Luminous gas phase

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. 

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). 

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). 

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... 

In the discharge of organic molecules, the ionization is not an accurate picture of the step that creates the luminous gas phase. First of all, the ionization energy... 

Ionization is the essential step in sustaining luminous gas phase but is not necessarily the primary step in initiating LCVD reactions. The scission of bonds occurs with a far greater frequency than the formation of ions. It was estimated that the concentration of neutral species in low-pressure plasma is usually five to six orders of magnitude higher than that of ions and electrons [8]. In other words, the scission of bonds does not occur as the consequence of the ionization of molecules but rather as the primary step to create luminous gas phase. 

Regardless of what term could represent the luminous gas phase created by an electrical discharge of gas or vapor, the total volume of luminous gas phase (glow) could be divided into many layers, in which the gas phase could be treated as a more or less uniform luminous gas phase. That is, the total luminous gas phase can be expressed by the onion layer structure, as indicated by Figure 3.7. In such an onion layer structure, only a relatively thin layer could be considered to be in plasma state or a state close to plasma state. 

Very important factors in LCVD are (1) the location of the critically important layer, i.e., the dissociation glow, in a glow discharge, and (2) the location of the substrate with respect to the onion layer structure, i.e., in which layer of an onion structure the substrate is placed. The location of the critical layer depends on what kind of discharge system is employed to create a luminous gas phase. In a strict sense, it is impossible to uniformly coat a substrate placed in a fixed position in a reactor, and the relative motion of a substrate to the onion layer structure of luminous gas phase is a mandatory requirement if high uniformity of coating is required. 

Adaptability of an LCVD process in an industrial scale operation greatly depends on the nature of the onion structure of the luminous gas phase that could be accommodated in the operation. The change of reactor size inevitably changes the basic onion layer structure of the luminous gas phase, which constitutes the main (often insurmountable) difficulty in the scale-up attempt by increasing the size of reactor. (The scale-up principle is discussed in Chapter 19.)... 

For the ease of comparison, let us term the most critical layer in a luminous gas phase as core. The meaning of the core depends on what kind of process with respect to the objective is considered. The reactor parameters, such as the distance... 

In microwave plasma, electrons adhere to the wall because the microwave energy propagates on surface. In this case, the inner surface of the vacuum chamber becomes the core of the luminous gas phase for LCVD. If the inner surface of a vacuum chamber is the main substrate surface on which LCVD coating is aimed, such as the case of the coating the irmer surface of a plastic bottle, the microwave discharge is the most efficient LCVD. 

When the system pressure of such a luminous gas phase is raised, it is generally considered that the electron temperature, 7)., ion temperature, T, and temperature of neutral species, Tji approach an equilibrium state (thermal equilibrium), and the plasma becomes hot at round lOOtorr. On the other hand, there is a type of glow discharge that is termed atmospheric-pressure glow discharge, which is claimed to be atmospheric low-temperature plasma. Obviously, such low-temperature plasma... 

The luminous gas phase created by a special mode of DC discharge recognized as the low-pressure cascade arc torch (LPCAT) provides an especially important case for understanding the fundamental aspects of the luminous gas phase. The luminous gas phase in form of luminous gas jet stream or torch are created by blowing out DC discharge into an expansion chamber in vacuum. The luminous gas jet of Ar mainly consists of photon-emitting excited neutral species of Ar, which is certainly not the plasma of classical definition. The core of LPCAT is the tip of injection nozzle however, it is not the core of electrical discharge. 

The luminous gas phase that can be characterized in the LPCAT very likely exists as remnant of electrical discharge plasma. The luminous gas phase in LPCAT provides an important foundation to elucidate the creation of chemically reactive species in LCVD, which is presented in Chapter 4. The details of LPCAT are given in Chapter 16. LPCAT provides an example that can distinguish the luminous gas phase from plasma, i.e., the luminous gas jet created by LPCAT is not in the state of plasma. In this book, the luminous gas phase is emphasized without being bound by the concept or definition of plasma. 

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