Crystalline materials defects

The im< e mode produces an image of the illuminated sample area, as in Figure 2. The imj e can contain contrast brought about by several mechanisms mass contrast, due to spatial separations between distinct atomic constituents thickness contrast, due to nonuniformity in sample thickness diffraction contrast, which in the case of crystalline materials results from scattering of the incident electron wave by structural defects and phase contrast (see discussion later in this article). Alternating between imj e and diffraction mode on a TEM involves nothing more than the flick of a switch. The reasons for this simplicity are buried in the intricate electron optics technology that makes the practice of TEM possible. 

The work on colour centres outlined in Section 3.2.3.1, much of it in the 1930s, and its consequences for understanding electrically charged defects in insulating and semiconducting crystalline materials, helped to stimulate ceramic researches in the electrical/electronic industry. The subject is enormous and here there is space only for a cursory outline of what has happened, most of it in the last 80 years. 

The incorporation of phosphorus yields fourfold-coordinated P atoms, which are positively charged, as phosphorus normally is threefold coordinated. This substitutional doping mechanism was described by Street [52], thereby resolving the apparent discrepancy with the so-called S N rule, with N the number of valence electrons, as originally proposed by Mott [53]. In addition, the incorporation mechanism, because charge neutrality must be preserved, leads to the formation of deep defects (dangling bonds). This increase in defect density as a result of doping explains the fact that a-Si H photovoltaic devices are not simple p-n diodes (as with crystalline materials) an intrinsic layer, with low defect density, must be introduced between the p- and n-doped layers. 

Apart from these, there are volume defects that cannot conveniently be described in any other terms. The most important of these consist of regions of an impurity phase—precipitates—in the matrix of a material (Fig. 3.39). Precipitates form in a variety of circumstances. Phases that are stable at high temperatures may not be stable at low temperatures, and decreasing the temperature slowly will frequently lead to the formation of precipitates of a new crystal structure within the matrix of the old. Glasses, for example, are inherently unstable, and a glass may slowly recrystallize. In this case precipitates of crystalline material will appear in the noncrystalline matrix. 

Optical elements, liquid crystalline materials in, 15 116—117 Optical emission spectra, 14 833-837 plutonium, 19 671—673 Optical emission spectroscopy (OES), archaeological materials, 5 742 Optical fiber(s), 13 391-392 24 618 defects in, 11 145 drawing of, 11 141-145 fabrication of, 11 135-141 health care applications for, 13 397 overcladding of, 11 144 remote measurements using, 14 234 in sensors, 22 270-271 sol-gel processing of, 11 144-145 strength of, 11 141-145 vitreous silica in, 22 444 Optical fiber sensors, 12 614-616 Optical germanium, 12 556... 

In most cases, however, polymers crystallize neither completely nor perfectly. Instead, they give semicrystalline materials, containing crystalline regions separated by adjacent amorphous phases. Moreover, the ordered crystalline regions may be disturbed to some extent by lattice defects. The crystalline regions thus embedded in an amorphous matrix typically extend over average distances of 10-40 nm. The fraction of crystalline material is termed the degree of crystallinity. This is an important parameter of semicrystalline materials. 

Experiments demonstrate that along crystal imperfections such as dislocations, internal interfaces, and free surfaces, diffusion rates can be orders of magnitude faster than in crystals containing only point defects. These line and planar defects provide short-circuit diffusion paths, analogous to high-conductivity paths in electrical systems. Short-circuit diffusion paths can provide the dominant contribution to diffusion in a crystalline material under conditions described in this chapter. 

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... 

B. Burton. Diffusional Creep of Poly crystalline Materials. Diffusion and Defect Monograph Series, No. 5. Trans Tech Publications, Bay Village, OH, 1977. 

The interfaces of importance in kinetic processes possess a wide range of structures and properties. In this appendix we classify and describe concisely the different types of crystalline materials interfaces relevant to kinetic processes. The different types of point and line defects that may exist in these interfaces are also described.1... 

In literature the defects in crystalline materials are called O-di-mensional or point defects, 1-dimensional, also called line defects or dislocations and 2-dimensional or packing defects. In this book we will confine ourselves to a brief description of some of the many kinds of defects. 

The transformation of the initial defective V0P04 is thus not a side effect but the central step enabling the active phase. The defect structure controlled by addition of promoters like Co, Ga, Fe and others will affect the partitioning between large crystalline material and still nanostructured VPO that is the reactive precursor to... 

An important modification of this model was performed by Wakai.33 The main assumptions are that the solution and precipitation reactions take place at line defects as kinks in steps formed at the grain boundaries (Fig. 16.4), and the spacing between kinks is small enough for the step to be considered as an ideal source or sink of solute particles. Thus, the solution and precipitation of crystalline materials at these steps produces their movement, and consequently strain and strain rate will have an expression analogous to Orowan s equation for dislocation movement ... 

It should be noted that the carrier mobility in nanowires is lower than that in bulk single-crystalline material due to possible scattering at wire and grain boundaries, uncontrolled impurities, and lattice defects. The overall effect of this additional scattering is taken into account by Matthiessen s rule (Ashcroft and Mermin, 1976b),... 

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