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921.
M. M. Eisl B. M. Reichl S. D. Böhmig H. Störi 《Fresenius' Journal of Analytical Chemistry》1994,349(1-3):194-196
Using a discrete model for the segregation kinetics based on the Darken theory of diffusion and on the regular solution model (RSM), one and two dimensional simulations of interface segregation in binary and ternary systems have been performed. The aim of the calculations was to investigate the kinetics of segregation sequences in ternary systems, the segregation kinetics in discrete systems during the initial phase and finally the kinetics of surface segregation in the region of a grain boundary intersected by the surface. 相似文献
922.
Christopher J. Booth John W. Goodby Judith P. Hardy Olwen C. Lettington Kenneth J. Toyne 《Liquid crystals》1994,16(6):925-940
The synthesis and characterization of seven novel (R)-2-(4-substituted-phenoxy)propanonitriles are described. The propanonitriles were prepared to evaluate their potential use as thermochromics and ferroelectric dopants, as well as to determine their twist sense properties. The materials exhibit smectic and chiral nematic phases of high thermal stability; the mesogenic behaviour of the nitriles is directly related to the type of two-ring core unit employed. The effects of the different molecular geometries and polarizabilities of the liquid crystalline cores on mesophase stability are discussed, particularly in relation to other members of this series. The chiral nematic phase of the propanonitriles is assigned as having a left-handed twist sense from contact preparation studies, and this is in agreement with rules relating absolute configuration and molecular structure to helical twist sense. 相似文献
923.
In this work, the electro-optic response of a 6CB liquid crystal layer is studied using a sensitive differential technique. The layer is held at a temperature just above the nematic to isotropic phase transition. Transverse magnetic (p) polarized light incident on the cell is coupled to guided modes in the liquid crystal layer using prism coupling. The modes manifest themselves as sharp dips in the reflectivity as the angle of incidence is scanned. When a low frequency sinusoidal voltage is applied to the cell, the resonant mode shapes and excitation angles are altered at a frequency which is twice that of the applied field, resulting in a modulation of the reflectivity for a given angle of incidence. By synchronous observation of the modulated signal, a differential signal is recorded. Comparing the data with modelling generated from multilayer optics theory, two effects are then quantified. The first of these is an induced birefringence, varying quadratically with applied voltage, which is well understood and can be expressed in terms of Landau-de Gennes theory. The second is a field induced perturbation in the imaginary part of the optical permittivity, δεi, which implies a modification of the light scattering properties of the liquid crystal. The measurement of the latter effect is, as far as we know, a novel one, being only made possible by the remarkable sensitivity of the synchronous differential technique. 相似文献
924.
R. D. Patel 《国际化学动力学杂志》1994,26(4):403-406
Pseudo-first-order rate constants and activation parameters have been measured for the solvolysis of 2-chloroquinoxaline in various aquo-organic mixtures using methanol, ethanol, and isopropanol as the organic solvent. Excellent linear correlations are found between lnk and the mol fraction of cosolvent and ln[H2O]. The medium effect on the rates of solvolysis is assessed by Grunwald–Winstein's mY correlationship. The estimated values of m (0.55–0.72) and the entropy of activation (148–212 J deg?1 mol?1) for the reactions are well in the range for a bimolecular aromatic substitution reactions. © 1994 John Wiley & Sons, Inc. 相似文献
925.
926.
The kinetics of the thermal decomposition reaction of gaseous 3,3,6,6-tetramethyl-1,2,4,5-tetroxane (ACDP) in the presence of n-octane was studied in the 403.2–523.2 K temperature range. This reaction yields acetone as the organic product. Under optimum conditions, first-order kinetics were observed, included when the S/V ratio of the Pyrex reaction vessel was increased by a nearly six-fold factor. In the range 443.2–488.2 K the temperature dependence of the rate constants for the unimolecular reaction in conditioned vessels is given by In k1/(s?1) = (31.8 ± 2.5) ? [(39.0 ± 2.5)/RT]. The value of the energy of activation in kcal/mol correspond to one O? O bond homolysis of the ACDP molecule in a stepwise biradical initiated decomposition mechanism. At the lower reaction temperatures as well in preliminary experiments participation of a surface catalyzed ACDP decomposition process could be detected. © 1994 John Wiley & Sons, Inc. 相似文献
927.
