The study of lithium insertion–deinsertion processes into composite graphite electrodes by in situ atomic force microscopy (AFM)

Li insertion–deinsertion into composite graphite electrodes, comprising synthetic graphite flakes (6 μm average size), polyvinylidene difluoride binder (PVdF), and copper current collectors, in commonly used alkyl carbonate solutions were studied by in situ atomic force microscopy (AFM). In this study, we were able to probe by in situ AFM the behavior of practical, composite graphite electrodes in ethylene carbonate–dimethyl carbonate (EC–DMC) solutions containing salts such as LiAsF₆ and LiPF₆ during entire lithiation–delithiation cycles. These in situ micro/nanomorphological studies could probe surface film formation on the graphite particles, as well as periodic volume changes in the graphite flakes during Li insertion–deinsertion cycles. These cyclic volume changes can explain the capacity fading of graphite electrodes upon prolonged cycling, in Li-ion batteries. While the overall morphology of these electrodes remains steady upon cycling in the appropriate solutions (in which the Li–C electrodes are efficiently passivated), there is a continuous problem in the extent of accommodation of the small volume changes in the graphite particles upon lithiation–delithiation, by the surface films. It is suggested that graphite electrodes fail during prolonged cycling due to small scale, continuous reactions of the active mass with solution species, which gradually increase their impedance and decrease the content of the lithium stored in the electrodes.     

Microstructural and gas separation properties of CVD modified mesoporous γ-alumina membranes

Chemical vapor deposition (CVD) was used to modify 4 nm pore, sol–gel derived, γ-alumina membranes supported on macroporous α-alumina. Aluminum oxide was deposited in the pores of the γ-alumina membrane by alternating additions of trimethylaluminum (TMA) and water vapor. By reducing the pore size, the permeance of non-condensable gasses was reduced much more than the permeance of condensable gasses due to capillary condensation or preference adsorption of water vapor. The modified membrane that exhibited the best separation properties had a water vapor permeance ranging from 1.5×10⁻⁶ to 3.0×10⁻⁷ mol/m² s Pa, an oxygen permeance ranging from 1.7×10⁻⁷ to 1.5×10⁻⁹ mol/m² s Pa, and a separation factor as high as 140 at room temperature. The microstructure of the pores contained some irregularities which were attributed to an atomic layer CVD (ALCVD) mechanism modified by homogeneous reactions. The effect of the modified ALCVD was higher permeances than would be expected. P-type zeolite membranes were also made and found to have similar separation properties to the more heavily modified γ-alumina membranes.     

Probability density functions of contact forces for cohesionless frictional granular materials

A theory is developed for the probability density functions of contact forces for cohesionless, frictional granular materials in quasi-static equilibrium. This theory is based on a maximum information entropy principle, with an expression for information entropy that is appropriate for granular materials. Entropy is maximized under the constraints of a prescribed stress and that the normal component of the contact force is compressive and that the tangential component of the contact force is limited by Coulomb friction. The theory results in a dependence of the probability density function for the tangential contact forces on the friction coefficient. The theoretical predictions are compared with results from discrete element simulations on isotropic, two-dimensional assemblies under hydrostatic stress. Good qualitative agreement is found for means and standard deviations of contact forces and the shape of the probability density functions, while the quantitative agreement is fairly good. Discrepancies between theory and simulations, such as the difference in shape of the probability density function for the normal force and the observed dependence on elastic properties of the exponential decay rate of tangential forces, are attributed to the fact that the method does not take into account any kinematics, which are essential in relation to elastic effects.     

Analysis of volatile organic compounds using gas chromatography

The focus of this review is the analysis of volatile organic compounds (VOCs) by gas chromatography (GC) in the field of environmental, food, flavour and fragrance, medical and forensic sciences. New trends in sample injection, separation and detection are covered, including multi-dimensional and high-speed GC. Attention is drawn to a growing interest in quality assessment. From the review, it is clear that it remains a challenge to generate multi-component gaseous standards of VOCs at ppbv and pptv.     

