Showing papers on "Graphite published in 1988"

Empirical interatomic potential for carbon, with application to amorphous carbon

Abstract: An empirical interatomic potential is introduced, which gives a convenient and relatively accurate description of the structural properties and energetics of carbon, including elastic properties, phonons, polytypes, and defects and migration barriers in diamond and graphite. The potential is applied to study amorphous carbon formed in three different ways. Two resulting structures are similar to experimental $a\ensuremath{-}\mathrm{C}$, but another more diamondlike form has essentially identical energy. The liquid is also found to have unexpected properties.

Graphite Fibers and Filaments

Abstract: Contents: Introductory Material on Graphite Fibers and Filaments.- Synthesis of Graphite Fibers and Filaments.- Structure.- Lattice Properties.- Thermal Properties.- Mechanical Properties.- Electronic Structure.- Electronic and Magnetic Properties.- High Temperature Properties.- Intercalation of Graphite Fibers and Filaments.- Ion Implantation of Graphite Fibers and Filaments.- Applications of Graphite Fibers and Filaments.- References.- Subject Index.

Immobilized enzyme electrodes

Abstract: Enzyme electrodes are disclosed which are capable of responding amperometrically to the catalytic activity of the enzyme in the presence of its respective substrate and comprising the enzyme immobilized or adsorbed onto the surface of an electrically conductive support member which consists of or comprises a porous layer of resin-bonded carbon or graphite particles, said particles having intimately mixed therewith, or deposited or adsorbed onto the surface of the individual particles prior to bonding to form said layer, a finely divided platinum group metal, thereby to form a porous, substrate layer onto which said enzyme is adsorbed or immobilized and comprising a substantially heterogeneous layer of resin-bonded carbon or graphite particles, with said platinum group metal dispersed substantially uniformly throughout said layer. The preferred substrate materials are resin bonded platinized carbon paper electrodes comprising platinized carbon powder particles having colloidal platinum adsorbed on the surface of the particles and bonded onto a carbon paper substrate using a synthetic resin, preferably polytetrafluoroethylene, as the binder. The preferred enzyme electrodes are glucose oxidase electrodes comprising glucose oxidase adsorbed or immobilized onto the surface of the substrate.

Formation of highly orientated graphite from polyacrylonitrile by using a two-dimensional space between montmorillonite lamellae

Abstract: Carbon materials have many attractive physical and chemical properties, because of a wide variety of chemical bonding, crystal structure and microtexture. Carbon properties can be created by selecting a preparation procedure and a raw material. For example, many new carbon materials such as exfoliated graphite, benzenederived fibre and diamond film have come into use recently. The possibility of creating a new carbon material by a new method still remains. Here we report a novel method of preparing a highly orientated graphite from polyacrylonitrile (PAN) by making use of the interlamellar opening of montmorillonite (MONT) as a two-dimensional space for carbonization. A MONT-PAN intercalation compound was prepared and heat-treated at 700 °C to produce carbon from PAN between MONT lamellae. The carbon was then released from MONT by acid treatment and subjected to further heat treatment at various temperatures up to 2,800°C. The interplanar spacing (d002) of the carbon treated at 2,800 °C was very close to that of ideal graphite crystal and the crystallite size, Lc and La, were ~40 nm and >1μm, respectively. The formation of such a highly orientated carbon is probably a consequence of the orientation of the two-dimensional carbon precursor produced between the lamellae of MONT. When PAN itself is heat-treated, it produces a three-dimensional carbon with a network of intertwined ribbons of stacked graphitic sheets.

First-principles calculation of highly asymmetric structure in scanning-tunneling-microscopy images of graphite

Abstract: Using a first-principles calculation, we have computed the charge density for states near ${E}_{F}$ that is related to the current density observable in scanning-tunneling-microscopy experiments for surfaces of hexagonal, rhombohedral, and a model stage-1 intercalated graphite. In hexagonal and rhombohedral graphite, the tunneling current is predicted to be considerably smaller at surface atomic sites which have nearest neighbors directly below them than at sites with no such neighbors. This asymmetry is explained by the particular symmetry of the wave functions at the Fermi surface of graphite near K\ifmmode\bar\else\textasciimacron\fi{} in the surface Brillouin zone. The calculated asymmetry is nearly independent of the polarity and decreases with increasing magnitude of the bias voltage. Our results show that no asymmetry is expected on surfaces of stage-1 AA stacked intercalated graphite. The dependence of the asymmetry on the tip-to-surface separation has also been evaluated using a tight-binding model for different bias voltages. For the surface of hexagonal graphite, our predictions have been confirmed by recent experiments.

