高速铁路工程中若干典型力学问题

高速铁路是一个复杂的系统工程,涉及一系列关键力学问题.本文讨论高速铁路工程中的3个关键固体力学问题:高速铁路轮轨滚动接触力学问题、高速列车关键结构部件疲劳问题和高速列车与线路结构动态相互作用问题;概述了其研究进展与工程应用;提出了今后进一步结合高速铁路工程需求的研究方向.     

discontinuous galerkin method for discontinuous temperature field problems

针对不连续温度场问题建立了一种间断Galerkin有限元方法,该方法的主要特点是允 许插值函数在单元边界上存在跳变. 在建立有限元方程时,通过在单元边界上引入数值通量 项和稳定性项来处理间断效应,并且数值通量可以直接由接触热阻的定义式导出. 数值算例 表明该方法可以很方便且准确地捕捉到结构内部由于接触热阻而引起的温度跳变,同时在局 部高梯度温度场的模拟方面也比常规连续Galerkin有限元方法效率明显要高. 该方法也为研 究由接触热阻引起的温度场与应力场之间的耦合问题提供了一种新的数值模拟手段.     

感应耦合等离子体刻蚀p-GaN的表面特性

研究了p-GaN材料经感应耦合等离子体(ICP)刻蚀后的表面特性,并用不同的方法对刻蚀表面进行处理.利用原子力显微镜(AFM)和X射线光电子能谱(XPS)对刻蚀样品进行分析,并在样品表面制作Ni/Au电极,进行欧姆接触特性的测试.实验结果表明了NaOH溶液处理表面对改善材料表面和欧姆接触特性是比较有效的. 关键词： GaN 感应耦合等离子刻蚀 表面处理 欧姆接触     

AlGaN/GaN肖特基结参数分析与电流运输机理研究

通过对AlGaN/GaN HEMT器件肖特基栅电流输运机理的研究,在变温下采用I-V法对AlGaN/GaN上的Ni/Au肖特基势垒高度和理想因子进行了计算. 通过对不同电流机理的分立研究,得到了更为准确的势垒高度值_b. 通过分析温度在300—550K之间肖特基反向泄漏电流的特性,得出结论：AlGaN材料的表面漏电不是HEMT器件反向泄漏电流的主要来源. 关键词： AlGaN/GaN异质结 肖特基结 理想因子     

Electrode contact study for SiGe thin film operated at high temperature

A study on the electrode contact of the sputtered SiGe thin film is reported for application of devices working at high temperature. Surface morphological characterization with optical microscope and AFM (atomic force microscope) together with the electrical characterization by TLM measurements (transmission line method) were performed before and after aging at 500 °C for 24 h using various sputtered multilayer electrodes, Ti/Au/Ti, Ta/Pt/Ta and Ti/Pt/Ti, on 300-nm B-doped SiGe thin film deposited by magnetron sputtering and furnace crystallisation at high temperature. After aging at 500 °C for 24 h, the Ti/Au/Ti multilayer electrodes seriously degraded to be non-ohmic contact, showing rough surface morphology. The Ti/Pt/Ti metal layers showed the lowest specific contact, resistivity before and after aging, 1.46 × 10⁻³ Ω cm² and 1.68 × 10² Ω cm² respectively.     

Electronic structure calculations for inhomogeneous systems: Interfaces,surfaces, and nanocontacts

The article gives an introduction into the application of density functional theory (DFT) to inhomogeneous systems. To begin with, we describe the interplay of specific materials at interfaces, resulting in structure relaxation and modifications of the chemical bonding. We address interfaces between YBa₂Cu₃O₇ and a normal metal, in order to quantify the intrinsic interface charge transfer into the superconductor. Moreover, we study the internal interfaces in a V₆O₁₃ battery cathode and the effects of ion incorporation during the charging and discharging process. The second part of the article deals with the influence of surfaces on the nearby electronic states. Here, we investigate a LaAlO₃/SrTiO₃ heterostructure in a thin film geometry. We particularly explain the experimental dependence of the electronic states at the heterointerface on the surface layer thickness. Afterwards, surface relaxations are studied for both the clean Ge(001) surface and for self‐assembled Pt nanowires on Ge(001). In the third part, we turn to atomic and molecular contacts. We compare the properties of prototypical Al nanocontact geometries, aiming at insight into the chemical bonding and the occupation of the atomic orbitals. Finally, the local electronic structure of a benzene‐1,4‐dithiol molecule between two Au electrodes is discussed as an example for a molecular bridge.     