Broadband characteristics investigation of waves in a left-handed miniaturized waveguide loaded with ISRRs

In this paper, broadband transmission characteristics of waves in a left-handed miniaturised rectangular waveguide (LHMRW) loaded with improved split ring resonators (ISRRs), which shows that the bandwidth of negative permeability is up to 5.6 GHz by an optimization design, are investigated. Some unexpected phenomena such as the discontinuous left-handed passband below the cutoff frequency of the dominant TE mode, which cannot be transformed into a continuous one by any impedance matching approach, are observed. The EM field distributions in the LHMRW are calculated so as to explain these phenomena. Results show that the LHMRW can be considered as an open ended resonant cavity instead of a normal transmission structure. In addition, the backward wave inside the time domain in the LHMRW is directly observed. PACS 78.70.Gq; 81.05.Zx; 84.40.Ba     

An ultra-small cavity resonator loaded with LHM and RHM layers

In this paper, an ultra-small cavity resonator (USCR) loaded with left-handed metamaterial (LHM) and right-handed material (RHM) layers is designed using a novel miniaturization approach. The resonant behavior is successfully observed, and the dimensions of the USCR are only 4.58 mm × 5.08 mm × 2.29 mm at the dominant resonance frequency of 10.3 GHz. Through the field distribution calculation, we confirmed that the miniaturization of the USCR arises from the left-handed property of the LHM. For a practical application, a miniaturized filter with overall length of 10.19 mm consisting of two USCRs is designed to confirm the frequency-selective characteristics. Results show that the filter has some narrow pass bands, which correspond to the resonant modes of the electromagnetic resonance in the USCR, and the insertion loss at the dominant resonance frequency of the USCR is as low as 0.65 dB. Moreover, the filtering characteristics of the filter can be controlled by changing its feeding loop positions in the USCR. PACS 78.70.Gq; 81.05.Zx; 84.40.Ba     

PRZEWALSKINIC ACID A,A NEW PHENOLIC ACID FROM SALVIA PRZEWALSKII MAXIM

From the root of Salvia przewalskii Maxim.a new phenolic acid,przewalskinic acid A was isolated and the structure was established by theanalysis of~(13)C-NMR,~1H-NMR and two-dimensional COSY experiments.     

基于云模型和证据理论的科技创新与持续发展能力评价

为科学合理地对企业科技创新与持续发展能力进行评估,文章从科技创新基础与现状情况、科技创新组织管理能力和科技创新持续发展能力三个方面入手建立企业科技创新与持续发展评估的指标体系,并针对指标的不确定性构建一种基于云模型和证据理论的科技创新与持续发展能力评价模型。首先应用云模型对评价指标转化为区间数并应用区间熵权法确定指标权重,其次运用证据理论对指标的置信度等进行计算得出评价对象的评估结果。最后以国网某省公司为例,分别应用基于云模型和证据理论的评估模型和区间证据推理方法进行评估研究,验证评价模型的可行性、准确性、科学性和合理性。研究结论为在企业科技创新与持续发展中应加强科技创新持续发展能力等方面的建设。     

正态分布和瑞利分布混合情形下的参数估计及分类问题

学者往往用单一的分布模拟和拟合杂波,如正态分布、瑞利分布和威布尔分布等。然而在实际中,雷达杂波由多种类型的杂波组成,单一分布通常不能精确刻画雷达杂波规律,因此,应用混合分布模型对雷达杂波数据建模更准确。本文考虑用正态分布和瑞利分布的混合分布拟合杂波,并应用矩估计方法和基于EM算法的极大似然估计方法估计模型参数,最后,应用最大后验概率分类准则验证2种估计方法的分类准确率。通过数据模拟,得出极大似然估计的效果和分类准确率都要优于矩估计的估计效果和分类准确率。