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991.
Summary A single domain nematic liquid crystal undergoes first- or second-order transitions when placed in an external field (Fréedericksz
transition). A new type of transition, giving rise to a periodic distortion of the mean molecular direction, within particular
ranges of the elastic-constant ratio, has been recently studied. In this paper we show that, in a particular situation involving
an electric and a magnetic field, both perpendicular to the director, and nonsymmetric boundary conditions, the periodic Fréedericksz
transition is forbidden for any value of the elastic-constant ratio.
Riassunto Un cristallo liquido nematico orientato in modo uniforme subisce una transizione orientazionale del primo o del secondo ordine quando è sottoposto ad un campo esterno (transizione di Fréedericksz). Recentemente è stato studiato un nuovo tipo di transizione che, per particolari valori del rapporto delle costanti elastiche, consiste in una distorsione periodica della direzione molecolare media. In questo lavoro si mostra che quando sono presenti due campi, elettrico e magnetico, incrociati e perpendicolari all’orientazione indisturbata del nematico, e le condizioni al contorno non sono simmetriche (ancoraggio forte/debole), non si ha piú transizione periodica di Freédericksz, per qualunque valore del rapporto tra le costanti elastiche.
Резюме Однодоменный нематический жидкий кристалл претерпевает переходы первого или второго порядка во внешнем поле (переход Фрпдерикса). Недавно был исследован новый тип перехода, вызывающий периодическую дисторсию среднего молекулярного направления в некоторых областях отношения упругих постоянных. В этой системе мы показываем, что для определен электрических и магнитных полей, перпендикулярных директоры, и несимметричных граничных условий периодический переход Фридерикса оказывается запрещенным для любых значений отношения упругих постоянных.相似文献
992.
993.
Joong-Hee Nam Yong-Hui Joo Jeong Ho Chang Myoung Pyo Chun 《Journal of magnetism and magnetic materials》2009,321(10):1389-1392
We present the synthesis, magnetic and UV spectrometry of NiZn-ferrite nanofiber. The single phase of spinel ferrite was obtained at 600 °C. The NiZn-ferrite fibers fabricated by an electrospinning process were formed as a polygonal grain growth with firing temperature in fiber matrix. It appeared that the saturation magnetization (MS) of NiZn-ferrite nanofiber was dependent on Ni/Zn molar ratio which is similar to that of the inverse spinel ferrites. The NiZn-ferrite fibers showed good DNA adsorption efficiency that can be modified and utilized for DNA separation with magnetic nanofiber as a novel material in clinical applications. 相似文献
994.
Junhua Xiao Gu Yuarf Weiqiang Huang John H. K. Lum K‐L Daniel Lee Albert S. C. Chan 《中国化学会会志》2001,48(5):929-932
Two nitrated analogs of distamycin were synthesized by a novel haloform reaction and the DCC/HOBT coupling reaction in a convenient and effective way with out amino group protection. Our results have demonstrated that these polyamides show DNA cleaving activity under long UV (LUV 365 nm) irradiation. Furthermore, it is an unexpected discovery that these analogs can prevent DNA dam age under short UV (SUV 302 nm). 相似文献
995.
DNA-modified electrodes (VII) 总被引:1,自引:0,他引:1
Qi Lu Daiwen Pang Shen Hu Jieke Cheng Xiongwei Cai Caihui Shi Bingwei Mao Hongping Dai 《中国科学B辑(英文版)》1999,42(4):425-432
Two kinds of DNA-modified electrodes were prepared by covalent and adsorptive immobilization of DNA onto self-assembled monolayers
of 2, 2’-dithiodiethanol on gold electrodes and characterized by cyclic voltammetry, X-ray photoelectron spectroscopy and
scanning tunneling microscopy. The results suggest that the methods are satisfactory for the immobilization of DNA on electrodes.
Project supported by the National Natural Science Foundation of China (Grant Nos. 39370213; 39770220; 29773034), State Key
Laboratory for Physical Chemistry of Solid Surfaces (Xiamen University), and Natural Science Foundation of Hubei Province
(Grant NO. 96J037). 相似文献
996.
The aim of this paper is to consider theoretically a Freedericksztransition for concentric toroidal layers of smectic C liquidcrystal arising from a simple geometric setup, thereby extendingthe results of Atkin & Stewart [Q. Jl Mech. Appl. Math.,47, 1994] who considered spherical layers of smectic C in theusual cone and plate geometry. Application of smectic continuumtheory leads, after suitable approximations are made, to a lineargoverning equilibrium equation which is satisfied by both thetrivial solution and a variable solution involving Bessel functions.We are able to determine the critical magnitude cH of the magneticfield H at which this variable solution exists, and a standardenergy comparison reveals that the variable solution is expectedto be more energetically favourable than the zero solution providedH > cH. Numerical examples of critical thresholds are given,which are comparable to those in the literature for nematics.The paper ends with a discussion section and some indicationof possible future work. 相似文献
997.
998.
999.
1000.
Electronic charge density distribution in molecular systems has been described in terms of the topological properties. After briefly reviewing methods of obtaining charge densities from X-ray diffraction and theory, typical case studies are discussed. These studies include rings and cage systems, hydrogen bonded solids, polymorphic solids and molecular NLO materials. It is shown how combined experimental and theoretical investigations of charge densities in molecular crystals can provide useful insights into electronic structure and reactivity. 相似文献