Defects disclinations, 408 screw dislocations

The helical structure of the c-director in the smectic C phase makes the defects different from those in the smectic C phase. As the Volterra process produces a screw dislocation, for example, along the z axis and the Burger vector b = d, it must be accompanied by a parallel wedge disclination in the c-director, in the form... 

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... 

To identify the antiferroelectric phase, texture observation of the homeotropic cells of racemic compounds is very effective. In the SmC phase, only the schlieren texture with four brushes is observable and that with two brushes is prohibited, because of the head-and-tail inequivalence of the C-director. In the SmCA phase, however, the schlieren texture with two brushes is sometimes seen, as shown in Figure 9.8 [18], [19]. The existence can be explained by taking into account a screw dislocation, as illustrated in Figure 9.9. The discontinuous change (7r-wall) of the C-director is compensated by the screw dislocation. This defect is a combined defect of a disclination and a dislocation, i.e., adispiration [18], [19]. 

Figure 9.9. Model structure of the two-brush defect i.e., dispiration, a combined defect of a wedge disclination and a screw dislocation.

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.

Spherulites showing concentric layers present a disclination radius or diameter, but this structure is due to a topological constraint and does not seem to be linked to liquid crystal growth. Very rapid growth of cholesteric phases often generates screw dislocations of the two types shown in Fig. 24i and j, and this has been filmed by Rault in p-azoxyanisol added to cholesterol benzoate [98, 99]. Slow growth does not result in the production of these defects. 

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). 

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