Face Of Graphite Research Articles

Facile synthesis of dense SiBCN coatings with excellent compactness, smoothness, and high-temperature inertness on defective graphite surfaces

SiBCN ceramics possess extreme high-temperature stability among engineering ceramics; however, their application is constrained by the difficulty in achieving large continuous products at low cost. This work exhibits the possibility of fabricating SiBCN continuous coatings on graphite substrates to improve their surface integrity, hardness, and inertness. The ceramic coatings were fabricated simply by spin coating, curing, and pyrolysis. The key innovations are the synthesis of the precursor, polyborosilazane (PBSZ), that derives continuous ceramic films after pyrolysis, and the finding that the xylene solution of PBSZ can achieve a very good spinning performance on various surfaces, especially the low-quality graphite surfaces with holes. The fabricated SiBCN coatings, characterized to be networks of Si–C, Si–N, C–C, and B–N bonds, possessed smooth and hard surfaces with roughness below 9 nm and a hardness of 7.3 GPa. The coating thickness can be well controlled in the range of 1.4 – 2.1 μm as the latter is linearly dependent on the concentration of PBSZ in the solution. Detailed observations of surface morphologies and physisorption experiments reveal that the coating surfaces are crack-free and highly dense, rendering a significant reduction in specific surface areas when a defective graphite face is coated.

Characterization and simulation of graphite edge surfaces for the analysis of carbonaceous material separation from sulfide ores by flotation

The efficient processing of base metal sulfide ores and sulfidic gold ores is sometimes compromised by the presence of carbonaceous material (CM). One common way of dealing with the CM problem is to pre-float the CM before the flotation of the sulfide minerals. Most CM exhibits some crypto crystalline graphitic structure. Although the graphite face surface has been studied extensively and is well described, the structural and surface properties of the graphite edge surface are more complex. In this study, the surface topography, graphitic structure, and functional features of the highly ordered pyrolytic graphite (HOPG) edge surface are reported to help provide a fundamental understanding of the behavior of CM in sulfide flotation separations. Surface structural defects, disordered atomic structure, and polar groups (C-O and C=O) were found from Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Raman, and X-ray Photoelectron Spectroscopy (XPS) analyses. The armchair and zigzag edge surfaces of graphite have been examined by ab-initio and Molecular Dynamics (MD) simulation to describe the experimental results, including wetting characteristics based on contact angle measurements. Polar oxygen species, such as C-O-C and C=O bonds, were identified during oxidation of the graphite edge surfaces by ab-initio simulation of the surface reaction with oxygen molecules. MD simulations indicated complete wetting of the simulated oxidized graphite edge surfaces by water, whereas previous experimental contact angle measurements at the HOPG edge surface (40°) revealed some hydrophobic character, which indicate the complex heterogenous nature of the edge surfaces.

Nanopore Surfaces Control the Shale Gas Adsorption via Roughness and Layer-Accumulated Adsorption Potential: A Molecular Dynamics Study

By performing molecular dynamic simulations, we study the methane adsorption on rough graphite surfaces prepared by rotating the graphite (0001) face with different tilt angles from 0° to 90°. It i...

Insights into adsorption, diffusion, and reactions of atomic nitrogen on a highly oriented pyrolytic graphite surface

To gain insight into the nitrogen-related gas-surface reaction dynamics on carbon-based thermal protection systems of hypersonic vehicles, we have investigated the adsorption, diffusion, and reactions of atomic nitrogen, N(4S), on the (0001) face of graphite using periodic density functional theory with a dispersion corrected functional. The atomic nitrogen is found to bind with pristine graphite at a bridge site, with a barrier of 0.88 eV for diffusing to an adjacent bridge site. Its adsorption energy at defect sites is significantly higher, while that between graphene layers is lower. The formation of N2 via Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms was also investigated. In the LH pathway, the recombinative desorption of N2 proceeds via a transition state with a relatively low barrier (0.53 eV). In addition, there is a metastable surface species, which is capable of trapping the nascent N2 at low surface temperatures as a result of the large energy disposal into the N-N vibration. The desorbed N2 is highly excited in both of its translational and vibrational degrees of freedom. The ER reaction is direct and fast, and it also leads to translationally and internally excited N2. Finally, the formation of CN from a defect site is calculated to be endoergic by 2.75 eV. These results are used to rationalize the results of recent molecular beam experiments.

