Crystal screw dislocation mechanism

D nucleation occurs when nuclei at the crystal surfaces act as sources of steps that allow for the further incorporation of growth units. In general, this mechanism accounts for the crystal growth observed at high supersaturations. The screw dislocation mechanism often accounts for growth at lower supersaturations. When the supersaturation is below the threshold for formation of 2-D nuclei, the presence of screw dislocations provides a source of steps for the addition of growth units in an infinite sequence of equidistant and parallel steps. 

A number of theories have been put forth to explain the mechanism of polytype formation (30—36), such as the generation of steps by screw dislocations on single-crystal surfaces that could account for the large number of polytypes formed (30,35,36). The growth of crystals via the vapor phase is beheved to occur by surface nucleation and ledge movement by face specific reactions (37). The soHd-state transformation from one polytype to another is beheved to occur by a layer-displacement mechanism (38) caused by nucleation and expansion of stacking faults in close-packed double layers of Si and C. 

Flinn et al. [30] describes an experimental impact technique in which <100)-oriented LiF single crystals ( 8 ppm Mg) are loaded in a controlled manner and the multiplication of screw dislocations is measured. The peak shear stress in this relatively soft material is 0.01 GPa. For shear impulses exceeding approximately 40 dyne s/cm, dislocation multiplication is adequately described by the multiple-cross-glide mechanism [(7.24)] with m = l/bL = (2-4) X 10 m, in reasonable agreement with quasi-static measurement [2]. 

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. 

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. 

Multilayer crystals with a central screw dislocation were commonly seen. A micrograph of a beautiful solution-grown multilayer crystal with regularly rotated terraces was presented by Keller [38]. This mechanism to multiply a single crystal layer into many crystal layers is important for the crystal growth from the melt to form spherulites. 

The screw-dislocation theory (sometimes referred to as the BCF theory because of its development by Burton, Cabrera, and Frank) is based on a mechanism of continuous movement in a spiral or screw of a step or ledge on the crystal surface. The theory shows that the dependence of growth rate on supersaturation can vary from a parabolic relationship at low supersaturations to a linear relationship at high supersaturations. In the BCF theory, growth rate is given by ... 

There is still considerable speculation on the cause of screw dislocations. Some may come from irregularities in foreign nuclei upon which many crystals form others may be started by inclusions of dirt or foreign atoms in the crystal. A strong mechanical disturbance may cause a grow-... 

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. ° ° ... 

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