Multiple compound layers

On the contrary, at x > x[f, there is a deficit of the B atoms because the reactivity of the A surface exceeds the flux of these atoms across the ApBq layer. Therefore, on reaching interface 1, each B atom is combined at this interface into the ApBq compound. In this case, there are no excessive B atoms for the formation of other compounds enriched in component A. Thus, none of compound layers located between A and ApBq can grow at the expense of diffusion of component B. This almost obvious result following in a natural way from the proposed physicochemical considerations is crucial for understanding the mechanism of formation of multiple compound layers. Perhaps, just its evident character is the main reason, firstly, why many researchers in the field have overlooked it and, secondly,... 

Note that even in those cases where multiple compound layers were present at the A-B interface, two layers were dominating. For example, G. Hillmann and W. Hofmann and O. Taguchi et al. observed the formation of all six intermetallics shown on the equilibrium phase diagram in the reaction zone between zirconium and copper, with two Cu-rich compounds occupying more than 90 % of the total layer thickness and layer-growth kinetics deviating from a parabolic law. When investigating... 

Reasons for the formation of multiple compound layers at the A-B interface... 

In some experimental works, the simultaneous presence of multiple compound layers was observed. Therefore, just as in the framework of diffusional considerations it is necessary to explain why the number of compound layers is in most cases so small, so in the framework of the physicochemical approach proposed it is necessary to explain why in certain cases the number of those layers is so large. There are a few reasons for the formation of multilayered structures at the A-B interface. These may be summarised as follows. 

During the whole course of annealing the A-B couple under pressure, contacts between initial and occurring phases may well be lost and renewed several times, giving rise to a hardly tractable microstructure of the A-B transition zone. Thus, in many cases the compound-layer formation actually takes place in a few independent couples. Though in each of those couples no more than two compound layers can grow under conditions of diffusion control, multiple compound layers will ultimately be seen between A and B. Evidently, the newly occurred layers can only grow at the expense of the former ones whose thickness must therefore decrease. 

Microstructures like those in Figs 3.8 and 3.9 are often employed to set off massive specimens with multiple compound layers against thin-film ones with one or two compound layers. For binary systems without any considerable solubility in the solid state, however, the dimensions of reaction couples play no role, if of course not of the order of lattice... 

In the diffusional stage of interaction of initial substances, inter-diffusion is only possible if a single compound layer is formed between A and B. With two compound layers, only monodiffusion in each of the layers takes place. With multiple compound layers, even monodiffusion in all the layers is not possible. It takes place only in two of them. 

In this model, OBPs participate in the selective transport of pheromone and other semiochemicals to their olfactory receptors. The selectivity of the system is likely to be achieved by layers of filters [ 16], i.e., by the participation of compartmentalized OBPs and olfactory receptors. It seems that OBPs transport only a subset of compounds that reach the pore tubules. Some of these compounds may not bind to the receptors compartmentalized in the particular sensilla. The odorant receptors, on the other hand, are activated by a subset of compounds, as indicated by studies in Drosophila, showing that a single OR is activated by multiple compounds [66]. If some potential receptor ligand reaches the pore tubules but are not transported by OBPs, receptor firing is prevented because the receptors are protected by the sensillar lymph. In other words, even if neither OBPs nor odorant receptors (ORs) are extremely specific, the detectors (olfactory system) can show remarkable selectivity if they function in a two-step filter. 

The thickness, x[f, of the ApBq layer is referred to as critical because the growth conditions for the layers of other compounds of a given multiphase system become indeed critical if x xj because all of them lose a source of the B atoms (actually, only substance B is such a source) and their growth at the expense of diffusion of the B atoms is stopped. This problem will be examined in more detail when analysing the process of simultaneous formation of two and multiple chemical compound layers. 

To understand the reculiarities of multiple layer formation, it suffices to consider the A-B binary system with three chemical compounds ApBq, ArBs and AiBn on the equilibrium phase diagram (Fig. 3.1). The scheme of analysis of the process of their occurrence at the A-B interface is analogous to that of two compound layers (see Chapter 2). First of all, the equations of partial chemical reactions taking place at phase interfaces must be written. These are as follows. 

To visualise the diffusing species, inert markers are to be embedded in each of compound layers. From Figs 3.2 and 3.6, it must be quite clear that one marker is insufficient to make far-reaching conclusions regarding the diffusing species in multiple layers. 

It seems relevant to remind once again that in the case of formation of a single-phase compound layer, the reverse (parabolic-to-linear) transition is impossible. From a physicochemical viewpoint, it is only possible during the simultaneous occurrence of two or more compound layers, as is indeed observed experimentally. Parabolic-to-linear growth kinetics are thus indicative of the formation of multiple layers of oxides, nitrides, sulphides, etc., even though some of them may be unindentifiable due to their extremely small thickness. 

The occurrence of two such points may be expected if two compound layers are growing simultaneously. If there are three or more compound layers, the kinetic dependence should in addition change considerably when some of those layers disappear after the others have reached their critical thickness with regard to components A and B (see Chapter 3). These peculiarities of multiple-layer formation would be taken into account in analysing the experimental data. 

The objective of the work described here was to examine whether a similar approach can be used to assess chemical uptake into the skin in vivo from contaminated soil. It is now well recognized that human skin contact with contaminated soil can represent an important route of exposure to toxic compounds in occupational, environmental, and recreational settings. Data on the dermal uptake of chemicals from soil, especially in vivo, are limited, however, and those that do exist may underrepresent the true risk. This is because the amount of soil applied to skin in these experiments (1) greatly exceeds the mass of soil adhering to skin during a typical exposure (U.S. Environmental Protection Agency, 2001) and (2) may have provided multiple soil layers that do not contribute equally to dermal absorption (Bunge and Parks, 1998). 

E. Menziani, B. Tosi, A. Bonora, P. Reschiglian and G. Eodi, Automated multiple development high-performance thin-layer chromatographic analysis of natural phenolic compounds , 7. Chromatogr. 511 396-401 (1990). 

M. T. Belay and C. E. Poole, Determination of vanillin and related flavor compounds in natural vanilla exti acts and vanilla-flavored foods by thin layer chromatography and automated multiple development , Chromatographia 37 365-373(1993). 

The multiplicity of responses makes thin-layer chromatography not particularly suited for pyrethrum analysis, either qualitative or quantitative. It did confirm, however, that the crude oleoresin contains several pyrethroid compounds in substantial quantity, as previously shown by gas chromatography work. 

Mass loss determinations refer to the total change resulting from reactant decomposition and usually include contributions from a mixture of product compounds, some of which would normally be condensed under conditions used for accumulatory pressure measurements. Such information concerned with the overall process is, however, often usefully supplemented by evolved gas analyses (EGA) using appropriate analytical methods. Sestak [130] has made a detailed investigation of the effects of size and shape of reactant container on decomposition kinetics and has recommended that the sample be spread as a thin layer on the surfaces of a multiple plate holder. The catalytic activity of platinum as a reactant support may modify [131] the apparent kinetic behaviour. 

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