19,421 research outputs found

    Mechanism of Seismic Liquefaction for Heterogeneous Soil

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    In addition to inducing uncertainty in the predicted response, natural spatial variability of soil properties affects the mechanical response of geotechnical structures. When a failure surface is involved in the response, this surface can deviate from its theoretical location to pass through weaker zones of material. For the case of seismically induced soil liquefaction, it has been found that a larger amount of excess pore water pressure is generated in a soil exhibiting small-scale variability of its properties than in the corresponding uniform soil having geomechanical properties equal to the average properties of the heterogeneous soil. An explanation for this important phenomenon is provided in this paper based on the results of centrifuge experiments and numerical simulations of heterogeneous and homogeneous saturated soil deposits subjected to seismic loads. It is demonstrated, based on a detailed analysis of the results, that partial drainage during the earthquake, consisting of pore water inflow from loose soil zones that liquefy first toward surrounding dense areas, may trigger softening of dilative soils. The water inflow effects in terms of volumetric strains leading to reduced cyclic resistance of dense sands are compared with results of specially designed cyclic triaxial tests reported by other researchers

    Enhanced efficiency of high-speed Si and Si-based PbSe MSM photodiodes with integrated photon-trapping holes in 800 ~ 1550 nm wavelengths

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    We demonstrate theoretically and experimentally that cylindrical-shaped hole array with a small depth and an appropriate period integrated on SOI substrate can enhance infrared absorption due to more bending of light and a higher back reflection. Si MSM photodiode with hole array with a depth of 250 nm exhibits a 3-fold improved EQE of 61%, and an ultrafast impulse response speed of 22 ps enabling a 3dB bandwidth up to 23.9 GHz. PbSe film with a thickness of 80 nm is integrated to broaden the response wavelength. More than 500% EQE enhancement of the Si-based PbSe photodiode with 150-nm-deep photon-trapping holes is achieved at 1550 nm compared to the device without hole structures. These photodiodes offer the potential to drastically improve the efficiency and bandwidth of Si-based detectors in wide band and reduce the cost of infrared sensing and communication system

    A fluorene-bridged double carbonyl/amine multiresonant thermally activated delayed fluorescence emitter for efficient green OLEDs

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    S. W. thanks the China Scholarship Council (201906250199) for support. D.S. acknowledges support from the Royal Academy of Engineering Enterprise Fellowship (EF2122-13106). E. Z.-C. thanks the Engineering and Physical Sciences Research Council (EP/W015137/1, EP/W007517) for support. X.-H. Z. acknowledges support from the National Natural Science Foundation of China (Grant No. 52130304, 51821002) and the Collaborative Innovation Center of Suzhou Nano Science & Technology.Herein, we report a fluorene-bridged double carbonyl/amine-based MR TADF emitter DDiKTa-F, formed by locking the conformation of the previously reported compound DDiKTa. Using this strategy, DDiKTa-F exhibited narrower, brighter, and red-shifted emission. The OLEDs with DDiKTa-F emitted at 493 nm and showed an EQEmax of 15.3% with an efficiency roll-off of 35% at 100 cd m−2.Publisher PDFPeer reviewe

    Integral abutment bridges

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    In recent years there has been renewed interest on integral abutment bridges (IABs), mainly due to their low construction and maintenance cost. Owing to the monolithic connection between deck and abutments, there is strong soil structure interaction between the bridge and the backfill under both thermal action and earthquake shaking. Although some of the regions where IABs are adopted qualify as highly seismic, there is limited knowledge as to their dynamic behaviour and vulnerability under strong ground shaking. To develop a better understanding on the seismic behaviour of IABs, an extensive experimental campaign involving over 75 shaking table tests and 4800 time histories of recorded data, was carried out at EQUALS Laboratory, University of Bristol, under the auspices of EU sponsored SERA project (Seismology and Earthquake Engineering Research Infrastructure Alliance for Europe). The tests were conducted on a 5 m long shear stack mounted on a 3 m 3 m 6 DOF earthquake simulator, focusing on interaction effects between a scaled bridge model, abutments, foundation piles and backfill soil. The study aims at (a) developing new scaling procedures for physical modelling of IABs, (b) investigating experimentally the potential benefits of adding compressible inclusions (CIs) between the abutment and the backfill and (c) exploring the influence of different types of connection between the abutment and the pile foundation. Results indicate that the CI reduces the accelerations on the bridge deck and the settlements in the backfill, while disconnecting piles from the cap decreases bending near the pile head

