Percolation threshold and morphology of composites of conducting carbon black/polypropylene/EVA

Conducting carbon black (CB), one of the intrinsic semi-conductors, was added into matrix polypropylene (PP) to prepare conducting composites by means of the melt processing method. Another component EVA was mixed into the composites in order to lower the percolation threshold. The percolation threshold of the ternary CB/PP/EVA composites was merely 3.8 vol%, while it was up to 7.8 vol% for the binary CB/PP composites without EVA. The conductivity of the ternary CB/PP/EVA composites was up to 10^–2 S/cm when the CB percentage was 5 vol%, while that of the binary CB/PP was lower than 10^–2 S/cm when the CB percentage was up to 10 vol%. DSC thermograms of the CB/PP/EVA composites showed that the melting peak shifted to low temperature with increasing CB content. The addition of CB and EVA resulted in the decrease of the crystallinity of PP in the ternary composites. The mechanical properties are also discussed. SEM and TEM were employed to study the morphology of the blend system. The results indicated that CB existed in the form of aggregations in the blend system. The smallest unit that formed a percolation network was grape-like aggregates with some small branches, which consisted of some CB particles, rather than the individual particles. This distribution was very valuable for forming conducting paths and for lowering the percolation value.     

Producing a compact long-lived plasma formation

A method of producing compact long-lived plasma formations (plasmoids) is described. The method is based on the effect of capture of a high-current relativistic electron beam under conditions of significant overcompensation of space charge of the beam by positive background ions. Requirements are formulated which are placed on the parameters of a plasma-beam system for the realization of this method, and numerical estimates are given of the overall electric charge and energy content of the electron component of a plasmoid. The dynamics of forming a plasmoid are investigated experimentally.Translated from Teplofizika Vysokikh Temperatur, Vol. 42, No. 6, 2004, pp. 843–849.Original Russian Text Copyright © 2004 by B. V. Alekhin, A. E. Dubinov, V. S. Zhdanov, V. G Kornilov, K. E. Mikheev, V. D. Selemir, N. V. Stepanov, V. I. Chelpanov, and O. A. Shamro.     

Optimal design of unit hydrographs using probability distribution and genetic algorithms

A nonlinear optimization model is developed to transmute a unit hydrograph into a probability distribution function (PDF). The objective function is to minimize the sum of the square of the deviation between predicted and actual direct runoff hydrograph of a watershed. The predicted runoff hydrograph is estimated by using a PDF. In a unit hydrograph, the depth of rainfall excess must be unity and the ordinates must be positive. Incorporation of a PDF ensures that the depth of rainfall excess for the unit hydrograph is unity, and the ordinates are also positive. Unit hydrograph ordinates are in terms of intensity of rainfall excess on a discharge per unit catchment area basis, the unit area thus representing the unit rainfall excess. The proposed method does not have any constraint. The nonlinear optimization formulation is solved using binary-coded genetic algorithms. The number of variables to be estimated by optimization is the same as the number of probability distribution parameters; gamma and log-normal probability distributions are used. The existing nonlinear programming model for obtaining optimal unit hydrograph has also been solved using genetic algorithms, where the constrained nonlinear optimization problem is converted to an unconstrained problem using penalty parameter approach. The results obtained are compared with those obtained by the earlier LP model and are fairly similar.     

基于UA302H数采器的虚拟测试系统设计

本文针对计算机辅助测试问题,阐述了集成测试和虚拟仪器的巨大优势,重点介绍了一套以UA302H数据采集器为A/D转换装置的虚拟测试系统的设计与应用情况,为普通用户自行开发虚拟测试系统提供了思路.     

振动式微机械陀螺的驱动模态的研究

本文针对的是振动式微机械陀螺自动增益驱动模态的研究.通常的振动式微机械陀螺的驱动模态采用交-直流电压变化的办法来获取振幅信号,然后通过反馈去控制驱动电压,从而达到恒幅振荡的效果.本文提出了一种直接由交流信号去控制驱动电压的方法,这就避免了由交-直流电压变化造成的延时,达到实时自动增益控制的效果.     

Sliding performance of polymer composites in liquid hydrogen and liquid nitrogen

Outstanding features favour the application of polymers and polymer composites in low-temperature technology. The booming hydrogen technology is a challenge for these materials, which are considered as seals and bearings in cryogenic pumps. In the present study, three types of thermoplastics, i.e., polyetheretherketone (PEEK), polyetherimide (PEI) and polyamide 6,6 (PA6,6), and one epoxy were considered as matrix materials. Micron-sized fillers, i.e., short carbon fibres, graphite flakes, and PTFE powders, were incorporated into these polymers together with nano-sized TiO₂ particles. Optimised compositions of each matrix were selected from our previous works at room temperature in order to be studied at very low temperature conditions. In particular, frictional tests were carried out with polymer composite pins against polished steel surfaces under constant load over a certain distance in liquid hydrogen and liquid nitrogen. Afterwards, worn surfaces were analysed by using scanning electron microscopy (SEM). It was found out that the tribological properties in liquid hydrogen are dominated by the matrix materials, in particular thermoplastics perform generally slightly better than thermosetting resins.     

Effect of synthesis temperature on density and microstructure of MgB2 superconductor at ambient pressure

The experimental thermodynamic of MgB₂ synthesis process and phase compositions have been investigated by diffraction thermal analysis (DTA) technology and X-ray diffraction. The fabrication of MgB₂ bulks and superconducting properties at the temperatures range from 600 to 800°C were reported. And microstructure of MgB₂ bulks were observed by scanning electron microscope (SEM). A method was developed to determine the porosity of MgB₂ and the highest density can be obtained in MgB₂ prepared at 650°C at ambient pressure. It is found that the vapor pressure of Mg increases remarkably at high temperature, leading to the high porosity in MgB₂ samples. MgB₂ bulk with good superconducting property and fine microstructure was synthesized at 750°C.