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Analysis of laser-beam absorptance and keyhole behavior during laser keyhole welding of aluminum alloy using a deep-learning-based monitoring system
No full textLaser welding of aluminum alloys is a highly complex and unstable process owing to the high reflectivity and high thermal conductivity of aluminum and vaporization of alloying elements. Laser-beam absorptance is one of the factors that greatly influences the process stability, but it is very difficult to measure with conventional monitoring systems. In this study, the laser-beam absorptance and keyhole behavior during the fiber laser welding of Al 5052-H32 was investigated using a deep-learning-based monitoring system. In this method, two synchronized coaxial high-speed cameras were used to simultaneously observe the top and bottom specimen surfaces. From the obtained images, top and bottom keyhole apertures were detected by using an objectdetection deep-learning model. Then, a ResNet-based deep-learning model was applied to the detected keyhole top and bottom apertures to predict the time-varying laser-beam absorptance inside the keyhole considering multiple reflections. By analyzing the monitoring results, we classified the keyhole behavior and absorption patterns into three types, and investigated the process stability and defect formation mechanisms. It was found that, when the keyhole opened constantly under excessive energy conditions, the keyhole size continued to increase and burn-through defects were generated if the aperture area exceeded the threshold values. Furthermore, process stability was improved and defect formation was suppressed by using helium as a bottom-side shielding gas under high-energy-density conditions
Catalytic Growth of Ultralong Graphene Nanoribbons on Insulating Substrates
No full textGraphene nanoribbons (GNRs) with widths of a few nanometers are promising candidates for future nanoelectronic applications due to their structurally tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability. However, the direct growth of micrometer-long GNRs on insulating substrates, which is essential for the fabrication of nanoelectronic devices, remains an immense challenge. Here, the epitaxial growth of GNRs on an insulating hexagonal boron nitride (h-BN) substrate through nanoparticle-catalyzed chemical vapor deposition is reported. Ultranarrow GNRs with lengths of up to 10 mu m are synthesized. Remarkably, the as-grown GNRs are crystallographically aligned with the h-BN substrate, forming 1D moire superlattices. Scanning tunneling microscopy reveals an average width of 2 nm and a typical bandgap of approximate to 1 eV for similar GNRs grown on conducting graphite substrates. Fully atomistic computational simulations support the experimental results and reveal a competition between the formation of GNRs and carbon nanotubes during the nucleation stage, and van der Waals sliding of the GNRs on the h-BN substrate throughout the growth stage. This study provides a scalable, single-step method for growing micrometer-long narrow GNRs on insulating substrates, thus opening a route to explore the performance of high-quality GNR devices and the fundamental physics of 1D moire superlattices
Narrowband Searches for Continuous and Long-duration Transient Gravitational Waves from Known Pulsars in the LIGO-Virgo Third Observing Run
No full textIsolated neutron stars that are asymmetric with respect to their spin axis are possible sources of detectable continuous gravitational waves. This paper presents a fully coherent search for such signals from eighteen pulsars in data from LIGO and Virgo's third observing run (O3). For known pulsars, efficient and sensitive matched-filter searches can be carried out if one assumes the gravitational radiation is phase-locked to the electromagnetic emission. In the search presented here, we relax this assumption and allow both the frequency and the time derivative of the frequency of the gravitational waves to vary in a small range around those inferred from electromagnetic observations. We find no evidence for continuous gravitational waves, and set upper limits on the strain amplitude for each target. These limits are more constraining for seven of the targets than the spin-down limit defined by ascribing all rotational energy loss to gravitational radiation. In an additional search, we look in O3 data for long-duration (hours-months) transient gravitational waves in the aftermath of pulsar glitches for six targets with a total of nine glitches. We report two marginal outliers from this search, but find no clear evidence for such emission either. The resulting duration-dependent strain upper limits do not surpass indirect energy constraints for any of these targets
Neutronic simulation of the CEFR experiments with the nodal diffusion code system RAST-F