    Bis-tridentate Ir(III) Phosphors and Blue Hyperphosphorescence with Suppressed Efficiency Roll-Off at High Brightness

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    Narrowband blue emitters are indispensable in achieving ultrahigh-definition OLED displays that satisfy the stringent BT 2020 standard. Hereby, a series of bis-tridentate Ir(III) complexes bearing electron-deficient imidazo[4,5-b]pyridin-2-ylidene carbene coordination fragments and 2,6-diaryloxy pyridine ancillary groups were designed and synthesized. They exhibited deep blue emission with quantum yields of up to 89% and a radiative lifetime of 0.71 μs in the DPEPO host matrix, indicating both the high efficiency and excellent energy transfer process from the host to dopant. The OLED based on Irtb1 showed an emission at 468 nm with a maximum external quantum efficiency (EQE) of 22.7%. Moreover, the hyper-OLED with Irtb1 as a sensitizer for transferring energy to terminal emitter v-DABNA exhibited a narrowband blue emission at 472 nm and full width at half-maximum (FWHM) of 24 nm, a maximum EQE of 23.5%, and EQEs of 19.7, 16.1, and 12.9% at a practical brightness of 100, 1000, and 5000 cd/m2, respectively

    Data-driven analysis of crustal and subduction seismic environments using interpretation of deep learning-based generalized ground motion models

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    Studies on understanding the regional seismological differences based on the variations in the characteristics of the ground motion waves recorded during seismic events have provided independent insights into the different seismic environments of the world. This study aims to showcase the differences between three of the major seismic environments of the world including Japanese subduction, Chilean subduction, and Californian crustal. The study is based on developing deep learning (DL)-based surrogate generalized ground motion models (GGMMs) and analyzing them to understand the patterns between the earthquake source parameters and the resulting ground motion waveforms’ engineering characteristics. The GGMMs are developed using long short-term memory (LSTM) based recurrent neural networks (RNNs), which are trained using six earthquake source and site parameters as the inputs and a 25 × 1 vector of amplitude-, duration-, and energy-based ground motion intensity measures (IMs). The GGMMs are trained and evaluated using carefully selected large datasets of ground motion records from the Japanese subduction, Chilean subduction, and Californian crustal sources (∼2000 records from each source). The models are developed in two settings: i) three independent GGMMs using the three datasets of each source, ii) one combined GGMM using the combined dataset. While the former provides individual surrogate models of the regional seismic environments and allows relative comparison among the three environments, the latter acts as a global seismic surrogate model and allows comparison in absolute terms. The seismic environments are investigated by analyzing the two types of GGMMs using explainable artificial intelligence (XAI) and game theory based Shapley explanations (SHAP). As the direct physical study of the seismic environments is not generally feasible/practical, the proposed GGMMs surrogating the process becomes a source of knowledge. By interpreting them, inferences about the seismic environments are derived. Results indicate the peculiar nature of the earthquakes arising from the three seismic backgrounds, further emphasizing the importance of conducting independent regional seismic hazard and risk analysis. In particular, the role of magnitude and rupture distance is observed to have a significantly different impact on the different IMs of the three different environments. The study further sets a novel basis to utilize advanced DL and XAI methods in understanding convoluted physics and engineering phenomena.</p

    Spectrally Tunable White Light-Emitting Diodes Based on Carbon Quantum Dot-Doped Poly(<i>N</i>‑vinylcarbazole) Composites

