Geometric Issues Research Articles

Hydrogen-Bonded Macrocyclic Supramolecular Systems in Solution and on Surfaces.

Cyclization into closed assemblies is the most recurrent approach to realize the noncovalent synthesis of discrete, well‐defined nanostructures. This review article particularly focuses on the noncovalent synthesis of monocyclic hydrogen‐bonded systems that are self‐assembled from a single molecule with two binding‐sites. Taking advantage of intramolecular binding events, which are favored with respect to intermolecular binding in solution, can afford quantitative amounts of a given supramolecular species under thermodynamic control. The size of the assembly depends on geometric issues such as the monomer structure and the directionality of the binding interaction, whereas the fidelity achieved relies largely on structural preorganization, low degrees of conformational flexibility, and templating effects. Here, we discuss several examples described in the literature in which cycles of different sizes, from dimers to hexamers, are studied by diverse solution or surface characterization techniques.

Geometrical Issues on the Structural Analysis of Transmission Electricity Towers Thanks to Laser Scanning Technology and Finite Element Method

This paper presents a multidisciplinary approach to reverse engineering and structural analysis of electricity transmission tower structures through the combination of laser scanning systems and finite element methodology. The use of laser scanning technology allows the development of both drawings and highly accurate three-dimensional geometric models that reliably reproduce geometric reality of towers structures, detecting imperfections, and particularities of their assembly. Due to this, it is possible to analyze and quantify the effect of these imperfections in their structural behavior, taking into account the actual geometry obtained, different structural models, and load hypotheses proposed. The method has been applied in three different types of metal electricity transmission towers with high voltage lines located in Guadalajara (Spain) in order to analyze its structural viability to accommodate future increased loads with respect that which are currently subjected.

Geometric issues in V-bending electromagnetic forming process of 2024-T3 aluminum alloy

As one of the high speed forming technologies, electromagnetic forming (EMF) has the ability to solve low formability problems in such lightweight alloys as aluminum and magnesium during the conventional forming process. However, one of the drawbacks of EMF is that shape deviation exists between the formed part and the desired part. In order to control this deviation, an in-depth study has been carried out so as to determine the causes of geometrical deviation during the EMF process and possible improvement methods. A series of V-bending electromagnetic forming experiments with rectangular samples of 2024-T3 were carried out using a flat spiral coil and V-shaped die at various voltages. Furthermore, not only the geometrical issues, especially the saddle issue, were quantitatively analyzed, but also a numerical model is established so as to analyze the origin and progression of geometric issues in the part. In the analysis, four major aspects were considered, namely the electromagnetic force distribution, the update of electromagnetic force distribution, the collision impact between sheet and die, and the duration of coil current pulse. It has been observed that electromagnetic force intensifies at the edges from which the distance to the eddy current center is shorter, and that the non-uniformity of force distribution deteriorates with increasing voltage, resulting in further non-uniformity of sheet deformation. In addition, the update of force distribution in sheet plays a useful role in obtaining the desired shape and a proper impact helps to weaken the saddle-like geometrical issue. Moreover, widening the duration of coil current pulse can suppress the rebound of the part in forming with a shallow die.

Effect of Cathode Contacting on Anode Supported Cell Performances and Degradation

Stacking Solid Oxide Fuel Cells implies the use of contacting layers, especially in the cathode compartments where ohmic losses are introduced at the interface between the electrode and the interconnect due to the high interfacial resistance and the lowered contact area caused by geometrical and porosity issues. Such losses compromise the current distribution over the electrodes strongly influencing the system performances in terms of power outputs. The contacting issues are weakly dependent on the operating conditions compared to other dissipative contributions related to the cells. This implies the need to solve them at the structural level in order to ensure high system performances. A dedicated testing setup has been used to test anode-supported cells with different contacting solutions. The contact geometry effect has been evaluated comparing the use of platinum meshes and stamped metallic bipolar plates (Crofer 22 APU) as current collectors. Additionally, the application of a La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) paste between the cathode and the interconnect allowed to estimate the beneficial effect of the contacting layer enhancing the current distribution at the cathode side of the cell. The system performances have been investigated by means of current-voltage curves. Electrochemical impedance spectroscopy measurements were carried out to distinguish the loss contributions within the system and study the degradation phenomena. Scanning electron microscopy equipped with energy dispersive X-ray spectroscopy and X-ray diffraction techniques were adopted as post-experiment characterization to investigate microstructural changes and elements migration in order to confirm the observed changes in performances.

