The nickel-catalyzed decarbonylation of unstrained diaryl ketones has been developed. The reaction is catalyzed by a combination of Ni(cod)2 and an electron-rich N-heterocyclic carbene ligand. High ...functional group tolerance and excellent yields (up to 98%) are observed. This strategy provides an alternative and versatile approach to construct biaryls using an inexpensive nickel catalyst.
Paleoanthropologists and vertebrate paleontologists have for decades debated the etiology of tooth wear and its implications for understanding the diets of human ancestors and other extinct mammals. ...The debate has recently taken a twist, calling into question the efficacy of dental microwear to reveal diet. Some argue that endogenous abrasives in plants (opal phytoliths) are too soft to abrade enamel, and that tooth wear is caused principally by exogenous quartz grit on food. If so, variation in microwear among fossil species may relate more to habitat than diet. This has important implications for paleobiologists because microwear is a common proxy for diets of fossil species. Here we reexamine the notion that particles softer than enamel (e.g., silica phytoliths) do not wear teeth. We scored human enamel using a microfabrication instrument fitted with soft particles (aluminum and brass spheres) and an atomic force microscope (AFM) fitted with silica particles under fixed normal loads, sliding speeds, and spans. Resulting damage was measured by AFM, and morphology and composition of debris were determined by scanning electron microscopy with energy-dispersive X-ray spectroscopy. Enamel chips removed from the surface demonstrate that softer particles produce wear under conditions mimicking chewing. Previous models posited that such particles rub enamel and create ridges alongside indentations without tissue removal. We propose that although these models hold for deformable metal surfaces, enamel works differently. Hydroxyapatite crystallites are “glued” together by proteins, and tissue removal requires only that contact pressure be sufficient to break the bonds holding enamel together.
•Concurrent topology optimization for multiscale dynamic composite structures.•A hybrid dimensional reduction method to estimate interval mean and variance.•A new robust topology optimization method ...under random-interval hybrid uncertainties.•Robust objective function defined as a weighted sum of mean and standard variance.
A new robust topology optimization method based on level sets is developed for the concurrent design of dynamic structures composed of uniform periodic microstructures subject to random and interval hybrid uncertainties. A Hybrid Dimensional Reduction (HDR) method is proposed to estimate the interval mean and the interval variance of the uncertain objective function based on a bivariate dimension reduction scheme. The robust objective function is defined as a weighted sum of the mean and standard variance of the dynamic compliance under the worst case. The sensitivity information of the robust objective function with respect to the macro and micro design variables can then be obtained after the uncertainty analysis. Several examples are used to validate the effectiveness of the proposed robust topology optimization method.
Distributed denial-of-service (DDoS) defense is still a difficult problem though it has been extensively studied. The existing approaches are not capable of detecting various types of DDoS attacks. ...In particular, new emerging sophisticated DDoS attacks (e.g., Crossfire) constructed by low-rate and short-lived benign traffic are even more challenging to capture. Moreover, it is difficult to enforce realtime defense to throttle these detected attacks since the attack traffic can be concealed in benign traffic. Software defined networking (SDN) opens a new door to address these issues. In this paper, we propose Reinforcing Anti-DDoS Actions in Realtime (RADAR) to detect and throttle DDoS attacks via adaptive correlation analysis built upon unmodified commercial off-the-shelf SDN switches. It is a practical system to defend against a wide range of flooding-based DDoS attacks, e.g., link flooding (including Crossfire), SYN flooding, and UDP-based amplification attacks, while requiring neither modifications in SDN switches/protocols nor extra appliances. It accurately detects attacks by identifying attack features in suspicious flows, and locates attackers (or victims) to throttle the attack traffic by adaptive correlation analysis. We implement RADAR prototype using open source Floodlight controller, and evaluate its performance under various DDoS attacks by real hardware testbed based experiments. We observe that our scheme can successfully detect and effectively defend against various DDoS attacks with acceptable overhead.
