Graphitic Regions Research Articles

Nanographite domains in carbon nanowalls

The nanostructure of carbon nanowalls (CNWs), composed of a stack of several graphite sheets, was investigated by transmission electron microscopy. Our detailed analysis revealed that numerous graphite regions with an average size of about 20 nm were formed in the CNWs. The formation of these regions originates from the introduction of lattice defects such as dislocations and the slight rotation of the graphite sheets. On this basis, it is concluded that the graphite regions, so-called “nanographite domains,” are the constitutional units of the CNWs.

A Study on Electrical Resistivity Behaviors of PAN-based Carbon Nanofiber Webs

The influences of various carbonization temperatures on electrical resistivity and morphologies of polyacrylonitrile (PAN)-based nanofiber webs were studied. The diameter size distribution and morphologies of the nanofiber webs were observed by a scanning electron microscope. The electrical resistivity behaviors of the webs were evaluated by a volume resistivity tester. From the results, the volume resistivity of the carbon webs was ranged from <TEX>$5.1{\times}10^{-1}\;{\Omega}{\cdot}cm$</TEX> to <TEX>$3.0{\times}10^{-2}\;{\Omega}{\cdot}cm$</TEX>, and the average diameter of the fiber webs was varied in the range of 310 to 160 nm with increasing the carbonization temperature. These results could be explained that the graphitic region of carbon webs was formed after carbonization at high temperatures. And the amorphous structure of polymeric fiber webs was significantly changed to the graphitic crystalline, resulting in shrinking the size of fiber diameters.

Curved-Surface Atomic Modeling of Nanoporous Carbon

We present a methodology for modeling atomically layered nanoporous carbonaceous solids. Our method models carbon atoms interacting on sets of analytically defined surfaces, in conjunction with reverse Monte Carlo techniques (RMC). Our approach allows the construction of large, physically realistic nanoporous models of low-density carbonaceous solids that are both physical and statistically consistent with experimental observations. To demonstrate the flexibility of our approach and to investigate different mesostructures, we have modeled two nanoporous structures of glassy carbon. One model is comprised of multiple surfaces with the same center of symmetry, and the other is based on sets of parallel surfaces. The resulting models contain highly curved graphitic regions that also resemble the glassy carbon microstructure observed in high-resolution electron microscopy.

Dimensionally and thermally stable polymer, containing disordered graphitic structure and adamantane

In an attempt to develop a low-k interlayer dielectric, adamantane-diphenyldiethynyl moiety containing oligomer is prepared. Oligomerization of 1,3,5,7-tetrakis[3/4-ethynylphenyl]adamantane (4) is accomplished by a Glaser–Hay oxidative coupling with 1,3,5-triethynylbenzene and phenylacetylene end-capping agent. The CHCl3 soluble oligomer is then thermally treated by step-curing at 200, 300, 380, and 450 °C for 30 min at each temperature under nitrogen flow to render a shiny void-free black polymer. TGA analysis indicates that the polymer is stable under nitrogen up to 500 °C with a marginal decomposition up to 800 °C. Solid-state 13C NMR, Raman scattering, and FTIR are used to characterize the structure of the polymer. The polymer consists of amorphous carbon networks with the adamantane moieties and nanosized graphitic regions (clusters), which are generated from the thermal crosslinking of the diphenyldiethynyl units. It shows a remarkably low linear coefficient of thermal expansion (∼25 ppm/°C), presumably due to the presence of the disordered graphitic structure. Its high density (∼1.21 g/cm3), refractive index (∼1.80 at 632 nm), and Young's modulus (∼17.0 GPa) are also consistent with the interpretation. This study reveals important details about the effect of microscopic structure on the macroscopic properties of the highly crosslinked polymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6909–6925, 2006

Combined Hydrogen Production and Storage with Subsequent Carbon Crystallization

We provide evidence of low-temperature hydrogen evolution and possible hydrogen trapping in an anthracite coal derivative, formed via reactive ball milling with cyclohexene. No molecular hydrogen is added to the process. Raman-active molecular hydrogen vibrations are apparent in samples at atmospheric conditions (300 K, 1 bar) for samples prepared 1 year previously and stored in ambient air. Hydrogen evolves slowly at room temperature and is accelerated upon sample heating, with a first increase in hydrogen evolution occurring at approximately 60 degrees C. Subsequent chemical modification leads to the observation of crystalline carbons, including nanocrystalline diamond surrounded by graphene ribbons, other sp2-sp3 transition regions, purely graphitic regions, and a previously unidentified crystalline carbon form surrounded by amorphous carbon. The combined evidence for hydrogen trapping and carbon crystallization suggests hydrogen-induced crystallization of the amorphous carbon materials, as metastable hydrogenated carbons formed via the high-energy milling process rearrange into more thermodynamically stable carbon forms and molecular hydrogen.

