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Empirical modeling and Monte Carlo simulation of secondary electron yield reduction of laser drilled microporous gold surfaces
No full textThis work investigates secondary electron yield (SEY) mitigation from a metal surface with a microporous array fabricated using the laser drilling technique. We propose a general empirical model to fit the experimentally measured SEY of a flat gold surface for normal and oblique incidences of primary electrons. Using this empirical model, we develop a two-dimensional Monte Carlo (MC) simulation scheme to determine the effective SEY of a microporous array. It is found that the SEY from a porous surface is significantly reduced compared to that of the flat surface. By taking into account all the generations of secondary electrons inside a well, our MC results are found to be in very good agreement with the experimental data. The dependence of the SEY on the aspect ratio of the micropores and porosity of the surface is examined. A simple empirical formula has been proposed to evaluate the effective SEY of the gold microporous array for pores of arbitrary aspect ratios
The growth and superconducting properties of RE-Ba-Cu-O single grains with combined RE elements (RE = Gd and Y)
No full textThe superconducting properties, melting temperatures and crystal growth rates of single grain, RE-Ba-Cu-O [(RE)BCO] bulk superconductors (where RE = a rare earth element or yttrium) decrease with the RE-element sequence of Nd, Sm, Eu, Gd, Dy and Y. The mechanical properties of these technologically important materials, on the other hand, however, improve in the same sequence. Consequently, one promising approach for optimising the balance between mechanical and superconducting properties of bulk (RE)BCO superconductors, or for adjusting growth rate, is the use of combinations of different rare earth elements. In this study, we explore combinations of Gd and Y in the formation of (Gd-Y)-Ba-Cu-O single grains. We describe the optimisation of the growth process for this multi-RE element system and use optical and scanning electron microscopy to study the microstructure of both non-superconducting (Gd-Y)2BaCuO5 [(Y-Gd)-211] phase inclusions and the (Y-Gd)Ba2Cu3O7-δ [(Y-Gd)-123] phase matrix itself. We demonstrate that (Gd-Y)-Ba-Cu-O single grains can be fabricated reliably and that they exhibit reasonably good superconducting properties. We observe that there is an increase in RE-211 particle size in this mixed rare earth system, which, ultimately, limits sample performance, and conclude that this may be a general disadvantage of this approach to the synthesis of single grains for high field engineering applications
The Marsh Lane Railway Viaduct: 2 Years of Monitoring with Combined Sensing and Surveying Technologies
No full textMarsh Lane viaduct is a typical example of a 19th century brick masonry railway arch in the UK. It frequently carries passenger trains to and from Leeds Station. This paper broadly discusses the sensing techniques and associated analysis procedures used to (i) identify the reasons for existing damage, (ii) quantify their impact on the dynamic response of the structure and (iii) measure degradation of the response over a period of one year. To identify existing damage, distortions in geometry of the structure are examined with new point cloud processing techniques. With the aid of limit analyses, these distortions are interpreted, and past support movements which may have caused the distortions are identified. Then, to measure the dynamic response of the bridge, quasi-distributed fibre optic strain sensing and digital image correlation displacement measurement techniques are used. These highlight the increased dynamic response around locations of existing damage, and point out to the global mechanisms of response that could propagate damage. Continuous fibre optic strain measurements between November 2017 and 2018 are then discussed to investigate the ongoing deterioration
Material Selection and Structural Optimization for Lightweight Truck Trailer Design
No full textThis article investigates options for lightweighting truck trailers through a combination of material selection and structural optimization. Critical chassis design load cases were established and a parametric finite element (FE) model of a typical European-style 13.5 m long truck trailer built from steel I-beams was developed. The model has been used to show that existing longitudinal steel I-beams could be reduced in weight by 28% (140 kg) through shape optimization alone. The model was expanded to analyze holistic composite trailer structures. It showed that up to 67% (1,326 kg) of weight could be saved by executing shape and material optimization in unison. The approach highlights that design through parametric analysis allows for many different structural configurations to be assessed in terms of both mechanical performance and material cost. This facilitates the construction of a theoretical design space of a lightweight chassis, clarifying the weight reduction limits that could be achieved with lightweight materials and structural optimization. The lightweight trailer chassis designs proposed here are also compared against a portfolio of shorter-term strategies for trailer lightweighting. These strategies are poised to have an increasingly important role in reducing the greenhouse gas emissions of the road freight industry
Numerical Modelling of Dynamic Resistance in a Parallel-Connected Stack of HTS Coated-Conductor Tapes
