Orientational defects concentration

The microstmcture and imperfection content of coatings produced by atomistic deposition processes can be varied over a very wide range to produce stmctures and properties similar to or totally different from bulk processed materials. In the latter case, the deposited materials may have high intrinsic stress, high point-defect concentration, extremely fine grain size, oriented microstmcture, metastable phases, incorporated impurities, and macro-and microporosity. AH of these may affect the physical, chemical, and mechanical properties of the coating. 

Show that regardless of the orientation of a straight dislocation line and its Burgers vector, there will exist a stress system that will convert the dislocation line into a helix whose axis is along the position of the original dislocation when the point-defect concentration is at the equilibrium value characteristic of the stress-free crystal. Use the simple line-tension approximation leading to Eq. 11.12. 

The first part of the right side of Eq. (1) gives the portion of the H-bonded OH groups with the concentration (1-Of) the second part gives the portion of the non H-bonded OH groups with the concentration 0F. The partial molar volume of H-bonded groups and the coefficient of thermal expansion is taken as ice like datas. Both properties of the orientation defects are adjusted. Spectroscopy cannot give informations on these constants. Therefore, the proof of the orientation defects assumption by the density is not very accurate. 

The summary of our simple approximation model of water is water consists at T< 100 °C of a network of H-bonded molecules. The co-operative mechanism of the angle dependence of H-bond energy has the consequence that the complete orientation defects of non H-bonded OH groups are not distributed statistically, they are concentrated by fissure plains of defects. The content of orientation defects at room T is about 12%. The number of molecules per idealized aggregate at room T is about 300 molecules (about 7 in one dimension). 

Different surface-sensitive techniques respond differently to the various kinds of disorder. The measurement of LEED beams is unique in largely filtering out all defects that are unrelated to the superlattice periodicity that defines the beams. Other techniques (including diffuse LEED) generally include contributions from all defects, for instance from adsorbates located at undesired steps and crystallite boundaries. Then only a reduction of the defect concentration can remove defect contributions from the experimental data. Rotational disorder of adsorbed molecules does not matter for techniques which measure only bond lengths, and not bond orientations. Thus, NEXAFS is more sensitive to such disorder than SEXAFS. 

Stretched polyacetylene with very low sp3 defect concentration has much higher conductivity and anisotropy than other materials. This point emphasizes the importance of oriented, low-defect systems if charge transport is principally along the conjugated chains. This discussion illustrates that no complete, consistent theory of transport in PA has yet been developed. 

The role of the a-c interface in Ion Beam Induced Epitaxial Crystallization (IBIEC) is demonstrated in Fig. 10.11, which shows that the regrowth rate is orientation dependent. The data shows that the rate is much lower (almost by a factor of 4) for (111) substrates relative to (100) substrates. These results suggest that the same interfacial defects that are responsible for thermal regrowth also are important in IBIEC, with the role of the ion beam being that of changing the average defect concentration. 

Anodic oxidation of graphitic materials in H2SO4 is also used as a test reaction to characterize their degree of orientation and defect concentration" ". Ether-solvated... 

Studies of the relaxation times showed [24,25] that the spin-lattice relaxation time was strongly dependent on the orientation of the external magnetic field relative to the molecular ion axis and that it was independent of the defect concentration. It was suggested that the dominant relaxation mechanism is intramolecular in nature. 

This information allows us to make tentative estimates of the concentration of orientational defects in pure ice, using an equation like (7.1), and of their mobility. It is clear from the energies involved that they should be much more numerous in pure ice than are the ion states. The energy barrier to proton motion is comparable in height to that for ion states but twice as wide, so that it is possible, and indeed turns out to be the case, that the anomalously high mobility of ionic states does not extend to orientational defects. Experimental information, derived from studies of the electrical properties of ice, is summarized for convenience in table 7.3. 

A real ice crystal at a finite temperature is, however, not perfect but contains an equilibrium concentration of point defects, as discussed in chapter 7. The most important of these in pure ice, from our present point of view, are the d- and L-orientational defects, since the product of their concentration and mobility is about 100 times greater than the same product for ion states, so that they provide the dominant relaxation mechanism. 

Since the d- and L-defects provide the mechanism by which the orientation of molecules can relax, their concentration and mobility will determine the relaxation time t, but will not influence since thermally generated D- and L-defects moving through the crystal always ultimately cause relaxation to the extent allowed by thermal equilibrium. Measurements of the temperature variation of T thus give information about the variation with temperature of the product of defect concentration and mobility, while the addition of impurities which introduce one or other type of defect allows further information on defect behaviour to be obtained. 

