Humans can naturally and effectively find salient regions in complex scenes. Motivated by this observation, attention mechanisms were introduced into computer vision with the aim of imitating this ...aspect of the human visual system. Such an attention mechanism can be regarded as a dynamic weight adjustment process based on features of the input image. Attention mechanisms have achieved great success in many visual tasks, including image classification, object detection, semantic segmentation, video understanding, image generation, 3D vision, multimodal tasks, and self-supervised learning. In this survey, we provide a comprehensive review of various attention mechanisms in computer vision and categorize them according to approach, such as channel attention, spatial attention, temporal attention, and branch attention; a related repository
https://github.com/MenghaoGuo/Awesome-Vision-Attentions
is dedicated to collecting related work. We also suggest future directions for attention mechanism research.
One of the challenging issues for Orthogonal Frequency Division Multiplexing (OFDM) system is its high Peak-to-Average Power Ratio (PAPR). In this paper, we review and analysis different OFDM PAPR ...reduction techniques, based on computational complexity, bandwidth expansion, spectral spillage and performance. We also discuss some methods of PAPR reduction for multiuser OFDM broadband communication systems.
Herein, we report a rhodium catalyzed directing‐group free regioselective C−H allylation of simple arenes. Readily available gem‐difluorinated cyclopropanes can be employed as highly reactive allyl ...surrogates via a sequence of C−C and C−F bond activation, providing allyl arene derivatives in good yields with high regioselectivity under mild conditions. The robust methodology enables facile late‐stage functionalization of complex bioactive molecules. The high efficiency of this reaction is also demonstrated by the high turnover number (TON, up to 1700) of the rhodium catalyst on gram‐scale experiments. Preliminary success on kinetic resolution of this transformation is achieved, providing a promising access to enantio‐enriched gem‐difluorinated cyclopropanes.
A rhodium catalyzed C(aryl)−C(allyl) bond‐forming reaction is achieved via a combination of C−H and C−C activation. Gem‐difluorinated cyclopropanes can be employed as highly reactive allyl surrogates, enabling C−H allylation of a variety of simple arenes under mild conditions.
•A novel refrigerant-based battery thermal management system is proposed.•Temperature distributions and boiling characteristics are predicted.•The maximum temperature is inversely correlated with ...refrigerant inlet velocity.•Temperature uniformity is predominantly affected by nucleate boiling heat transfer.
In this paper, a novel battery thermal management system (BTMS) using the dielectric, non-flammable HFE-7000 refrigerant is proposed for electric vehicles (EVs). Its thermal performance is studied both numerically and experimentally. The refrigerant flows and boils on the battery wall surfaces, which lowers the thermal contact resistance as well as enhances the heat transfer process. Therefore, the thermal performance of the battery module is improved. The results indicate that forced convection heat transfer of the liquid refrigerant is dominating in the control of the temperature rise in the battery module. The maximum battery temperature drops to 35.10°C at 0.3ms-1 inlet velocity and a 5C discharge rate. In contrast, the temperature uniformity between individual battery cells primarily depends on the nucleate boiling heat absorption and local perturbation of the two-phase turbulent flow. A temperature difference of no more than 3.71°C can be observed at 5C discharge rate and 0.1ms- 1. In addition, good agreement was found between the numerical results and experimental data.
The B4C/C(graphite) composites were fabricated by employing a pressureless sintering process. The pressureless sintered B4C/C(graphite) composites exhibited extremely low mechanical characteristics. ...The liquid silicon infiltration technique was employed for enhancing the mechanical property of B4C/C(graphite) composites. Since the porosity of the B4C/C(graphite) composites was about 25–38%, the liquid silicon was able to infiltrate into the interior composites, thereby reacting with B4C and graphite to generate silicon carbide. Thus, boron carbide, silicon carbide, and residual silicon were sintered together forming B4C-SiC-Si composites. The pressureless sintered B4C/C(graphite) composites were transformed into the B4C-SiC-Si composites following the silicon infiltration process. This work comprises an investigation of the microstructure, phase composition, and mechanical characteristics of the pressureless sintered B4C/C(graphite) composites and B4C-SiC-Si composites. The XRD data demonstrated that the pressureless sintered bulks were composed of the B4C phase and graphite phase. The pressureless sintered B4C/C(graphite) composites exhibited a porous microstructure, an extremely low mechanical property, and low wear resistance. The XRD data of the B4C-SiC-Si specimens showed that silicon infiltrated specimens comprised a B4C phase, SiC phase, and residual Si. The B4C-SiC-Si composites manifested a compact and homogenous microstructure. The mechanical property of the B4C-SiC-Si composites was substantially enhanced in comparison to the pressureless sintered B4C/C(graphite) composites. The density, relative density, fracture strength, fracture toughness, elastic modulus, and Vickers hardness of the B4C-SiC-Si composites were notably enhanced as compared to the pressureless sintered B4C/C(graphite) composites. The B4C-SiC-Si composites also manifested outstanding resistance to wear as a consequence of silicon infiltration. The B4C-SiC-Si composites demonstrated excellent wear resistance and superior mechanical characteristics.
The class activation maps are generated from the final convolutional layer of CNN. They can highlight discriminative object regions for the class of interest. These discovered object regions have ...been widely used for weakly-supervised tasks. However, due to the small spatial resolution of the final convolutional layer, such class activation maps often locate coarse regions of the target objects, limiting the performance of weakly-supervised tasks that need pixel-accurate object locations. Thus, we aim to generate more fine-grained object localization information from the class activation maps to locate the target objects more accurately. In this paper, by rethinking the relationships between the feature maps and their corresponding gradients, we propose a simple yet effective method, called LayerCAM. It can produce reliable class activation maps for different layers of CNN. This property enables us to collect object localization information from coarse (rough spatial localization) to fine (precise fine-grained details) levels. We further integrate them into a high-quality class activation map, where the object-related pixels can be better highlighted. To evaluate the quality of the class activation maps produced by LayerCAM, we apply them to weakly-supervised object localization and semantic segmentation. Experiments demonstrate that the class activation maps generated by our method are more effective and reliable than those by the existing attention methods. The code will be made publicly available.
