Scanning Direction Research Articles

Study on Wide-angle Scanning Characteristics of Hemispherical Array

A kind of hemispherical antenna array based on shape conformal is studied in this paper. In order to solve the polarization problem during beam scanning of the array, the method of phase compensation is adopted. The omni-directional scanning of 0−∘360∘ in azimuth direction and wide-angle scanning of −90to∘ 90∘ in elevation direction are realized. The gain of the array at each scanning angle is about 17 dB. To reduce the occlusion effect of array elements on radiation, the effective elements at different scanning angles are selected for thinned array arrangement. The power loss is reduced and the better scanning performance is achieved by using fewer elements.

Focus on software, data acquisition, and instrumentation

Focus on software, data acquisition, and instrumentation

Method for assessment of the apparent detector stack position in a whisk-broom sensor

In a whisk-broom sensor, such as the Advanced Baseline Imager (ABI) onboard the Geostationary Operational Environmental Satellite (GOES)-R series of geostationary satellites, the sensor optics projects the observed scene onto the sensor focal plane arrays (FPA), with the detectors arranged in a column perpendicular to the scan direction. An assessment system compares the images made by the sensor with known landmarks and calculates the image deviations from the landmark positions. These deviations are called navigation residuals, and in this work they are interpreted in a broad sense as deviations of the actual from the ideal optical paths. The deviations can be caused by several issues, such as a sensor pointing error, deficiency in sensor optics, and inaccuracy of detector locations on FPA. As the ABI sensor scans the Earth to make a full-disk image in 22 parallel swaths, we refer the landmark residuals from all swaths to the observing detector positions in the ABI detector column. This interpretation inverts the retrieval of navigation residuals and gives deviations of the image from the expected ideal position on an ideal FPA. As the errors of pointing and sensor optics can be assessed independently, the remaining deviations can be interpreted as these with respect to the ideal detector position map (in other words, to the pre-launch assessment). We call these deviations the apparent detector positions. Knowledge of these deviations will greatly simplify the navigation commissioning for future ABI sensors and similar devices. Using our archive of ABI image navigation data for ABI sensor on the GOES-16 satellite, we show the wealth of additional information about the sensor on-orbit condition that may be extracted with the proposed method. This algorithm can use any image navigation system that is sufficiently precise.

THE EFFECT OF OUT-OF-PLANE PATIENT SHIELDING ON CT RADIATION EXPOSURE AND TUBE CURRENT MODULATIONS: A PHANTOM STUDY ACROSS THREE VENDORS.

The aim of this study was to evaluate how out-of-plane patient shielding affects radiation exposure parameters and tube current modulation on different vendors' computed tomography (CT) scanners. Helical CT scans were performed using two homogenous phantoms to mimic patient attenuation. Four CT scanners from three vendors were investigated by varying the distance of the patient shield from the border of the imaging volume. Scans were performed with a shield placed before and after the localizer. Changes in volume computed tomography dose index (CTDIvol), dose-length product (DLP) and tube current-time products were studied. Out-of-field lead shield increased the CTDIvol and DLP values for each scanner at least for one scan setting when the shield was present in the localizer. The most notable changes were recorded with >1.3 pitch values when the shield was closest to the scanned volume (2.5cm), and the scan direction was towards the shield. The usage of patient shields in the localizer CT scans can disturb TCM even when placed 7.5cm away from the edge of the scan.

Marginal and internal fit of lithium disilicate endocrowns fabricated using conventional, digital, and combination techniques.

