Effects of electrolyte on the structure of pyrolytic graphite surfaces in anodic oxidation

Anodic oxidation effects on the structure of the basal and edge surfaces of pyrolytic graphite in alkaline electrolytes have been studied. Laser Raman spectroscopy, a gas-phase chemical modification method, coupled with X-ray photoelectron spectroscopy and secondary ion-mass spectroscopy techniques, were used. Anodic oxidation of the surfaces of pyrolytic graphite in alkaline electrolytes does not cause destruction of their surface structure, even at a higher level of treatment, unlike oxidation of acid electrolytes. In alkaline electrolytes, the number of hydroxyl groups added on the edge surface gradually increases with the increase in treatment level, whereas the number of carboxyl groups does not increase. It was found that anodic oxidation in alkaline electrolytes has a wider permitted range of treatment, in which hydroxyl groups can be added without destroying the edge surface structure, than that found in acid electrolytes. On the other hand, the number of hydroxyl groups added by treating with alkaline electrolytes is smaller than that with acid electrolytes. At a higher treatment level with acid electrolytes, oxidation occurs, even to a depth of 40 nm from the edge surface, whereas with alkaline electrolytes, oxidation occurs only at the surface. On the basis of these results, the effects of electrolytes on the adhesion between carbon fibres and epoxy resin matrix are discussed.     

Reorientation effects in vitreous carbon and pyrolytic graphite

Tubular specimens of vitreous carbon and of pyrolytic graphite, heated in nitrogen or helium for several minutes at 2850 to 3100° K by RF induction, developed a skin of crystalline carbon, nodular in appearance and apparently growing into the carbon from the surface. The vitreous carbon was isotropic, with no preferred orientation of crystallites; the pyrolytic graphite had a preferred orientation of basal planes at right angles to the axis of the specimen, so that curved surfaces were composed predominantly of edge atoms. X-ray diffraction studies of the skin indicated that it was partially graphitised and strongly orientated with basal planes parallel to the surface. The mechanism of the reorientation process is as yet not clear, but the driving force may be a change in surface free energy.     

Siliconated pyrolytic graphite

Siliconated pyrolytic graphite PG(Si) has been prepared by pyrolysis of a mixture of propane and silicon tetrachloride vapour at 1440 to 2025° C, a total gas pressure of 10 or 50 torr, and a partial pressure of silicon tetrachloride vapour of up to 13 torr. The amounts of silicon in PG(Si) are 0.02 to 4 wt %. The effects of the addition of silicon tetrachloride on the surface characteristics, the microstructure and the density of PG(Si) are examined.     

Siliconated pyrolytic graphite

The structural features of several samples of siliconated pyrolytic graphite (0.02 to 4 wt % silicon) prepared at various deposition temperatures, total gas pressures and partial pressures of silicon tetrachloride vapour have been examined by X-ray diffraction. The structural features investigated include interlayer spacing, ordering, intercrystallite porosity, preferred orientation and crystallite size. These features are affected by the preparation conditions. By the addition of silicon tetrachloride vapour, the ordering and the crystallite size of the graphite matrix are developed, the preferred orientation improved and the porosity diminished. The effects of the addition of silicon tetrachloride on the structural features are marked at high total gas pressures, at high partial pressures of silicon tetrachloride vapour and in the medium temperature range of 1600 to 1700° C.     

Interfacial debonding strength between the edge surfaces of pyrolytic graphite and epoxy resins

The edge surfaces of pyrolytic graphite have been treated by anodic oxidation and the interfacial debonding strength (IFDS) between the oxidized edge surfaces and epoxy resin measured. The unoxidized and oxidized edge surfaces were examined by X-ray photoelectron spectroscopy. The edge and epoxy resin surfaces after debonding test were examined by a field-emission scanning electron microscope. Carboxyl groups were expected to be added in great quantities by breaking the carbon-carbon bonds at the edge surface. On the other hand, hydroxyl groups could be added to edge carbon atoms at the edge surface without breaking the carbon-carbon bonds. The rise in IFDS with anodic oxidation does not correspond directly to the O/C ratios or the amount of carboxyl groups present on the edge surface. Hydroxyl groups added to the edge surface are considered to play an important role in improving the adhesion between the edge surface and epoxy resin through covalent bonds between hydroxyl and epoxy groups. In the case of epoxy resin used in this study, IFDS initially increased to 9 MPa with increasing -OH/C ratio up to about 0.02; thereafter it remains constant at this value. This strength is lower than those of PG and epoxy resin.     

Electron microscopy of pyrolytic graphite

A detailed study of heat-treated and deformed pyrolytic graphite has been made using transmission electron microscopy. Single-crystal characteristics are observed in material annealed at 3600° C. Tilt boundaries are shown to be kinks in the basal planes with some twin boundaries which can be described in terms of dislocations, as for natural crystals.     

Structural features of pyrolytic graphite

The structure of pyrolytic graphite has been examined on several samples deposited under various conditions at 1340 to 2310° C and 5 to 200 torr by means of an X-ray technique. The structural features investigated include interlayer spacing, ordering, preferred orientation, intercrystallite porosity, and crystallite size. These features are closely related to the microstructure and density. In particular, the temperature dependence of crystallite size explains three types of formation mechanism, the cause of the occurrence of a minimum density at a certain temperature, and the cause of a preferred orientation in pyrolytic graphite.     

Low-temperature magnetothermal conductivity of pyrolytic graphite

The basal plane thermal conductivity of three highly oriented pyrolytic graphite samples was measured in the temperature region between 0.36 and 4.2 K in various magnetic fields up to 23 kG. Analysis shows that the measured thermal conductivity is simply the sum of the electronic and the lattice contributions. The lattice thermal conductivity does not depend on the external magnetic field. The phonon mean free path is limited only by the crystallite boundaries. This evidence suggests that the electron-phonon interaction is very weak. Both magnitude and temperature dependence of the lattice thermal conductivity can be explained by Komatsu's semicontinuum model. The electronic thermal conductivity has a strong magnetic field dependence and is proportional to the absolute temperature. The magnitude of the electronic thermal conductivity can be related to the electrical resistivity by the free-electron Lorenz number.     

