Graphite Thin Films Research Articles

Development of A Novel Solid-State pH Sensor Based on Tin Oxide Thin Film

Abstract A solid-state pH sensor was fabricated using a transparent conductive tin oxide film on a glass substrate. The coating of the glass substrate was achieved by a novel simple chemical vapor deposition (CVD) procedure. The response time of the pH sensor was substantially reduced when a thin graphite film was deposited onto the tin oxide conductive film. The sensor slope was found to increase as the temperature of the solution was increased. The performance of the sensor was investigated in the pH range from 0.3 to 11.0. A straight-line calibration graph was achieved throughout the whole range tested, especially when the solution temperature was 80 ℃. The working pH range was found to decrease on the expense of the lower range as the temperature was decreased. Results obtained by the suggested sensor compares very well with conventional pH electrodes where the square of the correlation coefficient was 0.999.

Preparation and characterization of monolithic polyaniline–graphite composite actuators

We report here the fabrication and characterization of novel, monolithic electrochemical actuators based on polyaniline (PANI) and a micrometer-sized graphite powder. These PANI–graphite thin films have a graphite-rich layer that renders composite thin film conductive at all redox states. The asymmetric distribution of graphite also allows for a bending movement as the films are subjected to electrochemical oxidation and reduction. The unique asymmetric structure and high conductivity of the films allow the actuators to be operated at lower redox potentials, with larger bending angles and longer life cycles. Our study also shows that most of the bending movements occur during the transition between the emeraldine base and the pernigraniline base. Judging from the CV and from the current generated when the square wave potential is applied, we note that the actuators appear very stable and show no signs of degradation after 50,000 working cycles operated at 1 Hz. The life cycles of these actuators exceed 120,000 cycles (>33.6 h) in a 1.0 M CH 3SO 3H aqueous solution. As we extend the oxidation potential to greater than 0.8 V, the PANI–graphite thin film starts to degrade with time. These results reveal the optimum conditions under which the actuators should be operated.

Growth of Ni–In alloy nanowhiskers

Ni–In alloy nano-whiskers were synthesized by pulsed laser deposition and post-annealing of C–Ni–In mixture thin films. The growth mechanism of nano-whiskers is explained by the supersaturated liquid Ni–In alloys that were seeping out from a porous graphite thin film.

Graphitic carbon nitride thin films deposited by electrodeposition

Electrodeposition method was used to deposit the graphitic carbon nitride thin film on Si (100) substrate from a saturated acetonitrile solution of 2,4,6-trichloro-1,3,5-triazine and 2,4,6-triamino-1,3,5-trazine at room temperature. X-ray powder diffraction (XRD) results show that the d-values are in good agreement with the data of graphite-like carbon nitride calculated in the literature. X-ray photoelectron spectra (XPS) measurements indicate the elements in the deposited films were mostly of C and N (N/C=0.88), and N bonded with C in the forms of C–N and C=N bond. The Fourier transform infrared (FTIR) spectroscopy indicates that CN aromatic ring exists in the product. These results demonstrate that crystalline g-C 3N 4 was obtained.

Hydrogen content and density in nanocrystalline carbon films of a predominant diamond character

