Carbon Condensation Research Articles

Synthesis of high purity single-walled carbon nanotubes from ethanol by catalytic gas flowCVD reactions

Highly pure single-walled carbon nanotubes (SWNTs) with their contents>96% of the total products were achieved by chemical vapour deposition gas flow reactions usingethanol as the carbon source. The high purity SWNTs were formed under criticallycontrolled synthesis conditions and by applying a high hydrogen flow, under which thenon-catalytic condensation of carbon was completely suppressed. Purification of theproducts in acid at ambient conditions readily yielded over 99% pure SWNT products, asthe only impurities left in the products were iron particles of clean surface. Therefore, thepresent study demonstrates the full potential of the CVD gas flow reactions in continuousproduction of high quality SWNTs. Comparable syntheses were conducted usingother alcohols in place of ethanol and it was found that high alkyl alcohols likeisopropanol and hexanol produced more amorphous carbon while methanol produced nocarbon. The high yield growth of SWNTs was attributed greatly to the reactionchemistry of ethanol and the ‘right’ amount of hydrogen in the system, as discussed.

The Deep Impact oblique impact cratering experiment

The Deep Impact probe collided with 9P Tempel 1 at an angle of about 30° from the horizontal. This impact angle produced an evolving ejecta flow field very similar to much smaller scale oblique-impact experiments in porous particulate targets in the laboratory. Similar features and phenomena include a decoupled vapor/dust plume at the earliest times, a pronounced downrange bias of the ejecta, an uprange “zone of avoidance” (ZoA), heart-shaped ejecta ray system (cardioid pattern), and a conical (but asymmetric) ejecta curtain. Departures from nominal cratering evolution, however, provide clues on the nature of the impact target. These departures include: fainter than expected flash at first contact, delayed emergence of the self-luminous vapor/dust plume, uprange-directed plume, narrow early-time uprange ray followed by a late-stage uprange plume, persistence of ejecta asymmetries (and the uprange ZoA) throughout the approach sequence, emergence of a downrange ZoA at late times, detachment of uprange curved rays, very long lasting non-radial ejecta rays, and high-angle ejecta plume lasting over the entire encounter. The first second of crater formation most closely resembles the consequences of a highly porous target, while later evolution indicates that the target may be layered as well. Experiments also reveal that impacts into highly porous targets produce a vapor/dust plume directed back up the incoming trajectory. This uprange plume is attributed to cavitation within a narrow penetration funnel. The observed lateral expansion speed of the initial vapor plume downrange provides an estimate for the total vaporized mass equal to ∼ 5 m p (projectile masses) of water ice or 6 m p of CO 2. The downrange plume speed is consistent with the gas expansion added to the downrange horizontal component of the DI probe. Based on high-speed spectroscopy of experimental impacts, the observed delay in brightening of the DI plume may be the result of delayed condensation of carbon, in addition to silicates. Observed molecular species in the initial self-luminous vapor plume likely represent recombination products from completely dissociated target materials. The crater produced by the impact can be estimated from Earth-based observations of total ejected mass to be 130–220 m in diameter. This size range is consistent with a 220 m-diameter circular feature at the point of impact visible in highly processed, deconvolved HRI images. The final crater, however, may resemble an inverted sombrero-hat, with a deep central pit surrounded by a shallow excavation crater. Excavated distal material observed from the Earth was likely from the upper few meters contrasted with ballistic ejecta observed from the DI flyby, which included deep materials (10–30 m) within the diffuse plume above the crater and shallower (5–10 m) materials within the ejecta curtain.