The analysis of the electrical conductivity of graphite conductivity of graphite powders during compaction

Abstract: The conductivity of four graphite powders was measured as a function of the volume fraction of graphite (or air) along (axial) and across (transverse) the direction of compression. The conductivity varies by between two and four orders of magnitude during these measurements. The results are shown to fit an equation which expresses the conductivity of the mixture as a function of the conductivity of the graphite and two morphology parameters. One parameters is the critical volume fraction, at which the graphite-air mixture would become insulating (percolation threshold) and the other an exponent is a combination of the effective demagnetisation coefficients of the grains and the critical volume fraction. Electron micrographs of the powder grains are shown and volume orientation percentage measurements of the graphite using X-rays were made in order to try to understand the results better.

Carbon doping in molecular beam epitaxy of GaAs from a heated graphite filament

Abstract: Carbon doping of GaAs grown by molecular beam epitaxy has been obtained for the first time by use of a heated graphite filament. Controlled carbon acceptor concentrations over the range of 10 to the 17th-10 to the 20th/cu cm were achieved by resistively heating a graphite filament with a direct current power supply. Capacitance-voltage, p/n junction and secondary-ion mass spectrometry measurements indicate that there is negligible diffusion of carbon during growth and with postgrowth rapid thermal annealing. Carbon was used for p-type doping in the base of Npn AlGaAs/GaAs heterojunction bipolar transistors. Current gains greater than 100 and near-ideal emitter heterojunctions were obtained in transistors with a carbon base doping of 1 x 10 to the 19th/cu cm. These preliminary results indicate that carbon doping from a solid graphite source may be an attractive substitute for beryllium, which is known to have a relatively high diffusion coefficient in GaAs.

An Aluminum/Chlorine Rechargeable Cell Employing a Room Temperature Molten Salt Electrolyte

Abstract: A novel rechargeable electrochemical cell is described which employs an Al negative and graphite positive electrode in a room temperature molten salt electrolyte of 1.5:1 . The graphite positive electrode functions as a reversible intercalation electrode for chlorine, eliminating the need for separate anolyte and catholyte compartments. The cell possesses an average discharge voltage of 1.7V for currents of 1–10 mA/g graphite and over 150 cycles at 100% depth‐of‐discharge for positive electrode limited cells have been demonstrated. Improvements in the chlorine storage capacity of the positive electrode are needed to obtain satisfactory energy densities.

Structure of mesophase pitch-based carbon fibres

Abstract: The structure of five samples of commercially available carbon fibres with ultra-high modulus produced from mesophase pitch was studied by the complementary techniques of high resolution electronmicroscopy, X-ray diffraction and transverse magnetoresistance effect. The fibres with high strength and elongation to failure were found to be composed of turbostratic carbon structure, which was different from the three-dimensional graphite structure in ultra-high modulus carbon fibres. Transmission electron microscope examination revealed that the mesophase pitch-based fibres with high strength have a basic structure unit with folded sheets arranged nearly parallel to the fibre axis similar to those of high modulus carbon fibres produced from PAN. The present fold structure was suggested to contribute consequently to the lower graphitizability of the fibres and to the strong effects on the fibre strength. By controlling the microstructure, it is expected that the crystallographic as well as the mechanical properties could be improved significantly even from the same kind of precursor materials such as mesophase pitch.

Selectivity in Cinnamaldehyde Hydrogenation of Group-VIII Metals Supported on Graphite and Carbon

Abstract: Noble metal catalysts (Ir, Pt, Ru, Rh Pd) have been prepared with a very homogeneous dispersion on graphite (300 2 m 1 g) and carbon support (1400 m 2 g 1 ). On graphite the particles decorate the edges of the basal planes and there is an electron transfer to the metal. The cinnamaldehyde (CAL) hydrogenation into cinnamyl alcohol (COL) has been performed under 4MPa of H 2 in liquid phase. The factors affecting the selectivity to COL are: (i) the nature of the metal (0=Pd

