Reaction-barrier layer

In the second type of sample a buffer layer of the reaction product or a reaction-barrier layer is grown before growing the reactant layer. This geometry allows us to quantify the kinetics of the reaction separately from the nucleation. We can then examine the morphological development of the two moving interfaces and the effect of lattice misfit on this morphology. The expansion that occurs when the spinel forms can be readily acconuno-dated if a buffer layer is present forming a uniform layer... 

A similar effect was reported involving the addition of aluminum, also an active element, into Sn-9Zn [88]. Flip-chip solder bumps were produced on Cu termination pads by applying Sn-9Zn-0.5Al solder using a dipping method. During an extended annealing period at 150°C, A1 and Zn diffused to the Cu/solder interface to form a thin layer of AlCuZn intermetallic compound, but not Cu-Sn compounds, which can serve as a reaction barrier layer. 

Barrier Layers. Depending on composition, barrier layers can function simply as spatial separators or they can provide specified time delays by swelling at controlled rates or undergoing reactions such as hydrolysis or dissolution. Suitable barrier materials include cellulose esters and water-permeable polymers such as gelatin and poly(vinyl alcohol) (see Barrier polymers). 

There are various theories on how passive films are formed however, there are two commonly accepted theories. One theory is called the oxide film theory and states that the passive film is a diffusion-barrier layer of reaction products (i.e., metal oxides or other compounds). The barriers separate the metal from the hostile environment and thereby slow the rate of reaction. Another theory is the adsorption theory of passivity. This states that the film is simply adsorbed gas that forms a barrier to diffusion of metal ions from the substrata. 

Much of the difficulty in demonstrating the mechanism of breakaway in a particular case arises from the thinness of the reaction zone and its location at the metal-oxide interface. Workers must consider (a) whether the oxide is cracked or merely recrystallised (b) whether the oxide now results from direct molecular reaction, or whether a barrier layer remains (c) whether the inception of a side reaction (e.g. 2CO - COj + C)" caused failure or (d) whether a new transport process, chemical transport or volatilisation, has become possible. In developing these mechanisms both arguments and experimental technique require considerable sophistication. As a few examples one may cite the use of density and specific surface-area measurements as routine of porosimetry by a variety of methods of optical microscopy, electron microscopy and X-ray diffraction at reaction temperature of tracer, electric field and stress measurements. Excellent metallographic sectioning is taken for granted in this field of research. 

Tenet (iv). The influence of a barrier layer in opposition to the progress of reaction may be expected to rise as the quantity of product, and therefore the thickness of the interposed layer, is increased [35,37,38]. Thus, the characteristic kinetic behaviour of the overall process may be expected to include contributions from both geometric factors and the barrier effect, though in specific instances one or other of these may be dominant. 

Reaction of a solid] with a gas J L Reaction at immobile A surface <"l Interface control or Diffusion control No barrier layer Interface phenomena Geometric control... 

Barrier layer formation Diffusion across barrier control (reaction deceleratory)... 

Reaction of a solid with a liquid (or solute) As with solid and gas Diffusion in the liquid may be important, particularly where no barrier layer is formed... 

Reaction of a i solid with a solid] As with solid and gas Normally the immobility of reactants inhibits reaction and barrier layer for-1 tion is more common... 

The characteristic feature of solid—solid reactions which controls, to some extent, the methods which can be applied to the investigation of their kinetics, is that the continuation of product formation requires the transportation of one or both reactants to a zone of interaction, perhaps through a coherent barrier layer of the product phase or as a monomolec-ular layer across surfaces. Since diffusion at phase boundaries may occur at temperatures appreciably below those required for bulk diffusion, the initial step in product formation may be rapidly completed on the attainment of reaction temperature. In such systems, there is no initial delay during nucleation and the initial processes, perhaps involving monomolec-ular films, are not readily identified. The subsequent growth of the product phase, the main reaction, is thereafter controlled by the diffusion of one or more species through the barrier layer. Microscopic observation is of little value where the phases present cannot be unambiguously identified and X-ray diffraction techniques are more fruitful. More recently, the considerable potential of electron microprobe analyses has been developed and exploited. 

The properties of barrier layers, oxides in particular, and the kinetic characteristics of diffusion-controlled reactions have been extensively investigated, notably in the field of metal oxidation [31,38]. The concepts developed in these studies are undoubtedly capable of modification and application to kinetic studies of reactions between solids where the rate is determined by reactant diffusion across a barrier layer. 

Since the interposition of a barrier layer diminishes the effective contact between reactants, the nucleation step in solid + solid reactions is Usually completed very rapidly at temperatures conveniently used in studies of the bulk reaction and, accordingly, the rate processes are often deceleratory throughout. In addition to the progressive diminution in rate... 

The simplest kinetic law obeyed, when the surface area is constant and the diminution of reaction rate is a consequence of increasing thickness of the barrier layer, is... 

If diffusion across a barrier layer is rapid compared with the rate of reaction at the advancing interface, then the overall rate is determined by the interface step (Sect. 3.1). 

This account of the kinetics of reactions between (inorganic) solids commences with a consideration of the reactant mixture (Sect. 1), since composition, particle sizes, method of mixing and other pretreatments exert important influences on rate characteristics. Some comments on experimental methods are included here. Section 2 is concerned with reaction mechanisms formulated to account for observed behaviour, including references to rate processes which involve diffusion across a barrier layer. This section also includes a consideration of the application of mechanistic criteria to the classification of the kinetic characteristics of solid-solid reactions. Section 3 surveys rate processes identified as the decomposition of a solid catalyzed by a solid. Section 4 reviews other types of solid + solid reactions, which may be conveniently subdivided further into the classes... 