Coarse Columnar Structure of Transformation-Grown Ferrite in Pure Iron —On Wrought Iron and Sintered Iron—

When a nucleus of ferrite crystal is formed in cast-and-rolled or sintered iron, either of which contains less than approximately 50 ppm interstitial elements, namely, carbon and nitrogen, the ferrite crystal may grow into a coarse crystal joining many austenite crystals. Furthermore, such coarse ferrite crystals may compose a columnar macrostructure by unidirectional phase transformation under the condition accompanying a gradient of temperature. The formation of the macrostructure gives a maximum linear expansion of the sample that is numerically equal to the volume expansion at the same time. Even a small amount of carbon in pure iron can cause the condensation of carbon atoms or the formation of a fairly large number of minute cementite precipitates at ferrite/austenite phase boundaries. Both the condensation of carbon and the precipitation of cementite may cause the nucleation of new ferrite crystals, leading to the development of a fine-grained macrostructure. [doi:10.2320/matertrans.47.2449]

Effect of the carbon isotope composition on the properties of diamond

Parameters of the interatomic interaction potential for 12C and 13C carbon isotopes in the crystal lattice of diamond have been determined. Based on these data, the isotope dependence of the properties of diamond such as the Debye temperature, molar heat capacity, thermal expansion coefficient, vacancy formation energy, self-diffusion activation energy, surface energy, and longitudinal sound velocity is described. This approach is used for estimating a change in the bulk compression modulus of lithium crystals upon the passage from 7Li to 6Li. As the temperature increases, the isotope dependence of the heat capacity at constant volume vanishes; at a certain temperature, the isotope dependence of the thermal expansion coefficient changes from growth to decay. The expected isotope dependence of the parameters of phase transitions is predicted. It is shown that carbon condensates formed upon deposition from the gas phase must be enriched with the heavy isotope.

Nano-Diamond Synthesis in Strong Magnetic Field

Diamond is one of the five known allotropes of carbon. Diamonds are important due to their esthetic beauty and excellently remarkable properties. Single crystalline diamond has been synthesized by catalytic processes under high temperature and high pressure. Polycrystalline diamond has been prepared by various activated hot filament and plasma chemical vapor deposition methods. Industrial synthesis of diamonds has been limited by the extreme synthetic conditions, purity, crystallinity, size and high cost. Synthetic advancement for future industrial production will require better understanding and exploitation of the mechanism of formation. Current mechanistic uncertainty arises out of limited consideration of atomic scale dynamics. A new comprehensive model by Little addresses atomic scale electronic processes with nonclassical significance of the intermediary states. This new nonclassical, electronic mechanistic perspective identifies the creation, stabilization and condensation of carbon and metal intermediates for pathways from carbon precursors thru solvated carbon and carbides to sp3 carbon condensation as diamond. In order to test these mechanistic ideas and exploit the impact for better diamond synthesis, catalytic carbon condensation has been explored in strong magnetic field in an effort to influence various radicals and high spin metal and carbon intermediates along the pathway for more efficient diamond condensation. In this effort, static magnetic fields in excess of 15 T are observed to direct these carbon and metal intermediary microstates to promote nano-diamond nucleation and growth at atmospheric pressure and temperature of 900°C. The observed nano-diamond formation is consistent with the predicted faster kinetics due to lower potential energies of intermediates along pathways to diamond. The predicted stability of nano-diamond relative to nano-graphite also accounts for the observed nano-diamond in this work. This novel use of static magnetic field for diamond synthesis is compared with advancements since the first synthetics bulk diamond formation by scientists at both ASEA (Sweden) and GE Research Laboratory, Schenectady, NY, 50 years ago. On the basis of this predicted, invented, and intrinsic magnetic influence on the dynamics of carbon condensation and the promise for faster, feasible single crystal diamond formation, this discovery of the use of strong magnetic field for diamond production is asserted a major advancement during this 50-year period since the first successful synthesis.

Plasma CVD characterization of nanocarbon film growth

AbstractWe report on the characterization of d.c. discharge plasma‐enhanced chemical vapour deposition of nanocarbon film materials in a hydrogen–methane gas mixture. The effects of the gas composition and pressure on the current–voltage characteristics and optical emission spectra of the discharge plasma are studied. By varying the parameters we obtain various carbon thin‐film materials, the structure and composition of which are characterized qualitatively by Raman spectroscopy and electron microscopy. The data obtained by optical emission spectroscopy show the presence of H, H2, CH and C2 activated species in the discharge plasma. We assume that C2 dimers play a decisive role in nanostructured graphite‐like carbon film formation and carbon condensation in the gas phase. Copyright © 2004 John Wiley & Sons, Ltd.

Chemical models for simulating single-walled nanotube production in arc vaporization and laser ablation processes.

