巷道支护加钢筋网喷射混凝土破坏过程的数值模拟

确保喷射混凝土支护方式的有效性是喷射支护的关键,如何事先预测喷射混凝土的破坏模式是矿山建设工程中亟待解决的问题。采用三维真实破坏过程分析系统,对加钢筋网喷射混凝土结构在双向拉伸作用下的破坏过程进行数值模拟,直观再现了喷射混凝土结构中应力场分布及裂纹演化过程。分析其破坏机理,并提出了一些建议,对于工程实践有一定的指导作用。     

IPv4/IPv6协议过渡机制的实验研究

从IPv4向IPv6的过渡是网络发展的一大趋势，但存在很多技术问题亟待解决。为了使这一过渡尽可能地平滑，人们提出了很多解决方法。本文研究了几种主流的过渡机制，并就一个基于隧道代理的过渡方案给出其具体实现方法，并进行了实验。实验证明，该方案有效地实现了对现有IPv4网络向IPv6的升级，并保证了IPv6网络与IPv4网络的互联互通。     

Microstructure and electric property of MgO/Fe/MgO tri-layer films forming a nano-granular system

For tunneling magnetoresistance (TMR) devices using ferromagnetic nano particle films, the size, dispersion and number of nano particles are important factors. Relating to this, single layered Fe films (thickness: t = 0.5 - 10.0 nm) sandwiched between two MgO (2 nm thick) layers were fabricated by molecular beam epitaxy. By depositing at T_s = RT (room temperature), the Fe layer had an isolated island structure for less than 1 nm thick. Correspondingly, the negative magnetoresistance effect was observed, which is characteristic of TMR. By increasing T_s, the resistivity and the magnetoresistance (MR) ratio was increased. In this study, it was found that the optimal parameters for the growth of nano particle MgO/Fe/MgO based films are t = 0.5 - 1.0 nm and T_s = RT − 120 °C.     

Complementary tunneling transistor for low power application

The metal oxide semiconductor field effect transistor (MOSFET) is scaling to a “tunneling epoch”, in which multiple leakage current induced by different tunneling effects exist. The complementary Si-based tunneling transistors are presented in this paper. The working principle of this device is investigated in detail. It is found that the band-to-band tunneling current is be controlled by the gate-to-source voltage. Due to the reverse biased p-i-n diode structure, an ultra-low leakage current is achieved. The sub-threshold swing of TFET is not limited by kt/q, which is the physical limit of the MOSFET. Using the CMOS compatible processes, the complementary TFETs (CTFET) are fabricated on one wafer. From a circuit point of view, the compatibility between TFET and MOSFET enables the transfer of CMOS circuits to CTFET circuits.     

散射对量子阱隧穿电流的影响

本文从单带双谷模型出发研究了电子在量子阱内遭到散射时电子状态及隧穿几率的变化。弹性散射使用δ散射势来计算,非弹性散射则用虚散射势来描述。前者使隧穿电流峰向高电压端移动,后者减弱了电流峰的谐振强度。分别讨论了位于势阱和势垒层中的散射中心的不同散射作用。隧穿电流的变化趋势同中子辐照实验数据相吻合。     

One step towards bridging the materials gap: surface studies of TiO2 anatase

We present a short overview of surface studies on the main low-index surfaces of anatase, the technologically most interesting crystallographic form of titanium dioxide. Results are compared to the extensively investigated surfaces of TiO₂ rutile. The anatase (1 0 1) surface is stable in a (1×1) configuration. It exhibits twofold coordinated (bridging) oxygen atoms and fivefold coordinated Ti atoms with a density comparable to the one found on rutile (1 1 0). Step edges are terminated by fourfold coordinated Ti sites. In contrast to rutile (1 1 0), anatase (1 0 1) does not show a strong tendency for losing twofold coordinated oxygen atoms upon annealing in ultrahigh vacuum. The apparent lack of point defects is also reflected in the adsorption/desorption behavior of water and methanol. The anatase (1 0 0) surface has the second-lowest surface energy and tends to from a (1×2) reconstruction. A model with (1 0 1)-oriented microfacets agrees with the observed features in atomically-resolved STM images. The (0 0 1) surface forms a (1×4) reconstruction that is well explained by an ‘ad-molecule’ model predicted from density functional theory calculations. A (1×3) reconstruction was observed for the anatase (1 0 3) surface.     