HTTR環状炉心の炉心解析モデルの検討

It was shown from the annular core critical experiment of the HTTR that its excess reactivity values tend to be overestimated about 3%Δk/k by the HTTR core design method. Therefore, assessment of the calculation model for the annular core of the HTTR was systematically undertaken. The revised model was investigated which was based on the SRAC code system.It was concluded that the multiplication factor of the annular core is considerably affected by the three factors listed below:(1) The Benoist's anisotropic diffusion coefficients(2) The mesh interval in the whole core diffusion calculation(3) The mesh structure of graphite region in fuel lattice cells.The significantly large discrepancy previously reported was reduced down to 1.6%Δk/k by the revised annular core model.

Mathematical Model for Austenitization Kinetics of Ductile Iron

A mathematical model was developed in the current study to understand the progress of austenitization process in ductile irons. The austenitization time required to produce homogeneous austenite in a two-phase region of austenite and graphite has been estimated in terms of (a) time required for transformation of matrix to austenite and (b) time required for dissolution of graphite in austenite to attain uniform carbon content, which remains in equilibrium with graphite. The time required been related to the structural parameters of cast ductile iron-like radius of graphite nodule, radius of austenite cell, volume fraction of graphite, volume fraction of ferrite in cast matrix, and diffusion constant. The model was used to determine the minimum austenitization time required to achieve homogeneous austenite in three commercial ductile irons when austenitized at a temperature of 900 °C. The results were compared with those obtained. The uniformity of the carbon content in austenite of ductile iron was verified indirectly by measuring microhardness.

Effect of Expanded Graphite Lattice in Exfoliated Graphite Nanofibers on Hydrogen Storage

A graphite exfoliation technique, using intercalation of a concentrated sulfuric/nitric acid mixture followed by a thermal shock, has successfully exfoliated a herringbone graphite nanofiber (GNF). The exfoliated GNF retains the overall nanosized dimensions of the original GNF, with the exfoliation temperature determining the degree of induced defects, lattice expansion, and resulting microstructure. High-resolution transmission electron microscopy indicated that the fibers treated at an intermediate temperature of 700 degrees C for 2 min had dislocations in the graphitic structure and a 4% increase in graphitic lattice spacing to 3.5 A. The fibers treated at 1000 degrees C for 36 h were expanded along the fiber axis, with regular intervals of graphitic and amorphous regions ranging from 0.5 to >50 nm in width. The surface area of the starting material was increased from 47 m(2)/g to 67 m(2)/g for the 700- degrees C treatment and to 555 m(2)/g for the 1000- degrees C treatment. Hydrogen uptake measurements at 20 bar indicate that the overall hydrogen uptake and operative adsorption temperature are sensitive to the structural variations and graphitic spacing. The increased surface area after the 1000- degrees C treatment led to a 1.2% hydrogen uptake at 77 K and 20 bar, a 3-fold increase in hydrogen physisorption of the starting material. The uptake of the 700- degrees C-treated material had a 0.29% uptake at 300 K and 20 bar; although low, this was a 14-fold uptake over the starting material and higher than other commonly used pretreatment methods that were tested in parallel. These results suggest that selective exfoliation of a nanofiber is a means by which to control the relative binding energy of the hydrogen interaction with the carbon structure and thus vary the operative adsorption temperature.

Influence of the oxidation on the surface properties of silicon carbide

The effect of oxidation temperatures on the surface chemical properties of silicon carbide powders has been investigated. The various SiC powders have been firstly characterized using granulometry, X-ray diffraction, thermogravimetric analysis, diffuse reflectance infrared Fourier transform spectroscopy, 29Si and 13C MAS NMR spectroscopy. These materials seem to consist of a mixture of the 6H polytype, other polytypes β-SiC and some α-SiC. The experimental results suggest that the high-temperature treatment eliminates the graphitic region and leads to an enrichment in SiO x C y and especially SiO 2 units, without any important structural changes. Nitrogen and NH 3 gases adsorption data indicate that this conversion (from graphitic region to SiO x C y and SiO 2) gives rise to an important decrease of the specific surface area while the hydrophilic character of silicon carbide powders increases. The surface density of acidic sites on SiC treated at 350, 800 and 1000 °C represents respectively about 73, 79 and 91% of the total surface sites occupied by the adsorbing NH 3 molecules. These information concerning the dependence of both the hydrophilic–hydrophobic character and the extent of surface acidic sites with the oxidation conditions are confirmed by immersion calorimetry experiments. Finally the calorimetric data, especially the effect of the pH on the enthalpy of immersion of SiC into water, allow additional conclusions to be drawn concerning the dependence of the electrical double layer formation with pH.

