Function Of Specimen Thickness Research Articles

Highly Sensitive Detection of Microstructure Variation Using a Thickness Resonant Transducer and Pulse-Echo Third Harmonic Generation.

In nonlinear ultrasound testing, the relative nonlinear parameter is conveniently measured as a sensitive means of detecting and imaging overall variation of microstructures and damages. Compared to the quadratic nonlinear parameter (β'), the cubic nonlinear parameter (γ'), calculated as the third harmonic amplitude divided by the cube of the fundamental amplitude, has generally a higher value, providing better sensitivity in nonlinear parameter mapping. Since the third harmonic amplitude is about two orders of magnitude lower than the fundamental amplitude, efficient excitation and highly sensitive reception of third harmonic is very important. In this paper, we explore an odd harmonic thickness resonant transducer that meets the requirements for pulse-echo third harmonic generation (THG) measurements. We also address the problem of source nonlinearity that may be present in the measured amplitude of the third harmonic and propose a method to properly correct it. First, we measure γ' for a series of aluminum specimens using the through-transmission method to observe the behavior of γ' as a function of specimen thickness and input voltage, and examine the effects of various corrections such as attenuation, diffraction and source nonlinearity. Next, we apply the odd harmonic resonant transducer to pulse-echo THG measurements of precipitation heat-treated specimens. It is shown that such transducer is very effective in generation and detection of fundamental and third harmonics under finite amplitude toneburst excitation. The highly sensitive detectability of γ' are presented as a function of aging time, and the sensitivity of γ' is compared with that of β' and β'2.

Size-dependence of AM Ti–6Al–4V: Experimental characterization and applications in thin-walled structures simulations

Previous studies show that the properties of parts manufactured via additive manufacturing, such as selective laser melting, depend on local feature sizes like lattice wall thickness and strut diameter. Although size dependence has been studied extensively, it was not included in constitutive models for numerical simulations. In this work, flat dog-bone tensile specimens of different thicknesses were manufactured and tested under quasi-static conditions to characterize the size-dependent properties experimentally. It was observed that key mechanical properties decrease with specimen thickness. Through curve-fitting to experimental data, this work provides approximate analytical expressions for the material properties values as a function of specimen thickness, furnishing a phenomenological size-dependent constitutive model. The interpolating capability of the model is cross-validated with existing experimental data. Two numerical examples demonstrate the application of the size-dependent material model. The axial crushing of thin-walled lattices at varying wall thicknesses was simulated by the size-dependent material model and one that ignores size effects. Results show that ignoring size effects leads to overestimated peak crushing force and specific energy absorption. The two material models were also compared in the topology optimization of thin-walled structures. Results show that the size-dependent model leads to a more robust optimized design: having higher energy absorption and sustaining less material fracture.

Accurate Thickness Measurement Using Eddy Current System Based on Novel Transformer Model

Eddy current testing has been widely regarded as a popular tool for thickness measurement due to the advantages of non-contact, low cost and efficiency. Phase signature is believed to change linearly as a function of specimen thickness using the conventional transformer model, which, however, does not take into account the effect of eddy current diffusion and reflection. In this work, a novel transformer model is developed, and an accurate method for thickness measurement is proposed subsequently. Firstly, true skin depth is formulated with a simple but accurate function, and an equivalent thickness is introduced to characterize eddy currents considering the difference between the standard and true skin depth. Secondly, the use of the thickness of equivalent eddy currents develops the novel transformer model, and a novel method is figured out to improve the accuracy of thickness measurement. Basically, the difference between the true and standard skin depth is closely related to excitation frequency, liftoff distance and the electromagnetic properties of a specimen. Therefore, a correction parameter is needed and it should be re-determined for different cases. Finally, an eddy current system was built, and the experiments were carried out to evaluate the novel method. The results show that it outputs more accurate specimen thickness, especially for low conductive materials.

