Thiophenes from Dienediones

Our previous studies have demonstrate the usefulness of dienedione as versatile intermediates for the synthesis of cyclopentenone and arylfuran. The dienedione moiety may serve as a four carbon unit which can undergo consecutive nucleophilic addition reaction to form heterocycles. Herein we report our investigation on the addition of sulfur to the four carbon unit of dienedione for the synthesis of 2,5-disubstituted thiophenes. The regioselectivity of the reaction under various conditions will also be discussed.     

Transient laminar natural convection in a partitioned enclosure heated by uniform flux

Transient laminar natural convection in an enclosure partitioned by an adiabatic baffle is investigated numerically. The enclosure is heated by uniform heat flux from left wall and cooled from right wall which is kept at isothermal. A penalty finite element method with Newton-Raphson iteration algorithm and a backward difference scheme dealing with time term are adopted to solve governing equations. The effects of the baffle and Rayleigh number are found to be substantial on heat transfer mechanism during transient process. However, the variations of heat transfer mechanism occur mainly in the first one-third period of the time of transient, in spite of the present or not, or location of a baffle at the conditions of Rayleigh number being 10⁴ and 10⁶ in this study.Transiente, laminare freie Konvektion in einem von einer adiabaten Wand unterteilten Hohlraum wurde numerisch untersucht. Der Hohlraum wird von einem gleichmäßigen Wärmestrom von der linken Wand beheizt und von der rechten isothermen Wand gekühlt. Ein Finite-Element-Verfahren mit dem Newton-Raphson Iterationsalgorithmus und dem Differenzenverfahren mit Zeitterm sind übernommen worden, um bestehende Gleichungen zu lösen. Die Trennwand und die Rayleigh-Zahl sind für den Wärmeübergangsmechanismus während der transienten Phase von wesentlicher Bedeutung. Für den in dieser Studie untersuchten Bereich der Rayleigh-Zahl von 10⁴ und 10⁶ treten Veränderungen im Wärmeübergangsmechanismus hauptsächlich im ersten Drittel der transienten Phase auf und sind unabhängig von der Anwesenheit und Plazierung der Trennwand.     

Water States in Perfluorosulfonic Acid Membranes Using Differential Scanning Calorimetry

The water states in perfluorosulfonic acid membranes (Nafion®) were evaluated using low temperature differential scanning calorimetry (DSC) on both vapor and liquid penetrants. At low sorption levels, water sorbed in Nafion existed in the nonfreezable bound state until a critical value was reached. The critical, nonfreezable water content corresponded to 4.8 water molecules per sulfonate group. Beyond that critical value, additional sorbed water was partitioned between freezable and nonfreezable bound states. The freezable water was in intermediate or freezable bound water state with subzero fussion temperatures. The observed water fusion enthalpy for every additional gram of sorbed water was less than that of pure water. The partition coefficient (K) between the nonfreezable bound state and the freezable state water was estimated as 0.755. The critical nonfreezable water content (W _c ) and the partition coefficient for vapor permeate in Nafion were similar to those of liquid water penetrant. These findings allow one to estimate the bound and freezable water distribution in Nafion. The K value, in conjunction with the critical water content (W _c ), can be used as a quantitative indicator to characterize water states in ionomers. This model may serve as the basis to account for water transport, ionic conductivity, and proton transfer changes in various solid electrolyte membranes.     

低密度泡沫金激光-X射线转换特性模拟研究

在激光间接驱动惯性约束聚变中, 激光首先与黑腔壁高Z等离子体相互作用转换成强X射线辐射, 再通过高Z腔壁的X射线再辐射而在靶丸表面产生对称辐射以驱动其内爆, 改善腔中激光–X射线转换特性非常重要. 利用一维辐射流体程序模拟研究了低密度泡沫金对激光–X射线转换特性的影响, 结果表明: 在固定激光参数条件下, 随着Au材料密度降低, 激光–X射线转换效率提高, 当泡沫Au密度为0.1 g/cm³时, 转换效率相对提高19%; 同时, 金M带辐射份额随之减少; 对于发光区运动, 存在合适的泡沫Au密度使其得到有效抑制. 从能量平衡的角度分析了转换效率提高的原因: 在激光与低密度泡沫Au作用时, 转换为流体力学动能损耗的能量份额与固体Au相比有所降低, 因而相应的辐射能份额增加. 低密度泡沫Au改善激光–X射线转换特性是实现黑腔腔壁优化的一种途径, 模拟结果为进一步开展相应实验研究提供了依据. 关键词： 泡沫金 激光-X射线转换 辐射谱 等离子体运动     

一维时空分辨x射线辐射功率测量系统

采用狭逢、红光闪烁体、光纤阵列、条纹相机、像增强器等元件组成x射线辐射功率时空分辨测量系统，该系统具有对x射线能量响应平坦，可有效屏蔽本底光干扰的优点，在喷气Z pinch实验中成功获得内爆等离子体x射线辐射的轴向分布图像.文中对系统结构、参数和使用情况进行了介绍，对测量结果进行了讨论. 关键词： Z pinch x射线 时空分辨 图像     

Efficient Load Transfer to Polymer‐Grafted Multiwalled Carbon Nanotubes in Polymer Composites

Multiwalled carbon nanotubes (MWNTs) grafted with poly(methyl methacrylate) (PMMA) were synthesized by emulsion reactions and used as a reinforcement for commercial PMMA. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the applied tensile load on the composites was transferred to the PMMA‐grafted MWNTs, leading to a strain failure of the MWNTs rather than an adhesive failure between the MWNTs and the matrix. Dynamic mechanical analysis (DMA) data showed that the storage modulus at 20 °C of the PMMA composite containing 20 wt.‐% of the PMMA‐grafted MWNTs was significantly enhanced by ～ 29 GPa (or by ～ 1100 %) as compared with commercial PMMA.     

Synthesis and characterization of novel low‐dielectric cyanate esters

For the purpose of increasing the mobility of residual bisphenol A dicyanate ester (BADCY) during the final stage of curing and achieving a complete reaction of cyanate groups, a small quantity of monofunctional phenol was added to BADCY to form an imidocarbonate, or a small quantity of monofunctional cyanate esters was added to form cyanate ester copolymers. The proposed structures were confirmed with Fourier transform infrared, elemental analysis, mass spectrometry, and NMR spectroscopy. The thermal properties of the cured cyanate esters were measured with dynamic mechanical analysis, thermogravimetric analysis, and dielectric analysis. These data were compared with those for the cured BADCY resin. The cured modified cyanate esters exhibited a lower dielectric constant, a lower dissipation factor, and lower moisture absorption than the cured BADCY system. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2589–2600, 2004     

Weighted switching lifted digital redesign with state saturation for cascaded analogue systems

This paper presents the newly developed digital redesign controlscheme with a weighted switching strategy and lifted digitalcontrol, for a cascaded analogue system with state saturation.This method improves the generally poor transition responsecaused by state-saturation constraints. It replaces an existingor designed analogue controller with state saturation, by adigital controller with almost identical performance.