压力驱动膜过程中浓差极化的介电模型

提出了电导率呈指数型分布的浓差极化层的介电模型. 利用传递方程研究了压力驱动膜过程的介电谱, 并导出层内电导率分布与宏观测量的电容和电导之间定量关系的理论表达式, 得到介电谱高低频特征值与浓差极化模数之间的关系. 计算结果表明, 较之线性分布电导率, 指数型分布电导率能更好地描述浓差极化层的介电谱. 研究表明, 介电谱及其特征值和弛豫强度随浓差极化模数和层厚度显著变化. 对由分离膜、主体溶液和浓差极化层组成的复合体系的介电谱进行了数值模拟, 发现电导较之电容其频率依存关系对浓差极化模数的变化更为敏感, 上述研究为利用介电谱在线监测膜分离过程提供了有价值的理论参考.     

壳聚糖膜对水杨酸吸附及释放过程的实时介电谱法研究

制备了壳聚糖膜,并且测量了壳聚糖膜/水杨酸溶液、含水杨酸的壳聚糖膜/蒸馏水两个膜/液体系的介电谱,在100～700 kHz频率范围,发现两种体系都存在显著的介电弛豫现象,而且该弛豫的特征参数随壳聚糖膜在相应溶液相中浸泡时间的不同而变化.将该体系介电模型化、并利用在Maxwell-Wagner界面极化理论上建立的公式对介电谱进行解析,得到了体系中两相--壳聚糖膜相和溶液相的介电常数和电导率随时间的变化曲线.通过分析电导变化曲线得知,壳聚糖膜对溶液中的水杨酸有吸附现象,而含有水杨酸的壳聚糖膜在蒸馏水中可以缓慢释放出水杨酸;吸附过程包含的机制有氢键的形成、疏水力和扩散;而水杨酸的脱附过程则包含吸附过程几种机制的逆反应.理论分析和解析结果都验证了介电谱方法对吸附和释放过程实时监测的有效性.     

膜/液界面浓度极化现象的介电解析

对由强、弱荷电膜和溶液构成的膜／液非均匀体系，在10－107Hz频率范围进行了介电测量，在直流偏压下，该体系显示了二个显著的介电弛豫．利用具有电导率分布相，即浓度极化相的介电理论对强行电的离子交换膜的结果进行了介电解析，从实验上测得的介电参数求出了反映膜／液界面浓度极化层构造的参数，讨论了该体系产生介电弛像的原因、说明了介电理论的合理性，并提出了膜／液界面体系产生介电弛豫现象的可能机理．     

聚吡咯/聚苯乙烯混合膜/电解质溶液体系的介电谱解析电解质种类和浓度对膜透过性的影响

测量了混入聚吡咯粒子的聚苯乙烯混合膜在不同浓度的氯化钠、氯化钾、氯化钙及氯化镁体系的介电谱, 在40 Hz~11 MHz, 发现了显著的双介电弛豫现象(Double-relaxation phenomena). 基于Maxwell-Wagner理论讨论了弛豫产生的机制, 将体系进行了模型化, 并利用Hanai理论方法对介电谱进行了解析, 从获得的内部参数对混合后的膜荷电性质以及混合膜对离子的透过性进行了详细的讨论, 进而利用Donnan平衡理论推导的膜电位的表达式拟合, 获得了混合膜的固定电荷等参数.     

聚吡咯微粒掺入的聚苯乙烯膜在电解质溶液中的介电弛豫谱及其解析

在40 Hz～11 MHz频率范围测量了聚苯乙烯膜以及混入聚吡咯粒子的聚苯乙烯膜和电解质溶液构成的体系的介电谱, 发现了特异的弛豫现象: 纯的和掺入导电性聚吡咯后的聚苯乙烯膜分别显示出单一弛豫和双弛豫的不同模式的介电谱. 在Maxwell-Wagner界面极化概念基础上解释了该弛豫机制: 高、低频弛豫分别由膜-液界面极化和膜相本身的不均一性引起的. 将体系进行了模型化, 并利用Hanai理论方法对谱进行了解析, 获得了内部电性质的诸多参数. 对不同聚吡咯掺入量的膜/溶液体系的介电测量和解析结果表明, 电解质溶液的种类、浓度以及膜中混入聚吡咯的量都影响着膜相的介电响应. 这些结论为利用加入导电粒子改善绝缘高分子聚合物的电性质的研究以及制备既具有导电功能又使基体的力学性能得到提高的高分子复合物提供了重要的线索.     

