Methane plasma

Jiang, T. et al., Plasma methane conversion using dielectric-barrier discharges with zeolite, Catal. Today, 72, 229, 2002. 

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. 

Biomedical. Heart-valve parts are fabricated from pyrolytic carbon, which is compatible with living tissue. Such parts are produced by high temperature pyrolysis of gases such as methane. Other potential biomedical apphcations are dental implants and other prostheses where a seal between the implant and the living biological surface is essential. Plasma and arc-wire sprayed coatings are used on prosthetic devices, eg, hip implants, to achieve better bone/tissue attachments (see Prosthetic and BiOLffiDiCALdevices). 

G s-Ph se Synthesis. A gas-phase synthesis route to making fine, pure SiC having controllable properties has been described (78,79). Methane was used as a carbon source if required, and the plasma decomposition of three feedstocks, siUcon tetrachloride [10026-04-7] SiCl, dimethyl dichi orosilane, and methyltrichlorosilane [75-79-6] CH Cl Si, into fine SiC powders was investigated. 

Side reactions reduce the yield (99). Proposed processes for obtaining carbon disulfide from hydrogen sulfide and methane include a high temperature plasma (100) and low temperature operation with a catalyst and oxygen (101). 

Carbides may also be prepared, either by dhect carburizing, as in the case of steel, in which a surface carbide film dissolves into the subsuate steel, or by refractoty metal carbide formation as in die cases when one of the refractory metal halides is mixed with methane in the plasma gas. 

The result of the fast reactions in the ion source is the production of two abundant reagent ions (CH5+ and C2H5+) that are stable in the methane plasma (do not react further with neutral methane). These so-called reagent ions are strong Brpnsted acids and will ionize most compounds by transferring a proton (eq. 7). For exothermic reactions, the proton is transferred from the reagent ion to the neutral sample molecule at the diffusion controlled rate (at every collision, or ca. 10 9 s 1). 

Thus the reactant ions for chemical ionization formed in the methane plasma consists of approximately equal amounts of a strong gaseous Bronsted acid (CH5+) and ions which can act either as Lewis acids or Bronsted acids (C2H5+ + C3H5+). These reactant ions will effect the chemical ionization with an added substance by proton transfer or hydride ion transfer, both of which may be accompanied by fragmentation of the ion initially formed. 

Ohtake, N., and Yashikawa, M., Diamond Film Preparation by Arc Discharge Plasma Jet Chemical Vapor Deposition in the Methane Atmosphere, / Electrochem. Soc., 137(2) 717-722(1990)... 

A composite film of SiC and diamond was produced from tetramethylsilane, hydrogen, and methane in a microwave plasma on single-crystal silicon wafers. Volume fraction of the components can be adjusted by varying the gas composition. 

In ion beam deposition, hydrocarbon gas such as methane or ethyene is ionized into plasma by an ion source such as the Kaufman source [3]. The hydrocarbon ions are then extracted from the ion source and accelerated to form an ion beam. The ions and the unionized molecules condense on the substrate surface to form DEC coating. However, in this method, ionized ratio of precursor gases could hardly exceed 10 %. In order to obtain a better quality of DEC coatings. 

Conversion of Methane to Hydrogen and Carbon black by D.C. Plasma Jet... 

A thermal plasma system has been developed for the decomposition of methane. A schematic diagram of the experimental apparatus is shown in Fig. 1. The system consists primarily of D.C. plasma torch, plasma reactor and filter assembly. Plasma was discharged between a tungsten cathode and a copper anode using N2 gas. All the experiments were carried out at atmospheric pressure at 6 kW input electric power and N2 flow rate of 10 to 12 1/min. The feed gas (CH4) flow rates were varied from 3 to 15 1/min depending on the operating conditions, shown in Table. 1. 

Direct thermal decomposition of methane was carried out, using a thermal plasma system which is an environmentally favorable process. For comparison, thermodynamic equilibrium compositions were calculated by software program for the steam reforming and thermal decomposition. In case of thermal decomposition, high purity of the hydrogen and solidified carbon can be achieved without any contaminant. 

The methane conversion and hydrogen yield were investigated as a function of with respect to methane flow rate and both of the two were very high more than 90%. Particle size and sinface area of synthesized carbon were strongly dependent on methane flow rate. Hydrogen produced finm thermal plasma can be applied to fuel cell due to its high purity and carbon black can be applied for the synthesis of rubber industry. 

It could be concluded that thermal plasma process for methane decomposition is very effective for the production of high purity of the hydrogen as well as synthesis of the carbon black. 

In the plasma reactor dedicated for intrinsic material deposition (2 in Fig. 5), only hydrogen and silane are used, along with argon. A mixture of trimethylboron (5% TMB in H2), SiHa, and methane (CH4) is used in the / -plasma reactor (3 in Fig. 5). Diborane can also be used. A mixture of phosphine [PH3 (1% in H2)] and SIH4 is used in an n-plasma reactor (4 in Fig. 5). All gases are of 6.0 quality (99.9999% pure) if available from manufacturers, and otherwise as pure as possible. 

Ogata, A., Mizuno, K., Kushiyama, S. and Yamamoto, T. (1998) Methane Decomposition in a Barium Titanate Packed-Bed Nonthermal Plasma Reactor, Plasma Chem. Plasma Process 18, 363-73. 

Gulyaev and Polak [Kinetics and Catalysis, 6 (352), 1965] have studied the kinetics of the thermal decomposition of methane with a view toward developing a method for the commercial production of acetylene in a plasma jet. The following differential equations represent the time dependence of the concentrations of the major species of interest. 

Microwaves have been used to generate plasma in methane at 5-50 Torr. The radicals produced in such a system were then allowed to react over a nickel catalyst, affording a mixture of ethane, ethene, and ethyne [74],... 

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