Deposition Of Carbon Layers Research Articles

Structural, chemical and biological properties of carbon layers sputtered on polyethyleneterephtalate

Carbon layers on polyethyleneterephtalate (PET) backing were prepared by sputtering from graphite target. UV-VIS, Raman spectroscopy, RBS (Rutherford backscattering) and ERDA (Elastic Recoil Detection Analysis) techniques were used for the characterization of the layers. Surface morphology of the layers was determined by AFM technique and the adhesion of 3T3 mouse fibroblasts on the layers was studied in vitro. It was found that the properties of the deposited carbon layer depend on the sputtering time. The concentration of conjugated double bonds, fraction of amorphous hydrogenated carbon (a-C:H) containing oxygen and surface roughness are increasing functions of the sputtering time. The changes of the layer surface morphology with increasing sputtering time were also observed. For the sputtering times up to 30' the number of adhering 3T3 cells increases with increasing sputtering time. For longer sputtering times, however, the cell adhesion becomes lower probably due to unfavorable changes in roughness and morphology of the layer.

Analysis for hydrogen particle balance of plasma-wall system in the large helical device

The hydrogen particle balance of the plasma-wall system in the large helical device (LHD) is analyzed, using a zero dimensional model for plasma particles, neutrals in vessel and hydrogen inventory in wall. Based on the measurement of neutral gas pressure, plasma density and the pumping speed of the cryo-pumping system, it is found that the hydrogen retained in the wall desorbes with short and long time constant. The short term desorption is of order of 10 21 atoms with a time constant of a few minutes, which is much smaller than the wall pumping for one shot, 10 22 atoms. In a long time scale of about one experimental day, the wall absorbs significantly large amounts of hydrogen, up to 10 24 atoms. One of the possible reasons for the large wall pumping is a carbon deposition layer on the first wall surface. The effect of hydrogen retention on density control is also discussed.

Plasma deposition of carbon layer: Correlations between plasma parameters, film structure and properties

The links between plasma parameters, discharge, film structure and properties are not really yet understood for amorphous hydrogenated carbon layers (a-C:H) plasma deposition. Here, a-C:H layers are deposited in a dual radio-frequency–multipolar microwave plasma excited by distributed electron cyclotron resonance reactor at low CH 4 pressure. This study deals with the plasma analysis, the film characterization and with plasma parameters effect on a-C:H films deposition, structure and properties. The discharge analysis shows that CH 4 is decomposed in CH y1 + H radicals and that C 2H 2 y2 are present in the discharge. As at low pressure, recombination can only take place on the surface, C 2 H 2 y2 desorbs from the surface. Moreover, C 2 species are observed attributed to C 2 H 2 y2 dissociation. The evolution of film composition with plasma power shows that the proportion of sp 2 CC decreases in contrast with those of CH bonds which increases. From these observations, a phenomenological model for a-C:H deposition can be proposed. Finally, properties are correlated with the film structure and the effect of MW plasma power can then be given.

Thermo-oxidation to remove re-deposited layers and to release trapped hydrogen isotopes in HT-7 superconducting tokamak

Thermo-oxidation between 0.7 Pa and 32 Pa at wall temperatures of 400–470 K has been investigated in the HT-7 superconducting tokamak. During the oxidation process, deposited carbon layers were removed by the formation of CO and CO 2 and the incorporated hydrogen was released in the form of water molecules. A significant fraction of the injected oxygen was also absorbed on the walls. The overall amount of adsorbed oxygen is nearly linear with the filling pressure. The results of the thermo-oxidation experiment in HT-7 were similar to those in TEXTOR. High wall temperature and high oxygen pressure are helpful in removing carbon co-deposits and releasing trapped hydrogen.