X.T Zhang Y.C LiuZ.Z Zhi J.Y ZhangY.M Lu D.Z ShenW Xu X.W FanX.G Kong 《Journal of luminescence》2002,99(2):149-154
The properties of the excitonic luminescence for nanocrystalline ZnO thin films are investigated by using the dependence of excitonic photoluminescence (PL) spectra on temperature. The ZnO thin films are prepared by thermal oxidation of ZnS films prepared by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) technique. The X-ray diffraction (XRD) indicates that ZnO thin films have a polycrystalline hexagonal wurtzite structure with a preferred (0 0 2) orientation. A strong ultraviolet (UV) emission peak at 3.26 eV is observed, while the deep-level emission band is barely observable at room temperature. The strength of the exciton-longitudinal-optical (LO) phonon coupling is deduced from the temperature dependence of the full-width at half-maximum (FWHM) of the fundamental excitonic peak, decrease in exciton-longitudinal-optical (LO) phonon coupling strength is due to the quantum confinement effect. 相似文献
928.
S. I. Pesotskii R. B. Lyubovskii W. Biberacher M. V. Kartsovnik V. I. Nizhankovskii N. D. Kushch H. Kobayashi A. Kobayashi 《Journal of Experimental and Theoretical Physics》2002,94(2):431-433
Behavior of the de Haas-van Alphen (dHvA) oscillations depending on the angle between the magnetic field direction and the perpendicular to conducting layers in the quasi-two-dimensional organic metal α-(BETS)2 KHg(SCN)4 was studied in detail. The angular dependence of the dHvA oscillation amplitude exhibits a series of minima (at ±43.2°, ±64.6°, and ±72.0°) related to the “zero spin” effect, through which it is possible to estimate the splitting factor. An analysis of this value suggests that many-body interactions in the compound studied are either absent or at least radically weakened. 相似文献
929.
A. Thomasson S. Geffroy E. Frejafon D. Weidauer R. Fabian Y. Godet M. Nominé T. Ménard P. Rairoux D. Moeller J.P. Wolf 《Applied physics. B, Lasers and optics》2002,74(4-5):453-459
Continuous mapping of an ozone episode in Paris in June 1999 has been performed using a differential absorption lidar system.
The 2D ozone concentration vertical maps recorded over 33 h at the Champ de Mars are compiled in a video clip that gives access
to local photochemical dynamics with unprecedented precision. The lidar data are compared over the whole period with point
monitors located at 0-, 50-, and 300-m altitudes on the Eiffel Tower. Very good agreement is found when spatial resolution,
acquisition time, and required concentration accuracy are optimized. Sensitivity to these parameters for successful intercomparison
in urban areas is discussed.
Received: 11 February 2002 / Published online: 14 March 2002 相似文献
930.
V. S. Averbakh B. N. Bogolyubov Yu. A. Dubovoi Yu. M. Zaslavskii A. V. Lebedev A. P. Maryshev V. E. Nazarov K. E. Pigalov V. I. Talanov 《Acoustical Physics》2002,48(2):121-127
To excite seismic waves with a high coherence, powerful hydroacoustic radiators placed in a natural reservoir were used. Theoretical estimates and the test data demonstrate a high efficiency of the proposed method of seismic wave excitation. The calculations are in good agreement with the results of measurements. The results of phasing the radiation with the use of two monopole sources separated by a quarter-wave distance are presented. It is shown that the use of the proposed scheme of excitation makes it possible to control the radiation pattern while obtaining a high coherence of seismic waves. 相似文献