To compare the marginal and internal adaptation of endocrowns produced using conventional technique, digital technique, and combination (cast digitization) techniques using microcomputed tomography (micro-CT). Ten freshly extracted human mandibular molar teeth were prepared for all-ceramic endocrowns. A total of 40 Lithium Disilicate (IPS e.max) endocrowns were fabricated and grouped according to the impression and production technique to four groups: Group (CO): Conventional impression/heat pressed endocrowns (n =10), Group (CAD): Direct scanning of teeth/CAD-CAM endocrowns (n =10), Group (COMIO): Combination; Cast digitization using Intraoral scanner/CAD-CAM endocrowns (n =10), Group (COML): Combination; Cast digitization using laboratory scanner/CAD-CAM endocrowns (n =10). Micro-computed tomography was used to measure the marginal and internal gaps in 11 predetermined sites. Mean marginal and internal gaps were compared using analysis of variance and Scheffe's post hoc test. CO, CAD, COMIO, and COML groups showed significant differences in the mean marginal gap (150 ± 35 μm, 120 ± 27 μm, 110 ± 24 μm, 120 ± 29 μm, respectively p =0.013), gap at line angle (280 ± 70 μm, 130 ± 37 μm, 140 ± 54 μm, 130 ± 33 μm, respectively, p < 0.001), gap at cavity wall (210 ± 76 μm, 140 ± 43 μm, 140 ± 52 μm, 150 ± 44 μm, respectively, p =0.010) and gap at pulpal floor (500 ± 150 μm, 240 ± 58 μm, 260 ± 59 μm, 240 ± 64 μm, respectively, p <0.001). Digitally fabricated endocrowns showed superior marginal and internal fit compared to the conventionally fabricated endocrowns. Marginal and internal adaptation are detrimental factors for the success and survival of dental restorations including endocrowns. When compared with the conventional impressions and conventional production techniques, Digital workflow is more predictable and reliable as it reduces errors and improves the accuracy of fit.

Improvement of spatial resolution of elemental imaging using laser ablation-ICP-mass spectrometry.

Laser ablation-ICP-mass spectrometer (LA-ICPMS) now becomes one of the most principal analytical technique for mapping analysis for major to trace elements in rocks, minerals, functional materials, or biological tissue samples. In this study, imaging analysis was conducted with coupling of small volume cell and off-set laser ablation protocol to improve the spatial resolution. Combination of newly designed small volume cell and in-torch gas mixing protocols provides faster washout time of the signals (about 0.8s for reducing 238U being one part in a hundred, 1% level). This is very important to improve the spatial resolution in a direction of laser scanning. Moreover, combination of small distances between the laser-line scan (laser pitch distance) and preferential and total ablation of only biological tissue samples placed on glass substrate results in laser ablation of smaller areas than the size of laser ablation pit (shaving ablation). With the shaving ablation, laser-line scanning with narrower-band width (e.g., 2µm) can be achieved even by the laser beam of 8µm diameter. To demonstrate the practical usage of the present technique, imaging analysis of Gd-ethylenediamine tetra-methylene phosphonic acid-doped mouse bone was conducted. Preferential distribution of Gd at the edge of the apatite cell was more clearly identified by the present technique. Combination of the shorter washout system setup and the shaving ablation protocol enables us to improve the spatial resolution of the elemental imaging obtained with the LA-ICPMS technique.

Effect of Pre-Set Coating Thickness on the Quality of Electron Beam Cladding Forming of Aluminum Alloys

Numerical simulations combined with electron beam melting were adopted to study the effect of the thickness of the preset coating on the quality of electron beam melting and forming of aluminum alloys. The research was carried out using ANSYS19.0 finite element analysis software to perform the numerical simulation of the temperature field and stress field during the process of electron beam melting of the Ni60 coating on 6061 aluminum alloy. The results show that the thicker the preset coating, the higher the surface temperature and the greater the melting depth and width of the melt pool, but the smaller the substrate melting depth, and the highest surface temperature obtained was 2536.05 °C. When the coating thickness reached 1.5 mm, the substrate essentially showed no change; otherwise, the thicker the preset coating, the greater the residual stress, and the maximum residual stress on the coating surface along the scanning direction appeared at the position near the boundary. Moreover, the maximum residual stress along the depth direction occurred at the interface. The electron beam cladding experiments showed that the 0.5 mm thickness of the coating resulted in a cracking phenomenon at the interface with the substrate, and the surface of the molten layer had more defects such as pores and pits; the 1 mm thickness of the coating had a good metallurgical bond with the substrate, and the surface of the molten layer was dense and flat; the 1.5 mm thickness of the aluminum alloy substrate did not melt, and the surface of the molten layer had more cracks. The numerical simulation was essentially consistent with the electron beam cladding experiment, and the forming quality was better when the preset coating thickness was 1 mm.