Origin of the 2450 cm −1 Raman bands in HOPG, single-wall and double-wall carbon nanotubes

The second order Raman signals around the G′-band region of graphite and carbon nanotubes have been investigated at more than 15 excitation laser lines. Two distinct Raman bands have been observed around 2700 cm −1; a prominent one is due to the so-called G′-band and the other is a weak band around 2450 cm −1. Both two bands can be from the double resonance process involving two phonons around the K-point in the phonon dispersion of a two-dimensional graphite. The 2450 cm −1-band has exhibited little power dependence, whereas the intensity of G′-band has shown large photon energy dependence as already reported. The 2450 cm −1-band and the G′-band correspond to non-dispersive q = 0 and fully-dispersive q = 2 k, respectively. From the phonon dispersion and the corresponding phonon frequency, the 2450 cm −1-band can be assigned as an overtone mode of LO phonon (i.e. 2LO). This is revealed by calculated Raman spectra of graphite with proper electron–phonon matrix elements. The present study is the first report on the origin and assignment of the 2450 cm −1-band, which is based on the double resonance Raman scattering.

Interfaces between 4H-SiC and SiO2: Microstructure, nanochemistry, and near-interface traps

We report on electrical and microscopic investigations aimed to clarify the origin of near-interface traps (NITs) in metal–silicon dioxide–4H-silicon carbide structures. Using capacitance–voltage and thermal dielectric relaxation current (TDRC) analysis we investigated NITs close to the 4H-SiC conduction-band edge in differently prepared thermal and deposited oxides and found that the traps give rise to two characteristic TDRC signatures belonging to two groups of trap levels. The total trapped charge exceeds 1×1013cm−2. The observed density and energy distribution of these traps are nearly identical in all thermal and deposited oxides investigated, suggesting that the NITs belong to intrinsic defects at the SiO2∕SiC interface which are readily formed during oxide deposition or thermal oxidation of 4H-SiC. Using high-resolution electron microscopy combined with nanochemical analysis (electron energy-loss near-edge spectroscopy and energy-filtered transmission electron microscopy) we investigated the SiO2∕SiC interface in samples receiving reoxidation and did not find any indication of graphitic regions at or near the SiO2∕SiC interface or in the bulk silicon dioxide within a detection limit of 0.7nm. In addition, no amorphous carbon accumulation was observed near the SiO2∕SiC interface. The overall results strongly suggest that the NITs near the 4H-SiC conduction band are not related to carbon structures in the SiO2∕SiC interlayer.

Synthesis and characterization of carbon-enriched silicon oxycarbides

Polyhydridomethylsiloxane (PHMS) is a very practical and versatile source for a broad range of silicon oxycarbide (SiOC) and other silicon containing ceramic materials. PHMS modifications and crosslinking is conveniently achieved by dehydrocoupling and hydrosilylation reactions. The molecularly level mixed carbon content of the derived SiOC can be incrementally increased by reacting PHMS with 0–40 mol% of divinylbenzene (DVB) using 5 ppm of Pt catalyst relative to PHMS. With ceramic yields exceeding 80 wt.% and a dense microstructure, the derived carbon-enriched SiOCs exhibit significantly higher chemical stability compared with other high-carbon content polymer derived ceramics. Phase separation and nanocrystallinity are inhibited to at least 1200 °C for all the evaluated samples and the materials are completely amorphous. Very fine nanocomposite microstructures are obtained at 1450 °C, consisting of 5–10 nm graphite regions and amorphous SiOC domains with no significant porosity. Synthesis and microstructure characterization of the new carbon-enriched SiOC are discussed in comparison to the sole PHMS pyrolyzed product.