Road Safety and Pavement Management: a case study of Tanzania

The implementation of pavement management seems to ignore road safety, with its focus being mainly on infrastructure condition. Safety management as part of pavement management should consider various means of reducing the frequency of vehicle crashes by allocating corrective measures to mitigate accident exposure, as well as reduce accident severity and likelihood. However, it is common that lack of accident records and crash contributing factors impedes incorporating safety into pavement management. This paper presents a case study for the initial development of pavement management systems considering data limitations for 3000 km of Tanzania’s national roads. A performance based optimization utilizes indices for safety and surface condition to allocate corrective measures. A modified Pareto analysis capable of accounting for annual performance and of balancing resources to achieve good surface condition and low levels of safety was applied. Tradeoff analysis for the case study found the need to assign 30% relevance to condition and 70% to road safety. Safety and condition deficiencies were corrected within 5 years with the majority of improvements dedicated to surface treatments and some geometric corrections. Large investments for correcting geo-metric issues were observed in years two and three if more money was made available.

EXAFS investigation of UF4

Author(s): Tobin, JG; Booth, CH; Siekhaus, W; Shuh, DK | Abstract: A comparative structural study of UF4 and UO2, using extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge structure (XANES) techniques was conducted as part of the studies of 5f covalency in uranium compounds. This study confirmed the quality of the samples and provided additional insight into geometrical issues related to ionicity and covalency. It was observed that the local structure of UF4 from EXAFS was consistent with the long range structure derived from diffraction data.

Minimization of distortions during laser welding of ultra high strength steel

Ultra high strength steels are frequently used within the automotive industry for several components. Welding of these components is traditionally done by resistance spot welding, but to get further productivity and increased strength, laser welding has been introduced in the past decades. Fusion welding is known to cause distortions due to built in stresses in the material. The distortions result in geometrical issues during assembly which become the origin of low joint quality due to gaps and misfits. U-beam structures of boron steel simulating B-pillars have been welded with laser along the flanges. Welding parameters and clamping have been varied to create different welding sequences and heat input generating a range of distortion levels. The distortions have been recorded dynamically with an optical measurement system during welding. In addition, final distortions have been measured by a digital Vernier caliper. The combined measurements give the possibility to evaluate development, occurrence, and magnitude of distortions with high accuracy. Furthermore, section cuts have been analyzed to assess joint geometry and metallurgy. The results show that final distortions appear in the range of 0–8 mm. Distortions occur mainly transversely and vertically along the profile. Variations in heat input show clear correlation with the magnitude of distortions and level of joint quality. A higher heat input in general generates a higher level of distortion with the same clamping conditions. Section cuts show that weld width and penetration are significantly affected by welding heat input. The present study identifies parameters which significantly influence the magnitude and distribution of distortions. Also, effective measures to minimize distortions and maintain or improve joint quality have been proposed. Finally, transient finite element (FE) simulations have been presented which show the behavior of the profiles during the welding and unclamping process.

Geometric issues in reverse osmosis: numerical simulation and experimentation.

