共查询到20条相似文献,搜索用时 15 毫秒
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Maria Regina Alcantara Maria Veronica Marques Correia De Melo Vera Regina Paoli JosÉ Atilio Vanin 《Molecular Crystals and Liquid Crystals》2013,570(3-4):335-347
Lyotropic liquid crystals are prepared from racemic and optically active di-sodium N-lauroyl aspartate (SNLA). The optically active levo enantiomer is a suitable amphiphile for obtaining cholesteric lyotropic mesophases. In the present work, different types of mesophases, including cholesteric and nematic, were prepared and their type I or II magnetic behavior characterized by deuterium and 23Na NMR. The polarizing microscope textures were investigated, particularly with regards to magnetic field effects upon the samples. The usual chevron pattern was obtained for particular sample orientations and several characteristic effects were observed for the type I cholesteric lyomesophase in the first few minutes following removal of the sample from the magnetic field. Although the head group of di-sodium N-lauroyl aspartate possesses two negative charges, compositions are not too different from those reported for other amphiphile lyotropic systems. 相似文献
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E. H. Korte 《Molecular Crystals and Liquid Crystals》2013,570(1-2):151-160
An orientational order parameter for cholesteric liquid crystals is defined in analogy to nematic mesophases. The influence of this parameter on the selective reflexion and the optical rotatory dispersion is described. Its relative change with temperature is evaluated from ORD measurements for an induced cholesteric soIution and is found to be similar to that known for nematic liquid crystals. 相似文献
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The orientational order parameters fot two liquid crystal materials, 4-cyanophenyl 4-butylbenzoate and 4-cyanophenyl 4-pentylbenzoate, have been derived by measuring the change in the refractive index as function of temperature. The order parameters are compared with MaierSaupe theory, and the sharpness of the transitions has been shown using the Haller's plot. 相似文献
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Shoichi Kondo Masayuki Ishikawa Masakazu Fujiwara Isamu Ono Shigeru Mita 《Molecular Crystals and Liquid Crystals》2013,570(1):777-784
Abstract The orientational order parameters of six solutes with different shapes and sizes dissolved in two nematic solvents have been measured as a function of temperature. The parameter, Sxx-Syy is correlated with the molecular geometrical structure, while Szz is affected by not only the geometry but also the anisotropy in the polarizability. The molecular dynamics simulations are carried out using a realistic atom-atom potential for evaluating the ordering properties in the nematic phase. The results provide reasonable values of Szz corresponding to experimental ones for solutes and solvents, and also a prediction of uniaxiality for the nematic phase. 相似文献
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Vinay S. Sharma 《Molecular Crystals and Liquid Crystals》2016,633(1):80-90
Vinyl carboxy central group containing a novel chalconyl ester homologous series: RO.C6H4.CH ? CH.COO.C6H4.CH ? CH.CO.C6H4.I (para) have been synthesized and studied with a view to understand and establish the relation between thermotropic liquid crystalline (LC) behaviors and the molecular structure. Novel chalconyl ester homologous series consists of thirteen homologs (C1–C18). C1–C3 homologs are nonmesogenic, C4 homolog is enantiotropic nematogenic and the rest of the homologs (C5–C18) are enantiotropically smectogenic plus nematogenic. Phase transition temperatures and textures of LC phase were determined by an optical polarizing microscopy (POM) equipped with a heating stage. Cr-M/I, Sm-N, N-I transition curves in a phase diagram behaved in normal manner. Sm-N and N-I transition curves exhibited odd-even effect from C4–C10 or nearby C10 homolog. Textures of nematic phase are threaded or Schlieren and that of the smectic phase are of the type smectic A or C. Analytical and spectral data confirms the molecular structure of homologs. Thermal stabilities for smectic and nematic mesophases are 155.0 and 180.7, respectively, and their corresponding mesophaselengths are varied from minimum to maximum at 17.0°C–39.0°C as well as 15.0°C–30.0°C. Thus, it is middle ordered melting type series. The group efficiency orders derived on the basis of smectic and nematic thermal stabilities are as under: Sm: -C6H4.I > -C6H4.Cl > -C4H3S and N:- C6H4.I > -C6H4.Cl > -C4H3S from comparative study of structurally similar analogous series 相似文献
