Crystals dislocations

Frank, F.C. (1985) Some personal reminiscences of the early days of crystal dislocations, in Dislocations and Properties of Rea Materials (The Institute of Metals, London) p. 9. 

In his early survey of computer experiments in materials science , Beeler (1970), in the book chapter already cited, divides such experiments into four categories. One is the Monte Carlo approach. The second is the dynamic approach (today usually named molecular dynamics), in which a finite system of N particles (usually atoms) is treated by setting up 3A equations of motion which are coupled through an assumed two-body potential, and the set of 3A differential equations is then solved numerically on a computer to give the space trajectories and velocities of all particles as function of successive time steps. The third is what Beeler called the variational approach, used to establish equilibrium configurations of atoms in (for instance) a crystal dislocation and also to establish what happens to the atoms when the defect moves each atom is moved in turn, one at a time, in a self-consistent iterative process, until the total energy of the system is minimised. The fourth category of computer experiment is what Beeler called a pattern development... 

Mechanical engineering Physical metallurgy, crystal dislocation mobility... 

In these crystals, dislocation motion is divided into two regimes, above and below their Debye temperatures. Above their Debye temperatures, dislocation motion is thermally activated. The activation energies are equal to twice the band energy gaps, consistent with breaking electron-pair bonds (Figure 4.3). 

Crystal dislocations were invented (circa. 1930) by Orowan, Prandtl, and Taylor to explain why pure metal crystals are soft compared with homogeneous shear strengths calculated from atomic theory. They do this very well. However, roughly 15 years later (circa 1945) it was found that pure semiconductor crystals (e.g., Ge and Si) have hardnesses at room temperature comparable with calculated homogeneous shear strengths. Furthermore, it was known... 

Early in the history of crystal dislocations, the lack of resistance to motion in pure metal-like crystals was provided by the Bragg bubble model, although it was not taken seriously. By adjusting the size of the bubbles in a raft, it was found that the elastic behavior of the raft could be made comparable with that of a selected metal such as copper (Bragg and Lomer, 1949). In such a raft, it was further found that, as expected, the force needed to form a dislocation is large. However, the force needed to move a bubble is too small to measure. 

Nabarro, F. R. N. (1967) The Theory of Crystal Dislocations, Oxford University Press, Oxford... 

F.R.N. Nabarro. Theory of Crystal Dislocations. Clarendon Press, Oxford, 1967. 

The example in Fig. 13.4 is an extension of the model for the motion of a small-angle boundary by the glide and climb of interfacial dislocations (Fig. 13.3). Figure 13.4 presents an expanded view of the internal surfaces of the two crystals that face each other across a large-angle grain boundary. Crystal dislocations have... 

D.J. Dingley and R.C. Pond. On the interaction of crystal dislocations with grain boundaries. Acta Metall., 27(4) 667—682, 1979. 

Theories for even earlier forms of mutateable transmission of genetic information have involved clay minerals. Montmorillonite clay particles have been demonstrated to catalyse the condensation of nucleotides and we discussed in Section 8.2.8 the Cairns Smith hypothesis of how crystal dislocations could transmit some form of genetic information. Most hypotheses about clays have yet to be demonstrated experimentally, however. 

F.C. Frank Crystal dislocations. Elementary concepts and definitions. Phil. Mag. 42, 809... 

The puzzling thing about GaN is that it shouldn t work as a microelectronic material it is just too riddled with crystal dislocations. If GaAs has more than 1000 dislocations per square centimetre it is unable to function as an LED, yet GaN can have a billion dislocations per square centimetre and still work. How does GaN accomplish this mission impossible As yet there is no dear answer to this question although it has something to do with the interface between the p- and n-forms of the semiconductor. The p-GaN is obtained by adding traces... 

Theory of Crystal Dislocations, F.R.N. Nabarro. (65488-5) 18.95 Theory of Electromagnetic Wave Propagation, Charles Hcrach Papas. (65678-0) 6.95... 

In this paper Synchrotron Radiation X-Ray White-beam Topography (SRXWT) was used to investigate the defects of yttrium orthovanadate crystal, dislocations, inclusions and stress area were found in (001) and (100) wafers. Analysis showed that inclusions were the main cause of dislocations and stress area. Electronic Probe Microscopic Analysis (EPMA) indicated that these inclusions resulted from impurities appearing on the annealed (001) wafer, which revealed that there lied small amount of ferrum and aluminium. 

Several models for the creation of interfaces by dislocation reactions have been surveyed in [5], The basic idea is shown in fig.2. In the interior of one crystal dislocations of opposite sign move in opposite directions. They... 

The value of the diffusion coefficients of impurities and dopants in semiconductors can be modified by the presence of compensating impurities or of crystal dislocations so that the interpretation of diffusion measurements requires some judgment, ft must also be mentioned that as the diffusing species can be ions, the diffusion coefficient can be modified by an electric field. 

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