A comparative study of Cu–15Ni–8Sn alloy prepared by L-DED and L-PBF: Microstructure and properties

In this paper, dense Cu–15Ni–8Sn alloy blanks were fabricated by laser-directed energy deposition (L-DED). Then, we systematically compared the L-DED sample with the laser-powder bed fusion (L-PBF) Cu–15Ni–8Sn alloy sample prepared in our previous work in terms of their building rate, microstructure, and performance. The average grain size of the L-DED sample was about 23.3 ± 16.7 μm, and its microstructure mainly included fine dendrites with a width of 5.6 ± 1.2 μm. The Vickers hardness, yield strength, and elongation at break of the L-DED sample were 166 ±5HV1, 327 ±9 MPa and 23.9 ± 3.2%, respectively. The building rate of L-DED was about 16 times that of L-PBF. Due to the different laser power densities and different melting modes of L-DED and L-PBF, there were various preferred orientations along the laser scanning direction. Compared with the L-PBF sample, the sizes of grains, dendrites, and segregated phases of the L-DED sample coarsened, and its dislocation density decreased. Combined, these factors decreased its yield strength and were primarily responsible for the lower cooling rate of L-DED. The slow cooling rate alleviated the local thermal deformation in the L-DED sample, which reduced the possibility of stress concentration and increased its plasticity and work hardening rate.

Distribution and evolution of thermal stress in laser powder bed fusion: conduction mode versus keyhole mode

PurposeHigh temperature gradient induces high residual stress, producing an important effect on the part manufacturing during laser powder bed fusion (LPBF). The purpose of this study is to investigate the effect of the molten pool mode on the thermal stress of Ti-6Al-4V alloy during different deposition processes.Design/methodology/approachA coupled thermal-mechanical finite element model was built. The developed model was validated by comparing the numerical results with the experimental data in the maximum molten pool temperature, the molten pool dimension and the residual stress described in the previous work.FindingsFor the single-track process, the keyhole mode caused an increase in both the maximum stress and the high-stress area compared with the conduction mode. For the multitrack process, a lower tensile stress around the scanning track and a higher compressive stress below the scanning track were found in the keyhole mode. For the multilayer process, the stress along the scanning direction at the middle of the part changed from tensile stress to compressive stress with the increase in the deposition layer number. As the powder layer number increased, the stress along the scanning direction near the top surface of the part decreased while the stress along the deposition direction obviously increased, indicating that the stress along the deposition direction became the dominant stress. The keyhole mode can reduce the residual stress near the top of the part, and the conduction mode was more likely to produce a low residual stress near the bottom of the part.Originality/valueThe results provide a systematic understanding of thermal stress during the LPBF process.

Transformation from nano-ripples to nano-triangle arrays and their orientation control on titanium surfaces by using orthogonally polarized femtosecond laser double-pulse sequences

One-dimensional and two-dimensional laser-induced periodic structures (nano-ripples and nano-triangle arrays) are fabricated on titanium surfaces by direct scanning with orthogonally polarized and equal-energy femtosecond laser double-pulse sequences (OP pulses). Clear and regular nano-ripples and nano-triangle arrays are formed when the laser fluence is slightly higher than the ablation threshold of titanium and the time delay is approximately 2 ps or shorter. By changing only the scanning speed, we achieve the transformation from nano-ripples to nano-triangle arrays. The orientation of nano-ripples and that of one of the three sides of nano-triangle arrays are always perpendicular to the scanning direction rather than the laser polarization. The mechanism underlying this phenomenon is as follows. First, the nano-ripples are formed due to periodic energy deposition along the direction of surface waves (surface plasmon polaritons; SPPs), which is along the scanning direction during the scan process. Subsequently, the preformed nano-ripples irradiated with subsequent laser pulses undergo hexagonal convection flow and are transformed into nano-triangle arrays. The preformed nano-ripples perpendicular to the scanning direction have a positioning effect on the formation of nano-triangle arrays, as verified in two-step fast scan experiments. Large-area patterning of nano-triangle arrays producing three-directional structural color is demonstrated in this paper.

