Chemical Vapor Deposition Of Carbon Research Articles

Composition, mechanical properties and friction behavior of nickel/hydrogenated amorphous carbon composite films

Nickel/hydrogenated amorphous carbon composite films have been deposited on silicon and stainless steel substrates by combining sputter-deposition of metal and microwave plasma-assisted chemical vapor deposition of carbon from argon–methane mixtures of various concentrations. The composition and crystallographic structure of films were investigated as functions of the CH 4 concentration by Rutherford backscattering spectroscopy and X-ray diffraction techniques, respectively. The carbide phase, Ni 3C, was detected in Ni/C films deposited from a gas phase containing more than 8 vol.% of CH 4. The grain size of Ni and Ni 3C was determined as a function of the carbon content. The maximum magnitude of the compressive residual stresses was 0.6 GPa for films containing 25 at.% of carbon (Ni 3C). At carbon concentrations above 50 at.%, the stress magnitude diminished to 0.2 GPa. The hardness and elastic modulus of films deduced from nanoindentation measurements were studied as functions of the carbon content. The ball-on-disk tribological tests were conducted in room air at the temperature of 20 °C under a load of 1 N with a sliding speed of 50 mm/s. The friction coefficient was determined as a function of the carbon content in the films. The minimum value of 0.25 was obtained from films containing about 75 at.% of carbon.

Investigation of open-air laser-induced chemical vapor deposition of carbon on moving optical fibers

Using a CO2 laser as a heat source, carbon coatings have been successfully deposited on moving optical fibers in an open-air laser-induced chemical vapor deposition (LCVD) reactor. Applications include fusion splice recoat and in-line coating of optical fibers. The relationship between operating parameters and the carbon deposition temperature and rate was investigated. Results indicate that they are strongly dependent on the laser power density and the optical fiber's traverse velocity. In order to provide a deeper understanding of the fundamental principles that govern laser heating and the carbon LCVD processes, a heat transport model was developed to predict the fiber surface temperature during deposition. The surface temperatures obtained from experiments compared well with the predicted temperature. Based on the temperature calculations, various kinetic parameters, including the activation energy EA and pre-exponential factor k0 for the carbon deposition process, are determined. The optical fiber signal loss at 1550 nm induced by the LCVD process is observed to be less than 10–3 dB at low laser power density.

Phase composition and tribological properties of copper/carbon composite films

Copper/carbon composite films have been deposited on steel and silicon substrates by an hybrid technique combining sputter-deposition of copper and microwave plasma-assisted chemical vapor deposition of carbon from argon–methane mixtures of various concentrations. The composition of films was determined by Rutherford backscattering spectroscopy (RBS) and energy recoil detection analyses (ERDA). The crystallographic structure of films was identified by X-ray diffraction (XRD) techniques. The dry friction properties of films were investigated by ball-on-disk tribological tests conducted at room temperature in room air. Copper/carbon films containing 25–75 at.% of carbon were produced as the methane concentration in the gas phase was varied from 20% to 100%. These composite films consisted of copper crystallites and hydrogenated amorphous carbon (a-C:H) phase. The size of copper crystallites was in the range 2–20 nm. An abrupt increase in carbon content in the films was observed as the methane concentration in the gas phase increased from 60% to 70%. The tribological behavior tested in ball-on-disc geometry and microhardness of these Cu/a-C:H composite films are analyzed and discussed in terms of their microstructure, morphology and composition.

Characteristics of copper/carbon and nickel/carbon composite films produced by microwave plasma-assisted deposition techniques from argon–methane gas mixtures

Copper/hydrogenated amorphous carbon (a-C:H) and nickel/a-C:H composite films have been deposited on Si substrates by combining sputter-deposition of metal and microwave plasma-assisted chemical vapor deposition of carbon from argon–methane mixtures of various concentrations. The deposition rate of films was found to vary between 10 and 25 nm/min depending on the CH 4 concentration in the gas phase. The crystallographic structure of films was identified by X-ray diffraction (XRD) techniques. Diffraction peaks of the face-centered cubic (fcc) Cu phase were always detected in the XRD patterns of Cu/C films. Diffraction peaks of the fcc Ni phase appeared in the XRD patterns of Ni/C films deposited from gas mixtures containing less than 5% of CH 4. Diffraction peaks of Ni and Ni 3C phases were detectable in the XRD patterns of Ni/C films deposited from a gas phase containing 10% of CH 4. For Ni/C films deposited with a CH 4 concentration in the range 15–30%, the diffraction peaks in the XRD patterns were only ascribable to the Ni 3C phase. Beyond 30% of CH 4 in the gas phase, the structure of Ni/C films became progressively amorphous as the CH 4 concentration increased. The grain size of Cu, Ni and Ni 3C was determined as a function of the CH 4 concentration. The residual stresses in Cu/C films were very low and independent of the CH 4 concentration in the gas phase (or composition of films). For Ni–C films, the maximum value of compressive residual stresses was −0.6 GPa for films deposited with 20% of CH 4 in the gas phase (or for Ni/C films containing pure Ni 3C phase). The electrical resistivity of films determined by four point probe measurements as well as the hardness and elastic modulus of films deduced from nanoindentation measurements were studied as functions of the CH 4 concentration in the gas phase.

