Carbon containing gases

Plasma Carburizing. Plasma carburizing generates carbon atoms at the surface by ionization of a carbon-containing gas, eg, methane. The process is similar to that described for ion nitriding. Because the process is carried out in partial vacuum, there is less chance of oxidation. 

The deposition is carried out in a simple flow tube. The amount of carbon-containing gas is maintained at <5% of the overall gas composition to retard formation of non-diamond carbon. The addition of oxygen or oxygen compounds (air, H2O, CO2) enhances growth. 

As noted above, amorphous carbon films can be produced from carbon-containing gas phases (physical vapour deposition, PVD). They can also be produced from hydrocarbon-containing gases (chemical vapour deposition, CVD), Both PVD and CVD processes can be thermally-activated or can be plasma- and/or electric field-assisted processes (e.g., microwave assisted CVD and ion beam deposition). As a consequence a wide range of processes have been developed to form amorphous carbon films and a correspondingly complex nomenclature has evolved [70, 71],... 

The principal methods of gas activation are thermal and electrical much less common are chemical and photochemical activation. In the most commonly used thermal activation technique - the hot filament technique - a W or Ta wire is arranged in the immediate vicinity of the substrate to be coated by diamond (Fig. 1). The wire is heated until it reaches the temperature when H2 molecules dissociate readily. The gas phase is a mixture of a carbon-containing gas (e.g. methane, acetone or methanol vapor), at a concentration of a few per cent, and hydrogen. Upon the contact of the gas with the activator surface, excited carbon-containing molecules and radicals are produced, in addition to the hydrogen atoms. They are transferred to the substrate surface, where deposition occurs. Table 2 gives an indication of the hot-filament deposition process parameters. 

The growth of filamentous carbon along with the gas phase product analysis has been used to determine the influence of sulfur on the iron catalyzed decomposition of carbon containing gas mixtures at 600°C. Pretreatment of the metal in H2S was found to initially suppress the reactions leading to carbon deposition from the decomposition of CO/H2 mixtures. After a short time the activity was restored to approximately the same level as that exhibited by an unadulterated iron powder, suggesting that most of the sulfur atoms were being removed from the surface. The small residual fraction of adatoms did, however, induce modifications in the structural characteristics of the filamentous carbon deposit and also altered the reactivity pattern of the iron towards decomposition of a CO/C2H4/H2 mixture. 

During the reaction, the metal particles adopt well defined geometries with the carbon-containing gas molecules being adsorbed and decomposed on certain faces of the metal, this process being followed by diffusion of carbon atoms through the catalyst particle... 

Diamond deposition by the hot filament method consists of a carbon containing gas and hydrogen, which undergo dissociation by passing through a hot filament usually made of tungsten wire. The dissociated molecules then deposit on a substrate (at approximately 900°C) where a carbon matrix grows in the form of diamond. Deposition dynamics is described by the CVD process. 

The Chemical Vapor Reaction (CVR) Process (H. C. Starch) [9.21]. The process is based on chemical vapor reaction between a metal-containing compound (for example tungsten hexachloride) and a carbon-containing gas mixture (for example CH4/H2). The principle has been known for many years (see also Section 3.5.4), but has only been realized recently on a technical scale. The aim of the process is to produce a very fine (3-500 nm), homogeneous powder with a narrow particle size distribution. 

A large variety of carbon-containing gas species have been employed to synthesize diamond by CVD, These include methane, aliphatic and aromatic hydrocarbons, alcohols, ketones, amines, ethers, and carbon monoxide, with methane being the most frequently used reagent. In... 

Lan] studied transfer processes and phase boundary reactions during the carbon exchange between the carbon containing gas phase and solid Cu-Fe alloy with 4 at.% Cu at 900 to 1000°C in H2-CH4 mixtures. The diffusion coefficient, transition coefficient and the Biot index were determined. 

Carbon fibrils can be produced rather easily, e.g., by exposing supported, finely dispersed iron or nickel particles to reducing carbon containing gas flows. To this end, one has to produce first finely dispersed iron or nickel particles on a support material, such as alumina or silica. The desired catalyst can be prepared, e.g., by incipient wetness impregnation of the support material with a suitable metal salt solution or by means of homogeneous deposition-precipitation of the metal ions onto the carrier. 

According to Pumera (2009), there is also a problem connected with the features of CNTs synthesis. CNTs are typically grown from carbon-containing gas with the use of metallic catalytic nanoparticles. It is well documented that such nanoparticles remain in the CNTs even after extensive purification procedures, leading to two very significant problems (Pumera et al. 2007). It has been shown that such residual metallic impurities are electrochemically active even when intercalated within the CNTs and... 

Graphite reacts with iron to form iron carbide, FesC, usually by the direct solution of carbon in the molten iron. Iron carbide may also be formed from the reaction of iron with a carbon-containing gas. This reaction is known as case-hardening. 

A process to prepare carbon-modified Ti02, also active under visible-light irradiation, was developed heating of Ti02 powder either in a flow of carbon-containing gas (e.g., -hexane or ethanol) at 400 to 500°C or in ethanol vapor at 120°C in an antoclave). A portion of deposited carbon is thought to substitute oxygen in the anatase lattice on the particle surface. A small extent of carbon deposition, about 0.5 mass%, was found to result in only a small decrease in photoactivity, but was sufficient to make turbidity low and, as a consequence, practical water purification efficiency was markedly improved. 

The nanocomposites BSR/CNT with CNT content of 0.3 mass% have been used as the study object. CNT have been received in the Institute of Applied Mechanics of Russian Academy of Sciences by the vapors catalytic chemical deposition method (CVD), based on carbon - containing gas thermochemical deposition on nomnetallic catalyst surface. Two catalysts - Fe/Al Oj (CNT-fe) and C0/AI2O3 (CNT-co) - have been used for the studied CNT. The received nanotubes have diameter of 20 nm and lengfli of order of 2 mcm. 

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