20 Search Results

Abstract This paper reports on the use of machine learning to delineate data harnessed by fiber-optic distributed acoustic sensors (DAS) using fiber with enhanced Rayleigh backscattering to recognize vibration events induced by human locomotion. The DAS used in this work is based on homodyne phase-sensitive optical time-domain reflectometry (φ-OTDR). The signal-to-noise ratio (SNR) of the DAS was enhanced using femtosecond laser-induced artificial Rayleigh scattering centers in single-mode fiber cores. Both supervised and unsupervised machine-learning algorithms were explored to identify people and specific events that produce acoustic signals. Using convolutional deep neural networks, the supervised machine learning scheme achieved over 76.25%more » accuracy in recognizing human identities. Conversely, the unsupervised machine learning scheme achieved over 77.65% accuracy in recognizing events and human identities through acoustic signals. Through integrated efforts on both sensor device innovation and machine learning data analytics, this paper shows that the DAS technique can be an effective security technology to detect and to identify highly similar acoustic events with high spatial resolution and high accuracies.« less

This paper presents an integrated technical framework to protect pipelines against both malicious intrusions and piping degradation using a distributed fiber sensing technology and artificial intelligence. A distributed acoustic sensing (DAS) system based on phase-sensitive optical time-domain reflectometry (φ-OTDR) was used to detect acoustic wave propagation and scattering along pipeline structures consisting of straight piping and sharp bend elbow. Signal to noise ratio of the DAS system was enhanced by femtosecond induced artificial Rayleigh scattering centers. Data harnessed by the DAS system were analyzed by neural network-based machine learning algorithms. The system identified with over 85% accuracy in various externalmore » impact events, and over 94% accuracy for defect identification through supervised learning and 71% accuracy through unsupervised learning.« less

Hydrogen production from photoelectrochemical (PEC) water splitting using semiconductor photocatalysts has attracted great attention to realize clean and renewable energy from solar energy. The visible light response of WO₃ with a long hole diffusion length (~150 nm) and good electron mobility (~12 cm² V^–1 s^–1) makes it suitable as the photoanode. Yet, WO₃ suffers from issues including rapid recombination of photoexcited electron–hole pairs, photo-corrosion during the photocatalytic process due to the formation of peroxo-species, sluggish kinetics of photogenerated holes, and slow charge transfer at the semiconductor/electrolyte interface. Our report highlights the approaches to overcome these drawbacks of WO₃ photoanodes, including:more » (i) the manipulation of nanostructured WO₃ photoanodes to decrease the nanoparticle size to promote hole migration to the WO₃/electrolyte interface which benefits the charge separation; (ii) doping or introducing oxygen vacancies to improve electrical conductivity; exposing high energy crystal surfaces to promote the consumption of photogenerated holes on the high-active crystal face, thereby suppressing the recombination of photogenerated electrons and holes; (iii) decorating with co-catalysts to reduce the overpotential which inhibits the formation of peroxo-species; (iv) other methods such as coupling with narrow band semiconductors to accelerate the charge separation and controlling the crystal phase via annealing to reduce defects. These methods are reviewed with detailed examples.« less

TiO₂ nanorod (NR) array for perovskite solar cells (PSCs) has attached great importance due to superb power conversion efficiency (PCE) compared to traditional mesoporous TiO₂ film. A TiO₂ compact layer for the growth of TiO₂ NR array via spin-coating cannot meet the requirements for efficient NR based PSCs. Herein we have developed and demonstrated inserting an bifunctional extra-thin TiO₂ interlayer (5 nm) by atomic layer deposition (ALD) at the interface of FTO/TiO₂ compact layer to achieve an alleviated electron exchange and a reduced energetic barrier. Thus, an accelerated extraction of electrons from TiO₂ NR arrays via the compact layer tomore » FTO substrate is ready for improving the PSC efficiency. The thickness of the spin-coated TiO₂ compact layer on the ALD-deposited TiO₂ layer is spontaneously optimized. Finally, an outstanding efficiency of 20.28% has been achieved from a champion PSC with negligible hysteresis and high reliability. To the best of our knowledge, this is the first time to demonstrate the superiority of TiO₂-NR based PSCs withstanding in dry heat and thermal cycling tests. The results are of great importance for the preparation of efficient and durable PSCs for our real-world applications.« less

We report that chemical energy conversion/storage through water splitting for hydrogen production has been recognized as the ideal solution to the transient nature of renewable energy sources. Solid polymer electrolyte (SPE) water electrolysis is one of the most practical ways to produce pure H2. Electrocatalysts are key materials in the SPE water electrolysis. At anode side, electrode materials catalyzing the oxygen evolution reaction (OER) require properties. Among the reported materials, only iridium presents high activity and is more stable. In this article, an application overview of single iridium metal and its oxide catalysts; binary, ternary and multi-component catalysts of iridiummore » oxides; and supported composite catalysts for the OER in SPE water electrolysis is presented. Two main strategies to improve the activity of an electrocatalyst system, i.e., increasing the number of active sites and the intrinsic activity of each active site were reviewed with detailed examples. Finally, the challenges and perspectives in this field are also discussed.« less

