Strain-free situation

The opposed-flow geometry has some important differences, as well as benefits, compared with the burner-stabilized flat flame (e.g., Fig. 1.1). One is that the strain field can be varied by controlling the flow rate, ranging from an essentially strain-free situation to a flame extinction. As discussed subsequently, this flow configuration can be used experimentally for the accurate measurement of laminar burning velocities [238,438,448]. 

By assuming atomic radii to be specified by ionization radii, r0, the diatomic curves could, in principle, be used to calculate the dissociation energy of any covalent bond, irrespective of bond order, only from its interatomic distance. This promise is largely fulfilled, but the calculated energies are proportionately too low in virtually all cases. The obvious reason for this is that the calculation makes no allowance for steric interactions that occur in molecular environments. Because of this effect the (experimentally measured) interatomic distances used in the calculation do not refer to the strain-free situation modelled by ionization radii. 

Figure 11.18. Schematic diagrams of (a) the tilted interface shown in Figure 11.17, (b) a strain-free situation which requires a 13"-tilt of the diamond lattice, and (c) a highly strained case which exposes diamond 1, 1, 16 planes at the interface [306].

The situation is more complicated in homoadamantane (7) where a casual study of the molecular model would suggest that the molecule ought to assume two strain-free enantiomeric C2 conformations (6 and 6 ). Schleyer s force-field calculation (22) has predicted, however, that the achiral C2v conformer (7) will be more stable than the twisted conformers, and this has been borne out by a dynamic NMR study (23), as well as by CD spectral analyses of its mono- and C2 diketone derivatives (24). 

If crack propagation occurs by dissolution at an active crack tip, with the crack sides rendered inactive by filming, the maintenance of film-free conditions may be dependent not only upon the electrochemical conditions but also upon the rate at which metal is exposed at the crack tip by plastic strain. Thus, it may not be stress, per se, but the strain rate that it produces, that is important, as indicated in equation (8.8). Clearly, at sufficiently high strain rates a ductile fracture may be propagated faster than the electrochemical reactions can occur whereby a stress-corrosion crack is propagated, but as the strain rate is decreased so will stress-corrosion crack propagation be facilitated. However, further decreases in strain rate will eventually result in a situation where the rate at which new surface is created by straining does not exceed the rate at which the surface is rendered inactive and hence stress corrosion may effectively cease. 

Comparing the Cl-contents on the silica samples, pretreated at 973 K, with and without a pre-modification with HMDS, the reaction with strained siloxane bridges seems to be quite significant. However, in a real situation, the different reactions are competitive. HMDS modification creates a surface where the reaction with siloxane bridges can occur free of competition. Therefore, these values may overestimate the degree of reaction in a real situation. The scientific relevance of these data is concealed in the unambiguous proof that (1) chlorosilanes do react with strained... 

Thermal expansion of a semiconductor depends on its microstructure, i.e. stoichiometry, presence of extended defects, ffee-carrier concentration. For GaAs [24] it was shown that for samples of free-electron concentrations of about 1019 cm"3, the thermal expansion coefficient (TEC) is bigger by about 10% with respect to the semi-insulating samples. Different microstructures of samples examined in various laboratories result in a large scatter of published data even for such well known semiconductors as GaP or GaAs. For group III nitrides, compounds which have been much less examined, the situation is most probably similar, and therefore the TECs shown below should not be treated as universal values for all kinds of nitride samples. It is especially important for interpretation of thermal strains (see Datareview A 1.2) for heteroepitaxial GaN layers on sapphire and SiC. 

The described principle of equal force (stress) and added deformations (strains) equally applies to parallel layers of any kind, provided that their structure is isotropic. However, if any of the layers in the array is incompressible and softer than the rest, then it will expand laterally upon the force application. This is a familiar experience. When a sandwich or a layered cake is compressed, the filling sometimes leaks out from the sides, as shovm schematically in Figure 10.10. For such a situation. Equations (10.7) or (10.8) will not be an appropriate model. However, because the cellular layers retain their cross-sectional area, and because the free p>art of the expanded filling does not transmit any stress (theoretically), the stress-strain relationship of the array can still be calculated by accounting for the exuded material. 

The natural first question to ask is whether the crystal-liquid surface free energy can be measured experimentally by some method that is independent of nucleation kinetics. In gas-liquid nucleation studies, for example, it is routine to measure the surface tension of the liquid and to use its equality with the gas-liquid surface free energy to make predictions of nucleation rates and compare them with experiment. For the liquid-solid transition, the situation is quite different, however. This is true first because the surface tension and the surface free energy are no longer strictly equal due to the possible existence of strains in the crystal. The second reason is that measurements of liquid-solid free energies or interfacial tensions are by no means simple to devise or carry out, and so are available only in certain special cases. These limited experimental data are summarized in this section. 

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