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1.
《Tetrahedron》2006,62(2-3):381-389
Key features of the β-isocupreidine (β-ICD)-catalyzed asymmetric Baylis–Hillman reaction of aldehydes with 1,1,1,3,3,3-hexafluoroisopropyl acrylate (HFIPA) are presented. In addition, an improved method using azeotropically dried β-ICD is described.  相似文献   
2.
The balance between the rate and the selectivity of transport of various gas pairs in a series of polycarbonates has been examined. Replacement of the four available hydrogens on the aromatic rings of the bisphenol-A unit with CH3, Cl, or Br groups gives materials with a better balance of these two characteristics than the unsubstituted polycarbonate (PC). For example, using CH3 substitution increases the permeability to O2 by nearly a factor of four with no loss in O2/N2 selectivity compared with PC, while using Br substitution increases O2/N2 selectivity by 50% without any loss in O2 permeability compared with PC. While these substitutions affect the permeability through both its mobility and solubility components, the remarkable selectivity effects are caused primarily by changes in relative mobility since the changes in solubility characteristics are nearly the same for all gases. These substitutions alter chain motions, cohesion, and packing as discussed. The tetramethylbisphenol-A polycarbonate forms miscible blends with polystyrene. These blends show absolute permeability coefficients which are lower than additivity while the selectivity of transport is greater. These effects are a result of the interactions between the two polymers.  相似文献   
3.
Hidden magnetic excitations responsible for the Invar effect have been studied by polarized neutron scattering with polarization analysis. No low energy magnetic excitations could be found at the zone boundary, which excludes a simple weak ferromagnetism model for the Invar effect of the alloy. The result, however, suggests that there exist additional “dispersive” magnetic excitations which seem to develop along the magnon dispersions.  相似文献   
4.
以Zn(NO3)2· 6H2O和C6H12N4为原材料,采用二步水热法在碳纤维布上合成了形貌尺寸均匀的ZnO超细纳米线阵列。用 X 射线衍射(XRD)和扫描电镜(SEM)对其晶体结构和形貌进行了表征,利用恒流充放电测试等手段对其进行电化学性能测试。测试结果表明,材料表现出优异的电化学性能。在200 mA/g的电流密度下循环150次后,ZnO超细纳米线阵列仍然约有730 mAh/g的充放电比容量,库伦效率保持在95%以上。在1 200 mA/g的大倍率条件下,材料的充放电比容量依旧可达481 mAh/g左右,表现出十分良好的循环稳定性和可逆性能,是一种较为理想的锂离子电池负极复合材料。  相似文献   
5.
6.
席伟  陈广锋  管观洋 《应用光学》2018,39(1):100-106
高精度贴片机的视觉定位系统是提高贴片精度的关键。利用机器视觉技术分别对贴片元件和PCB板的位置及角度偏差进行检测。采用了以区域生长法为基础的元件中心和角度的计算方法,以最大内接矩形法计算了PCB板的中心和角度,从而得到了需要纠偏的位移和角度。实验验证元件中心的检测精度可达30 μm,算法处理总时间为63.6 ms,满足贴装元件定位检测系统对高速高精度的要求。  相似文献   
7.
We investigate the transport properties of a pair of Majorana bound states in a T-shaped junction, where two normal leads are coupled with an identical Majorana bound state. Both the scattering matrix and the recursive Green function method show that the peak value of the differential conductance (Gpeak) in units of e2/h and the shot noise Fano factor in the zero bias limit (F0), which are measured at the same lead and zero temperature, satisfy a linear relation as F0=1+Gpeak/2, independent of the magnitude or symmetry of the coupling strengths to the leads. Therefore, combined measurements of the differential conductance and shot noise in the T-shaped geometry can serve as a characteristic signature in probing Majorana bound states.  相似文献   
8.
One of the main differences between a low-pressure plasma treatment and an atmospheric pressure plasma treatment is that in atmosphere, the substrate material may absorb significant amount of water which may potentially influence the plasma treatment effects. This paper investigates how the moisture absorbed by aramid fibers during the atmospheric pressure plasma treatment influences the aging behavior of the modified surfaces. Kevlar 49 fibers with different moisture regains (MR) (0.5, 3.5 and 5.5%, respectively) are treated with atmospheric pressure plasma jet (APPJ) with helium as the carrier gas and oxygen as the treatment gas. Surface wettability and chemical compositions, and interfacial shear strengths (IFSS) to epoxy for the aramid fibers in all groups are determined using water contact angle measurements, X-ray photoelectron spectroscopy (XPS), and micro-bond pull out tests, respectively. Immediately after the plasma treatment, the treated fibers have substantially lower water contact angles, higher surface oxygen and nitrogen contents, and larger IFSS to epoxy than those of the control group. At the end of 30 day aging period, the fibers treated with 5.5% moisture regain had a lower water contact angle and more polar groups on the fiber surface, leading to 75% improvement of IFSS over the control fibers, while those for the 0.5 and 3.5% moisture regain groups were only 30%.  相似文献   
9.
本文研究了slide-away放电条件下的等离子体约束性能;分析了充气对等离子体约束性能以及反常多普勒共振不稳定性的影响.研究了等离子体密度的提升对slide-away放电过程中逃逸电子辐射行为的影响.研究结果发现:slide-away放电充气可以抑制逃逸电子反常多普勒不稳定性,但是使得等离子体约束状态变差,逃逸电子辐射增强.  相似文献   
10.
The authors theoretically investigate a triple-barrier ferromagnetic resonant-tunneling diode composing nonmagnetic couple wells and three GaMnN magnetic barriers, in which two kinds of antiparallel configurations are formed by changing the relative orientation of magnetization in the barriers. Based on a two-band model, the achievement of large magnetocurrents in the resonant bias regime is proposed under an optimal magnetization configuration. The magnetocurrents in both antiparallel configurations of triple-barrier system are much higher than that in double-barrier structure.  相似文献   
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