Defects buckling

Such changes in the defect population can be critical in device manufacture and operation. For example, a thin him of an oxide such as SiO laid down in a vacuum may have a large population of anion vacancy point defects present. Similarly, a him deposited by sputtering in an inert atmosphere may incorporate both vacancies and inert gas interstitial atoms into the structure. When these hlms are subsequently exposed to different conditions, for example, moist air at high temperatures, changes in the point defect population will result in dimensional changes that can cause the him to buckle or tear. 

Point defects. Point defects (Fig. 5.1) are limited to a single point in the lattice, although the lattice will buckle locally so that the influence of point defects may spread quite far. A Frenkel defect consists of a misplaced interstitial atom and a lattice vacancy (the site the atom should have occupied). For example, silver bromide, which has the NaCl structure, has substantial numbers of Ag+ ions in tetrahedral holes in the ccp Br array, instead of in the expected octahedral holes. Frenkel defects are especially common in salts containing large, polarizable anions like bromide or iodide. 

Figure 4 Scanning tunneling microscope image of a nearly defect-free Ge(00 1) showing the well-ordered c(4 x 2) - (2 x 1) domain pattern. Scan area is 40 x 40 nm2. Sample bias is —1.6V and tunneling current 1 nA. The inset shows the strong buckling near an SA step edge.

Fig. 14 HREM images of Nb-W-S nanotubes showing various tube closures (a) an irregular tube closure (b) a 90° wall-tip junction (c) another near 90° closure (the arrow points to the buckling defect in the tube wall) (d) an irregular closure with severe bending defects. (Reproduced with permission from ref. 41).

Fig. 3 Schematic representation of various types of failure associated with the mechanical instability of a film deposited on a substrate, (a) cracking of a thin film subjected to residual tensile stress, (b) plastic deformation of the substrate at the end of the crack, (c) deviation of the crack at the interface, (d) cracking of the substrate, (e) detachment and buckling (formation of a blister from an interface defect) of a film subjected to residual compressive stress, and (f) deviation of the crack through the thickness of the film (flaking).

Apart from the defect structures and Til, the structures we have mentioned are all essentially of the 6 6 coordination type. In the SnS (GeS) structure the distortion of the octahedral coordination groups is such as to bring three of the six original neighbours much closer than the other three, so that the structure could alternatively be described as consisting of very buckled 6-gon nets in which... 

Experimentally the situation was unclear because of STM experiments showing apparently symmetric dimers on an almost defect-free terrace of Si(001)(2xl), with tilting only near steps (Wiesendanger et al., 1990). However, recent temperature-dependent STM work has shown that on cooling to 120 K, the number of buckled dimers increases (Wolkow, 1992). It seems likely that the bistability of the asymmetric dimer results in flipping between... 

The relative strength of hollow-sphere foams lies between the theoretical performance of open- and closed-cell foams. The performance of optimized truss structures is similar to that of closed-cell foams and, for the Kagome truss, approaches the behavior of a Hashin-Shtrikman porous material. Honeycombs are the most efficient structures when loaded purely out-of-plane. However, plastic buckling can decrease its performance at low relative densities. Further, since honeycomb is highly anisotropic, any inplane loading results in severely reduced performance. Although the theoretical performance of closed-cell foams far exceeds that of open-cell foams, processing defects result in commercially available material that behaves similar to an open-cell material at low relative densities. Commercially available samples of other types of low-density metallic structures behave nearly as predicted. [17]... 

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