电视用液晶材料的研究进展

液晶电视由于其优异的显示性能将会越来越受到青睐。本文简要综述了近年来电视用液晶材料的研究进展,归纳总结了这些液晶材料的合成方法及其热性能、介电各向异性、双折射率、粘度等特性。     

2.45GHz下常用有机试剂复介电常数的测量与研究

1986年加拿大的R．Gedye和R．J．Giguere等发现了微波可以显著加快有机合成，他们发现用微波辐射可使反应速率和产率有不同程度的提高。由于微波作用机理的特殊性，微波化学对很多化学领域带来了冲击。但是，微波与化学反应体系之间相互作用的一些重大问题还未得到解决，如微波加热过程中化学反应系统的非线性反射、非均匀加热等。解决这类问题，首先必须了解化学反应过程中混合物的电学性质与磁学性质，而物质的宏观电学和磁学性质都用其介电常数和磁导率来描述。对于大部分是非磁性材料的有机试剂，微波与反应体系相互作用的特性集中体现在体系的等效复介电常数上。了解各种常用试剂的复介电常数，可以进一步了解各种化学试剂对微波的吸收和反射的情况。而很多试剂的复介电常数无法从现有文献中得到。本文利用谐振腔微扰法测定了在2．45GHz室温下各类常用有机试剂的复介电常数，结果显示：醇类试剂复介电常数实部与虚部都较大；酮类试剂复介电常数实部相对较大，而虚部较小；酸类试剂复介电常数实部和虚部都较小；烷烃和苯类试剂实部，虚部更小。同时，随着碳链增加，所有试剂的复介电常数的实部与虚部均有下降趋势。这些测试和分析结果将为微波辅助有机合成提供了有益参考。     

The dielectric properties of water in its different states of interaction

A tutorial on dielectric (relaxation) spectrometry of liquids is given in this article. Some methods of measuring complex (electric) permittivity spectra are briefly described. Results for water are presented and related to characteristic properties of the liquid structure and to models of the molecular dynamics, particularly as resulting from computer simulation studies. Dielectric spectra for aqueous solutions of low weight electrolytes, polyelectrolytes, small molecules, and polymers are discussed to illustrate effects of kinetic depolarization, structure saturation, as well as positive, negative, and hydrophobic hydration. Reference is also made to fluctuations in the hydrogen bond network of mixtures of water with liquids that are completely miscible with this unique solvent.     

Dielectric relaxation of polystyrene with side-chain- and chain-end-labels

Polystyrenes with different concentrations of side groups with cyano groups were prepared and complex dielectric constants were measured in the range of the glass transition temperature and the frequency range of 10^–2–10⁷ Hz.The GPC and DSC measurements showed that the molecular weight of these polystyrenes was about 10500 g/mole and the glass transition temperatures were 89.5°C for all samples.The dielectric relaxation spectra obtained for the side group polystyrene labels and also the chain-end polystyrene labels prepared before [9] were analyzed to find out the degree of coupling of the chain-end and side-group labels with the cooperative reorientation of the polymeric matrix. The analysis of the spectra was carried out using the analysis method developed by Mansour and Stoll [6].The results obtained showed that both end- and side-group labels are strongly coupled with the segmental reorientation and relax with relaxation times longer than that of the segments.The value of logf _m = (logf _m(label)) – logf _m(matrix)) was obtained from the recently designed comparison diagram suggested by Mansour and Stoll [6, 14]. The value of logf _m depends on the label length in the case of chain-end labels.It was surprising to find that the side groups relax slower than the segments by only 0.9 decades. These results obtained implied that the label relaxes through a multistep relaxation mechanism of the side and end groups and not through a diffusion mechanism of the whole chain. In addition, the effective lengths of the relaxing units were determined using the empirical equation obtained before in the case of rodlike molecules in polyisoprene [7].     

Thermal properties of sodium acetylacetonate

Sodium acetylacetonate was prepared by the interaction of acetyl acetone with sodium hydroxide. The thermal conductivity, phonon velocity, mean free path, Yong's modulus, and the thermal expansion coefficient were studied. The thermal conductivity of the material decreases with increasing temperature due to the thermal lattice scattering of phonons. The velocity of phonons is also decreased due to the perturbation of thermal phonons. The linear thermal expansion coefficient increases with temperature due to the weakness of the attractive forces between the small Na⁺ cations and bulkier acetylacetonate anions in the lattice.