This investigation is a synergistic combination of laboratory experimentation and numerical simulation to quantify the practical impact of geometric imperfections in the flow channels of a reverse osmosis (RO) system. To this end, carefully executed experiments are performed to quantify the fluid flow in a system containing feed spacers which are embedded in the RO membrane. In a complementary activity, numerical simulations were performed both for an ideal geometric situation (without embedments) and the actual geometric configuration including the embedments. It was found that the presence of unaccounted embedments affected the pressure drop predictions for the system by 14-19%. When account was taken of the embedments, the simulation results were found to be virtually coincident with the experimental results. This outcome suggests that deviations between experimental and simulation results encountered in the literature might well have been due to geometrical deviations of the type investigated here. The numerical simulation of the feedwater fluid flow was based on the often-used but unverified assumption that the velocity field experiences the geometric periodicity of the feed spacer. This assumption was lent support by results from a non-periodic simulation model and by the excellent agreement between the numerically based predictions and the experimental data.

Mapping Two Competing Grassland Species from a Low-Altitude Helium Balloon

This paper describes a method of low-altitude remote sensing in combination with in situ measurements (leaf area, spectroscopy, and position) to monitor the postfire canopy recovery of two competing grassland species. The method was developed in the Andes of Ecuador, where a tethered balloon with a digital camera was deployed to record a time series of very high spatial resolution imagery (nominal resolution=2 cm ) of an experimental plot covered by two competing species: 1) the pasture grass, Setaria sphacelata; and 2) the invasive southern bracken, Pteridium arachnoideum. Image processing techniques were combined to solve geometric issues and construct high-quality mosaics for image classification. The semiautomatic and object-oriented classification method was based on geometrical and textural attributes of image segments and showed promising results for detecting the invasive bracken fern in Setaria pastures (performance by area under the curve, AUC = 0.88). Valuable insights are given into vegetation monitoring applications using unmanned aerial vehicles, which produces a time series of species-specific maps, including foliage projective cover (FPC) and leaf area index (LAI). This new method constitutes an important and accessible tool for ecological investigations of competing species in pastures and validation of remote sensing information on mountain environments.

Using Web-Based Video as an Assessment Tool for Student Performance in Organic Chemistry

This article shows the potential for using video responses to specific questions as part of the assessment process in an organic chemistry class. These exercises have been used with a postbaccalaureate cohort of 40 students, learning in an online environment, over a period of four years. A second cohort of 25 second-year students taking the organic chemistry course onsite has been using these exercises over a period of three years. The questions in this exercise require the students to use a molecular model kit. The problems presented to the students involve standard concepts in organic chemistry relating to geometric issues with molecules or organic reactions. This method of assessment allows the instructor to see a student’s higher-order thinking on a particular topic to a greater degree than is generally afforded with clicker questions. Students are acclimated to this new environment for assessment through practice exercises that are evaluated and critiqued, but not counted towards a grade. A transitio...

Analysis of High Aspect Ratio through Silicon via (TSV) Diffusion and Stress Impact Profile during 3D Advanced Integration

