Optimizing the phase shift and the cut-on periodicity of phase plates for TEM

Images acquired with a phase plate often exhibit fringing and/or contrast reversal artifacts. The two basic parameters controlling the performance of the phase plate are phase shift and cut-on periodicity. We investigate theoretically and numerically the effect of these parameters on the image quality. The analysis covers not just the typical negative phase shift phase plates but also positive phase shift ones. The theoretical study derives formulas for calculating the optimal phase plate phase shift and for the maximum achievable contrast with a given specimen. Two figures of merit - fidelity and contrast - were defined and used to quantify the numerical results. Larger cut-on periodicities provide better performance with higher contrast and less artifacts in the images. Both, the theoretical results and the simulations indicate that positive phase shift phase plates generate higher contrast with better linearity and are free from contrast reversal artifacts. However, with such phase plates the amplitude and the phase contrast components are opposed to each other and the simulations show stronger fringing outside of objects. Based on these results it is difficult to predict if and to what extent the positive phase shift phase plates will be advantageous in practice. Two methods for reduction of fringing artifacts were compared—tapered phase plate and low-frequency amplification software filter. Overall the software solution produced better results and is much easier to implement than modifying the hardware of the phase plate to realize the taper.     

Effect of the phase shift due to dynamical scattering on the contrast of crystal structure images

The contrast of through-focus images of a niobium tungsten bronze, 2Nb₂O₅·7WO₃, taken by a 1 MV high resolution electron microscope, is discussed in terms of the transfer function, in which, in addition to the conventional phase aberration, the phase shift due to the dynamical scattering is taken into account. It is found that the phase shift may be advantagous in the formation of structure images, as long as the crystal very thin, i.e., a few nanometers in thickness. It is also made clear why the best defocus for optimum structure images differs from the Scherzer defocus, depending on the crystal thickness. An almost exact contrast reversal occurs at overfocus only in the images of extremely thin crystals, because the positive range of the transfer function is made narrower due to the above phase shift. The discussion is supported by the computer simulation of the image contrast.     

A high-speed precision digital phase meter with direct calculation of the signal phase

A programmed phase meter for the detection of small phase deviations is described. The device is designed for a remote vibration meter. The application of digital signal processing ensures the detection of differential or simple phase deviations at a specified carrier frequency of 10.7 MHz with a sensitivity of 10^–5 rad.__________Translated from Pribory i Tekhnika Eksperimenta, No. 2, 2005, pp. 88–91.Original Russian Text Copyright © 2005 by Goncharenko.     

Experimental investigation of zero phase shift effects for Coriolis flowmeters due to pipe imperfections

Theoretical investigations of a single, straight, vibrating, fluid-conveying pipe have resulted in simple analytical expressions for the approximate prediction of the spatial shift in vibration phase. The expressions have lead to hypotheses for real Coriolis flowmeters (CFMs). To test these, the flexural vibrations of two bent, parallel, non-fluid-conveying pipes are studied experimentally, employing an industrial CFM. Special attention has been paid on the phase shift in the case of zero mass flow, i.e. the zero shift, caused by various imperfections to the “perfect” CFM, i.e. non-uniform pipe damping and mass, and on ambient temperature changes. Experimental observations confirm the hypothesis that asymmetry in the axial distribution of damping will induce zero shifts similar to the phase shifts due to fluid flow. Axially symmetrically distributed damping was observed to influence phase shift at an order of magnitude smaller than the primary effect of mass flow, while small added mass and ambient temperature changes induced zero shifts two orders of magnitude smaller than the phase shifts due to mass flow. The order of magnitude of the induced zero shifts indicates that non-uniform damping, added mass as well as temperature changes could be causes contributing to a time-varying measured zero shift, as observed with some commercial CFMs. The conducted experimental tests of the theoretically based hypotheses have shown that simple mathematical models and approximate analysis allow general conclusions, that may provide a direct insight, and help increasing the benefit of time consuming numerical simulations and laboratory experiments.     

