Defect-induced melting in nematic liquid crystals

In this paper we employ a relatively simple theory to show how a nematic disclination line can act as a nucleation site for the growth of the isotropic phase. With this theory we are able to find analytical expressions for the critical temperature of nucleation and the behaviour of the core radius as a function of temperature. We are then able to compare these results with a previous numerical model of this effect.     

Defect-induced melting in nematic liquid crystals

In this paper we employ a relatively simple theory to show how a nematic disclination line can act as a nucleation site for the growth of the isotropic phase. With this theory we are able to find analytical expressions for the critical temperature of nucleation and the behaviour of the core radius as a function of temperature. We are then able to compare these results with a previous numerical model of this effect.     

Specifics of chemical bonding in zinc crystals

The theory of elasticity and crystal lattice dynamics methods were used to elucidate specific features of chemical bonding in zinc crystals, which extends the possibilities of solving materials science problems.     

Spectroscopic characterization of zinc oxide nanorods synthesized by solid-state reaction

Well-crystallized zinc oxide nanorods have been fabricated by single step solid-state reaction using zinc acetate and sodium hydroxide, at room temperature. The sodium lauryl sulfate (SLS) stabilized zinc oxide nanorods were characterized by using X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy and photoluminescence spectroscopy. The X-ray diffraction revealed the wurtzite structure of zinc oxide. The size estimation by XRD and TEM confirmed that the ZnO nanorods are made of single crystals. The growth of zinc oxide crystals into rod shape was found to be closely related to its hexagonal nature. The mass ratio of SLS:ZnO in the nanorods was found to be 1:10 based on the thermogravimetric analysis. Blue shift of photoluminescence emission was noticed in the ZnO nanorods when compared to that of ZnO bulk. FT-IR analysis confirmed the binding of SLS with ZnO nanorods. Apart from ease of preparation, this method has the advantage of eco-friendliness since the solvent and other harmful chemicals were eliminated in the synthesis protocol.     

Reversible repositioning of zinc atoms within single crystals of a zinc polycarboxylate with an open-framework structure

Upon dehydration at 220 degrees C under vacuum, an open-framework zinc tetrahydrofuran-2,3,4,5-tetracarboxylate undergoes reversible single crystal to single crystal conversion by some of the zinc atoms moving to different positions that are 3 A away. The reverse process occurs at room temperature by exposing dehydrated crystals to air.     

Aminotroponiminate alkyl and alkoxide complexes of zinc

Reaction of {(iPr)₂ATI} H((iPr)₂ATI = N-isopropyl-2-(isopropylamino)troponiminate) with dimethyl zinc in toluene afforded the methyl complex [{(iPr)₂ATI}Zn-Me]. Subsequent reaction of [{(iPr)₂ATI}Zn-Me] with different alcohols gave the alkoxide complexes [{(iPr)₂ATI}Zn-OR]₂ (R = iPr, tBu, Ph). These compounds, which were investigated by single crystal X-ray diffraction, are dimeric in the solid-state. In the solid-state the metal centers are bridged symmetrically by two μ-oxygen atoms, thus a flat Zn-O-Zn′-O′ plane is observed. [{(iPr)₂ATI}Zn-OR]₂ were used as catalysts for the copolymerization experiments of epoxides and CO₂.     

13C chemical shielding tensors in single crystals of tetraacetylethane

The orientations of the principal axes of the ¹³C chemical shielding tensor of the double-bonded C5 carbon nucleus in single crystals of 1,1,2,2-tetraacetylethane are essentially identical with those of an aromatic carbon nucleus. That observation and the similarities between the tensors of the carbonyl and the enolic carbon nuclei result from strong intramolecular hydrogen bonding.     

Synthesis of high surface area nanometer magnesia by solid-state chemical reaction

Nanometer MgO samples with high surface area, small crystal size and mesoporous texture were synthesized by thermal decomposition of MgC₂O₄ · 2H₂O prepared from solid-state chemical reaction between H₂C₂O₄ · 2H₂O and Mg (CH₃COO)₂ · 4H₂O. Steam produced during the decomposition process accelerated the sintering of MgO, and MgO with surface area as high as 412 m² · g⁻¹ was obtained through calcining its precursor in flowing dry nitrogen at 520°C for 4 h. The samples were characterized by X-ray diffraction, N₂ adsorption, transmission electron microscopy, thermogravimetry, and differential thermal analysis. The as-prepared MgO was composed of nanocrystals with a size of about 4–5 nm and formed a wormhole-like porous structure. The MgO also had good thermal stability, and its surface areas remained at 357 and 153 m²·g⁻¹ after calcination at 600 and 800°C for 2 h, respectively. Compared with the MgO sample prepared by the precipitation method, MgO prepared by solid-state chemical reaction has uniform pore size distribution, surface area, and crystal size. The solid-state chemical method has the advantages of low cost, low pollution, and high yield, therefore it appears to be a promising method in the industrial manufacture of nanometer MgO. Translated from Chinese Journal of Catalysis, 2006, 27(9): 793–798 (in Chinese)     

Electron paramagnetic resonance study of vanadyl doped in zinc maleate tetrahydrate single crystals

Spectroscopy of vanadyl doped zinc meleate tetrahydrate single crystals has been studied by EPR in the X-band frequencies (∼9.5 GHz) and optical spectra. It is found that vanadium enters the zinc host lattice substitutionally.     

