2‐Cyanoprop‐2‐yl dithiobenzoate mediated reversible addition–fragmentation chain transfer polymerization of acrylonitrile targeting a polymer with a higher molecular weight

The reversible addition–fragmentation chain transfer (RAFT) polymerization of acrylonitrile (AN) mediated by 2‐cyanoprop‐2‐yl dithiobenzoate was first applied to synthesize polyacrylonitrile (PAN) with a high molecular weight up to 32,800 and a polydispersity index as low as 1.29. The key to success was ascribed to the optimization of the experimental conditions to increase the fragmentation reaction efficiency of the intermediate radical. In accordance with the atom transfer radical polymerization of AN, ethylene carbonate was also a better solvent candidate for providing higher controlled/living RAFT polymerization behaviors than dimethylformamide and dimethyl sulfoxide. The various experimental parameters, including the temperature, the molar ratio of dithiobenzoate to the initiator, the molar ratio of the monomer to dithiobenzoate, the monomer concentration, and the addition of the comonomer, were varied to improve the control of the molecular weight and polydispersity index. The molecular weights of PANs were validated by gel permeation chromatography along with a universal calibration procedure and intrinsic viscosity measurements. ¹H NMR analysis confirmed the high chain‐end functionality of the resultant polymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1272–1281, 2007     

Implementation of some higher-order convection schemes on non-uniform grids

A generalized formulation is applied to implement the quadratic upstream interpolation (QUICK) scheme, the second-order upwind (SOU) scheme and the second-order hybrid scheme (SHYBRID) on non-uniform grids. The implementation method is simple. The accuracy and efficiency of these higher-order schemes on non-uniform grids are assessed. Three well-known bench mark convection-diffusion problems and a fluid flow problem are revisited using non-uniform grids. These are: (1) transport of a scalar tracer by a uniform velocity field; (2) heat transport in a recirculating flow; (3) two-dimensional non-linear Burgers equations; and (4) a two-dimensional incompressible Navier-Stokes flow which is similar to the classical lid-driven cavity flow. The known exact solutions of the last three problems make it possible to thoroughly evaluate accuracies of various uniform and non-uniform grids. Higher accuracy is obtained for fewer grid points on non-uniform grids. The order of accuracy of the examined schemes is maintained for some tested problems if the distribution of non-uniform grid points is properly chosen.     

Generation of high-power broad-band microwave pulses by picosecondoptoelectronic technique

A single-picosecond GaAs photoconductive switch is used to pulse excite a microwave resonant cavity, thus generating a variety of RF (radiofrequency) waveforms with picosecond synchronization. The length of the transmission line that connects the photoconductive switch and the cavity and the strength of input/output cavity coupling elements provide for continuous variation of the frequency distribution of the generated RF power. The generation of greater than 7-kW broadband microwave bursts is demonstrated     

Computers for symbolic processing

A detailed survey on the motivations, design, applications, current status, and limitations of computers designed for symbolic processing is provided. Symbolic processing computations are performed at the word, relation, or meaning levels, and the knowledge used in symbolic applications may be fuzzy, uncertain, indeterminate, and ill represented. Various techniques for knowledge representation and processing are discussed from both the designers' and users' points of view. The design and choice of a suitable language for symbolic processing and the mapping of applications into a software architecture are then considered. The process of refining the application requirements into hardware and software architectures is treated, and state-of-art sequential and parallel computers designed for symbolic processing are discussed     

Anchoring Properties of Nematic Liquid Crystals Studied Using the Attenuated Total Reflection Method

By using the attenuated total reflection method associated with the excitation of surface plasmons, the tilt angle of the liquid crystal director and its gradient at the surface are measured in a planar nematic cell as a function of the applied voltage. The surface anchoring anisotropy δπ of the liquid crystal and the surface elastic constant k_s, are found to be δπ = 0.288 erg/cm and k_s, = 9·12 × 10^-11 erg, respectively, when the boundary condition suggested by Barbero et al is used. The theoretical and experimental values obtained with this boundary condition and that of Mada are discussed. The results show that the boundary condition proposed by Barbero et al is in better agreement with the experiment.     

Reliability Modeling and Analysis of Communication Networks with Dependent Failures

This paper presents a new model to study the reliability of communication networks in which link failures are statistically dependent. The approach tries to identify and model explicitly the events that cause communication link failures. No conditional probabilities are needed, and so two major difficulties inherent to them, namely, an exponential number of conditional probabilities to deal with and a consistency requirement to satisfy, are avoided. For reliability computations, some existing algorithms for finding network reliability can be used with minor modifications and no significant increase in computational complexity.     

X射线聚束透镜原理及其应用

本文阐述了一种新型的Ｘ射线聚束装置─—Ｘ射线透镜的聚束原理。给出了透镜的基本设计方法，并对它应用于Ｘ射线光刻研究领域的前景作了展望。     

Nb2O5掺杂对ZnO压敏电阻器电性能的影响

本文研究了Ｎｂ＿２Ｏ＿５掺杂以及Ｎｂ＿２Ｏ＿５与ＺｎＯ煅烧对ＺｎＯ压敏电阻器电性能的影响。实验表明，Ｎｂ＿２Ｏ＿５的掺入使压敏电场减少，当Ｎｂ＿２Ｏ＿５含量为０．１％ｍｏｌ时，其压敏电场最小．非线性系数最大。煅烧温度越高，压敏电场越高，非线性系数越大。     

Bioaccumulation and excretion of arsenic compounds by a marine unicellular alga,polyphysa peniculus

Polyphysa peniculus was grown in artificial seawater in the presence of arsenate, arsenite, monomethylarsonate and dimethylarsinic acid. The separation and identification of some of the arsenic species produced in the cells as well as in the growth medium were achieved by using hydride generation–gas chromatography–atomic absorption spectrometry methodology. Arsenite and dimethylarsinate were detected following incubation with arsenate. When the alga was treated with arsenite, dimethylarsinate was the major metabolite in the cells and in the growth medium; trace amounts of monomethylarsonate were also detected in the cells. With monomethylarsonate as a substrate, the metabolite is dimethylarsinate. Polyphysa peniculus did not metabolize dimethylarsinic acid when it was used as a substrate. Significant amounts of more complex arsenic species, such as arsenosungars, were not observed in the cells or medium on the evidence of flow injection–microwave digestion–hydride generation–atomic absorption spectrometry methodology. Transfer of the exposed cells to fresh medium caused release of most cell–associated arsenicals to the surrounding environment.