Summary
In this paper, the solidification process of the phase change material (PCM) in the shell‐and‐tube latent‐heat thermal energy storage unit (LTESU) strengthened by fin is studied. For ...improving the strengthening effect of fins on the solidification performance of shell‐and‐tube LTESU, this paper proposes a novel connected‐Y‐shaped fin. The novel fin is inspired by the interconnection of the internal texture of the lotus leaf, and the branches of the two‐level Y‐shaped fins are connected into a whole by three metal annuluses. The numerical simulation method is applied to investigate the influence of the connected‐Y‐shaped fin on the solidification performance of PCM. The effects of annulus position and fin width on solidification performance are studied and analyzed. At the same time, the effects of the connected‐Y‐shaped fin on the solidification process under different wall temperatures and the radius ratio of outer to inner tube (γi) are investigated. The results indicated that the connected‐Y‐shaped fin can significantly enhance the discharging performance of LTESU. Compared with the traditional two‐level Y‐shaped fins, the optimized connected‐Y‐shaped fin can shorten the PCM solidification time by 53.9%. The optimum width ratio of the connected‐Y‐shaped fin should be 2. The optimum radius ratio of the outer annulus, middle annulus, and inner annulus to the outer tube should be 0.91, 0.64, and 0.41, respectively. The scope of application of the conclusion is that Ste = 0.139 − 0.496, and γi = 0.083 − 0.167.
Highlight
A novel connected‐Y‐shaped fin is proposed for the latent heat storage unit.
The influences of fin structure parameters on the solidification process are discussed.
Optimum fin parameters are recommended for maximizing the solidification rate.
The novel fin can decrease the solidification time of PCM by 53.9%.
This paper proposes a novel connected‐Y‐shaped fin to improve the performance of the latent heat storage unit, and discusses the influence of fin structure parameters on the solidification process. The optimal fin parameters are recommended to maximize the solidification rate. The results show that the solidification time of phase change material can be decreased by 53.9% by using the novel fin.
•Pre-cooling contributes significant effects on hot flow behaviour and post strength of quench sensitive aluminium alloys.•A unified model considers microstructure evolution and their mutual effect ...during hot forming with pre-cooling was proposed.•The new model enables to successfully predict the hot forming of high strength AA7075 requiring pre-cooling stage.
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Hot forming high strength aluminium alloys with a pre-cooling stage can overcome detrimental effects of the low-melting phases defect, and enhance hardening. The introduced pre-cooling brings significant changes in microstructures of the alloys at elevated temperatures, which directly affect the hot deformation and strength behaviour under the novel hot forming condition. In this study, systematic experimentation was performed to obtain the detailed effects of pre-cooling strategies on hot forming, and a physical-based unified material model considering the mutual effects of underlying microstructures evolution (precipitates, solute concentrations and dislocations) and mechanical properties (yield strength, work-hardening and ductility) was established. The developed model enables the precise prediction of hot flow behaviour, microstructure evolution (solute concentration, fraction of precipitates and dislocation density) and post strength (yield strength/hardness) of AA7075 conditioned by different pre-cooling strategies. The effectiveness of the developed model has been successfully supported and validated by corresponding experiments, including Gleeble thermal mechanical testing and transmission electron microscopy (TEM) observations for both precipitates and macro-properties. In addition, effects of the key parameters of pre-cooling, such as cooling rate and cooled temperature, on hot flow and post treated hardness were discussed based on both experimental and numerical results. It is found that the detrimental effect using a lower cooling rate, i.e. 5 K/s, is the induced coarse precipitates, which would alter the subsequent hot flow behaviour and deteriorate post strength. In addition, the lowest hardness in the formed alloy occurs within a holding temperature range of 598–623 K for AA7075. The developed model can provide direct guidance for process design and parameter optimisations for the hot forming processes of high strength AA7075 with pre-cooling.