In order to overcome the performances, dimensions and cost limit beyond the 22 nm technology node, three-dimensional (3-D) integration with through-silicon-vias (TSVs) has emerged as an effective solution. Another potential of the TSVs is their promises in enabling advanced multi-level chips, integrating heterogeneous CMOS technologies with emerging technologies such as MEMS and bio-chips.As many integration schemes include the need of high aspect ratio TSVs in order to increase silicon thickness for Die bow and warp limitations or decrease TSV diameter for density increase, TSV Metallization, particularly barrier and seed layer deposition, has become a critical process step of the integration. In a previous work, the investigation of a low temperature (200°C) MOCVD TiN film as a barrier layer to prevent copper diffusion was studied. From these studies, it comes out that even a 5nm thin TiN film withstand barrier efficiency thermal test on fullsheet deposition. This barrier shows up a different behavior while being integrated in the high aspect ratio TSV due to subsequent plasma treatment during the deposition. Actually, the plasma densification does not have the same efficiency through the sidewall of the 80 µm deep TSV thereby enabling a different material structure all along the profile of the via. Characterizations of the behavior of the barrier in the TSV then become a great challenge in order to handle the integration protocol both in horizontal and vertical scales. Working at this scale of topology makes standard diffusion methods limited to evaluate the intrinsic diffusion properties into the via. A localized diffusion methods using Tof-SIMS along the sidewall of the TSV was used to overcome these geometrical issues (figure 1). This technique allowed us to compare different type of barriers and the determination of the minimal thickness for the efficiency of these films as a barrier in the TSV.3-D stacking using TSV involves mechanical and thermal stresses in the Cu TSV itself as well as in the surrounding silicon substrate. As the Cu volume of the TSV becomes important compared to the size of the active components, stresses are induced extrinsically by the interaction of every metallurgical step of the TSV elaboration, including dielectric isolation, barrier/seed layer and copper filling depositions as well as subsequent copper anneals. Stresses can be induced intrinsically by thermal treatment due to the mismatch of thermal expansion between the via material and Si. Usually the X-rays diffraction (XRD) and the stress distribution from the variation of the curvature during the processing is used to measure the stress induced in thin films. Due to the depth of the structure, these methods are limited to measure the stress generated by the TSV.Micro Raman spectroscopy measurements allow measuring the stress induced by every single metallurgical step of the TSV elaboration. For the barrier layer step, the intrinsic stress generated into the silicon by different types of TiN barrier according to process parameters and subsequent plasma treatment were measured and compared (Figure 2). It comes out that some barriers induced a more compressive stress in the silicon and others seem to have a tensile behavior. Then the evolution of the stress of the TSV through different anneal conditions were studied to evaluate the stability of the via with the temperature.

Generation Of True Ortho‐Images Based On Virtual Worlds: Learning Aspects

AbstractMany of the theoretical concepts in photogrammetry are difficult to under‐stand by means of equations and figures. Furthermore, commercial software is not usually adapted to the requirements of teaching and learning. However, photogrammetric simulation software based on virtual worlds can be used to improve both learning and understanding of complex geometrical issues such as the generation of true ortho‐images in urban areas, and assists in training and skills development. This paper tackles image orthorectification by means of a photogrammetric simulation and its generation based on digital elevation/building/surface models, and the interior and exterior orientation parameters of the image sensor, namely, a digital camera. Internal processes such as masking and visibility analysis which are necessary to yield successful true ortho‐images are reported, with examples from both simple and complex urban environments.

A reflection on radiographic cephalometry: the evaluation of sagittal discrepancy

A critical review is presented of the basic properties and applications of cephalometry as a clinical tool with a focus on the evaluation of sagittal discrepancy. Diagnostic cephalometric assessments are subjective and not based on evidence. To assess individual skeletal and/or facial soft tissue form subjectively, selected norms are used. Norms have been developed for various ethnical groups to improve clinical applicability, but subjectivity remains. That subjectivity precludes application of a modern review system, making the present review a personal account. The cephalometric evaluation of sagittal discrepancy finds its historic origin in the Angle classification. Recent publications try to improve accuracy in classifying sagittal discrepancy. It remains unclear in what sense such efforts influence treatment decisions and/or treatment effect. Almost all selected landmarks are located on or dependent upon periosteal/endosteal bone image contours. Their homology is based on circumstantial reasoning and stability over time, which is implicitly assumed. However, implant growth studies and histological investigations show most landmarks to be unstable, as they are involved in displacement and bone remodelling. These landmarks are therefore heterologous when used for individual evaluation of change over time. Notwithstanding the above-indicated limitations, diagnostic cephalometric assessments are clinically useful and help to develop perceptions of balance and harmony and communication between colleagues and patients. There is no evidence-based method to prefer one particular diagnostic method. Landmark location accuracy and geometric issues do not play a decisive role. The subjective characteristic of diagnostic evaluations limits their power to size/shape comparisons. Structural superimposition is the valid biologically evidence-based method to provide advanced insight in individual growth and/or treatment changes and their variations.

