Chiral ferroelectric crystals with intriguing features have attracted great interest and many with point or axial chirality based on the stereocarbon have been successively developed in recent years. ...However, ferroelectric crystals with stereogenic heteroatomic chirality have never been documented so far. Here, we discover and report a pair of enantiomeric stereogenic sulfur‐chiral single‐component organic ferroelectric crystals, Rs‐tert‐butanesulfinamide (Rs‐tBuSA) and Ss‐tert‐butanesulfinamide (Ss‐tBuSA) through the deep understanding of the chemical design of molecular ferroelectric crystals. Both enantiomers adopt chiral‐polar point group 2 (C2) and exhibit mirror‐image relationships. They undergo high‐temperature 432F2‐type plastic ferroelectric phase transition around 348 K. The ferroelectricity has been well confirmed by ferroelectric hysteresis loops and domains. Polarized light microscopy records the evolution of the ferroelastic domains, according with the fact that the 432F2‐type phase transition is both ferroelectric and ferroelastic. The very soft characteristics with low elastic modulus and hardness reveals their excellent mechanical flexibility. This finding indicates the first stereosulfur chiral molecular ferroelectric crystals, opening up new fertile ground for exploring molecular ferroelectric crystals with great application prospects.
Following the discovery of the first ferroelectric chiral Rochelle salt more than 100 years ago, the first pair of stereogenic heteroatom sulfur‐chiral ferroelectric crystals is reported on this study. The findings provide a perspective for the development of heteroatomic chiral ferroelectric crystals with great application prospects.
Learning visual representations from web data has recently attracted attention for object recognition. Previous studies have mainly focused on overcoming label noise and data bias and have shown ...promising results by learning directly from web data. However, we argue that it might be better to transfer knowledge from existing human labeling resources to improve performance at nearly no additional cost. In this paper, we propose a new semi-supervised method for learning via web data. Our method has the unique design of exploiting strong supervision, i.e., in addition to standard image-level labels, our method also utilizes detailed annotations including object bounding boxes and part landmarks. By transferring as much knowledge as possible from existing strongly supervised datasets to weakly supervised web images, our method can benefit from sophisticated object recognition algorithms and overcome several typical problems found in webly-supervised learning. We consider the problem of fine-grained visual categorization, in which existing training resources are scarce, as our main research objective. Comprehensive experimentation and extensive analysis demonstrate encouraging performance of the proposed approach, which, at the same time, delivers a new pipeline for finegrained visual categorization that is likely to be highly effective for real-world applications.
In the context of fine-grained visual categorization, the ability to interpret models as human-understandable visual manuals is sometimes as important as achieving high classification accuracy. In ...this paper, we propose a novel Part-Stacked CNN architecture that explicitly explains the finegrained recognition process by modeling subtle differences from object parts. Based on manually-labeled strong part annotations, the proposed architecture consists of a fully convolutional network to locate multiple object parts and a two-stream classification network that encodes object-level and part-level cues simultaneously. By adopting a set of sharing strategies between the computation of multiple object parts, the proposed architecture is very efficient running at 20 frames/sec during inference. Experimental results on the CUB-200-2011 dataset reveal the effectiveness of the proposed architecture, from multiple perspectives of classification accuracy, model interpretability, and efficiency. Being able to provide interpretable recognition results in realtime, the proposed method is believed to be effective in practical applications.
Display omitted
•Cr(VI) was efficiently reduced to Cr(III) at alkaline pHs by UV/sulfite process.•Common ions and organic matters interfered with Cr(VI) reduction slightly.•Spontaneous precipitation ...of Cr(III) occurred in the presence of Ca2+ (>2ppm).•eaq− was identified as the dominant reactive species in UV/sulfite process.
