Crystalline solids grain structure

Moreover, most solids have a preferred crystalline shape and tend to preserve it or to revert to it. Hence, a solid, or an agglomerate of solids (grains, crystals) is not free to adjust to capillary forces, as postulated in Eqs. (56) and (57). The profile of a single crystal may look, for instance, like the line KLMN in Fig. 14, and the angle a usually is determined by the crystal structure. In a cubic crystal, frequently a = n/2. 

All crystalline solids exhibit defects in and departure from the ideal lattice structure, particularly at elevated temperature. Either lattice points remain unoccupied (vacancies) or lattice elements deposit between the regular lattice points (interstitial lattice points). These point defects determine material transport in a solid. In addition, there are a number of linear and face defects, dislocations, grain boundaries, and so forth, which although of importance to the mechanical properties of a solid are less significant for material transport. 

Amorphous soUds are free of defects most commonly found in crystalline solids, such as grain boundaries and stacking faults. Their homogeneous structure makes them stronger and more resistant to corrosion, and it improves their magnetic properties. Amorphons alloys are, however, thermodynamically unstable and on heating are liable to revert to the CTystalline state. Electron microscopic evidence indicates that the top layers of some alloy specimens (e.g. Pd4oNi4oP2o) are nanocrystalUne and defident in P. 

In the context of the crystalline solid state, a mechanical mixture (or physical mixture) is a mixture of crystals or crystallites of different phases (i.e., constituting different crystal structures) that are separated by grain boundaries. 

Here, two conclusions are important from this mechanism derived from metallurgical samples of magnetite/wiistite synthetic mixtures. First, the elemental iron is essential to reduce magnetite with hydrogen gas at low temperature. This elemental iron is produced from thermal decomposition of the wiistite mixture in the precursor. Stability and bulk distribution of the wiistite determine the abundance of reaction interfaces in the polycrystalline solid. The grain structure and porosity of the final catalyst is mainly predetermined by the disposition of these reaction centers representing the nuclei of the iron metal crystallines. Second, the reaction involves movement of all iron ions and allows a complete bulk restructuring of the solid at low temperature. The topochemistry of the reduction process will determine... 

Numerous chemical reactions or micro-structural changes in solids take place through solid state diffusion, i.e. the movement and transport of atoms in solid phases. In crystalline solids, the diffusion takes place because of the presence of defects. Point defects, e.g. vacancies and interstitial ions, are responsible for lattice diffusion. Diffusion also takes place along line and surface defects which include grain boundaries, dislocations, inner and outer surfaces, etc. As diffusion along linear, planar and surface defects is generally faster than in the lattice, they are also termed high diffiisivity or easy diffusion paths. Another frequently used term is short circuit diffusion. 

In most as-cast poly crystalline solids three distinct zones with different grain structures can be identified (Fig. 6.1), namely ... 

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