Heterogeneous Solid State Reactions

Heterogeneous solid state reactions occur when two phases, A and B, contact and react to form a different product phase C. A and B may be either chemical elements or compounds. We have already introduced this type of solid state reaction in Section 1.3.4. The rate law is parabolic if the reacting system is in local equilibrium and the growth geometry is linear. The characteristic feature of this type of reaction is the fact that the product C separates the reactants A and B and that growth of the product proceeds by transport of A and/or B through the product layer. 

the enhanced activity of the electrode can be attributed to the presence of both electrons and oxygen vacancies (mixed electronic and ionic conduction) in the electrode material. 

In addition to metals, other substances that are solids and have at least some electronic conductivity can be used as reacting electrodes. During reaction, such a solid is converted to the solid phase of another substance (this is called a solid-state reaction), or soluble reaction products are formed. Reactions involving nomnetaUic solids occur primarily in batteries, where various oxides (MnOj, PbOj, NiOOH, Ag20, and others) and insoluble salts (PbS04, AgCl, and others) are widely used as electrode materials. These compounds are converted in an electrochemical reaction to the metal or to compounds of the metal in a different oxidation state. 

Nonmetal electrodes are most often fabricated by pressing or rolling of the solid in the form of fine powder. For mechanical integrity of the electrodes, binders are added to the active mass. For higher electronic conductivity of the electrode and a better current distribution, conducting fillers are added (carbon black, graphite, metal powders). Electrodes of this type are porous and have a relatively high specific surface area. The porosity facilitates access of dissolved reactants (H+ or OH ions and others) to the inner electrode layers. 

Solid-state reactions between two phases of constant composition are called heterogeneous solid-state reactions. The mechanism of heterogeneous solid-state reactions 

In the case of substances having at least some solubility (e.g., 10 mol/L), the reaction often follows a scheme involving dissolved species  

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In this diagram, we classify the various types of heterogeneous solid state reactions that can occur. Note that both transformation to solid and fluid products are diagrammed. Also shown are variations where surface and/or volume nucleatlon Is Involved. 

Actually, there are three (3) major classifications for nuclei growth in heterogeneous solid state reactions. Restating, these are ... 

There are three (4) types of diffusion-controlled reactions possible for heterogeneous solid state reactions, viz-... 

Write equations for the four (4) primary types of heterogenous solid state reactions usualty encountered. Do not use those already given in Table 4-1. 

In heterogeneous solid-state reactions where the composition of both solid reactants does not change, the electrode s eqnilibrinm potential depends only on the nature of the two phases, not on their relative amonnts. Hence, dnring the reaction the potential does not change. It also remains constant when the cnrrent is interrupted after partial reduction or oxidation. 

Figure 1-5. Heterogeneous solid state reaction the formation of compound AB.

The increase A will occur at interface A/AB if LA/LR< 1, and it will occur at AB/B if La >Lr (Fig. 1-5). We conclude that parabolic rate laws in heterogeneous solid state reactions are the result of two conditions, the prevalence of a linear geometry and of local equilibrium which includes the phase boundaries. 

In heterogeneous solid state reactions, the phase boundaries move under the action of chemical (electrochemical) potential gradients. If the Gibbs energy of reaction is dissipated mainly at the interface, the reaction is named an interface controlled chemical reaction. Sometimes a thermodynamic pressure (AG/AK) is invoked to formalize the movement of the phase boundaries during heterogeneous reactions. This force, however, is a virtual thermodynamic force and must not be confused with mechanical (electrical) forces. 

Metal oxidation is a heterogeneous solid state reaction and starts in the same way as other heterogeneous reactions with nucleation and initial growth. This was discussed in Chapter 6. A time-dependent nucleation rate may dominate the overall growth kinetics of thin Films. Even under an optical microscope (i.e., in macroscopic dimensions), preferential sites of growth can still be discerned [J. Benard (1971)). This indicates that lateral transport on the surface (e.g., at sites where screw dislocations emerge) can possibly be more important for the initial reactive growth than transport across thin oxide layers. 

Chapters 6 and 7 dealt with solid state reactions in which the product separates the reactants spatially. For binary (or quasi-binary) systems, reactive growth is the only mode possible for an isothermal heterogeneous solid state reaction if local equilibrium prevails and phase transitions are disregarded. In ternary (and higher) systems, another reactive growth mode can occur. This is the internal reaction mode. The reaction product does not form at the contacting surfaces of the two reactants as discussed in Chapters 6 and 7, but instead forms within the interior of one of the reactants or within a solvent crystal. 

Figure 10-10. Representation of the chemical potential of A during the heterogeneous solid state reaction A+B = AB. a) Diffusion control, b) interface control at b2, c) rate control by rearrangement (relaxation) of A in B in zone A (B), d) simultaneous diffusion and interface control (bj).

The foregoing classification is not without ambiguity. For example, it is common practice to call the reaction A - B +C° (see Fig. 6-1) induced by decreasing the temperature a phase transformation. The similar (peritectoid) reaction C = a+fi (Fig. 12-2) induced by a temperature increase, however, is named a decomposition reaction. In addition, the isothermal reaction AO = A+j02, which occurs if the intensive variable fio2 is decreased so that AO decomposes, is called a metal oxide reduction. It is thus categorized as a genuine heterogeneous solid state reaction (the... 

Such transformations have been extensively studied in quenched steels, but they can also be found in nonferrous alloys, ceramics, minerals, and polymers. They have been studied mainly for technical reasons, since the transformed material often has useful mechanical properties (hard, stiff, high damping (internal friction), shape memory). Martensitic transformations can occur at rather low temperature ( 100 K) where diffusional jumps of atoms are definitely frozen, but also at much higher temperature. Since they occur without transport of matter, they are not of central interest to solid state kinetics. However, in view of the crystallographic as well as the elastic and even plastic implications, diffusionless transformations may inform us about the principles involved in the structural part of heterogeneous solid state reactions, and for this reason we will discuss them. 

For elemental solids and stoichiometric compound crystals, the primary influence of irradiation on their kinetic behavior is due to the increase in Acv(s Ac,). We would expect the enhancement in the component diffusion to be in proportion to the increase in the (average) defect concentrations, thus influencing all homogeneous, inhomogeneous, and heterogeneous solid state reactions. 

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