Typical Graphite Research Articles

An application of fractal geometry to complex microstructures: Numerical characterization of graphite in cast irons

It is demonstrated how the complexities of graphite shapes and distributions in cast irons can be described by fractal geometry. A selection of typical graphite morphologies, flake-like and spherulitic, is described unambiguously by two numbers, a shape factor, a, and fractal exponent, D. The meaning of these numbers is discussed and their possible correlations with growth kinetics and mechanical properties are briefly explored.

Pulsed-Laser Deposition and Characterization of Amorphous Diamondlike Carbon Films

ABSTRACTUnhydrogenated amorphous carbon films were deposited by KrF pulsed laser ablation of graphite under a laser power density of 8×108 W/cm2. The films were characterized using x-ray reflectivity, optical transmittance, spectroscopic ellipsometry and Raman spectroscopy. The physical, optical and bonding properties of the films have been investigated in detail as functions of substrate temperatures Ts (22-300°C) and post-annealing temperatures. Films deposited at Ts, <200°C possessed diamond-like properties with mass densities p=2.8-3.1 g/cm3 and optical bandgaps Eg,=1.5-2.2 eV. Above Ts,=200°C, the films showed typical graphite characteristics. The diamond-like films annealed at temperatures up to 750°C show excellent thermal stability.

Comparison of the physical, chemical and electrochemical properties of rayon- and polyacrylonitrile-based graphite felt electrodes

Two typical graphite felts, based on rayon or polyacrylonitrile (PAN) precursors, are used as eletrode materials for the vanadium-redox flow battery. The electrical resistivity, pore size and surface area of these felts are reported. The surface microstructure of the two felts is compared by using scanning electron microscopic SEM tehcniques. The differences in carbon-oxygen interaction behaviour between the two felts is investigated with X-ray photoelectron spectroscopic (XPS) techniques. The performance of vanadium-redox flow cells that employ either an untreated or thermally-treated graphite felt electrode is also evaluated. It is found that the electrical conductivity of the PAN-based felt is superior to that of its rayon-based counterpart. Although both felts have similar construction, the values of specific surface area are different. A difference in the surface microstructure is observed for the fibres from the two felts. XPS analysis reveals that the rayon-based felt reacts more easily with oxygen and forms CO carbon-oxygen groups, while the PAN-based felt is more resistance to oxidation and preferentially forms CO groups. It is believed that the more extensive oxygen interaction in the rayon felt is due to its microcrystalline structure.

Investigations of gas-flow patterns within a cylindrical glass tube having dimensions identical with those of a graphite furnace atomizer under the influence of forced convective flow

Gas-flow patterns within a cylindrical glass tube having the same dimensions as a typical graphite furnace atomizer were investigated by introducing two streams of argon gas seeded with a light-scattering medium of TiO2 particles into the open ends of the tube and viewing the scattering profiles of the particles. By placing a sample of iodine solution near the centre or ends of the cylindrical glass tube, further insight into gas-flow patterns is gained by examining the spatial distributions of the iodine vapour under the influence of convective flow. The TiO2 particle and iodine spatial profiles suggest that at room temperature the argon streams follow a laminar-flow pattern in the bulk-flow region between the ends of the cylindrical tube and the sample introduction hole; however, at the junction region where the two argon streams converge and discharge from the dosing hole, local turbulences or eddies may be generated owing to abrupt changes in gas-flow velocity and direction in the vicinity of an area (boundary layer) directly beneath the dosing hole.

Characterization of thin film n-CdS/ p-CdTe heterojunctions

p-Cadmium telluride films were prepared by the laser assisted vacuum evaporation technique, using oxygen as the dopant. The films formed at T s ∼ 593 K and an oxygen partial pressure of about 1 × 10 −5 torr showed low resistivity (2 × 10 2 ω cm) and high carrier concentration (2−3 × 10 16 cm −3). A typical graphite/ p-CdTe/ n-CdS/In heterojunction was fabricated and its dark and illuminated current-voltage characteristics were studied. A maximum efficiency of 7.3% was observed for a 1 cm 2 area cell. The stability of the cell was also studied for a period of about 8 months.

Preferential oxidation of carbon surfaces

It has been noticed in the context of various investigations that during the oxidation of carbon surfaces in cross flow particles may form which separate from the surface and are carried away by the flow. This process has been investigated in detail experimentally as well as theoretically. Results are presented giving in particular the rate of particle formation m ˙ p in dependence upon temperature for some typical artificial graphites. In the theoretical part relations are presented which specify the dependence of m ˙ p upon material properties and flow conditions. A comparison between theory and experiment shows acceptable agreement.

The influence of primary quinoline insolubles on the development of secondary quinoline insolubles in coal tar pitches

The development of secondary quinoline insolubles (2°QI) or mesophase has been studied by heat treating four different, commercially available coal tar binder pitches to a series of predetermined temperatures and hold times. The development of the 2°QI is discussed in terms of the shape and amount of the primary quinoline insolubles (1°QI) initially present in each pitch by examination of scanning electron micrographs of the extracted quinoline insolubles. The development of the 2°QI of these materials is compared with the development of 2°QI in a petroleum pitch where 1°QI is initially absent. Small, laboratory extruded graphite rods using each of the four binder pitches were produced in order to examine the influence of 2°QI development on typical graphite properties.

