Isothermal and adiabatic elastic tensors, corresponding to the slow quasi-static and ultra-fast loading regimes, respectively, are considered. Analyzing the database of computed elastic constants with a random forest regressor supervised learning algorithm we show that all elastic constants can be predicted accurately using L c , L a and OA as descriptors. Among the latter, OA is the one showing, by far, the strongest correlation with the elastic tensors. For such dense, non porous carbons, 3 of the 6 isothermal and 5 of the 6 adiabatic constants can even be accurately predicted using OA as the unique material descriptor. Calculation of the universal anisotropy index shows that isothermal tensors show more anisotropy than adiabatic ones, indicating that stress relaxation favors elastic anisotropy. Eventually, the Young's moduli and Poisson coefficients of six models of actual pyCs are presented and their longitudinal moduli compared to tensile measurements, showing relatively poor agreement. These results suggest that accounting for texture at a larger scale is required to capture pyC matrices elasticity. The developed supervised learning model is available for online use at https://web.ism.u-bordeaux.fr/rfr. [Display omitted] [ABSTRACT FROM AUTHOR]

From as small as single carbon dimers up to giant fullerenes or amorphous nanometer-sized particles, the large family of carbon nanoclusters holds a complex structural variability that increases with cluster size. Capturing this variability and predicting stable allotropes remains a challenging modelling task, crucial to advance technological applications of these materials. While small cluster sizes are traditionally investigated with first-principles methods, a comprehensive study spanning larger sizes calls for a computationally efficient alternative. Here, we combine the stochastic ab initio random structure search algorithm (AIRSS) with geometry optimisations based on interatomic potentials to systematically predict the structure of carbon clusters spanning a wide range of sizes. We first test the transferability and predictive capability of seven widely used carbon potentials, including classical and machine-learning potentials. Results are compared against an analogous cluster dataset generated via AIRSS combined with density functional theory optimizations. The best performing potential, GAP-20, is then employed to predict larger clusters in the nanometer scale, overcoming the computational limits of first-principles approaches. Our complete cluster dataset describes the evolution of topological properties with cluster size, capturing the complex variability of the carbon cluster family. As such, the dataset includes ordered and disordered structures, reproducing well-known clusters, like fullerenes, and predicting novel isomers. [Display omitted] [ABSTRACT FROM AUTHOR]

We report on various nanocarbons formed from a unique structural pattern containing two pentagons, three hexagons, and two heptagons, resulting from local rearrangements around a divacancy in pristine graphene, or nanotubes. This defect can be inserted in sheets or tubes either individually or as extended defect lines. Sheets or tubes containing only this defect as a pattern can also be obtained. These fully defective sheets, and most of the tubes, present a very pronounced rippled (wavy) structure and their energies are lower than other structures based on pentagons and heptagons published so far. Another particularity of these rippled carbon sheets is their ability to fold themselves into a two-dimensional porous network of interconnected tubes upon heat treatment as shown by hybrid Monte Carlo simulations. Finally, contrary to the common belief that pentagon/heptagon based structures are metallic, this work shows that this defect pattern should give rise to semimetallic conduction. [ABSTRACT FROM AUTHOR]

We present an extension of some popular hindered rotor (HR) models, namely, the one-dimensional HR (1DHR) and the degenerated two-dimensional HR (d2DHR) models, allowing for a simple and accurate treatment of internal rotations. This extension, based on the use of a variable kinetic function in the Hamiltonian instead of a constant reduced moment of inertia, is extremely suitable in the case of rocking/wagging motions involved in dissociation or atom transfer reactions. The variable kinetic function is first introduced in the framework of a classical 1DHR model. Then, an effective temperature and potential dependent constant is proposed in the cases of quantum 1DHR and classical d2DHR models. These methods are finally applied to the atom transfer reaction SiCl3+BCl3→SiCl4+BCl2. We show, for this particular case, that a proper accounting of internal rotations greatly improves the accuracy of thermodynamic and kinetic predictions. Moreover, our results confirm (i) that using a suitably defined kinetic function appears to be very adapted to such problems; (ii) that the separability assumption of independent rotations seems justified; and (iii) that a quantum mechanical treatment is not a substantial improvement with respect to a classical one. [ABSTRACT FROM AUTHOR]