Adsorption and Chromatographic Separation of Thiophene Derivatives on Graphitized Thermal Carbon Black

The thermodynamic characteristics of adsorption (TCA) of thiophene and its derivatives are determined under the conditions of equilibrium gas adsorption chromatography (GAC) on columns with graphitized thermal carbon black (GTCB) of Carbopack C HT grade. It is shown that the TCA values ​​ depend largely on the number and nature of the substituents in the main structural fragment. It is found that the surface of the GTCB is characterized by low structural selectivity for positional isomers in the series of thiophene derivatives. The effect of polar retention of an adsorbate on graphite from the gas phase due to additional intermolecular specific interactions of polar groups of the adsorbate with an easily polarizable surface of the base face of graphite is established and studied. Conclusions are reached on the applicability of the two-dimensional ideal gas model for describing the mobility of strongly polar molecules of thiophene derivatives on surfaces of graphite, and on the considerable limitations of this model when applied to the adsorption of thiophene molecules, along with its methyl and halogen derivatives.

Surface morphology and structural types of natural impact apographitic diamonds

External and internal morphologies of natural impact apographitic diamonds (paramorphoses) have been studied. The (0001) surface morphology of the paramorphoses reflects their phase composition and the structural relationship of its constituting phases. Growth and etch figures together with the elements of crystal symmetry of lonsdaleite and diamond are developed on these surfaces. The crystal size of lonsdaleite is up to 100 nm, and that of diamond is up to 300 nm. Two types of structural relations between graphite, lonsdaleite, and diamond in the paramorphoses are observed: the first type (black, black-gray, colorless and yellowish paramorphoses): the (0001) graphite face is parallel to the (100) lonsdaleite face and parallel to (111) diamond; the second type (milky-white paramorphoses): the (0001) graphite is parallel to the (100) lonsdaleite and parallel to the (112) diamond. The first type of the paramorphoses contains lonsdaleite, diamond, graphite or diamond, lonsdaleite, the second type of the paramorphoses contains predominantly diamond. The direct phase transition of graphite → lonsdaleite and/or graphite →diamond occurred in the paramorphoses of the first type. A successive phase transition graphite → lonsdaleite → diamond was observed in the paramorphoses of the second type. The structure of the paramorphoses of this type shows characteristic features of recrystallization.

Effect of the carbon nanomaterials structure on silica carbothermal reduction

“Carbon nanomaterial-SiO2” composites were synthesized using carbon nanofibers (CNF) and carbon nanotubes (CNT). Oligomethylhydridesiloxane (OMHS) was used as the SiO2 precursor. The CNT–SiO2 composite was shown to be thermally stable up to 1100–1200°C. The calcination temperature increase to 1300°C resulted in segregation of the CNT–SiO2 composite into individual components: CNT and SiO2 particles. In distinction to CNT+SiO2, the CNF–SiO2 system showed stronger interaction of the amorphous silica film with the end faces of graphite terminating at the external surface of CNF. Calcination of the CNF–SiO2 composite in an inert atmosphere at 1300°C for 1h did not lead to segregation of the components. Calcination of the CNF–SiO2 composite in an argon flow at 1400°C for 1h changed the phase composition of the sample. Silicon diffused to the interplane layers of graphite. In addition, the formation of silicon carbide was observed.