Vitreous carbon matrix for low-wear carbon/metal current collectors

Friction, wear, and resistivity are reported for composite specimens made of copper fibers in vitreous or glassy carbon matrix. The resistivity and friction are shown to be comparable to reported values for typical copper graphite specimens, and the wear is shown to be exceptionally low. The experiments reported show that sound composites can be made by polymerization and pyrolysis of an organic material to form a glassy-carbon matrix around copper alloy fibers. This is a radically different process from the techniques of forming such composites from powder, or by pressure infusion of the metal into a graphitic body. Although the vitreous carbon is hard, it does not appear to act as an abrasive on a soft material. Qualitative observations in handling the composite material indicate that it is tough, and this is attributed to the energy absorption by the metal fibers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fracture toughness of unidirectional graphite fibre reinforced/epoxy composite in mode ii (forward shear), using a thin tubular specimen

The fracture toughness of graphite fibre reinforced unidirectional composite in mode II has been determined using a thin tubular specimen, by the method developed by Giare and Campbell, accepted for publication in Engng Fracture Mech. The crack growth resistance at instability and the corresponding initial strain energy release rates were independent of the initial crack in the range of the crack length investigated. G IIC obtained by thin tubular torsion specimen is 42% greater as compared to end-cracked beam method. The end-cracked beam method underestimated the G IIC as reported by J. W. Gillespie Jr, L. A. Carlson and R. B. Pipes [ Compos. Sci. Technol. 27, 177–197 (1986)]. They presented a finite element analysis of the End-Notched Flexure test specimen for mode II interlaminar fracture testing of composite materials. They predicted, using finite elements, that data reduction schemes based upon beam theory underestimate G IIC by approximately 20–40% for typical graphite fibre composite test specimen. It is very encouraging that our results follow very close to their prediction. It is concluded that the thin tubular torsion specimen method is accurate as compared to the end cracked beam method. G IIC of graphite fibre reinforced epoxy composite is found to be half of the glass fibre reinforced/epoxy composites, for practically the same V t of fibres.

Finite element analysis of the end notched flexure specimen for measuring mode II fracture toughness

This paper presents a finite element analysis of the End Notched Flexure (ENF) test specimen for Mode II interlaminar fracture testing of composite materials. Virtual crack closure and compliance techniques employed to calculate strain energy release rates from linear elastic two-dimensional analysis indicate that the ENF specimen is a pure Mode II fracture test within the constraints of small deflection theory. Furthermore, the ENF fracture specimen is shown to be relatively insensitive to process induced cracks offset from the laminate midplane. Frictional effects are investigated by including the contact problem in the finite element model. A parametric study investigating the influence of delamination length, span, thickness and material properties is presented to assess the accuracy of beam theory expressions for compliance and strain energy release rate, G II . Finite element results indicate that data reduction schemes based upon beam theory underestimate G II by approximately 20–40% for typical unidirectional graphite fiber composite test specimen geometries. Consequently, an improved data reduction scheme which retains the simplicity of beam theory but also includes the accuracy of the finite element method is proposed.

Domain model of stage order and disorder in intercalation compounds.

A theoretical model of staging is presented which takes account of the domain structure of intercalation compounds. The model is able to treat stage order and disorder in a unified way. The staging phase transitions in which the stage index of the intercalation compound changes are predicted to proceed via stage-disordered states. If the domains are finite and can be considered to be independent, then the staging transitions are predicted to be continuous. The model is applied to the case of typical donor and acceptor graphite intercalation compounds. Other systems are also discussed. A model-independent scaling rule is proposed relating stage disorder to domain size, stage, temperature, in-plane density, and charge transfer, and a fundamental limit is set on the period of well-staged structures. The relationship between the present model and previous theories of staging is discussed. The implications of recent staging experiments are also considered.

Stage Order, Disorder, and Phase Transitions in Intercalation Compounds

A theoretical study of intercalation predicts stage-changing phase transitions to be continuous and to proceed via stage-disordered states. Detailed results are presented for typical donor and acceptor graphite intercalation compounds. Other systems are also discussed. A scaling rule relating stage disorder to domain size, stage, temperature, in-plane density, and charge transfer is proposed. A fundamental limit is set on the period of well staged structures. Experimental implications are discussed.

Neutron Irradiation Effects on Carbon and Graphite Cloths and Fibers

A series of cloth and fiber samples were irradiated to fluences of 3.5, 7.3, and 10 x 10/sup 21/ cm/sup -2/ at 470/sup 0/C. Dimensional changes of the fibers in the radial direction ranged from -19% to +33% and in the axial direction from -18% to -27%, roughly ten times greater than dimensional changes found for typical nuclear graphites. Despite these large dimensional changes, all but one of the 2-dimensional cloths remained essentially unchanged in overall physical appearance. The 3-dimensional cloths, on the other hand, deteriorated apparently because these types of weaves were less able to accommodate the large axial fiber shrinkages.

