Conv3D-Based Video Violence Detection Network Using Optical Flow and RGB Data.

Detecting violent behavior in videos to ensure public safety and security poses a significant challenge. Precisely identifying and categorizing instances of violence in real-life closed-circuit television, which vary across specifications and locations, requires comprehensive understanding and processing of the sequential information embedded in these videos. This study aims to introduce a model that adeptly grasps the spatiotemporal context of videos within diverse settings and specifications of violent scenarios. We propose a method to accurately capture spatiotemporal features linked to violent behaviors using optical flow and RGB data. The approach leverages a Conv3D-based ResNet-3D model as the foundational network, capable of handling high-dimensional video data. The efficiency and accuracy of violence detection are enhanced by integrating an attention mechanism, which assigns greater weight to the most crucial frames within the RGB and optical-flow sequences during instances of violence. Our model was evaluated on the UBI-Fight, Hockey, Crowd, and Movie-Fights datasets; the proposed method outperformed existing state-of-the-art techniques, achieving area under the curve scores of 95.4, 98.1, 94.5, and 100.0 on the respective datasets. Moreover, this research not only has the potential to be applied in real-time surveillance systems but also promises to contribute to a broader spectrum of research in video analysis and understanding.

Operando Identification of the Chemical and Structural Origin of Li-Ion Battery Aging at Near-Ambient Temperature.

Integrated with heat-generating devices, a Li-ion battery (LIB) often operates at 20-40 °C higher than the ordinary working temperature. Although macroscopic investigation of the thermal contribution has shown a significant reduction in the LIB performance, the molecular level structural and chemical origin of battery aging in a mild thermal environment has not been elucidated. On the basis of the combined experiments of the electrochemical measurements, Cs-corrected electron microscopy, and in situ analyses, we herein provide operando structural and chemical insights on how a mild thermal environment affects the overall battery performance using anatase TiO2 as a model intercalation compound. Interestingly, a mild thermal condition induces excess lithium intercalation even at near-ambient temperature (45 °C), which does not occur at the ordinary working temperature. The anomalous intercalation enables excess lithium storage in the first few cycles but exerts severe intracrystal stress, consequently cracking the crystal that leads to battery aging. Importantly, this mild thermal effect is accumulated upon cycling, resulting in irreversible capacity loss even after the thermal condition is removed. Battery aging at a high working temperature is universal in nearly all intercalation compounds, and therefore, it is significant to understand how the thermal condition contributes to battery aging for designing intercalation compounds for advanced battery electrode materials.

Engineering Titanium Dioxide Nanostructures for Enhanced Lithium-Ion Storage.

Various kinds of nanostructured materials have been extensively investigated as lithium ion battery electrode materials derived from their numerous advantageous features including enhanced energy and power density and cyclability. However, little is known about the microscopic origin of how nanostructures can enhance lithium storage performance. Herein, we identify the microscopic origin of enhanced lithium storage in anatase TiO2 nanostructure and report a reversible and stable route to achieve enhanced lithium storage capacity in anatase TiO2. We designed hollow anatase TiO2 nanostructures composed of interconnected ∼5 nm sized nanocrystals, which can individually reach the theoretical lithium storage limit and maintain a stable capacity during prolonged cycling (i.e., 330 mAh g-1 for the initial cycle and 228 mAh g-1 for the 100th cycle, at 0.1 A g-1). In situ characterization by X-ray diffraction and X-ray absorption spectroscopy shows that enhanced lithium storage into the anatase TiO2 nanocrystal results from the insertion reaction, which expands the crystal lattice during the sequential phase transition (anatase TiO2 → Li0.55TiO2 → LiTiO2). In addition to the pseudocapacitive charge storage of nanostructures, our approach extends the utilization of nanostructured TiO2 for significantly stabilizing excess lithium storage in crystal structures for long-term cycling, which can be readily applied to other lithium storage materials.

Enhancing p-Type Thermoelectric Performances of Polycrystalline SnSe via Tuning Phase Transition Temperature.