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    Electroluminescent white light-emitting diodes (WLEDs) are always of great interest for emerging display applications. Carbon-based quantum dots (CQDs) are the newest emerging nanoscale materials that can be employed for this purpose, owing to their broad and bright light emission properties. In the present work, highly luminescent CQDs with an emission quantum yield of 60% were prepared via a colloidal solvothermal method and subsequent silica gel column chromatography. The photoluminescence (PL) peak was located at 550 nm possessing yellow emission, with a full width at half-maximum of 98 nm and a relatively long lifetime of 10.23 ns through a single-exponential recombination pathway. CQDs were employed in an electroluminescent device architecture of an ITO/PEDOT:PSS/TFB/CQD:PVK/TPBi/LiF/Al structure and blended with poly(N-vinylcarbazole) (PVK) to evaluate their ability to reach white electroluminescent emission. Results confirmed a high external quantum efficiency (EQE) of 0.76% and a maximum luminescence of 774.3 cd·m–2. Tuning the ratio between CQDs and PVK from 1:10.25 to 1:5.75 resulted in a systematic shift in CIE x–y coordinates from 0.23–0.26 to 0.21–0.24, located close to the cool white region. The results of the present study can be considered a step forward in fabricating efficient WLEDs based on low-cost CQDs

    The effect of remnant CdSe layers on the performance of CdSeTe/CdTe photovoltaic devices

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    Thin film CdTe-based photovoltaic devices have achieved high efficiency above 22 %. The recent improvement in efficiency is due to Se alloying in the CdTe absorbers to form a CdSeTe/CdTe structure. The subsequent band gap grading increases the short circuit current density. The Se can be introduced by depositing a precursor thin film of either CdSe or a CdSeTe alloy and then diffusing the Se into the CdTe during the high temperature cadmium chloride activation process. Using CdSe is preferred because it is easier to control the Se concentration. However, during fabrication of the CdSeTe/CdTe devices, the CdSe thickness needs to be precisely controlled to prevent the retention of a CdSe remnant layer after the activation treatment. Retention of a remnant CdSe layer causes a dramatic reduction in device efficiency. In this work, we show that the reduction in efficiency is caused by a number of factors. The remnant CdSe layer is n-type which moves the position of the p-n junction. Also, it is widely thought that the CdSe remnants are photo-inactive. In this work, we clarify that the individual CdSe grains are actually highly photo-active. However, the grain sizes in the CdSe remnant and the adjacent CdSeTe layer are very small resulting in a high grain boundary area. Although the grain boundaries are passivated with chlorine, cathodoluminescence imaging and electrical measurements show that this is only partially effective. Also, EQE measurements show that the remnant CdSe causes parasitic absorption. Overall, the remnant CdSe layer causes a reduction in short circuit current density and device efficiency. The thickness of the CdSe precursor layer and the cadmium chloride activation process conditions must be precisely optimised to ensure that all the CdSe is consumed and inter-diffused to form the CdSeTe alloy for highest efficiency devices.</p

    Compositional transformation and impurity-mediated optical transitions in co-evaporated Cu2AgBiI6 thin films for photovoltaic applications

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    Quaternary copper-silver-bismuth-iodide compounds represent a promising new class of wide-bandgap (2 eV) semiconductors for photovoltaic and photodetector applications. In this study, vapor phase co-evaporation is utilized to fabricate Cu2AgBiI6 thin films and photovoltaic devices. The findings show that the properties of vapor-deposited films are highly dependent upon processing temperature, exhibiting increased pinhole density and transforming into a mixture of quaternary, binary, and metallic phases depending on the post-deposition annealing temperature. This change in phase is accompanied by an enhancement in photoluminescence (PL) intensity and charge-carrier lifetime, along with the emergence of an additional absorption peak at high energy (≈3 eV). Generally, increased PL is a desirable property for a solar absorber material, but this change in PL is ascribed to the formation of CuI impurity domains, whose defect-mediated optical transition dominates the emission properties of the thin film. Via optical pump terahertz probe spectroscopy, it is revealed that CuI impurities hinder charge-carrier transport in Cu2AgBiI6 thin films. It is also revealed that the predominant performance limitation in Cu2AgBiI6 materials is the short electron-diffusion length. Overall, the findings pave the way for potential solutions to critical issues in copper-silver-bismuth-iodide materials and indicate strategies to develop environmentally compatible wide-bandgap semiconductors
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