The influence of the type of quinoline insolubles on the quality of coal tar binder pitch

Seven pitches containing three types of quinoline insolubles, QI, were produced from a single coke oven coal tar. The tar was centrifuged into QI-rich and QI-lean fractions. A blend of the two fractions yielded a typical graphite electrode binder pitch. The QI content of the pitch derived from the QI-lean tar was increased through the addition of natural QI concentrate, or QI as a furnace carbon black, or QI as mesophase by means of heat treatment. The pitches were evaluated as binders by measuring the flexural strength of 19 mm diameter graphite electrodes. The results show that the typical electrode pitch is superior, and that increasing the QI content of the QI-lean material did not upgrade it. The results also show that the flexural strength of the derived graphite is not simply related to the coking value of a binder.

Mixed Sorption Models and Isotherms of the Cesium-Rubidium-Graphite System at High Temperatures

To determine fission product-graphite sorption behavior, an experimental and analytical study was conducted to obtain a model which would predict the vapor pressures of mixtures of sorbed fission product metals as a function of temperature and composition.The graphite sorbents studied were SP-1C, a very pure natural flake graphite, and TS-688, a typical needle-coke nuclear-grade graphite. Cesium and rubidium tagged with 137Cs and 86Rb were the sorbates, and the pseudo-isopiestic method was used to determine sorbate concentrations as a function of temperature and vapor pressure. Three models were examined for their capability in predicting binary mixed sorption behavior.Experiments showed the following: (a) The vapor pressure of a species varies inversely with the fraction of total sorbate metal present at a given sorbate concentration and temperature; (b) thermodynamic and FREVAP models were the most successful in the prediction of binary sorption behavior from single component isotherm data. It is concluded that if the departure from ideal adsorption behavior is not known, either model may be used in the calculation of fission product release.

High temperature creep of a POCO graphite

The creep behavior of POCO grade HPD-1 graphite was studied from 2200 to 2500°C in the as-received condition as well as after annealing at either 2750 or 2900°C. The apparent activation energy for creep, the stress dependence, the time dependence, creep curve shape, and recovery behavior are similar to those for the typical commercial polycrystalline graphites. However, there are several important differences. Creep strains to fracture are large, generally exceeding 30 per cent, and can exceed 70 per cent. Also, there is a reduction in cross-sectional area, by as much as 20 per cent, during high temperature deformation. Fracture stresses at high temperatures are high, sometimes reaching 18-20,000 psi, but the creep rates are relatively rapid at stresses of the order of 1/3 of the fracture stress. These differences are attributed to the small pore size of the POCO graphite and the uniform distribution of these pores.

High temperature creep of a Poco graphite

Abstract The creep behavior of POCO grade HPD-1 graphite was studied from 2200 to 2500°C in the as-received condition as well as after annealing at either 2750 or 2900°C. The apparent activation energy for creep, the stress dependence, the time dependence, creep curve shape, and recovery behavior are similar to those for the typical commercial polycrystalline graphites. However, there are several important differences. Creep strains to fracture are large, generally exceeding 30 per cent, and can exceed 70 per cent. Also, there is a reduction in cross-sectional area, by as much as 20 per cent, during high temperature deformation. Fracture stresses at high temperatures are high, sometimes reaching 18-20,000 psi, but the creep rates are relatively rapid at stresses of the order of 1/3 of the fracture stress. These differences are attributed to the small pore size of the POCO graphite and the uniform distribution of these pores.

Microstructure of carbon blacks in pitch-bonded graphites and structural changes produced by gas—graphite oxidation reactions

Carbon blacks have been found in binder regions of typical pitch-bonded graphites. These blacks appear to be part of the quinoline-insoluble fraction of the coaltar pitch binder used in graphite manufacture. Replication techniques and direct transmission electron microscopy have been used to determine the structure of the blacks and their disposition throughout the graphites as well as structural changes produced by thermal and electronically-excited oxidation reactions. Individual blacks consist of roughly-spherical arrangements of graphite crystallites with a radial orientation of c-axes. Outer crystallites appear so well joined that the blacks are quite resistant to thermal oxidation in contrast to the surrounding binder regions. However, the blacks are easily attacked by microwave-excited oxygen, and the oxidation process reveals minute impurity particles on and/or within the polygonal carbon-black structure.

GRAPHITE AND BORON NITRIDE (“WHITE GRAPHITE”): ASPECTS OF STRUCTURE, POWDER SIZE, POWDER SHAPE, AND PURITY

Graphite can be characterized by atomic structure and powder size, shape, and purity. A similar characterization can be applied to hexagonal boron nitride. Three typical graphites are studied to illustrate quality variation and some practical consequences. The properties of hexagonal boron nitride are described to distinguish it from the cubic form.