Classical nucleation theory pictures the homogeneous nucleation of a crystal as the formation of a spherical crystalline embryo, possessing the properties of the macroscopic crystal, inside a parent supercooled liquid. In this work we study crystal nucleation in moderately supercooled sulfur hexafluoride by umbrella sampling simulations. The nucleation free energy evolves from 5.2kBT at T=170 K to 39.1kBT at T=195 K. The corresponding critical nucleus size ranges from 40 molecules at T=170 K to 266 molecules at T=195 K. Both nucleation free energy and critical nucleus size are shown to evolve with temperature according to the equations derived from the classical nucleation theory. Inspecting the obtained nuclei we show, however, that they present quite anisotropic shapes in opposition to the spherical assumption of the theory. Moreover, even though the critical nuclei possess the structure of the stable bcc plastic phase, the only mechanically stable crystal phase for SF6 in the temperature range investigated, they are shown to be less ordered than the corresponding macroscopic crystal. Their crystalline order is nevertheless shown to increase regularly with their size. This is confirmed by a study of a nucleus growth from a critical size to a size of the order of 104 molecules. Similarly to the fact that it does not affect the temperature dependence of the nucleation free energy and of the critical nucleus size, the ordering of the nucleus with size does not affect the growth rate of the nucleus. [ABSTRACT FROM AUTHOR]

We report on a molecular simulation study of the homogeneous nucleation of CO2 in the supercooled liquid at low pressure (P=5 MPa) and for degrees of supercooling ranging from 32% to 60%. In all cases, regardless of the degree of supercooling, the structure of the crystal nuclei is that of the Pa3 phase, the thermodynamically stable phase. For the more moderate degree of supercooling of 32%, the nucleation is an activated process and requires a method to sample states of high free energy. In this work, we apply a series of bias potentials, which promote the ordering of the centers of mass of the molecules and allow us to gradually grow crystal nuclei. The reliability of the results so obtained is assessed by studying the evolution of the nuclei in the absence of any bias potential, and by determining their probability of growth. We estimate that the size of the critical nucleus, for which the probability of growth is 0.5, is ≈240 molecules. Throughout the nucleation process, the crystal nuclei clearly exhibit a Pa3 structure, in apparent contradiction with Ostwald’s rule of stages. The other polymorphs have a much larger free energy. This makes their formation highly unlikely and accounts for the fact that the nucleation of CO2 proceeds directly in the stable Pa3 structure. [ABSTRACT FROM AUTHOR]

We report on a computer simulation study of the early stages of the crystallization of molecular nitrogen. First, we study how homogeneous nucleation takes place in supercooled liquid N2 for a moderate degree of supercooling. Using the umbrella sampling technique, we determine the free energy barrier of formation for a critical nucleus of N2. We show that, in accord with Ostwald’s rule of stages, the structure of the critical nucleus is predominantly that of a metastable polymorph (α-N2 for the state point investigated). We then monitor the evolution of several critical nuclei through a series of unbiased molecular dynamics trajectories. The growth of N2 crystallites is accompanied by a structural evolution toward the stable polymorph β-N2. The microscopic mechanism underlying this evolution qualitatively differs from that reported previously. We do not observe any dissolution or reorganization of the α-like core of the nucleus. On the contrary, we show that α-like and β-like blocks coexist in postcritical nuclei. We relate the structural evolution to a greater adsorption rate of β-like molecules on the surface and show that this transition actually starts well within the precritical regime. We also carefully investigate the effect of the system size on the height of the free energy barrier of nucleation and on the structure and size of the critical nucleus. [ABSTRACT FROM AUTHOR]

