Microcrystallite

X-rays. The diffraction from this polycrystalline and disoriented fiber is the sum of the diffraction from all the randomly oriented microcrystallites, and it corresponds to a series of concentric rings, each with its characteristic (/-spacing. The intensity is uniform on a ring, but it varies among rings. This type of diffraction, commonly referred to as a powder pattern, is prevalent among minerals and polymers that have a low degree of polymerization. 

Fic. 2. (continued)—(b) an assembly of randomly oriented microcrystallites (left) diffracts to produce a series of concentric rings (right) (c) an assembly of partially oriented blocks of microcrystallites (left) diffracts to produce large arcs (right). 

When the fiber is stretched, longer blocks of unit cells might be facilitated, as shown in Fig. 2c. This falls under the second category, as the orientation of the large microcrystallites is no longer random, but somewhat tempered, Consequently, circles are trimmed down to arcs in the diffraction pattern in response to enhancement in orientation. 

The third category, shown in Fig. 2d, results when all of the long molecules or microcrystallites are aligned along the fiber axis, but they aggregate with little lateral ordering. This assembly, called an oriented fiber, diffracts to produce a series of layer lines that are perpendicular to the fiber axis. The intensity is nonuni-... 

Wood and Hill consider that the role of fluoride in these glasses is uncertain. Phase-separation studies suggest that the structure of the glass might relate to the crystalline species formed, in which case a microcrystallite glass model is appropriate. But other evidence cited above on the structure-breaking role of fluoride is compatible with a random network model. 

Yanata, Y, Suzuki, K and Oka, Y. (1993) Magneto-optical properties of CdMnSe microcrystallites in SiO glass prepared hy rfsputtering./. Appl. Phys., 73,4595-4598. 

It was also reported that microcrystallites of layered semiconductor Pblj were prepared in colloidal form. The spectrum of such a colloidal solution consisted of three absorption bands in the UV which were considerably bu hifted from the absorption threshold of macrocrystalline Pblj. These results were explained by carrier confinement in three differently sized crystallites, each a single layer ( 7 A) thick. However, complexes of Pbl2 with iodide have similar absorption bands, and it seems at the present time that additional experiments have to be carried out to ascertain the colloidal nature of the absorbing species. Size quantization was also reported for colloids of red Hgl2 in acetonitrile... 

A difference between microcrystallite-based ultrastructure and covalently-crosslinked systems is that microcrystallites melt at specific temperatures, allowing the polymer to be fabricated by heating at modest temperatures. Subsequent cooling of the system below the crystallization temperature allows the physical property advantages of the solid state to become manifest. Liquid crystallinity is also possible if some order is retained in the molten state. Crystalline order not only adds mechanical strength, it also provides opportunities for the appearance of other properties that depend on solid state order—such as electronic conductivity. 

Sweeney, W. V., D. Coucouvanis et al. (1973). ESR of spin 5/2 systems with axial symmetry and moderately large zero-field splittings. Application of line-shape calculations to the interpretation of randomly oriented microcrystallite spectra. J. Chem. Phys. 59 369-379. 

When the oxide is formed by anodizing in acid solutions and the sample is then left to rest at the OCP, some dissolution can occur. This process has been studied by a numbers of authors,70-75 especially in relation to porous oxides [cf. Section 111(4)]. It was found that pore walls are attacked, so that they are widened and tapered to a trumpet-like shape.70 71 Finally, the pore skeleton collapses and dissolves, at the outer oxide region. The outer regions of the oxide body dissolve at higher rates than the inner ones.9,19 The same is true for dissolution of other anodic oxides of valve metals.76 This thickness dependence is interpreted in terms of a depth-dependent vacancy concentration in the oxide75 or by acid permeation through cell walls by intercrystalline diffusion, disaggregating the microcrystallites of y-alumina.4... 

Microcrystallites of direct semiconductors usually show a simple exponential decay of the PL intensity P with time, with time constants r in the ps and ns range at RT. A similar simple exponential decay (r = 20ms at 2 K) is observed for PL from mesoporous silicon of high porosity, which shows a weak confinement effect... 

Alternatives to activated tungsten wire emitters are also known, but less widespread in use. Cobalt and nickel [44,47] as well as silver [48] can be electrochemi-cally deposited on wires to produce activated FD emitters. Mechanically strong and efficient emitters can be made by growing fine silicon whiskers from silane gas on gold-coated tungsten or tantalum wires of 60 pm diameter. [45] Finally, on the fracture-surface of graphite rods fine microcrystallites are exposed, the sharpness of which provides field strengths sufficient for ionization. [49]... 

Figure la. Examples of ED patterns a - Diffraction pattern of mosaic single crystals, microcrystallites in the film are a little disoriented around definite directions. 

Figure lb. DP of polycrystalline film with full disorientation s of microcrystallites. 

Where Jo and S are the current density of the primary beam and the area of the irradiated sample, Z is the wave length, Ohki the structure factor amplitude, Q the volume cell, Z a factor that takes the microstructure of sample into account (Zm - for a mosaic single crystalline film, Zt - for a texture film), t is the sample thickness, dhu the interplanar spacing, a represents the mean angular distribution of the microcrystallites in the film, p is a multiplicity factor (accounts for the number of reflections of coincidence), R is a horizontal coordinate of a particular reflection in DP from textures and (p is the tilt angle of the sample. In the case of polycrystalline films, a local intensity is usually measured and the corresponding relation is ... 

The most important question for the calculation of the structure amplitudes from the intensities is that for the validity of the kinematical approximation. Due to the strong interaction of fast electrons with matter the effects of dynamical scattering become more pronounced with increasing size of the microcrystallites in the film. In order to justify application of the kinematical equations it is necessary that the diffracted intensity is much less... 

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