On the possibility of ITER starting with full carbon

ITER is planned to start with Be as first wall armors, and W and CFC as divertor armors. Although Be is an excellent oxygen getter, its low melting point makes it unstable to use in a burning reactor. The performance of W armor in ITER-like plasma is still quite uncertain. Moreover, operating temperature window of W would be limited due to its brittleness at lower temperatures and significant grain growth at higher temperatures. Carbon is presently the most reliable plasma facing material, if we have to allow ITER plasma more flexibility and off-normal events, which should be unavoidable to obtain physics bases to attain burning plasma. Appropriate selection of divertor structure and fine tile alignment would reduce carbon deposition significantly. And if we could keep divertor surface temperature of the deposition area above 1000 K, tritium retention could be significantly reduced. We should not exclude the possibility of carbon as PFM even in a fusion reactor. Since the operating temperature of a fusion reactor is likely be above 800 K, tritium retention in carbon is not likely the problem, and erosion could be repaired by deposition of carbon layers between shots and the deposited layers must be graphitized by a succeeding shot.

Erosion of carbon deposition layer by hydrogen RF plasma

A carbon deposition layer was formed by sputtering method in hydrogen RF plasma at an RF power of 150 W and then eroded in a plasma at an RF power of 250 W. The sheath bias between plasma and the carbon deposition layer was measured to be about 10 V by Langmuir probe method. The erosion yield (C/ion) of carbon deposition layer was determined to be 0.31, which is one order of magnitude larger than that of isotropic graphite, 0.026. It was found that the plasma facing surface of the eroded carbon deposition layer became mesh-like and the inside of the layer became more porous. The atomic ratio of hydrogen to carbon (H/C) in the carbon deposition layer eroded for more than 6 h obviously decreased as compared to the initial value.

Hydrogen retention in carbon–tungsten co-deposition layer formed by hydrogen RF plasma

Carbon–tungsten co-deposition layers (C–W layers) were formed by sputtering method using hydrogen or deuterium RF plasma. The deposition rate of the C–W layer by deuterium plasma was faster than that by hydrogen plasma, where the increase of deposition rate of tungsten was larger than that of carbon. This indicates that the isotope effect on sputtering–depositing process for tungsten is larger than that for carbon. The release curve of hydrogen from the C–W layer showed two peaks at 400 °C and 700 °C. Comparing the hydrogen release from the carbon deposition layer and the tungsten deposition layer, it is considered that the increase of the release rate at 400 °C is affected by tungsten and that at 700 °C is affected by carbon. The obtained hydrogen retention in the C–W layers which have over 60 at.% of carbon was in the range between 0.45 and 0.16 as H / (C + W).

Detailed Tritium Distribution on the JET MK IIA Divertor Tiles

Tritium surface distributions on the plasma-facing surface and four sides of JET Mk IIA divertor tiles employed in the D-T operation phase of JET were measured by Tritium Imaging Plate Technique (TIPT). Tritium distribution on the plasma-facing surface was consistent with carbon deposition profiles and asymmetric in both poloidal and toroidal directions. The toroidal asymmetry was attributed to the alignment of the tiles preventing direct impact of flux lines to tile edges. Accordingly, no significant carbon deposition or tritium accumulation was observed on two sides facing the toroidal direction. As already reported, heavy codeposition retaining high levels of tritium was observed on the plasma-shadow area of the horizontal target tile surface and the bottom side of the vertical target tile of the inner divertor region where it was kept relatively cool by water coolant. In addition, TIPT has clearly distinguished at least two different carbon deposition layers with different tritium retention in poloidal direction, showing that the poloidal asymmetry on the horizontal target tiles is due to the different carbon deposition properties in the poloidal direction. All the results suggest that tritium retention in the divertor area, which was determined by the carbon/hydrocarbon distribution, correlates closely with divertor geometry and surface temperature.