Despite its relevance in many high temperature processes, the elastic behavior of single crystalline graphite is so far entirely undocumented away from room temperature conditions. In this work we present a molecular dynamics investigation of graphite's second order elastic tensor dependence on temperature from 300 to 4000 K, using a series of popular interatomic potentials for carbon. Data and analytic expressions are presented for the elastic tensor under the two limiting situations known as isothermal, or quasi-static, and adiabatic deformation conditions, prevailing in the limits of slow and large deformation rates, respectively. Independently from the potential, we identify a strong non-linearity of elastic constants with respect to temperature. We show that despite conserving an important elastic anisotropy whatever the temperature, the latter is being reduced by a factor of ∼ 5 when increasing temperature up to 4000 K. Also, we show that elastic anisotropy is about 20 times larger under isothermal conditions than under adiabatic conditions. Finally, we investigate the dynamics of the adiabatic to isothermal stress relaxations occurring right after ultra fast deformations such as those encountered under shock loading, showing that the relevant time and length scales are of the order of a few ps and nm, respectively, for deformations around a percent. [Display omitted] [ABSTRACT FROM AUTHOR]

An amorphous boron carbide ceramicis prepared via hot wall chemicalvapor deposition at 1000 °C using a BCl3/CH4/H2mixture. Its elemental composition is assessed byelectron probe microanalysis (EPMA) and its structure studied by Ramanspectroscopy, transmission electron microscopy (TEM), both X-ray diffraction(XRD) and neutron diffraction, 11B magic angle spinningnuclear magnetic resonance (MAS NMR), X-ray absorption spectroscopy(XAS), and ab initio modeling. The atomic structure factor and pairdistribution function derived from neutron diffraction data are comparedto those deduced from an atomistic model obtained by a liquid quenchab initio molecular dynamics simulation. The good agreement betweenexperimental data and simulation shows that the as-prepared materialis essentially made of a random arrangement of icosahedra (B12, B11C, and B10C2) embedded in anamorphous matrix rich in trigonal (BC3or BC2B) and tetrahedral (CB4) sites. The existence of trigonal boron environmentsis clearly confirmed by a peak at 50 ppm in both the experimentaland simulated 11B MAS NMR spectra, as well as a 190.0 eVcomponent in the XANES-B(1s) spectrum. The intericosahedral linearC–B–C chains observed in crystalline B4Care absent in the as-processed material. Free hexagonal carbon andB4C crystallites appear in the ceramic when heat-treatedat 1300 °C/2 h/Ar, as evidenced by high-resolution TEM and Ramanspectroscopy. Comparing the pair distribution functions of the heat-treatedmaterial with the crystalline B4C model allows confirmingthe apparition of C–B–C chains in the material. Indeed,two new peaks located at 1.42 and 2.35 Å can only be attributedto a first-neighbor distance between the B and C atoms in the chainand a second-neighbor distance between a chain-boron atom and an icosahedron-boronatom, respectively. [ABSTRACT FROM AUTHOR]

Abstract: We present a microstructural investigation of two different pyrocarbons (PyCs), belonging respectively to the rough laminar (RL) and smooth laminar (SL) families. The structure of the materials is analyzed in terms of their pair distribution functions (PDFs) as determined from neutron diffraction experiments. The data are correlated with polarized light optical microscopy, high resolution transmission electron microscopy, Raman microspectrometry, X-ray diffraction, He pycnometry, elastic recoil detection analysis and secondary ion mass spectrometry. Results show that the two materials may be clearly distinguished using the PDF data above a 15Å distance. The PDF of the more ordered carbon (the RL PyC) is also compared to data obtained from an image-guided atomistic reconstruction, showing excellent agreement. [Copyright &y& Elsevier]