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CaCu3Ti4O12 (CCTO) has challenged for the last few years the scientific community due to its large dielectric constant, which is almost temperature and frequency independent, from 100 K to 400 K and from 1 kHz to 1 MHz, respectively. This makes the material desirable for many electronic applications. However, the dissipation factor is very large, with tan δ values, at room temperature and 1 kHz, higher than 0.1.In our work we report how the addition of TeO2 lowers the dielectric loss and, although there is a decrease of dielectric constant of doped samples relatively to the undoped one, high dielectric constant values are still being reached. The sample of doped CCTO with 1.5% of TeO2 by weight, presents, at room temperature and 60 kHz, a large dielectric constant, over 3000, and a dissipation factor around 0.09, which represents a decrease on tan δ over 30% relatively to the CCTO undoped sample. Two relaxation processes were identified for all the samples, one at MHz region and the other one at low frequency region (< 1 kHz). DC bias voltage was applied up to 40 V and a strong dc bias influence on the low frequency region was observed both at dielectric and impedance responses of the undoped sample, which was much weaker than the dc bias effects on the 4% Te doped sample.Dielectric measurements will be discussed and correlated with the samples' microstructure, supported on internal barrier layer capacitance (IBLC) and surface barrier layer capacitance (SBLC) models. 相似文献
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P. W. Mokruchnikov 《Crystal Research and Technology》1999,34(9):1169-1173
This paper regards the interaction of free and forced convection in the Czochralski configuration. A relationship between the crystal rotation rate and growth parameters at which the crystallization front remains flat, at which the crystallization front remains almost flat, has been studied. This relationship is compared with experimental data. 相似文献
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采用液相法制备了纳米ZnO,在沉淀过程中采用乙醇-水混合溶液,煅烧过程中加入碱金属硝酸盐介质,以研究两者对粒径的调控作用和机理.并利用XRD、TEM、BET和PL等测试手段进行产物表征.结果表明:相同条件下,采用NaNO_3和LiNO_3熔盐辅助煅烧后,产物粒径较直接煅烧明显减小;同量的LiNO_3相对于NaNO_3,其产物粒径小于后者;当醇水比例增加时,由于介电常数的减小,产物粒径也明显减小.最后讨论了醇水体系和熔盐介质粒径调控的机理. 相似文献
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以坐标系二次旋转为理论基础研究了PMN-0.33PT晶体三方相的常用压电模量d15、d24、d31、d33、d36和机电耦合系数k15、k24、k31、k33、k36在三维空间的变化规律.通过MATLAB编程求得以上各系数关于坐标系旋转角度的函数表达式,绘制出了它们的三维空间分布图,并一一求得这些系数的最大值,以及与最大值相对应的旋转角.发现除d33和k33外,d31、d36、k31和k36也在空间变化显著.d31和k31旋转后的最大值比原坐标系下的数值分别扩大了15倍和6倍,对应的旋转角分别为(-4.5°/90°)、(22.5°/270°).d36 和k36在原坐标系下不存在,经旋转后的最大值可分别达到1340 pC/N 和0.73.该研究结果对PMN-0.33PT晶体在压电传感器、换能器中的应用具有重要的理论价值. 相似文献
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This paper presents the results of a dielectric spectroscopy study of freeze-dried lactose with a range of moisture contents. Dielectric properties were measured over a wide range of frequency (10−1 to 106 Hz) and temperature (−120 °C to 120 °C). Four relaxation processes were analysed with respect to moisture content and corresponding relaxation mechanisms were suggested. Two processes (γ and β) were observed in the sub-Tg range of temperature and another two processes were observed near to and above the glass transition temperature, Tg. The relatively high-frequency γ-process was ascribed to the mobility of pendant hydroxymethyl groups and exhibited only a weak dependence on moisture content. The most moisture sensitive process was the second sub-Tg (Johari-Goldstein) β-process, whereby the relaxation time changed by 2 orders of magnitude as the moisture was increased by 7%. Also the third process (α-relaxation, near Tg) was sensitive to moisture content and was in good agreement with DSC data measured for freeze-dried lactose. The fourth process was a proton percolation process at the micro-crystals formed at the surface of amorphous particles during heating at the temperatures higher than Tg and shows the moisture dependence. 相似文献
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Influence of boron concentration on the oxidation-induced stacking fault ring in Czochralski silicon crystals 总被引:1,自引:0,他引:1
E. Dornberger D. Gr f M. Suhren U. Lambert P. Wagner F. Dupret W. von Ammon 《Journal of Crystal Growth》1997,180(3-4):343-352
The influence of the boron doping level in the range of 1 × 1015-2 × 1019 cm−3 on the position of the oxidation-induced stacking fault ring (R-OSF) in silicon crystals has been investigated by experiments and numerical simulation. For low boron-doped crystals, the position of the R-OSF is described by a critical value Ccrit defined by the ratio of the pull rate and the temperature gradient in the crystal at the solid/liquid interface. Boron concentrations higher than 1017 cm−3 shift the position of the R-OSF towards the wafer center without change of growth parameters. The critical value Ccrit converts into a function Ccrit(CB, depending linearly on the boron concentration CB. Crystal-originated particles (COP) and gate oxide integrity (GOI) yield distributions which are consistent with the R-OSF pattern. A low COP density and a high GOI yield are observed outside the ring; a high COP density and a medium GOI yield in the inner region bordered by the ring. It is assumed that boron atoms modify the thermodynamical properties of vacancies and self-interstitials. 相似文献
