A discrete dislocation analysis of fatigue crack growth

Cyclic loading of a plane strain mode I crack under small scale yielding is analyzed using discrete dislocation dynamics. The dislocations are all of edge character, and are modeled as line singularities in an elastic solid. At each stage of loading, superposition is used to represent the solution in terms of solutions for edge dislocations in a half-space and a non-singular complementary solution that enforces the boundary conditions, which is obtained from a linear elastic, finite element solution. The lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation are incorporated into the formulation through a set of constitutive rules. An irreversible relation between the opening traction and the displacement jump across a cohesive surface ahead of the initial crack tip is also specified, which permits crack growth to emerge naturally. It is found that crack growth can occur under cyclic loading conditions even when the peak stress intensity factor is smaller than the stress intensity required for crack growth under monotonic loading conditions; however below a certain threshold value of ΔKI no crack growth was seen.

Discrete dislocation plasticity analysis of crack-tip fields in polycrystalline materials

Small scale yielding around a mode I crack is analysed using polycrystalline discrete dislocation plasticity. Plane strain analyses are carried out with the dislocations all of edge character and modelled as line singularities in a linear elastic material. The lattice resistance to dislocation motion, nucleation, interaction with obstacles and annihilation are incorporated through a set of constitutive rules. Grain boundaries are modelled as impenetrable to dislocations. The polycrystalline material is taken to consist of two types of square grains, one of which has a bcc-like orientation and the other an fcc-like orientation. For both orientations there are three active slip systems. Alternating rows, alternating columns and a checker-board-like arrangement of the grains is used to construct the polycrystalline materials. Consistent with the increasing yield strength of the polycrystalline material with decreasing grain size, the calculations predict a decrease in both the plastic zone size and the crack-tip opening displacement for a given applied mode I stress intensity factor. Furthermore, slip-band and kink-band formation is inhibited by all grain arrangements and, with decreasing grain size, the stress and strain distributions more closely resemble the HRR fields with the crack-tip opening approximately inversely proportional to the yield strength of the polycrystalline materials. The calculations predict a reduction in fracture toughness with decreasing grain size associated with the grain boundaries acting as effective barriers to dislocation motion.

Effects of KI encapsulation in single-walled carbon nanotubes by Raman and optical absorption spectroscopy.

The effect of KI encapsulation in narrow (HiPCO) single-walled carbon nanotubes is studied via Raman spectroscopy and optical absorption. The analysis of the data explores the interplay between strain and structural modifications, bond-length changes, charge transfer, and electronic density of states. KI encapsulation appears to be consistent with both charge transfer and strain that shrink both the C-C bonds and the overall nanotube along the axial direction. The charge transfer in larger semiconducting nanotubes is low and comparable with some cases of electrochemical doping, while optical transitions between pairs of singularities of the density of states are quenched for narrow metallic nanotubes. Stronger changes in the density of states occur in some energy ranges and are attributed to polarization van der Waals interactions caused by the ionic encapsulate. Unlike doping with other species, such as atoms and small molecules, encapsulation of inorganic compounds via the molten-phase route provides stable effects due to maximal occupation of the nanotube inner space.

Interaction of corticosterone and nicotine in regulation of prepulse inhibition in mice

The aim of the present Study was to investigate if different levels of circulating corticosterone (CORT) modulate the effect of nicotine on prepulse inhibition (PPI), a measure of sensorimotor gating that is disrupted in schizophrenia and other mental illnesses. Four groups of mice were investigated: sham-operated, adrenalectomized (ADX) and implanted with a cholesterol pellet, ADX and implanted with a 10 mg CORT pellet, or ADX and 50 mg, of CORT. Different CORT levels or doses of nicotine did not significantly affect startle responses. Baseline PPI was significantly reduced in mice implanted with the highest dose of CORT. In ADX mice implanted with cholesterol, nicotine treatment influenced PPI depending on the prepulse intensity. In ADX mice implanted with 50 mg of CORT, treatment with 10 mg/kg of nicotine caused a significant increase in PPI at all prepulse intensities. Binding studies showed that corticosterone treatment had significantly affected nicotinic acetylcholine receptor (nAChR) density in the mouse brain. Treatment with 50 mg CORT decreased I-125-epibatidine binding in the globus pallidus and I-125-alpha-bungarotoxin binding in the claustrum. These results suggest a possible interaction of corticosterone and nicotine at the level of the alpha4- and alpha7-type nAChR in the regulation of PPI. In situations of high circulating levels of corticosterone, nicotine may be beneficial to restore disruption of PPI. (C) 2004 Elsevier Ltd. All rights reserved.