闪锌矿MTe (M=Zn/Mg)的几何结构、弹性性质、电子结构和光学性质

采用基于密度泛函理论(DFT)框架下广义梯度近似(GGA)平面波超软赝势(PP-PW)方法, 计算了闪锌矿型MTe (M=Zn/Mg)的几何结构、弹性性质、电子结构和光学性质. 同时采用杂化密度泛函调准了带隙. 结果表明, 立方相ZnTe和MgTe均为直接带隙半导体材料. 所得晶格参数、弹性常数及体模量与实验数据基本吻合. 由弹性常数推导出ZnTe、MgTe的德拜温度分别为758、585 K. 研究了MTe的复介电函数、折射率、反射率和能量损失系数等光学性质, 并基于电子能带结构和态密度对光学性质进行了解释.     

平面结构非均匀体系的介电驰豫─—组成相的数目与电导对弛豫的影响

非均匀体系在０．１ｋＨｚ～１０ＭＨｚ频率范围显示出显著的介电弛豫,为了弄清介电弛豫的原因以及该弛豫与非均匀体系界面数目的关系,本文从介电的观点在理论及实验两方面讨论了典型的非均匀体系的例子：膜／溶液体系。特别地,对频率域的介电弛豫谱与组成相的结构及浓度的关系进行了分析,并在Ｍａｘｗｅｌｌ－Ｗａｇｎｅｒ机理的基础上现象论地解释了弛豫的原因。非均匀体系的介电弛豫性质具有下列特征：（１）介电弛豫数目等于体系内界面种类的数目;（２）特征弛豫频率紧密地依存于溶液相的电导。     

壳聚糖微球悬浊液的介电谱研究: 弛豫机制的解释和内部信息的获得

结合DLVO理论和壳聚糖微球的特点, 提出了适合实验数据的电动力学模型, 并分析了浓厚分散系的界面极化弛豫的微观机制. 进而测量了不同粒径壳聚糖微球悬浊液的介电谱, 发现在10～100 MHz频率范围内均出现明显的介电弛豫现象. 利用上述模型合理解释了该弛豫现象以及微球粒径对界面极化弛豫的影响, 结果也证明了我们提出的模型的适当性. 此外, 利用Hanai方法, 通过介电参数准确计算出了各悬浊液的内部信息, 并且分析了这些实时信息的合理性. 研究结果从实验和理论两方面展示了介电谱方法在即时获取壳聚糖内部信息上的独到优势.     

球壳粒子悬浮系模型化研究——内部相参数对介电谱影响的数值模拟

在Hanai理论基础上对球壳粒子悬浮系的介电模型进行了模拟研究,通过用C++的复数类对理论公式的程序化,建立了介电谱的介电参数与体系内部相参数的关系.所得的解析解可方便地模拟介电弛豫谱依不同相参数的变化曲线,计算并分析了内部参数对介电谱的模式以及介电参数的影响因素.     

介电常数对Stokes频移时间相关性的影响

基于连续介质理论,利用溶剂化能计算了香豆素水溶液单球孔穴模型的Stokes频移响应函数. 将水的介电谱实验数据拟合进而得到介电常数理论公式, 并基于此计算了相关函数. 在考虑了延迟时间后, 相关函数的理论计算结果与实验吻合. 考虑到低频响应在溶剂化驰豫中的重要性, 考察了低频极化对响应函数的影响.     

Improved Approximation of Water Dielectric Permittivity for Calculation of Hamaker Constants

Due to the highly polar nature with a multipeak absorption spectrum of water, the contribution of the relaxation in the microwave and infrared regions to the water dielectric spectrum is significant. The old data obtained by the Cauchy plot analysis of the parameters of the single-relaxation representation of water dielectric spectrum produce the discrepancy in the Hamaker constants computed by the complete continuum theory. New data are obtained by the direct fitting of the single-relaxation model to the complete water dielectric spectrum. The Hamaker constants computed using the improved approximate and the complete spectra for water permittivity are in good agreement. The Hamaker function of quartz-water-quartz and quartz-water-air systems computed using the improved approximation for water and the Cauchy plot approximation for quartz also agrees with that computed using the complete spectrum for both liquid water and crystalline quartz. The new data are to be used, instead of the old Cauchy plot analysis data, in the calculation of the van der Waals interaction across water films based on the available simplified expressions. Copyright 2000 Academic Press.     

