Defects interface structure

Distribution and parametrization of charge defects Interface structure and dynamics... 

HREM images can be used for studying defects, local structure and chemical variations in crystals, as well as interfaces. 

Crystal/crystal interfaces possess more degrees of freedom than vapor/crystal or liquid/crystal interfaces. They may also contain line defects in the form of interfacial dislocations, dislocation-ledges, and pure ledges. Therefore, the structures and motions of crystal/crystal interfaces are potentially more complex than those of vapor/crystal and liquid/crystal interfaces. Crystal/crystal interfaces experience many different types of pressures and move by a wide variety of atomic mechanisms, ranging from rapid glissile motion to slower thermally activated motion. An overview of crystal/crystal interface structure is given in Appendix B. 

In the case under consideration different physical structures were realized due to the formation of the polymer network in the surface layers the filler surface, as usually happens in filled systems. As is known79, this induces considerable changes in the structure of the material. It is also possible that in these conditions a more defective network structure is formed. These results show that even the purely physical factors influencing the formation of the polymer network in the interface lead to such changes in the relaxation behavior and fractional free-volume that they cannot be described within the framework of the concept of the iso-free-volume state. It is of great importance that such a model has been devised for a polymer system that is heterogeneous yet chemically identical. 

Figure 5. Schematic arrangement of the surface of a partly crystallized E-L TM amorphous alloy such as Pd-Zr. A matrix of zirconia consisting of the two polymorphs holds particles of the L transition metal (Pd) which are structured in a skin of solid solution with oxygen (white) and a nucleus of pure metal (black). The arrows indicate transport pathways for activated oxygen either through bulk diffusion or via the top surface. An intimate contact with a large metal-to-oxide interface volume with ill-defined defective crystal structures (shaded area) is essential for the good catalytic performance. The figure is compiled from the experimental data in the literature [26, 27].

Most relevant for the oxygen transport should be the defective crystal structure of both catalyst components. The defective structure and the intimate contact of crystallites of the various phases are direct consequences of the fusion of the catalyst precursor and are features which are inaccessible by conventional wet chemical methods of preparation. Possible alternative strategies for the controlled synthesis of such designed interfaces may be provided by modem chemical vapor deposition (CVD) methods with, however, considerably more chemical control than is required for the fusion of an amorphous alloy. 

Tunneling in multilayered LB films is defect-mediated via trap sites within the conduction band of the molecules (Poole conduction), or by Schottky emission between widely spaced trap sites (Poole Frenkel conduction) in thicker samples [13]. With good molecular conductors the current from molecular conduction should dominate the small contribution from tunneling. However, the conduction mechanism between adjacent layers is not always obvious, due to the complexity of the interface structure. 

In this and the following chapter, we will describe the most important simple (binary) crystal structures found in ceramic materials. You need to know the structures we have chosen because many other important materials have the same structures and because much of our discussion of point defects, interfaces, and processing will use these materials as illustrations. Some, namely FeSi, TiOi, CuO, and CU2O, are themselves less important materials and you would not be the only ceramist not to know their structure. We include these oxides in this discussion because each one illustrates a special feature that we find in oxides. These structures are just the tip of the topic known as crystal chemistry (or solid-state chemistry) the mineralogist would have to learn these, those in Chapter 7, and many more by heart. In most examples we will mention some applications of the chosen material. 

If charging, however, involves a change in the interface composition that is too great to be accomplished solely by defect motions, the result may be a voltage-dependent "interface structure", such as we have described for competitive chemisorption. 

This condition of local equilibrium implies that point defects associated with mass transport within an oxide scale must be created and/or annihilated at gas/scale and/or scale/substrate interfaces. Therefore, the interface reactions must be expressed to account for this required interface action of point defect creation/annihilation. But this interface action depends also on the structure of the interface. A description of the interface structure is thus needed before considering how interface action and interface structure interact to simultaneously achieve, or fail to achieve, the relative displacement of the metal and/or oxide lattices and the migration of the scale/substrate interface. 

Pieraggi B, Rapp R A and Hirth J P (1995), Role of interface structure and interfacial defects in oxide scale growth , Oxid Met, 44(1/2), 63-79. 

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