Grain Boundary Sliding and Slip Transmission in High Purity Aluminum

Despite its significance in polycrystalline materials, there have been few experimental investigations of the activity of grain boundary sliding (GBS) and the relationship between GBS and slip transmission at grain boundaries. The present work addresses this knowledge gap by the characterization of full-field strain and microstructural information in an experimental system of high-purity (99.99%) columnar aluminum subjected to uniaxial tension at 190 °C. High-resolution, full-gage strain fields were characterized on an unloaded specimen by distortion-corrected and stitched scanning electron microscope-enabled digital image correlation (SEM-DIC). Alignment between the lower-resolution electron backscatter diffraction (EBSD) and higher-resolution strain fields was significantly improved by clustering of strain data within an EBSD-defined boundary mantle. Grain boundary sliding was investigated at select boundaries, and it was determined that GBS magnitude profiles can have large gradients along a single boundary and vary significantly between boundaries. Using a geometric compatibility factor (m′) to quantify favorability of slip transmission, the two grain boundaries that exhibited the largest average GBS magnitude experienced contiguous slip on moderately well aligned slip systems, although the exact nature of this slip activity, whether transmission or nucleation, remains under investigation.

     

Analysis of current voltage characteristics of MWIR homojunction photodiodes for uncooled operation

Dark currents n⁺/ν/p⁺ Hg_0.69Cd_0.31Te mid wave infrared photodiodes were measured at room temperature. The diodes exhibited negative differential resistance at room-temperature, but with increasing leakage currents as a function of reverse bias. The current–voltage characteristics were simulated and fitted by incorporating trap assisted tunneling via traps and Shockley–Read–Hall generation recombination process due to dislocations in the carrier transport equations. The thermal suppression of carriers was simulated by taking energy level of trap (E_t), trap density (N_t) and the doping concentrations of n⁺ and ν regions as fitting parameters. Values of E_t and N_t were 0.78E_g and ～6–9 × 10¹⁴ cm^?3 respectively for most of the diodes. Variable temperature current voltage measurements on variable area diode array (VADA) structures confirmed the fact that variation in zero bias resistance area product (R₀A) is related to g–r processes originating from variation in concentration and kind of defects that intersect a junction area.     

An improved Hardy-Sobolev inequality and its application

For , a bounded domain, and for , we improve the Hardy-Sobolev inequality by adding a term with a singular weight of the type . We show that this weight function is optimal in the sense that the inequality fails for any other weight function more singular than this one. Moreover, we show that a series of finite terms can be added to improve the Hardy-Sobolev inequality, which answers a question of Brezis-Vazquez. Finally, we use this result to analyze the behaviour of the first eigenvalue of the operator as increases to for .

     

Spontaneous assembly of a hydrogen-bonded tetrahedron

Triphenylamine ortho-tricarboxylic acid (1) has been synthesized and the crystal structure reported. This molecule is shown to spontaneously self-assemble into a hydrogen-bonded tetrahedron. Furthermore, Electrospray Ionization Mass Spectroscopy shows evidence for the stability of such aggregates from an ethanol/water solution.     

A novel approach to combinatorial library design

We address the problem of designing a general-purpose combinatorial library to screen for pharmaceutical leads. Conventional approaches focus on diversity as the primary factor in designing such libraries. We suggest making screening libraries out of a set of pharmaceutically relevant scaffolds, with multiple analogs per scaffold. The rationale for this rests on the fact that even though the hit-rate in active series is much higher than in the database as a whole, often a large fraction of the compounds in active series are inactive. This is especially true when the series has not been optimized for the target under study. We introduce the concept of hit-rate within a series and use historic screening data to arrive at a crude estimate for it. We then use simple probability arguments to show that 50-100 compounds are required in each series in order to be nearly certain of finding at least one active compound in each true active series for any given target.     

High-efficiency coupling structure for a single-line-defect photonic-crystal waveguide

We present the design and fabrication of a planar structure for coupling light from a multimode feed waveguide into a single-line-defect photonic-crystal waveguide. Finite-difference time-domain calculations predict a coupling efficiency of greater than 90%, and preliminary experimental results indicate successful coupling through a single-line-defect photonic-crystal waveguide. Device design, fabrication, and characterization are presented.     

Symmetry-breaking in local Lyapunov exponents

Integrable dynamical systems, namely those having as many independent conserved quantities as freedoms, have all Lyapunov exponents equal to zero. Locally, the instantaneous or finite time Lyapunov exponents are nonzero, but owing to a symmetry, their global averages vanish. When the system becomes nonintegrable, this symmetry is broken. A parallel to this phenomenon occurs in mappings which derive from quasiperiodic Schr?dinger problems in 1-dimension. For values of the energy such that the eigenstate is extended, the Lyapunov exponent is zero, while if the eigenstate is localized, the Lyapunov exponent becomes negative. This occurs by a breaking of the quasiperiodic symmetry of local Lyapunov exponents, and corresponds to a breaking of a symmetry of the wavefunction in extended and critical states. Received 25 October 2001 / Received in final form 8 December 2001 Published online 2 October 2002 RID="a" ID="a"e-mail: r.ramaswamy@mail.jnu.ac.in     

A segregated finite element formulation of Navier-Stokes equations under laminar conditions

This paper describes the application of the Taylor-Galerkin based finite element method to the calculation of incompressible viscous flows. The fundamental concepts and characteristics of the formulations and the associated solution methodology used are described in technical detail. This scheme is second-order accurate for velocity and pressure in time as well as in space. The three test cases chosen for the performance evaluation of this formulation are the standing vortex problem, the lid-driven cavity flow at Reynolds numbers up to 5000, and the sudden expansion problem at Reynolds numbers up to 800. These results are provided to illustrate the quality of the computed transient solutions in two dimensions under laminar conditions.     

Oxidative cyclization and fragmentation of steroidal alcohols induced by ceric ammonium nitrate

Oxidative cyclizations of steroidal alcohols (1a–1d) using ceric ammonium nitrate (CAN) in aqueous acetonitrile or aqueous acetic acid gave the corresponding cyclic ethers (2a–2d), whereas the tertiary alcohol (4) gave secosteroid (5).