Chemical kinetic models for the nucleation and growth of clusters and single-walled carbon nanotube (SWNT) growth are developed for numerical simulations of the production of SWNTs. Two models that involve evaporation and condensation of carbon and metal catalysts, a full model involving all carbon clusters up to C80, and a reduced model are discussed. The full model is based on a fullerene model, but nickel and carbon/nickel cluster reactions are added to form SWNTs from soot and fullerenes. The full model has a large number of species--so large that to incorporate them into a flow field computation for simulating laser ablation and arc processes requires that they be simplified. The model is reduced by defining large clusters that represent many various sized clusters. Comparisons are given between these models for cases that may be applicable to arc and laser ablation production. Solutions to the system of chemical rate equations of these models for a ramped temperature profile show that production of various species, including SWNTs, agree to within about 50% for a fast ramp, and within 10% for a slower temperature decay time.

Change of internal stress of carbon superhard condensates at a process of annealing

Superhard carbon coatings with thickness from 5 nm up to 10 μm were obtained by vacuum-arc deposition of carbon plasma on sublayers from stainless steels, titanium and silicon under the temperature from 50 up to 150 °C. Correlation dependences between conditions of formation of carbon condensates and change of internal stresses in them at a process of annealing in a range of temperatures from 200 up to 475 °C were examined. It was shown that the character of internal stresses changing at a process of annealing of carbon condensates had proof correlation with the parameters of deposition (the temperature of sublayer, duration of a pulse of discharge) and it could be used for identification of coatings and for control of their properties. It was determined that superthin layers of carbon obtained by impulse vacuum-arc method effectively protected a surface from oxidation in an atmosphere of air up to 425 °C. Researches of surface structures of coatings by use of scanning probe microscopy (SPM) were carried out. It was classified as fractal and an explanation of mechanism of change of internal stress value in carbon coatings at a process of annealing was given.

Comparative study of removal of volatile organic compounds by cryogenic condensation and adsorption by activated carbon fiber

This paper describes studies on two abatement techniques, namely: adsorption by activated carbon fiber (ACF) and condensation at cryogenic temperatures for controlling emissions of volatile organic compounds (VOCs). The breakthrough analysis was carried out on commercially ACF packed in a tubular reactor under dynamic adsorption conditions. The breakthrough time was found to significantly decrease with increase in VOC concentration from 5000 to 50,000 ppm. The regeneration of ACF was carried out by electrical (dc ∼50 V) heating. A temperature range of 120–150 °C and regeneration time of 45–60 min were typically required for the complete regeneration of ACF pre-equilibrated with 8000 ppm of VOC. The VOC removal by condensation was carried out in a vertical tubular condenser using liquid nitrogen (LN) as a coolant medium. The studies showed that adsorption was effective if VOCs concentrations in the pollutants laden gas were in parts per million (ppm) levels, whereas the removal by cryogenic condensation was found to be effective at the relatively higher VOCs concentrations levels (>1%). The simulation results from the mathematical models developed to understand the separation process in each of two methods studied were found to be in good agreement with the experimental data.

Synthesis and properties of plasma-deposited carbon condensates

Structural data, thermal characteristics, and theoretically calculated binding energies are reported for a graphite condensate obtained by carbon deposition from plasma. It is demonstrated that this condensate can be effectively used in self-propagating high-temperature synthesis processes.

In situ plasma diagnostics for chemical vapor deposition of nano-carbon thin film materials

We report on the study of nano-carbon film deposition by in situ measuring of the optical emission and electrical parameters of the d.c. discharge in a methane–hydrogen gas mixture. By changing the deposition process parameters over a wide range, we obtain a variety of carbon thin film materials, the structure and composition of which are qualitatively characterized by a number of methods including the Raman and electron microscopy. The obtained results show the presence of H-, H 2-, CH-, and C 2-activated species in the discharge plasma. These species play a decisive role in the formation of nano-carbon graphite-like films including carbon nanotubes, and in carbon condensation in gas phase. We propose a model that describes the formation of graphitic nanostructured carbon films in the C 2-dimers enriched plasma.