High-Current ESD Test of Advanced Triple Junction Solar Array Coupon

Testing was conducted on an advanced triple junction (ATJ) coupon that was part of a risk reduction effort in the development of a high-powered solar array design by Space Systems/Loral, LLC (SSL). The ATJ coupon was a small, four-cell, two-string configuration that has served as the basic test coupon design used in previous SSL environmental aging campaigns. The coupon has many attributes of the flight design; e.g., substrate structure with graphite face sheets, integrated by-pass diodes, cell interconnects, room-temperature vulcanizing grout, and wire routing. The objective of this test was to evaluate the performance of the coupon after it is subjected to secondary arc testing at two string voltages (100 and 150 V) and four array currents (1.650, 2.000, 2.475, and 3.300 A). An external test circuit, unique to SSL solar array design, was built that simulates the effect of missing cells and strings in a full solar panel with special primary arc (PA) flashover circuitry. A total of 73 PAs were obtained that included seven temporary sustained arcs (TSAs) events. The durations of the TSAs ranged from $50~\mu \text{s}$ to 2.75 ms. All TSAs occurred at a string voltage of 150 V. Post-test large area pulsed solar simulator, Dark $I$ – $V$ , and by-pass diode tests showed that no degradation occurred due to the TSA events. In addition, the post-test insulation resistance measured was $>50~\text{G}\Omega $ between cells and substrate. These test results point toward a robust design for application to a high-current, high-power mission.

The Nature of Graphene Surfaces As Determined from the Wettability Studies of Basal and Edge Planes

Electrochemical studies of two-atom thick graphene sheet have attracted significant attention from the research community due to the attractive properties of its basal and edge surfaces. The graphene edge surface contains dangling bonds, unsaturated valencies, and can favorably attach to a plethora of functional groups. It has been demonstrated that the graphene edge surface has several orders of magnitude higher specific capacitance, rapid electron transfer rates and much stronger electrocatalytic activity than those of graphene basal surfaces. The graphene basal plane has been known to possess a high conductivity. Coupling these properties of the graphene basal and edge surfaces presents graphene or high quality single crystal graphite as potential electronic material. But the utilization of this versatile material in aquatic conditions is still limited due to unknown key properties of the basal and edge surfaces such as interaction of these surfaces with water and the isoelectric point. In this paper, we have tried to throw some light on the wettability characteristics of graphite basal and edge surfaces using carbon nanoparticles (fullerene and fullerol) and other characterization techniques. Highly oriented pyrolytic graphite (HOPG) is used as a model surface for fundamental studies of the wetting characteristics of graphite surfaces. HOPG is a highly anisotropic material, exhibiting different properties on its face and edge surfaces. The essential aim of this work was to provide surface chemistry information for HOPG surfaces. Fundamental wetting studies were accomplished through contact angle measurements of the as-received HOPG single crystal, which was characterized by its distinct face and edge surfaces. To study the effect of functional groups, contact angle measurements were repeated on the HOPG face and edge surfaces oxidized with different concentrations of hydrogen peroxide. Further, Raman spectroscopic studies were performed on the as-received and oxidized HOPG basal and edge surfaces. Atomic force microscopy (AFM) was also used a tool to image the HOPG surfaces and for surface force measurements to determine the iso-electric point (IEP) of the face and edge surface of HOPG. Zeta potential measurements of graphite powder in solutions of varying pH were also done to determine its iso-electric point and compare to the values reported in literature for graphitic carbon materials. Further AFM was used to study the interaction of the fullerene and fullerol nanoparticles on the graphite face and edge surfaces. The zeta potential measurements of the above mentioned carbon nanoparticles were also carried out to confirm that the interaction was due to hydrophobic or hydrophilic forces and not due to electrostatic interactions. In addition molecular dynamics simulations were also carried out to verify the interactions of the carbon nanoparticles with the graphite surfaces. Results indicate interesting charge properties of the graphite basal and edge surfaces and that the basal surface is highly hydrophobic whereas the edge is less hydrophobic (not completely hydrophilic), which makes it a promising electronic material for aquatic use.

Molecular dynamics simulation of copper nano-cluster melting on the graphite substrate

The melting processes of different-sized copper nano-clusters supported on graphite (0001) plane are investigated by the molecular dynamics method. In this work, the melting point is predicted through the caloric curve. The simulation results show that the melting point of the supported copper nano-cluster is higher than that of the isolated one with the same Cu atoms. In the heating process, the copper nano-particle will adhere to the (0001) face of graphite with its (111) face. Pair analysis results show that the copper atoms close to the graphite can keep with order arrangement even when the temperature is higher than the melt point of the isolated nano-cluster.