Controlling the growth of CVD carbon from methane on transition metal substrates

A hot-wall chemical vapor deposition (CVD) method was employed to grow crystalline graphitic carbon on transition metal substrates. Temperature dependence of the CVD carbon growth and the influence of methane concentration (carbon source) on the growth rate of carbon were systematically investigated. The effect of the substrate on the growth rate and carbon crystallinity was also investigated. These factors, temperature, methane concentration, and type of substrate, influenced the growth of CVD carbon. The carbon growth pattern appears to start with the most crystalline graphitic carbon being produced on the surface of the Fe and Ni and becomes more amorphous as the coating thickness increases.

Media tribology design to improve flyability at low flying height

As the head media separation (HMS) continues to decrease with increasing areal density, maintaining good flyability performance is crucial to ensure the robustness of the head-disk-interface. Carbon wear and lubricant pickup on the slider surface are found to be the main contributors to flyability degradation. The effects of carbon overcoat and lubricant on load/unload flyability were investigated at the near-contact flying height by either reducing the environmental pressure or slowing down the disk rotating speed. Changing lubricant process and X-1P additive amount can affect lubricant pickup. In order to reduce the head wear, a thin layer of softer functional carbon layer was applied on the top of plasma-enhanced chemical vapor deposition carbon. However, carbon and lubricant modification has some limitations on improving flyability as HMS continues to drop. As an alternative approach to minimizing slider pickup at low flying height, landing the head in the laser textured contact-start-stop zone serves the function of cleaning the head and removing head smear.

Characterization of CVD carbon films for hermetic optical fiber coatings

Thin carbon films are being studied as hermetic coatings for optical fibers used in harsh environment applications. This work details an experimental study of pyrolytic carbon films grown in a cold walled chemical vapor deposition (CVD) reactor. The films are grown from methane, acetylene, propane and butane precursors on stationary 3-mm quartz rods. Results are presented at 15 and 600 torr, total pressure for substrate temperatures between 1050 and 1750 K. Hydrocarbon precursor gas concentrations are varied between 10 and 100% in N 2 dilutant. Auger electron spectroscopy (AES) shows mostly carbon composition, with trace levels of Cr and O 2 impurities. X-Ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) reveal small amounts of SiC at the carbon–glass interface, but could not resolve a continuous transition layer. The thickness, microstructure and surface roughness are analyzed using environmental scanning electron microscopy (ESEM). ESEM reveals an increase in deposition rate with temperature and concentration for all hydrocarbon species and a noticeable change in surface morphology with deposition temperature and hydrocarbon species. Raman spectroscopy is used to relate film microstructure to deposition conditions, providing non-intrusive evaluation of crystalline size and hermetic properties.

Plasma enhanced chemical vapour deposition carbon nanotubes/nanofibres—howuniform do they grow?

The ability to grow carbon nanotubes/nanofibres (CNs) with a high degree ofuniformity is desirable in many applications. In this paper, the structuraluniformity of CNs produced by plasma enhanced chemical vapour deposition isevaluated for field emission applications. When single isolated CNs were depositedusing this technology, the structures exhibited remarkable uniformity in terms ofdiameter and height (standard deviations were 4.1 and 6.3% respectively of theaverage diameter and height). The lithographic conditions to achieve a high yieldof single CNs are also discussed. Using the height and diameter uniformitystatistics, we show that it is indeed possible to accurately predict the average fieldenhancement factor and the distribution of enhancement factors of the structures,which was confirmed by electrical emission measurements on individual CNs in anarray.

Controlled Growth and Applications of Carbon Nanotubes

This article reviews recent results in the field of nanotube research obtained at EPFL. We show in particular that with catalytic deposition techniques such as chemical vapor deposition carbon nanotubes with controlled properties can be obtained. We then discuss the functionalization of these CVD-grown nanotubes for the realization of fibers and composites, as well as their use in field emission devices.

