Measurement of spherical aberration,beam tilt and image rotation in tem with a focussed probe

When a crystalline specimen in a transmission microscope is illuminated by a small focussed probe parallel to a zone axis, electrons diffracted at large angles by planes in higher order Laue zones (HOLZ) are imaged as a circle surrounding the direct beam. The diameter of this HOLZ image circle is directly related to the spherical aberration coefficient C_s of the objective lens. For the Philips EM430 at voltages between 50 and 300 kV, the 〈111〉 zone axis in silicon was used to measure an averaged C_s of 2.12 ± 0.04 mm. A displacement of the HOLZ image circle relative to the central probe image is attributed to a small tilt of the beam axis induced by lens interactions when switching between image and diffraction modes. This tilt could affect the quality of lattice images, where we require that both the beam and zone axes should be parallel with the optic axis. Even in an aberration-free lens, multiple diffracted images are observed when the probe is focussed and imaged in a plane below the specimen. This effect is proposed as a convenient method for measuring the direction and sign of g in two-beam defect analysis.     

The importance of beam alignment and crystal tilt in high resolution electron microscopy

The relative influences of crystal tilt and beam alignment on high-resolution electron-microscopic imaging have been investigated. With the use of contrast transfer theory in generalised dimensionless form, the major effect of slight beam misalignment has been shown to be the introduction of an antisymmetric phase shift in the diffracted beams so that the presence of any such misalignment cannot be detected by the standard diagnostic tool of high-resolution electron microscopy, namely the optical diffractogram. Specific image simulations, at 100 and 500 keV, for materials of both small and large unit cells (SnO₂ and Ti₂Nb₁₀O₂₉ respectively) show, however, that even slight beam tilt can have a marked effect on the images of crystalline materials, causing considerable spurious detail and a loss of expected symmetry. The various options for ensuring accurate beam and crystal alignment are briefly reviewed, and some aspects of the alignment problems are demonstrated using some recent experimental images recorded at 500 kV.     

Thickness dependence of signal/background ratio of inner-shell electron excitation loss in eels

The influence of the specimen thickness on the core-edge loss intensity has been studied using the relative specimen thickness (t_R) as the standard rule for the specimen thickness in EELS. The core-edge loss intensity monotonously increased with the increasing relative specimen thickness, 0 < t_R ? 1, and monotonously decreased with increasing relative specimen thickness, t_R ? 1, behind the maximum of core-edge intensity at t_R ? 1. “Optimum specimen thickness” to get the highest core-edge intensity is suggested for the thickness of t_R = 1. The thickness-dependent factor T(t_R) should be considered in a practical measurement in EELS.     

Wavelength- and energy-dispersive Electron Probe Microanalysis (EPMA) measurements with non-perpendicular incidence of the electron beam

A database of 416 wavelength‐ and energy‐dispersive EPMA measurements on tilted specimens of NiAl, TiO₂ and Ti₃Al is presented. The analyses were performed between 10 and 30 kV and the tilt angles were varied between 0° and 60° in seven steps. The necessary hardware modifications for the specimen holder are discussed, as well as the various focusing techniques used in the measurements. A comparison between the experimental data, the calculations of our proza 96t program and the results of Monte Carlo simulations shows that up to 50° tilt the predictions of our software are more than satisfactory. At larger tilt angles some deviations become noticeable. The Monte Carlo simulations appear to produce deviations at a somewhat earlier stage already, for reasons as yet unknown.     

High-angle triple-axis specimen holder for three-dimensional diffraction contrast imaging in transmission electron microscopy

Electron tomography requires a wide angular range of specimen-tilt for a reliable three-dimensional (3D) reconstruction. Although specimen holders are commercially available for tomography, they have several limitations, including tilting capability in only one or two axes at most, e.g. tilt-rotate. For amorphous specimens, the image contrast depends on mass and thickness only and the single-tilt holder is adequate for most tomographic image acquisitions. On the other hand, for crystalline materials where image contrast is strongly dependent on diffraction conditions, current commercially available tomography holders are inadequate, because they lack tilt capability in all three orthogonal axes needed to maintain a constant diffraction condition over the whole tilt range. We have developed a high-angle triple-axis (HATA) tomography specimen holder capable of high-angle tilting for the primary horizontal axis with tilting capability in the other (orthogonal) horizontal and vertical axes. This allows the user to trim the specimen tilt to obtain the desired diffraction condition over the whole tilt range of the tomography series. To demonstrate its capabilities, we have used this triple-axis tomography holder with a dual-axis tilt series (the specimen was rotated by 90° ex-situ between series) to obtain tomographic reconstructions of dislocation arrangements in plastically deformed austenitic steel foils.     