SnSe emerges as a new class of thermoelectric materials since the recent discovery of an ultrahigh thermoelectric figure of merit in its single crystals. Achieving such performance in the polycrystalline counterpart is still challenging and requires fundamental understandings of its electrical and thermal transport properties as well as structural chemistry. Here we demonstrate a new strategy of improving conversion efficiency of bulk polycrystalline SnSe thermoelectrics. We show that PbSe alloying decreases the transition temperature between Pnma and Cmcm phases and thereby can serve as a means of controlling its onset temperature. Along with 1% Na doping, delicate control of the alloying fraction markedly enhances electrical conductivity by earlier initiation of bipolar conduction while reducing lattice thermal conductivity by alloy and point defect scattering simultaneously. As a result, a remarkably high peak ZT of ∼1.2 at 773 K as well as average ZT of ∼0.5 from RT to 773 K is achieved for Na0.01(Sn1-xPbx)0.99Se. Surprisingly, spherical-aberration corrected scanning transmission electron microscopic studies reveal that NaySn1-xPbxSe (0 < x ≤ 0.2; y = 0, 0.01) alloys spontaneously form nanoscale particles with a typical size of ∼5-10 nm embedded inside the bulk matrix, rather than solid solutions as previously believed. This unexpected feature results in further reduction in their lattice thermal conductivity.

Liquid Precursor-Assisted Chemical Vapor Deposition of One-Dimensional van der Waals Material Nb2Se9: Tunable Growth for Room-Temperature Gas Sensors.

In this study, Nb2Se9, a one-dimensional (1D) material with van der Waals (vdWs) bonding, was synthesized by chemical vapor deposition (CVD). A liquid precursor was used to overcome the difficulty of controlling the length and density of Nb2Se9 by CVD due to the high melting point of Nb. Growth proceeded horizontally in a nano-ribbon shape on the substrate in the [100] direction, which had the most stable bonding distance, resulting in a preferred orientation of the (010) plane on the out-of-plane axis. Unlike that grown by conventional mechanical or chemical exfoliation, the nanoscale Nb2Se9 grown by CVD was uniform and did not have contaminants, such as dispersants, on its surface, meaning it could effectively induce reactions such as gas adsorption and desorption. It exhibited high sensitivity to NO2 gas adsorption at room temperature (27 °C), and its behavior was confirmed in a high-humidity environment. For the first time, this study demonstrated the possibility of synthesizing a vdWs bonding-based 1D material by CVD, which is expected to be widely used in a variety of low-dimensional materials and devices.

High Breakdown Current Density in Quasi-1D van der Waals Layered Material Ta2NiSe7.

We synthesized ternary composition chalcogenide Ta2NiSe7, a quasi-one-dimensional (Q1D) material with excellent crystallinity. To utilize the excellent electrical conductivity property of Ta2NiSe7, the breakdown current density (JBD) according to thickness change through mechanical exfoliation was measured. It was confirmed that as the thickness decreased, the maximum breakdown voltage (VBD) increased, and at 18 nm thickness, 35 MA cm-2 of JBD was measured, which was 35 times higher than that of copper, which is commonly used as an interconnect material. By optimization of the exfoliation process, it is expected that through a theoretical model fitting, the JBD can be increased to about 356 MA cm-2. It is expected that the low-dimensional materials with ternary compositions proposed through this experiment can be used as candidates for current-carrying materials that are required for the miniaturization of various electronic devices.

Bi-MOF derived micro/meso-porous Bi@C nanoplates for high performance lithium-ion batteries.

Micro/meso-porous Bi@C nanoplates are synthesized by pyrolyzing Bi-based metal-organic frameworks (MOFs) prepared by a microwave-assisted hydrothermal method to overcome huge volume expansion and pulverization of anode materials during battery operation. The Bi@C nanoplates are composed of ∼10-50 nm Bi nanoparticles in an amorphous carbon shell. The material shows very high capacity (556 mA h g-1) after 100 cycles at 100 mA g-1 and good cycling performance. Moreover, the Bi@C nanoplates perform well at high current densities and have excellent cyclic stability; their capacity is 308 mA h g-1 after 50 cycles and 200 mA h g-1 after 1000 cycles at 3000 mA g-1. The outstanding performance of this anode is due to the nanosized Bi and amorphous carbon shell. The nanosized Bi reduces the diffusion length of Li ions, while the amorphous carbon shell improves the electrical conductivity of the anode and also restrains the pulverization and aggregation of the metal during cycling. The proposed hierarchical micro/meso-porous materials derived from MOFs are a new type of nanostructures that can aid the development of novel Bi-based anodes for LIBs.

