Asymmetric Catalytic Formal 1,4‐Allylation of β,γ‐Unsaturated α‐Ketoesters: Allylboration/Oxy‐Cope Rearrangement

A highly enantioselective formal conjugate allyl addition of allylboronic acids to β,γ‐unsaturated α‐ketoesters has been realized by employing a chiral Ni^II/N,N′‐dioxide complex as the catalyst. This transformation proceeds by an allylboration/oxy‐Cope rearrangement sequence, providing a facile and rapid route to γ‐allyl‐α‐ketoesters with moderate to good yields (65–92 %) and excellent ee values (90–99 % ee). The isolation of 1,2‐allylboration products provided insight into the mechanism of the subsequent oxy‐Cope rearrangement reaction: substrate‐induced chiral transfer and a chiral Lewis acid accelerated process. Based on the experimental investigations and DFT calculations, a rare boatlike transition‐state model is proposed as the origin of high chirality transfer during the oxy‐Cope rearrangement.     

Multi-hierarchical nanosheet-assembled chrysanthemum-structured Na<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C as electrode materials for high-performance sodium-ion batteries

The structure and morphology of sodium vanadium phosphate (Na₃V₂(PO₄)₃) play a vital role in enhancing the electrochemical performance of sodium-ion batteries due to the inherent poor electronic conductivity of the phosphate framework. In order to improve this drawback, a new chrysanthemum-structured Na₃V₂(PO₄)₃/C material has been successfully assembled with multi-hierarchical nanosheets via a hydrothermal method. Continuous scattering nanosheets in chrysanthemum petals are beneficial in reducing energy consumption during the process of sodium ion diffusion, on which the carbon-coated surface can significantly increase overall conductivity. The as-prepared sample exhibits outstanding electrochemical performance due to its unique structure. It rendered a high initial specific capacity of 117.4?mAh?g^?1 at a current density of 0.05 C. Further increasing the current density to 10 C, the initial specific capacity still achieves 101.3?mAh?g^?1 and remains at 87.5?mAh?g^?1 after 1000 cycles. In addition, a symmetrical sodium-ion full battery using the chrysanthemum-structured Na₃V₂(PO₄)₃/C materials as both the cathode and anode has been successfully fabricated, delivering the capacity of 62?mAh?g^?1 at 1?C and achieving the coulombic efficiency at an average of 96.4% within 100 cycles. These results indicate that the new chrysanthemum-structured Na₃V₂(PO₄)₃/C can provide a new idea for the development of high-performance sodium-ion batteries.     

Li原子修饰笼型Si6团簇的结构和储氢性能

利用杂化密度泛函B3LYP方法, 在6-311+G(d, p)基组水平上对Si6和Li修饰的Si6团簇的几何结构和电子性质及储氢性能进行模拟计算和理论研究. 结果表明, Si6团簇最低能量构型为笼型结构, 纯Si6团簇不能有效吸附氢分子. Li原子的引入显著改善了Si6团簇的储氢能力. 以两个Li原子端位修饰Si6团簇为载体, 其氢分子的平均吸附能为1.692~2.755 kcal/mol, 每个Li原子周围可以有效吸附五个氢分子, 储氢密度可达9.952wt%. 合适的吸附能和较高储氢密度表明Li修饰Si6团簇有望成为理想的储氢材料.     

Improvement in tribological properties of atmospheric plasma-sprayed WC-Co coating followed by Cu electrochemical impregnation

The WC-Co coating obtained by atmospheric plasma spraying (APS) was modified by Cu electrochemical impregnation. The copper has infiltrated into and filled up the pores in WC-Co coating. The tribological properties of the coating against the stainless steel ball as sliding pairs were investigated with a ball-on-disc (BOD) configuration in air at room temperature. The as-prepared samples were characterized by means of optical microscope, scanning electron microscope and X-ray diffraction. It was found that the frictional behavior of the WC-Co coating followed by Cu electrochemical impregnation was superior to that of WC-Co coating. The wear mechanism of the WC-Co coating followed by Cu electrochemical impregnation was microcutting, whilst that of a WC-Co coating was the fatigue wear. The improvement in tribological properties of the WC-Co coating followed by Cu electrochemical impregnation was attributed to the formation of self-lubricating Cu film on the wear surface which induces the transformation of wear mechanism.     