TRANSPORT PROPERTIES OF TEMPLATE-SYNTHESIZED GOLD AND CARBON NANOTUBE MEMBRANES

We discuss the fabrication of gold and carbon nanotubes prepared by the template method. The gold nanotubes are prepared via electroless deposition of Au onto the pore walls of a nanoporous polycarbonate filtration membrane. Carbon nanotube membranes (CNMs) are prepared by doing chemical vapor deposition of carbon within the pores of a microporous alumina template. The pores in these filtration membranes act as templates for the nanotubes. We have shown that by controlling the Au deposition time, Au nanotubes that have effective inside diameters of molecular dimensions (<1 nm) can be prepared. These membranes are a new class of molecular sieves and can be used to separate molecules based on size. Furthermore, we describe fundamental investigations of electro osmotic flow (EOF) in carbon nanotube membranes. In addition, an electrochemical derivatization method was used to attach carboxylate groups to the nanotube walls; this enhances the anionic surface charge density, resulting in a corresponding increase in the EOF rate in the CNM.

Characterization of plasma-enhanced chemical vapor deposition carbon nanotubes by Auger electron spectroscopy

Plasma-enhanced chemical vapor deposition (PECVD) is a versatile technique for growing well-aligned, precisely patterned, multiwalled carbon nanotubes directly on substrates. We report on the characterization of PECVD deposited nanotubes using Auger Electron Spectroscopy (AES); we believe that this is the first comprehensive AES study of nanotubes and the effect of the deposition process on the substrate. The nanotubes contained well-crystallized graphitic carbon, in contrast to the amorphous/disordered carbon byproduct which is condensed on the substrate surface. By adjusting the deposition gas ratios, we show, using depth-profiled composition analysis, that it is possible to eliminate the unwanted amorphous carbon on the substrate surface. However, a 5 nm interfacial layer, which contained the plasma species, was always present on the substrate surface due to its exposure to the plasma. We could prevent the formation of this interfacial layer by shielding areas of the substrate from the plasma to achieve truly byproduct free deposition. This technique has allowed us to fabricate promising microelectronic field emission devices using vertically aligned carbon nanotubes.

Electroosmotic flow in template-prepared carbon nanotube membranes.

Carbon nanotube membranes (CNMs) were prepared by doing chemical vapor deposition of carbon within the pores of a microporous alumina template. Electroosmotic flow (EOF) was driven across the CNMs by allowing the membrane to separate two electrolyte solutions and using an electrode in each solution to pass a constant ionic current through the nanotubes. EOF was investigated by measuring the flux of a probe molecule (phenol) across the CNM. The as-synthesized CNMs have anionic surface charge, and EOF is in the direction of cation migration across the membrane. Measurements of the rate of EOF as a function of applied transmembrane current provided the zeta potential. The effect of pH on zeta provided the pK(a) for the surface acidic sites responsible for this anionic charge; the acidic-site density was also determined. An electrochemical derivatization method was used to attach carboxylate groups to the nanotube walls; this enhances the anionic surface charge density, resulting in a corresponding increase in the EOF rate. Electrochemical derivatization was also used to attach cationic ammonium sites to the nanotube walls to yield CNMs that show EOF in the opposite direction of the as-synthesized or carboxylated membranes.

The role of back-mixing in the decomposition of methane

Mechanisms proposed by Roscoe and Thompson as well as by Dean were used to investigate the influence of back-mixing on the decomposition of methane. Increasing degrees of back-mixing were simulated by decreasing numbers of vessels in series. It is shown that back-mixing has a significant effect on the decomposition of methane; the rate may be increased by more than one order of magnitude. The results are important for the simulation of chemical vapor deposition of carbon from methane. This process is usually performed in flow reactors at reduced pressure and low flow rates, which implies that back-mixing or axial dispersion have to be considered.

Well-aligned carbon nanotube array membrane and its field emission properties

Ensembles of aligned and monodispersed carbon nanotubes (CNTs) can be prepared by templating method which involves fabrication of porous anodic aluminum oxide (AAO) template, control of catalytic iron particle size and chemical vapor deposition of carbon in the cylindrical pores of AAO. Here we show that template-synthesized CNTs can be fabricated as well-aligned nanoporous CNTs membrane, which can be directly used as an electron field emitter. A low threshold electric field of 2—4 V/μm and maximum emission current density of ～12 mA/cm 2 are observed. The results also show that the electron emission current is a function of the applied electrical field and the Fowler-Nordheim (F-N) plot almost follows a linear relationship which indicates a Fowler-Nordheim tunneling mechanism, and the field enhancement factor estimated is about 1100—7500. The simple and convenient approach should be significant for the development of nanotube devices integrated into field emission displays (FEDs) technology.

