Graphite Fragments Research Articles

Metal-containing carbon clusters

The results of injected-ion drift-tube studies of the structures and isomerization of metal-containing carbon clusters are described. Three basic types of geometry are observed: metal-containing rings, metal-containing graphitic fragments and metallofullerenes. The nature of these isomers depends on the electronic properties of the metal. While only a few metals and stoichiometries have been examined so far, it is possible to begin to identify periodic trends in both the structures and properties.

Structure and stability of molecular carbon: Importance of electron correlation.

The electronic structure of medium-size molecular carbon is determined by a variety of methods, including density functional, quantum chemistry, and quantum Monte Carlo approaches. We demonstrate (i) significant differences between the mean-field methods in determining the minimum energy structure of a set of ${\mathrm{C}}_{10}$ and ${\mathrm{C}}_{20}$ isomers, and (ii) the crucial importance of an accurate account of electron correlation which enables us to predict, e.g., that of the available geometries for ${\mathrm{C}}_{20}$ the relaxed graphite fragment (bowl) is lower in energy than either the ring or fullerene isomers.

Combined influence of an inert gas environment and a mechanical action on a graphite surface

The adsorption and reactivity properties of graphite wear fragments produced in a tribocontact depend on the gaseous environment surrounding the contact which conditions the type of wear (fatigue or abrasive wear) and the surface reactivity. The determination of the specific area of the wear fragments, using a nitrogen adsorption method (Brunauer, Emett and Teller method), permits to reach a description of the surface structure. We carried out on a pin on disc tribometer located in a vacuum chamber wear experiments on two samples of polycrystalline graphites, under different inert gaseous environments. We observed that the size of wear fragments (as well as the friction coefficient) mainly depends on the orientation of the crystallites. This mechanical phenomenon principally depends on the nature of the environment and on the normal load. Depending on these conditions, the powder can have different colorations, and its specific area shows variations from 140 m 2 g −1 to 285 m 2 g −1. Wear particles produced under inert environment present many active sites (edge sites) but their reactivity depends on their size and specific area. The most reactive can exhibit pyrophoric properties, when exposed to the ambient air.

Characterization of vapour plume species and deposition residues resulting from pulsed laser ablation of a graphite/epoxy composite

A modified time-of-flight mass spectrometer fitted with a special collection stage for carbon-coated transmission electron microscope specimen grids is used to monitor laser-pulse ablation products from graphite/epoxy composite targets. Scanning electron microscopy observations show ablation damage to consist of matrix pyrolysis, fibre fracture and spallation of fragments which include elemental hydrogen, carbon epoxide and acetylene groups. Transmission electron microscope examination of specimen grids showed a variety of crystals and polycrystalline hexagonal graphites having a wide range of shapes including spheres and faceted polyhedra and platelets, textured flake structures, and microrosettes. These observations lend some credibility to a model for laser-shock and pyrolysis effects which create molecular plume fragments and deposition fragments of hexagonal graphite.

Nitric acid oxidation of carbon fibers and the effects of subsequent treatment in refluxing aqueous NaOH

Nitric acid oxidation effectively created acidic functional groups on PAN-based high-strength carbon fibers. The acidic capacities of carbon fibers increased with oxidation time. Typically, 60 minutes of oxidation created 60 μeq acidic functional groups on each gram of carbon fibers. Nitric acid oxidation also caused tensile strength decreases and fiber weight losses. By treating fibers in refluxing aqueous NaOH after nitric acid oxidation, a weakly bound layer of partially oxidized graphitic fragments was removed and this caused higher weight losses. This treatment also removed a source of interference in later analyses. The effects of aqueous NaOH treatment after oxidation on the mechanical properties (tensile strength, Izod IS, IFSS, and ILSS) of fibers and their composites were also investigated.