DC discharge plasma studies for nanostructured carbon CVD

A synthesis of carbon films by d.c. discharge plasma-enhanced chemical vapor deposition using a hydrogen–methane gas mixture was investigated by optical emission spectroscopy and by measurements of current–voltage dependencies. The effects of gas composition and pressure on the characteristics of d.c. discharge in the methane–hydrogen gas mixture are studied. Variation of the deposition process parameters over a wide range allows us to obtain various carbon thin film materials, whose structure and composition were qualitatively characterized by Raman spectroscopy and electron microscopy. The data of optical emission spectroscopy show the presence in the discharge plasma of H, H 2, CH and C 2 activated species, which play a decisive role in nanostructured graphite-like carbon film formation and carbon condensation in the gas phase. We propose a model for the formation of graphitic nanostructured carbon films in plasma containing C 2 dimers.

LOW HYDROGEN CONTENT HYDROGENATED AMORPHOUS CARBON FILMS USING DIRECT CURRENT GLOW DISCHARGE PLASMA CVD

We report on the growth of hydrogenated amorphous carbon (a-C:H) films, using a simple dc glow discharge plasma chemical vapor deposition method. A variety of complementary techniques were employed to analyze the properties of these films. a-C:H films having high fraction of tetrahedrally ( sp 3) bonded carbon atoms (~ 70%) and low hydrogen content (< 5at.%) were obtained. These films were characterized by a high optical bandgap ~ 2.7 eV, and refractive index (at 600nm) ~ 2.2. Growth of metastable diamond structure in a-C:H is attributed to energetic condensation of carbon containing ions in ( sp 3) configuration.

Ablation and carbon deposition induced by UV laser irradiation of polyimide: Application to the metallization of VIAs in high density printed circuit boards

Polyimides are known to exhibit large ablation rates upon irradiation with excimer laser due to their high absorbance in the UV and low fluorescence yield. We have studied different regimes of laser ablation according to the fluence and studied the structures resulting from carbon products deposition. For fluences larger than the polyimide ablation threshold, but lower than the carbon one, the development of one structure is the dominant process, whereas large ablation rates lead to polyimide etching above the carbon ablation threshold. The deposition of a carbon layer on the walls of ablated slits has in particular been investigated using an original experimental technique. Optical microscopy and MEB have shown that this carbon layer covered the main height of the ablated holes whereas a threshold (bare polyimide) for carbon condensation was evidenced at the bottom of the ablated hole. Raman spectroscopy and conductivity measurements have shown that the carbon phase is mainly graphitic. A dependence of the carbon condensation threshold on the slit width has been evidenced and discussed in relation with the angle of ejection of ablation debris. A procedure has been developed to render the bare polyimide of the threshold region conductive. At last the conductive properties of the walls of the ablated holes have been exploited to perform an electrolytic metallization. Implications for the production of interconnection vertical interconnections (VIAs) in high density printed circuits are addressed.

Condensation of Carbon in Radioactive Supernova Gas

The chemistry of carbon molecules leading to the formation of large carbon-bearing molecules and dust in the interior of an expanding supernova is explored and the equations governing their abundances are solved. A steady state between production and destruction is set up early and evolves adiabatically as the supernova evolves. Simple solutions for that steady state limit yield the abundance of each linear carbon molecule and its dependence on the C/O atomic ratio in the gas. Carbon dust condenses from initially gaseous C and O atoms because Compton electrons produced by the radioactivity cause dissociation of the CO molecules, which would otherwise form and limit the supply of C atoms. The resulting free C atoms enable carbon dust to grow faster by C association than its destruction by oxidation for various C/O ratios. Nucleation for graphite growth occurs when linear molecules transition to ringed C n molecules. We survey the dependence of the abundances of these molecules on the C/O ratio and on C n several other kinetic rate parameters. The concept of ii population control ˇˇ is signi—cant for the maximum sizes of carbon particles grown during supernova expansion. Interpretation of presolar micrometer-sized carbon solids found in meteorites and of infrared emission from supernova is relaxed to allow O to be more abundant than C, but the maximum grain size depends upon that ratio.