Nanocrystalline carbon films possessing a prevailing diamond or graphite character, depending on substrate temperature, can be deposited from a methane hydrogen mixture by the direct current glow discharge plasma chemical vapor deposition method. While at a temperature of ∼880 °C, following the formation of a thin precursor graphitic film, diamond nucleation occurs and a nanodiamond film grows, at higher and lower deposition temperatures the films maintain their graphitic character. In this study the hydrogen content, density and nanocrystalline phase composition of films deposited at various temperatures are investigated. We aim to elucidate the role of hydrogen in nanocrystalline films with a predominant diamond character. Secondary ion mass spectroscopy revealed a considerable increase of the hydrogen concentration in the films that accompanies the growth of nanodiamond. It correlates with near edge x-ray adsorption spectroscopy measurements, that showed an appearance of spectroscopic features associated with the diamond structure, and with a substantial increase of the film density detected by x-ray reflectivity. Electron energy loss spectroscopy showed that nanocrystalline diamond films can be deposited from a CH4/H2 mixture with hydrogen concentration in the 80%–95% range. For a deposition temperature of 880 °C, the highest diamond character of the films was found for a hydrogen concentration of 91% of H2. The deposition temperature plays an important role in diamond formation, strongly influencing the content of adsorbed hydrogen with an optimum at 880 °C. It is suggested that diamond nucleation and growth of the nanodiamond phase is driven by densification of the deposited graphitic films which results in high local compressive stresses. Nanodiamond formation is accompanied by an increase of hydrogen concentration in the films. It is suggested that hydrogen retention is critical for stabilization of nanodiamond crystallites. At lower deposition temperatures an excess of hydrogen in the deposited layers helps to prevent the densification of the films and accumulation of microstresses and consequently the films maintains its graphitic character. At higher temperatures the hydrogen content in the films is relatively low and the film maintains its graphitic character.

Surface morphology of 6H-SiC after thermal diffusion

Diffusion of aluminum into 6H-SiC has been carried out in the temperature range of 1800-2100°C. Aluminum carbide (Al4C3) was used for a p-type impurity source; the diffused surface exhibited good stoichiometry and surface morphology. A thin-layer graphite film was developed to protect the wafer surface from deterioration during the high-temperature diffusion process. A high-resolution optical microscope (HROM) and atomic force microscopy (AFM) were employed to evaluate the surface morphology of the diffused samples. The protective graphite layer significantly decreased the surface roughness. X-ray photoelectron spectroscopy (XPS) was used to identify the Si/C ratio near the surface regions. Very little surface graphitization occurred during diffusion. In addition, secondary ion-mass spectroscopy (SIMS) was used to investigate the influence of the thin graphite film on the diffusion properties in SiC. There were no significant differences in doping profiles in the samples with and without the graphite film.

Microstructure and phase composition evolution of nano-crystalline carbon films: Dependence on deposition temperature

Nano-crystalline carbon films possessing a prevailing diamond or a graphite character, depending solely on substrate temperature and deposition time, can be deposited from a methane–hydrogen mixture by the direct current glow discharge plasma chemical vapor deposition method. In this study we investigate the evolution of nano-crystalline carbon films deposited in the 800–950 °C temperature range onto silicon substrates aiming to enlight the physicochemical processes leading to the formation of nano-diamond films. While at a deposition temperature of ∼880 °C the formation of a thin precursor graphitic film is followed by deposition of a film of diamond character, at higher and lower temperatures the films maintain their graphitic character. The morphology of the films and their growth rate vary with deposition temperature: slower growth rates and higher film roughness are obtained at lower temperatures suggesting the importance of kinetic effects during the growth process. For deposition times longer than ∼60 min, similar morphologies are obtained irrespectively of the deposition temperature. A preferred spatial alignment of the basal planes of the graphitic film at the interface with the silicon substrate was determined. The alignment was found to differ with deposition temperature: at 800 and 880 °C the alignment occurs along the graphitic â axis perpendicular to the silicon substrate, while at 950 °C the ĉ axis is aligned perpendicular to the silicon substrate. However, it was determined that for films a few hundred nm thick close to the evolving surface the films display a preferred alignment of the basal planes vertical to the surface, irrespectively of their orientation at the interface. The reason for this alignment is suggested to be associated with a stress relaxation mechanism in the graphitic films. It was determined that film growth is accompanied by the evolution of large local stresses which obtain a maximum value for the films deposited at 880 °C. The relaxation of these stresses is suggested to lead to the transformation of the graphitic material into the diamond phase. The narrow range of temperatures (880+/−10 °C) which enables the formation of the diamond phase indicates the importance of hydrogen adsorption/desorption processes in the nucleation and growth of the nano-crystalline diamond films. The morphological evolution of the films was analyzed by atomic force microscope. By electron diffraction and high-resolution transmission electron microscopy the phase composition of the films and their microstructure were examined. The alignment of the graphitic films within the near-surface region of the evolving films as a function of the deposition time and temperature was investigated by angle-resolved near edge x-ray absorption fine structure measurements. Raman spectroscopy was applied to determine the presence of stresses within the films and their phase composition.