Investigation of InP/InGaAs superlattice-emitter bipolar transistor with multiple negative-differential-resistance regions

The sequential tunneling behavior in InP/InGaAs superlattice-emitter bipolar transistors is demonstrated by theoretical analysis and experimental results. The tunneling mechanism in the InP/InGaAs superlattice structures is analyzed by theoretical calculation. Due to the weak coupled tunneling mechanism, the interesting multiple negative-differential-resistances (NDRs) resulting from the creation and extension of high-field domain in the superlattice are observed at room temperature. Experimentally, the transistor performances, including a high current gain of 454, a low collector–emitter offset voltage of 80 mV, and a pronounced multiple NDRs, are achieved. The proposed structures may provide good potential for signal amplifiers and multiple-valued logic circuit applications.     

Specific contact resistance and carrier tunneling properties of the silver metal/porous silicon/p-Si ohmic contact structure

The ohmic properties of the Ag metal/porous silicon/p-type silicon (Ag metal/PS/p-Si) contact structure have been investigated by measuring the specific contact resistance (ρ_c) and current–voltage (I–V) characteristics, respectively. The characterization of the ρ_c of the contact structure by the extrapolation technique based on the three-point probe (TPP) method shows that the ohmic properties of the resultant contact structure are excellent with ρ_c  1.01 × 10⁻⁶ Ω cm². The relationship between the specific contact resistance and impurity doping level is also discussed. The tunneling theory for the heavily doped semiconductor surface is used to explain the ohmic behavior of the contact structure. The carrier injection properties of the contact structure were examined by measuring the I–V curves as a function of temperature. It shows that under low forward bias voltages, the I–V curves is exhibiting a slight curvature at the origin which indicates that the current transport mechanism is determined by the space charge limited currents or field-induced tunneling process across the potential barrier at the Ag metal/porous silicon interface. The carrier injection across the Schottky potential barrier is used to explain the observed current transport mechanism of the contact structure.     

Role of Simulation in Construction Engineering and Management

Construction simulation is the science of developing and experimenting with computer-based representations of construction systems to understand their underlying behavior. This branch of operations research applications in construction management has experienced significant academic growth over the past two decades. In this paper, the author summarizes his views on this topic as per his Peurifoy address, given in October 2008. The paper provides an overview of advancements in construction simulation theory as reported in literature. It then summarizes the key factors that contribute to successful deployment of simulation in the construction industry, and the key attributes of problems that make them more amenable for simulation modeling as opposed to other tools. The paper then provides an overview of long-term simulation initiatives leading to the next generation of computer modeling systems for construction, where simulation plays an integral role in a futuristic vision of automated project planning and control.     

3D Modeling of an NATM Tunnel in High K0 Clay Using Two Different Constitutive Models

This paper studies the accuracy of the three-dimensional finite-element predictions of a displacement field induced by tunneling using new Austrian tunneling method (NATM) in stiff clays with high K0 conditions. The studies are applied to the Heathrow express trial tunnel. Two different constitutive models are used to represent London Clay, namely a hypoplastic model for clays and the modified Cam-clay (MCC) model. Good quality laboratory data are used for parameter calibration and accurate field measurements are used to initialize K0 and void ratio. The hypoplastic model gives better predictions than the MCC model with satisfactory estimate for the displacement magnitude and slightly overestimated width of the surface settlement trough. Parametric studies demonstrate the influence of variation of the predicted soil behavior in the very-small-strain to large-strain range and the influence of the time dependency of the shotcrete lining behavior.