Point defects and dislocations

The physical properties of tellurium are generally anistropic. This is so for compressibility, thermal expansion, reflectivity, infrared absorption, and electronic transport. Owing to its weak lateral atomic bonds, crystal imperfections readily occur in single crystals as dislocations and point defects. 

First, I shall describe the hydrogenation method I used and then consider the passivation of surface states and that of bulk dangling bonds, including grain boundaries, dislocations and point defects. 

INTERACTION OF DISLOCATIONS AND POINT DEFECTS 3.7.1 Dislocation Loops... 

Recrystallization occurs when a crystalline material is plastically deformed at a relatively low temperature and then heated [1]. The as-deformed material possesses excess bulk free energy resulting from a high density of dislocations and point-defect debris produced by the plastic... 

For the weathering of trace minerals from the solid matrix, the dissolution occurs selectively on spots where the mineral is exposed to the surface. These mineral surfaces are usually not smooth, but show dislocations (screw, jump, step dislocations) and point defects (vacant sites, interstitial sites) (Fig. 23 left). Dissolved ions are immediately transported from the surface into solution, so that no gradient can develop. Since the total concentrations of trace minerals in the solution are low, no equilibrium can be reached. In the following the dissolution of trace minerals is called surface-controlled. 

P. Omling, E. R. Weber, L. Montelius, H. Alexander and J. Michel, Electrical properties of dislocations and point defects in plastically deformed silicon , Phys. Rev. B, 32, 6571 (1985). 

Fig. 9. SEM images of etched cylinder forming PS-b-PEO (PEO removed) in rectangular trenches (60 nm depth and width as shown) of various widths. The white circles show various defects present including grain boundaries, dislocations and point defects. See text for further details.

Most metals and alloys used by engineers are polycrystals. In addition to dislocations and point defects, their surfaces are crisscrossed by grain boundaries that separate regions of different orientation and, in case of alloys, by phase boundaries that separate different metallic phases. The atoms located at these defects are less solidly bound in the crystal lattice, and therefore they are favorite sites for chemical or electrochemical attack. The phenomenon is used in metallography to visualize the microstructure of metals and alloys. Figure 3.34 shows a micrograph of a polycrystalline copper surface that has been subjected to electrochemical dissolution. The grain boundaries are clearly visible. 

Precipitates on grain boundaries can be a major factor in inhibiting grain growth, since the migration of a boundary is hampered when it is pinned by second-phase particles. Inclusions also affect the mechanical properties of the ceramic via interactions with dislocations and point defects, and precipitate stress fields may influence the further segregation of other species dissolved within the grains. 

Both dislocations and point defects are nearly always present to some degree in real metallic crystals. The concept of a geometrically perfect crystal is useful when explaining certain properties of crystals, but there are other properties that can only be adequately explained by consideration of the defects. ... 

Virtually all minerals contain defects. In addition to point defects (e.g., vacancies that exist in a thermodynamically determined equilibrium number, impurities etc ), macroscopic minerals contain line defects (dislocations), and planar defects such as stacking foults, antiphase boundaries and twins. Intergrown layers of different structure or composition, and polytypic disorder also may be present. 

Although point defects certainly occur in nanoparticles (and unusual coordination sites are probably common in some very small nanoparticles), it is generally agreed that nanocrystals do not contain dislocations or other extended defects because the energetics of these features are significant and diffusion distances are small. So (in the absence of deformation), given that all big crystals start out small, where do dislocations and planar defects in macroscopic materials come from ... 

We have seen above that the kinetics of mineral dissolution is well explained by transition-state theory. The framework of this theory and kinetic data for minerals have shown that dissolution is initiated by the adsorption of reactants at active sites. Until now these active sites have been poorly characterized nevertheless, there is a general consensus that the most active sites consist of dislocations, edges, point defects, kinks, twin boundaries, and all positions characterized by an excess surface energy. Also these concepts have been strongly supported by the results of many SEM observations which have shown that the formation of crystallographically controlled etch pits is a ubiquitous feature of weathered silicates. 

