Solid state reactions growth

Fej04 . A similar correspondence between theory and practice has been found for growth of Fej04 by the solid state reactions from FeO and Fe, , between 600 and 1 200°C. The growth rate of FeO is within 10% of the theoretical rate expected from Fe lattice diffusion, calculated according to the Wagner theory . 

The Pb02/PbOx border slowly penetrates into the metal, but only at a very slow rate as a solid-state reaction. Cracks are formed when the oxide layer exceeds a given thickness, on account of the growth in volume when lead becomes converted into lead dioxide (Table 7). Underneath the cracks the corrosion process starts again and again. As a whole, the corrosion proceeds at a fairly constant rate. It never comes to a standstill, and a continually flowing anodic current, the corrosion current is required to re-establish the corrosion layer. 

Solar energy, 6, 488 surface modified electrodes, 6, 30 Sol-Gel process fast reactor fuel, 6, 924 Solid state reactions, 1, 463-471 fraction of reaction, 1, 464 geometric, 1, 464 growth, 1, 464 nucleation, 1, 464 rate laws, 1,464 Solochrome black T metallochromic indicators, 1,555 Solubility... 

Melt growth Vapor growth Hydrothermal synthesis Solid-state reactions... 

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

We have already dealt with two of these. Section 2 dealt with formation of a phase boundary while we have just completed Section 4 concerning nuclei growth as related to a phase boundary. We will consider diffusion mechanisms in nuclei and diffusion-controlled solid state reactions at a later part of this chapter. 

Although we have covered mechanisms relating to solid state reactions, the formation and growth of nuclei and the rate of their growth in both heterogeneous and homogeneous solids, and the diffusion processes thereby associated, there exist still other processes zifter the particles have formed. These include sequences in particle growth, once the particles have formed. Such sequences include ... 

Sintering of particles occurs when one heats a system of particles to an elevated temperature. It Is caused by an interaction of particle surfaces whereby the surfaces fuse together and form a solid mass. It Is related to a solid state reaction In that sintering is governed by diffusion processes, but no solid state reaction, or change of composition or state, takes place. The best way to illustrate this phenomenon is to use pore growth as an example. 

If a precipitate is allowed to undergo Ostwald ripening, or is sintered, or is caused to enter into a solid state reaction of some kind, it will often develop into a distribution which has a size limit to its growth. That is, there is a maximum, or minimum limit (and sometimes both) which the particle distribution approaches. The distribution remains continuous as it approaches that limit. The log-probability plot then has the form shown in 5.8.2. on the next page. 

Solid state reactions are also very common in producing oxide materials and are based on thermal treatment of solid oxides, hydroxides and metal salts (carbonates, oxalates, nitrates, sulphates, acetates, etc.) which decompose and react forming target products and evolving gaseous products. Solid-state chemistry states that, like in the case of precipitation, powder characteristics depend on the speed of the nucleation of particles and their growth however, these processes in solids are much slower than in liquids. 

Table 1 Nuclei growth models for solid state reactions. Values possible for intercalation into a layered host are highlighted in bold ...

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. 

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. 

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). 

In Chapter 11, growth morphologies are dealt with and the question is raised as to which conditions make the moving phase boundaries morphologically stable or unstable during solid state reactions. One criterion for instability is met if the interface moves against the flux direction of the rate determining (slow) reaction partner. 

The crystal defects of the host lattice structure aid in the incorporation of chromophores. By increasing those defects, reactants can diffuse more easily through the product layers and the pigment is formed faster. The presence of mineralizers can also positively affect the solid-state reaction (24). A mineralizer is a compound that facilitates crystal growth during solid-state reactions by providing a local environment that makes the movement of reactants through the solids mixture easier. The incorporation of the chromophore into the host lattice usually results in the formation of a substitution, or less often an addition compound. 

Anhy2drous calcium sulfoaluminate is formed by calcination of lime, gypsum and bauxite. The active expansive ingredient, C A S is formed by solid-state reaction from mixtures of compounds composed of calcium oxide, aluminum oxide, sulfur trioxide gas formed during the calcination of gypsum, and bauxite. Crystal growth of CSAs is encouraged to proceed at a slow rate to preserve the potential force of expansion for extended periods [76],... 

In the solid-state reaction, nucleation and growth have a fundamental role, because, in essence, the solid-state reaction is a phase transformation. In this type of reactions, nucleation and growth follow similar principles as those previously analyzed in Section 3.1 the principal difference being the increased role of diffusion in solid-state reactions [30],... 

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