Defects subgrains

All real surfaces will contain defects of some kind. A crystalline surface must at the very least contain vacancies. In addition, atomic steps, facets, strain, and crystalline subgrain boundaries all can be present, and each will limit the long-range order on the surface. In practice, it is quite difficult to prepare an atomically flat surface. 

Extended defects interrupt the continuity of the crystal, generating crystal subgrains whose dimensions depend, in a complex fashion, on the density of extended defects per unit area. Table 4.1 gives examples of reported dislocation densities and subgrain dimensions in olivine crystals from the San Carlos perido-tite nodules (Australia). Assuming a mean dislocation density within 1.2 X 10 and 6 X 10 cm , Kirby and Wegner (1978) deduced that a directional strain pressure of 35 to 75 bar acted on the crystals prior to their transport to the surface by the enclosing lavas. 

Subgrain boundaries (sub-GBs) are major and basic defects in metallic materials, particularly in constructional materials, and have been well investigated, because they strongly affect the mechanical properties of the materials. In Si multicrystals, sub-GBs have also recently become major defects, since the grain size has increased as a result from improving the growth technique. In this section, we present the results of investigations of sub-GBs in metallic materials, which can be applied to Si multicrystals used in solar cells. 

A.E. Romanov, P. Fini and J.S. Speck, Modeling the extended defect evolution in lateral epitaxial overgrowth of GaN subgrain stability ,/. Appl. Pbys., 93, 106-114... 

A more obvious but perhaps underappreciated problem with surface roughness is the existence of defect sites on a surface, i.e., sites that would not be exposed on a perfectly smooth surface. This type of defect is separate from classical defects like stacking faults, subgrain boundaries and dislocations, and is due just to non-uniform expression of the substrate structure in an uneven surface (Fig. 9) such as could occur with the local development of vicinal faces. As surface characterization methods are generally poor except in the case of a small suite of oxides and silicates, this effect has probably not been fully considered to date. For example, it is possible to imagine a low roughness (hkl) surface that is entirely terminated by small faces with other (hkl) orientations, so that the exposed surface functional groups differ both in density and orientation from what is expected. 

Small-angle boundaries are ones across which the difference in orientation is small. They are also called subgrain boundaries. There are two general classes of small-angle houndaries tilt boundaries, which comprise an array of edge dislocations, and twist boundaries, which comprise two or more arrays of screw dislocations. (The reader who is unfamiliar with the line defects known as dislocations will find a clear and readable description in Reference [6].)... 

Tafel slope values are spread over a wide range from less than 30mV to more than 100 mV depending on ill-identified parameters. The role of metallurgical factors such as dislocations and subgrain boimdaries was interpreted as favoring the catalytic mechanism (30 mV), whereas annealed iron with a low density of structural defects would obey the consecutive mechanism (40 mV) [61,62]. 

Larger etch pit densities of VCo.88 than of VCo,s3 form the subgrain boundaries characterized by the presence of substructure such as antiphase boundaries due to the formation of an ordered compound (150). The hardness of NbC decreases with carbon content and the hardness anisotropy of NbCo.8 is less pronounced than that of NbCo.9 (Fig. 11), which would be due to (a) deviation from stoichiometry of the crystal and (b) ordering of carbon vacancies. A high-resolution electron microscopy (HRFM) study gives very detailed information about defect order... 

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