The fact that the dielectric relaxation of ice is accurately described by a simple Debye curve implies that only a single mechanism is involved. Bjerrum (1951) discussed the possible reorientation mechanisms in ice—either ion-state motion or orientational defect motion—and concluded that the latter was the relevant process. As we shall see later, his conclusion is correct for pure ice, where the greater concentration of L- and D-defects more than makes up for their lower mobility, compared with ion... 

The contributions of ion states and orientational defects to are very nearly independent. Since the intrinsic concentration of L- and D-defects is lo cm , their concentrations will not be affected for impurity concentrations less than 10 cm , so all... 

For concentrations of anunonia or hydrogen fluoride less than about 10 cm the contribution made to by orientational defects is constant and has a sign determined primarily by the mobility ratio which is not well known from other... 

In this formalism the mobility is determined partly by the value of AW (which occurs as AW ) and partly by the scattering of ion-state waves by lattice vibrations in the form of phonons. Because the ion-state band is so narrow and the difference between proton vibrational levels is so large compared with typical phonon energies, it is not possible for (k) to be scattered to a new state (k ) by emission or absorption of a single phonon. Instead, so that energy and momentum can be conserved, scattering must occur by the simultaneous emission and absorption of a pair of phonons of nearly equal energy. The analysis is therefore rather complicated but, if we assume that orientational defects are present in sufficient concentration that polarization effects do not block ion paths, the... 

Fig. 3.3. Illustration of the main proton transfer mechanisms (a) defect mechanism in a densely packed structure (b) loosely packed structure with a high concentration of mobile species (c) quasi-liquid state with a proton jump contribution In (a) the conductivity is favoured by intrinsic (interstitial rabbits) or extrinsic (impurity elephant) point defects. An orientation defect (hippopotamus in the wrong orientation) can also favour disorder of rabbits (Oj for Zr02 CaO, H for KHSO4) (b) the tree sublattice is a perfectly stable loosely packed structure and a high rabbit disorder can exist without affecting the host lattice (e.g. NH4 in p-AljOj) (c) only the mobile species sublattice is considered here these entities are moving with different speeds in different directions and some are hopping such may be the image of a quasi liquid or surface liquid (V205.nH20, HUP).

The peculiar prohibition on bend results in the appearance of new types of defects in LC polymers [54]. If we assume that only splay is realized in conditions of elastic equilibrium, then in the plane approximation, the disclinations can be point singularities relative to which the director is oriented over concentric circles. When the prohibition on bend is removed, the combined effect of the two types of bending deformation can result in extensive defects C striped textures) with linear disclinations. 

The undoped samples exhibit lower defect concentrations than conventional polyacetylene. Crystallinity is about 80 % and the density is 0.9 g/cm /15, 16, 18/. SEM pictures of strech-aligned samples show a fibrillar morphology with a partial orientation parallel to the stretching direction and typical fibrillar diameters of 50 - 100 nm /40/ (see chapter 6). ... 

The mathematical representation of the elastic behavior of oriented heterogeneous solids can be somewhat improved through a more appropriate choice of the boundary conditions such as proposed by Hashin and Shtrikman [66] and Stern-stein and Lederle [86]. In the case of lamellar polymers the formalisms developed for reinforced materials are quite useful [87—88]. An extensive review on the experimental characterization of the anisotropic and non-linear viscoelastic behavior of solid polymers and of their model interpretation had been given by Hadley and Ward [89]. New descriptions of polymer structure and deformation derive from the concept of paracrystalline domains particularly proposed by Hosemann [9,90] and Bonart [90], from a thermodynamic treatment of defect concentrations in bundles of chains according to the kink and meander model of Pechhold [10—11], and from the continuum mechanical analysis developed by Anthony and Kroner [14g, 99]. 

Theoretical studies of diffusion aim to predict the distribution profile of an exposed substrate given the known process parameters of concentration, temperature, crystal orientation, dopant properties, etc. On an atomic level, diffusion of a dopant in a siUcon crystal is caused by the movement of the introduced element that is allowed by the available vacancies or defects in the crystal. Both host atoms and impurity atoms can enter vacancies. Movement of a host atom from one lattice site to a vacancy is called self-diffusion. The same movement by a dopant is called impurity diffusion. If an atom does not form a covalent bond with siUcon, the atom can occupy in interstitial site and then subsequently displace a lattice-site atom. This latter movement is beheved to be the dominant mechanism for diffusion of the common dopant atoms, P, B, As, and Sb (26). 

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