LPCAT processing

Another potential mode of LPCAT processing is that the integrated cascade arc generator, such as that shown in Figure 16.2, is placed in a vacuum chamber held by a robot arm. In this mode, LPCAT jet could scan over a complex shaped substrate by the robotic operation. 

In the LPCAT process, only an inert gas such as Ar exists in the cascade arc generator, and DC voltage is applied between the cathode and the anode. Therefore, it is a DC discharge of Ar, but it occurs under much higher pressure than in most low-pressure DC discharges, and the gas travels very fast in one direction in the generator. The basic process of ionization of Ar takes place in the cascade arc generator, which can be depicted as follows. 

The argon emission intensity showed a linear dependence on the combined parameter, W FM), i.e., the total energy applied to the carrier gas in the LPCAT process, as described by Eq. (16.5), can be expressed by this combined experimental parameter, W FM)c ... 

Whether any of these characteristic features is an advantage or a disadvantage entirely depends on the objectives to be accomplished by the process. However, these two characteristic features indicate that LPCAT process is better suited for the surface treatment by excited neutrals of Ar than the surface coating. 

Figures 30.18 and 30.19 show the changes in the longest water boiling time, which still pass the tape test, of primer-coated TPOs with the arc current and plasma exposure time of LPCAT treatment, respectively. From Figure 30.18 it is evident that durable bonding of the primer to TPOs was obtained with argon and methane LPCAT treatments at a lower arc current. Since the arc current represents energy input in the LPCAT process, plasma treatment conducted at a lower arc current may prevent the overtreatment on the TPO and thus give better adhesion results.

In LPCAT process, only an inert gas, such as Ar, exists in the cascade arc generator, and DC voltage is applied... 

The process, therefore, is not an alternative means to carry out conventional plasma processes, and the adaptation of the process should be done with careful identification of the specific goal that cannot be attained by other conventional plasma processes. The major features of the process that could distinguish LPCAT processes from other conventional plasma processes are ... 

LPCAT is a luminous CVD process with decoupled ionization process, i.e., the chemically reactive species are created by the neutral species-impact dissociation of molecules, but not by ion-impact or electron-impact dissociations. Because of this aspect, LPCAT provides important information pertinent to the nature of the creation of chemically reactive species in LCVD and will be discussed in some detail in the following chapters. 

In general plasma polymerization processes it has been established that the deposition rate and properties of a plasma polymer primarily depend on the value of the normalized energy input parameter WjFM, as described in Chapter 8. In LPCAT polymerization processes, as described in Chapter 16, the deposition rate of a plasma polymer primarily depends on the value of the normalized energy input parameter, which is given by W FM)J FM). In this composite parameter, W is the power input applied to arc column, FM) is the mass flow rate of carrier gas (argon), and FM) is the mass flow rate of monomer that is injected into the cascade arc torch. The quantity of W FM)J FM) can be considered as the energy, which is transported by carrier gas plasma, applied to per mass unit of monomers. 

In comparison with conventional electrical discharge processes, LPCAT is a very different process in that its activation of carrier gas and the creation of polymerizable species by the activated carrier gas are temporally and spatially separated. When discharge power is applied to the cascade arc generator, the plasma of carrier gas (usually argon) is produced in the cascade arc column and the luminous gas phase is blown into a vacuum chamber where monomers are introduced. The deactivation of the reactive species, some of which lead to the creation of polymerizable species in the luminous gas phase, occurs within the relatively narrow beam of an argon luminous gas jet. The higher the flow rate of Ar, the narrower is the beam and the longer the luminous gas flame. 

The plotting deposition rates of different monomers with different feed rates in Normalized DR) versus W FM)cI FM)t coordinates would provide the most objective comparison of their tendencies with regard to deposition in CAT LCVD. In LCVD, it is very important that the polymerization (material formation) is atomic rather than molecular processes, implying that the depositing entities are fragmented species of the original monomer molecule. Therefore, the deposition rate in LPCAT polymerization is determined largely by the type of atoms contained in the monomer structure rather than by molecular structures. 

LPCAT polymerization or coating could be considered more or less the same as the plasma polymerization or coating by other conventional plasma processes, except that the kinetic pathlength of growth is short. The ultrathin layers prepared by LPCAT polymerization have the general characteristics of plasma polymers, i.e., amorphous (noncrystalline), high concentration of the dangling bonds (free radicals... 

The ideal surface modification of powders is the process that breaks down the aggregates and modifies the surface of the primary particles simultaneously. LPCAT treatment could be very close to this ideal situation. The supersonic velocity of reactive species breaks up the existing aggregates to a significant level, if not completely, allowing the chemically reactive species to interact with the primary particle surface. 

In such a process, particles are dropped into the horizontal LPCAT jet a few centimeters away from the nozzle, and blown to the downstream of the expansion chamber and separated from the gas stream. Figure 16.20 depicts the principle of the LPCAT powder treatment process. Figure 16.21 depicts the schematics of the system, and Figure 16.22 shows a pictorial view of the system. Such a system could treat a large amount of powders in relatively short time, e.g., 10 kg of powders could be treated in less than 1 h, while conventional plasma treatment of powder generally... 

As seen in Fig. 3, the LPCAT flame is relatively narrow implying that a uniform luminous gas phase is not created in the expansion chamber. Consequently, the treatment that can be achieved by an LPCAT is governed by the line of sight process, regardless of whether the substrate touches the luminous gas flame or not, and limited to a relatively small area that is exposed to the flame or near the tip of flame. When a substrate is placed along the line of the jet stream, the well-identifiable flame is destroyed, and gaseous species scatter in the downstream of the substrate. The scattered species could cause surface treatment effects however, their extents are much smaller than that by the jet. 

LPC Acyhransferase. Some evidence exists for a fatty acyl exchange process between acyl-CoA and the sn-2 position of PC (Griffiths et al, 1988b Stymne et al., 1990), and involves lysophosphatidylcho-lineacyltransferase (LPCAT) (EC 2.3.1.23). Operation of this system permits particular enrichment of the acyl-CoA pool in Cig polyunsaturated fatty acyl residues. LPCAT, as is the case with CPT, appears to possess lower activities in those oilseeds which have a relatively poor capacity to produce polyunsaturated oils. Thus, the relative contribution and importance of these systems in TAG synthesis vary with the plant (Stymne etal., 1990)... 

---