Structure and electronic transport in graphene wrinkles

Wrinkling is a ubiquitous phenomenon in two-dimensional membranes. In particular, in the large-scale growth of graphene on metallic substrates, high densities of wrinkles are commonly observed. Despite their prevalence and potential impact on large-scale graphene electronics, relatively little is known about their structural morphology and electronic properties. Surveying the graphene landscape using atomic force microscopy, we found that wrinkles reach a certain maximum height before folding over. Calculations of the energetics explain the morphological transition and indicate that the tall ripples are collapsed into narrow standing wrinkles by van der Waals forces, analogous to large-diameter nanotubes. Quantum transport calculations show that conductance through these "collapsed wrinkle" structures is limited mainly by a density-of-states bottleneck and by interlayer tunneling across the collapsed bilayer region. Also through systematic measurements across large numbers of devices with wide "folded wrinkles", we find a distinct anisotropy in their electrical resistivity, consistent with our transport simulations. These results highlight the coupling between morphology and electronic properties, which has important practical implications for large-scale high-speed graphene electronics.     

高性能铝基复合材料的设计与加工技术

从铝基复合材料微观结构特征与性能关系角度,回顾了承载结构用典型铝基复合材料在体系设计、制备与成型加工研究方面的成果与进展,同时简要介绍了近期铝基复合材料为实现性能突破所开展的研究探索,最后根据铝基复合材料的发展历程与现有研究的特点对铝基复合材料今后的研究应用进行了展望。     

Numerical and modeling issues in application of CFD to flow in a simplified plenum relevant to a prismatic VHTR

Computational Fluid Dynamics (CFD) calculations have been performed for turbulent flow inside a plenum model that resembles a section of the lower plenum of a typical helium-cooled prismatic Very High Temperature Reactor (VHTR). Different Reynolds Averaged Navier-Stokes (RANS) based turbulence models are employed to investigate the capability in capturing unsteady large scale coherent structures due to vortex shedding at two different Reynolds numbers. A grid convergence study is conducted with those models which were able to capture unsteady vortex shedding. The non-linear interaction of mesh quality, turbulence model and numerical scheme lead to flow regime changes with significantly different unsteady behaviors. This makes it difficult to assess numerical and modeling uncertainty using the procedures available in the literature. Some remedies to overcome this difficulty are recommended. The numerical uncertainty in the local values of velocity components at selected locations inside the plenum, as well as the uncertainty associated with derived quantities such as wall shear stress at critical locations are calculated and reported. Since there are no experiments corresponding to the present cases simulated, the current analysis can be considered as a blind application of the proposed uncertainty estimation procedures.     

The tricriteria flowshop scheduling problem

Multicriteria flowshop scheduling problems have been one of the most attractive subjects in recent years. In the multicriteria flowshop scheduling literature, a very limited number of studies have been performed on problems which include a tardiness criterion. In this paper a multicriteria (tricriteria) two-machine flowshop scheduling problem with a tardiness criterion is tackled. The objective is to minimise a weighted sum of total completion time, total tardiness and makespan. An integer programming model is proposed for the problem which belongs to NP-hard class. The modified NEH (Nawaz, Enscore and Ham) algorithm, a tabu search-based heuristic method, random search and the EDD rule (the earliest due date rule) are used to solve problems with up to 2,500 jobs. A computational analysis is conducted to evaluate the performance of the heuristics. The analysis shows that the heuristics are quite efficient, and the performance of the tabu search based heuristic is the best of all in terms of solution quality.     

Investigation of the effect of ethanol additives on the structure of low-pressure ethylene flames by photoionization mass spectrometry

Molecular beam mass spectrometry with photoionizationby synchrotron radiation was used to study the structure of flat premixed flames of C₂H₄/O₂/Ar and C₂H₄/C₂H₅OH/O₂/Ar at a pressure of 30 torr (4 kPa) with equivalence ratios ϕ = 1 and 2. It was shown that the replacement of part of ethylene with ethanol in the starting mixture lowered the concentration of a number of intermediate flame species, including soot precursors C₃H₃ and C₆H₆. These data are important for the development, analysis, and further improvement of detailed kinetic mechanisms for combustion of mixtures of hydrocarbon and oxygenated fuels.     