The role of tribo-layers on the sliding wear behavior of graphitic aluminum matrix composites

This study provides a systematic investigation of the role played by the tribo-layers that form on the contact surfaces during the sliding wear of graphitic cast aluminum matrix composites. The graphitic composites include A356 Al–10% SiC–4% Gr and A356 Al–5% Al 2O 3–3% Gr that are being developed for cylinder liner applications in cast aluminum engine blocks. Three main wear regimes, namely, ultra-mild, mild and severe wear were determined. At nearly all sliding speeds and loads in the mild wear regime a protective tribo-layer was formed. By increasing the speed and load the tribo-layer covered a larger proportion of the contact surface and became more compact and smoother. The hardness of the tribo-layers increased with the applied load and speed and reached values as high as 800 kg/mm 2. The tribo-layers were removed by extrusion process at the onset of severe wear. The topmost part of the tribo-layer consisted of iron-rich layers. The rest of the tribo-layer consisted of fractured SiC and Al 3Ni particles and thin graphite films, which were elongated over long distances in the direction of sliding, thus, reducing shear stresses transmitted to the subsurface regions. It was shown that because of the thicker and more stable tribo-layers on the contact surfaces of graphitic composites, than that of non-graphitic composites and the A356 Al alloy, the graphitic composites displayed a transition from mild-to-severe wear at load and sliding speed combinations, which were considerably higher than those of the A356 aluminum alloy and the non-graphitic A356 Al–20% SiC composite. A negative effect of the hard constituents in the tribo-layers was the scuffing damage that they inflicted on the counterface.

Microstructural Characterization of C60‐Doped Zirconia

Zirconia powders doped with C60 molecules were prepared from an aqueous solution of zirconium oxynitrate dihydrate, C60 and C16TMA, and sintered at 600°C under 5.5 GPa for 2 h. C60 was found to be retained in the sintered specimens by HRTEM, and carbon was observed to be uniformly dispersed by the SEM‐EDX analysis. HRTEM observations of the sintered specimens exhibited the formation of ZrO2 crystal grains covered with thin graphitic or amorphous carbon films.

Characterization of elastic properties of hard carbon and boron nitride films using the Brillouin light scattering technique

The elastic properties of thin graphitic carbon and hexagonal boron nitride films have been determined by Brillouin light scattering from thermally excited surface acoustic phonons. The carbon films have been grown by plasma beam deposition at 420°C. The BN films have been deposited by r.f.-magnetron sputtering and simultaneous ion plating. In the case of amorphous carbon a layered substructure has been found with a rather compliant intermediate layer at the interface. The stiffness of the films increases with increasing ion energy. As for carbon the BN coatings exhibit a pronounced elastic anisotropy indicating a preferred orientation of the hexagonal planes perpendicularly to the surface. The comparison of the Young's moduli determined by load indentation and by Brillouin light scattering allows the conclusion that the material is composed of two or more components of different stiffness.

Preparation and structure of nitrogen-substituted, highly oriented graphite thin film prepared by chemical vapor deposition

Highly-oriented, nitrogen-substituted graphite (C/N graphite) thin films were prepared by chemical vapor deposition (CVD) using pyrrole as a starting material and palladium deposited single crystal sapphire as a substrate material whose temperature was kept at 1273K. The interlayer spacing of C/N graphite films, determined by x-ray diffraction, was close to that of single-crystal graphite. The orientation of crystalline of C/N graphite films, evaluated by rocking angle measurement, was higher compared with that of highly oriented pyrolytic graphite. XPS and Raman spectroscopy were also measured for the characterization of these films.