Elastic, dielectric and optical constants of 4'-pentyl-4-cyanobiphenyl

4'-n-Pentyl-4-cyanobiphenyl (5CB) is a room temperature nematic liquid crystal with a high positive dielectric anisotropy and a high chemical stability. Many experimental results concerning the elastic and dielectric constants of 5CB are available in the literature, although there is often no satisfactory agreement between the experimental data obtained by different groups, especially as far as the dielectric constants are concerned. Furthermore, no detailed investigation of the temperature dependence of the elastic and dielectric constants close to the nematic-isotropic transition temperature T_NI has yet been reported. In this paper, we report the measurement of the elastic and dielectric constants of 5CB, and the temperature behaviour close to T_NI has been investigated in detail. The experiment consists in the measurement of the director deformation induced by an electric field using simultaneously both a dielectric and an optical method. The simultaneous use of these two methods provides an indirect check on the reliability of the measurements. Special attention has been devoted to control possible sources of uncertainty. In particular, the effects of finite anchoring energy and of finite pretilt angle have been considered. The temperature dependence of the anisotropy of the refractive indices is also obtained in the experiment.     

Elastic,dielectric and optical constants of 4'-pentyl-4-cyanobiphenyl

4'-n-Pentyl-4-cyanobiphenyl (5CB) is a room temperature nematic liquid crystal with a high positive dielectric anisotropy and a high chemical stability. Many experimental results concerning the elastic and dielectric constants of 5CB are available in the literature, although there is often no satisfactory agreement between the experimental data obtained by different groups, especially as far as the dielectric constants are concerned. Furthermore, no detailed investigation of the temperature dependence of the elastic and dielectric constants close to the nematic-isotropic transition temperature T _NI has yet been reported. In this paper, we report the measurement of the elastic and dielectric constants of 5CB, and the temperature behaviour close to T _NI has been investigated in detail. The experiment consists in the measurement of the director deformation induced by an electric field using simultaneously both a dielectric and an optical method. The simultaneous use of these two methods provides an indirect check on the reliability of the measurements. Special attention has been devoted to control possible sources of uncertainty. In particular, the effects of finite anchoring energy and of finite pretilt angle have been considered. The temperature dependence of the anisotropy of the refractive indices is also obtained in the experiment.     

First-principles study of structural,elastic, electronic and optical properties of perovskites XCaH3 (X = Cs and Rb) under pressure

The stability, structural parameters, elastic constants, electronic and optical properties of perovskites CsCaH₃ and RbCaH₃ were investigated by the density functional theory. The calculated lattice parameters are in agreement with previous calculation and experimental data. The energy band structures, density of states, born-effective-charge and Mulliken charge population were obtained. The perovskites CsCaH₃ and RbCaH₃ present a direct band gap of 3.15 eV and 3.27 eV at equilibrium. The top of the valence bands reflects the s electronic character for both structures. Furthermore, the absorption spectrum, refractive index, extinction coefficient, reflectivity, energy-loss spectrum, and dielectric function were calculated. The origin of the spectral peaks was interpreted based on the electronic structures. The static dielectric constant and refractive index are indeed, inverse proportional to the direct band gap.     

The calculation of hamaker constants from liftshitz theory with applications to wetting phenomena

The physical signifance of the function ? (iξ) for a dielectric material is discussed together with the rationale for the construction of ? (iξ) from dielectric data. The utility of refractive index/wavelength data in the visible region in providing the crucial unltraviolet relaxation frequency and oscillator strength is demonstrated. A variety of real systems is analysed to obtain the data necessary for the construction of ?(iξ). Hamaker constants for these materials are computed.The use of these Hamaker constants in describing wetting phenomena is discussed with examples.     

Sensitivity of metal nanoparticle surface plasmon resonance to the dielectric environment