On the preparation of specimens from multifilamentary Nb3Sn superconducting wires for transmission electron microscopy

A set of techniques for preparing transmission electron microscope specimens from sections of thin (≤ 1 mm overall diam.) multifilamentary wires (filaments being approximately 5 μm diam.) are described. Results illustrating the successful uniform thinning of the several phases present in ‘bronze-route’ Nb₃Sn are shown.     

Experimental determination of time lag due to phase shift on a flexible pipe conveying fluid

A finite element model of a flexible tube conveying fluid is developed in MATLAB^© based on the principle of virtual work, using a three-node isoparametric beam element. Finite element equations are formulated by applying Galerkin technique on the coupled equations of pipe conveying fluid. The present element developed is capable to model three-dimensional flexible tubes by including curved geometry, effects of damping, velocity and gyroscopic effects. The external excitation applied at the middle of the tube in the lateral direction produced a time lag between the lateral responses, which were measured at two equidistant points from the excitation point. This is due to the Coriolis effect, and the same is simulated using the developed code. An experiment, supported with a robust error analysis, is also conducted on a straight polyurethane tube conveying water, subjected to a sinusoidal excitation at the center between the clamped supports. The measured time responses are compared with the numerical values predicted by the code. The time lags for both cases are obtained from the temporal shift along the time axis, between the zero crossing points of the time–response curves. The results obtained agreed well. The code can be used to predict the time lag, which is correlated to the mass flow rate. The proposed method will help to design Coriolis mass flow meters for existing pipelines, without altering the system.     

减速器上箱体强度分析与计算

减速器是重型装备传动装置中的主要部件,而减速器的箱体是齿轮系统的承载体,承受较大的负荷并产生一定的应力,箱体由于强度不足会出现裂纹,从而导致失效。由于箱体的形状及受力都十分复杂,用传统的理论解析方法进行力学分析非常困难,而有限元作为数值模拟的强大工具,可为复杂结构的设计计算提供有力支撑。本文以某重型装备减速器的上箱体为对象,     

On implementation of the finite difference lattice boltzmann method with internal degree of freedom to edgetone

The lattice Boltzman method (LBM) and the finite difference-based lattice Boltzmann method (FDLBM) are quite recent approaches for simulating fluid flow, which have been proven as valid and efficient tools in a variety of complex flow problems. They are considered attractive alternatives to conventional finite-difference schemes because they recover the Navier-Stokes equations and are computationally more stable, and easily parallelizable. However, most models of theLBM orFDLBM are for incompressible fluids because of the simplicity of the structure of the model. Although some models for compressible thermal fluids have been introduced, these models are for monatomic gases, and suffer from the instability in calculations. A lattice BGK model based on a finite difference scheme with an internal degree of freedom is employed and it is shown that a diatomic gas such as air is successfully simulated. In this research we present a 2-dimensional edge tone to predict the frequency characteristics of discrete oscillations of a jet-edge feedback cycle by theFDLBM in which any specific heat ratio γ can be chosen freely. The jet is chosen long enough in order to guarantee the parabolic velocity profile of a jet at the outlet, and the edge is of an angle of α=23°. At a stand-off distancew, the edge is inserted along the centerline of the jet, and a sinuous instability wave with real frequency is assumed to be created in the vicinity of the nozzle exit and to propagate towards the downstream. We have succeeded in capturing very small pressure fluctuations resulting from periodic oscillation of the jet around the edge.     

Electron holography of thin amorphous carbon films: measurement of the mean inner potential and a thickness-independent phase shift

The phase shift induced by thin amorphous carbon films with thicknesses between 1 and 16 nm was measured by electron holography in a transmission electron microscope. Large phase shifts Delta phi are observed as the thickness of the amorphous C films decreases which cannot be described by the well-known equation Delta phi = CE V0t (V0: mean inner Coulomb potential of the material, t: sample thickness). Data plotted in a Delta phi vs. t diagram can be well-fitted by a modified equation Delta phi=CE V0t + phi add. The mean inner Coulomb potential of the amorphous carbon with a density of 1.75 g/cm3 was determined to be 9.09 V which is consistent with previous experimental data for amorphous carbon with a higher density. The thickness-independent phase offset phi(add) of 0.497 rad is large for amorphous carbon under the given experimental conditions. We suggest that a surface-related electrostatic potential is responsible for the thickness-independent contribution phi add.     