Reactive and nonreactive ion mixing of Ni films on carbon substrates

Abstract: The influence of nonreactive and reactive ion beam mixing on the interfacial chemistry, morphology, and adhesion of Ti films on glassy carbon, a‐face pyrolytic graphite, and c‐face pyrolytic graphite was examined and compared to thermal processing. Nonreactive ion beam mixing was performed with 180‐keV 84Kr+ at doses of 1 to 5×1016 Kr/cm2. Reactive ion beam mixing was performed with 150‐keV 28Si+ at doses of 1 to 5×1017 Si/cm2. Vacuum annealing was performed at 1000 °C for 1 h. Both 84Kr+ and 28Si+ implantation induced substantial intermixing between the Ti films and each of the carbon substrates. The 1000 °C anneal produced little interfacial reaction. Ion beam mixing produced adhesion increases of up to 1000× while no adhesion enhancement was observed following the anneal. Adhesion increases were attributed primarily to mechanical interlocking for both the 28Si+ and the 84Kr+ implantation. There were indications of ion induced film/substrate chemical bonding (Ti–C and Ti–O–Si‐C), but this bonding did not appear to greatly influence the observed adhesion. Ion induced topographical changes included void and bubble formation, cracking, erosion, and carbon aggregation. Topographical modifications were at a minimum on the glassy carbon specimens and at a maximum on the c‐face pyrolytic graphite specimens. Film characterization was performed using secondary ion mass spectrometry, Auger electron spectroscopy, x‐ray photoelectron spectroscopy, scanning electron microscopy with energy dispersive x‐ray analysis, and x‐ray diffraction. Adhesion was examined using a scratch test.

Thin films of molybdenum and tungsten disulphides by metal organic chemical vapour deposition

Abstract: Thin films of MoS2 and WS2 have been prepared on various substrates (glass, quartz, LiF, MgO, mica, molybdenum, gold, platinum, aluminium, copper, steel, graphite, MoSe2) by metal-organic chemical vapour deposition using sulphur or hydrogen sulphide and the hexacarbonyls of the transition metals as volatile components. The deposition technique is described here. The thin layers have been examined by X-ray powder diffraction and scanning electron microscopy. Time-resolved microwave conductivity measurements were used to detect the photoactivity of these materials.

The Synergistic Effects of Graphite on the Friction and Wear of MoS2 Films in Air

Abstract: Literature data are interpreted to formulate an original mechanism of MoS2 film degradation by tribooxidation and elucidate the synergistic effects of graphite in extending the wear life of burnished or bonded MoS2 layers. It is shown that the known but previously unexplained reduction in hexagonal platelet displacement in the direction of tangential shear and blister formation within the films are probably caused by oxidation. The molar volume reduction during the conversion of MoS2 into its stable oxide results in roughening of the interlaminar surfaces by oxidative etch-pitting at the slip steps and at the basal plane defect sites of MoS2 crystallites. It is proposed that interlocking of the pitted crystallites, combined with the formation of SO2 trapped and frictionally heated along with other gases in the coalesced free volume regions, is responsible for reduced shear, the formation of blisters and catastrophic delamination of the films. It is further suggested that the much slower oxidizing graphite...

Reactive and kinetic properties of carbon monoxide and carbon dioxide on a graphite surface

Abstract: Temperature-programmed desorption (TPD) results after chemisorption of carbon monoxide (CO) and carbon dioxide (CO,) on polycrystalline graphite are presented. CO adsorbs onto graphite with a very low sticking coefficient. After CO Chemisorption, CO (mass 28 amu) desorbs in two temperature regions, between 400 and 700 K and between 1000 and 1300 K, and COz (mass 44 amu) desorbs below 950 K. The intensity of the COz signal is less than 1 order of magnitude lower than the CO intensity. After C02 adsorption the major desorption product is CO at high temperatures (1000 < T (K) < 1300), whereas a small amount of COz desorbs around 450 K. The adsorption of a CI6Oz and C'*Oz mixture leads to a nearly total oxygen scrambling of the C02 desorbed. A mechanism for CO and COz interconversion on the graphite surface is presented in terms of surface oxide species, mainly lactones and semiquinones, and their relative stability. Assignments of the TPD features are proposed accordingly. Reaction studies on the C02 gasification of clean graphite and the CO disproportionation (Boudouard reaction) have been performed. A good agreement is found between the activation energies obtained and the desorption energies calculated from the analysis of the TPD results.

