Interfacial defect boundary

Figure 3.1. Schematic representation of lamellar crystal. Ic - lamellar thickness, 1 — interfacial defect boundaries alternated by amorphous regions, D, — die average coherent length of the crystals. [Adapted, by permission, from Balta-Calleja, F J Ezquerra, T A,...

This chapter has argued for the critical role of interfacial defects in solids. We have favored an artificial classification into three primary types of interfacial defects, namely, surfaces, stacking faults and twins, and grain boundaries. In each of these cases, it has been argued that a continuum description of the energetics of such... 

A crystalline defect refers to a lattice irregnlarity having one or more of its dimensions on the order of an atomic diameter. Classification of crystaUine imperfections is frequently made according to the geometry or dimensionality of the defect. Several different imperfections are discussed in this chapter, inclnding point defects (those associated with one or two atomic positions) linear (or one-dimensional) defects and interfacial defects, or boundaries, which are two-dimensional. Imparities in solids are also discussed, because impurity atoms may exist as point defects. Finally, techniques for the microscopic examination of defects and the structnre of materials are briefly described. 

Interfacial defects are boundaries that have two dimensions and normally separate regions of the materials that have different crystal structures and/or crystallographic orientations. These imperfections include external surfaces, grain boundaries, phase boundaries, twin boundaries, and stacking faults. 

Furthermore, the surfaces of chain-folded layers (Figure 14.13) are considered interfacial defects, as are also boundaries between two adjacent crystalline regions. 

As discussed above, lipid membranes are dynamic structures with heterogeneous structure involving different lipid domains. The coexistence of different kinds of domains implies that boundaries must exist. The appearance of leaky interfacial regions, or defects, has been suggested to play a role in abrupt changes in solute permeabilities in the two-phase coexistence regions [91,92]. 

There are different criterion of how to classify solid-solid interfaces. One is the sharpness of the boundary. It could be abrupt on an atomic scale as, for example, in III-IV semiconductor heterostructures prepared by molecular beam epitaxy. In contrast, interdiffusion can create broad transitions. Surface reactions can lead to the formation of a thin layer of a new compound. The interfacial structure and composition will therefore depend on temperature, diffusion coefficient, miscibility, and reactivity of the components. Another criterion is the crystallinity of the interface. The interface may be crystalline-crystalline, crystalline-amorphous, or completely amorphous. Even when both solids are crystalline, the interface may be disturbed and exhibit a high density of defects. 

Our conclusions thus are that whenever the boundary concentrations C(0) and C(L) are fixed in value by the interfacial reactions, the current is inversely proportional to L and the concentration is a linear function of the position x within the oxide. Considering xjL to be a dimensionless normalized position within the oxide, we can conclude further that the defect concentration is a linear function of this normalized position. It requires only a few more easy steps (cf. Sect. 1.13) to show that the corresponding oxide layer thickness would increase as the square root of the time under these conditions. 

Another contribution to variations of intrinsic activity is the different number of defects and amount of disorder in the metallic Cu phase. This disorder can manifest itself in the form of lattice strain detectable, for example, by line profile analysis of X-ray diffraction (XRD) peaks [73], 63Cu nuclear magnetic resonance lines [74], or as an increased disorder parameter (Debye-Waller factor) derived from extended X-ray absorption fine structure spectroscopy [75], Strained copper has been shown theoretically [76] and experimentally [77] to have different adsorptive properties compared to unstrained surfaces. Strain (i.e. local variation in the lattice parameter) is known to shift the center of the d-band and alter the interactions of metal surface and absorbate [78]. The origin of strain and defects in Cu/ZnO is probably related to the crystallization of kinetically trapped nonideal Cu in close interfacial contact to the oxide during catalyst activation at mild conditions. A correlation of the concentration of planar defects in the Cu particles with the catalytic activity in methanol synthesis was observed in a series of industrial Cu/Zn0/Al203 catalysts by Kasatkin et al. [57]. Planar defects like stacking faults and twin boundaries can also be observed by HRTEM and are marked with arrows in Figure 5.3.8C [58],... 

Figure 36. Defect concentration and conductance effects for three different thicknesses Li L2 Lj. The mesoscale effect on defect concentration (l.h.s.) discussed in the text, when L < 4J, is also mirrored in the dependence of the conductance on thickness (r.h.s.). If the boundary layers overlap , the interfacial effect previously hidden in the intercept is now resolved. It is presupposed that surface concentration and Debye length do not depend on L. (Both can be violated, c , at sufficiently small L because of interaction effects and exhaustibility of bulk concentrations.)36 94 (Reprinted from J. Maier, Defect chemistry and ion transport in nanostructured materials. Part II. Aspects of nanoionics. Solid State Ionics, 157, 327-334. Copyright 2003 with permission from Elsevier.)...

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