Highly translucent dental resin composites through refractive index adaption using zirconium dioxide nanoparticles and organic functionalization

ObjectivesFor dental resin composites, high translucency is important. Therefore, the aim of the study was to create a biocompatible and highly translucent resin-based composite, and to investigate the effect of material thickness on translucency. MethodsA biocompatible ORMOCER® resin matrix was reinforced with dental glass powder as fillers. To reach a high translucency, refractive index matching of the matrix and fillers was done in the two ways: (1) Highly refractive ZrO2 nanoparticles were incorporated into the resin. (2) The resin was modified via addition of 4-Methylthiophenol. The corresponding refractive indices were acquired on an Abbe refractometer (n = 5). In both cases, the dental glass powder was added and translucency of the resulting minifilled and nanohybrid composites were measured using spectral photometry (n = 5). Additionally, the translucency of the experimental composites was determined as a function of specimen thickness in the range 10 μm–2 mm (n = 5). One-way ANOVA was performed to determine the significant differences in various optical parameters among different amounts of modifications and thicknesses at α = 0.05. Furthermore, cytotoxicity tests (extract and direct contact tests) were conducted according to ISO 10993 to classify the biocompatibility of the composites (n = 6). ResultsThe translucency values of the composites with 47 wt.-% dental glass powder and a specimen thickness of 2 mm, could be increased from 26% up to 71% by increasing the refractive index of the matrix through incorporating ZrO2 nanoparticles. Moreover, it can also be increased to 67% via addition of 4-Methylthiophenol. Further results showed that the translucency significantly depended on the sample thickness following an exponential function. The effect of all tested parameters was significant among the materials (p < 0.001). The composites did not show any cytotoxic effect. SignificanceHighly translucent and biocompatible resin composites were developed. They show attractive properties for the use as dental enamel material in direct and indirect restorations.

Measurement of atomic electric fields and charge densities from average momentum transfers using scanning transmission electron microscopy

This study sheds light on the prerequisites, possibilities, limitations and interpretation of high-resolution differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM). We draw particular attention to the well-established DPC technique based on segmented annular detectors and its relation to recent developments based on pixelated detectors. These employ the expectation value of the momentum transfer as a reliable measure of the angular deflection of the STEM beam induced by an electric field in the specimen. The influence of scattering and propagation of electrons within the specimen is initially discussed separately and then treated in terms of a two-state channeling theory.A detailed simulation study of GaN is presented as a function of specimen thickness and bonding. It is found that bonding effects are rather detectable implicitly, e.g., by characteristics of the momentum flux in areas between the atoms than by directly mapping electric fields and charge densities. For strontium titanate, experimental charge densities are compared with simulations and discussed with respect to experimental artifacts such as scan noise. Finally, we consider practical issues such as figures of merit for spatial and momentum resolution, minimum electron dose, and the mapping of larger-scale, built-in electric fields by virtue of data averaged over a crystal unit cell. We find that the latter is possible for crystals with an inversion center. Concerning the optimal detector design, this study indicates that a sampling of 5mrad per pixel is sufficient in typical applications, corresponding to approximately 10×10 available pixels.

Self-consistent absorption correction for quantitative energy- dispersive X-ray spectroscopy of InGaN layers in analytical transmission electron microscopy

A new method of absorption correction for energy-dispersive X-ray spectroscopy in a transmission electron microscope is tested on InGaN samples. We simulate the effective k-factor for the In L line with respect to Ga L or Ga K and plot this as a function of the Ga K/L intensity ratio, which can be directly measured from experimental spectra. This basically performs an internal self-consistency check in the quantification using differently absorbed X-ray lines, which is in principle equivalent to an absorption correction as a function of specimen thickness but has the practical advantage that neither specimen thickness nor density or mass-thickness of the specimens need actually be measured.

Calibration of thickness-dependent k-factors for germanium X-ray lines to improve energy-dispersive X-ray spectroscopy of SiGe layers in analytical transmission electron microscopy

We show that the accuracy of energy-dispersive X-ray spectroscopy can be improved by analysing and comparing multiple lines from the same element. For each line, an effective k-factor can be defined that varies as a function of the intensity ratio of multiple lines (e.g. K/L) from the same element. This basically performs an internal self-consistency check in the quantification using differently absorbed X-ray lines, which is in principle equivalent to an absorption correction as a function of specimen thickness but has the practical advantage that the specimen thickness itself does not actually need to be measured.

A HAADF investigation of AlAs/GaAs interfaces using SuperSTEM

Z-contrast imaging using a high-angle annular dark field detector can be used to characterise III-V heterostructures. GaAs/AlAs heterostructures were grown using MBE and prepared for TEM using a cross-sectional method. SuperSTEM 1 was used to investigate both the GaAs-on-AlAs and the AlAs-on-GaAs interfaces as a function of specimen thickness. The analysis of the images showed that the apparent interface widths varied with thickness in an unexpected manner. The GaAs-on-AlAs widths remained constant with thickness while the AlAs-on-GaAs interface widths increased. The actual interfacial width can be a result of either surface stepping during MBE growth or inter-diffusion of the column III atoms. To assist the interpretation of this result a series of interfacial models were created and explored using a frozen phonon multislice simulation. The models consisted of terraced, vicinal and diffused interfaces. The results show that while the width of the diffused interface reduces with specimen thickness, the widths of the stepped interfaces vary in a more complex manner. This variability is due in part to the different channeling depths of the Ga and Al columns, but the measured width is also affected by the depth of the material before the first step and whether GaAs or AlAs is on top. These effects can cause the apparent interfacial width to vary markedly from the actual width. Even measuring the width itself is non-trivial.