Structural and Thermodynamic Understandings in Mn-Based Sodium Layered Oxides during Anionic Redox.

A breakthrough utilizing an anionic redox reaction (O2-/On-) for charge compensation has led to the development of high-energy cathode materials in sodium-ion batteries. However, its reaction results in a large voltage hysteresis due to the structural degradation arising from an oxygen loss. Herein, an interesting P2-type Mn-based compound exhibits a distinct two-phase behavior preserving a high-potential anionic redox (≈4.2 V vs Na+/Na) even during the subsequent cycling. Through a systematic series of experimental characterizations and theoretical calculations, the anionic redox reaction originating from O 2p-electron and the reversible unmixing of Na-rich and Na-poor phases are confirmed in detail. In light of the combined study, a critical role of the anion-redox-induced two-phase reaction in the positive-negative point of view is demonstrated, suggesting a rational design principle considering the phase separation and lattice mismatch. Furthermore, these results provide an exciting approach for utilizing the high-voltage feature in Mn-based layered cathode materials that are charge-compensated by an anionic redox reaction.

Fabrication and scanning-angle temperature dependence of metal-based, optical resonant scanners with PZT actuation.

Metal-based, optical scanning devices with a large mirror size of 1 mm(2) and a wide scanning angle of over 90 degrees were fabricated. A high optical scanning angle (90 degrees ) was achieved at a resonance frequency of 7.33 kHz and a driving voltage of 80 V (peak-to-peak) in ambient air without vacuum packaging. We compared the performance of metal-based, optical scanners driven by PZT bulk ceramic and aerosol-deposited PZT thick films over the temperature range from -20 degrees C to 80 degrees C in an environmental chamber.

Na+/Vacancy Disordered P2-Na0.67Co1-xTixO2: High-Energy and High-Power Cathode Materials for Sodium Ion Batteries.

Although sodium ion batteries (NIBs) have gained wide interest, their poor energy density poses a serious challenge for their practical applications. Therefore, high-energy-density cathode materials are required for NIBs to enable the utilization of a large amount of reversible Na ions. This study presents a P2-type Na0.67Co1-xTixO2 (x < 0.2) cathode with an extended potential range higher than 4.4 V to present a high specific capacity of 166 mAh g-1. A group of P2-type cathodes containing various amounts of Ti is prepared using a facile synthetic method. These cathodes show different behaviors of the Na+/vacancy ordering. Na0.67CoO2 suffers severe capacity loss at high voltages due to irreversible structure changes causing serious polarization, while the Ti-substituted cathodes have long credible cycleability as well as high energy. In particular, Na0.67Co0.90Ti0.10O2 exhibits excellent capacity retention (115 mAh g-1) even after 100 cycles, whereas Na0.67CoO2 exhibits negligible capacity retention (<10 mAh g-1) at 4.5 V cutoff conditions. Na0.67Co0.90Ti0.10O2 also exhibits outstanding rate capabilities of 108 mAh g-1 at a current density of 1000 mA g-1 (7.4 C). Increased sodium diffusion kinetics from mitigated Na+/vacancy ordering, which allows high Na+ utilization, are investigated to find in detail the mechanism of the improvement by combining systematic analyses comprising TEM, in situ XRD, and electrochemical methods.

Dielectric characteristics of barium strontium titanate films prepared by aerosol deposition on a Cu substrate.

Polycrystalline barium strontium titanate (Ba(x)Sr(1-x)TiO(3), BST) films were directly prepared on copper substrates using an aerosol deposition method (ADM) at room temperature. Electric properties, such as the dielectric constant and tunability, were investigated as a function of the Ba content in Ba(x)Sr(1-x)TiO(3) (x = 1.0, 0.6, 0.4) films. At a frequency of 100 kHz, the dielectric constant of the as-deposited BST films were 100, 201, and 72 for x = 1.0, 0.6, and 0.4, respectively. After annealing in N(2) ambient at 500 degrees C, the dielectric constant of the films on Cu electrodes were 190, 400, and 170 at x = 1.0, 0.6, and 0.4, respectively. Moreover, the BST (x = 0.6) film showed a high tunability of 30% under an applied electric field of 300 kV cm(-1).