Currently, the available single crystal GaN substrates are limited by the size and point defects that make them unsuitable for mass production [1]. An alternative to native GAN substrates is to use free-standing GaN templates prepared by hydride vapor phase epitaxy (HVPE), which has a typical thread dislocation (TD) density of 105-106 cm-2, however, the high price limits their availability [2],... 

At low temperatures, where diffusion in the solid state is unimportant, dislocations move principally by the process of slip (or glide) and may interact with other dislocations which move either in the same or in intersecting planes. Various kinds of lattice imperfections are introduced by such movement, and we shall discuss their identity in this subsection. At higher temperatures dislocation may anneal out by a process of annihilation resulting from slip. Moreover, since diffusion of individual species is now easier, important kinds of interaction between line and point defects are possible. These phenomena are also outlined below. 

Defect structure of neutron-irradiated Fe-Cr alloys contains free vacancies, SIA, VC, pure dislocation loops, dislocation loops decorated by Cr atoms, and vacancy-Cr complexes as well as Cr precipitates depending on the irradiation regime (Malerba et al. 2008). The CD model in our study is close to the model proposed by Christien and Barbu (2004), where the CD simulations are first performed for the free vacancies, SIAC, and point defect clusters and then for the precipitates, taking into account the steady state values of the free point defects concentrations obtained in the first step. In addition, we take into account the Cr-effect on the SIA diffusivity according to the density functional theory (DFT) calculations (Terentyev et al. 2008). 

In the majority of mechanical applications of materials, their surfaces experience contact with another material and take the external load before the bulk of the material is influenced. In some cases, surface interactions influence the bulk (e.g., propagation of cracks dislocations or point defects from the surface in depth). In many cases, only the outermost surface layer is affected by the surface contact with no detectable changes in the bulk of the material. This is like a storm that is frightening and destructive on the ocean surface, but does not have any influence on deep-water life. We are primarily concerned in this review with that kind of interaction. The surface layer thickness affected by external mechanical forces ranges from nanometers to microns. Thus, in our case, the definition of surface is different from the one used by surface scientists, that is, physicists and chemists. We introduce here an engineering definition of surface the outermost layer of the material that can be influenced by physical and/or chemical interaction with other surfaces and/or the environment. In this chapter, we consider only mechanical effects, but both mechanical and chemical interactions are possible and their synergy can lead to mechanochemical alteration of a material surface. 

On the other hand materials deform plastically only when subjected to shear stress. According to Frenkel analysis, strength (yield stress) of an ideal crystalline solid is proportional to its elastic shear modulus [28,29]. The strength of a real crystal is controlled by lattice defects, such as dislocations or point defects, and is significantly smaller then that of an ideal crystal. Nevertheless, the shear stress needed for dislocation motion (Peierls stress) or multiplication (Frank-Read source) and thus for plastic deformation is also proportional to the elastic shear modulus of a deformed material. Recently Teter argued that in many hardness tests one measures plastic deformation which is closely linked to deformation of a shear character [17]. He compared Vickers hardness data to the bulk and shear... 

Next, somewhat more detail is given for the line and point defects. The latter will be the major topic in connection with their occurrence in computer simulations displayed in Sect. 5.3.4. Figure 5.90 gives a schematic representation of the two basic dislocations. The cubes of the illustrated crystals represent the motifs. In the screw... 

The study of defects in liquid crystal systems is rooted in the understanding of defects in the solid state. For instance, crystals are rarely perfect and usually contain a variety of defects, e.g., point defects, line defects, or dislocations, and planar defects such as grain boundaries. In addition to these typical imperfections of the solid state, liquid crystals can also exhibit defects known as disclinations. These defects are not usually found in solids and result from the fact that mesophases have liquid-like structures that can give rise to continuous but sharp changes in the orientations of the molecules, i.e., sharp changes in orientation occur in the director field. 

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