Discrete dislocation plasticity analysis of size effects in single crystals

The effect of size and slip system configuration on the tensile stress-strain response of micron-sized planar crystals as obtained from discrete dislocation plasticity simulations is presented. The crystals are oriented for either single or symmetric double slip. With the rotation of the tensile axis unconstrained, there is a strong size dependence, with the flow strength increasing with decreasing specimen size. Below a certain specimen size, the flow strength of the crystals is set by the nucleation strength of the initially present Frank-Read sources. The main features of the size dependence are the same for both the single and symmetric double slip configurations.

Molecular modeling on human CCR5 receptors and complex with CD4 antigens and HIV-1 envelope glycoprotein gp120

AIM: To investigate the interaction between human CCR5 receptors (CCR5) and HIV-1 envelope glycoprotein gp120 (HIV-1 gp120) and HIV-1 receptor CD4 antigens (CD4). METHODS: The structurally con served regions (SCR) of human CCR5 was built by the SYBYL/Biopolymer module using the corresponding transmembrane (TM) domain of bacteriorhodopsin (bR) as the template. The coordinates for amino-ter minal residue sequence, and carboxyl-terminal residue sequence, extracellular and cytoplasmic loops were generated using LOOP SEARCH algorithm. Subsequently the structural model was merged into the complex with HIV-1 gp120 and CD4. RESULTS: Human CCR5 interacted with both an HIV-1 gp120 and CD4. The N-terminal residues (especially Met1 and Gln4) of human CCR5, contacted with CD4 residues, mainly 7Nith one span (56 - 59) of CD4 in electrostatic interaction and hydrogen-bonds. The binding sites of human CCR5 were buried in a hydrophobic center surrounded by a highly basic periphery. On the other hand, direct interatomic contacts were made between ? CCR5 residues and 6 gp120 amino-acid residues, which included van der Waals contacts, hydrophobic interaction, and hydrogen bonds. CONCLUSION: The interaction model should be helpful for rational design of novel anti-HIV drugs.

Structural and functional characterization of the human CCR5 receptor in complex with HIV gp120 envelope glycoprotein and CD4 receptor by molecular modeling studies

The entry of human immunodeficiency virus (HIV) into cells depends on a sequential interaction of the gp120 envelope glycoprotein with the cellular receptors CD4 and members of the chemokine receptor family. The CC chemokine receptor CCR5 is such a receptor for several chemokines and a major coreceptor for the entry of R5 HIV type-1 (HIV-1) into cells. Although many studies focus on the interaction of CCR5 with HIV-1, the corresponding interaction sites in CCR5 and gp120 have not been matched. Here we used an approach combining protein structure modeling, docking and molecular dynamics simulation to build a series of structural models of the CCR5 in complexes with gp120 and CD4. Interactions such as hydrogen bonds, salt bridges and van der Waals contacts between CCR5 and gp120 were investigated. Three snapshots of CCR5-gp120-CD4 models revealed that the initial interactions of CCR5 with gp120 are involved in the negatively charged N-terminus (Nt) region of CCR5 and positively charged bridging sheet region of gp120. Further interactions occurred between extracellular loop2 (ECL2) of CCR5 and the base of V3 loop regions of gp120. These interactions may induce the conformational changes in gp120 and lead to the final entry of HIV into the cell. These results not only strongly support the two-step gp120-CCR5 binding mechanism, but also rationalize extensive biological data about the role of CCR5 in HIV-1 gp120 binding and entry, and may guide efforts to design novel inhibitors.

Complementary metal-oxide-semiconductor-compatible and self-aligned catalyst formation for carbon nanotube synthesis and interconnect fabrication

We have for the first time developed a self-aligned metal catalyst formation process using fully CMOS (complementary metal-oxide-semiconductor) compatible materials and techniques, for the synthesis of aligned carbon nanotubes (CNTs). By employing an electrically conductive cobalt disilicide (CoSi 2) layer as the starting material, a reactive ion etch (RIE) treatment and a hydrogen reduction step are used to transform the CoSi 2 surface into cobalt (Co) nanoparticles that are active to catalyze aligned CNT growth. Ohmic contacts between the conductive substrate and the CNTs are obtained. The process developed in this study can be applied to form metal nanoparticles in regions that cannot be patterned using conventional catalyst deposition methods, for example at the bottom of deep holes or on vertical surfaces. This catalyst formation method is crucially important for the fabrication of vertical and horizontal interconnect devices based on CNTs. © 2012 American Institute of Physics.