Chemical reduction of Cr(VI) to Cr(III) followed by Cr(III) precipitation is a widely employed strategy to mitigate Cr(VI) pollution from industrial effluents. Nevertheless, most of the available reduction processes are feasible at acidic pHs only, and very few technologies are capable of reducing Cr(VI) at alkaline pHs. Herein, we demonstrated that the UV/sulfite process is very promising for alkaline Cr(VI) remediation, including the Cr(VI) reduction to Cr(III) and simultaneous Cr(III) precipitation. In this process Cr(VI) reduction followed near zero-order kinetics, declining with an increase of pH (5–10) but boosting with increasing sulfite concentration. The co-existing Cl− and SO42− in water exerted negligible effect on Cr(VI) reduction, whereas the reduction kinetics was improved in the presence of citrate, EDTA or humic acid possibly due to their complexation with Cr(III). Similarly, the presence of borate buffer would significantly inhibit Cr(VI) reduction to Cr(III) as well as the final Cr(III) removal during precipitation. Fortunately, the presence of calcium ions even at trace level would favor Cr(III) precipitation and result in one-step removal of the total Cr at alkaline pH. The mechanism of Cr(VI) reduction was probed through irradiation manipulation and N2O addition, and the results suggested that excitation of sulfite is essential for alkaline Cr(VI) reduction, and eaq− is the dominant reactive species in the UV/sulfite process.
Covalent organic frameworks (COFs) are one type of porous organic materials linked by covalent bonds. COFs materials exhibit many outstanding characteristics such as high porosity, high chemical and ...thermal stability, large specific surface area, efficient electron transfer efficiency, and the ability for predesigned structures. These exceptional advantages enable COFs materials to exhibit remarkable performance in photocatalysis. Additionally, the activity of COFs materials as photocatalysts can be significantly upgraded by ion doping and the formation of heterojunctions. This paper summarizes the latest research progress on COF‐based materials applied in photocatalytic systems. Initially, typical structures and preparation methods of COFs are analyzed and compared. Moreover, the essential principles of photocatalytic reactions over COFs‐based materials and the latest research developments in photocatalytic hydrogen production, CO2 reduction, pollutants elimination, organic transformation, and overall water splitting are indicated. At last, the outlook and challenges of COF‐based materials in photocatalysis are discussed. This review is intended to permit instructive guidance for the efficient use of photocatalysis based on COFs in the future.
In this review, typical structures and preparation methods of covalent organic framework (COFs) are introduced. Subsequently, the essential principles of photocatalytic reactions over COFs‐based materials and the latest research developments in photocatalytic hydrogen production, CO2 reduction, and pollutant elimination are discussed. Finally, challenges and prospects for COFs‐based materials applied in photocatalysis are proposed.
Shear viscosity in a gluon gas Xu, Zhe; Greiner, Carsten
Physical review letters,
05/2008, Letnik:
100, Številka:
17
Journal Article
Recenzirano
Odprti dostop
The relation of the shear viscosity coefficient to the recently introduced transport rate is derived within relativistic kinetic theory. We calculate the shear viscosity over entropy ratio eta/s for ...a gluon gas, which involves elastic gg-->gg perturbative QCD (PQCD) scatterings as well as inelastic gg<-->ggg PQCD bremsstrahlung. For alpha_{s}=0.3 we find eta/s=0.13 and for alpha_{s}=0.6, eta/s=0.076. The small eta/s values, which suggest strongly coupled systems, are due to the gluon bremsstrahlung incorporated.
Ever since the outbreak of the COVID-19 epidemic, various public health control strategies have been proposed and tested against the coronavirus SARS-CoV-2. We study three specific COVID-19 epidemic ...control models: the susceptible, exposed, infectious, recovered (SEIR) model with vaccination control; the SEIR model with shield immunity control; and the susceptible, un-quarantined infected, quarantined infected, confirmed infected (SUQC) model with quarantine control. We express the control requirement in metric temporal logic (MTL) formulas (a type of formal specification languages) which can specify the expected control outcomes such as "the deaths from the infection should never exceed one thousand per day within the next three months" or "the population immune from the disease should eventually exceed 200 thousand within the next 100 to 120 days". We then develop methods for synthesizing control strategies with MTL specifications. To the best of our knowledge, this is the first paper to systematically synthesize control strategies based on the COVID-19 epidemic models with formal specifications. We provide simulation results in three different case studies: vaccination control for the COVID-19 epidemic with model parameters estimated from data in Lombardy, Italy; shield immunity control for the COVID-19 epidemic with model parameters estimated from data in Lombardy, Italy; and quarantine control for the COVID-19 epidemic with model parameters estimated from data in Wuhan, China. The results show that the proposed synthesis approach can generate control inputs such that the time-varying numbers of individuals in each category (e.g., infectious, immune) satisfy the MTL specifications. The results also show that early intervention is essential in mitigating the spread of COVID-19, and more control effort is needed for more stringent MTL specifications. For example, based on the model in Lombardy, Italy, achieving less than 100 deaths per day and 10000 total deaths within 100 days requires 441.7% more vaccination control effort than achieving less than 1000 deaths per day and 50000 total deaths within 100 days.