Effect of disturbances on the dynamic performance of a wavy-tilt-dam mechanical seal

A three-dimensional theoretical model is developed to study the effect of disturbances on the dynamic performance of a wavy-tilt-dam (WTD) mechanical seal. Parametric study on the WTD seal faces is presented. The results show that the wave amplitude is the significant parameter that affects the hydrodynamic effect and the floating ring's tracking property. When cavitation takes place in the film, the tilt of the WTD floating ring will increase the hydrodynamic force. The optimal parameters for the WTD seal working more stable are α=1, k around 10 and w_d=0.15B–0.20B.     

The secondary fluorescence correction for X-ray microanalysis in the analytical electron microscope

A general formulation for the secondary fluorescence correction is presented. It is intended to give an intuitive appreciation for the various factors that influence the magnitude of the secondary fluorescence correction, the specimen geometry in particular, and to serve as a starting point for the derivation of quantitative correction formulae. This formulation is primarily intended for the X-ray microanalysis of electron-transparent specimens in the analytical electron microscope (AEM). The fluoresced intensity, I^Y_X, is expressed relative to the primary intensity of the fluorescing element, I_Y, rather than to that of the fluoresced element, I_X, as has been customary for microanalysis. The importance of this choice of I_Y as a reference intensity for the electron-transparent specimens examined in the AEM is discussed. The various factors entering the secondary fluorescence correction are grouped into three factors, representing the dependencies of the correction on specimen composition, X-ray fluorescence probability and specimen geometry. In principle, an additional factor should be appended to account for the difference in detection efficiencies of the fluoresced and fluorescing X-rays; however, this factor is shown to be within a few per cent of unity for practical applications of the secondary fluorescence correction. The absorption of secondary X-rays leaving the specimen en route to the detector is also accounted for through a single parameter. In the limit that the absorption of secondary X-rays is negligible, the geometric factor has the simple physical interpretation as the fractional solid angle subtended by the fluoresced volume from the perspective of the analysed volume. Studies of secondary fluorescence in the published literature are compared with this physical interpretation. It is shown to be qualitatively consistent with Reed's expression for secondary fluorescence in the electron probe microanalyser and with the specimen-thickness dependence of the Nockolds expression for the parallel-sided thin foil. This interpretation is also used to show that the ‘sec α’ dependence on specimen tilt in the latter expression is erroneous and should be omitted. The extent to which extrapolation methods can be used to correct for secondary fluorescence is also discussed. The notion that extrapolation methods, by themselves, can be used to correct for secondary fluorescence is refuted. However, extrapolation methods greatly facilitate secondary fluorescence correction for wedge-shaped specimens when used in conjunction with correction formulae.     

Interpretation of the fading of diffraction patterns from organic substances irradiated with 100 keV electrons at 10-300 K

The fading curves of Debye-Scherrer ring intensities have been recorded for evaporated films of some aliphatic and aromatic compounds at temperatures between T_s=14-66 K and room temperature. The lowest real specimen temperature T_s was calibrated by the condensation of different gas atoms and depends on the current density used, which has to be raised with increasing resistivity to radiation damage. The fading curves for the lowest specimen temperature were compared with a theory of Clark et al. modified by a multiple-hit model. By fitting the theory to experimental results, values of the cross-section σ for a single hit, the mean number m of hits necessary to destroy a sensitive unit and the ratio b of the damaged to the undamaged structure amplitude of a unit cell can be established. There are considerable differences in these quantities for different compounds. The temperature dependence of σ for tetracene and other compounds can be explained by an activation energy of the order of a few tens of one meV.     

Erosion damage of polymeric material by slurry

An attempt was made to reveal the erosion mechanism of a plastic material in slurry containing glass beads of approximately 176 μm in diameter. The experimental observations were interpreted in terms of parameters such as the impact velocity V_p, the impact angle α of the beads and the striking efficiency η. A theoretical flow analysis for a solid-liquid two-phase flow was made and it was found that the trajectories of the particles curved as they approached the specimen, indicating that the actual surface onto which the particles were impinging was much narrower than that which would be expected for sand erosion in an air stream. In the slurry erosion the impact velocity and impact angle differed greatly at different positions of the specimen. This is possibly due to the differences in the density and viscosity of liquid and air. Two distinctly different types of erosion were observed: a typical brittle behaviour occurred near a stagnant point, after an initial incubation period, whereas at a distance from the stagnant point only a slight surface roughness was produced. These two different types of erosion produced a clear boundary. Erosion rates depended on the positions of the specimen and were found to be proportional to (V_p sin α)^2.6. This suggested that the normal component of the impact velocity of the particle determined the erosion rate of the plastic material due to slurry.     