On the Photophysics of Carotenoids: A Multireference DFT Study of Peridinin

We present a quantum-mechanical investigation of the photophysics of a specific carotenoid, peridinin, which is present in light-harvesting complexes. The fundamental role played by the geometry in determining the position and character of its low-lying singlet electronic states is investigated using a multireference DFT approach in combination with a continuum solvation model. The main photophysical properties of peridinin appear to be governed by the lowest two singlet excited states, as no evidence points to an intermediate S* state and the energies of the upper excited states are too high to allow their population with excitation in the visible range. These two excited states (S1, 2(1)A(g)(-) and S2, 1(1)B(u)(+)) are highly connected through the conjugation path here characterized by the value of the bond length alternation (BLA). The S1 and S2 states present distinct natures for small BLA values, whereas for larger ones they become more similar in terms of both brightness and dipolar character and their energies become closer. The geometrical issue is thus of fundamental importance for a correct interpretation of the spectroscopic signatures of peridinin.

Geometrical issues in the continuum mechanics of defective solid crystals

Geometrical issues in the continuum mechanics of defective solid crystals

Microparticles confined to a nematic liquid crystal shell

A seminal paper [D. R. Nelson, Nano Lett., 2002, 2, 1125.] has proposed that a nematic coating could be used to create a valency for spherical colloidal particles through the functionalization of nematic topological defects. Experimental realizations however question the complex behaviour of solid particles and defects embedded in such a nematic spherical shell. In order to address the related topological and geometrical issues, we have studied micrometer-sized silica beads trapped in nematic shells. We underline the mechanisms that strongly modify the texture of the simple (particle-free) shells when colloidal particles are embedded. Finally, we show how the coupling between capillarity and nematic elasticity offers new ways to control the valence and directionality of shells.

Effect of pre-impingement length and misalignment in the hydrodynamics of multijet impingement atomization

The multijet impingement atomization strategy is proposed as a competitive one in designing atomizers with relatively simple geometries, producing sprays from the impact of multiple jets at low-flow rate conditions (<1.5 l/min). Most theoretical studies have been made for the spray produced by the impact of two jets. However, there are two important geometric issues related with the atomizer design, scarcely approached in the literature, which are the pre-impingement length of jets before impact and jet misalignment. This work analyses the influence of these parameters on the structure of the liquid sheet as well as on drop size. First, the liquid sheet structure has two hydrodynamic regimes: closed-rim and open-rim. The results evidence that higher pre-jet-impingement distances influence only the open-rim regime, namely, producing shorter liquid sheet breakup lengths, smaller drop sizes, and more droplets. The jet misalignment elongates the liquid sheet in the closed-rim regime, and anticipates breakup in the open-rim regime.

Harnack inequalities for hypoelliptic evolution operators: geometric issues and applications

Consideriamo Equazioni alle Derivate Parziali lineari del secondo ordine in forma di somma di quadrati di campi vettoriali di Hormander piu un termine di drift in un dominio assegnato. Dimostriamo che una disuguaglianza di Harnack vale in ogni sottoinsieme compatto dell'insieme raggiungibile denito in termini dei compi vettoiali che definiscono l'Equazione alle Derivate Parziali considerata. Applichiamo quindi le disuguaglianze di Harnack per dimostrare stime asintotiche dal basso per la densita congiunta di un'ampia classe di processi stocastici. Analoghe stime dall'alto sono dimostrate per mezzo del Calcolo di Malliavin.

