N，N－二甲基对甲苯胺引发N－对甲苯基马来酰亚胺光聚合的引发机理

Ｎ，Ｎ－二甲基对甲苯胺引发Ｎ－对甲苯基马来酰亚胺光聚合的引发机理叶克强，董建华，丘坤元，冯新德（北京大学化学系，北京，１００８７１）关键词光引发聚合机理，电荷转移复合物，激基复合物，荧光淬灭，Ｎ，Ｎ－二甲基对甲苯胺，Ｎ－对甲苯基马来酰亚胺Ｎ，Ｎ－二甲...     

Rayleigh–Benard convection in a chemical equilibrium gas (simulation of surface detonation wave initiation)

The problem of the Rayleigh–Benard convection for a chemical equilibrium gas is solved numerically. The gas is assumed to be incompressible, and the layer boundaries are assumed to be flat, isothermal, and free from shear stress. The Boussinesq model with the coefficient at the buoyancy term depending on the transverse coordinate is used. The resultant nonlinear system of equations is solved by a previously developed numerical method based on the spectral representation of vorticity and temperature fields. According to the idea of splitting, analytical formulas are first used to take into account the linear increase in disturbances, and then the nonlinear convective transfer is calculated by the finite-difference method. Various convection modes are obtained: stationary, periodic, quasi-periodic, and stochastic convection.     

弄潮儿潮头立——评谭践的《岁月无悔》

通过对《岁月无悔》的文本分析,解读作为改革者和启蒙者的李增文,阐释了他的改革理想和启蒙理念;从官方话语的积极性和推动性的角度诠释了官方话语对李增文的正面意义和必要的存在;在《岁月无悔》的叙事中,谭践进行了多样化的艺术探索。《岁月无悔》是当代报告文学发展的一个风向标。     

Neural networks to study invariant features of protein folding

Protein secondary structures result both from short-range and long-range interactions. Here neural networks are used to implement a procedure to detect regions of the protein backbone where local interactions have an overwhelming effect in determining the formation of stretches in α-helical conformation. Within the framework of a modular view of protein folding we have argued that these structures correspond to the initiation sites of folding. The hypothesis to be tested in this paper is that sequence identity beside ensuring similarity of the three-dimensional conformation also entails similar folding mechanisms. In particular, we compare the location and sequence variability of the initiation sites extracted from a set of proteins homologous to horse heart cytochrome c. We present evidence that the initiation sites conserve their position in the aligned sequences and exhibit a more reduced variability in the residue composition than the rest of the protein. Received: 24 April 1998 / Accepted: 4 August 1998 / Published online: 11 November 1998     

Impact initiation of explosives and propellants via statistical crack mechanics

A statistical approach has been developed for modeling the dynamic response of brittle materials by superimposing the effects of a myriad of microcracks, including opening, shear, growth and coalescence, taking as a starting point the well-established theory of penny-shaped cracks. This paper discusses the general approach, but in particular an application to the sensitivity of explosives and propellants, which often contain brittle constituents. We examine the hypothesis that the intense heating by frictional sliding between the faces of a closed crack during unstable growth can form a hot spot, causing localized melting, ignition, and fast burn of the reactive material adjacent to the crack. Opening and growth of a closed crack due to the pressure of burned gases inside the crack and interactions of adjacent cracks can lead to violent reaction, with detonation as a possible consequence.This approach was used to model a multiple-shock experiment by Mulford et al. [1993. Initiation of preshocked high explosives PBX-9404, PBX-9502, PBX-9501, monitored with in-material magnetic gauging. In: Proceedings of the 10th International Detonation Symposium, pp. 459-467] involving initiation and subsequent quenching of chemical reactions in a slab of PBX 9501 impacted by a two-material flyer plate. We examine the effects of crack orientation and temperature dependence of viscosity of the melt on the response. Numerical results confirm our theoretical finding [Zuo, Q.H., Dienes, J.K., 2005. On the stability of penny-shaped cracks with friction: the five types of brittle behavior. Int. J. Solids Struct. 42, 1309-1326] that crack orientation has a significant effect on brittle behavior, especially under compressive loading where interfacial friction plays an important role. With a reasonable choice of crack orientation and a temperature-dependent viscosity obtained from molecular dynamics calculations, the calculated particle velocities compare well with those measured using embedded velocity gauges.     

论新形势下的体育教育改革对学生创新能力的影响

创新是一个民族进步和发展的灵魂,时代的发展呼唤着创新教育。体育教育必须以创新为基础,在体育教学中,发展学生的思维能力,拓展学生的思维空间,体现素质教育的重要性。     

原子转移自由基聚合及其催化引发体系研究进展

原子转移自由基聚合(ATRP)作为一种有效的"活性"/可控聚合可对聚合物进行分子设计,制备结构和相对分子质量可控的各类聚合物,具有潜在而广泛的研究价值。本文综述了ATRP的研究进展,特别是对传统ATRP催化引发体系、RATRP催化引发体系、AGET ATRP催化引发体系、SR&NI ATRP催化引发体系、ICAR ATRP催化引发体系、ARGET ATRP催化引发体系、杂化或双金属催化体系等的催化引发机理进行了详细的介绍。并综述了ATRP聚合中各种实施方法如本体聚合法、溶液聚合法、悬浮聚合法、乳液聚合法等的研究现状。     

On critical conditions for detonation initiation by shock reflection from obstacles

A series of experiments supported by numerical simulations are reported on the interaction between a planar incident shock and a single obstacle. The test mixtures used were stoichiometric hydrogen and oxygen diluted with either argon or nitrogen at sub-atmospheric pressures. The main aim of the study was to determine the conditions under which a reflected detonation was generated. Observed critical conditions are compared with a simple predictive criterion based on the ratio of auto-ignition delay time behind an ideal reflected shock to the acoustic transit time across the face of the obstacle. Received 7 December 2001 / Accepted 3 March 2002 Published online 8 July 2002     

串联EFP隔板与前级装药距离对后级装药影响的研究

隔板是破-破型串联战斗部中的重要组成部分,隔板的材料,形状,角度和与前级装药距离等参数对后级EFP的形成以及头部、尾部速度都有较大的影响。采用了LS-DYNA三维数值模拟的方法,在隔板的材料,形状和顶角确定的情况下,选用合适的起爆延时间隔时间,模拟研究隔板与前级装药的距离对后级EFP速度及成型的影响。最终得出隔板与前级装药距离H和速度差百分数的函数关系,H在1~5 mm之间,后级EFP头尾部速度下降较小,形状较为稳定;H在5~15 mm之间,后级EFP头尾部速度下降较大,成型效果不好;而当H为15 mm,后级EFP头部发生了严重变形,头尾部速度大大下降。因此隔板与前级装药间最佳距离范围为1~5 mm。     

基于动态密钥的SIP授权认证机制的设计与实现

会话初始化协议（ SIP）广泛应用在嵌入式移动终端设备中,但由于SIP协议本身特点使得其在复杂、开放的网络应用环境中容易被攻击者模仿、篡改。虽然已经提出了很多增强SIP协议安全性的认证机制,但由于认证过程计算性能消耗大,不适合在嵌入式设备中使用。在传统Http认证的基础上,提出了一种基于动态密钥的认证机制,以较低的计算代价改进了SIP协议的安全性,对SIP协议在嵌入式设备中普及有一定的实用价值。