We report on a theoretical study of the gas-phase decomposition of boron trichloride in the presence of hydrogen radicals using ab initio energetic calculations coupled to TST, RRKM, and VTST-VRC kinetic calculations. In particular, we present an addition-elimination mechanism (BCI3 + H → BHCl2 + Cl) allowing for a much more rapid consumption of BCI3 than the direct abstraction reaction (BCI3 + H → BCl2 + HCl) considered up to now. At low temperatures, T ≤ 800 K, our results show that a weakly stabilized complex BHCl3 is formed with a kinetic law compatible with the consumption rate measured in the former experiments. At higher temperatures, this complex is not stable and then easily eliminates a chlorine atom. Our work also shows that a very similar mechanism, involving the same intermediate and sharing the same transition state, allows for the elimination of HCl. A dividing coefficient between these two elimination pathways is obtained from both a potential energy surface based statistical analysis and an ab initio molecular dynamics transition path sampling simulation. It finally allows partitioning of the global consumption rate of BCI3 in terms of the formation of (i) BHCI3, (ii) BHCI2 + Cl through a H addition/Cl elimination mechanism, (iii) BCl2 + HCl through a H addition/ HCl elimination mechanism, and (iv) BCl2 + HCl through direct abstraction. [ABSTRACT FROM AUTHOR]

A method for the generation of atomistic models of dense nanotextured carbons is presented. This method is based on the statistical analysis of high resolution transmission electron microscopy images and their three-dimensional (3D) extension through image synthesis under constraint. The resulting 3D images then serve as an external potential bringing the atoms to settle preferentially on the black areas during a conventional simulated annealing simulation. Application of this method to the case of two laminar pyrocarbons, differing in their degree of disorder, highlights the promising nature of this approach. [ABSTRACT FROM AUTHOR]

The self-referential (SR) method incorporating thermodynamic integration (TI) [Sweatman et al., J. Chem. Phys. 128, 064102 (2008)] is extended to treat systems of rigid linear bodies. The method is then applied to obtain the canonical ensemble Helmholtz free energy of the α-N2 and plastic face centered cubic phases of systems of hard and Lennard-Jones dumbbells using Monte Carlo simulations. Generally good agreement with reference literature data is obtained, which indicates that the SR-TI method is potentially very general and robust. [ABSTRACT FROM AUTHOR]

The self-referential method [M. B. Sweatman, Phys. Rev. E 72, 016711 (2005)] for calculating the free energy of crystalline solids via molecular simulation is combined with thermodynamic integration to produce a technique that is convenient and efficient. Results are presented for the chemical potential of hard sphere and Lennard-Jones face centered cubic crystals that agree well with this previous work. For the small system sizes studied, this technique is about 100 times more efficient than the parameter hopping technique used previously. [ABSTRACT FROM AUTHOR]

This article focuses on polymorphism. Despite many experimental and theoretical studies, it still remains impossible to predict which crystalline variety or polymorph will form upon cooling a liquid or a solution. This issue is of both fundamental and technological interest. Since polymorphs may have very different physical properties (e.g., solubility in water), it is of great importance, e.g., for the pharmaceutical industry to know which polymorph of a drug molecule will be obtained. In this work, researchers show that the crystallization of nitrogen takes place in accord with the empirical Ostwald's rule of stages.

Reactive molecular dynamics (MD) simulations, especially those employing acceleration techniques, can provide useful insights on the mechanism underlying the transformation of buried organic matter, yet, so far, it remains extremely difficult to predict the time scales associated with these processes at moderate temperatures (i.e., when such time scales are considerably larger than those accessible to MD). We propose here an accelerated method based on flux sampling and kinetic integration along a 1D order parameter that can considerably extend the accessible time scales. We demonstrate the utility of this technique in an application to the dehydration of crystalline cellulose at temperatures ranging from 1900 K to 1500 K. The full decomposition is obtained at all temperatures apart from T = 1500 K, showing the same distribution of the main volatiles (H2O, CO, and CO2) as recently obtained using replica exchange molecular dynamics. The kinetics of the process is well fitted with an Arrhenius law with Ea = 93 kcal/mol and k0 = 9 × 1019 s−1, which are somehow larger than experimental reports. Unexpectedly, the process seems to considerably slow down at lower temperatures, severely departing from the Arrhenius regime, probably because of an inadequate choice of the order parameter. Nevertheless, we show that the proposed method allows considerable time sampling at low temperatures compared to conventional MD. [ABSTRACT FROM AUTHOR]