Substantial advances in enantioconvergent C(sp3)–C(sp3) bond formation reactions have been made in recent years through the use of transition-metal-catalyzed cross-coupling reactions of racemic ...secondary alkyl electrophiles with organometallic reagents. Herein, we report a general process for the asymmetric construction of alkyl–alkyl bonds adjacent to heteroatoms, namely, a nickel-catalyzed enantioconvergent reductive hydroalkylation of olefins with α-heteroatom phosphorus or sulfur alkyl electrophiles. Including the use of readily available olefins, this reaction has considerable advantages, such as mild reaction conditions, a broad substrate scope, and good functional group compatibility, making it a desirable alternative to traditional electrophile–nucleophile cross-coupling reactions.
Divergent synthesis of fluorine‐containing scaffolds starting from a suite of raw materials is an intriguing topic. Herein, we report the solvent‐controlled rhodium‐catalyzed tunable arylation of ...1‐bromo‐2,2‐difluoroethylene. The selection of the reaction solvents provides switchable defluorinated or debrominated arylation from readily available feedstock resources (both arylboronic acids/esters and 1‐bromo‐2,2‐difluoroethylene are commercially available). This switch is feasible because of the difference in coordination ability between the solvent (CH2Cl2 or CH3CN) and the rhodium center, resulting in different olefin insertion. This protocol allows the convenient synthesis of monofluoroalkenes and gem‐difluoroalkenes, both of which are important scaffolds in the fields of medicine and materials. Moreover, this newly developed solvent‐regulated reaction system can be applied to the site‐selective dechlorinated arylation of trichloroethylene. Overall, this study provides a useful strategy for the divergent synthesis of fluorine‐containing scaffolds and provides insight into the importance of solvent selection in catalytic reactions.
We report a solvent‐controlled rhodium‐catalyzed tunable arylation of 1‐bromo‐2,2‐difluoroethylene, which allows the synthesis of monofluoroalkenes and gem‐difluoroalkenes from readily available feedstocks. This study provides a useful strategy for the divergent synthesis of fluorine‐containing scaffolds and sheds light on the importance of solvent selection in catalytic reactions.
Latent heat thermal energy storage (LHTES) using phase change materials (PCM) has been considered a promising technique for improving the energy efficiency of thermal systems. However, a LHTES unit ...often suffers from low power density, e.g., low energy charging rates, because of the low thermal conductivity of common PCM like paraffin. Close-contact melting (CCM) has long been studied as a special melting process featuring very high heat transfer rates as a result of the low thermal resistance across the thin melt film between the PCM and the heating surface. It was shown in the literature that utilization of CCM could reduce the charging time of a well-designed LHTES container by even 4 times. Towards increasing the power density of LHTES units, this review paper focuses on the progress of CCM studies, including fundamental mechanisms, applications, and enhancement methods. Available theoretical models, experimental observations, as well as numerical approaches were reviewed systematically. The research gaps between investigations and applications were discussed, and the main challenges on both mechanistic understanding and utilization of CCM were also outlined. Last, some recommendations for future researches on CCM as a pathway for realizing rapid energy charging of PCM-based LHTES units were proposed.
•Close-contact melting (CCM) is deemed to be a pathway for highly efficient melting.•A review of CCM studies for charging of latent heat thermal storage was performed.•CCM can reduce charging time by 4 or more times in a well-designed container.•Main challenges and approaches on cold CCM applications were identified.•Directions towards realizing CCM for rapid charging of latent heat were outlined.
PDF
