Screw dislocation model

By use of the proper experimental conditions and Ltting the four models described above, it may be possible to arrive at a reasonable mechanistic interpretation of the experimental data. As an example, the crystal growth kinetics of theophylline monohydrate was studied by Rodriguez-Hornedo and Wu (1991). Their conclusion was that the crystal growth of theophylline monohydrate is controlled by a surface reaction mechanism rather than by solute diffusion in the bulk. Further, they found that the data was described by the screw-dislocation model and by the parabolic law, and they concluded that a defect-mediated growth mechanism occurred rather than a surface nucleation mechanism. 

A screw dislocation model that predicts preferred growth along a defined dislocation. Here,... 

Crystal growth by the layer growth mechanism describes the formation of steps (i.e., layers) by two different mechanisms—2-D nucleation and screw dislocation. The model for 2-D nucleation was developed by Volmer[ ° l and Stranski. The screw dislocation model was first described by Burton, Cabrera, and Frank (BCF). The details of the derivations for these models have been summarized in a number of other references. ° ° ... 

Young, R. J. (1988) Screw dislocation model for yield of polyethylene, Mater. Forum, 11, 210-216. 

The issue of blocking of a threading dislocation is investigated here on the basis of screw dislocation models, following the general approach outlined by Freund (1990) for a buried layer and by Freund (1990) for interaction of 60 degree dislocations in an equi-biaxial strain field in a film with a free surface. The model system is illustrated in Figure 7.5. A film of thickness h is bonded to a relatively thick substrate. The film supports a mismatch shear strain e = An interface misfit dislocation with its... 

Fig. 7.9. Illustration of the results established as critical conditions for formation of two arrays of periodic interface misfit dislocations, one array orthogonal to the other, at the interface between a strained epitaxial film and its substrate. The lower curve is a plot of the result (7.21) for insertion of the last dislocation necessary to complete one of the arrays, the other being already complete, and the upper curve is the equivalent result (7.22) based only on mean strain measures and the critical thickness condition for insertion of an isolated misfit dislocation. The graphs are based on the screw dislocation model with a mismatch strain 7m = 0.01.

Although the screw dislocation model is attractive in its simplicity and the semiquantitative agreement of observed yield stress with theoretical values, major conceptual problems remain. In a tensile configuration, stress is transmitted to crystallites via taut tie ehains. It is difficult to conceive how randomly distributed tie chains could apply the critical shear stress necessary to activate screw disloca-... 

There are two layer-spreading models. In these models, the crystal surface is atomically flat except at screw dislocations or steps of a partially grown surface layer. If there are screw dislocations, growth would continue on the screw... 

These results indicate that in the present linear elastic model, the limiting velocity for the screw dislocation will be the speed of sound as propagated by a shear wave. Even though the linear model will break down as the speed of sound is approached, it is customary to consider c as the limiting velocity and to take the relativistic behavior as a useful indication of the behavior of the dislocation as v — c. It is noted that according to Eq. 11.20, relativistic effects become important only when v approaches c rather closely. 

R.M. Thomson and R.W. Balluffi. Kinetic theory of dislocation climb I. General models for edge and screw dislocations. J. Appl Phys., 33(3) 803—817, 1962. 

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. 

M.P. Dewald, W.A. Curtin Multiscale modelling of dislocation/grain boundary interactions. II. Screw dislocations impinging on tilt boundaries in Al. Phil. Mag. 87, 4615 1641 (2007)... 

Figure 13.27. Surface models for crystal growth (a) mononuclear growth, (b) polynuclear growth, and (c) screw dislocation growth. Along the step a kink site is shown. Adsorbed ions diffuse along the surface and become preferentially incorporated into the crystal lattice at kink sites. As growth proceeds, the surface step winds up in a surface spiral. Often the growth reaction observed occurs in the sequence c, a, b. (From Nielsen, 1964.) (d) Salient features and elementary processes at surfaces.

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