Solid state reaction, internal

The oxidation and hydrolysis of Fe leads to Fe " oxides either directly or via soluble green rust complexes, solid green rusts or Fe(OH)2. The latter convert to the oxides either by a solid state reaction or a via solution (reconstructive) transformation. Generally, where there is a difference between the structure of the precursor and that of the final oxide, a via solution process seems more likely, but internal rearrangement during topochemical oxidation to the new phase, may also take place. 

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. 

A third type of internal solid state reaction (see later in Fig. 9-12) is characterized by two (solid) reactants A and B which diffuse into a crystal C from opposite sides. C acts as a solvent for A and B. If the reactants form a stable compound AB with each other (but not with the solvent crystal C), an internal solid state reaction eventually takes place. It occurs in the solvent crystal at the location of maximum supersaturation of AB by internal precipitation and subsequent growth of the AB particles. Similar reactions can be observed on a crystal surface which, in this case, plays the role of the solvent matrix C. Surface transport of the reactants leads to a product band precipitated on the surface at some distance from each of the two reactants and completely analogous to the internal reactions described before. In addition, internal reactions have also been observed if (viscous) liquids are chosen as the reaction media (C). 

There is still another type of internal solid state reaction which we will discuss and it is electrochemical in nature. It occurs when an electrical current flows through a mixed conductor in which the point defect disorder changes in such a way that the transference of electronic charge carriers predominates in one part of the crystal, while the transference of ionic charge carriers predominates in another part of it. Obviously, in the transition zone (junction) a (electrochemical) solid state reaction must occur. It leads to an internal decomposition of the matrix crystal if the driving force (electric field) is sufficiently high. The immobile ionic component is internally precipitated, whereas the mobile ionic component is carried away in the form of electrically charged point defects from the internal reaction zone to one of the electrodes. 

Under this criterion, an internal solid state reaction is the extreme of a reaction with morphologically unstable boundaries. In the following sections, the different types of internal solid state reactions will be discussed and treated quantitatively [H. Schmalzried, M. Backhaus-Ricoult (1993)]. 

The common feature of the internal reactions discussed so far is the participation of electronic defects. In other words, we have been dealing with either oxidation or reduction. We now show that reactions of the type A+B = AB can take place in a solvent crystal matrix as, for example, the formation of double oxides (CaO +Ti02 = CaTi03) in which atomic (ionic) but no electronic point defects are involved. Although many different solvent crystal matrices can be thought of (e.g., metals, semiconductors, glasses, and even viscous melts and surfaces), we will deal here mainly with ionic crystal matrices in order to illustrate the basic features of this type of solid state reaction. 

Figure 9-12. a) Scheme of the internal solid state reaction CaO +Ti02 = CaTi03 in the matrix crystal NiO. Concentration profiles and precipitate are indicated, b) Photograph of cross section with internal reaction zone (T = 1340 C, t = 413 h reaction time). 

The types of internal solid state reactions discussed in this section can have interesting technical applications. Since these reactions are localized, the introduction of AB precipitates into a C matrix can strongly influence such local properties as the mechanical, electrical, or optical properties inside a crystal. 

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. 

Accumulation of the product may increase the rigidity of the medium, diminish the rate of the reaction, and ultimately limit the chemical yield of a solid-state reaction. In a recent study on the photocyclization of three chloro-substituted a-oxoamides (57-59, Scheme 36) by Hashizume et al. it was speculated that limited chemical yields may be attributed to increased internal strain caused by the... 

The correctness of new ideas and theories in scientific research is commonly evaluated by their fruitfulness in interpretation of problems accumulated in one or another field and by their capability to predict unknown trends and effects. The other general criteria that are applied to new theories are simplicity, internal consistency, experimental reliability (verifiability), and compliance with previous theories. The thermochemical approach to the kinetics of solid-state reactions used in this study meets these criteria. 

Kooy, C. (1965). Material transport in solid state reactions. In Fifth International Symposium on the Reactivity of Solids, Munich. Ed. G. M. Schwab. Elsevier, London, pp. 21-8. 

Many solid-state reactions give a parabolic rate law for the growth of an internal phase, and such a parabolic rate law is taken as evidence that the reaction is diffusion-controlled. The units of k are the same as the units of the diffusion coefficient, m s Generally, one can write ... 

As the outer scale advances into the metal, the Cr203 internal oxide islands are engulfed by NiO and a solid-state reaction ensues to form NiCr204 ... 

To better understand the axial expansion characteristics of compositions in CaZr4(P04)6-MgZr4(P04)6 system, high temperature XRD was used to determine lattice parameter fluctuations upon heating. Powder samples were made by solid-state reaction methods. The compositions CaZr4(P04)e, (Cao.8.Mgo.2)Zr4(P04)6, and (Cao.7,Mgo.3)Zr4(P04)6 were selected for analysis. Samples were mixed with a pure Pt powder (for use as an internal standard) and scanned with a Scintag 0-0 fully-automated diffractometer equipped with a... 

The morphology of the internal oxidation zone and stabihty of transformation front were studied in [29]. Solid-state reactions with formation of two-phase zones were analyzed in [3, 30, 31]. 

C. Kooy, Material Transport in Solid State Reactions, in 5th International Symposium on Reactivity of Solids, Elsevier Publishing Co., Amsterdam, 1965, p. 21. 

Solid State Ionics, International Journal on Diffusion and Reactions, Elsevier, Amsterdam. 

Crystal structure, crystal defects and chemical reactions. Most chemical reactions of interest to materials scientists involve at least one reactant in the solid state examples inelude surfaee oxidation, internal oxidation, the photographie process, electrochemieal reaetions in the solid state. All of these are critieally dependent on crystal defects, point defects in particular, and the thermodynamics of these point defeets, especially in ionic compounds, are far more complex than they are in single-component metals. I have spaee only for a superficial overview. 

An example of a journal hovering between broad and narrow spectrum is Journal of Alloys and Compounds, subtitled an interdiciplinary journal of materials science and solid-state chemistry and physics. One which is more restrictively focused is Journal of Nuclear Materials (which I edited for its first 25 years). Ceramics has a range of journals, of which the most substantial is Journal of the American Ceramic Society. Ceramics International is an example of an international journal in the field, while Journal of the European Ceramic Society is a rather unusual instance of a periodical with a continental remit. More specialised journals include Solid State Ionics Diffusion and Reactions, and a new Journal of Electroceramics, started in 1997. 

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