Chemical vapor deposition of carbon from methane at various pressures, partial pressures and substrate surface area/reactor volume ratios

Carbon deposition from methane was studied as a function of substrate length at a temperature of 1100 °C, total pressures ranging from 90 to 10 kPa at a constant methane partial pressure of 10 kPa, methane pressures ranging from 5 to 20 kPa, and residence times of 0.55 s and 1.1 s using substrates with surface area/volume ratios of 0.78, 1.65 and 3.55 mm−1. Various total pressures at constant methane pressure have significant influences on the deposition profile of carbon, but they are mainly caused by different boundary conditions. The methane pressure reference suggests saturation adsorption (Langmuir - Hinshelwood kinetics) upto a pressures of about 15 kPa. Increasing surface area/volume ratios leads to decreasing surface related deposition rates, because the deposition chemistry is changed.

Thermal diffusivity measurements of carbon materials using optical beam deflection

Abstract Optical beam deflection (OBD) is a non-contact, non-destructive method of evaluation of materials. It is a powerful tool for the thermal characterization of carbon materials, regardless of the morphology, geometry, thickness and the anisotropy of the sample. The theory of determining the thermal diffusivity of materials by OBD is summarized in detail. The efficacy of using OBD to measure thermal diffusivities is experimentally verified by a study of metallic samples. Finally, the ability of OBD to distinguish anisotropic chemical vapor deposition (CVD) carbon from isotropic CVD carbon is demonstrated by measuring the thermal diffusivities of samples as they are heat treated at a series of temperatures reaching beyond the graphitization temperature.

Chemistry and kinetics of chemical vapor deposition of pyrolytic carbon from methane

The paper summarizes various recent results on chemical vapor deposition of carbon from methane in the low temperature regime around 1100 °C. The following topics are discussed: gas phase reactions, surface/deposition reactions, interactions of gas phase and surface/deposition reactions, hydrogen inhibition, influence of temperature. Based on the results a simplified reaction scheme is presented which was successfully used to simulate the influence of temperature on carbon deposition. This simulation as well as the influence of the substrate surface area/reactor volume-ratio (3rd parameter of CVD) are treated in other papers of the conference.

The role of the substrate surface area/reactor volume ratio in chemistry and kinetics of chemical vapor deposition

Chemical vapor deposition of carbon from methane was used as a test reaction to investigate the influence of the substrate surface area/reactor volume ratio, [A S /V R ], on the deposition kinetics. Experiments were performed at an ambient pressure of about 100 kPa and a temperature of 1100 °C using methane at a partial pressure of 10 kPa and a methane/hydrogen mixture (P CH4 =17.5kPa, P H2 =2.5kPa). The [A S /V R ]-ratio was varied from 1.8 to 10, 20, 40 and 80 cm -1 . It is shown that surface related deposition rates are not constant, but decrease with increasing [A S /V R ]-ratio. This result indicates that any kinetics can be determined by changing this ratio, which is called the third parameter of CVD. Consequences are discussed.

Electron field emission and structural properties of carbon chemically vapor-deposited films

Abstract Low-voltage electron field emission was obtained for carbon films grown by the chemical vapor deposition (CVD) method in hydrogen–methane plasma activated by a d.c. discharge. We found that the electron field emission properties were improved by increasing the density of structural defects and non-diamond carbon inclusions in polycrystalline diamond films and, for the first time, we found that completely non-diamond CVD carbon films displayed the best field emission characteristics. The threshold electric field for the completely non-diamond CVD carbon film cathodes was as low as 1.5 V μm −1 , the emission current reached 1 mA cm −2 , and the emission site density exceeded 10 6 cm −2 at the field of 4 V μm −1 . Based on RHEED, HRTEM, Raman and cathodoluminescent data of CVD films grown on silicon substrates at various methane percentages in the gas mixture, we propose a general mechanism for cold electron emission from materials containing graphite-like carbon.

Metal-Nanocluster-Filled Carbon Nanotubes: Catalytic Properties and Possible Applications in Electrochemical Energy Storage and Production

Ensembles of highly aligned and monodisperse graphitic carbon nanotubules have been prepared via the template method using chemical vapor deposition of carbon within the pores of alumina membranes. Tubules with diameters of 200 nm have been prepared, and smaller diameters are possible. Free-standing aligned carbon-tubule membranes are formed by this template method. These novel carbon tubule membranes can be filled with nanoparticles of electrocatalytic materials (i.e., Pt, Ru, Pt/Ru), which can then be used to electrocatalyze O2 reduction and methanol oxidation as well as the gas-phase catalysis of hydrocarbons. Hence, these membranes have potential applications in fuel cell development. Smaller, highly ordered graphitic-carbon tubules can also be prepared within the template-synthesized carbon tubules, using Fe nanoparticles as catalysts. In these novel tube-in-tube structures, both the outer and the inner tubules are electrochemically active for Li+ intercalation, suggesting possible applications such ...