TEM analyses of surface ridge network in an ion-irradiated graphite thin film

A ridge network induced by He +-irradiation on a graphite surface was examined by in situ and ex situ transmission electron microscopy. It was found that the ridge structure appeared in a few seconds from the start of irradiation and developed to the full network structure within a minute, where the ion dose corresponds to less than 0.01 dpa. The analyses of electron diffraction patterns showed that the crystallographic relationship between the matrix and ridges was similar to those of twins as previously reported, but the ridges consisted of curled graphitic sheets rather than a pair of twin bands. We have to conclude from the present results that the formation was affected by collective electronic excitations associated with the high density of energy deposition near the surface, rather than the displacement damage or the accumulation of implanted gas.

Generation of 3D hydrocarbon thin films via organic molecular cluster collisions

Thin film growth through the consecutive impact of energetic ethyne molecular clusters with a non-rigid, hydrogen-terminated diamond (111) surface in vacuum has been studied using molecular dynamics simulations. A second-generation version of the reactive empirical bond-order potential for hydrocarbons developed by Brenner, that has been modified to include long-range van der Waals interactions, is used in the simulations. The velocities considered are in the hyperthermal region and are comparable with those that can trigger shock-induced chemistry in energetic materials. The resulting film structure is predicted to be significantly different from diamond, graphite, or diamond-like amorphous-carbon thin films. The evolving film morphology is discussed in detail and the results compared with single cluster impacts involving a comparable number of molecules.

Size effect in the in-plane electrical resistivity of very thin graphite crystals

In order to investigate the effect of film thickness of thin graphite crystals on the electrical properties, we prepared thin graphite films by cleaving a kish graphite (KG) crystal with the rrr value of 32.3. The values of the thickness of the graphite films were ranging from around 300 to 1000A. Keeping these graphite films free from strain, we measured the temperature dependence of the in-plane resistivity between liquid helium and room temperatures. The experimental results could be expressed by a simple two band model and the Sugihara's theory for lattice vibration in thin-carbon films. By using these expressions, we estimated the overlap energy E0 of conduction and valence bands for the thin graphite films with various thicknesses. It was found that E0 decreases with decreasing film thickness. We also estimated the reciprocal of the relaxation time due to lattice defects or surface scattering l/τi and that due to lattice vibration l/τi for the thin films with various thicknesses. Consequently, it was found that the effect of l/τi increases rapidly, as the film thickness decreases. KEYWORDS graphite, thin film, resistivity, size effect, band overlap energy

Development of life prediction model and improvement of a die steel for hot forging die : Miyahara, M., Nomoto, K., Umino, M., Sera, T., Kurokawa, Y. and Okada, Y. Sumitomo Metals (Apr. 1996) 48 (2), 76–84

In hot forging, water-based-graphite lubricants are sprayed on the die surface to provide a thin adherent graphite film for the desired metal flow. The film formation depends on the spread of the lubricant droplets and their dry-off. With dryoff-times being very small, the spreading is essentially isothermal. Therefore, the fundamentals of spreading droplets are investigated through droplet mechanics and the similarity principles. Relations are derived between the spray parameters and the non-dimensional spread coefficient using single droplet experiments. These relations together with measured rheological properties are used to predict spread ratio for the various lubricant dilution ratios using validated numerical models.

The petrologic case for a dry lower crust

Fluid pressure in the crust may be controlled by different mechanisms according to depth, temperature, and the mineralogy of the host rocks. Where rocks are fluid‐saturated, fluid pressure may approach lithostatic or hydrostatic pressure depending on the ductility of the wall rocks and the connectivity of pores and fractures. However, if the host rocks contain minerals formed at temperatures higher than those currently prevailing, they will react with fluids to produce hydrated (or carbonated) retrograde minerals, and the fluid pressure will be limited by thermodynamic equilibrium between high‐grade reactant minerals and retrograde products. The thermodynamically constrained parameter, water fugacity, may have a value of tens to hundreds of bars in the lower crust. In practice, this means that for typical igneous or high‐grade metamorphic rocks now occurring in stable lower crust, notional fluid pressures are substantially (1 to 3 orders of magnitude) lower than lithostatic. No free, connected fluid phase can be present in deep stable crust, and alternative explanations must be sought for the relatively high electrical conductivity of such rocks. The proposal that high lower crustal conductivity is due to thin grain boundary films of graphite is also unlikely to be generally true because films of sufficient thickness would be readily visible on broken surfaces of hand specimens. An alternative explanation of the discrepancy between laboratory and field measurements of the conductivity of high‐grade rocks is that laboratory measurements are not normally made under appropriate conditions of rock‐buffered fluid pressure.