Melt Crystallization of Constrained Polyethylene Chains in Uniaxially Drawn Film and Solution-Grown Lamellae

When a uniaxially drawn polyethylene film was coated by evaporation of carbon in a vacuum, the deposited carbon layer fixed the texture and molecular orientation. Polyethylene single crystals were fixed at the fold surface on a carbon pedestal by carbon deposition. Since molecular chains of these samples were anchored on the carbon pedestal, their translational displacement on the pedestal, and longitudinal sliding along their axis, were largely restricted even when their thermal motions were activated at high temperatures. Consequently, the original molecular direction and the stacked lamellar morphology in drawn film, and the lozenge shape of polyethylene single crystals, were retained in the molten state. On crystallizing from melt, the stacked lamellar structure was reconstructed in the drawn film as double orientation was performed, and the c axis was parallel to the drawing direction, as the b axis was aligned preferentially parallel to the film surface. In single crystals, constrained chains crystallized in such a way that the b axis was kept in the lamellar plane, and their chain axis was tilted toward the long axis of the lozenge (the a axis) from the lamellar normal with an angle of 30°−54°.

Long distance coupling of lower hybrid waves in JET plasmas with edge and core transport barriers

Efficient coupling of lower hybrid (LH) waves in conditions close to those foreseen in ITER has been obtained in advanced scenario plasmas in the JET tokamak. Up to 3 MW of lower hybrid current drive (LHCD) power has been coupled at a distance between the separatrix and the launcher of 11 cm, in the presence of edge localized mode activity. The key to the improved LH wave coupling is local control of the Scrape-Off-Layer (SOL) density through gas injection in the region magnetically connected to the launcher. This increases the electron density in front of the launcher so as to improve the coupling of the LH waves, i.e. reduce the reflected power in the launcher. The average power reflection coefficient was 5.7% with gas injection, at 11 cm distance between the separatrix and the launcher. A change in the gas injection design has made the gas puffing more efficient, making the use of D2 injection possible as an alternative to CD4. At similar injected electrons/s rates, D2 gives higher electron density in the SOL than CD4, resulting in better LH coupling with D2. The possibility of using D2 instead of the earlier used CD4 is an encouraging result in view of ITER operation, as CD4 may not be compatible in ITER due to the problem of tritium retention in deposited carbon layers.

Deuterium concentration in deposited carbon layers in Tore Supra

Deposited carbon layers observed in the tokamak Tore Supra have been sampled in order to quantify their deuterium content using D(3He,p)4He nuclear reaction analysis. The results show two trends: the D/C ratio in wetted deposited carbon layers is less than 1% and the D concentration depth profile decreases rapidly from the first micrometers of the plasma facing sub-surface to the rear surface, while the D/C ratio in shadowed deposited carbon layers reaches about 10% and shows a more constant profile along the probed depth. Using the information about D concentrations and of the maximum thickness of the different samples and with the estimated deposited areas, an estimated D inventory is found to be about 1×1023D, which is too low to account for the D retention expected for a whole long plasma discharge campaign.

Raman spectroscopy and X-ray diffraction studies of some deposited carbon layers in Tore Supra

Carbon deposited layers collected in Tore Supra on the neutralizers, on the toroidal pumped limiter and on the vertical outboard limiter have been analysed by X-ray diffraction and Raman spectroscopy. Both techniques show that these samples are highly disordered, mainly consisting of graphite-like nanoparticles. They are not similar to amorphous or hydrogenated amorphous carbons: this is consistent with the low amount of deuterium found trapped inside. Their structure can be related to their vicinity to plasma, particle bombardment inducing high temperature and preventing full amorphization. Samples exposed to the highest fluency in the machine show the largest degree of disorder, with probably a minor contribution of three-dimensional disorder, which may be due to the formation of bonds with deuterium.