Anomalous Defects and Dynamic Failure of Armor Ceramics

The ballistic performance of state‐of‐the‐art silicon carbide armor material can exhibit a fairly wide variability in certain test configurations, which, it is proposed, may be due to the presence of large (&gt;0.1 mm), rare defects, termed, herein, “anomalous” defects. SiC rubble resulting from ballistic tests was examined, as were quasi‐static test samples. Ballistic fragment fracture surfaces revealed large carbonaceous defects that seemed to affect fracture path and mode. Low‐strength biaxial flexure samples demonstrated similar defects (&gt;0.1 mm) as failure origins. Carbonaceous defects similar in appearance but smaller in size were also found at the fracture origins of SiC bend bars. Frequently, alumina inclusions were found within the carbonaceous discontinuities. These alumina inclusions may cause the graphitic regions to form during sintering. The random distribution of such large, rare carbonaceous discontinuities from sample‐to‐sample, as well as batch‐to‐batch variability, may explain high ballistic variability for SiC armor ceramics.

Conducting fluorocarbon coatings for organic light-emitting diodes

Conducting fluorocarbon coatings (CFx) have recently been used as an anode buffer layer in organic light-emitting diode (OLEDs) for enhancement of stability and carrier injection. The effect of CFx on OLEDs was related to its conductivity. We found the resistivity of poorly conducting (about 1010 Ω cm) CFx coatings can be substantially decreased to 105 Ω cm by either near UV or Ar ion irradiation. OLEDs were prepared on untreated and treated CFx for performance comparison. The UV treatment of CFx improved the device performance, while the Ar ion treatment led to deterioration. X-ray photoelectron spectroscopy analysis showed the UV treatment created graphitic regions leading to the higher conductivity of the CFx layer but the underlying indium tin oxide (ITO) anode remained intact. In contrast, argon ion bombardment caused damage to the ITO anode which contributed to poor device performance, although the CFx conductivity was similarly increased.

Laser‐Induced Direct Lithography for Patterning of Carbon with sp3 and sp2 Hybridization

AbstractA new method of laser‐induced lithography for direct writing of carbon on a glass surface is described, in which deposition occurs from a transparent precursor solution. At the glass–solution interface where the laser spot is focused, a micro‐explosion process takes place, leading to the deposition of pure carbon on the glass surface. Transmission electron microscopy (TEM) analysis shows two distinct co‐existing phases. The dominant one shows a mottled morphology with diffraction typical of cubic (sp3) diamond. The other region shows an ordered array of graphene sheets with diffraction pattern typical of sp2‐bonded carbon. The sp3 crystallites range in size from 9 to 30 Å and are scattered randomly throughout the sample. A UV Raman spectrum shows a broad band at the location of the expected diamond peak, together with a peak corresponding to the graphite region. We conclude that the patterned carbon is composed of a mixture of nanocrystalline sp3 and sp2 carbon forms.

Inelastic neutron scattering in spectroscopic studies of hydrogen on carbon-supported catalysts-experimental spectra and computed spectra of model systems

Inelastic neutron scattering spectroscopy has been used to observe and characterise hydrogen on the carbon component of a Pt/C catalyst. INS provides the complete vibration spectrum of coronene, regarded as a molecular model of a graphite layer. The vibrational modes are assigned with the aid of ab initio density functional theory calculations and the INS spectra by the a-CLIMAX program. A spectrum for which the H modes of coronene have been computationally suppressed, a carbon-only coronene spectrum, is a better representation of the spectrum of a graphite layer than is coronene itself. Dihydrogen dosing of a Pt/C catalyst caused amplification of the surface modes of carbon, an effect described as H riding on carbon. From the enhancement of the low energy carbon modes (100–600 cm −1) it is concluded that spillover hydrogen becomes attached to dangling bonds at the edges of graphitic regions of the carbon support.