Single- and double-ionization potentials of polycyclic aromatic hydrocarbons and fullerenes by photon and electron impact

Single- and double-ionization appearance potentials of 21 polycyclic aromatic hydrocarbons (PAHs) were determined by measurement of photoion yield curves using a synchrotron radiation source. Analogous measurements were made by electron impact on three PAHs. The origin of differences, of up to 4 eV, between appearance potentials for double ionization by photon and electron impact is discussed. The ratio of the double- to single-ionization potentials of PAHs was found to be 2.65 ± 0.06 for photon impact, significantly lower than for electron-impact and charge-stripping measurements. An electrostatic model, which considers that PAHs ionize like fragments of graphite, is applied to the PAHs studied and to data on the fullerenes C60 and C70. The validity and limitations of the model are discussed and its analytical and predictive propensities concerning multiple-ionization potentials and processes are demonstrated.

Small carbon rings: dissociation, isomerization, and a simple model based on strain

Injected ion drift tube techniques have been used to examine the properties of carbon rings with 10–36 atoms. Previous studies have shown that the monocyclic ring is the dominant isomer for annealed clusters with 10–36 atoms, while for unannealed clusters a bicyclic ring first appears at n ≈ 22 and becomes the dominant isomer for n > 30. A detailed study of the annealing of the bicyclic ring has been performed for clusters with 24–36 atoms. Activation energies for the bicyclic to monocyclic transition are around 2.4eV and show a slight systematic decrease with increasing cluster size. The fragmentation of carbon cluster ions containing 6–30 atoms has been studied and dissociation energies estimated. A simple model for the strain energy of the carbon rings accounts for many of their physical properties, including isomerization to monocyclic rings, their fragmentation patterns, and dissociation energies. From simple estimates of their stabilities, planar graphite fragments should be lower in energy than the monocyclic rings for medium sized clusters ( n > 30), but little, if any, of this isomer is observed even after annealing. The low abundance of the graphite fragments is attributed to the large activation energy (induced by strain energy) for their formation from monocyclic rings.

Parameter Optimization for the Arc-Adlc-Deposition Process

ABSTRACTOne method used by the industry for the deposition of thin carbon films is the dc cathodic arcion plating process. The evaporation of a graphite cathode by a high energy arc beam results in the production of carbon atoms, ions and macroscopic graphitic fragments. All these particles influence the film growth, the structure and therefore the film properties. The incorporation of the macroscopic graphitic fragments can be avoided by the use of mechanical or magnetic filters in front of the cathode. These filters prevent the direct bombardment of the substrates with particles. This particle separation involves decreased substrate current densities and this means lower deposition temperature (< 200°C) and growth rates. With the addition of inert (argon) and reactive gases (nitrogen and methane) and the use of a bias voltage on the substrate economical deposition rates of 2-4 μm/h and good adhesion on various materials can be reached.This paper describes the production of amorphous hydrogen and hydrogen-free carbon films by a modified cathodic arc evaporation process using a simple mechanical filter for particle reduction. It shows the influence of some process parameters on the optimization of the process stability and process efficiency. Some characteristic properties of these coatings are shown by an example.

On the way to fullerenes: molecular dynamics study of the curling and closure of graphitic ribbons

Sparked by the synthetic breakthrough of Kratschmer, Huffman, and co-workers yielding macroscopic amounts of research on the fullerenes (discovered 5 years earlier3) has continued to accelerate. Despite the pace of this research, the mechanisms leading to the facile formation of highly symmetric fullerene cages in the chaotic conditions of the carbon arc remain to be ~larified.~ In this Letter, following ideas advanced by Smalley, Curl, Kroto, and co-workers,eE we use molecular dynamics (MD) simulations to study the behavior of graphitic fragments thought to condense in the cooling carbon vapor prior to fullerene formation. These fragments are treated as ribbons as an idealization of their probable low-symmetry, far-from-circular shapes. At the conditions present in the carbon arc, we find that such ribbons can exhibit large thermal fluctuations from planarity without fragmentation. These fluctuations, when sup plemented by the curvature induced by pentagons, often cause these isolated strips spontaneously to form open-ended hollow carbon structures. Once formed, these hollow structures represent good fullerene precursors. Graphitic ribbons are modeled using a bond-order type empirical hydrocarbon potentiallo which accurately predicts the bonding and energetics of solid diamond lattices and graphite sheets, as well as hydrocarbon molecules, while still allowing reactions to occur. The reactive aspect of the potential is important both to model correctly the reactive edges of these ribbons and to allow for any possible fragmentation at high temperature. Although this potential has already been shown to provide a good model for a wide variety of properties of fullerenes and related structure~,~~-~~ to assess further its reliability for the present study we examined the energetics of the inversion of the corranulene molecule (Cal0). This molecule is similar in shape to one-third of a buckminsterfullerene cluster. Recent studies have shown that corranulene can rapidly fluctuate between two curved minima at room temperature14-a behavior that is related to the ribbon motions discussed below. First principles calculations yield an energy difference between the minimum-energy curved structures and a planar intermediate of 8.8-1 1.0 kcal/mol.l5J6 Our empirical potential predicts a difference of 10.3 kcal/mol, in excellent agreement with these calculations.

