Screw dislocation density

This equation shows that in contrast to the steady state current density I ss (c.f. eq. (5.28)) the current density transient depends on the screw dislocation density, Ndis /A. 

A major breakthrough in the understanding of basic concepts of crystal growth has been achieved in model systems with single crystal substrate faces, either free of screw dislocations (quasi-perfect) or with a low screw dislocation density. 

The investigation of the spiral growth mechanism of faces with a low screw dislocation density represents the first quantitative confirmation of Frank s theory. First, the parabolic dependence of the normal growth rate on the supersaturation was... 

Figure 16. Steady state current-overvoltage dependence in an i vs. 17 plot on an Ag (100) face in 6M AgNOa at 45°C. Screw-dislocation density 1-5-5 x 10 disl. cm. Overvoltage is IR corrected.

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 NEB method has been applied successfiilly to a wide range of problems, for example studies of diffusion processes at metal smfaces, multiple atom exchange processes observed in sputter deposition simulations, dissociative adsorption of a molecule on a smface, diffusion of rigid water molecules on an ice Di siuface, contact formation between metal tip and a smface, cross-slip of screw dislocations in a metal (a simulation requiring over 100,000 atoms in the system, and a total of over 2,000,000 atoms in the MEP calculation), g d diffusion processes at and near semiconductor smfaces (using a plane wave based Density Fimctional Theory method to calculate the atomic forces). In the last two applications the calculation was carried out on a cluster of workstations with the force on each image calculated on a separate node. 

Line defects in crystals such as screw or edge dislocations are quite common in solid substances. The typical length of all dislocation in 1 m3 of annealed material amounts to 1010 m (10 km in 1 cm3). The so-called dislocation density is defined as ... 

The possibility that the core of a dislocation could be empty was first recognised by Frank [1], If the strain energy density arising from a dislocation is sufficiently large, it may become energetically favourable to remove the material near the core and place it in an unstrained environment far from any dislocations. This process creates additional surface area around the core of the dislocation. The equilibrium radius of the hollow core of a screw dislocation is given by... 

Chapter 5). The measured overpotential dependence of the steady state current density of the transients (c.f. Fig. 6.15) indicates a multilayer growth mode generated by screw dislocations [4.62, 4.63, 4.68, 4,69]. It can be assumed that the screw dislocations of a real Au.Qikl) substrate are inherited by the growing ultra-thin Ag film, determining the growth mechanism under these experimental conditions. 

The pyramid slopes as a function of the overpotential can easily be investigated on crystal faces intersected by only a few screw dislocations using the following polarization routine [5.9, 5.72]. A short cathodic current pulse with a relatively high amplitude is applied to a crystal face, which is initially flattened by growth at extremely low current density. As a result, well-separated regular growth pyramids appear at the... 

The transient behavior of Ag(lll) and Ag(lOO) crystal faces intersected by only one screw dislocation (A disl = 1) has been investigated in the system standard Ag(M0/AgNO3 [5.83]. A typical current density transient obtained on an Ag(lll) face is shown in Fig. 5.42. A linear i vs. t dependence is obtained for the initial part of the transient (Fig. 5.42b) in agreement with eq. (5.32). 

Figure 5.42 Current density transient obtained on an Ag(lll) crystal face intersected by one screw dislocation in the standard system Ag(lll)/AgN03 at 7i = - 0.25 mV and / f = - 1.15 mV [ 5.83]. (a) Overall transient (b) i vs. t plot of the initial part of the transient in (a).

Figure 5.45 Overall exchange current density o, Ag/Ag+ as a function of the step density Ls(rj) of a Ag(lOO) crystal face intersected by few screw dislocations in the standard system Ag(100)/AgNO3 [5.29]. Different step densities Ls,(rj) are obtained by electrochemical growth at different overpotentials I77I.

---