Improved Zernike‐type phase contrast for transmission electron microscopy

Zernike phase contrast has been recognized as a means of recording high‐resolution images with high contrast using a transmission electron microscope. This imaging mode can be used to image typical phase objects such as unstained biological molecules or cryosections of biological tissue. According to the original proposal discussed in Danev and Nagayama (2001) and references therein, the Zernike phase plate applies a phase shift of π/2 to all scattered electron beams outside a given scattering angle and an image is recorded at Gaussian focus or slight underfocus (below Scherzer defocus). Alternatively, a phase shift of ‐π/2 is applied to the central beam using the Boersch phase plate. The resulting image will have an almost perfect contrast transfer function (close to 1) from a given lowest spatial frequency up to a maximum resolution determined by the wave length, the amount of defocus and the spherical aberration of the microscope. In this paper, I present theory and simulations showing that this maximum spatial frequency can be increased considerably without loss of contrast by using a Zernike or Boersch phase plate that leads to a phase shift between scattered and unscattered electrons of only π /4, and recording images at Scherzer defocus. The maximum resolution can be improved even more by imaging at extended Scherzer defocus, though at the cost of contrast loss at lower spatial frequencies.     

Seismic performance evaluation of steel moment resisting frames through incremental dynamic analysis

Earthquake hazards effect significant damage to structures and cause widespread failure throughout buildings. Moment resisting frames are widely used as lateral resisting systems when sufficient ductility is to be met. Generally three types of moment resisting frames are designed in practice namely Special, Intermediate and Ordinary Moment Frames, each of which has certain level of ductility. Comparative studies on the seismic performance of these three different types of structure are performed in this study. Analytical models of connections are employed including panel zone and beam to column joint model. Incremental dynamic analysis is then utilized to assess the structural dynamic behavior of the frames and to generate required data for performance based evaluations. Maximum annual probability of exceeding different limit states may reveal the superiority of a ductile structure in which a greater behavior factor is employed. Special moment resisting frames are expected to perform better once a certain level of ductility is to be met but the amount of superiority may be the subject of investigation especially from a performance based design standpoint.     

三角形荷载作用下烟囱圆板基础环壁内的上部弯矩分析

烟囱圆板基础在三角形荷载作用下,按《烟囱设计规范》（GB50051—2002）计算底板反力p,用此值计算环壁以内底板上部两个正交方向单位宽度的弯矩设计值时,发现与实际受力相差较大。本文根据有限元程序分析圆板在三角形荷载作用下的实际弯矩。同时,通过进一步的分析比较,建议采用本文所述的实用计算方法进行计算。与规范计算公式相比,本文方法可减少配筋。     

复杂环境中短波天线的MoM/FDTD混合法分析

利用电磁场数值计算方法中的矩量法/时域有限差分（MoM/FDTD）混合方法,分析了复杂环境中短波天线的电磁辐射性能,建立了短波双极天线的自由空间模型。采用矩量法和时域有限差分法相结合,对其模型进行仿真计算,并将计算结果和已有的文献进行比较,验证了混合方法计算结果的可行性、准确性。总结了该模型对短波天线远区场影响的有益经验,为改善短波天线性能提供了设计参考。     

轴系摩擦力矩测试系统的设计与实现

对轴承摩擦力矩的产生机理进行归纳、总结、研究,基于力矩测试基本方法,提出了一种简单有效的测试方法,通过机械结构及软硬件的设计,完成了一套轴系摩擦力矩测试系统,经用于某型号轴系的检验,可满足测试要求。     

磁悬浮控制力矩陀螺框架系统谐波减速器的迟滞建模

为了抑制双框架磁悬浮控制力矩陀螺(DGMSCMG)框架伺服系统中谐波减速器固有的迟滞特性对系统精度的影响,提出了一种基于Preisach模型的谐波减速器迟滞特性建模方法。首先,使用一阶回转曲线法采集谐波减速器的柔轮输出力矩与扭转角,获得建立谐波减速器迟滞模型的实验数据,其中谐波减速器柔轮的输出力矩是在不使用力矩传感器的条件下用系统动力学模型估计得到的;然后,使用Preisach模型对谐波减速器柔轮输出力矩与扭转角迟滞关系进行建模;最后,采用将模型离散化的数字型实现方法辨识模型中的权重函数,并给出模型的离散递归算法使模型利于简易化编程与进一步的在线控制。实验结果显示,谐波减速器的迟滞模型误差不超过0.005°,MSE值不超过(0.000 83%)°。结果显示了所述建模方法的正确性和实用性。     

多连杆悬架系统中铰接点载荷的计算方法

考虑衬套的六向非线性刚度,建立了汽车多连杆悬架系统的数学模型。基于悬架连杆和车轮托架等力-位移静平衡方程,给出了悬架各铰接点力的计算公式。以五连杆后悬架为例,利用未考虑衬套连接及同时考虑衬套线刚度和扭转刚度的两种模型,分别计算了各模型在汽车小载荷工况和大载荷工况下各铰接点的受力。计算结果表明,当悬架系统受到一些大载荷工况作用时,考虑衬套六向非线性刚度的模型能更准确地计算出各铰接点的力和力矩。因此,该方法可为五连杆后悬架铰接点载荷计算提供了一套准确的建模与计算方法,计算得到的载荷可用作悬架系统各元件的强度计算和疲劳试验中的载荷输入。     

几何图形的矩标识

简单介绍惯性矩理论，提出惯性比概念，讨论用最小包络矩形和惯性比来唯一标识一个几何图形的可能性，给出了证明和算例。     

小型弹箭惯性矩的测定

叙述了三线摆的结构原理 ,介绍了该摆小角与大角摆动周期的计算方法。利用三线摆的结构原理可以测量小型弹箭、子弹头、飞机模型及其他构件的惯性矩。当 R/ L <1 / 30 ,φ0 ≤ 60°时 ,该装置的测量精确度可优于 1 %。     

Hydrogen diffusivity enhancement in alumina pellets due to impregnated palladium

Single-pellet moment technique was used for the evaluation of effective diffusivities and adsorption equilibrium constants of hydrogen, carbon dioxide, and helium in pure alumina and palladium-impregnated alumina pellets. It was found that hydrogen was strongly adsorbed on palladium of Pd-alumina pellets, and significant enhancement of hydrogen diffusivity was observed due to impregnated palladium at 40°C. On the other hand, for carbon dioxide and helium, effective diffusivities obtained in Pd-alumina pellets were smaller than the corresponding values in pure alumina due to higher tortuosity factor values of Pd-alumina pellets.