Field emission enhancement by graphitic nano-scale channels through ta-C layers

A key issue in field emission (FE) from CVD diamond and other carbon films is to understand the role that non-diamond inclusions and conduction pathways in the material have on the FE process. FE is significantly enhanced, apart from geometric enhancements due to rough surface morphologies, by the presence of non-diamond phases embedded in the diamond matrix. These phases appear in the form of grain-boundaries, graphitic inclusions, graphite nano-particles and others. It is, however, not clear in most reports, what the relative contribution of surface morphology and the presence of conductive channels to improved emission is. To investigate the possibility of FE originating from graphitic regions in a perfectly smooth material, we performed ion-irradiation to selectively transform diamond-like carbon (sp 3 bonding) into graphite like (sp 2 bonding) material. Single ion tracks were formed by implanting high-energy single ions at extremely low doses through a thin smooth amorphous tetrahedral–amorphous–carbon (ta-C) layer. The layers were thin enough for the ions to penetrate through them and stop in the substrate. N-doped conductive and un-doped ta-C films were used and the FE results were compared. Four ion-implantation schemes were used: (a) 1 GeV U ions at a low dose; (b) sub MeV Xe ions at a low dose; both were meant to form well separated single ion tracks through the material; (c) sub MeV C ions at a high dose, for complete material transformation; and (d) un-implanted sample as a control. Field emission turn-on conditions were carefully studied for these, employing a very slow voltage ramp-up, to avoid undesired vacuum electric discharges, known to result in a ‘conditioning’ affect on FE. Field emission seems to initiate at the lowest applied field for samples that include nano-scale graphitized channels threading all the way through the layer, as realized by single ion implantations. A 40% reduction in the field required for emission onset is found for all samples with single ion tracks compared to the un-implanted sample. However, the sample implanted for full coverage transformation shows no emission up to at least 100 V/μm. The observed improvement in FE is shown to be consistent with field enhancement calculations due to conductive pathways in the material.

Cathode effects on a relativistic magnetron driven by a microsecond e-beam accelerator

Experiments have been performed on a relativistic magnetron driven at e-beam accelerator peak parameters: voltage = -0.4 MV, current = 16 kA, and pulselength = 0.5 /spl mu/s. The magnetron is a six-vane device operating at about 1 GHz with extraction from two cavities. For equal power in both extraction waveguides, the peak microwave power of this device is between 200 and 300 MW. Microwave pulse-shortening limits pulselengths to the range of 10-100 ns. Time-frequency analysis of microwave emission indicates operation at about 1.03 GHz, close to the pi mode frequency identified from cold tests and the three-dimensional MAGIC code. Two cold cathodes were tested: 1) an emitting aluminum knob in the vane region with no endcap and 2) an extended cathode with a graphite fiber emission region in the vanes and endcap outside the vanes. Electron endloss current has been measured for the two cathodes. With no endcap, the cathode exhibited endloss current fraction up to 50% of the total; with one endcap, the cathode reduced the endloss current fraction to as little as 12%. Both cathodes produced peak total-electronic efficiency in the range of 14%-21%.

Effect of activation on the surface chemistry of carbons from polymer precursors

Carbonized chars prepared from polyacrylonitrile, polyethyleneterephthalate and cellulose were activated in a steam/nitrogen flow to ca. 50% burn-off (b.o.). The surface chemistry of these carbons has been characterized by X-ray photoelectron spectroscopy (XPS), adsorption from benzene–methanol binary liquid mixture, and potentiometric acid–base titration. The activation of the various polymer-based chars, in spite of the treatment similarity (temperature, burn-off) results in different physical and chemical surface properties. It has been confirmed by XPS analysis, that the elevated temperature of the activation not only promotes the relative extension of the graphitic regions by removal of noncarbon atoms and part of the amorphous carbon, but also helps on the further development of graphitic regions. The carbons contain both hydrophobic and hydrophilic sites on their surface already in their carbonized form, but the activated ones show a more hydrophobic character. The carbons exhibit an amphoteric surface, containing both acidic and basic surface groups. The titration curves show a hysteresis when performed upward and downward. Suspensions of the activated samples exhibit a basic pH. Both the increased hydrophobic property and the basicity can be attributed to extended graphitic regions. Part of the oxygen atoms forming basic functional groups may contribute to basicity, as well.

Reconstructions of 6H-SiC(0001) Surfaces Studied by Scanning Tunneling Microscopy and Reflection High-Energy Electron Diffraction

Surface reconstructions and surface decomposition of 6H-SiC(0001) covered with Si were observed using scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). The 3×3 structure terminated with Si atoms was obtained by annealing at 1050°C; this changed to a mixture of the graphite 1×1 and SiC 6×6 by annealing at 1300°C. The graphite 1×1 consisted of two types of graphite lattices, rotated 30° with respect to the SiC lattice and along the SiC lattice. The SiC 6×6 was terminated with C atoms. The RHEED spots due to the double diffraction between the SiC (02) spot and the graphite spot rotated by 30° with respect to the SiC lattice were observed. The thickness of the graphite layer and the area of the graphite region increased by successive annealing. String-shaped structures were formed along the steps at 1450°C. Further annealing up to 1800°C resulted in the formation of a thick graphite layer and amorphous carbon.