Onion layer structure

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.)... 

The apparent discrepancy described above is due in part to the fact that sample collection is done at different layer of an onion layer structure because substrates are placed on the fixed locations on the reactor wall. These factors associated with the size of a reactor are important in an attempt to increase the size of reactor in order to scale up the processing capability. Since the onion layer structure of the luminous gas phase changes with the size of the reactor, the straightforward increase of reactor size to cope with a larger substrate or larger numbers of substrates to increase the production rate often has disastrous consequences. 

As introduced previously, type 2 ABC triblock copolymer micelles are formed by triblock copolymers containing an insoluble A block while the B and C blocks are soluble in the considered solvent. The insoluble blocks can be located either between the two soluble blocks (BAC structure) or at one end of the triblock (ABC or ACB structures). Micelles of the latter type were discussed above for, e.g., PS-P2VP-PEO pH-responsive micelles and are indeed considered as core-shell-corona, onion, or three-layer structures since the heterogeneity in the micellar corona is observed in the radial direction (Fig. 18). Micelles formed by BAC triblock copolymers are different from the previous case because they can give rise in principle to a heterogenous corona in the lateral dimension (Fig. 18). This could induce the formation of noncentrosymmetric micelles as discussed in Sect. 7.3. 

Nested fullerene-type structures with other inorganic layered materials have been investigated recently. Tenne et al (1992) have shown that MoSj is formed in a nested (onion-like) structure when MoOj is reduced by HjS in the gas phase at 1070-1220K (Feldman et al, 1995). Similar results have been obtained with WSj. The idea seems to be more general, being applicable to other inorganic layered structures, as... 

Coarser spray droplets yield larger granules which tend to a raspberry coalesced, rather than an onion skin layered, structure. This effect is diminished the larger the ratio of granule size to droplet size. 

Figure 4.21 A sufficiently smooth surface provided, electron irradiation can cause the formation of a graphitic layer on diamond. Irregularities, on the other hand, give rise to onion-like structures ( Elsevier 1996).

Figure 4.30 shows the effect of temperature on the structure and a pitch fiber, with an onion skin structure, is preferred to a radial type structure. Possible cross sectional microstmctures of mesophase carbon fibers are given in Figure 4.34 and modification of the flow profile during extrusion can produce a less flow-sensitive product and higher tensUe strength. The lines within each section depict carbon layers, which are at least preferentially parallel to the fiber axis. 

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