On the calculation of magnetic fields generated by coils

The edge effect in electron microscopical images is proposed to be measured in length units by using a ratio of the edge or near-to-edge signal enhancement or depletion integrated over all this feature along a line-scan across an edge, to the mean signal away from the edge, giving the result in length units. This approach is applied to quantification of the edge effect in Auger electron images, which is examined by using specimens with sharp edge terminated overlayer terraces, both chemically homogeneous and heterogeneous. The following combinations of overlayer/substrate materials were used: Si/Si of three different overlayer thicknesses, W/W and W/Si, and line-scans across the edges were recorded in both the low- and the high-energy Auger electrons mode, i.e. Si LMM and Si KLL or W NOO and W MNN. The experimental results are presented for the 3-, 10- and 20-keV primary electron energies. Owing to a low signal-to-noise ratio in the measured data, basic relations between the effect appearance and the experimental conditions were revealed only: on both Si and W homogeneous specimens with a surface step, the edge enhancement is the dominating subphenomenon while at the W terrace edge on the Si substrate, the ‘penetration’ of the radiation characteristic to both areas separated by the step, to the neighbouring feature, is observed as the most significant effect. The quantification has shown that the effect is, first of all, proportional to the step height, amounting to one-third to up to the full height, while the material dependence was weak, equally to the dependence on the Auger electron energy. The primary electron energy dependence is increasing in accordance with expectation. The results indicate that the effect cannot be modelled simply by the interaction volume cut by the surface step but phenomena such as subsurface electron channelling along the sidewall have to be taken into account.     

On calculation of the DGS diagrams of an angle transducer

The acoustic path of an angle transducer is calculated in the Kirchhoff approximation for a reflector of the flat-bottom-hole type. The formula obtained can be used for constructing DGS diagrams. In the limiting case of the far-field zone, an asymptotic formula is obtained for the acoustic path confirming the validity of using analogous relationships, which were heuristically obtained earlier. Numerical calculations of the DGS diagram of an angle transducer are performed, the results of which are compared to those obtained via the recalculation of the diagram for a normal transducer.     

On the finite element calculation of stress distributions in arches

In previous papers the properties of a number of cylindrical shell finite elements have been investigated by applying them to circular line arches. More recent work has shown the need for a more detailed study of the accuracy with which stress distributions (as opposed to displacements) can be calculated. Such a study is described in the present paper. The most convenient element of those tested, able to calculate stresses accurately in arches of all proportions, is found to be thestrain element, introduced by earlier research and previously shown to be superior to all those tested when used for calculating displacements and natural frequencies. It is suggested that a similar conclusion may apply to the corresponding cylindrical shell element.     

轴心压杆稳定性验算在起重机设计规范中的变化

通过比较新旧2种设计规范在轴心压杆稳定性计算上的变化,结合稳定性研究的最新成果,充分肯定了这种变化的合理性和科学性。在直观分析稳定系数曲线趋势图的基础上,指出这种变化在实际运用中可能导致的差异,提出了新规范在实际运用中应注意的问题,有助于工程技术人员正确运用新规范和更好地把握其实质。     

Determination of lattice parameters from multiple CBED patterns: A statistical approach

This study deals with the uncertainty of the measurement of lattice parameters by CBED using the kinematic approximation. The analysis of a large number of diffraction patterns acquired on a silicon sample at 93 K with a LaB₆ TEM without energy filter shows the presence of both the systematic and the random parts of errors. It is established that random errors follow the normal statistical distribution and that the precision quantified by the relative standard deviation is about 3–4×10⁻⁴ for lattice parameter measurements made from single pattern. The error sources are analyzed, different ways of enhancement are reviewed, and a new approach is proposed. It is shown that both accuracy and precision can be simply improved by taking into account multiple patterns analysis for the determination of the actual voltage, the single lattice parameter “a” or the complete set of lattice parameters. The precision of about 1.5–2×10⁻⁴ can be reached using a minimum of three HOLZ line patterns for the single “a” parameter and about 5×10⁻⁴ for the complete set of lattice parameters using six diffraction patterns. The use of multiple patterns also allows overcoming the non-uniqueness of solution linked to the CBED studies.     