Thermal Behaviour of Deposited Carbon Layers on the Neutraliser Plates in Tore Supra

The temperature measured on the surface of particle neutralisers of Tore Supra, i.e. the target plates under the toroidal pump limiter that channel particles into the pumping ducts, is significantly higher than expected because of thick carbon deposits formed on those plates. With growing deposit thickness their temperature and cooling time constants increase. Lower hybrid current drive heating and gas puffing seem to enhance the layer growth. The deposits form at first isolated hillocks giving rise to an enhanced luminance in the near infrared range. Later on, they may form continuous films (with black-body like spectra) which may flake off again distorting the spectra. The thermal behaviour of thick continuous films can be described by a radiative model. The conducted power flux density to the neutralisers for which this model is in accordance with the measurements is of the order of 0.1 MW/m2 in ohmic discharges. This requires characteristic scrape-off layer decay lengths of 7 to 8 mm as far as 5 cm outside the last closed magnetic flux surface. The thermal diffusivities deduced with this model from observations in Tore Supra and from laboratory experiments show values 2 to 3 orders of magnitude below the values characteristic for carbon fibre composites.

In situ oxygen plasma cleaning of a PECVD source for hard disk overcoats

Reducing the thickness of the diamond-like carbon protective overcoat to a thickness of about 2–3 nm is one major key to increase the recording density of magnetic disk drives. Plasma-enhanced chemical vapor deposition (PECVD) deposited carbon layers have been shown to be denser and harder than those produced by conventional sputter deposition. One key problem of PECVD deposited carbon is the contamination of the carbon film by particles produced inside the carbon source after long-time operation. This particle production limits the runtime of the source drastically. To avoid this particle generation the source was cleaned by an intermittent in situ oxygen plasma process. The cleaning efficiency was investigated by recording the pressure change inside the source during the cleaning process caused by the CO production. The ratio of the cleaning time and the deposition time shows no significant dependence on the deposition time. An almost linear increase of the ratio with the acetylene flow was observed. This results from a higher deposition rate at higher acetylene flow, leading to a higher contamination inside the source. A strong dependence of the cleaning rate on the oxygen flow in the cleaning process was measured. More oxygen leads to a strong decrease of the needed cleaning time. Adding Ar gas to the oxygen discharge shows no improvement of the needed cleaning time. The cleaning process seems to be dependent only on the amount of reactive oxygen species in the discharge .

Amorphous carbon layer deposition on plastic film by PSII

Plasma-source ion implantation (PSII) is successfully employed without an external radio frequency plasma source to form a thin amorphous carbon layer on the surface of polyethylene terepthalate (PET) film by applying pulsed high negative voltage (∼10 μs pulse width, 300–900 pulses/s, −10 kV) to a sample immersed in C 2H 2 gas with and without Ar. This pulsed voltage condition is found to itself generate a plasma surrounding the substrate and PET film to form a thin carbon layer. The oxygen barrier characteristics of the PET were improved remarkably by this PSII processing, and laser Raman, X-ray photoelectron and Fourier-transform infrared spectroscopy spectra revealed that the thin amorphous carbon layer was found to consist primarily of graphite crystal with the characteristics of diamond-like carbon. The ratio of sp 2 (graphite) to sp 3 (diamond) populations was found to vary according to the gas pressure of C 2H 2 and the addition of Ar, which affects on the oxygen barrier characteristics.

Growth of carbon nanotubes by chemical vapor deposition

Abstract The growth behavior of carbon nanotubes (CNT) deposited from C2H2 by thermal CVD method was investigated. Nickel particles of diameter ranging from 15 to 90 nm were used as the catalyst. CNTs were deposited in various environments of N2, H2, Ar, NH3 and their mixtures to investigate the effect of the environment on the CNT growth behavior. The deposition was performed at 850°C in atmospheric pressure. In pure N2 environment, thick carbon layer deposition occurred on the substrate without CNT growth. The Ni particles encapsulated by the carbon deposition could not work as the catalyst in this condition. However, the growth of CNT was enhanced as the H2 concentration increased in the mixture of N2 and H2 environment. In pure H2 environment, randomly tangled CNTs could be obtained. The growth of CNT was much enhanced when using NH3 as the environment gas. Vertically aligned CNTs could be deposited in NH3 environment, whereas the CNT growth could not be obtained in the mixture of N2 and H2 environment of the same ratio of N/H. These results were discussed in terms of the passivation of the catalyst caused by the excessive deposition of carbon on the catalyst surface. For the deposition of the CNT, the decomposition rate of C2H2 should be controlled to supply carbon for nanotube growth without passivation of the catalyst surface by excessive carbon deposition. The present work showed that the composition of environment gas significantly affects the reaction kinetics in the CNT growth. It is also noted that nitride surface layer formation on Ni catalyst in NH3 environment can affect the CNT growth behavior.