Large volume scanning laser induced fluorescence measurement of a bluff-body stabilised flame in an annular combustor

This study outlines a variant of three-dimensional OH planar laser-induced fluorescence and its application in characterising a single bluff body stabilised flame inside a 12 burner annular combustor. In this variant of the method a relatively large volume was scanned slowly in order to calculate the full three-dimensional Flame Surface Density (FSD) distribution. The method used a combination of two scanning directions to overcome bias errors associated with laser sheet positions close to the flame edges. The source of this bias error was confirmed numerically through a complimentary synthetic PLIF study, which was also used to refine the experimental setup. The bias error resulted in a reduction of FSD magnitude, although the method was still capable of capturing the flame structure. This was demonstrated by comparing the reconstructions from the two independent scan directions. Combining the data from both directions overcame the bias, and allowed flame asymmetry due to the confinement to be assessed. The FSD was used to determine the heat release rate of the flame with varying local azimuthal angle for different downstream regions. This highlighted the highly asymmetric structure, produced by the asymmetric confinement.Graphical abstract

Accuracy comparison of intraoral versus laboratory scanners used in the contemporary dental practice

The aim of this in vitro study was to compare the quality of the data obtained by intraoral and by laboratory 3D scanners. An artificial jaw with an acrylic tooth prepared for a full metal ceramic crown was used as a master model of the prosthetic field. A reference model was created by scanning this field by a coordinate measuring machine (CCM). The acrylic master model was scanned by four scanners using three different methods and six digital models were obtained. They were divided in three groups - direct intraoral scanning (DIS), laboratory stone scanning (LSS) and laboratory conventional impression scanning (LCIS). Each scan was saved in the STL file format. Using a computer program, each of the six digital models were compared with the reference model created by the CCM and the results were displayed as color maps. The minimal acceptable deviation was defined as the deviation from the digital standard within the ± 0.02 mm limits. Thus, the LSS group exhibited an average of 13.5% points within these limits, followed by the LCIS group with 17.25%. The result for the DIS group was 44.75%.

Cyclic Voltammetric Experiment-Simulation Comparisons of the Complex Zr4+ to Zr0 Reduction Mechanism at a Molybdenum Electrode in LiF-CaF2 Eutectic Molten Salt

Nuclear energy represents an important option for generating largely clean CO2-free electricity. Zirconium is a fission product in the nuclear reaction that needs to be extracted from irradiated fuels used in Gen-IV molten salt reactors. The present investigation addresses the electrochemical reduction of solution soluble Zr4+ (soln) to surface confined Zro (s−c) at a molybdenum electrode in a LiF—CaF2 eutectic molten salt at 840 °C using DC cyclic, square-wave and AC voltammetry. Cyclic voltammograms simulated by the reaction scheme: Zr4+ (soln) + 4e− → Zro (soln); Zro (soln) ↔ Zr*(s−c); Zr*(s−c) → Zr4+*(s−c) + 4e−; Zr4+*(s−c) → Zr4+ (soln); Zr*(s−c) + Zr4+ (soln) ↔ 2Zr2+ (soln); Zr2+ (soln) ↔ Zr2+*(s−c) and Zr2+*(s−c) → Zr4+*(s−c) + 2e− provided excellent agreement with experimental data over the scan rate range of 50 to 500 mV s−1. The interpretation of the simulation is that the reduction of Zr4+ (soln) to Zro (metal) takes place via a transiently soluble Zro (soln) in an overall 4-electron essentially reversible diffusion-controlled process having a reversible formal potential (Eo f) of −1.22 V (vs Pt). A minor oxidation process observed at −0.455 V (vs Pt) on the reversing the potential scan direction is simulated via the reaction step Zr(s−c) + Zr4+ (soln) ↔ 2Zr2+ (s−c) followed by Zr2+ (s−c) → Zr4+ (sol) + 2e−. The sharply rising initial component where reduction of Zr4+ (sol) commences, contains evidence of a nucleation-growth mechanism associated with the electrocrystallisation of zirconium metal. This initial rapid growth of current is not fully accommodated in the simulations, but all features found beyond the peak potential are supported by the theory. A comparison with theory based on a direct reduction of Zr4+ (soln) to the metallic state having unit activity is provided. It is proposed that an analogous mechanism applies at a Ni electrode, except that a Ni-Zr alloy formation occurs instead of Zr metal.