Characterization of composite carbon coatings deposited by dc cathodic arc technique

Carbon films deposited by a modified dc cathodic arc technique were characterized by several analytical methods. The coatings consist of two constituents originating from process characteristics of the technique applied. Small fragments of graphite are embedded in a matrix material of an amorphous structure with diamond-like short-range order. The coatings exhibit friction and wear reducing properties.

On the two modes of operation of monolithic Ag-C brushes

The previously observed transition from low-temperature (mode I) to high-temperature (mode II) behavior in silver-graphite brushes can be explained either by changes of constriction resistance through the temperature dependence of electrical resistivity and hardness at negligible film resistivity and one to three contact spots, or by loss of graphite lubrication within the interfacial film. These two interpretations were tested by controlling the contact spot temperature: (1) by heating in an oven and (2) by local heat input through friction. Correlated studies of contact resistance, friction, wear rate, wear chip size, wear chip microstructure, and calculations of contact spot temperatures favor the second hypothesis. Micrographic evidence suggests that this occurs through desorption of water vapor from the graphite. It is found that primary wear particles form due to the wedge mechanism and by cutting by graphite fragments steeply inclined to the interface. Silver fragments may consolidate into secondary wear particles.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Seizure resistance of cast aluminium alleys containing dispersed graphite particles of differing sizes

Seizure resistance of the cast graphite particle‐aluminium composite alloys, containing graphite particles of varying sizes has been studied using a Hohman wear tester. The size of the spheroidal graphite particles was varied from 30 µm to 400 µm, and in one case 80 µm size flake graphite was used to observe the effect of shape of graphite. When the graphite content of graphitic aluminium alloys is more than 2 per cent, these alloys can be self‐mated under condition of boundary lubrication without seizing. The size and shape of the graphite particles had no significant effect on the seizure resistance of these alloys, in the range of conditions investigated in this study. This is attributed to the extensive deformation and fragmentation of graphite due to the low yield strength of the aluminium matrix and the low flow stress of the graphite particles. During wear, the deforming aluminium matrix accentuates the deformation and fragmentation of subsurface graphite particles and causes them to come to the mating surface, thus providing continuous lubrication and preventing seizure. Even after a short run‐in period, a continuous layer of graphite is observed on the mating surfaces of graphite particle‐aluminium composite alloys. This layer persists even after extensive wear deformation.

Seizure resistance of cast aluminium alloys containing dispersed graphite particles of various sizes

The seizure resistance of cast graphite-aluminium composite alloys containing graphite particles of various sizes was studied using a Hohman wear tester. If the graphite content is more than 2% these alloys can be selfmated without seizure under conditions of boundary lubrication. The size and shape of the graphite particles had no significant effect on seizure resistance. Owing to the extensive deformation and fragmentation of graphite, the low yield strength of the aluminium matrix and the low flow stress of the graphite particles, a continuous layer of graphite is formed on the mating surfaces even after a short running-in period. This layer persisted even after extensive wear deformation.

