Glow discharge acetylene

Hot RF and - DC plasma, are discharge, plasma jets Oxy-acetylene flames Low pressure microwave plasma, holt filament. Low pressure DC or RF glow discharge Thermal decomposition... 

The identification of all new radicals has been closely linked to the production of the appropriate species in the laboratory, mainly by glow discharges in acetylene and other molecules. Likewise, the detection of new interstellar ions has depended on laboratory work, typically involving glow discharges of a novel type designed by De Lucia et al. (1983). The newly discovered radicals are C3N, C3H, and C4H reported... 

Figure 1.3 clearly demonstrates the luminous gas phase created under the influence of microwave energy coupled to the acetylene (gas) contained in the bottle. This luminous gas phase has been traditionally described in terms such as low-pressure plasma, low-temperature plasma, nonequilibrium plasma, glow discharge plasma, and so forth. The process that utilizes such a luminous vapor phase has been described as plasma polymerization, plasma-assisted CVD (PACVD), plasma-enhanced CVD (PECVD), plasma CVD (PCVD), and so forth. 

When an organic vapor rather than an inert gas is used in the same discharge reactor, a nearly completely different phenomenon occurs, in which deposition of material is an aspect. Deposition of material constitutes the foundation of LCVD. In an LCVD environment, the composition of the gas phase changes continuously as deposition proceeds. This difference could be further illustrated by examples for glow discharge of argon and of acetylene. 

In contrast to this situation, the glow discharge of acetylene in a closed system extinguishes in a few seconds to few minutes depending on the size of the tube and the system pressure. This is because acetylene forms deposit and coats the wall of the reactor. In this process of LCVD (plasma polymerization) of acetylene, very little hydrogen or any gaseous species is created, and the LCVD of acetylene acts as a vacuum pump. When the system pressure decreases beyond a certain threshold value, the discharge cannot be maintained. 

Table 7.16 Elemental Analysis of Glow Discharge Pol5miers of Acetylene with N2, CO, and H2O...

Figure 12.5 Pressure pg in a glow discharge versus flow rate Fq for acetylene (Q) and ethylene ( ).

The distribution of polymer deposition observed in the plasma polymerization of acetylene at different flow rates (and different system pressures under plasma conditions) is shown in Figure 20.2. It should be noted that acetylene is the fastest polymerizing hydrocarbon and the system pressure decreases on the inception of glow discharge. In this particular configuration of reactor, the monomer does not pass the radio frequency coil, and presents a typical case in which the creation of chemically reactive species occurs at the boundary where the monomer meets the luminous gas phase, i.e., activation by luminous gas, not by ionization. 

The exact mechanism of propylene glow discharge polymerization is not known. The presence of a terminal acetylene (presumable propyne) in the gaseous products of propylene polymerization was indicated by the interaction of the cold trap gaseous condensates with 1% alcoholic or ammonlacal AgNO sol.(22) after the polymerization was over. An immediate formation of an explosive silver acetyllde was detected. Intermediate formation of propyne is also indicated by the IR spectra of the yellow relatively less volatile liquid left in the cold trap after the polymerization was over. Sharp but weak absorptions at 3310 cm l and 1270 cm are indications of a substituted acetylenic compound. The IR spectra of the yellow liquid also points to the presence of mono, dl- and trl-substituted aromatic compounds in the mixture (i.e. sharp absorption bands at 3080 cm l, 1640 cm l, 920 cm , 810 cm and a multiple band in the 1000-1120 cm l region are observed). The NMR spectra in deuterated acetone indicated the presence of an aromatic nucleus in the yellow liquid obtained from the cold trap. The formation of aromatic compounds can be explained if a propynyllc Intermediate is involved. 

Monomers Not Polymerizable by Plasma Initiation. When styrene and a-methy1styrene were subjected to plasma treatment, the monomers became yellowish and only trace amounts of insoluble films were formed. The discoloration was intensified and extensive formation of dark films were observed if carbon tetrachloride was added as the solvent. No post-polymerization was detectable for these monomers. Generally styrene and a-methylstyrene readily undergo thermal polymerization. However, no thermal polymerization was possible for these monomers after having been subjected to plasma treatment for one minute or less. It has been demonstrated from the emission spectra of glow discharge plasma of benzene (6) and its derivatives (7 ) that most of the reaction intermediates are phenyl or benzyl radicals which subsequently form a variety of compounds such as acetylene, methylacetylene, allene, fulvene, biphenyl, poly(p-phenylenes) and so forth. It is possible that styrene and a-methylstyrene also behave similarly, so that species from the monomer plasma are poor initiators for polymerization. 

In the gas phase above petroleum oil fractions at reduced pressures, the products formed were dependent more on gas pressure and type of discharge than on the type of oil135). At pressures of 100 torr in an arc, hydrogen and carbon were the main products. As the pressure was reduced, acetylene was formed and at 12 torr the amount of acetylene reached a maximum value which was 22% of the products. In a glow discharge methane, ethylene and other olefins were formed, the maximum value of ethylene being 22% of the products. 

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