An Optimal Procedure for the Design of Discrete Constrained Lens Antennas with Minimized Optical Aberrations. Part III: Three-Dimensional Architectures with an Extended Field of View

In this paper, an innovative procedure is proposed for the design of three-dimensional discrete lens antennas characterized by an extended field of view. While in a companion paper the design procedure was based on the definition of multifocal constrained lenses and on their evolution in rotationally symmetric ones, in this paper, lenses are assumed from the beginning to be rotationally symmetric and are derived by enforcing minimized optical aberrations specifically for the largest scanning directions. It is shown that, for discrete lenses exhibiting a feeding array with a cross section, projected in a plane perpendicular to the main lens axis, larger as compared to the back lens cross section, there are significant improvements (15–20%) in terms of maximum aberrations and, at the same time, similar or slightly improved accommodation in terms of volume can be obtained as compared to the architectures considered in the companion paper. Because of this property, the proposed lens antennas may be particularly useful in emerging applications requiring an extended field of view.

Microscale residual stresses in additively manufactured stainless steel: Computational simulation

Metal additive manufacturing (AM) has attracted much attention in recent years due to its ability of producing parts with complex geometry. Unfortunately, the large temperature gradient during fabrication leads to residual stresses which undesirably result in distortion and even crack of as-built parts. A computational framework is used to study how residual stresses form and evolve in AM parts at the length scale of individual grains, including a multi-physics thermal-fluid flow model, a phase field model for grain growth and a crystal plasticity finite element model. First, this framework is validated by comparing the lattice strain with experimental results in different grain families in two samples made of 316L stainless steel, which were produced by laser powder-bed-fusion with two different sets of process parameters. The relationship between residual stress, plastic strain and grain orientation near the top surface of the samples is then investigated. The residual stresses are observed to form during laser scanning due to compression followed by tension around the molten pool, thus the shape of the molten pool has a significant influence on the residual stress distribution. Redistribution of the plastic deformation during cooling stage is predicted and the residual stresses with greater magnitude occur along the laser scanning direction. This work provides useful insight into the mechanism of microscale residual stress generation and evolution in AM parts.

The Atacama Cosmology Telescope: Modeling bulk atmospheric motion

Fluctuating atmospheric emission is a dominant source of noise for ground-based millimeter-wave observations of the cosmic microwave background (CMB) temperature anisotropy at angular scales $\ensuremath{\gtrsim}0.5\ifmmode^\circ\else\textdegree\fi{}$. We present a model of the atmosphere as a discrete set of emissive turbulent layers that move with respect to the observer with a horizontal wind velocity. After introducing a statistic derived from the time-lag dependent correlation function for detector pairs in an array, referred to as the pair-lag, we use this model to estimate the aggregate angular motion of the atmosphere derived from time-ordered data from the Atacama Cosmology Telescope (ACT). We find that estimates derived from ACT's CMB observations alone agree with those derived from satellite weather data that additionally include a height-dependent horizontal wind velocity and water vapor density. We also explore the dependence of the measured atmospheric noise spectrum on the relative angle between the wind velocity and the telescope scan direction. In particular, we find that varying the scan velocity changes the noise spectrum in a predictable way. Computing the pair-lag statistic opens up new avenues for understanding how atmospheric fluctuations impact measurements of the CMB anisotropy.

Feasibility study of the photofission technique for radiological characterization of 220-L concrete-lined nuclear waste drums using 7 or 9 MeV linacs

Photofission is a promising technique for the radiological characterization of nuclear waste packages with dense and voluminous matrices. The goal of this paper is two-fold. First, we assess the performance of the photofission technique for the radiological characterization of a 220-L nuclear waste mock-up drum with a concrete liner and different uranium or plutonium samples and segmented hotspot detection using a 9 MeV linear electron accelerator (linac). Second, we build an MCNP6 numerical model of the measurement setup and validate it against the experimental data. Furthermore, using the validated MCNP6 numerical model at 9 MeV we assess the performance of the photofission technique at 7 MeV and with virtual cases, representing different homogeneous matrix materials that fill the entire volume of the mock-up drum. Results of this study indicate that the minimal detectable masses of depleted uranium and 94% 239Pu samples obtained with a linac operated at 9 MeV and 100 Hz during 10 min are 0.539 and 0.336 grams respectively. The sensitivity for hotspot detection indicated a 6-times difference in the counting statistics in both scanning directions between the extremes and center position. The photofission rate with a linac operated at 7 MeV is poorer than that at 9 MeV by 15-times for all studied characterization cases. However, the characterization performance at 7 MeV could still be sufficient for actinide mass determination in concrete, plastic and metallic waste matrices, which fill the entire volume of a 220-L drum.

