万变的网络,永恒的安全--谈网络安全体系的构建

本文阐述了构建安全体系结构的意义,以及安全体系结构所包括的主要内容.重点以企业网为例介绍了应当如何根据自身实际,以安全体系为指导,构建其安全防护体系.     

On the compression and streaming of concentric mosaic data for free wandering in a realistic environment over the Internet

In this paper, we describe a system for wandering in a realistic environment over the Internet. The environment is captured by the concentric mosaic, compressed via the reference block coder (RBC), and accessed and delivered over the Internet through the virtual media (Vmedia) access protocol. One of the key contributions of the paper is the proposal of the RBC concentric mosaic coder. The RBC coder not only compresses the huge dataset of the concentric mosaic very efficiently, but also produces a well-organized bitstream that can be accessed just-in-time (JIT). To reconstruct a virtual view, only a portion of the RBC bitstream needs to be accessed and decoded. This greatly reduces the memory and computation requirement of the viewer compared with first decoding the entire concentric mosaic data set and then rendering from the decoded data. Our second contribution is the employment of the Vmedia protocol to deliver the compressed concentric mosaic bitstream just-in-time over the Internet. Only the bitstream segments corresponding to the current view are streamed over the Internet. The delivered bitstream segments are managed by a local Vmedia cache, so that frequently used bitstream segments do not need to be streamed over the Internet repeatedly, and a RBC bitstream larger than the memory capacity can be easily handled. Combining RBC and Vmedia, a concentric mosaic interactive browser is developed through which the user can freely wander in a realistic environment, e.g., rotate around, walk forward/backward, and sidestep, even under a tight bandwidth.     

A fracture criterion for three-dimensional crack problems

A criterion for predicting the growth of three-dimensional cracks is developed on the basis of the strain energy density concept which has been used successfully for treating two-dimensional crack problems. Fracture is assumed to initiate from the nearest neighbor element located by a set of spherical coordinates (r, θ, φ) attached to the crack border. The new fracture surface is described by a locus of these elements whose locations correspond to the strain energy function, dW/dV, being a minimum. The function dW/dV is found to be singular of the type 1/r and is of quadratic form in the three stress intensity factors k₁, k₂ and k₃ expressed through the strain energy density factor S. It is postulated that unstable crack propagation initiates from a region where S reaches a critical value S_cr = r₀(dW/dV)_cr. The locations of failure lying on the fracture surface is determined by holding (dW/dV)_cr = S_min/r₀ constant. The quantity S_min stands for the value of S minimized with respect to θ and φ and r₀ is a radial distance measured from the crack border.

An example of failure prediction for an embedded elliptical crack subjected to both normal and shear loads is presented. According to the S-criterion, fracture initiation takes place at the ends of the minor axis. An unexpected result is that for a narrow elliptical crack and Poisson's ratio of 1/3 the lowest failure load occurs when the uniaxial tensile load makes an angle of approximately 60° with the crack surface and is in the plane of the major axis. This is in contrast to the expectation that the lowest critical load occurs when the uniaxial tension is perpendicular to the crack surface. In the limit as the elliptical crack becomes increasingly narrower, the result reduces to the two dimensional line crack case of Mode I and III loading. The S-criterion is also applied to the failure prediction of three dimensional cracks under compressive loads.     

Optimized microwave dielectric properties of Co- and Ca-substituted Mg0.6Zn0.4TiO3

The system of (1 − y)(Mg_0.6Zn_0.4)_1−xCo_xTiO₃-yCaTiO₃ was investigated to optimize its microwave dielectric properties by adopting appropriate contents of Co and Ca and by controlling sintering conditions. The effect of Co substitution was to enhance densification and Qf value, while the addition of CaTiO₃ resulted in increases of dielectric constant and TCF. As an optimal compositions, 0.93(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃-0.07CaTiO₃ successfully demonstrated a dielectric constant of 23.04, a Qf of 79,460 GHz and a TCF value of +1.4 ppm/°C after firing at a relatively lower sintering temperature of 1200 °C. The increase of sintering temperature beyond 1200 °C tended to degrade overall microwave dielectric properties presumably due to Zn volatilization as evidenced by the presence of a Zn-deficient phase (MgTi₂O₅) at 1400 °C. An attempt to establish the correlation between microstructure characteristics and dielectric properties was made in this dielectric system where the extensive range of firing temperature up to 1400 °C was evaluated.     

弯管塑性极限载荷分析

利用Von-Mises屈服准则,推导出内压和扭矩联合载荷以及内压、弯矩与扭矩联合载荷作用下弯管的塑性极限载荷方程,经过简化后可得到任一单个载荷、两个或3个载荷共同作用下的塑性极限载荷.有关实验验证了理论的正确,为充分挖掘弯管的塑性极限承载能力提供了理论依据,也为研究有缺陷弯管塑性极限载荷提供了方法.     

Chitosan and <Emphasis Type="Italic">β</Emphasis>-cyclodextrin microspheres as pulmonary sustained delivery systems

Chitosan and β-cyclodextrin were used to prepare microspheres with theophylline for pulmonary delivery by spray drying method. The characteristics, mucociliotoxicity, permeation rate and drug release were studied. The drug entrapments of microspheres Ⅰ, Ⅱ and Ⅲ were from 35.70% to 21.09% and 13.33%, while yields and encapsulation efficiencies were higher than 45% and about 90% respectively. The microspheres possessed low tap densities （0.34-0.48 g/cm^3）, appropriate diameters （3.35-3.94 μm） and theoretical aerodynamics diameters （2.20-3.04 μm）. SEM images showed the microspheres were spherical with smooth or wrinkled surface surfaces. FT-IR demonstrated theophylline had formed hydrogen bonds with chitosan and fl-cyclodextrin. The microspheres could effectively reduce the ciliotoxicity and easy to penetrate the memberine. The in vitro release of the microspheres was related to the ratio of drug/polymer and microspheres Ⅱ had a prolong release, providing the release of 72.00% in 12 h. The results suggestes that chitosan/β-cyclodextrin microspheres Ⅱ are a promising carrier as sustained release for pulmonary delivery.     

带二次流增推尾喷管的脉冲爆震发动机数值模拟研究

为提升现有脉冲爆震发动机增推喷管性能,对带二次流尾喷管发动机分别在定常流和爆震多循环过程进行了数值模拟,研究了二次流尾喷管的流场并计算了其推进性能。结果表明,定常流动中,随着主流压力的升高,注入二次流对尾喷管推进性能的提升幅度逐渐降低;当爆震室内压力很高时,二次流的瞬时流量会自动降低或者停止注入,当爆震室压力较低时,二次流流量又会自动恢复;单管注入二次流对PDE平均推力和平均单位推力的提升率分别达到2.63%和0.36%;双管注入二次流达到5.96%和0.75%。     

Iridium thin film as a bottom electrode for high dielectric (Ba,Sr)TiO3 capacitors

Abstract

We propose Ir thin films as new electrode materials for high dielectric BST capacitors. Ir was found to be superior to Pt in a number of aspects such as resistivity, adhesion and surface roughness. The Pt/BST/Ir/SiO₂/Si capacitors showed leakage currents as low as Pt/BST/Pt/SiO₂/Si ones, but higher capacitance resulted. For endurance properties with +5V unipolar pulse trains, the dielectric constant of BST films on Ir decreased by only 10% below its initial value after switching of 10⁹ cycles while that on Pt degraded by 30% after 10⁸ cycles. Ir bottom electrode effects on BST film properties were well explained by the formation of IrO₂ phases on the surface of Ir electrodes.