Superpermeable membrane for particle control in divertor: the effect of impurity deposition

The effect of impurity (stainless steel (SS) components, carbon) deposition onto niobium membrane surface on the membrane permeability to hydrogen particles is investigated with a plasma device. The deposition of SS components onto the upstream surface of the membrane at the membrane temperature ( T M)<800°C decreases plasma driven permeation (PDP) rate but increases molecule gas driven permeation (GDP) rate. In the case of carbon deposition at T M<800°C, PDP rate is reduced while GDP rate remains almost unchanged. It is argued that deposited SS components layer enhances the absorption/release of hydrogen molecules, on the other hand, deposited carbon layer lets hydrogen molecules freely pass through but traps suprathermal hydrogen particles. The impurities deposition effects on PDP rate disappear at T M≥800°C. It appears to be due to the dissolution of the impurities deposited onto the upstream surface into the membrane bulk within the measurements.

Theory of long term gain growth in traveling wave tubes

The properties of the attenuator materials used in traveling wave tubes (TWT) are a major factor in determining the gain of the tube. Carbon is a commonly used attenuator material which is typically deposited on the dielectric rods used to support the helix slow wave structure in TWTs. The deposited carbon layers are susceptible to ion bombardment induced damage during operation and outgassing of the tube, which increases the electrical resistivity of the layer and reduces the rf loss in the tube. The total gain in the tube grows during this process until the bombardment ceases or the layer damage saturates. A simple theory is developed that relates this gain change to the gas evolved in the tube that results in the particle bombardment of the layers during operation. The model accurately describes the observed time dependent behavior of the gain, and the time constant for outgassing the tube is found for several tubes ranging from S to Ka band. The time constant increases significantly for smaller, high frequency tubes with a low conductance and poor pumping speeds. The model shows that stabilization of the gain occurs after several time constants have passed, which may take hundreds to thousands of hours.

Oriented recrystallization of pre-oriented thin polymer films

The effects of a thin vacuum evaporated carbon film on the subsequent melting and recrystallization behavior of pre-oriented thin polymer films were studied by transmission electron microscopy and electron diffraction. It is found that a thin vacuum deposited carbon layer can keep the orientation of the pre-oriented thin polymer films after a complete melting and recrystallization. This process is repeatable. The recrystallization of the polymers can be performed even with the polymer films heated to a temperature more than 100°C above their melting temperatures and kept at that temperature for more than 10 h.

Fullerenes as nuclei of carbon black particles

The existing models for the structure and inception processes of carbon black particles are analyzed taking into account the experimental data on the cluster formation. A new model of the formation of carbon black particles is proposed. The inference is made that single-layer fullerenes being the minimum possible closed carbon black particles serve as nuclei of large-sized carbon black particles. These particles grow via the deposition of carbon layers on the surface of fullerenes with defects. The defects fulfill the role of seeds that give rise to the front of the layer growth. The defect-free fullerenes do not take part in the carbon black formation due to the thermodynamic stability as a result of structural perfection. The primary carbon black particles—globules—are many-layer formations with a hole in the center whose size and configuration match the size and configuration of the initial fullerene nucleus. The layers in the form of concentric spheroids inserted into each other are composed of distorted and defect carbon networks. The flatest parts of several outer layers of carbon black particles are the regions of the coherent X-ray scattering.