Adsorption and separation of isomeric methyl- and dimethylaminoadamantanes on graphitized thermal carbon black

The thermodynamic characteristics of adsorption (TCA) of isomeric molecules of methyl- and dimethylaminoadamantanes on the surface of the basis face of graphite were determined experimentally and calculated by the molecular statistical method. A relationship between the geometric structure of adsorbate molecules and the values of their TCA on graphitized thermal carbon black was established. The data obtained were used for the gas chromatographic identification of the amination products of a mixture of Z,E-isomers of 1,4-dimethyladamantane and 1,3-dimethyladamantane.

Molecular statistical calculation of thermodynamic characteristics of adsorption of polymantane molecules on the basis face of graphite

The thermodynamic characteristics of adsorption (TCA) of polymantane molecules and their some derivatives on the basis face of graphite were calculated for the first time in terms of the atom-atomic approximation of the semiempirical molecular statistical theory of adsorption. The graphite surface exhibits high structural selectivity to isomeric polymantanes. A model for adsorption of cage molecules on the planar surface was proposed. The model is based on the idea that contributions of atoms of the adsorbate molecule to the total adsorption energy can be discriminated according to the distance of these atoms from the surface of a solid. Advantages and limitations of using the data on adsorption of molecules of isomeric alkanes, including rotamers, for the analysis of equilibrium TCA values of isomeric polymantanes on graphite are discussed. The possibility of separation of representatives of the polymantane hydrocarbon family by gas adsorption chromatography on columns with graphitized thermal carbon black was suggested on the basis of the calculated TCA values and logarithmic retention indices.

Adsorption of haloadamantanes on the basal face of graphite

The graphite surface exhibits different structural selectivity to isomeric haloadamantanes depending on the kind and number of halogen atoms in the molecule. In the series of fluoro-, chloro-, bromo-, and iodoadamantanes, the selectivity of separation of the respective isomers on columns packed with graphitized thermal soot increases. The equilibrium adsorption characteristics of haloadamantanes (adsorption equilibrium constants, heats and entropies of adsorption), calculated within the framework of molecular-statistical theory, agree well with the experimental values and allow comprehensive description of the adsorbed state of these compounds on the graphite surface. The results of molecular-statistical calculations allow evaluating the possibility of haloadamantane separation on columns with graphitized thermal soot under the conditions of equilibrium gas-adsorption chromatography.

Features of the energy distribution of secondary particles upon the ion bombardment of graphite

The secondary emission of carbon atoms from the (0001) plane of graphite nanocrystallites bombarded with argon ions with energies of 1 and 10 keV and the incidence angle α = 45° is investigated. The unusual oscillating energy distributions of secondary C+ ions with main maxima E max in the range of 40–60 eV and peaks corresponding to the energies E 1 ≈ 20, 30, 70, 80, and 100 eV have been revealed. The C+ ion yield decreases, the energy spectrum increases, and the maximum E max shifts to larger energies E 1 with increasing emission angle (with respect to the normal to the surface). The secondary-ion emission from the (0001) face of graphite is numerically simulated with allowance for the charge exchange of secondary ions to obtain a qualitative explanation of the observed results.

Adsorption of nitrobenzene, aniline and nitroanilines on the surface of a basal face of graphite

Thermodynamic characteristics of the adsorption of nitrobenzene, aniline, and isomeric nitro-anilines are determined experimentally and from molecular statistics in the range of the closest packing of a graphite surface according to equilibrium gas adsorption chromatography. It is found that graphitized thermal carbon black is characterized by high selectivity upon the chromatographic separation of isomeric nitro-anilines (retention increases in the series of ortho, meta, and para isomers). Using the atom-atomic approximation of the semi-empirical molecular statistical theory of adsorption, parameters are calculated for the potential function of the pair intermolecular interaction of the NO2 group in nitrobenzene and the N atom in aniline with the C atoms in a basal face of graphite. The effect of the NH2 and NO2 groups on the parameters of adsorption for isomeric nitroanilines as a result of direct polar conjugation is demonstrated and must be assumed when performing molecular statistical calculations.