Intracellular pH is an important parameter associated with cellular behaviors and pathological conditions. Quantitative sensing pH and monitoring its changes by near-infrared (NIR) fluorescence ...imaging with high resolution in living systems are essential but challenging due to the lack of effective probes. To achieve good adaptability, in this study, a class of resolution-tunable ratiometric NIR fluorescent probes, which possess a stable NIR hemicyanine skeleton bearing different substituents, are rationally designed and synthesized, enabling detection through noninvasive optical imaging of organisms. Based on the protonation/deprotonation of the hydroxy group, a marked NIR emission shift provides a ratio signal in response to pH. Meanwhile, two states exhibit good photostability, sensitivity and reversibility, conducive to longtime monitoring of persistent pH changes without disturbance of other biological active species. Among the series, NIR-Ratio-BTZ modified with an electron-withdrawing substituent of benzothiazole exhibited the largest emission shift of about 76 nm from 672 to 748 nm with the pH environment changing from acidic to basic, which could be considered as a good candidate for high resolution pH imaging in live cells, tissues and organisms. Moreover, NIR-Ratio-BTZ has an ideal pK a value (pK a ≈ 7.2) for monitoring the minor fluctuations of physiological pH near neutrality. The ratiometric fluorescence measurement is beneficial to ensure the accuracy of quantitative measuring pH changes, as well as the real-time monitoring pH-related physiological effects both in living cells and living mice. The results demonstrate that NIR-Ratio-BTZ is a highly sensitive ratiometric pH probe in vivo, giving it potential for biological applications.
Crystal structure, electronic structure, elastic anisotropy, and phonon of P63/mmc-Mg17Ce2 and I4/m-Mg41Ce5 have been thoroughly studied by employing density functional theory (DFT) in conjunction ...with density functional perturbation theory (DFPT). The relaxed lattice parameters a and c, equilibrium cell volumes V0 agree with the available experimental reports. The individual elastic stiffness constants Cij's were obtained via “energy-strain” and “stress-strain” techniques. We indicate that both P63/mmc-Mg17Ce2 and I4/m-Mg41Ce5 should be stabilized mechanically. Polycrystalline aggregates, brittle or ductile characteristics, Poisson's ratio, Debye temperature and the integration of elastic wave velocities along various crystal orientations were also derived incidentally. The anisotropy of the directional linear compression and Young's modulus of P63/mmc-Mg17Ce2 and I4/m-Mg41Ce5 were evaluated, it means that both P63/mmc-Mg17Ce2 and I4/m-Mg41Ce5 possess a certain degree of elastic anisotropy under equilibrium lattice constants. Particularly, the phonon dispersions, phonon density of states, and InfraRed, Raman and Silent vibrational modes of both systems are reported for the first time. Based on our calculated phonon vibrations, it is also shown that both the P63/mmc structure of Mg17Ce2 and the I4/m structure of Mg41Ce5 should be stabilized dynamically.
•Both P63/mmc-Mg17Ce2 and I4/m-Mg41Ce5 should be stabilized mechanically.•Both the P63/mmc structure of Mg17Ce2 and the I4/m structure of Mg41Ce5 should be stabilized dynamically.•The Raman and Infrared active phonon modes at Г point are assigned by group theory.
Over the last decades, scientists have endeavored to develop nanoscopic machines and envisioned that these tiny machines could be exploited in biomedical applications and novel material fabrication. ...Here, a visible‐/near‐infrared light‐driven nanomotor based on a single silicon nanowire is reported. The silicon nanomotor harvests energy from light and propels itself by the self‐electrophoresis mechanism. Due to the high efficiency, the silicon nanowire can be readily driven by visible and near‐infrared illumination at ultralow light intensity (≈3 mW cm−2). The experimental study and numerical simulation also show that the detailed structure around the concentrated reaction center determines the migration behavior of the nanomotor. Importantly, due to the optical resonance inside the silicon nanowire, the spectral response of the nanowire‐based nanomotor can be readily modulated by the nanowire's diameter. Compared to other methods, light controlling potentially offers more freedom and flexibility, as light can be modulated not only with its intensity and direction, but also with the frequency and polarities. This nanowire motor demonstrates a step forward to harness the advantages of light, which opens up new opportunities for the realization of many novel functions such as multiple channels communication to nanorobots and controllable self‐assembly.
A visible/near‐infrared‐light‐driven nanomotor is developed based on a silicon‐nanowire solar cell and the electrophoretic mechanism. The morphology influence is experimentally and theoretically investigated, which offers a new protocol for motion maneuvering. The spectral engineering to modulate optical resonance inside the silicon wire provides a new dimension for the light‐controlled nanorobots and sheds light on their future biomedical applications.
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