Infrared spectroscopy of organometallic ions in the gas phase: from model to real world complexes

Gas phase mid-infrared spectroscopy of molecular ions can nowadays be performed with high performance mass spectrometers coupled to free electron lasers (FEL). The wide and continuous tunability of highly intense FELs in the mid-infrared region can be exploited for performing infrared multiple photon dissociation (IRMPD) spectroscopy of molecular ions. This review will focus on gas phase IRMPD spectroscopic investigations aiming at probing the structure and the reactivity of transition metal complexes. The performance of infrared spectroscopy for characterizing the coordination mode of polydentate ligands and the spin state of the metal will be illustrated. Infrared spectroscopy has also been exploited to probe the reactivity of metal complexes, and a special attention will be given to the infrared spectroscopy of reactive intermediates.     

On the calculation of characteristics of echo signals of transverse and longitudinal waves from reflectors with flat surfaces

On the basis of an earlier developed model on scattering of transverse waves at a crack in the form of a strip, exact and approximate (estimated) scattering indicatrices for transverse and longitudinal elastic waves are studied, taking into account the influence of their transformation at the flat free surface of a reflector into other wave types on the change in the echo-signal amplitude. The limits of applicability of the estimated indicatrices for mirror and back reflections are numerically studied. The exact results for longitudinal waves are compared to those obtained by the Kirchhoff method.     

基于单测头误差相位偏移法的嵌入式角位移传感器自校准研究

针对嵌入式角位移传感器长期使用出现精度损失问题,结合传感器结构特点提出一种基于单测头误差相位偏移法的自校准方法。首先,依据圆周空间封闭性原则,基于傅里叶变换分析传感器误差特性;然后,以起始测量值序列作为误差偏移起始基准,根据误差特性等间隔变换误差序列并解算误差与起始基准的函数关系,构建传感器误差模型;最后,搭建在线校准实验平台进行验证实验。实验结果表明,单个对极内误差结果与测量误差特性一致,整周测量误差大幅度降低,误差峰峰值由127.80″降低至5.90″;该方法校准效率相比于比较校准法提升了80.69%,校准后误差分布集中,同时具有良好的测量稳定性。本文所述自校准方法在不依赖高精度基准器具前提下,有效解决了传感器精度损失问题,对实现嵌入式角位移传感器自校准具有重要的应用意义。     

On the calculation of autoresonance vibroimpact systems perturbed by random forces

The investigation into autoresonance vibroimpact systems is continued. As an example, the system containing the sensor for impact pulses is selected. The main purpose of the study is to the calculation procedure for systems in the presence of small random perturbations. The corresponding calculation procedure based on the determination of fluctuation corrections obtained by the averaging method is given. Examples of calculations for two types of perturbations, namely, small random forces of the white noise type and the forces with a short correlation time are presented. The solutions have a clear mechanical interpretation.     

On the analysis of models of strongly nonlinear vibroimpact processes in equipped lattice systems

Compound objects comprising a central structure with an elementary lattice structure and additional tools that interact with the central structure via systematic impacts of system elements are considered. Hypotheses of impact interaction can be either classical (Newtonian) or more realistic ones that take into account finite impact durations. Such structures can be investigated with analytical time-frequency techniques of vibroimpact process analysis, which are applied on going from differential motion to operator equations and, in describing periodic processes, to less general integral equations of periodic regimes. Defining relationships are derived, systems of operator equations are constructed, and examples are given.