Application of Planck-Rosseland-Gray Model for High-Enthalpy Arc Heaters

An upgraded ARCFLO computer code using a radiation model named Planck-Rosseland-Gray(PRG) model is developed. The developed method is capable of analyzing both an air and a carbonaceous gas flow in the constrictor of an arcjet wind tunnel. In the first part of this paper, the method is tested against the experimental data for air. The testing is carried out by modifying the turbulence parameters employed in the ARCFLO code. Radiation calculation is made by accounting for electron thermal nonequilibrium. Agreements between experimental data and calculation are obtained by choosing a set of the turbulence parameter appropriately. In the second part, a carbonaceous gas flow is preliminary calculated with the turbulence parameter so chosen. The depth of carbon condensation is evaluated approximately. The characteristic data such as mass averaged enthalpy, etc., are presented for typical operation conditions.

Removal of ethylacetate vapor from waste gases by a trickle-bed air biofilter

Biofilter system is a relatively new process that has been proven to be more cost-effective than traditional technologies such as carbon adsorption, liquid scrubbing, condensation, thermal incineration, and catalytic incineration for removing low-strength volatile organic compounds from waste gases. The trickle-bed air biofilter (TBAB) performance for ethylacetate (EA) removal was evaluated under different influent loadings. In the pseudo-steady states, the elimination capacity increased, but the removal efficiency decreased with increased influent loading. More than 95 and 90% removal efficiencies could be achieved for EA loadings below 490 and 810 g m −3 h −1, respectively. The TBAB appears to be very effective for controlling EA emission under low to high loading conditions, and the effectiveness could be maintained over 190 days of laboratory operation.

Condensation of carbon in radioactive supernova gas.

Chemistry resulting in the formation of large carbon-bearing molecules and dust in the interior of an expanding supernova was explored, and the equations governing their abundances were solved numerically. Carbon dust condenses from initially gaseous carbon and oxygen atoms because energetic electrons produced by radioactivity in the supernova cause dissociation of the carbon monoxide molecules, which would otherwise form and limit the supply of carbon atoms. The resulting free carbon atoms enable carbon dust to grow faster by carbon association than the rate at which the dust can be destroyed by oxidation. The origin of presolar micrometer-sized carbon solids that are found in meteorites is thereby altered.

Fullerenes and other structures of carbon synthesized in a carbon plasma jet under helium flow

The plasma chemical reactor based on a carbon plasma jet was created for the production of fullerenes. This reactor has the changing temperature zone and provides deposition of soot under different conditions. X-ray powder diffraction investigation is carried out for identification of substances obtained in different parts of the installation. The fullerene crystals and an amorphous phase were found in the chamber. The amorphous fraction is probably composed mainly of the atomic aggregations similar in size and coordination to the fullerene molecules. The residual graphite and its polytype modifications were also found. This residual graphite is better crystallized than the initial one, therefore, it can be considered as a product of carbon crystallization from plasma but not as the particles of the initial graphite seized from the central electrode. The fullerene and amorphous fractions were not found on the external electrode which was cooled by water and where very fast condensation of carbon from plasma takes place. The sample taken from this electrode is composed mainly from a new x-graphite phase which can be considered a result of this fast condensation. This last conclusion is also supported by the fact that this x-graphite was also found on the walls of the chamber where the fast condensation also must take place, but it was not found in the flying soot fraction.

A Mathematical Model of the Carbon Arc Reactor for Fullerene Synthesis

A mathematical model of the carbon arc process for the synthesis of fullerenes (C 60 , C 70 ) is developed. The two-dimensional model solves for the velocities, temperature, and total concentration of carbon species. The net emission coefficient method is used for the radiation term. The carbon species conservation equations consider the evaporation of carbon from the anode, cathode surface deposition, and carbon condensation. The thermodynamic and transport properties are calculated as a function of temperature and carbon mass fraction, using the method of Chapman–Enskog. Erosion rates used by the model are determined experimentally. Calculated fields of the velocities, temperatures, carbon mass fraction and current intensity are presented. Comparison is made of the behavior of the arc at 1 and 4 mm interelectrode gaps, and between operation in argon and in helium. The results of simulations provide a justification for the higher yields observed in helium compared to the argon case.