STEM-based mass spectroscopy of supported Re clusters

Annular dark-field imaging in a scanning transmission electron microscope (STEM) is known to enhance contrast of small supported particles of heavy elements. At high scattering angles (∼ 100 mrad), this imaging technique can be used quantitatively to measure absolute particle size by comparing measured and theoretical elastic scattering cross-sections. Here, we report quantitative size measurement of small rhenium particles supported on thin graphite films. Statistical measurement of absolute cross-sections of prepared Re-6 clusters show good agreement with theoretical cross-sections, within a ±2 atom random experimental error. This is the first report of such measurements using angles sufficiently high that coherent effects can be neglected. Our experiments confirm the exceptional stability of the Re-6 organometallic compound relative to other compounds with 7, and 8-Re atoms. Details of the high angle annular dark-field STEM experiments are presented including image processing/analysis, annular dark-field detector calibration, and sources of error in the absolute intensity measurements. We discuss the future possibility of individual atomic sensititity.

Size Effect inthe In-plane Electrical Resistivity of Very Thin Graphite Crystals

In order to investigate the effect of film thickness of thin graphite crystals on the electrical properties, we prepared thin graphite films by cleaving a kish graphite (KG) crystal with the rrr value of 32.3. The values of the thickness of the graphite films were ranging from around 300 to 1000Å. Keeping these graphite films free from strain, we measured the temperature dependence of the in-plane resistivity between liquid helium and room temperatures. The experimental results could be expressed by a simple two band model and the Sugihara's theory for lattice vibration in thin-carbon films. By using these expressions, we estimated the overlap energy E0 of conduction and valence bands for the thin graphite films with various thicknesses. It was found that E0 decreases with decreasing film thickness. We also estimated the reciprocal of the relaxation time due to lattice defects or surface scattering l/τi and that due to lattice vibration l/τi for the thin films with various thicknesses. Consequently, it was found that the effect of l/τi increases rapidly, as the film thickness decreases. KEYWORDS graphite, thin film, resistivity, size effect, band overlap energy

Dimensional Control of Carbon Materials by Template Carbonization Technique

A template carbonization technique was developed for synthesizing novel carbon materials. This technique utilizes the inorganic substances whose openings or pores are controlled at nanometer level. The first attempt was to prepare thin graphite films by the carbonization of organic polymer in the two-dimensional opening between the lamellae of a layered clay. Then, this technique was applied to the preparation of one-dimensional carbon such as carbon tubes by using an anodic aluminum oxide film as one-dimensional template, and uniform hollow nanotubes with open ends were obtained. Furthermore, a well-defined nano space in zeolite was utilized for the synthesis of novel porous carbon materials. The present review introduces the one- and three-dimensional approaches using the anodic oxide film and zeolite as a template and describes the structure of the resultant carbon materials prepared from these template techniques.

Preparation of thin film graphite-like carbons intercalated by metallic potassium

Abstract A thin film of graphite-like carbons was prepared on TM/Al2O3 support (TM = Co and/or Fe). Then CTM/Al2O3 precursors were intercalated with metallic potassium vapour. The KCTM/Al2O3 materials obtained were very active catalysts in the ammonia synthesis reaction. In the resulting KCTM/ Al2O3 catalysts, the presence of a KC8, KC9, KC16 and KC24 intercalates mixture was found. The formation of intercalates is coupled with the changing of the graphite-like carbons orientation from a flat structure in the CTM/Al2O3 precursor to rather filamentous needle-shaped or regular blocks in the KCTM/Al2O3 catalyst. The intercalated compounds of potassium and graphite thin film might facilitate transport of 2s electrons of potassium via a carbon graphite-like network towards very active supported transition metals.