While it is inherently probable that product formation will be most readily initiated at sites of effective contact between reactants (A IB), it is improbable that this process alone is capable of permitting continued product formation at low temperature for two related reasons. Firstly (as discussed in detail in Sect. 2.1.1) the area available for chemical contact in a mixture of particles is a very small fraction of the total surface (and, indeed, this total surface constitutes only a small proportion of the reactant present). Secondly, bulk diffusion across a barrier layer is usually an activated process, so that interposition of product between the points of initial contact reduces the ease, and therefore the rate, of interaction. On completion of the first step in the reaction, the restricted zones of direct contact have undergone chemical modification and the continuation of reaction necessitates a transport process to maintain the migration of material from one solid to a reactive surface of the other. On increasing the temperature, surface migration usually becomes appreciable at temperatures significantly below those required for the onset of bulk diffusion within a product phase. It is to be expected that components of the less refractory constituent will migrate onto the surfaces of the other solid present. These ions are chemisorbed as the first step in product formation and, in a subsequent process, penetrate the outer layers of the... 

Fig. 20. Schematic representation of the solid + solid reaction A + B -> AB in which constituents of the relatively mobile reactant (A) are transported to the outer surfaces of the product phase (AB) and rate is controlled by diffusion of constituents of A and/ or B across the barrier layer AB.

The kinetic principles operating during the initiation and advance of interface-controlled reactions are identical with the behaviour discussed for the decomposition of a single solid (Chaps. 3 and 4). The condition that overall rate control is determined by an interface process is that a chemical step within this zone is slow compared with the rate of arrival of the second reactant. This condition is not usually satisfied during reaction between solids where the product is formed at the contact of a barrier layer with a reactant. Particular systems that satisfy the specialized requirements can, however, be envisaged for example, rate processes in which all products are volatilized or a solid additive catalyzes the decomposition of a solid yielding no solid residue. Even here, however, the kinetic characteristics are likely to be influenced by changing effectiveness of contact as reaction proceeds, or the deactivation of the catalyst surface. 

The relative displacement rates of the interfaces AIAB and ABIB in any particular system will, of course, depend on the relative migration velocities of all mobile participants across the barrier layer and reaction will continue while appropriate reactant constituents remain available. Such migrant entities travel by the most efficient route and therefore overall rates of such reactions are frequently sensitive to the concentration of imperfections in the product crystal lattice [1171]. One possible... 

Further investigations of spinel formation reactions are to be found in the literature [1], but the above representative selection illustrates a number of typical features of these rate processes. Following migration of cations from one constituent onto the surfaces of the other, the process is limited by the rate of diffusion across a barrier layer. While obedience to a particular kinetic expression is sometimes reported, the data available are not always sufficiently precise to enable the fit found to be positively... 

Since, in both these reactions (i.e. KI or Rbl and Agl), product formation occurs on both sides of the original contact interface, it is believed that there is migration of both alkali metal and silver ions across the barrier layer. Alkali metal movement is identified as rate limiting and the relatively slower reaction of the rubidium salt is ascribed to the larger size and correspondingly slower movement of Rb+. The measured values of E are not those for cation diffusion alone, but include a contribution from... 

Two product barrier layers are formed and the continuation of reaction requires that A is transported across CB and C across AD, assuming that the (usually smaller) cations are the mobile species. The interface reactions involved and the mechanisms of ion migration are similar to those already described for other systems. (It is also possible that solid solutions will be formed.) As Welch [111] has pointed out, reaction between solids, however complex they may be, can (usually) be resolved into a series of interactions between two phases. In complicated processes an increased number of phases, interfaces, and migrant entities must be characterized and this requires an appropriate increase in the number of variables measured, with all the attendant difficulties and limitations. However, the careful selection of components of the reactant mixture (e.g. the use of a common ion) or the imaginative design of reactant disposition can sometimes result in a significant simplification of the problems of interpretation, as is seen in some of the examples cited below. 

Owing to their strong bond on Ru(OOOl), mixed COa 0.55 V, the shift of the equilibrium between water and adsorbed OHad/Oad towards the latter increases the density of the respective species in the intermixed adlayer, which increases the repulsions between the adsorbed species and hence leads to more weakly bound OHad/Oad and COad species. These latter species are less stable against COOHad or CO2 formation, because of the reduced reaction barrier ( Brpnsted-Polanyi-Evans relation [Bronstedt, 1928]), and can support a reaction via (14.9) or (14.12), respectively, at low rates. (Note that the total density of the adlayer does not need to remain constant, although also this is possible.)... 

The rate of chemical attack will depend on the concentration according to the order of the reaction (i.e. in a zero-order reaction the rate is independent of concentration, in a first-order reaction the rate depends linearly on concentration, and in second-order reaction the rate depends on the square of concentration). Increasing the concentration, therefore, provides a means of acceleration. Remember, however, that chemical attack on plastics is a liquid-solid and not a liquid-liquid reaction, such that the reaction laws only hold if there is free movement of all chemical species with no limitations due to diffusion or transport and no barrier layers. Since this is rarely the case, temperature is preferred as a means of acceleration. 

---