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Spherical Crystals (spherical + crystal)
Selected AbstractsCharge-density analysis of the ground state of a photochromic 1,10-phenanthroline zinc(II) bis(thiolate) complexACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010Stephan Scheins The charge density of the title compound was determined at 90,K, using a spherical crystal of 150,µm diameter. The proper treatment of the Zn atom in the pseudo-tetrahedral environment is considered in detail. A satisfactory refinement is only obtained when anharmonic Gram,Charlier parameters are included as variables in the refinement. A successful combined anharmonic/multipole refinement indicates a small polarization of the 4s shell in the anisotropic environment. One of the two toluenethiols is approximately ,-stacked with the phenanthroline ligand. A bond path is found connecting the two ligands. In addition the Zn,S bond to this ligand is slightly extended compared with the same bond to the second toluenethiol. A separate photocrystallographic and theoretical study indicates the long wavelength emission of the title compound to be due to a ligand-to-ligand charge transfer (LLCT) from a toluenethiol to the phenanthroline ligand. The charge-density results do not provide a basis for deciding which of the thiole ligands is the source of the transferred electron density. This result is in agreement with the theoretical calculations, which show comparable oscillator strengths for charge transfer from either of the ligands. [source] The minimum crystal size needed for a complete diffraction data setACTA CRYSTALLOGRAPHICA SECTION D, Issue 4 2010James M. Holton In this work, classic intensity formulae were united with an empirical spot-fading model in order to calculate the diameter of a spherical crystal that will scatter the required number of photons per spot at a desired resolution over the radiation-damage-limited lifetime. The influences of molecular weight, solvent content, Wilson B factor, X-ray wavelength and attenuation on scattering power and dose were all included. Taking the net photon count in a spot as the only source of noise, a complete data set with a signal-to-noise ratio of 2 at 2,Å resolution was predicted to be attainable from a perfect lysozyme crystal sphere 1.2,µm in diameter and two different models of photoelectron escape reduced this to 0.5 or 0.34,µm. These represent 15-fold to 700-fold less scattering power than the smallest experimentally determined crystal size to date, but the gap was shown to be consistent with the background scattering level of the relevant experiment. These results suggest that reduction of background photons and diffraction spot size on the detector are the principal paths to improving crystallographic data quality beyond current limits. [source] Preparation of Whisker ,-Spodumene Glass,CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2006Hu Anmin ,-spodumene glass,ceramics with a whisker-like microstructure were prepared from the following materials (in wt%): 64.5 SiO2, 18.0 Al2O3, 4.2 Li2O, 4 ZrO2, and 8 MgF2. Scanning electron microscope (SEM) analysis showed that phase separation in the base glass leads to the formation of a primary crystal phase of MgF2 that promotes the formation of spherical ,-spodumene. Whisker spodumene crystals surrounded by spherical crystals are observed at 720°C after 1 h, and the whisker crystals grow at the cost of spherical crystals with increasing temperature and time. The flexural strengths of the glass,ceramics reach a maximum of 228 MPa after heat treatment at 850°C for 1 h. [source] Preparation and characterization of polyurethane,gold nanocomposites prepared using encapsulated gold nanoparticlesPOLYMER INTERNATIONAL, Issue 7 2010Chao-Ching Chang Abstract Gold nanoparticles (GNPs) have been widely studied due to their unique properties. Although many research groups have developed the synthesis of GNPs using various polymers as stabilizing or reducing agents, the effects of GNPs on the structures and properties of polymer matrices have been less reported. We propose a new design for the preparation of polyurethane,gold (PU,Au) nanocomposites. 11-Mercapto-1-undecanol-coated GNPs acted as the chain extenders and reacted with isocyanates to form covalent bonds between PU and GNPs. PU,Au nanocomposites were successfully synthesized, and the effects of multifunctional GNPs on the structures, morphology and properties of poly(ester urethane) were investigated. Scanning electron microscopy images suggested the GNPs can be dispersed uniformly in the PU matrix. Maltese-cross of spherical crystals was observed in the PU,Au nanocomposites, and the size of the crystals decreased with an increase in gold content. As the gold content increased, the thermal decomposition temperature and the temperature of the maximum decomposition rate increased. The glass transition temperature, crystal melting temperature and melting enthalpy of the soft segment also increased progressively. The results showed that multifunctional GNPs concentrated hard segments and resulted in an increase of heterogeneous nucleation, phase separation and elasticity. Copyright © 2010 Society of Chemical Industry [source] The Influence of Shear Stress on Crystallization in an Ultrasound LevitatorCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 10 2007S. Sacher Abstract Industrial precipitation processes often use chemical agents to influence crystal morphology and size distribution. This experimental study deals with the investigation of physical parameters including an alternative method to affect crystal growth, thus, avoiding the presence of additives as intrinsic impurities. The influence of shear stress acting on growing crystals within a droplet is investigated in an ultrasound levitator. An ultrasound levitator enables the suspension of a single droplet against gravity and the study of containerless precipitation with specific mechanical forces acting on crystals. The levitator is used as a three-phase reactor with precipitation from the gas and liquid, and as a reactor for precipitation from two different solutions. Calcium carbonate is used as a model system. The variation of temperature and the amount of applied shear stress leads to different amounts of calcium carbonate morphologies. An increase in the shear stress results in more rounded or spherical crystals. The intensity of the shear stress also influences the particle size distributions of the precipitated crystals, i.e., with increasing shear stress, particle size distributions are shifted to smaller sizes. [source] | |||||||||||||