High absorption capacity and broad absorption bandwidth electromagnetic wave (EMW) absorption materials (namely, EMW absorbers) are highly desirable due to the interference with electronics and harms on human beings’ health. In search for rational design on nanostructured absorbers, we have synthesized and demonstrated the rod-shape composites with Fe-containing magnetic nanoparticles (Fe₃O₄, Fe₃C and Fe NPs) embedded into nano-porous carbon (NPC) through pyrolysis of Fe-based metal-organic frameworks (MOFs). The morphologies, compositions, and graphitization degree of the Fe-MOFs derived magnetic NPC nanorods can be effectively controlled via adjusting the pyrolysis temperatures. The graphitization level has a significant influence on the permittivity ofmore » the composites upon variation of pyrolysis temperatures, thereby a tunable electromagnetic wave (EMW) absorption is observed. Consequently, the resulting magnetic NPC nanorods obtained at pyrolysis temperature of 600 and 700 °C exhibit the most remarkable EMW absorption performance with a strong reflection loss of -52.9 dB and broad effective bandwidth (f_e) of 4.64 GHz at 3.07 mm. With a thickness of 3.5 mm, the f_e for the magnetic NPC nanorods at 600 °C covers the whole X-band from 7.92 to 12.48 GHz. Finally, the noticeable EMW absorption performances have been greatly enhanced compared to those reported Fe₃O₄ based absorbers, owing to the synergy of multiple components and the porous structures inherited from MOFs.« less

Silicon carbon (Si/C) materials are promising anode candidates for high performance lithium ion batteries (LIBs). However, serious volume expansion and solid electrolyte interface formation limited their actual capacity during lithiation and delithiation. In the present study, an innovative and low-cost synthetic approach was developed for synthesizing carbon-rich silicon-containing polymer-derived ceramics from poly(dimethylsilyene)diacetylenes (PDSDA) and its feasibility to be used as anodes was demonstrated. The attained PDCs@800 °C exhibited a high specific capacity upto 883 mAhg^-1 at 400 mAg^-1, with >99% coulombic efficiency (CE), and 90% capacity retention even after 500 cycles, setting a new record for PDCanode materials in LIBs.more » The high specific capacity was attributed to the incessant Si/C network which delivered consistent conductance and a stable solid electrolyte interphase (SEI). Here, this study opens the door to explore and apply well-designed ceramic materials derived from tailored polymers as high performance anodes for lithium ion batteries.« less

Abstract We report a general chemical approach to synthesize strongly ferromagnetic rare‐earth metal (REM) based SmCo and SmFeN nanoparticles (NPs) with ultra‐large coercivity. The synthesis started with the preparation of hexagonal CoO+Sm ₂ O ₃ (denoted as SmCo‐O) multipods via decomposition of Sm(acac) ₃ and Co(acac) ₃ in oleylamine. These multipods were further reduced with Ca at 850 °C to form SmCo ₅ NPs with sizes tunable from 50 to 200 nm. The 200 nm SmCo ₅ NPs were dispersed in ethanol, and magnetically aligned in polyethylene glycol (PEG) matrix, yielding a PEG‐SmCo ₅ NP composite with the room temperature coercivity ( Hmore » _c ) of 49.2 kOe, the largest H _c among all ferromagnetic NPs ever reported, and saturated magnetic moment ( M _s ) of 88.7 emu g ⁻¹ , the highest value reported for SmCo ₅ NPs. The method was extended to synthesize other ferromagnetic NPs of Sm ₂ Co ₁₇ , and, for the first time, of Sm ₂ Fe ₁₇ N ₃ NPs with H _c over 15 kOe and M _s reaching 127.9 emu g ⁻¹ . These REM based NPs are important magnetic building blocks for fabrication of high‐performance permanent magnets, flexible magnets, and printable magnetic inks for energy and sensing applications.« less

We report here a general chemical approach to synthesize strongly ferromagnetic rare-earth metal (REM) based SmCo and SmFeN nanoparticles (NPs) with ultra-large coercivity. The synthesis started with the preparation of hexagonal CoO+Sm₂O₃ (denoted as SmCo-O) multipods via decomposition of Sm(acac)₃ and Co(acac)₃ in oleylamine. These multipods were further reduced with Ca at 850 °C to form SmCo₅ NPs with sizes tunable from 50 to 200 nm. The 200 nm SmCo₅ NPs were dispersed in ethanol, and magnetically aligned in polyethylene glycol (PEG) matrix, yielding a PEG-SmCo₅ NP composite with the room temperature coercivity (H_c) of 49.2 kOe, the largest H_cmore » among all ferromagnetic NPs ever reported, and saturated magnetic moment (M_s) of 88.7 emu g^-1, the highest value reported for SmCo₅ NPs. The method was extended to synthesize other ferromagnetic NPs of Sm₂Co₁₇, and, for the first time, of Sm₂Fe₁₇N₃ NPs with H_c over 15 kOe and M_s reaching 127.9 emu g^-1. These REM based NPs are important magnetic building blocks for fabrication of high-performance permanent magnets, flexible magnets, and printable magnetic inks for energy and sensing applications.« less