Quantitative EFTEM mapping of near physiological calcium concentrations in biological specimens

Although electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) provides high sensitivity for measuring the important element, calcium, in biological specimens, the technique has been difficult to apply routinely, because of long acquisition times required. Here we describe a refinement of the complementary analytical technique of energy-filtered transmission electron microscopy (EFTEM), which enables rapid imaging of large cellular regions and measurement of calcium concentrations approaching physiological levels. Extraction of precise quantitative information is possible by averaging large numbers of pixels that are contained in organelles of interest. We employ a modified two-window approach in which the behavior of the background signal in the EELS spectrum can be modeled as a function of specimen thickness t expressed in terms of the inelastic mean free path λ. By acquiring pairs of images, one above and one below the Ca L 2,3 edge, together with zero-loss and unfiltered images, which are used to determine a relative thickness ( t/ λ) map, it is possible to correct the Ca L 2,3 signal for plural scattering. We have evaluated the detection limits of this technique by considering several sources of systematic errors and applied this method to determine mitochondrial total calcium concentrations in freeze-dried cryosections of rapidly frozen stimulated neurons. By analyzing 0.1 μm 2 areas of specimen regions that do not contain calcium, it was found that the standard deviation in the measurement of Ca concentrations was about 20 mmol/kg dry weight, corresponding to a Ca:C atomic fraction of approximately 2×10 −4. Calcium concentrations in peripheral mitochondria of recently depolarized, and therefore stimulated and Ca loaded, frog sympathetic neurons were in reasonable agreement with previous data.

The response to shock loading of a glass-fibre–epoxy composite: effects of fibre orientation to the loading axis

The shock response of a two-dimensional glass-fibre–epoxy composite has been investigated in terms of orientation of the fibres to the loading axis. Results show that when shocked in the through thickness orientation (fibres normal to the loading axis), the resultant shock pulses are similar to many other monolithic materials, and the shock velocity–particle velocity is linear. Examination of the spall response as a function of specimen thickness and the release velocities has also led us to suggest that the shock pulse generates damage behind it, which accumulates as a function of pulse duration. In contrast, when shocked along a fibre direction, the shock pulses have a strongly ramped nature, which steepens to a shock at higher stress amplitudes. Analysis of the wave velocity–particle velocity behaviour has led us to suggest that a low amplitude stress pulse is transmitted along the fibres, with the higher amplitude remainder being transmitted in the ‘matrix’—i.e. the epoxy binder and fibres normal to the loading axis between them. Such a response has been observed in other fibre-based composites, and it is believed that this is due to the structural configuration of such materials.

Quantitative Morphology Study of Cu-Neutralized Poly(styrene-ran-methacrylic acid) Ionomers: STEM Imaging, X-ray Scattering, and Real-Space Structural Modeling

Our previous study showed quantitative correspondence between the size of ionic aggregates in a Cu-neutralized poly(styrene-ran-methacrylic acid) ionomer (Cu-SMAA) measured from scanning transmission electron microscopy (STEM) images and the sizes determined from X-ray scattering data interpreted using the Yarusso−Cooper model. Here we compare the average sample volume per ionic aggregate (Vp) from STEM images and X-ray scattering data from Cu-SMAA. The relationship between the 3D morphology of hard spheres as defined by the Yarusso−Cooper parameters (R1, RCA, Vp) and the projected 2D images obtained by STEM was elucidated by modeling. These simulations show that a significant number of the ionic aggregates in Cu-SMAA are obscured due to overlap effects and provide a quantitative relationship between the number of spheres in 3D and the number of features in 2D projections as a function of specimen thickness. Upon measuring the local thickness of a Cu-SMAA specimen using electron energy loss spectrometry (EELS), we find that the Vp determined from STEM images strongly agrees with that determined from X-ray scattering data. Thus, quantitative correspondence of both the size and number density of ionic aggregates has been obtained from direct imaging and X-ray scattering of Cu-SMAA. These data provide compelling evidence that the Yarusso−Cooper scattering model presents an excellent description of the nanoscale morphology of this solvent cast ionomer. Interpretation of this morphological data indicates that the ionic aggregates likely include both ionic and nonionic species. This comprehensive characterization provides a foundation on which a systematic evaluation of the factors that influence the nanoscale morphology, and consequently the macroscopic properties, can be accomplished.

