Kink-type "defects

Fig. 7.1 Several types of defects generated on carbon nanotube wall and silicon dioxide substrate after argon atom collision. Single vacancy (purple circle), kink (complex) defect (black circle), carbon chemisorption and doping on Si02 substrate (orange circle) [38]...

A silicon surface, no mater how well it is prepared, is not perfectly flat at the atomic scale, but has surface defects such as surface vacancies, steps, kinks sites, and dopant atoms. The dissolution of the surface is thus not uniform but modulated at the atomic scale with higher rates at the defects and depressed sites. The micro roughness of the surface will increase with the amount of dissolution due to the sensitivity of the reactions to surface curvature associated with the micro depressed sites. These sites, due to the higher dissolution rates, will evolve into pits and eventually into pores. Depending on the condition, a certain amount of dissolution is required before the initiation of pores on all types of materials. 

Figure 3.16. Some simple defects found on a low-index crystal face 1, the perfect flat face, a terrace 2, an emerging screw dislocation 3, the intersection of an edge dislocation with the terrace 4, an impurity adsorbed atom 5, a monatomic step in the surface, a ledge 6, a vacancy in the ledge 7, a kink, a step in the ledge 8 an adatom of the same type as the bulk atoms 9, a vacancy in the terrace 10, an adatom on the terrace. (From Ref. 12, with permission from Oxford University Press.)...

The simplest model of a surface with defects is the terrace-ledge-kink model. In this model only 4 types of defects are assumed present in the surface ... 

The catalyst particle is usually a complex entity composed of a porous solid, serving as the support for one or more catalytically active phase(s). These may comprise clusters, thin surface mono- or multilayers, or small crystallites. The shape, size and orientation of clusters or crystallites, the extension and arrangement of different crystal faces together with macrodcfects such as steps, kinks, etc., are parameters describing the surface topography. The type of atoms and their mutual positions at the surface of the active phase or of the support, and the type, concentration and mutual positions of point defects (foreign atoms in lattice positions, interstitials, vacancies, dislocations, etc.) define the surface structure. 

The complexity of the problem is increased by the fact that the point defects can be located at various sites, terraces, edges, steps, and kinks [11] and that they can be isolated, occur in pairs, or even in clusters . Furthermore, the concentration of the defects is usually low, making their detection by integral surface sensitive spectroscopies very difficult. A microscopic view of the metal/oxide interface and a detailed analysis of the sites where the deposited metal atoms or clusters are bound become essential in order to rationalize the observed phenomena and to design new materials with known concentrations of a given type of defects. 

For the plasma sample PT0X-40P, D = 650 X and G = 1.25%. By contrast, the comparable radiation samples have values of 550 X and 0.7%, respectively. The longitudinal dimensions for both samples are considerably shorter than the extended length of the microfibril, which is of the order of 10 X. The most likely interpretation for this observation is that the type of defects suggested by the kink model ( 7) must exist in the chain direction. 

Even the cleanest of all the substrates shows areas where the periodic surface potential is perturbed. These sites can be generally called defects. Defects are generally classified into two main subclasses point defects, like corners, kinks, impurities or missing atoms and extended defects, like dislocations and steps. The type, concentration and characteristics of defects depend on several factors but the nature of the oxide and the history of the sample are no doubt the most important ones. In this section, two of the most commonly found MgO defects21,126 — low coordinated anion sites (steps and corners) and oxygen vacancies — will be considered with special emphasis on their interaction with metal atoms. 

Figure 2.2. One type of screw dislocation giving rise to (a) atomic steps at the surface and (b) the slip plane that produces the dislocation and, ultimately, the defects at the surface (steps and kinks).

F. 23. Various types of contacts between two chain molecules of which both have the same conformational defect, i.e. a kink... 

Ghavamian and Ochsner [43] were analyzed the effect of defects on the buckling behavior of single and multiwalled CNT based on FE method considering three most likely atomic defects including impurities, vacancies (carbon vacancy) and introduced disturbance. The results demonstrate that the existence of any type kinks in CNTs structure, conducts to lower critical load and lower buckling properties. 

Mutual solid solubilities were found to be about 5 a/o Y in Mo silicides and vice versa. The solubility of Mo in the defect AlB2-type of YSij 5 was of a special kind, first extending in the direction of (Mo Yi )Si, up to 7 a/o Mo, then revealing a kink where Si/Mo substitution starts, the Y concentration now remaining constant at 33 a/o Y. 

Zero-dimensional defects or point defects conclude the list of defect types with Fig. 5.87. Interstitial electrons, electron holes, and excitons (hole-electron combinations of increased energy) are involved in the electrical conduction mechanisms of materials, including conducting polymers. Vacancies and interstitial motifs, of major importance for the explanation of diffusivity and chemical reactivity in ionic crystals, can also be found in copolymers and on co-crystallization with small molecules. Of special importance for the crystal of linear macromolecules is, however, the chain disorder listed in Fig. 5.86 (compare also with Fig. 2.98). The ideal chain packing (a) is only rarely continued along the whole molecule (fuUy extended-chain crystals, see the example of Fig. 5.78). A most common defect is the chain fold (b). Often collected into fold surfaces, but also possible as a larger defect in the crystal interior. Twists, jogs, kinks, and ends are other polymer point defects of interest. 

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