We summarize and reanalyze observations bearing on missing galactic baryons, where we propose a consistent picture for halo gas in L L* galaxies. The hot X-ray-emitting halos are detected to 50-70 ...kpc, where typically Mhot(<50 kpc) ∼ 5 × 109 M , and with density n ∝ r−3/2. When extrapolated to R200, the gas mass is comparable to the stellar mass, but about half of the baryons are still missing from the hot phase. If extrapolated to 1.7R200-3R200, the ratio of baryon to dark matter approaches the cosmic value. Significantly flatter density profiles are unlikely for R < 50 kpc, and they are disfavored but not ruled out for R > 50 kpc. For the Milky Way, the hot halo metallicity lies in the range 0.3-1 solar for R < 50 kpc. Planck measurements of the thermal Sunyaev-Zel'dovich (SZ) effect toward stacked luminous galaxies (primarily early type) indicate that most of their baryons are hot, are near the virial temperature, and extend beyond R200. This stacked SZ signal is nearly an order of magnitude larger than that inferred from the X-ray observations of individual (mostly spiral) galaxies with M* > 1011.3 M . This difference suggests that the hot halo properties are distinct for early- and late-type galaxies, possibly due to different evolutionary histories. For the cooler gas detected in UV absorption line studies, we argue that there are two absorption populations: extended halos, and disks extending to ∼50 kpc, containing most of this gas, and with masses a few times lower than the stellar masses. Such extended disks are also seen in 21 cm H i observations and in simulations.
The recycled fine-powder (RFP), produced during the recycling process, will induce a serious impact on the environment with improper disposition. A potential green way to reuse RFP is to add it as ...supplementary cementitious material in concrete. The effects of RFP on the hydration, microstructure, shrinkage and mechanical properties of ultra-high performance engineered cementitious composites (UHP-ECC) with different replacement ratios up to 50% were investigated. The hydration kinetics were compared among the different replacement ratios using the isothermal calorimetry, which demonstrated an accelerating effect of RFP to the hydration of UHP-ECC matrix. The phase development was quantified by the thermal gravimetric analysis and proved the pozzolanic effect of RFP. The compressive and tensile properties of UHP-ECCs were obtained at 3, 7 and 28 days, respectively, to trace their development along the curing ages. The addition of RFP significantly reduced the autogenous shrinkage of UHP-ECC. Besides, the single fiber pullout test was investigated to quantify the influence of RFP at the fiber level. The environmental scanned electron microscope analysis was conducted to study the morphology of PE fiber at the fracture surface.
•Up to 50% of cement was replaced by recycled fine powder (RFP) without significant loss in mechanical properties•Effects of RFP on the hydration, microstructure, shrinkage and mechanical properties of UHP-ECC were investigated•A linkage of micro-mechanical scale at the fiber level and to macro-mechanical scale at the composite level was established.
Here we present a modular, chemo‐, regio‐, and stereoselective synthesis of fully‐substituted and configuration‐defined alkyl vinyl ethers (AVEs) using simple chemical feedstocks. The distinctive ...approach involves the chemo‐ and regioselective functionalization of the CF2 unit in gem‐difluorinated cyclopropanes with O−H and C−H nucleophiles in a specific order. The resulting highly functionalized cyclopropanyl ethers then undergo a stereoselective ring‐opening process to produce fully‐substituted and configuration‐defined AVEs. These AVEs are rarely accessible through conventional methods and are easily transformable. Mechanistic experiments indicate that the success of this method relies on the use of dual‐functional copper catalysis, which is involved in both the functionalization of the CF2 unit and the subsequent ring‐opening process.
The synthesis of highly‐substituted alkyl vinyl ethers in a well‐defined configuration is challenging. Now, a modular, chemo‐, regio‐, and stereoselective synthesis of fully‐substituted and configuration‐defined alkyl vinyl ethers (AVEs) enabled by dual‐functional Cu catalysis is reported. It also represents a novel ring‐opening pattern involving the cleavage of the C1−C3 bond in gem‐difluorinated cyclopropanes.
Widely distributed across the globe, water wave energy is one of the most promising renewable energy sources, while little has been exploited due to various limitations of current technologies mainly ...relying on electromagnetic generator (EMG), especially its operation in irregular environment and low frequency (<5Hz). The newly developed triboelectric nanogenerator (TENG) exhibits obvious advantages over EMG in harvesting energy from low-frequency water wave motions, and the network of TENGs was proposed as a potential approach toward large-scale blue energy harvesting. Here, a review is given for recent progress in blue energy harvesting using TENG technology, starting from a comparison between the EMG and the TENG both in physics and engineering design. The fundamental mechanism of nanogenerators is presented based on Maxwell's displacement current. Approaches of water wave energy harvesting by liquid-solid contact electrification TENG are introduced. For fully enclosed TENGs, the structural designs and performance optimizations are discussed, based on which the TENG network is proposed for large-scale blue energy harvesting from water waves. Furthermore, the energy harvested by TENG from various sources such as water wave, human motion and vibration etc, is not only new energy, but more importantly, the energy for the new era – the era of internet of things.
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•Recent progress in blue energy harvesting with TENG technology is reviewed.•The fundamental physics mechanism of nanogenerators is the Maxwell's displacement current.•The TENG network is proposed for large-scale blue energy harvesting.
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