External Surfaces and Grain Boundaries

Frequently the most important planar defects in a crystal are the external surfaces. These may dominate chemical reactivity, and solids designed as catalysts or, for example, as filters must have large surface areas in order to function. Rates of reaction during corrosion are frequently determined by the amount of surface exposed to the corrosive agent. (A further discussion of surface physics and chemistry, although fascinating, is not possible within the scope of this volume.) 

Most metals and ceramics in their normal states are polycrystalline. Polycrystalline solids are composed of many interlocking small crystals, often called grains (Fig. 3.33). The surfaces of the grains that make up the solid are often similar to the external surfaces found on large crystals. 

The nature of a surface will depend upon which atoms are exposed. For example, the surface of a crystal with the sodium chloride structure might consist of a mixture of atoms, as on 100 (Fig. 3.34a), or of just one atom type, as on 111 (Fig. 3.34b and 3.34c). However, it must be remembered that no surface is clean and uncontaminated unless it is prepared under very carefully controlled conditions. Absorbed gases, especially water vapor, are invariably present on a surface in air, which leads to changes in chemical and physical properties. 

The behavior of polycrystalline materials is often dominated by the boundaries between the crystallites, called grain boundaries. In metals, grain boundaries prevent dislocation motion and reduce the ductility, leading to hard and brittle mechanical properties. Grain boundaries are invariably weaker than the crystal matrix, and 

Grain boundaries have a significant effect upon the electrical properties of a polycrystalline solid, used to good effect in a number of devices, described below. In insulating materials, grain boundaries act so as to change the capacitance of the ceramic. This effect is often sensitive to water vapor or other gaseous components in the air because they can alter the capacitance when they are absorbed onto the ceramic. Measurement of the capacitance allows such materials to be used as a humidity or gas sensor. 

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Figure 3.24 Surfaces and boundaries in a crystal (a) the external surface (b) grain boundaries (c) a twin plane (d) an antiphase boundary and (e) a crystallographic shear plane...

All real crystals have atoms which occupy external surface sites and which do not possess the correct number of nearest neighbors as a consequence, Thus, a surface is a scat of energy and is characterized by surface tension. Furthermore, internal surfaces exist, grain boundaries and twin boundaries across which atoms are incorrectly positioned. In a crystal of reasonable size—say 1 cubic centimeter, these two-dimensional defects, called surface defects, contain only about 1 atom in 106, a rather small fraction. Even so, surfaces are important attributes of solids. 

If mass is transported from the external surface to the pore then the external surface will contract decreasing both surface area and grain boundary area. The differential energy change in the external region (dE )... 

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