Dislocation in crystal

Fig. VII-8. (a) Screw dislocation (from Ref. 115). (b) The slip that produces a screw-type dislocation. Unit slip has occurred over ABCD. The screw dislocation AD is parallel to the slip vector. (From W. T. Read, Jr., Dislocations in Crystals, McGraw-Hill, New York, 1953, p. 15.)...

Extraterrestrial dust particles can be proven to be nonterrestrial by a variety of methods, depending on the particle si2e. Unmelted particles have high helium. He, contents resulting from solar wind implantation. In 10-)J.m particles the concentration approaches l/(cm g) at STP and the He He ratio is close to the solar value. Unmelted particles also often contain preserved tracks of solar cosmic rays that are seen in the electron microscope as randomly oriented linear dislocations in crystals. Eor larger particles other cosmic ray irradiation products such as Mn, Al, and Be can be detected. Most IDPs can be confidently distinguished from terrestrial materials by composition. Typical particles have elemental compositions that match solar abundances for most elements. TypicaUy these have chondritic compositions, and in descending order of abundance are composed of O, Mg, Si, Ee, C, S, Al, Ca, Ni, Na, Cr, Mn, and Ti. 

By 1969, when a major survey (Thompson 1969) was published, the behaviour of point defeets and also of dislocations in crystals subject to collisions with neutrons and to the eonsequential collision cascades had become a major field of researeh. Another decade later, the subjeet had developed a good deal further and a highly quantitative body of theory, as well as of phenomenological knowledge, had been assembled. Gittus (1978) published an all-embracing text that eovered a number of new topics chapter headings include Bubbles , Voids and Irradi-ation(-enhanced) Creep . 

Read,W.T. Dislocations in crystals. New York McGraw-Hill Book Company 1953. 

The concept of dislocations was theoretically introduced in the 1930s by E. Orowan and G. I. Taylor, and it immediately played an essential role in the understanding of the plastic properties of crystalline materials, but it took a further twenty years to understand fully the importance of dislocations in crystal growth. As will be described in Section 3.9, it was only in 1949 that the spiral growth theory, in which the growth of a smooth interface is assumed to proceed in a spiral step manner, with the step serving as a self-perpetuating step source, was put forward [7]. 

The most commonly encountered distribution of dislocations in crystals grown from solution or vapor phase (dilute ambient phases) by natural nucleation and without seed may be observed as dislocation bundles starting from the center of a crystal and running nearly perpendicular to the habit faces. In addition to these dislocations, smaller dislocation bundles originating from inclusions may be observed. See Figs. 6.1(e) and 6.5 for examples. 

As early as 1829, the observation of grain boundaries was reported. But it was more than one hundred years later that the structure of dislocations in crystals was understood. Early ideas on strain-figures that move in elastic bodies date back to the turn of this century. Although the mathematical theory of dislocations in an elastic continuum was summarized by [V. Volterra (1907)], it did not really influence the theory of crystal plasticity. X-ray intensity measurements [C.G. Darwin (1914)] with single crystals indicated their mosaic structure (j.e., subgrain boundaries) formed by dislocation arrays. Prandtl, Masing, and Polanyi, and in particular [U. Dehlinger (1929)] came close to the modern concept of line imperfections, which can move in a crystal lattice and induce plastic deformation. 

Read, W.T. (1953) Dislocations in Crystals, McGraw Hill, New York... 

Dislocations in crystals tend to move in response to forces exerted on them. In general, an effective driving force is exerted on a dislocation whenever a displacement of the dislocation causes a reduction in the energy of the system. Forces may arise in a variety of ways. 

Can this model also be applied to ceramic superconductors After extensive correspondence and a literature search involving scanning tunneling electronmicroscopy and screw dislocations in crystals, I decided to drop this subject, mainly because it exceeds the level of this book. It can, however, be concluded that superconductivity in ceramic materials is based on a different mechanism. 

W. T. Read. Dislocations in Crystals. New York McGraw-Hill, 1953. 

In the decomposition of the cubic form (above 240 C) they arc spherical, randomly distributed throughout the crystals. The activation energy of their growth is 17 1 kcal/mol above the tran point. The dislocation in crystals during their decomposition was revealed etching the surface of crystals with ethanol [79bJ. 

Frenkel and Kontorova were not the hrst ones to use the model that is now associated with their names. However, unlike Dehlinger, who suggested the model [99], they succeeded in solving some aspects of the continuum approximation. Like the PT model, the FK model was first used to describe dislocations in crystals. Many of the recent applications are concerned with the motion of an elastic object over (or in) ordered [100] or disordered [101] structures. The FK model and generalizations thereof are also increasingly used to understand the friction between two solid bodies. 

LEV 97] LEVINE L.E., THOMSON R., X-ray scattering by dislocations in crystal. General theory and application to screw dislocations ,4cto Cryst A, vol. 53, p. 590-602,1997. 

The local translational and orientational order of atoms or molecules in a sample may be destroyed by singular points, lines or walls. The discontinuities associated with the translational order are the dislocations while the defects associated with the orientational order are the disclinations. Another kind of defect, dispirations, are related to the singularities of the chiral symmetry of a medium. The dislocations were observed long after the research on them began. The dislocations in crystals have been extensively studied because of the requirement in industry for high strength materials. On the contrary, the first disclination in liquid crystals was observed as early as when the liquid crystal was discovered in 1888, but the theoretical treatment on disclinations was quite a recent endeavor. 

Screw dislocation in crystal surface and movement of particle into kink. 

Incontrovertible evidence for the occurrence of dislocations in crystals comes from the application of techniques such as optical, electron. X-ray, and field-ion microscopy. The reader is referred to comprehensive reviews of the full details of the various methods of study (see, for example, references 7, 12, 47, 48). In this section we shall adumbrate the most important exjierimental bases for the principles enunciated in Section II, and pay especial attention to some of the methods that have proved valuable in systems to be diseussed more fully in Section IV. 

There are several ways in which X-ray diffraction may be used (81) to detect dislocations in crystals. However, basically they all make use of the fact that any local bending of the lattice associated with a dislocation results in a change in the reflection conditions and the X-rays are scattered differently in this region. A reduction in the intensity diffracted from the crystal in this region is therefore brought about, and a dark line is produced on the photograph. 

As remarked in chapter 1, the nematic state is named for the threads that can be seen within the fluid under a microscope (fig. 1.1.6(a)). In thin films sandwiched between glass plates these threads can be seen end on. A typical example of the texture in a plane film of thickness about 10 /tm between crossed polarizers - the structures a noyaux or schlieren textures - is given in fig. 1.1.6(6). The black brushes originating from the points are due to line singularities perpendicular to the layer. In analogy with dislocations in crystals, Frank proposed the term disinclinations , which has since been modified to disclinations in current usage. 

These are exactly analogous to screw dislocations in crystals. There is a spiral arrangement of the smectic layers around the dislocation line L which is normal to the layers. The associated deformation is given by... 

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