Experimental study of the U-238(S-36,3-5n)(269-271)Hs reaction leading to the observation of (270)Hs

The deformed doubly magic nucleus (270)Hs has so far only been observed as the four-neutron (4n) evaporation residue of the reaction Mg-26+Cm-248, where a maximum cross section of 3 pb was measured. Theoretical studies on the formation of (270)Hs in the 4n evaporation channel of fusion reactions with different entrance channel asymmetry in the framework of a two-parameter Smoluchowski equation predict that the reactions Ca-48+Ra-226 and S-36+U-238 result in higher cross sections due to lower reaction Q values, in contrast to simple arguments based on the reaction asymmetry, which predict opposite trends. Calculations using HIVAP predict cross sections for the reaction S-36+U-238 that are similar to those of the Mg-26+Cm-248 reaction. Here, we report on the first measurement of evaporation residues formed in the complete nuclear fusion reaction S-36+U-238 and the observation of (270)Hs, which is produced in the 4n evaporation channel, with a measured cross section of 0.8(-0.7)(+2.6) pb at 51-MeV excitation energy. The one-event cross-section limits (68% confidence level) for the 3n, 4n, and 5n evaporation channels at 39-MeV excitation energy are 2.9 pb, while the cross-section limits of the 3n and 5n channel at 51 MeV are 1.5 pb. This is significantly lower than the 5n cross section of the Mg-26+Cm-248 reaction at similar excitation energy.

Dynamic properties and asymmetry of tension and compression of metal matrix composites under impact loading

The mechanical behaviors of 2124, Al-5Cu, Al-Li and 6061 alloys reinforced by silicon carbide particulates, together with 15%SiCw/6061 alloy, were studied under the quasi-static and impact loading conditions, using the split Hopkinson tension/compression bars and Instron universal testing machine. The effect of strain rate on the ultra tensile strength (UTS), the hardening modulus and the failure strain was investigated. At the same time, the SEM observations of dynamic fracture surfaces of various MMC materials showed some distinguished microstructures and patterns. Some new characteristics of asymmetry of mechanical behaviors of MMCs under tension and compression loading were also presented and explained in details, and they could be considered as marks to indicate, to some degree, the mechanism of controlling damage and failure of MMCs under impact loading. The development of new constitutive laws about MMCs under impact loading should benefit from these experimental results and theoretical analysis.

Two-Phase Flow In Fuel Cells In Short-Term Microgravity Condition

A visual observation of liquid-gas two-phase flow in anode channels of a direct methanol proton exchange membrane fuel cells in microgravity has been carried out in a drop tower. The anode flow bed consisted of 2 manifolds and 11 parallel straight channels. The length, width and depth of single channel with rectangular cross section was 48.0 mm, 2.5 mm and 2.0 mm, respectively. The experimental results indicated that the size of bubbles in microgravity condition is bigger than that in normal gravity. The longer the time, the bigger the bubbles. The velocity of bubbles rising is slower than that in normal gravity because buoyancy lift is very weak in microgravity. The flow pattern in anode channels could change from bubbly flow in normal gravity to slug flow in microgravity. The gas slugs blocked supply of reactants from channels to anode catalyst layer through gas diffusion layer. When the weakened mass transfer causes concentration polarization, the output performance of fuel cells declines.

Novel Mechanical Behavior Of Zno Nanorods

A novel stress-strain relation with two stages of linear elastic deformation is observed in [0 0 0 1]-oriented ZnO nanorods under uniaxial tensile loading. This phenomenon results from a phase transformation from wurtzite (WZ, P6(3)mc space group) to a body-centered tetragonal structure with four-atom rings (denoted as BCT-4) belonging to the P4(2)/mnm space group. The analysis here focuses on the effects of nanorod size and temperature on the phase transformation and the associated mechanical behavior. It is found that as size is increased from 19.5 to 45.5 angstrom, the critical stress for nucleation of the transformation decreases by 25% from 21.90 to 16.50 GPa and the elastic moduli of the WZ- and BCT-4-structured nanorods decrease by 24% (from 299.49 to 227.51 GPa) and 38% (from 269.29 to 166.86 GPa), respectively. A significant temperature effect is also observed, with the critical stress for transformation initiation decreasing 87.8% from 17.89 to 2.19 GPa as temperature increases from 300 to 1500 K. (c) 2007 Elsevier B.V. All rights reserved.

Lattice type of fracture model for concrete

Concrete is usually described as a three-phase material, where matrix, aggregate and interface zones are distinguished. The beam lattice model has been applied widely by many investigators to simulate fracture processes in concrete. Due to the extremely large computational effort, however, the beam lattice model faces practical difficulties. In our investigation, a new lattice called generalized beam (GB) lattice is developed to reduce computational effort. Numerical experiments conducted on a panel subjected to uniaxial tension show that the GB lattice model can reproduce the load-displacement curves and crack patterns in agreement to what are observed in tests. Moreover, the effects of the particle overlay on the fracture process are discussed in detail. (C) 2007 Elsevier Ltd. All rights reserved.