No full textDynamic resistance is observed in type-II superconductors carrying a DC transport current while simultaneously exposed to an alternating magnetic field. The appearance of a non-zero resistance is attributed to the interaction between the transport current and moving fluxons. This effect is relevant to many superconductor applications such as high-Temperature-superconductor (HTS) flux pumps, DC/AC magnets, synchronous machines, and persistent current switches. Here, we present a finite element method (FEM) analysis of both the time averaged dynamic resistance and the instantaneous current sharing behaviour in a cable comprised of a stack of four YBCO thin films connected in parallel. Numerical modelling was performed using the H-formulation method implemented in the commercial software COMSOL. The model employs experimentally measured values of the angular dependence of the critical current Ic(B, θ) and the flux creep exponent n(B, θ). A single threshold field is observed, above which a finite dynamic resistance is observed in all tapes simultaneously. The time-Averaged dynamic resistance of individual tapes tends to be larger for the exterior tapes than the interior tapes, but this difference decreases as the total transport current in the cable increases. We attribute this to shielding currents flowing in the exterior tapes during the majority of the cycle, which displace net DC current into the interior tapes. However, the relative proportion of DC transport current flowing in the exterior and interior tapes is also observed to vary periodically once per half cycle of the applied field. This is due to the periodic trapping of return screening currents in the interior tapes
Second harmonic generation in defective hexagonal boron nitride
No full textDetermining the role of defects in materials can be an important task both for the fundamental understanding of their influence on material properties and for future applications. In this work, we studied the influence of defects on the second harmonic generation (SHG) in hexagonal boron nitride (h-BN). We characterized the sample by photoluminescence imaging and spectroscopy, showing strong and sharp photoluminescence emission at visible range from h-BN flakes due to single defect states. By doing second harmonic imaging, we found strong emission from the h-BN flakes that correlates spatially with the photoluminescence imaging. By doing polarization-resolved SHG, we found deviations from the expected polarization pattern in pristine h-BN samples. We also characterized the nonlinear optical susceptibility of h-BN with defects with a value of one order of magnitude larger than for pristine h-BN, which highlights the role of defects in the efficiency of SHG. Therefore defect engineering could be used as a potential tool for nonlinear optical signal enhancement
Localized Nanoresonator Mode in Plasmonic Microcavities
No full textSubmicron-thick hexagonal boron nitride crystals embedded in noble metals form planar Fabry-Perot half-microcavities. Depositing Au nanoparticles on top of these microcavities forms previously unidentified angle- A nd polarization-sensitive nanoresonator modes that are tightly laterally confined by the nanoparticle. Comparing dark-field scattering with reflection spectroscopies shows plasmonic and Fabry-Perot-like enhancements magnify subtle interference contributions, which lead to unexpected redshifts in the dark-field spectra, explained by the presence of these new modes
Multi-Stage 8 × 8 Silicon Photonic Switch Based on Dual-Microring Switching Elements
No full textWe demonstrate the first multi-stage 8 × 8 silicon photonic switch with switching elements based on dual add-drop microrings with a compact footprint of 4 mm2. This device leverages co-design of the switch architecture and the switching elements with a well-balanced set of performance metrics. The switching elements are designed to have a 3-dB optical passband of 165 GHz, exhibiting off-and on-resonance losses of 0.67 dB and 2 dB, respectively. Full characterization of all switch paths shows an end-to-end on-chip loss between 4.4 and 9.6 dB, with worst-case crosstalk leakage averaged at-16 dB. Owing to the efficient doped waveguide thermo-optic phase shifters, the device features a tuning efficiency of 48.85 GHz/mW. The reconfiguration time of the switch fabric is measured to be 1.2 μs and 0.5 μs at the rise and fall edge, respectively. The dual-microring switching element together with the multi-stage architecture preserves an end-to-end passband over 55 GHz. We validate the switch performance with optical paths of varying numbers of on-and off-resonance switching elements-less than 2 dB power penalties are obtained for all data routings at 32 Gbps
Relative limitations of increasing the number of modulation levels in computer generated holography
No full textPhase and amplitude spatial light modulators (SLMs) capable of both binary and multi-level modulation are widely available and offer a wide range of technologies to choose from for holographic applications. While the replay fields generated with multi-level phase-only SLMs are of a significantly higher quality than those generated by equivalent binary phase-only SLMs, evidence is presented in this letter that this improvement is not as marked for amplitude SLMs, where multi-level devices offer only a small benefit over their binary counterparts. Heuristic and numerical justifications for this are discussed and conclusions drawn
Automatic Detection and Visualisation of Metastatic Bone Disease
No full text© 2020, Springer Nature Switzerland AG. This paper presents a novel method of finding and visualising metastatic bone disease in computed tomography (CT). The approach we suggest locates disease by comparing trabecular bone density between symmetric bony regions. Areas of strong difference could indicate metastatic bone disease as bone lesions either increase or decrease bone density. Our detection method is completely automatic and only requires raw CT data as input. Results are visualised in an interactive 3-dimensional viewer which displays a polygonal mesh of the bone structure overlaid with colour combined with resliced CT data. Diseased regions are clearly highlighted in both the mesh and in the resliced CT data. We test our method on both healthy and diseased CT data to demonstrate the validity of the technique. Experimental results show that our method can detect metastatic bone disease, although further work is needed to improve the robustness of the technique and to decrease false positives