Defects Dislocations

As in crystals, defects in liquid crystals can be classified as point, line or wall defects. Dislocations are a feature of liquid crystal phases where tliere is translational order, since tliese are line defects in tliis lattice order. Unlike crystals, tliere is a type of line defect unique to liquid crystals tenned disclination [39]. A disclination is a discontinuity of orientation of tire director field. 

In the last chapter we examined data for the yield strengths exhibited by materials. But what would we expect From our understanding of the structure of solids and the stiffness of the bonds between the atoms, can we estimate what the yield strength should be A simple calculation (given in the next section) overestimates it grossly. This is because real crystals contain defects, dislocations, which move easily. When they move, the crystal deforms the stress needed to move them is the yield strength. Dislocations are the carriers of deformation, much as electrons are the carriers of charge. 

Obviously this model is very simplified, compared with the real crystal which contains many defects, dislocations and entanglements. In particular, it neglects many aspects of the true three-dimensional nature of the lamella which one may have thought to be important the influence of the stacking of folds, which is... 

The metal surface is polycrystalline and has a rather complex profile. Because of different crystallite orientations at the surface, different crystaf faces are exposed, such as smooth fow-index faces and stepped high-index faces. Surface texture where a particufar kind of face is predominant can devefop in individual cases. Microcracks and various lattice defects (dislocations, etc.) will also emerge at the surface. 

At smooth metal electrodes that have been subjected to annealing, the number of different crystallographic defects (dislocations, kinks, etc.) emerging at the surface is between 10 and lO cm. This number is small relative to the total number of surface atoms (which is on the order of 10 cm ). In the literature, attempts have been described to determine the catalytic activity of electrodes having an artihcially boosted number of surface defects. These experiments gave no unambiguous results in some cases some increase, in other cases some decrease in activity was observed. 

The lattice imperfections play an important role in reactions in solid state. The reactivity of solids are due to the defect or fault in the lattice. The more perfect a crystal is, the smaller is its reactivity. The defect may be point defect, dislocations, stacking faults, bulk defects etc. 

Surface Defects, Dislocations that exist in the bulk of crystal can extend onto the surface. Dislocation density is defined as the number of dislocations that cut through a unit area. in metals is usually on the order of 10 cm . This dislocation density can be reduced by annealing. In a well-annealed crystal, is in the range 10 to 10 cm . Screw dislocation free surfaces (about 0.01 mm ) can be produced by... 

The usefulness of quadrupolar effects on the nuclear magnetic resonance c I 7 yi nuclei in the defect solid state arises from the fact that point defects, dislocations, etc., give rise to electric field gradients, which in cubic ciystals produce a large effect on the nuclear resonance line. In noncubic crystals defects of course produce an effect, but it may be masked by the already present quadrupole interaction. Considerable experimental data have been obtained by Reif (96,97) on the NMR of nuclei in doped, cubic, polycrystalline solids. The effect of defect-producing impurities is quite... 

Figure 4.25 Growth of the planar defects (dislocation loops) as a function of annealing conditions. The total area covered by the defects saturates at about 1.8% of the total surface area studied by TEM.

Structure Crystal structures, Point defects, Dislocations Crystal structures, Defect reactions, The glassy state Configuration, Conformation, Molecular Weight Matrices, Reinforce- ments Biochemistry, Tissue stracture... 

Whereas in good-conducting doped or polymeric dyes ft-or -type conductivity can be explained without difficulty by analogy with inorganic semiconductors, the p- and -type photoconductivity in insulating (intrinsic) dye films cannot be explained in this manner. It is necessary to take into consideration the existence of defect states (lattice defects, dislocations, impurities etc.) distributed at different depths in the forbidden zone between valence and conduction band these defect states are able to trap electrons and holes, respectively, with different probability 10,11,88),... 

In contrast to fluids, crystals have a greater number of control parameters crystal structure, strain and stress, grain boundaries, line defects (dislocations), and the size and shape of crystallites, etc. These are all relevant to kinetics. Treatments that go beyond transport and diffusion in this important field of physical chemistry are scarce. 

Surface Defects. Dislocations that exist in the bulk of crystal can extend onto the surface. Dislocation density Nd is defined as the number of dislocations that cut... 

At the late stage of lamella orientation, classical topological defects (dislocations and disclinations) dominate [40, 41] (Fig. 8h and Fig. 9), and their movement and annihilation can be followed in Fig. 8h-i and Fig. 9. The latter presents an example of the apparent topological defect interactions and their transformations. Displayed are two dislocations of PMMA, which have an attractive interaction due to their opposite core sign. Therefore, in the next annealing step the dislocation is shifted... 

A domain wall under an external electric field moves in a statistical potential generated by their interaction with the lattice, point defects, dislocations, and neighboring walls. Reversible movement of the wall is regarded as a small displacement around a local minimum. When the driven field is high enough, irreversible jumps above the potential barrier into a neighboring local minimum occur (see Figure 1.23). 

An extension of the kinetic theory on cases when a mechanical pressure interacts with kinetic processes inside solid volume and on interfaces has wide application interests. The elastic deformations in solid are presented from influence of external forces and from presence of internal defects of crystal structure point defects (vacancy, intersite atoms, complexes of atoms, etc.), extended defects (dislocations and inner interfaces in polycrystals), and three-dimensional defects (heterophases crystals, polycrystals). 

Based on conceptions of work [105] the specific dielectric relaxation in PPX with M nanoparticles is supposed to be connected with reorientation of dipoles in polymer environment of M nanoparticles that accompanies the electron transfer between M nanoparticles of percolation cluster. Dipole centers in PPX are (Tv-units of polymer chains on a surface of lamellar PPX crystallites. Such centers are characteristic, in particular, for extended polymer defects (dislocations, grain boundaries, interfaces between amorphous and crystalline areas) where, most probably, M nanoparticles are formed. 

These results indicate that the radiation induced defects such as some point defects, dislocations and lattice distortions have no influence on the protonic conduction. However, the electronic conduction is modified by sub-band annihilation in gap between valence and conduction bands after neutron irradiation [2, 6, 7],... 

Samples presently available are rich in extended defects, dislocations and grain boundaries, and the thermodynamics of growth at low temperatures suggests high densities of at least some of the point defects. 

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