高增益砷化镓光导开关中的光致电离效应

研究了光致电离在高增益本征砷化镓(GaAs)光导开关(PCSS)中的效应.在高增益本征砷化镓光导开关中,各级流注发展由三个过程组成:光致电离、畴电子崩(DEA)和载流子碰撞电离雪崩生长.光致电离在本征砷化镓绝缘区中产生局部高载流子密度区域,提供了允许畴存在的局域环境.光致电离包括激光触发和流注的复合辐射两种情形.分析了光导开关的最优触发激光条件.通过计算机数值模拟,计算了在距离流注表面y≤30 μm的范围内,平均光生载流子密度n(t=0)乘以该局域的特征尺度满足偶极畴成核条件:n(t=0)·y>1012 cm-2;探讨了流注的复合辐射在流注周围产生非平衡载流子的规律;发现了触发区域沿电场方向的长度阈值LEC,触发区域的特征长度LE必须满足条件:LE≥LEC.     

A model study of quark-number susceptibility at finite chemical potential and temperature

In this Letter, we try to give a direct method for calculating the quark-number susceptibility (QNS) at finite chemical potential and temperature. In our approach the QNS is given by a formula which solely involves (the dressed quark propagator at finite chemical potential μ and temperature T). From this the QNS at finite chemical potential and temperature is calculated in the framework of rainbow-ladder approximation of the Dyson–Schwinger approach using the meromorphic quark propagator proposed in [M.S. Bhagwat, M.A. Pichowsky, P.C. Tandy, Phys. Rev. D 67 (2003) 054019]. It is found that the QNS χ(μ,T) has a singularity when μ comes close to a critical value μ₀, and the susceptibility as a function of T becomes discontinuous at some values of T when μ is near μ₀. At high temperature the QNS approaches the ideal quark gas result, while at very small temperature (T<40 MeV) the susceptibility equals zero. For all values of μ we studied, the susceptibility shows a rapid increase near T=120–140 MeV, which could be regarded as the signal of a crossover.     

Luminescence properties of the Eu-doped porous glass and spontaneous reduction of Eu to Eu

The luminescent properties of Eu³⁺-doped porous glass were investigated and reduction of Eu³⁺ to Eu²⁺ was observed when the Eu³⁺-doped porous glasses were heat treated in air. The absorption, emission and excitation spectra of Eu³⁺-doped porous glasses were measured and the results significantly depended on the treating temperature. The integral intensity of Eu³⁺ and Eu²⁺ ions at different temperatures was also investigated to investigate this reduction process. Electron spin resonance (ESR) spectra of the samples were also measured, which also confirmed the reduction effect. A possible mechanism was discussed to explain this process.     

The relaxations of temporal bond polarizabilities of methylviologen adsorbed on the Ag electrode by 514.5 nm excitation: a Raman intensity study

An algorithm to elucidate the temporal bond polarizabilities from the surface enhanced Raman (SERS) intensities was employed to the case of methylviologen (MV) adsorbed on the Ag electrode. This enables us to obtain the properties of its SERS mechanisms and the effect of its adsorption. The analysis shows that the charge transfer and electromagnetic mechanisms involving in this MV SERS system possess different relaxation times for its various temporal bond polarizabilities. The physics is that the process involved in the charge transfer mechanism will take longer time than that involved in the electromagnetic mechanism since it needs more time to redistribute the charges during relaxation. The time division between these two mechanisms is figured out to be around 3 ps for this system. Adsorption also enhances the relaxation of the temporal bond polarizabilities, in general. The adsorption effect is indicated by the temporal bond polarizabilities close to the final stage of relaxation. They are, in fact, the quantities parallel to the bond electronic densities in the molecular orbital (MO) concept. For comparison, the case of MV solid was also analyzed. Copyright © 2008 John Wiley & Sons, Ltd.     

Deposition of Ag nanoparticles on porous anodic alumina for surface enhanced Raman scattering substrate

Ag nanoparticles were exclusively deposited inside the pores of the porous anodic alumina (PAA) template through the deposition cycle including the incubation and the subsequent reduction of Ag(NH₃) . Both the density and size of the produced Ag nanoparticles increased as the deposition cycle number increased. A field‐emission scanning electron microscopeand an ultraviolet‐visible spectrometer were applied, respectively, to study the morphology and the extinction spectra of the Ag nanoparticles. The optimum deposition number was found from the scanning electron microscope (SEM) analysis. Surface enhanced Raman scattering (SERS) spectra of p‐aminothiophenol recorded on the Ag–PAA substrates prepared under increasing number of deposition cycles, manifested an enlarging trend of peak intensity. A point‐by‐point SERS mapping of p‐aminothiophenol on the Ag–PAA substrate was acquired to characterise the homogeneity of the substrate. Copyright © 2008 John Wiley & Sons, Ltd.