Defects disclinations, 328-9 edge dislocations

Ideal graphite does not exist and the ideal crystal forms invariably contain defects, such as vacancies due to a missing atom, stacking faults and disclination as depicted in Figure 2.18. Other defects include screw and edge dislocations (Figure 2.19). Edge defects find some... 

The defects that can occur in BCP nanopatterns can take several forms and it is beyond the scope of this chapter to detail these in full, however, it is worth providing a general overview. They take the form of many structural defects in other systems and can be broadly described as dislocations and disclinations and a good review is provided elsewhere (Krohner and Antony, 1975). In the simplest explanation, a dislocation is a defect that affects the positional order of atoms in a lattice and the displacement of atoms from their ideal positions is a symmetry of the medium Screw and edge dislocations representing insertion of planes or lines of atoms are typical of dislocations. For a discUnation the defects (lines, planes or 3D shapes) the rotational symmetry is altered through displacements that do not comply with the symmetry of the environment. Kleman and Friedel give an excellent review of the application of these topics to modern materials science (Kleman and Friedel, 2008). 

A very plausible alignment mechanism involves movement of defects. Here, forces on defect structures in an electric field are described. The actual material contains a large number of defects in many configurations, so a full analysis would be prohibitively complex. However, it is instructive to consider instead some simple ca.ses a general wall defect, a +1/2, -1/2 pair of parallel disclination lines, and two compound edge dislocations with opposing Burgers vector. 

This simple treatment of liquid crystalline defects is only applicable to nematics, and the detailed appearance of disclination lines will differ from the simple structures described above because of differences between the elastic constants for splay, twist and bend. In smectic phases, defects associated with positional disorder of layers will also be important, and some smectic phase defects such as edge dislocations have topologies similar to those described for crystals. The defect structures of liquid crystals contribute to the characteristic optical tex-... 

Figure 16. Creation of defects in a smectic A phase, (a, a ) Creation of a right-handed screw dislocation, (b, b ) Creation of an edge dislocation, (c, c ) Creation of a stack of nested conic layers, as observed along focal conics, (d, d, d") Creation of a disclination from a planar cut surface limited by a line L a +jt separation of lips S] and S2 is followed by the addition of matter and relaxation.

This texture distribution is common, but corresponds to a rather schematic model, indicating that high-energy defects, such as disclinations, are found mainly in the vicinity of the isotropic transition. Other situations are observed in thick preparations of cholesterics, for example, where planar domains can be interrupted by walls of vertical layers (Fig. 34 c and d), due to edge dislocations disjoining into disclina-tion pairs (see Fig. 5 d). Despite these par-... 

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