Free radicals and graphite

The basal faces of crystalline graphite do not react with molecular oxygen at ambient or moderately raised temperatures. Free radicals, however, react with exposed graphene layers of graphites and carbon materials. Such reactions can be exploited for the functionalization of carbon surfaces. Disordered carbons, e.g. activated carbons, are slowly oxidized also at room temperature when water vapor is present. Surface oxides are formed, and some CO2 is gradually released in this aging reaction. It was found that nitrogen doping of carbons accelerates considerably their aging and increases also their catalytic activity in other oxidation reactions with O2. Evidence is presented that this activation of O2 molecules is due to transfer of electrons from the N-doped carbons to adsorbed O2. The electrons come from nitrogen atoms bound substitutionally in the graphene layers. Superoxide ions, O2-, are created that react with water producing hydroxyl radicals, OH

Adsorption of isomeric aryl- and diadamantane molecules on the surface of graphitized thermal carbon black

The thermodynamic characteristics of adsorption of isomeric molecules of 1,1′-, 1,2′-, and 2,2′-diadamantanes, 1- and 2-phenyladamantanes, and 1-cyclohexyladamantane on the surface of graphitized thermal carbon black (GTC) were experimentally determined. The influence of the molecular structure of adsorbates on their adsorption on the basis face of graphite was considered. A high selectivity of the GTC surface for the gas chromatographic separation of mixtures of the considered structural isomers was shown.

Thermodynamic characteristics of the adsorption of isomeric molecules of fluoroadamantanes on the surface of the basal face of graphite

The thermodynamic characteristics of adsorption (TCA) for isomeric molecules of mono-, di-, tri-, and tetrafluoroadamantanes were determined for the first time experimentally and by molecular statistics on the surface of graphitized thermal black (GTB). The parameters of atom-atom potential function of pair intermolecular interaction (φ(r)) for F atoms included in fluoroadamantanes with C atoms on the basal face of graphite were calculated for the first time within an atom-atom approximation of the semi-empirical molecular statistical theory of adsorption. The adsorption non-equivalence of F atoms in various positions of an adamantane framework, a consequence of the mutual effect of atoms in a framework molecule, was determined. On the basis of the definite TCA values, the conclusion was drawn as to the possibility of isolation of isomeric fluoroadamantanes under the conditions of gas-adsorption chromatography on GTB.

On the Influence of Surface Topography on the Electric Double Layer Structure and Differential Capacitance of Graphite/Ionic Liquid Interfaces

Molecular simulations reveal that the shape of differential capacitance (DC) versus the electrode potential can change qualitatively with the structure of the electrode surface. Whereas the atomically flat basal plane of graphite in contact with a room-temperature ionic liquid generates camel-shaped DC, the atomically corrugated prismatic face of graphite with the same electrolyte exhibits bell-shaped behavior and much larger DCs at low double-layer potentials. The observed bell-shaped and camel-shaped DC behavior was correlated with the structural changes occurring in the double layer as a function of applied potential. Therefore, the surface topography clearly influences DC behavior, suggesting that attention should be paid to the electrode surface topography characterization in the studies of DC to ensure reproducibility and unambiguous interpretation of experimental results. Furthermore, our results suggest that controlling the electrode roughness/structure could be a route to improving the energy den...

An experimental and molecular-statistical study of the adsorption of the iodobenzene, 2-iodothiophene, and isomeric iodoadamantane molecules on the graphite basal face surface

The thermodynamic characteristics of adsorption of iodobenzene, 2-iodothiophene, and 1- and 2-iodoadamantanes on the surface of graphitized thermal carbon black were determined experimentally. The influence of the special features of the molecular structure of the adsorbates on the thermodynamic characteristics of adsorption was studied. The atom-atom approximation of the semiempirical molecular-statistical theory of adsorption was used to calculate the thermodynamic characteristics of adsorption of the adsorbates using the newly determined potential function parameters of pair intermolecular interaction (φ(r)) of I with C atoms of the basal graphite face. For the example of isostructural monohalogenated benzenes, thiophenes, and adamantanes, a comparative analysis of the contributions of the F, Cl, Br, and I atoms to the thermodynamic characteristics of adsorption was performed for the nonspecific adsorption of these compounds on a plane graphitized carbon black surface.