Optimization of technological parameters in plasma chemical etching of quartz single crystals

Main technological parameters of the process of local plasmochemical etching of single-crystal quartz were optimized. The etching was performed in a gas mixture of CF₄ and H₂ under radio frequency (RF, 13.56 MHz) discharge excitation. The scientific experiment design by the Taguchi matrix method was used to examine the effect of chamber pressure, RF generator power, negative bias applied to the substrate holder, and hydrogen flow rate on the rate of the etching process. The experimental results made it possible to evaluate for the first time the influence exerted by the technological parameters on the etching rate. It was shown that the influence exerted by the technological parameters in the conditions under study decreases in the following order: pressure in the reaction chamber, bias potential, RF power, hydrogen flow rate.     

Synthesis and characterization of some single crystals of thiourea urea zinc chloride

Thiourea Urea Zinc (II) Chloride (TUZC), a new semiorganic non-linear optical material has been synthesized. The solubility studies have been carried out at room temperature. Single crystals of different proportions of TUZC have been grown by slow evaporation of saturated aqueous solution at room temperature. The FTIR and UV spectral bands have been compared with urea, thiourea and bis Thiourea Zinc Chloride (BTZC). The TG curve showed a two steps mass loss on heating the compound between 30 and 800 °C corresponding to two exothermic DTA peaks at 181–183 and 250–252 oC.     

Concerted molecular displacements in a thermally-induced solid-state transformation in crystals of DL-norleucine

Martensitic transformations are of considerable technological importance, a particularly promising application being the possibility of using martensitic materials, possibly proteins, as tiny machines. For organic crystals, however, a molecular level understanding of such transformations is lacking. We have studied a martensitic-type transformation in crystals of the amino acid DL-norleucine using molecular dynamics simulation. The crystal structures of DL-norleucine comprise stacks of bilayers (formed as a result of strong hydrogen bonding) that translate relative to each other on transformation. The simulations reveal that the transformation occurs by concerted molecular displacements involving entire bilayers rather than on a molecule-by-molecule basis. These observations can be rationalized on the basis that at sufficiently high excess temperatures, the free energy barriers to concerted molecular displacements can be overcome by the available thermal energy. Furthermore, in displacive transformations, the molecular displacements can occur by the propagation of a displacement wave (akin to a kink in a carpet), which requires the molecules to overcome only a local barrier. Concerted molecular displacements are therefore considered to be a significant feature of all displacive transformations. This finding is expected to be of value toward developing strategies for controlling or modulating martensitic-type transformations.     

Determination of chemical shift parameters from zeeman nuclear quadrupole resonance in single crystals

The components of the chemical shift tensor of the ³⁵Cl nuclei in paradichlorobenzene single crystal have been determined directly from Zeeman-split NQR spectra, with the magnetic field applied parallel to the principal axes of the electric-field-gradient tensor. The results are compared with values obtained by NMR in the pure quadrupole regime.     

Artificial neural network model for the evaluation of chemical kinetics in thermally induced solid-state reaction

The artificial intelligence technique is utilized to improve evaluation of thermally induced solid-state reaction kinetics. A general regression neural network (GRNN) model was applied to directly determine the kinetic triplets, i.e., activation energy, pre-exponential factor, and mechanism model. The effect of number of heating rate on prediction performance of the GRNN model was assessed based on the estimation indictors. The obtained kinetic triplets based on the triple heating rates were considered to be accepted. The prediction ability of the GRNN model was very robust at more than three heating rates. The relative errors for kinetic parameters derived from five heating rates were within ±?4%, and the cognition rates for mechanism models were up to 99.6%. The developed GRNN model was successfully applied in the high-temperature synthesis of Li₄Ti₅O₁₂/C composites. It is expected that the model also could be extended to estimate the kinetics of other solid-state reactions.

     

Tunneling in photochemical solid-state H-transfer.: Temperature dependence of the H-abstraction rate in phenazine-doped fluorene single crystals

The reaction rate has been measured in the temperature range 50 <T < 300 K for photochemical H-abstraction by phenazine in its excited triplet state doped in fluorene single crystals. Typical features for a tunneling process like the approach to a constant low-temperature rate, a non-Arrhenius temperature-dependent rate and a substantial deuterium effect are demonstrated. Under specific conditions the nuclear motion promoting the reaction can be assigned as a vibrational oscillator with the fundamental energy of 146 cm⁻¹. Problems and possible ways towards final specification of the reaction-promoting nuclear motion are discussed.     

Site effects in controlling the chemical reactivity in crystals: solid-state photochromism of N-(n-propyl)nitrospiropyrane

The dynamics of decay of the photoproducts of the two non equivalent molecules of N-(n-propyl) nitrospiropyrane in the crystalline state is significantly different due to the effect of the specific site where each of the molecules is located in the crystal latice.     

A pulsed NMR investigation of F chemical shift anisotropy in single crystals of fluorapatite

Measurement of the ¹⁹F NMR frequency versus orientation in single crystals of fluoropatite Ca₅F(PO₄)₃ about two perpendicular crystal axes has allowed determination of all principal components of the nuclear screening tensor, which was found to be axially symmetric with σ_∥ - σ_⊥ ≈ −84 ppm. An explanation of the negative value for σ_∥ - σ_⊥ is suggested by the bonding arrangement most consistent with the crystal structure.