Evolution of temperature and residual stress behavior in selective laser melting of 316L stainless steel across a cooling channel

PurposeThe current investigation aims at observing the influence of the cooling channel on the thermal and residual stress behavior of the selective laser melting (SLM)316L uni-layer thermo-mechanical model.Design/methodology/approachOn a thermo-mechanical model with a cooling channel, the effect of scanning direction, parallel and perpendicular and scan spacing was simulated. The effect of underlying solid and powder bases was evaluated on residual stress profile and thermal variables at various locations.FindingsThe high heat dissipation of solid base due to high cooling rates and steep thermal gradients can reciprocate with smaller melt pool temperature and melt pool size. Given the same scan spacing, residual stresses were found lower when laser scanning was perpendicular to the cooling channel. Moreover, large scan spacing was found to increase residual stresses.Originality/valueCooling channels are increasingly being used in additive manufacturing; however, their effect on the residual stress behavior of the SLM component is not extensively studied. This research can serve as a foundation for further inquiries into the impact of base material design such as cooling channels on manufactured components using SLM.

Laser incidence angle influence on energy density variations, surface roughness, and porosity of additively manufactured parts

The use of additive manufacturing (AM) technologies for fabricating structural and functional parts is rapidly rising due to their transformative design flexibility and customizability. However, maintaining the uniformity and consistency of the fabricated part across the build plate is a critical challenge. One of the main sources of variability across the build plate is laser spot elongation in laser beam powder bed fusion (LB-PBF) AM which typically occurs when the beam moves from the center of the plate to the edge at different incidence angles. This laser spot elongation may then result in the loss of laser power density which can potentially cause elongated melt pools, poor dimensional accuracy, and formation of lack of fusion and more gas entrapped pores in the final part. Here, we report both geometrical calculations and experimental evidence to show the behavior of the laser in the different areas of the build plate. A pulsed laser is first used to show and calculate the geometrical footprint of the beam elongations at different incidence angles. Then, the surface roughness, melt pool size, and porosity variations are investigated for varying laser interaction cross-section across the build plate by manufacturing AlSi10Mg (an aluminum alloy) parts and correlating them to theoretical calculations. It is found that for given optimized process parameters at the center of the build plate, as the beam goes farther away from the center, it becomes elongated. As the power density falls, melt pool width becomes wider, melt pool depth becomes shallower (depending on the elongation orientation with respect to the scan direction), and the number of lack of fusion defects increases. This work provides fundamental insight into possible sources of variations across the build plate in an LB-PBF process that may help design new strategies to compensate for such variations to enhance uniformity and reliability of AM parts.

Controlling the parameters of focused ion beam for ultra-precise fabrication of nanostructures

At present, the focused ion beam method is an effective technique for nanoscale profiling of a solid surface and prototyping of micro- and nanoscale structures. The article reveals the results of experimental studies on improving the accuracy and resolution of nanoscale profiling of the surface of solids with a focused ion beam. Investigations of the regularities of the influence of the focused ion beam current, beam dwell time and overlap on the parameters of nanoscale structures and the surface profile have been carried out. The influence of the FIB parameters on the deviation of the structure profile from the specified by the template was estimated. Experimental studies have been carried out to determine the influence of the direction of scanning of the ion beam by the template on the magnitude of the error that occurs when the structure of the graphic template is transferred to the substrate. The optimal relationships between the FIB current and the dimensions of the structures being formed have been determined, thus making it possible to ensure the highest accuracy and rate of formation of nanoscale structures. The results can be used to optimize the choice of the ion-beam milling parameters to achieve the maximum accuracy of reproduction of the given sizes of structures.