Quantification and thickness correction of EFTEM phosphorus maps

We describe a method for correcting plural inelastic scattering effects in elemental maps that are acquired in the energy filtering transmission electron microscope (EFTEM) using just two energy windows, one above and one below a core edge in the electron energy loss spectrum (EELS). The technique is demonstrated for mapping low concentrations of phosphorus in biological samples. First, the single-scattering EELS distributions are obtained from specimens of pure carbon and plastic embedding material. Then, spectra are calculated for different specimen thicknesses t, expressed in units of the inelastic mean free path λ. In this way, standard curves are generated for the ratio k 0 of post-edge to pre-edge intensities at the phosphorus L 2,3 excitation energy, as a function of relative specimen thickness t/λ. Thickness effects in a two-window phosphorus map are corrected by successive acquisition of zero-loss and unfiltered images, from which it is possible to determine a t/λ image and hence a background k 0-ratio image. Knowledge of the thickness-dependent k 0-ratio at each pixel thus enables a more accurate determination of the phosphorus distribution in the specimen. Systematic and statistical errors are calculated as a function of specimen thickness, and elemental maps are quantified in terms of the number of phosphorus atoms per pixel. Further analysis of the k 0-curve shows that the EFTEM can be used to obtain reliable two-window phosphorus maps from specimens that are considerably thicker than previously possible.

Phonon Scattering in High-Resolution Electron Microscopy

It is generally accepted that the lattice fringe contrast in experimental high-resolution electron microscope (HREM) images is lower than that in image simulations by a factor of between 2 and 6 [1]. One likely reason for this discrepancy is the contribution phonon, or thermal diffuse, scattering makes to lattice images. Electrons scattered by phonons are visible as diffuse scattering between the Bragg spots in the diffraction pattern and the loss in intensity of the Bragg spots is modeled by the Debye-Waller factor in image simulations. However, the additive effect of this diffuse scattering on images is not taken into account by image simulations. The amount of phonon scattering as a function of specimen thickness has been measured both for Si by convergent beam diffraction [2] and for ZrB12 by electron holography [3] and in both cases found to contribute about 10% of the total intensity at a thickness typically used in high resolution imaging.

Development of a new analytical electron microscopy technique to quantify the chemistry of planar defects and to measure accurately solute segregation to grain boundaries

A new technique of analytical transmission electron microscopy called ConceptEM has been developed for determining highly accurately small amounts of solute or dopant atoms incorporated into well-defined planar defects such as stacking faults, grain boundaries or interfaces. The method is based on recording series of analytical spectra taken with different electron beam diameters on the same position centred above a defect that is orientated either edge-on or slightly inclined with respect to the electron beam. It can be applied to energy-dispersive X-ray spectroscopy or electron energy-loss spectroscopy and necessitates only a nano-probe modus but no scanning unit. Reliability and accuracy have been tested numerically under various conditions using simulations for a specific geometry, as a function of specimen thickness, material, acceleration voltage, collection angle, random beam displacements and solid solubility. The accuracy has been found to be substantially better (by factors of 5-10) than that of any other current standard technique based on single measurements. Our calculations suggest an accuracy in the determination of the Gibbsian solute excess at a special grain boundary down to +/- 1% of a monolayer, i.e. around +/- 0.1 atoms nm(-2) under typical experimental conditions, with a maximum error about twice as large. The parameter limiting a straightforward analysis is found to be the solid solubility, which itself, however, can be measured accurately by the technique so that it can be taken into account quantitatively and the above-stated precision is retained.

Lorentz Microscopy Study on the Magnetization Reversal Process in Co-Zr-O Nanogranular Magnetic Films

The domain structure and the magnetization reversal process in Co/sub 71/Zr/sub 13/O/sub 16/ granular thin film, governed by a perpendicular anisotropy, are reported. Pattern of maze domains is observed, and the configuration is refined by the surface magnetic charges with function of specimen thickness. The wall energy is reduced by the low effective anisotropy and the exchange constant mediated by the amorphous layer. The magnetization reversal process is characterized by two magnetization rotation and one Barkhausen jump processes. The Barkhausen jumps easily take place at "Y" junctions of the stripe domains, where the distribution of the surface charges is irregular; the magnetization rotations are nonuniform in order to meet the requirement of the decrease in the demagnetization energy. 360/spl deg/ domain walls are found after Barkhausen jumps, indicating some pinning sites in the film.

