Formation of compound layers

Direct chemical reaction between elementary substances A and B clearly ceases after the formation of compound layers ApBq and ArBs, a few crystal-lattice units thick, which separate the reacting phases from each other. Subsequently, four partial chemical reactions take place at the layer interfaces. These are as follows ... 

In any binary system, the sequence of formation of compound layers is governed by the rate of chemical transformations (partial chemical reactions) at phase interfaces. 

From a theoretical viewpoint, predicting the sequence of layer occurrence at the A-B interface would present no difficulties if the values of all the chemical constants entering a system of differential equations like (3.27) were known. For any multiphase binary system A-B, these values are determined by the physical-chemical properties of the elements A and B and their compounds. With their dependence on those properties established, the sequence of formation of compound layers would readily be predicted from the system of equations (3.27) or similar. Unfortunately, the theory of reaction diffusion has not yet reached this stage of its development. 

Definitely, more experimental work is desirable with thin films, but even already existing data provide sufficient evidence for rejecting the erroneous idea about the simultaneous formation of compound layers, with some being invisible due to their small thickness. 

This result is of importance for practice because too rapid formation of compound layers at the interface of reacting substances is often undesirable. An example is the occurrence of thick layers of intermetallic compounds during welding or brazing dissimilar metals, whose presence is... 

If the major constituents of a solid alloy in contact with a liquid alloy are highly soluble in the latter without formation of compounds, progressive attack by solution is to be expected. If, on the other hand, a stable inter-metallic compound is formed, having a melting point above the temperature of reaction, a layer of this compound will form at the interface and reduce the rate of attack to a level controlled by diffusion processes in the solid state. By far the most serious attack, however, occurs in the presence of stresses, since in this case the liquid alloy, or a product of its reaction with the solid alloy, may penetrate along the grain boundaries, with resultant embrittlement and serious loss of strength. 

A further decrease in the X Me ratio, to 4, leads to linkage of the octahedral units by sharing more than one ligand so as to achieve coordination saturation. Sharing of two vertexes (two comers of the each octahedron) leads to the formation of compounds with layered-type structures. 

Oxidative UPD involves the oxidation of species to form an atomic layer where the precursor contains the element in a negative oxidation state. A classic example is the formation of oxide layers on Pt and Au, where water is oxidized to form atomic layers of oxygen. Halide adsorption can be thought of similarly, where a species such as I oxidatively adsorbs on a metal surface as the halide atom. In that case, a bulk film is not formed at more positive potentials, but the diatomic is generated and diffuses into solution. With respect to compound formation, oxidative UPD from a sulfide solution is a good example ... 

There are a number of papers in the literature concerning the formation of compound semiconductor diodes by electrodeposition, the most popular structure being a CdS-CdTe based photovoltaic. CdS was generally deposited first on an ITO on glass substrate, followed by a layer of CdTe, usually by codeposition [51, 204-213],... 

Initially, a thin layer flow cell (Fig. 19) was used in this group to study the EC ALE formation of compounds [158] and in studies of electrochemical digital etching [312,313], Wei and Rajeshwar [130] used a flow cell system to deposit compound semiconductors as well, however, the major intent of that study was to form superlattices by modulating the deposition of CdSe and ZnSe. Their study appears to be the first example of the use of a flow electrodeposition system to form a compound semiconductor superlattice. 

Figure 5 Model for the formation of a layer of the active compound on a vanadia-niobia catalyst.

Although Wolfs indicated that the catalyst particles are covered by a skin of bismuth molybdate, Batist (112) recently found bismuth, molybdenum, and iron in the surface layers of multicomponent catalysts. Additional data are needed to determine if multicomponent catalysts gain their activity as a result of the formation of compounds such as bismuth iron molybdate, or by surface enhancement of an active component such as 7-phase bismuth molybdate, or by creation of low-energy electronic transitions. Of course, due to their complexity, all of these factors may be important. 

Consider first the main characteristic features of formation of the layers of chemical compounds, common to solid-solid, solid-liquid and solid-gas systems (Chapters 1 to 4). Then, the effect of dissolution of a solid in the liquid phase of a solid-liquid system or of its evaporation into the gaseous phase of a solid-gas system on the growth kinetics of a chemical compound layer will be analysed in Chapter 5. Thus, under the conditions of occurrence of a chemical reaction its product will be assumed to be solid and to form a continuous compact layer adherent at least to one of the initial phases. 

The idea about the summation of the times of consecutive steps of the examined solid-state process is of primary importance for understanding the peculiarities of multiphase growth of compound layers in binary heterogeneous systems. Moreover, even in the case of formation of a single compound layer, this idea makes it possible to reveal a few aspects of reaction... 

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,... 

It is easy to notice that during the same nine seconds three B atoms could have displaced from substance B across the AB layer to interface 1, if the chemical transformations at interface 1 (including also external diffusion of the B atoms to phase A through interface 1) would occur instantaneously or if the excessive B atoms (there are two such atoms in the case under consideration) would be used in the formation of the layers of other compounds of the same binary system. In the examined case of a single compound, this possibility of diffusion of excessive B atoms from interface 2 to interface 1 is not realised because the diffusion path is closed up until the full completion of chemical transformations at interface 1. However, the existence of such a possibility must be borne in mind when analysing the multiple layer growth. 

At x< x f2, there is an excess of diffusing A atoms since the reactivity of the B surface towards these atoms is less than their flux across the ApBq layer. The excessive A atoms can be used in the formation of the layers of other chemical compounds of a given binary system enriched in component B in comparison with ApBq, if present on the equilibrium phase diagram. 

A schematic diagram illustrating the growth process of the layers of two chemical compounds ApBq and ArBs, with p, q, r and s being positive numbers, at the A B interface is shown in Fig. 2.1. Note that the lines showing the distribution of the concentration of components A and B in the phases involved in the interaction are parallel to the distance axis since (/) the formation of the layers of chemical compounds which have narrow, if any, ranges of homogeneity is considered and (ii) initial substances are assumed to be mutually insoluble. 

Like the case of formation of the layer of a single chemical compound, it is assumed that the time of diffusion is directly proportional to both the increase of the thickness of a given compound layer and its existing total thickness, whereas the time of chemical transformations is directly proportional to the increase of the thickness of the layer and is quite independent of its total thickness (see Section 1.3). Hence,... 

If initial phases are chemical compounds, not elementary substances, the growth of the layers of two new chemical compounds in a quasibinary system takes place as a result of counter diffusion of the same-type ions or atoms of smaller size. The common ion usually does not take active part in the layer-growth process. This does not mean, however, that its presence has no effect on the mechanism of formation of the layers. The Rb2AgI3 and RbAg4J5 layers are known to form in the Rbl-AgI system. 5 5 Their formation is due to the following partial chemical reactions ... 

In fact, chemical transformations taking place at the interfaces between reacting phases are responsible for the occurrence of the barriers to diffusing atoms at the critical values of the thickness of the ApBq and ArBs layers. Their rate is also decisive in determining the sequence of formation of the layers of those compounds in the A-B reaction couple. 

The second apparent factor influencing the mobility of the atoms and hence the sequence of compound-layer formation is atomic radii of reacting elements. Clearly, the direct juxtaposition of the melting points to decide which compound has a greater chance to occur first is only justified if the atomic radii are identical or close for both elements, as is the case with titanium and aluminium, the atomic radius being 0.146 nm and 0.143 nm, respectively.152 153 Similarly, the juxtaposition, with the same purpose, of the atomic radii is valid only if the melting points of both elements are close. An example of this kind is the Al-Mg binary system already considered in Section 2.8.3 of Chapter 2. 

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