在单脉冲激波管中研究TiCl_4裂解反应速率

在单脉冲激波管中,用对比速率法研究了TiCl_4的裂解。从自由基清扫剂环已烷C_6H_(12)的诱发裂解中产生的C_2H_4的测量,确定反应的机制,并测量得到了TiCl_4裂解初始过程的反应速率常数:k(TiCl_4→TiCl_3•+Cl•)=10~(16.4)exp(-(41700)/T)sec~(-1)。

振荡流中圆柱受力测量研究

本文叙述了测量振荡流中圆柱受力大小和性质的方法和步骤。给出了测量结果,并首次给出这种力的频谱特性。

苏联低温等离子体主要研究单位一览

<正> 低温等离子体由于能提供一般矿物燃料所不能提供的高温、可控气氛、高纯度,且容易实现自动化控制,因此,许多国家都在化工、冶金、机械加工、半导体等工艺过程中开展了大量的等离子体技术应用研究,有的已成为成熟的工艺,苏联西伯利亚储藏着极丰富的矿物燃料资源,但距离工业发达的苏联欧洲部分遥远,据估计,如果苏联欧洲部分的工业全部用

使用任意非正交曲线坐标解变截面圆轴扭转问题的数值计算

<正> 1.基本方程及其数值求解由于讨论的是轴对称问题,因此取柱坐标系中的θ坐标为任意曲线坐标系(q~1,q~2,q~3)中的q~2坐标,在轴纵剖面Z—R平面上取任意非正交曲线坐标q~3和q~1,则变截面圆轴扭转问题以无量纲应力函数ψ表达的求解方程为:

选择CO_2激光器输出谱线的一个简单方案

本文提出了一个用控制锗窗温度的办法实现CO_2激光器选频的简单方案。定量地讨论了锗窗温度变化对腔频的影响及所引起的其相干反射极大的频移,并结合介质增益分析了它们对激光输出谱线的影响。实验证实该选频方案是切实可行的。

有限电导率的等离子体柱在径向外力作用下的不稳定性问题

从磁流体力学方程出发,討論了处于定态的等离子体柱的不稳定性問題。在等离子体是不可压縮的、电导率是有限的并且等离子体受到径向外力的作用等假定下,导出了相应的色散关系。針对Curzon等人的实驗数据进行的計算表明,磁粘性v_m对不稳定性增长率ω的影响很小,可以略去不計。Green和Niblett曾經提出过一个公式,这个公式夸大了v_m对ω的影响,得出的結果与实驗不符,所以是不对的。

Asymptotic solution of mode-iii crack in damaged softening materials

In this paper, a constitutive model of elasticity coupled with damage suggested by Lemaitre et al, [1] is used. The macroscopic stress-strain response of the model includes two stages: strain hardening and strain softening. The basic equation is derived for the anti-plane shear problem. Several lowest order asymptotic solutions are obtained, and assembled for the crack-tip fields.

On the invariant representation of spin tensors with applications

The invariant representation of the spin tensor defined as the rotation rate of a principal triad for a symmetric and non-degenerate tensor is derived on the basis of the general solution of a linear tensorial equation. The result can be naturally specified to study the. spin of the stretch tensors and to investigate the relations between various rotation rate tensors encountered frequently in modern continuum mechanics. A remarkable formula which relates the generalized stress conjugate to the generalized strain in Hill's sense. to Cauchy stress, is obtained in invariant form through the work conjugate principle. Particularly, a detailed discussion on the time rate of logarithmic strain and its conjugate stress is made as the principal axes of strain arc not fixed during deformation.

液滴热毛细迁移问题的研究进展

液滴或气泡的迁移现象无论是在流体力学的基础研究中,还是在材料加工,化学工程等实际应用中都是一个很重要的课题.在微重力环境中,如果液滴或气泡所在的母液中外加一个温度场,则液滴或气泡就会由于表面张力分布的不均匀而发生迁移运动.这种运动被称为Marangoni迁移或热毛细迁移运动.综述了液滴或气泡的热毛细迁移问题历史研究中理论分析,数值模拟以及实验方面的主要结果,阐述了该问题的研究发展过程.目前液滴迁移问题的研究,理论分析解还只限于线性及弱非线性的定常问题,数值模拟工作已经得到了在热对流作用较小时液滴的非定常迁移过程,对于热对流影响很大的情况(Marangoni数大于100)则尚未得到与实验中观测到的相一致的理论结果.在总结前人研究的基础上,给出了热对流作用较大时液滴热毛细迁移非定常问题的最新的数值模拟结果,并对该问题在此情况下产生的新变化给予了分析.最后,分析了当前研究中所存在的问题并进一步展望液滴热毛细迁移问题未来的发展方向.