Layered mass geometry: a novel technique to overlay seeds and applicators onto patient geometry in Geant4 brachytherapy simulations

A problem faced by all Monte Carlo (MC) particle transport codes is how to handle overlapping geometries. The Geant4 MC toolkit allows the user to create parallel geometries within a single application. In Geant4 the standard mass-containing geometry is defined in a simulation volume called the World Volume. Separate parallel geometries can be defined in parallel worlds, that is, alternate three dimensional simulation volumes that share the same coordinate system with the World Volume for geometrical event biasing, scoring of radiation interactions, and/or the creation of hits in detailed readout structures. Until recently, only one of those worlds could contain mass so these parallel worlds provided no solution to simplify a complex geometric overlay issue in brachytherapy, namely the overlap of radiation sources and applicators with a CT based patient geometry. The standard method to handle seed and applicator overlay in MC requires removing CT voxels whose boundaries would intersect sources, placing the sources into the resulting void and then backfilling the remaining space of the void with a relevant material. The backfilling process may degrade the accuracy of patient representation, and the geometrical complexity of the technique precludes using fast and memory-efficient coding techniques that have been developed for regular voxel geometries. The patient must be represented by the less memory and CPU-efficient Geant4 voxel placement technique, G4PVPlacement, rather than the more efficient G4NestedParameterization (G4NestedParam). We introduce for the first time a Geant4 feature developed to solve this issue: Layered Mass Geometry (LMG) whereby both the standard (CT based patient geometry) and the parallel world (seeds and applicators) may now have mass. For any area where mass is present in the parallel world, the parallel mass is used. Elsewhere, the mass of the standard world is used. With LMG the user no longer needs to remove patient CT voxels that would include for example seeds. The patient representation can be a regular voxel grid, conducive to G4NestedParam, and the patient CT derived materials remain exact, avoiding the inaccuracy of the backfilling technique. Post-implant dosimetry for one patient with 125I permanent seed implant was performed using Geant4 version 9.5.p01 using three different geometrical techniques. The first technique was the standard described above (G4PVPlacement). The second technique placed patient voxels as before, but placed seeds with LMG (G4PVPlacement+LMG). The third technique placed patient voxels through G4NestedParam and seeds through LMG (G4NestedParam+LMG). All the scenarios were calculated with 3 different image compression factors to manipulate the number of voxels. Additionally, the dosimetric impact of the backfilling technique was investigated for the case of calcifications in close proximity of sources. LMG eliminated the need for backfilling and simplified geometry description. Of the two LMG techniques, G4PVPlacement+LMG had no benefit to calculation time or memory use, actually increasing calculation time, but G4NestedParam+LMG reduced both calculation time and memory. The benefits of G4NestedParam+LMG over standard G4PVPlacement increased with increasing voxel numbers. For the case of calcifications in close proximity to sources, LMG not only increased efficiency but also yielded more accurate dose calculation than G4PVPlacement. G4NestedParam in combination with LMG present a new, efficient approach to simulate radiation sources that overlap patient geometry. Cases with brachytherapy applicators would constitute a direct extension of the method.

Commissioning of MRI‐only based treatment planning procedure for external beam radiotherapy of prostate

In radiotherapy, target tissues are defined best on MR images due to their superior soft tissue contrast. Computed tomography imaging is geometrically accurate and it is needed for dose calculation and generation of reference images for treatment localization. Co-registration errors between MR and computed tomography images can be eliminated using magnetic resonance imaging-only based treatment planning. Use of ionizing radiation can be avoided which is especially important in adaptive treatments requiring several re-scans. We commissioned magnetic resonance imaging-only based procedure for external radiotherapy, treatment planning of the prostate cancer. Geometrical issues relevant in radiotherapy, were investigated including quality assurance testing of the scanner, evaluation of the displacement of skin contour and radiosensitive rectum wall, and detection of intraprostatic fiducial gold seed markers used for treatment localization. Quantitative analysis was carried out for 30 randomly chosen patients. Systematic geometrical errors were within 2.2 mm. The gold seed markers were correctly identified for 29 out of the 30 patients. Positions of the seed midpoints were consistent within 1.3 mm in magnetic resonance imaging and computed tomography. Positional error of rectal anterior wall due to susceptibility effect was minimal. Geometrical accuracy of the investigated equipment and procedure was sufficient for magnetic resonance imaging-only based radiotherapy, treatment planning of the prostate cancer including treatment virtual simulation.