Defect-environment interactions

The critical factors governing defect-related failures are the severity of the defect and the environmental conditions with which the defect may interact to produce failure. A defect, if sufficiently severe, may cause failure in and of itself, without contributions from environmental conditions. Defect-environment interactions are more common, however. Unquestionably, equipment can operate satisfactorily with components containing significant defects simply because there are no environmental conditions with which the defect may interact to produce failure. 

Hwang SJ, Beaty TH, Panny SR, Street NA, Joseph JM, Gordon S, McIntosh t, Francomano CA (1995) Association study of transforming growth factor alpha (TGF)-environment interaction in a population-based sample of infants with birth defects. Am J Epidemiol, 141 629-636. 

FIGURE 5.3. The model used in the two-region calculation of defect energies. In Region I, which contain a few hundred atoms forming the immediate environment of the defect d, interactions are evaluated explicitly. Between this and the outer Region II, whose contributions are treated by bulk dielectric methods, lies the interface Region Ila. 

As mentioned above, it is reasonable to assume that this tetrahedral V species forms at defect sites (hydroxyl nests) in the zeolite framework, but is stabilized by this interaction in a well defined environment through V-O-Si bonds. As indicated by the characterization data, the local coordination of vanadium must be different from that found for well dispersed vanadium sites on silica. This stabilization probably limits the unselective metal-bonded propane or propylene adsorption, in agreement with the role of adsorbate bonding on the selection of partial and total oxidation pathways of ethane on vanadium supported on silica (76) and in agreement with IR evidence (Fig. 

The importance of interactions amongst point defects, at even fairly low defect concentrations, was recognized several years ago. Although one has to take into account the actual defect structure and modifications of short-range order to be able to describe the properties of solids fully, it has been found useful to represent all the processes involved in the intrinsic defect equilibria in a crystal (with a low concentration of defects), as well as its equilibrium with its external environment, by a set of coupled quasichemical reactions. These equilibrium reactions are then handled by the law of mass action. The free energy and equilibrium constants for each process can be obtained if we know the enthalpies and entropies of the reactions from theory or... 

The difference in reactivity was also found for the paramagnetic surface defects -(=Si-0-)3Si radicals [16]. Since the observed effects are due to the difference in the structure of the nearest environment of the surface silicon atom, it is most pronounced when this atom acts as an active site. This difference should cease with an increase in the number of chemical bonds that separate the active site and surface silicon atom of the solid with which it is linked. They are almost absent for the (=Si-0-)3Si-CFl2- CF[2 radical in which the active site is localized on the terminal carbon atom [16]. For this reason, it is desirable to have a probe in the immediate contact with a lattice silicon atom. The Si-H group fits best these requirements. Such groups can be obtained upon the interaction of the silyl-type radicals with the hydrogen or deuterium molecules (cf. Section 6.3). The IR band due to the stretching vibrations of the Si-Fl bonds obtained upon the hydrogenation of silyl radicals ... 

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