浅谈变电运行中等电位倒闸操作的应用

倒闸操作是变电运行的重点和难点工作之一,倒闸操作的正确性直接关系到操作人员的人身安全及设备的安全运行,如果操作不当,将会导致整个电力系统处于崩溃的边缘,其重要性不言而喻。分析了倒闸操作的基本原理和存在的问题,并探讨了变电运行中等电位倒闸操作的具体程序。     

Damping of vinyl acetate–n‐butyl acrylate copolymers

A photopolymerization process at room temperature was devised to copolymerize vinyl acetate (VAc) and n‐butyl acrylate (BA) mainly to prepare rubber‐like damping sheet bearing pressure‐sensitive adhesive property in this study. The investigations using both the differential scanning calorimeter and rheometric dynamic analysis show the existence of two glass transition temperatures for each copolymer. The scanning electron microscopic pictures reveal that the degree of microphase separation increases with increasing annealing time at 70°C. It was suggested that the rubbery domain (formed by the PBA blocks) disperses in the glassy domain (constituted by the PVAc blocks), making an effective damping entity. Excellent damping was observed for the copolymer samples, with the tanδ peak values as high as 1.76–1.80 at a certain temperature range and with tanδ> 0.3 at quite wide temperature ranges. In addition, the copolymers containing more VAc tend to have the higher damping. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1396–1403, 2004     

智能手机点击劫持攻击检测方法研究

点击劫持通过欺骗用户点击经过伪装的界面元素达到攻击目的。移动互联网环境下,智能手机的屏幕特征、手势识别、HTML5支持度高等特性成为点击劫持新的利用点。深入分析并实验验证智能手机脆弱特性,在此基础上提出点击劫持在智能手机上的潜在攻击方案。进而设计并实现一套有针对性的检测方案。该方案从静态页面和动态行为两个角度提取攻击特征,并进行基于规则的量化评估与组合判定。实验结果表明该方案可以有效地降低传统页面特征检测方案的漏报和误报情况。     

Polyimide foams with ultralow dielectric constants

To explore ultralow dielectric constant polyimide, the crosslinked polyimide foams (PIFs) were prepared from 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (BTDA), 4,4′‐oxydianiline (ODA), and 2,4,6‐triaminopyrimidine (TAP) via a poly(ester–amine salt) (PEAS) process. FTIR measurements indicated that TAP did not yield a negative effect on imidization of PEAS precursors. SEM measurement revealed the homogeneous cell structure. Through using TAP as a crosslinking monomer, the mechanical properties of PIFs could be improved in comparison with uncrosslinked BTDA/ODA based PIF. The crosslinked PIFs still exhibited excellent thermal stability with 5% weight loss temperatures higher than 520°C. In the field with frequency higher than 100 Hz, the dielectric constants of the obtained PIFs ranged from 1.77 to 2.4, and the dielectric losses were smaller than 3 × 10^?2 at 25–150°C. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1734–1740, 2006     

Effect of incipient removal of hydrogen through palladium membrane on the conversion of methane steam reforming: Experimental and modeling

This paper presents a mathematical model based on the reaction rate expressions to describe the displacement of methane conversion in the steam reforming. The effect of several parameters including weight hourly space velocity (WHSV), load-to-surface ratio, reaction pressure, hydrogen partial pressure in permeate side and reaction temperature were investigated. Simulation and experimental results showed that a conversion higher than 80% could be achieved in a palladium membrane reactor at reaction temperature of 500 °C relative to 850 °C in a conventional fixed bed reactor (FBR). Besides, the yield of CO (<2%) in membrane reactor was much lower than that (>50%) in the FBR, which indicated the significant depression of CO production in use of membrane reactor.     

Crystallization and morphological features of syndiotactic polystyrene induced from glassy state

Spherulitic growth rates and microstructure of syndiotactic polystyrene (sPS) cold-crystallized isothermally at various temperatures, T_c (115–240 °C), have been investigated by small-angle light scattering (SALS), optical microscopy and transmission electron microscopy. The derived activation energy for sPS chain mobility at the crystal growing front is 5.4 kJ/mol, which is relatively lower than that of isotactic polystyrene, 6.5 kJ/mol. In addition, the Hv scattering invariant (Q_Hv) measured by SALS on the crystallized sPS samples displays a pronounced minimum at 150 °C. Despite a wide range of T_c used, however, the sample crystallinity estimated by Fourier transformation infrared spectroscopy remains unchanged. Prior to crystallization, the correlation length derived from the Vv patterns on the basis of Debye–Bueche model is ca. 1.13 μm regardless of T_c used. Interconnected domains with a width of ca. 1.8±0.5 μm are readily observed in all the crystallized samples under phase contrast microscopy and the phase-separated structure is conserved within sPS spherulites whose diameters are increased with increasing T_c.

