Microcrystals structures

M.M. Harding and B.M. Kariuki, Microcrystal Structure Determination of AIPO4-CHA using Synchrotronradiation. Acta Crystallogr., Sect. C Cryst. Struct. Commun., 1994, 50, 852-854. 

M.J. Gray, J.D. Jasper, A.P. Wilkinson, and J.C. Hanson, Synthesis and Synchrotron Microcrystal Structure of an Aluminophosphate with Chiral Layers containing Lambda tris(ethylenediamine)cobalt(III). Chem. Mater., 1997, 9, 976-980. 

Souza, M. M., and Borsali, R (2004). Rodlike cellulose microcrystals Structure, properties, and applications. 

Lima, M.M., Borsali, R., 2004. Rod-like cellulose microcrystals structure, properties and applications. Macromol. Rapid Commun. 25 (7), 771—787. 

De Boer, B. G. Stein, G.D. A metal cluster generator for gas-phase electron diffraction and its application to bismuth, lead, and indium variation in microcrystal structure with size. Surf. Set 1981,106, 84—94 Yokozeki, A. Stein, G.D. Production and electron diffraction studies of silver metal clusters in the gas phase. Appl.Phys. 1978, 49, 2224-2230. 

Physical structures of activated carbon. One of major roles of activated carbons as a support of catalyst is to increase the dispersion and stability of active components, to provide more active sites, which has close relationship with its pore structure and surface area. The amorphous structmes of activated carbons determine their developed micropores. There are some interspaces formed during activation process through ehminating the carbons of carbon compounds and non-organic component filling in the pore between microcrystal and parts of carbons in the microcrystal structure. These micropores have some shapes, such as plane slit, cylindrical, V-shape, cone, inkpot and distortions of those shapes etc. The adsorp>-tion abilities of activated carbon are closely related with the micropores structure and their distribution. The distribution of macropores, medium pore and micropores in activated carbon are listed in Table 6.3. 4... 

M. M. de Souza lima, R. Borsali, Rodlike Cellulose Microcrystals Structure, Properties, and Applications. Macromolecular Rapid Communications. 25(7), 771-87 (2004). 

Progress in deducing more structural details of these fibers has instead been achieved using NMR, electron microscopy and electron diffraction. These studies reveal that the fibers contain small microcrystals of ordered regions of the polypeptide chains interspersed in a matrix of less ordered or disordered regions of the chains (Eigure 14.9). The microcrystals comprise about 30% of the protein in the fibers, are arranged in p sheets, are 70 to 100 nanometers in size, and contain trace amounts of calcium ions. It is not yet established if the p sheets are planar or twisted as proposed for the amyloid fibril discussed in the previous section. 

In general, the R factor is between 0.15 and 0.20 for a well-determined protein structure. The residual difference rarely is due to large errors in the model of the protein molecule, but rather it is an inevitable consequence of errors and imperfections in the data. These derive from various sources, including slight variations in conformation of the protein molecules and inaccurate corrections both for the presence of solvent and for differences in the orientation of the microcrystals from which the crystal is built. This means that the final model represents an average of molecules that are slightly different both in conformation and orientation, and not surprisingly the model never corresponds precisely to the actual crystal. 

Berry CR (1967) Structure and optical absorption of Agl microcrystals. Phys Rev 161 848-851... 

The structure of metallic deposits is determined primarily by the size, shape (faceting), type of arrangement, and mutual orientation of the crystallites. Two factors may influence the orientation and spatial alignment of the microcrystals in electrocrystallization the field direction (or direction of the electric current) and the nature of the substrate. The deposits are said to have texture when the crystallites are highly oriented in certain directions. Epitaxy implies that the lattice is altered under the influence of the substrate. 

Emission spectra at these points are shown in Figure 8.2d. The band shapes were independent of the excitation intensity from 0.1 to 2.0 nJ pulse . The spectrum of the anthracene crystal with vibronic structures is ascribed to the fluorescence originating from the free exdton in the crystalline phase [1, 2], while the broad emission spectra of the pyrene microcrystal centered at 470 nm and that of the perylene microcrystal centered at 605 nm are, respectively, ascribed to the self-trapped exciton in the crystalline phase of pyrene and that of the a-type perylene crystal. These spectra clearly show that the femtosecond NIR pulse can produce excited singlet states in these microcrystals. 

Fig. 2.4. Microphotographs of sintered ZnO films with different structures a - structure consists of microcrystals connecting each other by thin crystal bridges b - lace structure is characterized by variety of branch thickness. Magnification 2 1(H.

Thus, the whole complex of existing experimental data indicates that the major part of polycrystalline contacts in vacuum sintered polycrystalline oxides are provided by bridges of open type. Moreover, the vacuum sintering at moderate temperatures 300 - 350°C leads to formation of a unified pattern (see Fig. 2.4, b) which cannot be disjoint into specific microcrystals and connecting bridges [37, 40]. The structure of adsorbents obtained presents a complex intertwining of branches of various thickness. 

Let us dwell on existing key models describing chemisorption induced response of electric conductivity in semiconductor adsorbent. Let us consider both the stationary values of electric conductivity attained during equilibrium in the adsorbate-adsorbent system and the kinetics of the change of electric conductivity when the content of ambient atmosphere changes. Let us consider the cases of adsorption of acceptor and donor particles separately. In all cases we will pay a special attention to the issue of dependence of the value and character of signal on the structure type of adsorbent, namely on characteristics of the dominant type of contacts in microcrystals. 

The information obtained has made it possible to propose a mechanism of interaction between RGMAs and Au/ZnO structures based on the concept of some change in the nature of the interaction between the metal microcrystal applied and the substratum as its size increases. 

The structure of a polycrystalline electrode depends on its preparation. Usually toe rough electrodes are prepared by electrochemical deposition of a given metal onto a suitable substrate. Microcrystals present in polycrystaUine samples are randomly oriented on the surface. Most likely, not only basal but also higher MiUer-index planes should be considered in anticipating toe final structure of the electrode surface. It was shown that the stmcture of the platinized platinum surface depends strongly on toe platinization conditions, e.g., on toe concentration of the platinization... 

In the case of thicknesses larger than mentioned above the intensities must be calculated according to the more general many-beam theory. The calculation should include summation over different groups of crystals having a certain distributions of thickness and orientation. A method based on the matrix formulation of the many-beam theory was developed for partly-oriented thin films and have been successfully applied samples [2]. The main problem in using direct many-beam calculation is to find the distribution functions for size and orientation of the microcrystals. However, it is not always possible to refine these functions in the process of intensity adjustment. Additional investigation of the micro-structure by electron microscopy is very helpful in such case. 

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