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Fei Duan 《Journal of Non》2011,357(5):1494-1497
The role of calcium oxide (CaO) was investigated in the crystallization of BaO-SrO-TiO2-SiO2 glass. The CaO dopant altered the oriented growth facets of crystal unit cells in the glass-ceramics. The crystallites are acicular in micron scale in the samples having as small as 1% CaO dopant, but are long granular in the nanometer scale in the sample without CaO. The glass-ceramics which have nano crystallites are observed with a less crystallization density qualitatively, a higher transmissivity in the wavelength range of 200-2600 nm, and 30% lower piezoelectric coefficient, d33, at 9.5 ± 0.3 pC/N than those glass-ceramics having CaO in the study. 相似文献
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Jie-Jun Wu Yusuke KatagiriKazuki Okuura Da-Bing LiHideto Miyake Kazumasa Hiramatsu 《Journal of Crystal Growth》2009,311(14):3801-3805
A 4–6 μm thick a-plane (1 1 2¯ 0) AlN was grown on r-plane sapphire substrate by low-pressure hydride vapor phase epitaxy (LP-HVPE), using a direct growth without any nitridation and buffer layer, a single-step nitridation growth, a two-step nitridation growth and a two-step buffer growth method. For the two-step buffer growth procedure, smoother surface is observed with the lower full widths at half maximum (FWHM) of X-ray rocking curves (XRC) compared with the other two kinds of nitridation procedures. A smaller FWHM of in-plane XRC peak anisotropy features are reversed, which is consistent with the smaller in-plane stress anisotropic distribution in a-plane AlN, when the two-step nitridation or buffer growth method is used. In four kinds of initial growth procedures, the two-step buffer method is the suitable method for the growth of a-plane AlN by HVPE with the high crystal quality and more isotropic distribution. 相似文献
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《Journal of Non》2006,352(42-49):4665-4671
Dielectric relaxation properties of six benzene-derivative liquids were studied in the temperature range above 80 K and in the frequency range (0.1 Hz)–(1 MHz); 3-isopropylphenol and 2-phenyl-1-propanol with hydroxyl group within their substituents, 3-isopropylaniline and dl-1-phenylethylamine with amine group, and isopropylbenzene and sec-butylbenzene only with alkyl group. Heat capacities were additionally measured of sec-butylbenzene in the range 10–300 K. The β relaxation process as well as the α one was found to be present in all the substances. A striking contrast was found between the substituent dependences of α and β glass transition temperatures Tgα and Tgβ. While the Tgα was considerably high in the substances with hydroxyl group, a little lower in those with amine group than with hydroxyl group, and relatively low in those with only alkyl group as the substituent, the Tgβ was essentially the same being in the range 73–77 K irrespective of the presence/absence of hydroxyl or amine groups within the substituents of a limited size of isopropyl group. In 2-phenyl-1-propanol and sec-butylbenzene the substituents of which have hydroxyl or methyl groups, respectively, extruding beyond the size, the bulkiness appears to raise the Tgβ to 100–105 K. The character is discussed based on the heterogeneous structure originating from short-range ordering in molecular aggregation. 相似文献
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Heterojunction diodes fabricated by plasma enhanced chemical vapor deposition of hydrogenated amorphous carbon (a-C:H) and fluorine-doped amorphous carbons (a-C:H:F) on p-type silicon are analyzed in terms of their electronic and photovoltaic properties. Their structural and optical properties were identified by Raman spectroscopy, X-ray photoelectron spectroscopy, ellipsometry, and UV-VIS transmittance. The nature of the heterojunction is confirmed by the rectifying current-voltage characteristic of carbonaceous deposits/p-Si junction. The diodes show a behavior dependent on the amount of the fluorine content. The photovoltaic behavior of the junction is investigated as a function of both fluorine incorporation and thermal treatment of the a-C:H:F films after the deposition. Better photovoltaic effects were observed from annealed a-C:H:F heterojunction structures. The optical and structural characterization performed on films after the thermal treatment indicates that this behavior is most likely due to an extended graphitization. 相似文献