Essential experimental parameters for quantitative structure analysis using spherical aberration-corrected HAADF-STEM

The accuracy of quantitative analysis for Z-contrast images with a spherical aberration (C_s) corrected high-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) using SrTiO₃(0 0 1) was systematically investigated. Atomic column and background intensities were measured accurately from the experimental HAADF-STEM images obtained under exact experimental condition. We examined atomic intensity ratio dependence on experimental conditions such as defocus, convergent semi-angles, specimen thicknesses and digitalized STEM image acquisition system: brightness and contrast. In order to carry out quantitative analysis of C_s-corrected HAADF-STEM, it is essential to determine defocus, to measure specimen thickness and to fix setting of brightness, contrast and probe current. To confirm the validity and accuracy of the experimental results, we compared experimental and HAADF-STEM calculations based on the Bloch wave method.     

Electron beam damage of behenic acid films

The rate of fading of electron diffraction patterns of behenic acid monolayer crystals as well as multilayer crystals was measured at 100 kV at room temperature to investigate the dependence of beam damage on specimen thickness. The diffracted intensities for monolayers and double layers decreased nearly exponentially with electron exposure; however, the intensities for multilayers were unchanged during initial electron exposures, often increased temporarily and then decreased with electron exposure. The critical dose, D_e, defined as the dose at which the diffracted intensity falls to 1/e of its initial value, was 1.0 electrons/Ų for the monolayers, 1.8 electrons/Ų for the double layers and more for multilayers. These results lead to the conclusion that D_e for behenic acid increases nearly linearly with specimen thickness in the range of about 25–100 Å for dose rate of 0.1–2 electrons/Ų min.     

Basic questions related to electron-induced sputtering in the TEM

Although the theory of high-angle elastic scattering of fast electrons is well developed, accurate calculation of the incident-energy threshold and cross section for surface-atom sputtering is hampered by uncertainties in the value of the surface-displacement energy E_d and its angular dependence. We show that reasonable agreement with experiment is achieved by assuming a non-spherical escape potential with E_d=(5/3) E_sub, where E_sub is the sublimation energy. Since field-emission sources and aberration-corrected TEM lenses have become more widespread, sputtering has begun to impose a practical limit to the spatial resolution of microanalysis for some specimens. Sputtering can be delayed by coating the specimen with a thin layer of carbon, or prevented by reducing the incident energy; 60 keV should be sufficiently low for most materials.     

Automated high-throughput electron tomography by pre-calibration of image shifts

Electron tomography is a versatile method for obtaining three‐dimensional (3D) images with transmission electron microscopy. The technique is suitable to investigate cell organelles and tissue sections (100–500 nm thick) with 4–20 nm resolution. 3D reconstructions are obtained by processing a series of images acquired with the samples tilted over different angles. While tilting the sample, image shifts and defocus changes of several µm can occur. The current generation of automated acquisition software detects and corrects for these changes with a procedure that incorporates switching the electron optical magnification. We developed a novel method for data collection based on the measurement of shifts prior to data acquisition, which results in a five‐fold increase in speed, enabling the acquisition of 151 images in less than 20 min. The method will enhance the quality of a tilt series by minimizing the amount of required focus‐change compensation by aligning the optical axis to the tilt axis of the specimen stage. The alignment is achieved by invoking an amount of image shift as deduced from the mathematical model describing the effect of specimen tilt. As examples for application in biological and materials sciences 3D reconstructions of a mitochondrion and a zeolite crystal are presented.     

Investigation of the overall friction coefficient in single-pass scratch test

Single-pass scratch test on bilinear elastic–plastic materials with a conical indenter was simulated using a three-dimensional finite element model. The influence of the interfacial friction coefficient μ_s and the apical angle α of the indenter on the induced maximum tangential force F_T, normal force F_N, and the overall friction coefficient μ=F_T/F_N, were systematically studied. It was found that the induced tangential force is greater than the normal force when the apex is small and vice versa when the apex is large. The tangential force increases with μ_s, but the normal force decreases with μ_s. The overall friction coefficient μ was found to increase linearly with μ_s and tangent of the attack angle of the indenter. The relationship between the adhesion frictional component (μ_a), the plowing frictional component (μ_p), and the interfacial friction coefficient μ_s was analyzed. An analytical model for the overall friction coefficient μ was also developed based on the interaction between the indenter and the specimen and compared to the numerical results. The model was found to yield a good agreement with the finite element simulation results.     