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Zusammenfassung Natriumacetylacetonat wurde durch die Wechselwirkung zwischen Acetylaceton und Natriumhydroxid gewonnen. Es wurde die Wärmeleitfähigkeit, die Phononengeschwindigkeit, die mittlere freie Wegstrecke, das Elastizitätsmodul und der Wärmeausdehnungskoeffizient untersucht. Die Wärmeleitfähigkeit der Substanz nimmt wegen der thermischen Gitterstreuung der Phononen mit steigender Temparatur ab. Die Geschwindigkeit der Phononen nimmt auf Grund der Perturbierung der thermischen Phononen ebenfalls ab. Der lineare Wärmeausdehnungskoeffizient steigt mit zunehmender Temperatur an, was auf die schwachen Anziehungskräfte zwischen den kleinen Na⁺ Ionen und den massigen Acetylacetonat-Anionen im Gitter zurückzuführen ist.

     

Sol Gel Processing of Low Permittivity,Fiber-Reinforced SiO2-Polyimide Substrates

Low permittivity ceramic substrates with a sandwich structure consisting of a porous, fiber-reinforced SiO₂ core and two thin polymer plates were fabricated by sol-gel processing, and polymer infiltration. The rheological behavior of an aqueous colloidal SiO₂ sol, containing short SiO₂ fibers was studied as a function of particle loading, fiber loading and gelation time. Short SiO₂ fibers were introduced to limit drying shrinkage and thus minimize substrate cracking during drying. After the psuedoplastic sol was tape cast and sintered at 1150°C for 2 h, a polyimide solution was infiltrated into the porous SiO₂ core. Permittivities ranging from 1.81 to 3.08 could be obtained by using 32–42% SiO₂, 5–50% polyimide and 63 to 8% porosity. The substrate strength was increased from 1.93 MPa to 3.83 MPa after polyimide infiltration.     

由聚苯胺粒子组成的电流变液的研究

由经碱处理的掺杂态聚苯胺（ＰＡｎ）制得高介电常数的半导体ＰＡｎ粒子，将其悬浮于电绝缘油可组成电流变液，讨论了电流变（ＥＲ）液的静态屈服应力（τ＿ｓ），电流密度等性能与ＰＡｎ的介电常数（ε＿ｐ），导电率（σ），体积分数和应用电场强度的关系。对导电率相同的ＰＡｎ粒子，用氨水处理的ＰＡｎ粒子ε＿ｐ较用ＮａＯＨ液处理的高，前者在σ足够低如σ≤１．０×１０￣（－７）／ｃｍ时，ＥＲ液的τ＿ｓ随偶极系数的平方（β￣２）的增大而呈现非线性增加；后者ＥＲ液的τ＿ｓ随β￣２的增大出现一最大值．结果表明：由聚苯胺粒子可组成高电流变活性的无水ＥＲ液．     

Measurement of moisture in grains at extreme temperatures

Summary This research has demonstrated that the dielectric method can be used successfully for grain moisture measurements for diverse grain types over wide ranges of temperatures if the grain moisture content is below certain (grain-specific) threshold values. These high-moisture limits were estimated. Optimum temperature correction coefficients for 149 MHz moisture measurements were determined for grain samples at different moisture levels. This information should be very helpful for grain moisture meter manufacturers and for moisture meter users who need to determine grain moisture contents at temperature extremes to be able to market grain more efficiently.     

Dielectric studies of binary mixtures of n -propyl alcohol and ethylenediamine

Dielectric constant (ε’) and dielectric loss (ε") of n-propyl alcohol (PA), ethylenediamine (EDA) and their binary mixtures, for different mole fractions of ethylenediamine have been experimentally measured at 11.15 GHz microwave frequency. Values of density (ρ), viscosity (η) and square refractive index (n _D ² ) of binary mixtures as well as those of pure liquids are reported. Excess square refractive index, viscosity and activation energy of viscous flow have also been estimated. These parameters have been used to explain the formation of complexes in the system.     

Dielectric relaxation of electrolyte solutions in acetonitrile

Complex permittivity spectra in the frequency range 0.95v (GHz)89 for acetonitrile and its solutions of LiBr, NaI, NaClO₄, and Bu₄NBr at 25°C show one Debye equation for the neat solvent whereas the superposition of a Debye process for the solute and a Cole-Cole distribution for the solvent is necessary to account for the dielectric relaxation behavior of the solutions. The reorientation of bulk acetonitrile is diffusive and only weakly coupled to viscosity. The number of solvent molecules irrotationally bound to the electrolyte is in good agreement with conventional solvation numbers for all electrolytes, when kinetic depolarization is assumed to be negligible. The solute relaxation process is dominated by the formation kinetics and reorientation of contact ion pairs. There is evidence for solvent-shared ion pairs in dilute NaClO₄ solutions.