Crystallization spherical

Spherulite formation by geometrical selection may rarely be seen on crystals with isotropic Habitus. Native arsenic. As, occurs in a confeito-like form, and is a type of spherulite grown through the geometrical selection of rhombohedral crystals. Spherical aggregation of calcite crystals with nail-head Habitus is also observed. Semi-spherical aggregates of platy barite crystals known as desert rose are shown in Fig. 8.6. 

Figure 16.7 (a) Sintering of two abutting single-crystal spherical particles of differing... 

Micellar solutions are isotropic microstructured fluids which form under certain conditions. At other conditions, liquid crystals periodic in at least one dimension can form. The lamellar liquid crystal phase consists of periodically stacked bilayers (a pair of opposed monolayers). The sheetlike surfactant structures can curl into long rods (closing on either the head or tail side) with parallel axes arrayed in a periodic hexagonal or rectangular spacing to form a hexagonal or a rectangular liquid crystal. Spherical micelles or inverted micelles whose centers are periodically distributed on a lattice of cubic symmetry form a cubic liquid crystal. 

After an aqueous dispersion of monodispersed spherical colloids was injected into the cell, a positive pressure was applied through the glass tube to force the solvent (water) to flow through the channels. The beads were accumulated at the bottom of the cell, and crystallized into a three-dimensional opaline lattice under continuous sonication. So far, we have successfully applied this approach to assemble monodispersed colloids (both polystyrene beads and silica spheres) into ccp lattices over areas of several square centimeters. This method is relatively fast opaline lattices of a few square centimeters in area could be routinely obtained within several days. This method is also remarkable for its flexibility it could be directly employed to crystallize spherical colloids of various materials with diameters between 200 nm and 10 pm into three-dimensional opaline lattices. In addition, this procedure could be easily modified to crystalhze spherical colloids with diameters as small as 50 nm. ... 

H. Ren, Y. H. Fan, S. Gauza, and S. T. Wu, Tunable-focus flat hquid crystal spherical lens, Appl. Phys. Lett. 84, 4789 (2004). 

In the connnonly used atomic sphere approximation (ASA) [79], the density and the potential of the crystal are approximated as spherically synnnetric within overlapping imifiBn-tin spheres. Additionally, all integrals, such as for the Coulomb potential, are perfonned only over the spheres. The limits on the accuracy of the method imposed by the ASA can be overcome with the fiill-potential version of the LMTO (FP-LMTO)... 

Figure B3.3.10. Contour plots of the free energy landscape associated with crystal niicleation for spherical particles with short-range attractions. The axes represent the number of atoms identifiable as belonging to a high-density cluster, and as being in a crystalline environment, respectively, (a) State point significantly below the metastable critical temperature. The niicleation pathway involves simple growth of a crystalline nucleus, (b) State point at the metastable critical temperature. The niicleation pathway is significantly curved, and the initial nucleus is liqiiidlike rather than crystalline. Thanks are due to D Frenkel and P R ten Wolde for this figure. For fiirther details see [189].

Dissolve 8 8 g. (9 0 ml.) of cyclohexanone in 50 ml. of glacial acetic acid, add 8 ml. of phenylhydrazine, and boil the solution under reflux for 5 minutes. Cool the solution, when the tetrahydrocarbazole will crystallise out. Filter at the pump, drain well, and recrystallise either from aqueous ethanol or (better) from aqueous acetic acid. The recrystallisation should be performed rapidly, for the tetrahydrocarbazole undergoes atmO" spheric oxidation in hot solutions after recrystallisation, the compound should be dried in a vacuum desiccator and not in an oven. Repeated recrystallisation should be avoided. The tetrahydrocarbazole, after thorough drying, is obtained as colourless crystals, m.p. 118° yield of recrystallised material, 11 g. 

The disks are assumed to lie in the same plane. While this picture is implausible for bulk crystallization, it makes sense for crystals grown in ultrathin films, adjacent to surfaces, and in stretched samples. A similar mathematical formalism can be developed for spherical growth and the disk can be regarded as a cross section of this. 

In bulk-crystallized polymers, lamellae are often organized into spheruHtes, spherical stmctures which grow outward from a point of nucleation, typically to about 0.01 mm in diameter. SpheruHtes are in some ways similar to the grain stmcture in metals. They can make a polymer brittle and also reduce transparency. 

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