Based on the above facts, we conclude that the presence of a Q_Hv minimum is ascribed to the resultant events of the two competitive transitions i.e. liquid–solid crystallization, and liquid–liquid demixing resulting from the spinodal decomposition (SD). At lower T_c, the unstable SD transition overwhelms the crystallization. Despite the low chain mobility, the coarsening process driven by the interfacial energies has reached a certain level before crystalline nucleation takes place. At higher T_c, on the other hand, cold crystallization becomes the dominant process due to the enhanced chain mobility, leading to the suppression of ongoing SD coarsening process. At an intermediate T_c range, comparable competition of the phase separation and crystallization prohibits the development of ordered symmetry within spherulites, giving the presence of Q_Hv minimum.     

Identification of organochlorines and organobromines in coals

Four Chinese bituminous coals were extracted with CS₂, n-hexane, benzene, methanol, acetone, tetrahydrofuran (THF) and THF/methanol (1: 3 vol/vol) mixed solvent sequentially. The resulting 28 extracts were analyzed with GC/MS. Six organochlorines (OCs) and two organobromines (OBs) were identified in eight extracts from the coals. Our experiments provide, for the first time, the information on the molecular structure of OCs and OBs in coals.     

浅析计算机网络信息安全问题及对策

随着IT应用在生活中的日益普及,互联网已经覆盖了175个国家和地区的数亿台计算机,用户数量超过lO亿计算机技术广泛应用于各行各业,向着深度和广度不断地发展,构成了各自的网络,为资源共享、信息交换和分布处理提供了良好的环境,企业上网、政府上网、学校用网以及网上购物等计算机的应用表现了一个网络化的社会已经呈现在我们的面前、因此,在增加网络实用性的同时,系统的脆弱性不可避免的显露了出来,使计算机网络面临严重的安全问题。!     

LIQUID PHASE TRANSPORT PROPERTIES IN A HIGH PRESSURE PACKED COLUMN

A mathematical model of fluid flow and mass transfer in a packed bed was derived and used to evaluate the liquid phase axial dispersion and mass transfer coefficients under high pressure conditions. The least-squares method was used to evaluate the rate parameters from experimental breakthrough curves, and the agreement between the concentration curves predicted from rate parameters and those measured experimentally was good. Experiments were performed at 20 and 200°C with water as a solvent and nonporous soda-lime glass beads as packing. Although the axial dispersion coefficient was independent of temperature and pressure, the mass transport parameters were found to be pressure dependent.     

Manipulating the microstructure and rheology in polymer‐organoclay composites

A series of nanocomposites prepared by melt‐blending of cloisite‐based organoclays with poly(ethylene‐vinylacetate) (EVA) and neutralized poly (ethylene‐methacrylic acid) (EMA) copolymers were investigated via DSC, small‐angle X‐ray scattering (SAXS), and rheological techniques. SAXS results indicated partial clay exfoliation in all samples. In both EMA and EVA systems, the nominal melting temperature T_m and bulk crystallinity are not significantly affected by the presence of organoclays, suggesting that clay particles are predominantly confined in the amorphous phase. In rheological measurements (above T_m), the EVA‐clay system demonstrated a solid‐like rheological behavior under the small‐strain oscillatory shear, yet it was able to yield and flow under a steady shear, which is the characteristic of physical crosslinking. In contrast, the EMA‐clay system exhibited a melt‐like rheological behavior, where the influence of organoclay on the thermorheological behavior of the EMA composite was quite minimal. We propose that the carbonyl groups of vinylacetate in EVA interact with the clay surface, resulting in a strong physically crosslinking like interaction in the melt. On the other hand, the interaction between EMA and clay is weak because of repulsion between carboxyl anions and negatively charged clay surface.