Superfluid onset in4He films adsorbed on pyrolytic graphite

Third-sound velocities and superfluid onsets have been studied for⁴He films adsorbed on pyrolytic graphite. The present onset data are compared with previous experiments on graphite substrates. The third-sound onset data are found to be significantly different from superfluid mass-flow and heat-flow data. A value of 3.19±0.52×10^–9 g cm^–2 K^–1 is obtained for the superfluid areal density at onset divided by the transition temperature. This value is consistent with the Kosterlitz-Thouless-Nelson picture of the phase transition in a two-dimensional superfluid.     

Infrared ellipsometry of highly oriented pyrolytic graphite

We study the anisotropic response of highly oriented pyrolytic graphite (HOPG) in mid-infrared range, utilizing the potential of spectroscopic ellipsometry. We aim at the parameters of the infrared-active vibrations of E_1u and A_2u symmetry, and examine the phenomena related to the strong uniaxial anisotropy of HOPG. We explain the appearance of the weakly polar out-of-plane vibration in the elipsometric spectra taken on cleaved planes. We find a detectable Fano-type behavior of the in-plane phonon.     

Modification of pyrolytic graphite surface with plasma irradiation

The effects of oxygen plasma treatment on the basal and edge surfaces of pyrolytic graphite have been investigated and compared with those of electrochemical treatment on their surfaces. The structural changes caused by the plasma and electrochemical treatments have been characterized by laser Raman and X-ray photoelectron spectroscopies. For the basal surface the plasma treatment causes the slight destruction of the graphitic structure, whereas for the edge surface it causes no marked destruction of the graphitic structure unlike the electrochemical treatment. The amount of surface oxygen both on the basal and edge surfaces increases after the plasma treatment. In particular, for the edge surface, a high surface oxygen concentration can be attained in a short treatment time. In addition, carbonyl-type groups appear to dominate in the plasma-treated basal and edge surfaces over -O- type groups. It can be concluded that oxidation by the plasma treatment is restricted to the vicinities of the surfaces and does not proceed to the internal structure of graphite, unlike the electrochemical treatment.     

The effect of extrinsic defects in pyrolytic graphite on the a-axis resistivity

A contactless r.f. resistivity technique is shown to be well suited to the measurement of room temperature a-axis resistivities in lamellar compounds, particularly those of graphite. The method is used to investigate the relative damage induced by different techniques employed in cutting of samples.Nato exchange visitor, University of Nancy, France.     

Suggestions regarding thermal diffusivity measurements on pyrolytic graphite and pyrolytic boron nitride by the laser pulse method

The laser pulse method can be successfully applied to the measurement of thermal diffusivity of isotropic materials subject to some assumptions. For anisotropic materials, this method is applicable to the measurement of principal thermal diffusivity only on the condition that there is no difference in direction between the principal axis and that of the temperature gradient. After analyzing the heat conduction process in an anisotropic solid, it has been shown that large errors in the measurement of thermal diffusivity would exist if the direction of the principal axis deviates inconspicuously from that of the temperature gradient. The experimental results of thermal diffusivity of highly oriented pyrolytic graphite (HOPG) samples with various deviation angles have been compared with the analytical results. The laser pulse method is not applicable to measurements on semitransparent pyrolytic boron nitride (PBN). We adopted a two-layer composite sample to measure the thermal diffusivity of PBN in the c direction and a particular graphite-PBN composite sample has been prepared which has a very low thermal resistance at the interface. The thermal diffusivity and thermal conductivity of PG (below 2300°C) and PBN (below 1000°C) are given.Invited paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Regularities of pyrolytic boron nitride coating formation on a graphite matrix

The kinetics and structure of chemical vapour-phase deposition of boron nitride ceramics on a graphite matrix from the mixture BCl₃/NH₃/N₂ have been investigated for a wide range of conditions (temperature 1300–2100°C, pressure 130–2600 Pa, at varying partial pressures of the components). The growth-rate was found to vary non-linearly with the consumption rates of the reagents. The content of hexagonal components in the BN-ceramics rose, while the turbostrate microphase content diminished with temperature. The dependences of the concentrations of hexagonal, turbostratic and amorphous fractions in BN-ceramics on the reaction mixture composition and total pressure have been determined.     

Infrared reflection spectroscopy of thin films on highly oriented pyrolytic graphite

The properties of highly oriented pyrolytic graphite (HOPG) as a substrate for external reflection infrared spectroscopy in the mid-infrared region were investigated. Clean HOPG substrates, physisorbed hydrocarbon multilayers, and chemisorbed monolayers of p-substituted aryl radicals on HOPG were used as samples, and the experimental spectra were compared and complemented with the results of spectral simulations. From reflectivity measurements of clean HOPG surfaces with polarized light as a function of the light incidence angle and the frequency, the anisotropic optical constants n (refractive index) and k (absorption index) were determined for in-plane and out-of-plane directions with respect to the graphite basal plane. These constants express the semimetallic properties of HOPG, indicated by an intermediate reflectivity between a typical metal and a dielectric substrate and by asymmetric, distorted peak shapes in adsorbate film spectra, which represent a transition state between symmetrical, positive absorptions on metals and inverted, negative peaks on dielectric substrates. Regarding spectral sensitivity and surface selection rules, HOPG behaves much like a metal and is therefore an equally suitable substrate for external reflection infrared (IR) measurements.