Revision of cavitation erosion database and analysis of stainless steel data

Cavitation erosion data have been accumulated in our laboratory for about 32 years since 1970. The database was constructed as electronic data in MS Excel files. The data files are able to offer quick search in terms of the test material, test method and test conditions from among 859 data. In this study, 131 data since 2003 were newly added to the database constructed in our previous study. The stainless steel data were analyzed, including various stainless steels such as ferritic, austenitic, duplex and martensitic stainless steels. Vibratory cavitation test results for different stainless steels, obtained with varying test conditions of frequency, amplitude and attachment of specimen, were converted analytically to obtain average erosion rates under assumed standardized conditions of a stationary specimen test with 1 mm standoff distance, and with frequency and amplitude as specified by ASTM G32. The average of erosion rate under the standardized condition (ASTM G32, stationary specimen method, standoff distance 1 mm) was determined for different stainless steels. The erosion resistance was defined as a reciprocal of erosion rate, and the correlation between erosion resistance and hardness of the specimen after erosion test was better than with the other mechanical properties. The erosion resistance is equal to 2.6E?07 × (HV × F_mat)^2.4 (HV; Vickers hardness, F_mat; material factor), and the correlation coefficient is 0.98. It was concluded that the erosion resistance of different stainless steels could be estimated with high reliability from the material hardness and the material factor.     

Applicability of ultrasonic technique for evaluation of elastic plastic fracture toughness of high manganese steel at low temperatures

A study on the evaluation of elastic-plastic fracture toughness, J_IC, by ultrasonic technique is described for a newly developed high manganese steel at low temperature. In order to see the applicability of the ultrasonic technique based on pulse echo method at low temperature, special attention was paid to detect change point of ultrasonic echo due to the onset of stable crack growth. The J_IC values are evaluated by the ultrasonic method (which needs single specimen) and by stretched zone method (which needs several specimens) for compact tension (CT) specimens and three-point bend (3PB) specimens. The temperature dependence of these J_IC values of theCT specimens by the ultrasonic method show almost the same values to 3PB specimens over the temperature range tested. The J_IC values of 3PB specimens by the stretched zone method show slightly higher values than those of theCT specimens at low temperature. The J_IC values evaluated by the ultrasonic method give more conservative values than those evaluated by the stretched zone method for bothCT and 3PB specimens.     

Initiation and growth of buckling in the biaxial diagonal tensile test on steel sheet

Initiation and growth of buckling, a main cause of surface deflection in large steel press-formed components such as automobile outer panels, is simulated by the biaxial diagonal tensile test on square steel sheet specimens. The effects of the properties of the steel sheets and the biaxial loading conditions on the initiation and growth of buckling are examined by the tests and by FEM analysis.It is found that both the initiation and the growth of buckling are strongly affected by the yield strength of the material, σ_s, and by the tensile stress in the breadth direction, p_Y, and that buckling is restrained when p_Y/σ_s is high: this is because the compressive stress at the center of the specimen, the compressive stress area, and the development of the buckling area are largely affected by these factors.     

In situ topography measurement in the SEM

An automatic technique for measuring heights in situ in the SEM based on a combination of stereo-metric and focusing methods has been developed. A pair of images of a surface element on the specimen is obtained by tilting the beam electrically in a manner such that the plane containing the tilt axis is coincident with the focal plane of the final lens. Cross-correlation is used to determine the parallax between the image pair which is then used to iteratively correct the height of the tilt axis by changing the lens focus. As a result, the lens focus tracks the specimen topography. With an appropriate specimen surface containing high resolution features for image correlation, the technique is capable of maintaining both its lateral and vertical resolutions over several decades of height displacement up to 100 μm. In an experimental system based on a commercial electron-optical column, spot, line profile, and three-dimensional measurements have been demonstrated.     

Plastic deformation of copper single crystals by wedge indentations

Plastic deformations due to wedge indentation of copper single crystals with three different orientations are quantitatively assessed by analyses of deformed grid patterns on the specimens. Deformed regions, which are subdivided into six regions in terms of slip traces, are found to extend for 10–13 times the contact site size between the wedge and the specimen. The distortions near the contact site show that the deformations are generally characterized by typical features of strains and material rotation such as negative ?_x (normal strain in the indentation direction) in the deep region, positive ?_x in the shallow region, negative γ_xy(engineering shear strain) in the intermediate region and rotation contracting the material in the indentation direction. Although such distortions are affected by the feasibility of cross slip and the orientation dependence of the elasticity, they indicate that the overall material flow is similar to that in polycrystalline metals.     

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.