The effect of a surface layer capping p-InGaN/p-GaN superlattices on the contact to p-GaN

In this study, the influence of the surface layer (p-InGaN or p-GaN) capping p-InGaN/p-GaN superlattices (SLs) on the contact to p-type GaN was investigated. It was found that the specific contact resistance (_ρc)$\left({\rho }_c\right)$ to p-type GaN is lower when using p-InGaN as the surface layer. The lowest value of _ρc${\rho }_c$ was 1.99×10⁻⁴ Ω cm² at room temperature. It was also found that low temperature growth of the p-GaN layers in the SLs is beneficial for lowering the ohmic contact resistance. Unlike Ni/Au deposited directly on p-GaN (without the strained p-InGaN/p-GaN SLs), Ni/Au deposited on p-InGaN/p-GaN SLs produces ohmic behavior even before annealing.     

Experimental studies of influence of the turbulent flow structure on temperature distribution in a compact heat exchanger

Results of experimental investigation of temperature distribution over the surface of a complex heat exchanger (the Frenkel packing type) are presented. Measurements were carried out in the air flow between two sheets with triangular corrugations directed at 90° to each other. Measurements were carried out by the microthermocouples glued on the heated outer surface. The effect of Reynolds numbers, a gap between corrugated sheets, and substitution of one corrugated sheet by the smooth one on temperature distribution over the heat exchanger surface in the turbulent air flow is analysed. According to the performed experiments, there is a significant effect of a gap and applied perturbations on the type of temperature distribution over the perimeter of a heated cell.     

Laterally inhomogeneous barrier analysis of the methyl violet/p-Si organic/inorganic hybrid Schottky structures

In this work, we have investigated the electrical characteristics, such as current-voltage (I-V) and capacitance-voltage (C-V) measurements, of identically prepared crystal violet/p-Si Organic/Inorganic (OI) Schottky structures formed by evaporation of organic compound solution to directly p-Si semiconductor substrate. It has been seen that the crystal violet organic dye thin film on the p-Si substrate has exhibited a good rectifying behavior. The barrier heights (BHs) and ideality factors of all devices have been calculated from the electrical characteristics. Although the diodes were all identically prepared, there was a diode-to-diode variation: the effective barrier heights ranged from 0.6 ± 0.1 to 0.8 ± 0.1 eV, and the ideality factor from 1.6 ± 0.4 to 3.5 ± 0.4. The barrier height versus ideality factor plot has been plotted for the OI devices. Lateral homogeneous BH was calculated as a value of 0.7 eV from the observed linear correlation between BH and ideality factor, which can be explained by laterally inhomogeneities of BHs. The values of barrier height and acceptor doping concentration yielded from the reverse bias C-V measurements ranged from 0.7 ± 0.1 to 1.3 ± 0.1 eV and from (4.7 ± 0.8) × 10¹⁴ to (8.1 ± 0.8) × 10¹⁴ cm⁻³, respectively. The mean barrier height and mean acceptor doping concentration from C-V characteristics has been calculated 1.0 eV and 5.9 × 10¹⁴ cm⁻³, respectively. It has been seen that the mean BH value of 0.7 eV obtained for the Al/methyl violet/p-Si contact is significantly larger than BH values of the conventional Al/p-Si Schottky diodes. Thus, modification of the interfacial potential barrier for metal/Si diodes has been achieved using a thin interlayer of the methyl violet organic semiconductor; this has been ascribed to the fact that the methyl violet interlayer increases the effective barrier height by influencing the space charge region of Si.     

Analysis of protein folds using protein contact networks

Proteins are important biomolecules, which perform diverse structural and functional roles in living systems. Starting from a linear chain of amino acids, proteins fold to different secondary structures, which then fold through short- and long-range interactions to give rise to the final three-dimensional shapes useful to carry out the biophysical and biochemical functions. Proteins are defined as having a common ‘fold’ if they have major secondary structural elements with same topological connections. It is known that folding mechanisms are largely determined by a protein’s topology rather than its interatomic interactions. The native state protein structures can, thus, be modelled, using a graph-theoretical approach, as coarse-grained networks of amino acid residues as ‘nodes’ and the inter-residue interactions/contacts as ‘links’. Using the network representation of protein structures and their 2D contact maps, we have identified the conserved contact patterns (groups of contacts) representing two typical folds — the EF-hand and the ubiquitin-like folds. Our results suggest that this direct and computationally simple methodology can be used to infer about the presence of specific folds from the protein’s contact map alone.