The elastic properties of graphene as described by the reactive empirical bond order potential are studied through uniaxial tensile tests calculations at both zero temperature, with a conjugate gradient approach, and room temperature, with molecular dynamics simulations. A perfect linear elastic behavior is observed at 0 K up to ≈0.1% strain. The Young’s modulus and Poisson’s ratio obtained with this potential are of ≈730 GPa and 0.39, respectively, with little chirality effects. These values differ significantly from former estimations, much closer to experimental values. We show that these former values have certainly been obtained by neglecting the effect of atomic relaxation, leading to a severe inaccuracy. At larger strains, an extended apparent linear domain is observed in the stress–strain curves, which is relevant to Young’s modulus calculations at finite temperature. Our molecular dynamics simulations at 300 K have allowed obtaining the following, chirality dependent, apparent Young’s moduli, 860 and 761 GPa, and Poisson’s ratios, 0.12 and 0.23, for armchair and zigzag loadings, respectively. [ABSTRACT FROM AUTHOR]

We report on the structure of carbon–ceramic interface models obtained by quenching a carbon liquid in between two identical crystalline surfaces of aluminium nitride usingab initiomolecular dynamics simulations. Two kinds of neutral and non-polar surfaces [and] were studied. The models obtained in both cases show typical disordered graphitic carbon domains with well-defined anisotropic nanotextures, while a bulk carbon system quenched in the same conditions leads to a nearly isotropic amorphous solid. The graphene layers of the nanocomposite withsurfaces develop parallel to the latter, showing weak interface bonding. On the contrary, they develop normally to thesurfaces, with many covalent bonds connecting each graphene layer to both surfaces. Finally, by looking at the mobility of carbon atoms during the quench simulations, we show that carbon actually gets bonded tosurfaces much before the temperature reaches its melting point. This indicates that the solidification of the carbon phase is strongly catalysed by the presence of surfaces. It does not happen whensurfaces are used. In this case, no clear distinction can be observed between the self-diffusivity coefficients of carbon in the nanocomposite and those in the bulk carbon system, whatever the temperature. [ABSTRACT FROM PUBLISHER]

This paper presents an ab initio study of the B/C/Cl/H gas phase mechanism, featuring 10 addition-elimination reactions involving BHiClj (i + j ≤ 3) species and a first description of the chemical interaction between the carbon-containing and boron-containing subsystems through the three reactions BCl3 + CH4 ⇌ BCl2CH4 + HCl, BHCl2 + CH4 ⇌ BCl2CH3 + H2, and BCl2 + CH4 ⇌ BHCl2 + CH3. A reaction mechanism is then proposed and used to perform some illustrative equilibrium and kinetic calculations in the context of chemical vapor deposition (CVD) of boron carbide. Our results show that the new addition-elimination reaction paths play a crucial role by lowering considerably the activation barrier with respect to previous theoretical evaluations; they also confirm that BCl2CH3 is an important species in the mechanism. [ABSTRACT FROM AUTHOR]

Abstract: The formation of the hydration shell of chloride ion at room temperature and at a polar stratosphere temperature has been studied by Monte Carlo simulations. Gibbs free energy, entropy and formation work of Cl−(H2O)n, n=1,…,23 clusters are calculated via the bicanonical ensemble method. Pair distribution functions are calculated in n=70,500 systems. A combined model has been applied: the interaction between the water molecules is described by ST2 potential (F.H. Stillinger, A. Rahman, J. Chem. Phys. 60 (1974) 1545), whereas the description of ion–water interaction is based on SPC model (H.J.C. Berendsen, J.P.M. Postma, W.F. van Gunsteren, J. Hermans, in: B. Pullman (Ed.) Intermolecular Forces, Reidel, Dordrecht, 1981, p. 331). The combined model that has been applied includes explicitly the polarization of the ion in the field of the molecules and the polarization of the molecules in the field of the ion. Ion–water model parameters have been fitted to reproduce the experimental values of free energies and entropies corresponding to the first water molecules association reactions to a cluster in vapour as well as pair distribution functions in bulk phase. The anion hydration shell is thermodynamically stable and remains so in supersaturated vapour. In polar stratosphere conditions, nearly all ions are hydrated by 7–10 water molecules. [Copyright &y& Elsevier]