Electrodynamic simulations of gold nanoparticle spectra were used to investigate the sensitivity of localized surface plasmon band position to the refractive index, n, of the medium for nanoparticles of various shapes and nanoshells of various structures. Among single-component nanoparticles less than 130 nm in size, sensitivities of dipole resonance positions to bulk refractive index are found to depend only upon the wavelength of the resonance and the dielectric properties of the metal and the medium. Among particle plasmons that peak in the frequency range where the real part of the metal dielectric function varies linearly with wavelength and the imaginary part is small and slowly varying, the sensitivity of the peak wavelength, lambda, to refractive index, n, is found to be a linearly increasing function of lambda, regardless of the structural features of the particle that determine lambda. Quasistatic theory is used to derive an analytical expression for the refractive index sensitivity of small particle plasmon peaks. Through this analysis, the dependence of sensitivity on band position is found to be determined by the wavelength dependence of the real part, epsilon', of the particle dielectric function, and the sensitivity results are found to extend to all particles with resonance conditions of the form, epsilon' = -2chin(2), where chi is a function of geometric parameters and other constants. The sensitivity results observed using accurate computational methods for dipolar plasmon bands of gold nanodisks, nanorods, and hollow nanoshells extend, therefore, to particles of other shapes (such as hexagonal and chopped tetrahedral), composed of other metals, and to higher-order modes. The bulk refractive index sensitivity yielded by the theory serves as an upper bound to sensitivities of nanoparticles on dielectric substrates and sensitivities of nanoparticles to local refractive index changes, such as those associated with biomolecule sensing.     

Acetonitrile + nitromethane: An example of ideal solvent mixtures

Densities, viscosities, static dielectric constants and refractive indices of acetonitrile + nitromethane mixtures in the temperature range between 5 and 45°C are reported. Density and viscosity data, analyzed in terms of molar excess volumes and molar excess fluidities, show a weak interactions field between acetonitrile and nitromethane molecules. Lorentz-Lorenz molar refractivities and Kirkwood correlation factors, obtained from refractive indices and dielectric constants data, respectively, indicate that such weak interactions can be atributed to an antiparallel alignment between neighboring dipoles of unlike solvent molecules. Such specific features do not significantly perturb the local structures of the pure components so that acetonitrile + nitromethane mixtures can be considered, in a first approximation, one of the few ideal solvent mixtures for which the physical properties can be determined by applying the additivity rules to the pure components.     

Polyarylene Ether Nitrile and Titanium Dioxide Hybrids as Thermal Resistant Dielectrics

With the expanding application of capacitors, thermal resistant dielectric materials are in high demand due to the increasing harsh environments where the capacitors are needed and the heat generated by the capacitors. Herein, we present polyarylene ether nitrile and titanium dioxide hybrids which can be used as thermal resistant dielectrics for these capacitors. Phthalonitrile modified titanium dioxide (TiO_2-CN) and phthalonitrile end-capped polyarylene ether nitrile (PEN-Ph) are firstly prepared. After being cast into TiO_2-CN/PEN nanocomposite films,these composites self-crosslink upon heating at 320℃ for 4 h, forming the polyarylene ether nitrile and titanium dioxide hybrids (TiO_2-PEN).Improved dielectric constants which are stable from room temperature to 200℃ of these hybrids are observed, indicating the potential application of the hybrids as thermal resistant dielectrics.     

Single oscillator model and refractive index dispersion properties of ternary ZnO films by sol gel method

The optical constants of the NiMnZnO thin films prepared by spin-coating method were determined using transmittance and reflectance spectra. The optical band gap of the NiMnZnO thin films was found to be in the range of 3.20–3.35 eV. The refractive index dispersion of the NiMnZnO films obeys the single oscillator model. The dispersion energy and oscillator energy values of the NiMnZnO films were found to be in the range of 6.46–9.85 and 4.82–5.54 eV, respectively. The real and imaginary parts of the dielectric constants of the NiMnZnO films were determined. The reflectance spectra of the NiMnZnO films give a peak and the presence of this peak is due to the photoexcitation process, in which, the electrons are excited from valence band to the conduction band. The obtained results suggest that the optical constants and refractive dispersion energy parameters of the films are controlled by the molar ratios of NiMnZnO films.     

Study of dielectric constants of binary composites at microwave frequency by mixture laws derived from three basic particle shapes

Powder mixture rules derived from the filler particles with the shapes of sphere, cylinder or rod, and lamella or disk with random distributions are studied in this paper. Three ceramic powders of fillers with dielectric constants of 10, 20, and 36, respectively, are adopted. The experimental dielectric constants of ceramic dispersions in the polyethylene matrix at microwave frequency are compared to those obtained by using different mixing laws. The mixing rules are also adopted to estimate the dielectric constants of pure ceramics from the measured dielectric constants of composites with various concentrations. The theory for the error of estimation is studied. In contrast to the traditional concept of obtaining the best curve fitting of mixture rule with the experimental data, this study conclude a very important concept on the powder mixture rule, that is, the most adequate mixture law for estimating the dielectric constants of pure ceramics requires both good curve fitting and potential of less error.