Boundary orientation

The principal applications of REELS are thin-film growth studies and gas-surface reactions in the few-monolayer regime when chemical state information is required. In its high spatial resolution mode it has been used to detect submicron metal hydride phases and to characterize surface segregation and difRision as a function of grain boundary orientation. REELS is not nearly as commonly used as AES orXPS. 

It is known from experiment that the boundary energy and diffusivity are a function of the grain boundary orientation angle and often show minima at certain specific orientations [Q. Ma, R.W. Balluffi (1993) A.N. Aleshin, etal. (1977)]. This... 

Fig. 22a—c. Microtome section of an intersperujitic crack path in bulk coarse spherulitic PP 1120 a. Figure b indicates as an SEM-micrograph taken from the surface of the specimen the interspherulitic craze formation prior to the cracking process, c shows a site of shear along a spherulite boundary oriented under an angle of about 60° to the horizontal crack direction... 

Figure 2 A high-resolution TEM photomicrograph of the amorphous altered layer (lower left) developed on crystalhne lahradorite (hulk material, upper right) after dissolution at pH 1. The hlurry lattice fringes at the interface reflect the varying boundary orientation with respect to the ultrathin section. Interface thickness is 0.5-2 nm. Energy filtered (EE) TEM was also used to chentically characterize the alteration zone, which was found to he depleted in Ca, Na, K, and Al, and enriched in H, O, and Si. The sharp structural interface shown here and the sharp chemical interface observed with EFTEM are interpreted by the authors to indicate that the alteration layer is formed by dissolution-precipitation. Such amorphous altered layers are often high in porosity and yield high BET surface areas (reproduced by permission of Springer from Phys.

Figure 3.40 Wedge fringes at a grain boundary (a) bright-held image and (b) schematic illustration of grain boundary orientation in a specimen that generates the fringes in the bright-held image. (Reproduced with permission from M. von Heimandahl, Electron Microscopy of Materials, Academic Press, New York. 1980 Elsevier B. V.)...

Brooks and coworkers [136,141] measured drop electrophoretic mobilities in ATPSs. They were surprised to discover that the sign of the droplet mobilities was opposite to that predicted from the phosphate partition and the Donnan potential. They also found mobility to be directly proportional to drop radius, which is a contradiction of standard colloid electrokinetic theory [144]. Levine [140] and Brooks et al. [141] hypothesized that a dipole potential at the phase boundary oriented in a way that reverses the potential gradient locally is responsible for the paradox of the sign of electrophoretic mobilities of ATPS droplets. 

Most corrosion costs are due to corrosion in electrolytes, or wet corrosion. Local differences in a given metal can be chemical or mechanical in nature, for example, impurities such as oxides and other inclusions, grain boundaries, orientation of grains, differences in composition of the microstructure, localized stresses, and scratches and nicks. Highly polished surfaces are used in special cases. Pure and smooth zinc will only corrode slowly in pure hydrochloric add, but pure metals are not often used because they are expensive and have relatively low strength. 

The first attempt at a theory of negative which we are aware of (we do not dignify our simple picture presented in [17] as a theory ) was a letter from P.K. Currie communicated to us by K.F. Wissbrun (December 14,1979). Currie s analysis was based on the Leslie-Ericksen theory for MLC nematics and is of debatable relevance to polymeric liquid crystals. Nevertheless, he does conclude that a negative is possible and would occur for a narrow range of boundary orientations. This analysis is available to interested parties from G. Kiss. In MLC nematics, may change from negative to positive as the shear rate increases [75]. 

The importance of surface orientation on the performance of guest-host displays was recognized by White and Taylor, who investigated displays having both parallel and perpendicular boundary orientations. In this section we will look more closely at the textures and electro-optical properties of guest-host layers having these two types of boundary orientation. 

The cholesteric texture occurring under the conditions of parallel boundary orientation is a featureless, twisted planar... 

Fig. 10. Intensity versus voltage curves for a guest-host mixture under conditions of homotropic and parallel boundary orientation. Arrows in the region of hysteresis indicate the direction that the curves were traced. Curves are for a guest-host mixture containing 2.0% D16 and 3.7%...

The cholesteric texture occurring with perpendicular boundary orientation, on the other hand, is not a uniform texture. Under the microscope the entire field of view is filled with right-and left-handed spirals. This cholesteric texture is known as the scroll texture and appears very much like an end-on view of a bundle of rolled-up scrolls. Capacitance measurements show that the cholesteric helical axis in this texture is still predominantly perpendicular to the plane of the layer. An exact analysis of the structure of this texture, even for the case of equal elastic constants, seems difficult to achieve. Important from the applications point of view is that the scroll texture is adopted without disclinations immediately after a display element is turned off and that this structure is essentially nonscattering. Homeotropic boundary orientation therefore solves the problem of display after-images. 

Figure 11 shows the dependence of the critical unwinding voltage Vf on the cell thickness-to-pitch ratio for a cholesteric layer with perpendicular boundary orientation. For these measurements two reusable cells, having thicknesses of 7.8 and 10.9 ym, were filled with various mixtures of the nematic liquid crystal RO-TN-103 and the chiral substance CB 15.We used the Cano wedge technique to determine the pitch of the mixtures and found the simple inverse relationship... 

Fig. 11. Measured dependence of the critical unwinding voltage Vf in volts upon the cell thickness-to-pitch ratio. Homeotropic boundary orientation. Dashed line is theoretical curve for infinite medium without boundaries. Host substance is RO-TN-103.

Calculations incorporate beam size and foil thickness effects, and allow for slight imprecision in boundary orientation and beam location. Strictly, several spectra are taken to produce a composition profile across the boundary. Quality of input spectra and profile (best if several near-boundary spectra are included) affect outcome. 

Fig. 2.52 Representative HRTEM photographs of boundaries oriented a parallel and b perpendicular to the applied load direction, indicating that the grain-boundary film thickness decreased after superplastic deformation, under compression ((->) applied stress direction during deformation) [51], With kind permission of John Wiley and Sons...

Grain-boundary grooving experiments were performed on E5 (210) twist boundaries. Planar boundaries, oriented 90° to the surface, were annealed in air using various times and temperatures. Atomic force microscopy was used to characterize the resultant grooves. Use of the Mullins approach led to the expression ... 

The volume change associated with the annihilation of vacancies located at grain boundaries oriented normally to the plane of a thin film was discussed in Section 1.8.6. Although a volume change occurs at any site in a thin film where a vacancy is annihilated, this process need not always result in the generation of an internal stress. Provide a simple mechanistic justification for each of the following statements. 

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