Quantitative electron holography of biased semiconductor devices

Off-axis and in-line electron holography have been used to determine the electricalproperties of a silicon p–n junction. Specimens were prepared for transmission electronmicroscopy (TEM) by focused ion beam (FIB) thinning and examined in a biasing holder,with holograms recorded as a function of specimen thickness and applied reverse bias. Thedata revealed the important role played by the surfaces of the thin TEM sample whichaffect the electrostatic potential distribution within the specimen. The presence ofelectrically inactive layers, corresponding to the FIB-damaged amorphous surfaceand an additional crystalline layer, was inferred from the experimental results.Such effects must be taken into account if electron holography is to be used in areproducible and quantitative way for the examination of semiconductor devices.

The phonon contribution to high-resolution electron microscope images

The amount of phonon scattering as a function of specimen thickness is determined for a clean silicon sample, free from amorphous surface layers, by measuring the diffuse scattering in energy-filtered convergent-beam diffraction patterns. It is found that for a 25 nm thick sample, only 7.5% of the intensity scattered to less than 18 nm −1 is phonon scattered. This means that in a typical high-resolution sample most of the diffuse scattering is caused by surface amorphous layers rather than phonon scattering.

Damage evolution in low velocity impacted unreinforced vinyl ester 411-350 and 411-C50 resin systems

Damage evolution in plaques made of vinyl ester resin systems was investigated as a function of specimen thickness, impact energy level and matrix material. Dow DERAKANE vinyl ester 411-350 and 411-C50 resin systems, which have low viscosity and are ideally suited for low-cost liquid processing techniques like vacuum assisted resin transfer molding (VARTM), were considered for the low velocity instrumented impact testing. Characterization of damage evolution was undertaken using optical microscopy and analysis of impact load histories recorded during the impact event. Radial cracking, perforations at the point of impact (in the form of a truncated cone), and damage resulting from the support constraints were identified as the dominant failure characteristics in both resin systems. Radial cracking, which originated from the bottom surface, was operative in all failed specimens and was attributed to the catastrophic failure due to extensive flexural tensile strength losses. For specimens that could deflect significantly, radial cracking and support-constraint-induced damage were the operative failure mechanisms. Radial cracking and through-thickness shearing led to failure in stiffer plaques. The DERAKANE 411-350-vinyl ester resin system was found more damage resistant than the 411-C50 system.

Effect of specimen thickness on fatigue crack growth rate

Fatigue tests were performed on the compact tension (CT) specimens of Type 304 stainless steel and Inconel 718. To investigate the effects of specimen thickness on crack tip deformation and fatigue crack growth rate (FCGR), specimens of different thickness were used. In the analysis, the elastic plastic fracture mechanics (EPFM) parameter known as the cyclic J-integral, Δ J was adopted to observe the local plasticity at the crack tip and compared with the linear elastic fracture mechanics (LEFM) parameter known as the stress intensity factor range, Δ K. The results show that FCGR is a function of specimen thickness, the effect of which is accelerated as specimen thickness increases. Therefore, it is thought that not only applied stress level but also specimen thickness should be taken into account in the measurement of FCGR, which is not considered in ASTM E 647 (ASTM E 647, 1995. Standard test method for measurement of fatigue crack growth rates.).

The use of multiple-scattering data to enhance small-angle neutron scattering experiments.

Multiple scattering of neutrons by the inhomogeneities responsible for small-angle neutron scattering (SANS) during the passage of the beam through the specimen can be used to provide valuable information about the shape of the objects and the absolute value of the contrast between the scattering particles and the matrix. The neutrons emerging from the specimen are classified into those that have been scattered n times. The index n ranges from zero to infinity. The remnant of the incident beam is the group of neutrons for which n equals zero. Each group contributes separately to the scattering profile. The small-angle scattering cross section is independent of the neutron wavelength for n = 1 only. Thus collection of data as a function of specimen thickness and of neutron wavelength will provide a number of different profiles describing the same physical situation. Simultaneous analysis of these profiles provides absolute values of the cross section for scattering into the small-angle region and of the cross section for removal of neutrons from the small-angle region. So that the method can be used generally, a profile function that is a very good approximation to those in the literature is introduced. The implications for time-of-flight SANS are discussed.