Get PDFCEFR is a small core-size sodium-cooled fast reactor (SFR) using high enrichment fuel with stainless-steel reflectors, which brings a significant challenge to the deterministic methodologies due to the strong spectral effect. The neutronic simulation of the start-up experiments conducted at the CEFR have been performed with a deterministic code system RAST-F, which is based on the two-step approach that couples a multi-group cross-section generation Monte-Carlo (MC) code and a multi-group nodal diffusion solver. The RAST-F results were compared against the measurement data. Moreover, the characteristic of neutron spectrum in the fuel rings, and adjacent reflectors was evaluated using different models for generation of accurate nuclear libraries. The numerical solution of RAST-F system was verified against the full core MC solution MCS at all control rods fully inserted and withdrawn states. A good agreement between RAST-F and MCS solutions was observed with less than 120 pcm discrepancies and 1.2% root-mean-square error in terms of l e ft and power distribution, respectively. Meanwhile, the RAST-F result agreed well with the experimental values within two-sigma of experimental uncertainty. The good agreement of these results indicating that RAST-F can be used to neutronic steady-state simulations for small core-size SFR, which was challenged to deterministic code system. (C) 2022 Korean Nuclear Society, Published by Elsevier Korea LLC
Metabolic engineering of Escherichia coli W3110 for efficient production of homoserine from glucose
No full textEfficient microbial cell factory for the production of homoserine from glucose has been developed by iterative and rational engineering of Escherichia coli W3110. The whole pathway from glucose to homoserine was divided into three groups, namely, glucose transport and glycolysis ('up-stream'), TCA and glyoxylate cycles ('midstream'), and homoserine module (conversion of aspartate to homoserine and its secretion; 'down-stream'), and the carbon flux in each group as well as between the groups were accelerated and balanced. Altogether, ~18 genes were modified for active and consistent production of homoserine during both the actively-growing and non-growing stages of cultivation. Finally, fed-batch, two-stage bioreactor experiments, separating the growth from the production stage, were conducted for 61 h, which gave the high titer of 110.8 g/L, yield of 0.64 g/g glucose and volumetric productivity of 1.82 g/L/h, with an insignificant amount of acetate (<0.5 g/L) as the only noticeable byproduct. The metabolic engineering strategy employed in this study should be applicable for the biosynthesis of other amino acids or chemicals derived from aspartic acid
A review on triterpenoids from plant sources as potential antiviral agents
No full textViral infections are considered as leading a health issue globally. Numerous numbers of biologically active anti-viral agents have been identified from plants and other organisms. Particularly, terpenoids are a major component of the plant secondary metabolites and a complexity of these structures is accompanied by the potency of their biological activities. It is believed that most of the terpenoids possess the bioactivity against viral infections and cancer diseases. Hence, affected by the pressing a need elevated by the spreading of seriously life-threaten viruses, this review highlights the importance of terpenoids and their activity as antiviral agents that can be employed to treat current lethal diseases such as HIV, H1N1, SARS-CoV and HSV
Controlling Wettability of Electrodes for Enhanced Multi-Phase Electrochemical Reactions
No full textMulti-phase electrochemical reactions are academically and practically interesting subjects. Examples include hydrogen evolution and CO2 reduction reactions, which can be utilized for the efficient storage and utilization of excess renewable electricity. In these reactions, the interface between solid, liquid, and gas phases play a critical role in the efficiency and stability of electrodes/catalysts. However, most conventional studies have been focused on the development of efficient electrocatalysts, whereas less attention has been paid to the engineering of electrode wettability. In this talk, I will present our recent achievements in the development of extremely bubble-repellent (superaerophobic) electrodes using porous hydrogels for enhanced hydrogen evolution reactions. The porous and hydrophilic nature of hydrogels can impart extremely bubble-repellent properties to the underlying electrodes, regardless of their types and morphologies, and minimize the blocking of catalytically active electrode surfaces. As a result, hydrogel-coated electrodes exhibit significantly enhanced performance for hydrogen evolution reactions by facilitating the removal of gas bubbles from the electrode surface. Lastly, I will also briefly review recent promising results on the engineering of electrode wettability for CO2 reduction reactions
