Luminous gas

The recorded chronology of the coal-to-gas conversion technology began in 1670 when a clergyman, John Clayton, in Wakefield, Yorkshire, produced in the laboratory a luminous gas by destmctive distillation of coal (12). At the same time, experiments were also underway elsewhere to carbonize coal to produce coke, but the process was not practical on any significant scale until 1730 (12). In 1792, coal was distilled in an iron retort by a Scottish engineer, who used the by-product gas to illuminate his home (13). 

Gas transport properties for the products of combustion of the common fuels, fired at normal excess air at or nearfull boiler load, may be obtained from Tables 23.1-23.4. Non-luminous gas radiation has a small overall effect in the convective section, typically 2-5 per cent of total convection. It may therefore be neglected for a conservative calculation. 

Cool Flame Ignition - A relatively slow self sustaining, barely luminous, gas phase reaction of the sample or its decomposition products with an oxidant. Cool flames are visible only in a darken area. 

The spectra of the alkali metals are illustrated by Fig. 13. A lithium salt in the non-luminous gas-flame furnishes two sharply defined spectral lines, a weak yellow line, Lip, of wave-length 6104, and a bright red line, Li , of wave-length 6708. The presence of nearly a millionth of a milligram of lithium can be detected in this... 

Acknowledgements - We wish to thank Robert Dibble at Sandia Livermore for suggesting the test using the LED source, Marcus Alden at Chalmers University for his help during the earlier experimental phases of this work, and Donald R. White of this laboratory, who suggested the use of a high frequency pulsed laser for luminous gas diagnostics. 

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. 

Any chemical reaction that yields polymeric material can be considered polymerization. However, polymerization in the conventional sense, i.e., yielding high enough molecular weight materials, does not occur in the low-pressure gas phase (without a heterogeneous catalyst). With a heterogeneous catalyst, polymerization is not a gas phase reaction. Therefore, the process of material deposition from luminous gas phase in the low-pressure domain might be better represented by the term luminous chemical vapor deposition (LCVD). Plasma polymerization and LCVD (terms explained in Chapter 2) are used synonymously in this book, and the former... 

Figure 1.3 Pictorial view of the luminous gas phase created in a bottle, courtesy of Sidel.

The common denominator factor that has not been emphasized but deserves its own identity is the luminous gas phase from which the material deposition occurs. The key issues are how the luminous gas phase is created in the low-pressure electrical discharge and how chemically reactive species are created in the luminous gas phase. In this chapter we focus on the domain of CVD that functions only under the influence of the luminous gas phase by using the term luminous chemical vapor deposition (LCVD). 

In a plasma polymerization, the substrate is generally not heated, nor is the vapor heated. The chemical activation is done by the interaction of gas phase molecules with plasma (luminous gas) or by the generation of plasma of the starting material. In other words, activation of the starting material occurs in the vapor (plasma) phase, and the substrate is merely the collector of the product unless the substrate is used as an electrode. 

After a long reaction time, polymers with exceptionally high molecular weight can be synthesized by plasma-induced polymerization. Since only brief contact with luminous gas phase is involved, plasma-induced polymerization is not considered to be LCVD. However, it is important to recognize that the luminous gas phase can produce chemically reactive species that trigger conventional free radical addition polymerization. This mode of material formation could occur in LCVD depending on the processing conditions of LCVD, e.g., if the substrate surface is cooled to the extent that causes the condensation of monomer vapor. 

No deposition of materials occurs in most cases however, the deposition of plasma polymer could occur depending on the nature of substrate polymer. Such a deposition of materials can be viewed as PP of organic vapors, which emanated from the substrate, by the interaction with plasma. Because the major player is the luminous gas phase, the surface treatment is included in this book under the term luminous chemical vapor treatment (LCVT). 

Comparing the terms plasma chemical vapor deposition and luminous chemical vapor deposition, the dilference exists in the meaning of plasma and luminous gas and its implications to the nature of chemical reactions that occur in the gas phase. Without referring the details of the difference, however, the process could be described either plasma polymerization (plasma CVD) or luminous CVD in all practical purposes. 

These characteristics of glow in an LCVD reactor cast some serious questions regarding the nature of glow and the domain of plasma in a reactor. It is certain that one cannot intuitively assume that the luminous gas phase (glow) in glow discharge is plasma, while plasma has characteristic glow. 

Figure 3.3 Change of the intensity and location of luminous gas phase depending on the discharge power and the system pressure of Ar DC discharge. Left column 25mtorr, right column lOOmtorr. Top row 3 W, middle row 10 W, bottom row 15W.

There is no direct indication where glow exists according to and Hq, that is, Tq and can be measured both in dark space and in glow. The calculated Debye length decreases nearly linearly with the distance from the cathode covering the dark space and luminous gas phase, i.e., the value alone does not indicate where is plasma. 

The sharp increase of electron density, roughly 3 cm away from the cathode, can be taken as a clear indication that beyond this point there can be no electrical neutrality, i.e., it is impossible to accumulate large number of positively charged ions near the anode. The luminous gas phase in this space cannot be considered as plasma. Thus the domain of the luminous gas phase extends beyond the domain of plasma or the state that is close to the plasma state. The space in which and... 

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