Glide activation energies

Figure 4.3 Glide activation energies for various covalent crystals versus their minimum energy gaps. The correlation coefficient is 0.95. Without the point for GaP, it would be much higher. 

Figure 5.12 Glide activation energies from high temperature data vs. energy band gaps. Note that the data for the homopolar crystals (C, SiC, Si, and Ge) lie quite close to the correlation line, while the data for The heteropolar crystals show some scatter. The reason why GaP Is an exception is not known. Also, note that the slope of the correlation line is two. 

Figure 6.3 Plot of the glide activation energy against the band gap, E, for the Group 14 elements. Reprinted with permission from J.J. Gilman, Science, 261, 1436 (1993). 1993 American Association for the Advancement of Science.

Dislocation motion in covalent crystals is thermally activated at temperatures above the Einstein (Debye) temperature. The activation energies are well-defined, and the velocities are approximately proportional to the applied stresses (Sumino, 1989). These facts indicate that the rate determining process is localized to atomic dimensions. Dislocation lines do not move concertedly. Instead, sharp kinks form along their lengths, and as these kinks move so do the lines. The kinks are localized at individual chemical bonds that cross the glide plane (Figure 5.8). 

Minute amounts of added material can change the strength greatly. For example, added carbon atoms in iron can act as dislocation traps and halt the gliding motion. Work-hardening of metals is a process whereby many of the dislocations intersect and collide with one another, thereby becoming partially immobilized. The movement of dislocations can be studied at various temperatures and the activation energy found. At 1500°C plastic flow can be seen, even in diamond. 

The activation energy of the process controlling the BDT in sapphires, derived from the strain rate variation of T, is approximately 3.2 eV, close to that for dislocation glide. This was obtained by the Eq. (2.1) ... 

Fig. 6.16. Glide speed of an isolated threading dislocation in a SiGe/Si(100) film versus inverse temperature from in situ transmission electron microscope observations. The slope of the line fitted to the data provides an estimate of the activation energy according to (6.29). Adapted from Nix et al. (1990).

In 1996, Duesbery and Jobs [45] determined that in usual stress conditions, dislocations should belong to the glide set. Using Peierls barriers deduced from atomistic computations, these authors calculated the kink pair activation energies... 

A number of models for the mechanical strength have been developed based on the concept of ionicity to predict the hardness of several compounds [35—38]. It is anticipated that covalent bonds take the responsibility for increasing the hardness. The hardness, or the activation energy required for plastic gliding, was related to the bandgap Eq, which is proportional to the inverse bond length in a fashion with the power index n varying from 2.5 to 5.0. 

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