Magnetic recording medium

Abstract: PURPOSE:To obtain a magnetic recording medium which has excellent durability, good electrostatic charge characteristic and can be produced without increasing stages by incorporating 1-10ptswt fine non-flocculated graphite powder having <=1mum grain size into 100ptswt magnetic powder CONSTITUTION:The magnetic recording medium is produced by coating a magnetic coating compd which is prepd by replacing coating compd with the fine graphite powder to be dispersed in single particles without being flocculated, onto a substrate such as ordinarily used polyester film and drying the coating This fine graphite powder is required to disperse to the single particles, to have <=1mum grain size and to be incorporated at 1-10ptswt per 100ptswt magnetic powder into the above-mentioned coating compd The magnetic characteristics change partially and the degraded performance as the recording medium and the deteriorated durability thereof are caused if the fine graphite powder is flocculated The magnetic recording medium obtd in such a manner is small in the coefft of friction and is excellent in the wear resistance and durability

Scanning tunneling microscopy of the local atomic structure of two-dimensional gold and silver islands on graphite.

Abstract: Small, two-dimensional islands of silver and gold formed in situ by evaporation onto graphite cleaved in ultrahigh vacuum are imaged atom by atom with a scanning tunneling microscope. These islands contain ordered regions of roughly 50 atoms in rectangular lattices, incommensurate with the substrate lattice, that are not close packed as in the bulk fcc structure. In one series of images, the shorter lattice spacing remained constant at 2.35 +- 0.15 A whereas the longer decreased from 4.05 +- 0.1 to 3.5 +- 0.1 A over a period of roughly 10 min.

Effect of testing conditions and microstructure on the sliding wear of graphite fibre/PEEK matrix composites

Abstract: The sliding friction and wear behaviour of unreinforced polyetheretherketone (PEEK) matrix and its unidirectional continuous and two-dimensional woven graphite fibre-reinforced composites were investigated. The operating wear mechanisms, as evinced by scanning electron microscopy of the worn surfaces, and the coefficients of friction and the wear rates changed considerably with the fibre reinforcement form and orientation. Sliding wear rates, on account of their extreme sensitivity to the microstructure of the interacting surfaces at the sliding interface, were found to be a function of not only the surface roughness, but also of the sliding time. Complex interactions arising due to the effects of the testing parameters such as fibre orientation, sliding velocity, contact pressure and interface temperature were characterized for the neat matrix and the two composite systems. The wear rates of the two-dimensional woven composites were almost an order of magnitude lower than those of the unidirectional fibre composite or the unreinforced matrix.

Structure and properties of highly crystallized graphite films based on polyimide Kapton

Abstract: Highly crystallized graphite films were prepared by heat treatment of carbonized polyimide films (Kapton) at temperatures of 2700 and 3050° C. Interlayer spacing d002 at room temperature, and electrical resistivity, magnetoresistance and Hall coefficient at room and liquid nitrogen temperatures were measured. All of these data indicate high crystallinity of the graphitized Kapton films obtained. For the graphite films heat treated at 3050° C mean-square mobilities were estimated from the magnetoresistance data at 1 T to be 0.91 m2 V−1 sec−1 at room temperature and 2.3 m2 V−1 sec−1 at liquid nitrogen temperature; the value at liquid nitrogen temperature corresponds to that for a pyrolytic graphite heat treated at 3200° C for 1 h (PG 3200). Magnetic field dependence of Hall coefficient at liquid nitrogen temperature for this sample also agrees well with that for PG 3200. Scanning electron micrographs on the surfaces show that the present graphite films consist of grains of large crystallites, and grain size increases as the crystallinity of the material improves.

Electroconductive resin composition

Abstract: An electroconductive resin composition comprising (i) 100 parts by weight of thermoplastic resins and/or thermosetting resins and (ii) 15 to 150 parts by weight of (a) carbon black having a DPB oil absorption amount of 400 ml/100 g or more and (b) expanded graphite having an average particle size of 40 μm or more, wherein the expanded graphite content is 40 to 90% by weight of the total amount of the carbon black and the expanded graphite.

Chemical erosion of graphite by hydrogen impact: A summary of the database relevant to diamond film growth

Abstract: It is now established that diamond can be grown by vapor deposition, a condition where graphite is the more thermodynamically stable state. This supposed contradiction of thermodynamics is explained by the recognition that hydrogen is more likely to chemically erode graphite than diamond. In most diamond growth environments, hydrogen gas must be kept above a certain concentration (>95% of the mixture) to deposit films with nearly no graphitic component. This paper discusses the chemical interactions of hydrogen with graphite. The discussion is primarily based on the database collected from studies in the fusion research community. The studies cover interactions using both energetic and low‐energy atomic hydrogen. Kinetic models have also been developed which can explain reasonably well the observed interaction characteristics. The majority of the interaction studies used graphite as the target material for hydrogen; a few were also carried out with carbon and diamond films.