Seizure Resistance of Cast Aluminum Alloys Containing Dispersed Graphite Particles of Different Sizes

Seizure resistance of the cast graphite particle-aluminum composite alloys, containing graphite particles of varying sizes has been studied using a Hohman wear tester. The size of the spheroidal graphite particles was varied from 30 μm to 400 μm, and in one case 80 μm size flake graphite was used to observe the effect of shape of graphite. When the graphite content of graphitic aluminum alloys is more than 2 percent, these alloys can be self-mated under condition of boundary lubrication without seizing. The size and shape of the graphite particles had no significant effect on the seizure resistance of these alloys, in the range of conditions investigated in this study. This is attributed to the extensive deformation and fragmentation of graphite due to the low yield strength of the aluminum matrix and the low flow stress of the graphite particles. During wear, the deforming aluminum matrix accentuates the deformation and fragmentation of subsurface graphite particles and causes them to come to the mating surface, thus providing continuous lubrication and preventing seizure. Even after a short run-in period, a continuous layer of graphite is observed on the mating surfaces of graphite particle-aluminum composite alloys. This layer persists even after extensive wear deformation.

Uranmineralisering i det vittrade underlaget till den underkambriska sandstenen vid Kalmarsund

Abstract In 1957 a radioactive mineralization was found just north of the village Mönsterås on the east coast of southern Sweden opposite the island of Öland. There the bedrock consts of Pre-Cambrian granite and porphyry covered by Cambrian sandstone. The radioactive minerals are situated in the weathered layer under the sandstone and they consist of black coalrich fragments assumed to be thucholite. In the same area breccia with galena, fluorite and calcite is found As the contents of lead in the thucholite bearing arkose is high too we suppose that there exists a genetical relationship between the two types of mineralization. At another place more to the south, similar mineralizations were found and from earlier deep borings at Öland natural gas with helium and graphitic fragments in the quartzite under the Cambrian sandstones have been described.

Capacitive Energy and the Ionization of Aromatic Hydrocarbons

Large polynuclear aromatic molecules ionize like fragments of graphite, and their ionization potentials approach the sum of the graphite work function vgr and the electrostatic work of charging a conductor the size and shape of the molecule. The capacitive work for a single charge is u=e2/2C, where C is the molecular capacitance. In the limit of large size, the work to remove n electrons is Wn=nvgr+n2u. This limiting law implies a relation among successive ionization potentials and the electron affinity, which is well satisfied by anthracene and phenanthrene, and moderately well by naphthalene and even benzene. For larger molecules, C, and therefore u, can be estimated classically by assuming the conducting region is bounded by the carbon skeleton, and computed by analog measurements on metallic models; results are tabulated for a series of aromatics. The relation between the limiting law and more detailed quantal formulations is discussed, and the magnitude and trend of some expected deviations are explored with the help of a simple free electron picture.

Notes on Ancient Boulder-clay of Natal

T here is largely developed in the Colony of Natal a formation which seems to be in all essential particulars identical with Mr. Bain’s claystone porphyry of the Cape of Good Hope. This formation flanks a long range of sandstone hills, which runs from the Tugela river frontier, at a distance of some six or eight miles from the sea, across the Umgeni river, and through the Berea hills to the mouth of the Umbilo. It also crops out extensively near Maritzburg, and stretches thence, in one direction, over the Umgeni and Umvoti to the Tugela valley between Greytown and the Biggersberg; and in the other direction over the Umlasi and Umkomasi towards the opposite frontier. In the latter course it goes onwards to the St. John’s river and to the further districts of the Cape. The deposit itself consists of a greyish-blue argillaceous matrix, containing fragments of granite, gneiss, graphite, quartzite, greenstone, and clay-slate. These imbedded fragments are of various size, from the minute dimensions of sand-grains, up to vast blocks measuring 6 feet across, and weighing from 5 to 10 tons. They are smoothed, as if they had been subject to a certain amount of attrition in a muddy sediment; but they are not rounded like boulders that have been subjected to sea-breakers. The fracture of the rock is not conchoidal, and there is manifest in its substance a rude disposition towards wavy stratification. The general appearance is that of a clay which has been deposited by aqueous action