Atomistic modeling of disordered yet textured carbons is notoriously difficult even though it can prove extremely helpful in rationalizing structure–property relationships for this class of materials. In this work we introduce a polygranular image-guided atomistic reconstruction method, which allows building models with fine-tuned values of the in-plane (L a) and out-of-plane (L c) coherence lengths, and of the orientation angle (OA). Applying a parametric study of grain size and orientation distribution, a database of 210 models is presented with parameters spanning domains characteristic of high and medium textured pyrolytic carbons: 1.5–8 nm, 2–5.5 nm and 25–110°, for L c , L a and OA, respectively. A machine learning model based on a random forest regression shows that these three measurable properties can be accurately predicted from a limited set of microscopic information characterizing the distribution of local atomic environments in the models. Finally, the computed diffraction properties and high-resolution transmission electron microscopy images of a series of six models, that best match the properties of a set of well-characterized pyrocarbons, are extensively compared to experimental data, showing excellent agreement and drastically improving over former modeling studies on high textured pyrocarbons, in addition to providing the first atomistic model of a medium textured pyrocarbon. [Display omitted] [ABSTRACT FROM AUTHOR]

We report on a successful attempt to introduce Al 2 O 3 and AlN nanoparticles in the matrix of a C/C composite via (i) aqueous synthesis of AlOOH nanoparticles in a predensified C/C composite preform, (ii) ceramization of AlOOH into either Al 2 O 3 or AlN, and (iii) final pyrocarbon densification. We show that we were able to synthesize a uniform and thin layer of high aspect ratio (nanoneedle) AlOOH particles at the surface of pyrocarbon coated fibers. This morphology was conserved upon transformation to alumina, however, coalescence into larger compact grains (thus discontinuous) was observed after nitridation. Nevertheless, the ceramic introduction did not alter the microstructure of the resulting composite after full densification. While most of the ceramic/carbon interfaces showed a parallel arrangement of graphene layers at the surface of the ceramic particles, characteristic of weak bonding, a carbon/AlN interface showed a perpendicular arrangement, suggesting strong covalent bonding. [ABSTRACT FROM AUTHOR]

Detailed knowledge regarding the nature of and mechanisms causing neutron irradiation damage in graphite remains a scientific and technological challenge, particularly at high irradiation doses. Using electrons as a surrogate for neutron irradiation, we develop a time-dependent atomistic reconstruction strategy fed by a time series of high-resolution transmission electron microscopy (HRTEM) images, to monitor damage propagation in a graphite grain up to a dose of about one displacement per atom ( i.e. well beyond the conventional irradiation simulations based on molecular dynamics). The reduction in crystalline order and the development of interlayer bonding observed in the models with increasing irradiation time induce significant modifications of the elastic constants and thermal conductivity. Homogenizing these properties to the case of isotropic polycrystalline graphite we are able to reproduce the increase in Young's modulus and decrease in thermal conductivity observed experimentally for reactor graphites with increasing dose. Further validation of the models is provided via a comparison of simulated and experimental data from irradiated material such as: HRTEM images, carbon K-edge electron energy loss spectra, dose rate and stored energies. [ABSTRACT FROM AUTHOR]

We report on an in-situ monitoring of graphite oxidation using a high temperature environmental scanning electron microscope. A morphological transition is clearly identified around 1040 K between hexagonal pits at low temperatures and circular pits at high temperatures, with apparently no change in the kinetic law. A kinetic Monte Carlo model allows rationalizing these findings in terms of the competitive oxidation of armchair and zig-zag edge sites and provides an estimate of the rate laws associated to these two events. Extended to three dimensions, the model also explains the 'in-depth' transition between the stepwise hexagons and the hemispheres observed by atomic force microscopy. [ABSTRACT FROM AUTHOR]