Reduced kinetic mechanism for combustion of synthesis gas at elevated temperatures and pressures

The key combustion reactions of synthesis gas at elevated initial temperatures (T ₀ = 500–700 K) and pressures (p = 10–30 atm) are identified by analyzing the kinetic mechanism. A reduced mechanism of the oxidation reactions of synthesis gas consisting of 14 elementary reactions involving 13 species is proposed which adequately describes the results of experimental data on the burning velocities of mixtures of synthesis gas with oxygen and inert diluents at T ₀ = 300–700 K, p = 10–30 atm, and ratios CO/H₂ = 0.05–0.95, and satisfactorily predicts the flame structure and the dependence of the flammability limits on the initial temperature at atmospheric pressure.     

High-frequency graphene voltage amplifier

While graphene transistors have proven capable of delivering gigahertz-range cutoff frequencies, applying the devices to RF circuits has been largely hindered by the lack of current saturation in the zero band gap graphene. Herein, the first high-frequency voltage amplifier is demonstrated using large-area chemical vapor deposition grown graphene. The graphene field-effect transistor (GFET) has a 6-finger gate design with gate length of 500 nm. The graphene common-source amplifier exhibits ～5 dB low frequency gain with the 3 dB bandwidth greater than 6 GHz. This first AC voltage gain demonstration of a GFET is attributed to the clear current saturation in the device, which is enabled by an ultrathin gate dielectric (4 nm HfO(2)) of the embedded gate structures. The device also shows extrinsic transconductance of 1.2 mS/μm at 1 V drain bias, the highest for graphene FETs using large-scale graphene reported to date.     

Systematic mesh development for 3D CFD simulation of fixed beds: Single sphere study

To develop and validate meshes for computational fluid dynamics (CFD) simulations of transport in fixed beds, a single particle is often used as a test case. We present results for drag coefficient (C_D) and heat transfer Nusselt number (Nu) for flow past a sphere, focusing on high flow rates typical of industrial steam reformers (400 < Re < 20,000). Over this range, good predictions of C_D were obtained using large eddy simulation (LES) to capture vortex shedding and wake dynamics, with a mesh refined downstream from the sphere. The small time-steps and high cell count required make this too expensive for fixed beds. Nu can be accurately calculated using a Reynolds-averaged Navier-Stokes (RANS) method with shear-stress transport (SST) k-ω closure provided the mesh at the particle surface is fine enough and covers most of the boundary layer. Single sphere simulations of heat transfer are more useful for fixed bed mesh development than drag coefficient calculations.     

基于RBF神经网络和多信息融合技术的往复压缩机状态评估研究

为及时、准确地捕捉设备运行的异常信息,提出基于径向基函数神经网络（Radial Basis Function Neural Network,简称RBFNN）和多信息融合技术的设备状态评估方法;利用往复压缩机不同测点特征值的趋势变化特点,研究神经网络分类技术,优选敏感特征参数,对不同测点特征值的趋势变化特点进行分类与组合,建立往复压缩机状态评估流程,实现设备运行异常状态的预警评估.     

Calculation of the thermodynamic interaction parameter ρ C C,Ni , in the austenite of the Fe-C-Ni system

In the study, the interaction model has been considered, which assumes that the radius of interaction between carbon and nickel atoms in austenite corresponds to the nearest neighborhood of an octahedral interstice by lattice sites, and the interaction radius between two carbon atoms corresponds to two nearest coordination shells formed by octahedral interstices in the lattice. The three-particle potential of approach of C-Ni-C atoms is taken equal to zero. The pair potential of interaction between carbon atoms in austenite is taken to be equal to that obtained from the data of Mössbauer spectroscopy. In this model, the magnitude of the secondorder thermodynamic cross-interaction parameter ρ _C ^{C, Ni} in the austenite at a temperature of 1273 K has been calculated. The results of the calculation (ρ _C ^{C, Ni} = 11.4) satisfactorily agree with the experimental value (ρ _C ^{C, Ni} = 11.1).