Dispersion of carbon nanotubes: Mixing, sonication, stabilization, and composite properties

Advances in functionality and reliability of carbon nanotube (CNT) composite materials require careful formulation of processing methods to ultimately realize the desired properties. To date, controlled dispersion of CNTs in a solution or a composite matrix remains a challenge, due to the strong van der Waals binding energies associated with the CNT aggregates. There is also insufficiently defined correlation between the microstructure and the physical properties of the composite. Here, we offer a review of the dispersion processes of pristine (non-covalently functionalized) CNTs in a solvent or a polymer solution. We summarize and adapt relevant theoretical analysis to guide the dispersion design and selection, from the processes of mixing/sonication, to the application of surfactants for stabilization, to the final testing of composite properties. The same approaches are expected to be also applicable to the fabrication of other composite materials involving homogeneously dispersed nanoparticles. © 2012 by the authors; licensee MDPI, Basel, Switzerland.

Dynamic technology leadership: The adaptive role of the CTO

OVERVIEW: The importance of the chief technology officer role is widely accepted, particularly in today's turbulent, global conditions. However, not enough is known about the key activities of CTOs or the factors that influence their priorities. Thirty in-depth interviews conducted with the CTOs in global firms identified key activities: aligning technology and corporate strategy and business models, determining technology entry and exit points, and preparing business cases to secure funding for technology development. The research also showed that priority areas for CTOs are related to technology transition points-major contextual and business discontinuities that impact the focus of the CTO. We conclude that the determination of priorities at these technology transition points is highly idiosyncratic and closely related to whether the CTO functions more or less strategically.

Modeling fatigue crack growth in crystalline solids with discrete dislocation plasticity

Analyses of crack growth under cyclic loading conditions are discussed where plastic flow arises from the motion of large numbers of discrete dislocations and the fracture properties are embedded in a cohesive surface constitutive relation. The formulation is the same as used to analyse crack growth under monotonic loading conditions, differing only in the remote loading being a cyclic function of time. Fatigue, i.e. crack growth in cyclic loading at a driving force for which the crack would have arrested under monotonic loading, emerges in the simulations as a consequence of the evolution of internal stresses associated with the irreversibility of the dislocation motion. A fatigue threshold, Paris law behaviour, striations, the accelerated growth of short cracks and the scaling with material properties are outcomes of the calculations. Results for single crystals and polycrystals will be discussed.

Carbon nanotube growth for through silicon via application.

Through silicon via (TSV) technology is key for next generation three-dimensional integrated circuits, and carbon nanotubes (CNT) provide a promising alternative to metal for filling the TSV. Three catalyst preparation methods for achieving CNT growth from the bottom of the TSV are investigated. Compared with sputtering and evaporation, catalyst deposition using dip-coating in a FeCl2 solution is found to be a more efficient method for realizing a bottom-up filling of the TSV (aspect ratio 5 or 10) with CNT. The CNT bundles grown in 5 min exceed the 50 μm length of the TSV and are multi-wall CNT with three to eight walls. The CNT bundles inside the TSV were electrically characterized by creating a direct contact using a four-point nanoprober setup. A low resistance of the CNT bundle of 69.7 Ω (297 Ω) was measured when the CNT bundle was contacted midway along (over the full length of) the 25 μm deep TSV. The electrical characterization in combination with the good filling of the TSV demonstrates the potential use of CNT in fully integrated TSV applications.

抗新霉素基因对草鱼肾细胞的磷酸钙共沉淀转移

<正> 自1973年 Graham 和 Van der Eb 首次成功地应用 DNA-磷酸钙共沉淀法转化哺乳动物培养细胞以来,又提出了电脉冲法;显微注射法;磷脂载体法及细菌原生质体融合法等多种培养细胞基因转移方法。然而,目前的研究多局限于哺乳动物细胞,鱼类培养细胞方面的报道却很少。建立鱼类培养细胞基因转移的方