Boundary defects

To further characterize the event it is first necessary to identify critical features of the initial configuration that will strongly influence the process. For powder compacts, the most obvious features are the morphological characteristics of the powders, their microstructures, and the porosity of the compact. For solid density samples, the grain structure, grain boundaries, defect level, impurities, and inclusions are critical features. 

Grain boundary defects are primarily responsible for the operation of zinc oxide (ZnO) varistors, a shortened form of variable resistor. The varistor behaves like an insulator or poor semiconductor at lower electrical field strengths, but at a critical breakdown voltage the resistance decreases enormously and the material behaves like an electrical conductor (Fig. 3.36). When a varistor is connected in parallel with electrical equipment, negligible power flows through it under normal low... 

Point defects have zero dimension line defects, also known as dislocations, are onedimensional and planar defects such as surface defects and grain boundary defects have two dimensions. These defects may occur individually or in combination. 

We distinguish between classical, modified, specific, and grain boundary defect configurations. Figure 22 presents examples of simulated and measured classical... 

Section 3.1. Phonon scattering by impurity atoms, the grain boundary, defects, etc. should help to decrease the thermal conductivity. 

Materials Research Strategy Boundary Defect Chemistry... 

Quite often the mobilities determined for a given molecule, e.g. rabrene or pentacene, differ for different devices in different laboratories, reflecting the problems related to the extraction of the mobility data from the electrical characteristics. Typically, these characteristics are not only defined by the mobilities but also by the contact resistance and of course the presence of domain boundaries defects and impirrities in the organic semiconductor. The determination of the true intrinsic mobility of an organic semiconductor is still a challenge, which has only been overcome in a very few cases. 

Those that affect diffusivity at grain boundaries by modification of boundary defects. 

To proceed, consider two finite planar networks, a regular Euclidean triangular lattice (interior valence v — 6, but with boundary defect sites of valence v = A and v = 2) of dimension d — 2, and a fractal lattice... 

At constant current STM-REBIC and CITS images were also acquired after thermal treatment of the nc-Si films. The topography shows the same features as in the as-implanted samples while the STM-REBIC images do not show any contrast. The contrast of the cell boundaries in the CITS images is also removed after the annealing of the films. These results indicate that the thermal treatment induces an enhancement in conductivity of the samples due to a recrystallization process and a change in the cell boundaries defect distribution and structure. 

Volknann M., Hagenbeck R., and Waser R., Grain-boundary defect chemistry of acceptor-doped titanates Inversion layer and low-field conduction, J. Am. Ceram. Soc., 80, 2301-2314, 1997. 

S. Longhi, Tunneling escape in optical waveguide arrays with a boundary defect, Phys. Rev. E 74 (2006) 026602. 

The formation of the polycrystal nucleus with defects distributed among the crystal grains with low atomic density and change interatomic distances. In addition, the grain boundaries are non-equilibrium and contain a great number of grain-boundary defects. 

Choi J, JeongHK, Snyder MA, StoegerJA,MaselRl, Tsapatsis M. Grain boundary defect elimination in a zeolite membrane by rapid thermal processing. Science 2009 325 590-593. 

The same analysis was performed on the Type II defect simulation for each of the three energy minima highlighted in Fig. 5b (i.e.. A, B, and C). These minima are related to the presence of the outward boundary defect (see Fig. 3) the inward boundary defect appears to have little influence on binding. Within sigma of each minimum, all structures below an RMSD cutoff of 0.2 nm were clustered. This resulted in 9,885 structures in 11 clusters for minimum A, 41,203 structures in 23 clusters for minimum B, and 14,710 structures in 9 clusters for minimum C. The central cluster conformations of the clusters with the top three weights calculated for each of the minima are shown in Fig. 7. 

The efficiency of dye-sensitized solar cell has reached as much as 11% [48], but values higher than 15% are required to make them competitive with Si-based solar cells. Electrons transport time is delayed in the nanotube by the presence of grain boundaries, defects, and trap sites. This is an important limitation in manufacturing long Ti02 nanotube for efficient dye-sensitized... 

The mobility of faceted boundaries is also dependent on boundary defects. In the case of BaTi03, 111 twins induced and enhanced abnormal grain growth. On the other hand, dislocations at boundaries in SrTiOs did not enhance the boundary mobility, unlike the enhanced mobility of solid/liquid interfaces by dislocations.In this case, however, the boundary was not fully faceted ( 35% faceted). According to a recent investigation in BaTiOs, it seems that dislocations can also enhance the boundary mobility if the boundary is well faceted. 

Figure 17.3 Permeances and separation factors of c-oriented MFI membranes for xylene (A and B), butane (C and D) and hexane (E and F). (A, C, E) transport properties of conventionally calcined membranes and (B, D, F) transport properties of membranes treated by rapid thermal processing. Reprinted from J. Choi, H.-K. Jeong, A. Snyder, J. A. Stoeger, R. E Masel and M. Tsapatsis, Grain boundary defect elimination in a zeolite membrane by rapid thermal processing. Science, 325, 590-593, 2009, with permission of AAAS.

M. Vollman, R. Waser Grain Boundary Defect Chemistry of Acceptor-Doped Titanates Space Charge Layer Width J Am.Ceram.Soc. 77[1] 235-43 (1994)... 

While high-resolution transmission electron miCToscopy (TEM) is a powerful technique, it samples only a tiny fraction of the grain boundary area, potentially allowing some grain boundary defects to be missed. A definitive statement as to the absence of grain boundary phases cannot be made—the phases could be too thin or too sparse to detect. For electrically conducting materials, IS averages over the entire sample and thus provides a valuable complement to TEM. 

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