Luminous chemical vapor deposition

Luminous Chemical Vapor Deposition and Interface Engineering, HirotsuguYasuda... 

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... 

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). 

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. 

The terms luminous chemical vapor deposition and plasma polymerization are used synonymously in this book. Dealing with mechanism of reactions that lead to formation of solid deposition, PP is used according to the traditional use of the term. When dealing with the formation of reactive species and other operation and processing aspects, LCVD is preferentially used. 

Creation of Chemically Reactive Species in Luminous Chemical Vapor Deposition... 

The photon-emitting species are vitally important in luminous chemical vapor deposition (LCVD) process, and the location of the luminous gas phase indicates where actions occur within the interelectrode space. On the other hand, whether any particular photon-emitting species is primarily responsible for LCVD is a dilferent issue. As is described in Chapter 5 for the growth and deposition mechanisms, many chemically reactive species, such as various forms of free radicals that do not emit photons, are major reactive species that carry the growth reactions. No single species could be identified as the precursor or chemically reactive species for the process. 

ROLE OF FREE RADICALS IN LUMINOUS CHEMICAL VAPOR DEPOSITION... 

In luminous chemical vapor deposition (LCVD), irradiation with photons at various energy levels and the deposition of materials that contain free radicals occur concurrently. The balance of these processes is closely related to the chemical structure of the monomer and its characteristic behavior in the luminous gas phase. The investigation of this balance by ESR provides valuable information pertinent to the growth and the deposition mechanisms in LCVD. The details of this aspect are described in Chapter 7, which deals the chemical structure of the monomer (starting material for LCVD). 

Because of the unique growth mechanism of material formation, the monomer for plasma polymerization (luminous chemical vapor deposition, LCVD) does not require specific chemical structure. The monomer for the free radical chain growth polymerization, e.g., vinyl polymerization, requires an olefinic double bond or a triple bond. For instance, styrene is a monomer but ethylbenzene is not. In LCVD, both styrene and ethylbenzene polymerize, and their deposition rates are by and large the same. Table 7.1 shows the comparison of deposition rate of vinyl compounds and corresponding saturated vinyl compounds. 

Table 7.2 Classification of Organic Compounds in Luminous Chemical Vapor Deposit...

The unit of normalized deposition rate is m, and normalized deposition rate decreases with the total surface on which deposition occurs. This aspect can be conceived as loading factor of luminous chemical vapor deposition (LCVD). Mass balance in a reactor (flow system) can be established as... 

As mentioned in Chapter 9, many reactions occur simultaneously in luminous chemical vapor deposition (LCVD) systems. In order to understand material formation in the luminous gas phase one should recognize the nature of other processes as well because it is nearly impossible to single out one particular process by eliminating others completely. 

As described in Chapters 3, and 4, chemically reactive species in luminous chemical vapor deposition (LCVD) are created mainly by molecular dissociation. However, in the dilfused luminous gas phase that constitutes the major volume of the luminous gas phase in most practical configurations of LCVD, the actual creation of chemically reactive species occurs at the tip of glow that contacts with the freshly fed monomers. Thus, the size and shape of glow within a reactor is critically important however, these factors depend on the size and shape of the reactor. 

The deposition in a luminous chemical vapor deposition (LCVD) system occurs on surfaces that are either in contact with luminous gas phase (glow) or in the vicinity of glow. The amount of polymer deposition is influenced by three important geometrical factors of LCVD reactor. These are the relative position of polymer deposition with respect to (1) the location of electric energy input, (2) the monomer flow, and (3) the position within a reactor. The system pressure, which determines the mean free path of gaseous species, has a great influence on the distribution of polymer deposition. In general, the lower the system pressure, the wider or more even is the distribution [1,2]. 

A luminous chemical vapor deposition (LCVD) layer could be used as transition phase between metal and polymer (coatings or films). The deposition of LCVD layer on metals can be done by the cathodic LCVD, in which the metal to be incorporated is used as the cathode of DC discharge. This method provides an excellent adhesion of LCVD nanofilm to the metal as well as the adhesion of coatings that will be applied on the surface of the LCVD layer [1,2]. This method is described in some detail in Part IV. 

Developing a reactor for a luminous chemical vapor deposition (LCVD) operation to deal with large numbers of materials to be coated, probably the most difficult problem encountered is how to hold the substrates, place them in appropriate position, and move them in the luminous gas phase. In order to coat large number of substrate uniformly, the movement of substrate within the luminous gas phase is a mandatory requirement, with the exception of direct current (DC) cathodic LCVD. The difficulty progressively increases as the size of the substrate decreases, and it becomes virtually impossible to hold as the size reaches millimeters or less. For instance, the surface of small particulate matters cannot be treated or coated by the conventional modes of LCVD in which substrates are held by some kind of holder. 

An ultrathin layer of plasma polymer of trimethylsilane (TMS) has been utilized in the corrosion protection of aluminum alloys by means of system approach interface engineering (SAIE) [1 ]. SAIE by means of low-temperature plasmas utilizes low-temperature plasma treatment and the deposition of a nanolilm by luminous chemical vapor deposition (LCVD). This approach does not rely on the electrochemical corrosion-protective agents such as six-valence chromium, and hence the process is totally environmentally benign. 

The hybrid process of IVD/cathodic luminous chemical vapor deposition (LCVD) showed great advantage of providing improved corrosion protection by environmentally benign process [3-5]. Table 32.1 shows the comparison of typical... 

GENERAL PRINCIPLES IN MEMBRANE APPLICATION OF LUMINOUS CHEMICAL VAPOR DEPOSITION AND LUMINOUS GAS TREATMENT... 

Luminous chemical vapor deposition (LCVD) and luminous gas treatment (LGT), which does not yield the primary deposition, could be used in the preparation and modification of membrane and barrier [1]. The term primary deposition refers to the direct deposition of material from the luminous gas (LCVD) in contrast to secondary deposition that results from the deposition of ablated material in LGT. It should be emphasized, however, that both methods are nanofilm technologies and require the substrate membrane on which LCVD nanofilm is deposited or the surface is modified. Accordingly, their use should be limited to special cases where such a nanofilm could be incorporated into membrane or the LGT of surface is warranted. 

Application of Luminous Chemical Vapor Deposition in Biomaterials... 

The imperturbability of a surface is a vitally important factor, in the author s view, for the biological compatibility of artificial surfaces. The imperturbability of a surface could be attained by (1) molecular configuration of macromolecule, such as the case of poly(oxyethyle) (POE), or (2) immobile network structure in the top surface, such as the case of the tight network of luminous chemical vapor deposition (LCVD) nanofilm. 

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