Crystals generation

Figure 18. SEM images with increasing magnifications of (Aussln crystals, generated in a monolayer of dendrimers.

Apart from deliberate or accidental introduction of tiny seed crystals to the system, and productive interactions between existing crystals and quasi-crystalline embryos or clusters in solution, the most influential mode of new crystal generation in an industrial crystalliser is contact secondary nucleation between the existing crystals themselves, between crystals and the walls or other internal parts of the crystalliser, or between crystals and the mechanical agitator. Secondary nucleation rates (in m-3 s 1) are most commonly correlated by empirical relationships such as ... 

Extended defects interrupt the continuity of the crystal, generating crystal subgrains whose dimensions depend, in a complex fashion, on the density of extended defects per unit area. Table 4.1 gives examples of reported dislocation densities and subgrain dimensions in olivine crystals from the San Carlos perido-tite nodules (Australia). Assuming a mean dislocation density within 1.2 X 10 and 6 X 10 cm , Kirby and Wegner (1978) deduced that a directional strain pressure of 35 to 75 bar acted on the crystals prior to their transport to the surface by the enclosing lavas. 

Stack of lamellar crystals generated by spiral growth at one or more screw dislocations. Note The axial displacement over a full turn of the screw (Burgers vector) is usually equal to one lamellar thickness. 

SGX-CAT maintains a direct T1 network connection from the Advanced Photon Source in Illinois to SGX San Diego. Database inquiries are handled over this link. Interactions with the database occur in three ways. An extensive web-based system is used for data entry and retrieval. For crystals generated by external users of the beamline, upload of an electronic spreadsheet transfers the required crystal data attributes to the SGX LIMS. For automated operations, such as crystal screening and data collection, custom scripts place the computed results directly into the database. 

Vibrational Wavenumbers (cm ) and Intensities for Surface Methyl Groups on Metal Single Crystals Generated by Thermal or Photolytic... 

FIGURE 8.11. (A, B) SEM images of (100)-oriented colloidal crystals generated by templating 1.0-jxm polystyrene beads against a 2D array of 4-i.im wide square pyramidal pits. The crystals belong to die same sample that happened to stick to the bottom (A) and top (B) substrate when die top glass substrate was separated from the cell. 

Figure 4.22 The transmission of an intense beam of light through a nonlinear crystal generates a dc voltage across it.

Evaporative crystallizers generate supersaturation by removing solvent from the mixture, thereby increasing the solute concentration. They may be operated under vacuum, and in those circumstances it is necessary to have a... 

Chiral crystals generated from non-chiral molecules have served as reactants for the performance of so-called absolute asymmetric synthesis. The chiral environments of such crystals exert asymmetric induction in photochemical, thermal and heterogeneous reactions [41]. Early reports on successful absolute asymmetric synthesis include the y-ray-induced isotactic polymerization of frans-frans-l,3-pentadiene in an all-frans perhydropheny-lene crystal by Farina et al. [42] and the gas-solid asymmetric bromination ofpjp -chmethyl chalcone, yielding the chiral dibromo compound, by Penzien and Schmidt [43]. These studies were followed by the 2n + 2n photodimerization reactions of non-chiral dienes, resulting in the formation of chiral cyclobutanes [44-48]. In recent years more than a dozen such syntheses have been reported. They include unimolecular di- r-methane rearrangements and the Nourish Type II photoreactions [49] of an achiral oxo- [50] and athio-amide [51] into optically active /Mactams, photo-isomerization of alkyl-cobalt complexes [52], asymmetric synthesis of two-component molecular crystals composed from achiral molecules [53] and, more recently, the conversion of non-chiral aldehydes into homochiral alcohols [54,55]. 

K. Hirahara, K. Suenaga, S. Bandow, H. Kato, T. Okazaki, H. Shinohara, and S. Iijima, One-dimensional metallofullerene crystal generated inside single-walled carbon nanotubes, Phys. Rev. Lett. 85 5384—5387 (2000). 

In a presentation by Ciurczak [44], it was observed that pure d- and Z-amino acids gave identical NIR spectra, while racemic crystals generated quite different spectra. A paper presented by Ciurczak [45] in 1986 outlines work later completed by Buchanan et al. [46]. In this work, varying percentages of d- and Z-valine were mixed physically and scanned by NIR. The spectra were identical except for particle size-induced baseline shifts. These mixtures were then dissolved and recrystallized as recemic crystals. These new samples were scanned by NIR obvious qualitative and quantitative differences were observed. 

Figure 3.42 Electron intensity oscillation in the thickness direction of a specimen. A wedge-shaped crystal generates dark and bright fringes in the image because of the continuous change in thickness of the wedge.

We have claimed that the hydrate generated in the ternary system of H2, CO2 and water (without THF) can be regarded as the pure CO2 hydrate crystal (s-1) from Raman spectroscopic study. The gas hydrate crystal generated in H2 + CO2 + THF + water mixed system is the s-II. The D-cage of s-ll is somewhat smaller than that of s-I. Therefore, it is possible that the H2 molecule occupies the D-cage to generate H2 + CO2 + THF mixed hydrate crystal. 

Control of crystallization processes requires modulation of either nucleation or growth or, as is most often the case, both modes of crystal development simultaneously. Each operation must be evaluated to determine which of these process objectives is most critical, from the point of view of overall outcome, to determine whether nucleation or growth should be the dominant phase. Much of the literature is focused on nucleation for the obvious reason that the number and size of nuclei initially formed can dominate the remainder of the operation. However, it is generaUy agreed that nucleation can be difficult to control, since there are several factors that can play a role in the conditions for nucleation onset, nucleation rate, and number of crystals generated before growth predominates. 

The presence of the new solvate created many complications for the development of the drug. It became impossible to crystallize the original nonsolvate crystals using the original MeOH/IPAC solvent system. Crystals generated by Options 1, 2, or 3 were all solvate. In order to avoid the complication of the solvate issue, the original solvent system, MeOH/IPAC, was abandoned. 

The presence of several molecules bound together in a molecular crystal generates an external force field and new sequences of vibrations are created, the external modes. These run down from about 80 cm", see below, and again the lowest attainable frequency is a function of the number of oscillators. The number of molecules in a crystal is generally very large and the dispersion curve of the external modes becomes truly continuous, the very lowest frequencies are available and the density-of-states is not composed of sharp individual transitions. However, the spectral features assoeiated with the TAMs remain sharp sinee the original wavevectors retain their significance. 

Thermotropic liquid crystals and also lyotropic liquid crystals generate functional molecular assemblies. lyotropic liquid crystalline phases are exhibited by amphiphilic molecules in appropriate solvents. They form nano-segregated structures because the molecular structures consist of hydrophilic and hydrophobic components. In Chapter 6, Gin and co-workers describe how lyotropic liquid crystals may be used to form functional materials. Lyotropic liquid crystals can act as templates for inorganic materials, ion conductors, catalysts, drug delivery systems, and nanofilters. 

In Kukushkin [39] it was demonstrated that stresses in the crystal generate a vacancy flux, which, in the case under consideration, is proportional to the radial component of the elastic stress tensor. This vacancy flux can be represented by the following expression ... 

Figure 6.12 Crystal Mounting The delicacy of crystals generated from biological macromolecule crystallisation requires that individual crystals for X-ray crystallography be mounted in a sealed capillary separated from crystal mother liquors by an air pocket. The capillary is mounted in the goniometer head of a four circle diffractometer (see Fig. 6.15).

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