Microstructure and mechanical properties of electron beam weld joints of a Zr41Ti14Cu12Ni10Be23 bulk metallic glass with Zr

The electron beam welding technique was used to join Zr41Ti14Cu12Ni10Be23 bulk metallic glass (BMG) to crystalline pure Zr. Compositional, microstructural, and mechanical property variations across the welded interface were evaluated. It is shown that a crystalline layer develops close to the welding interface. Transmission electron microscopy of this layer indicates the crystalline phase to be tetragonal with lattice parameters close to that reported for Zr2Ni. However, the composition of this phase is different as it contains other alloying additions. The interface layer close to the bulk metallic glass side contains nanocrystalline Zr2Cu phase embedded in the glassy matrix. Nanoindentation experiments indicate that the hardness of the crystalline layer, although less than the bulk metallic glass, is more than the Zr itself. Commensurately, tensile tests indicate that the failure of the welded samples occurs at the Zr side rather than at the weld joint.

Effect of Dilatation on the Elasto-Plastic Response of Bulk Metallic Glasses Under Indentation

Elasto-plastic response of bulk metallic glasses (BMGs) follows closely the response of granular materials through pressure dependent (or normal stress) yield locus and shear stress induced material dilatation. On a micro-structural level, material dilatation is responsible for stress softening and formation of localized shear band, however its influence on the macro-scale flow and deformation is largely unknown. In this work, we systematically analyze the effect of material dilatation on the gross indentation response of Zr-based BMG via finite element simulation. The strengthening/softening effect on the load-depth response and corresponding stress-strain profiles are presented in light of differences in elastic-plastic regimes under common indenters. Through comparison with existing experimental results, we draw conclusions regarding selection of suitable dilatation parameters for accurately predicting the gross response of BMGs

The role of defects in carrier type reversal in bismuth doped Ge-Se glasses by photoluminescence spectroscopy

The role played by defects in bringing out n-type conduction in Ge20Se80-xBix and Ge20Se70-xBixTe10 glasses is using investigated photoluminescence (PL) spectroscopy. It was found that for both the systems, the compositions at lower Bi content exhibit luminescence with fine features associated while the compositions that show n-type conduction do not exhibit luminescence. The identification of the associated fine features, carried out by deconvoluting the experimental spectra, reveals that Bi addition brings out a relative diminishing in D+ defects as compared to D- ones. The study gives an overall indication for the role played by native defects in bringing out n-type conduction in Bi-doped glasses.

Network Coding for TCP Throughput Enhancement over a Multi-Hop wireless Network

The poor performance of TCP over multi-hop wireless networks is well known. In this paper we explore to what extent network coding can help to improve the throughput performance of TCP controlled bulk transfers over a chain topology multi-hop wireless network. The nodes use a CSMA/ CA mechanism, such as IEEE 802.11’s DCF, to perform distributed packet scheduling. The reverse flowing TCP ACKs are sought to be X-ORed with forward flowing TCP data packets. We find that, without any modification to theMAC protocol, the gain from network coding is negligible. The inherent coordination problem of carrier sensing based random access in multi-hop wireless networks dominates the performance. We provide a theoretical analysis that yields a throughput bound with network coding. We then propose a distributed modification of the IEEE 802.11 DCF, based on tuning the back-off mechanism using a feedback approach. Simulation studies show that the proposed mechanism when combined with network coding, improves the performance of a TCP session by more than 100%.

A Simulation Based Study and Analysis of Double Gate Tunnel FET Performance for Low Stand-By Power Applications

The conventional metal oxide semiconductor field effect transistor (MOSFET)may not be suitable for future low standby power (LSTP) applications due to its high off-state current as the sub-threshold swing is theoretically limited to 60mV/decade. Tunnel field effect transistor (TFET) based on gate controlled band to band tunneling has attracted attention for such applications due to its extremely small sub-threshold swing (much less than 60mV/decade). This paper takes a simulation approach to gain some insight into its electrostatics and the carrier transport mechanism. Using 2D device simulations, a thorough study and analysis of the electrical parameters of the planar double gate TFET is performed. Due to excellent sub-threshold characteristics and a reverse biased structure, it offers orders of magnitude less leakage current compared to the conventional MOSFET. In this work, it is shown that the device can be scaled down to channel lengths as small as 30 nm without affecting its performance. Also, it is observed that the bulk region of the device plays a major role in determining the sub-threshold characteristics of the device and considerable improvement in performance (in terms of ION/IOFF ratio) can be achieved if the thickness of the device is reduced. An ION/IOFF ratio of 2x1012 and a minimum point sub-threshold swing of 22mV/decade is obtained.

Structural basis for the functional and inhibitory mechanisms of beta-hydroxyacyl-acyl carrier protein dehydratase (FabZ) of Plasmodium falciparum

The beta-hydroxyacyl-acyl carrier protein dehydratase of Plasmodium falciparum (PfFabZ) catalyzes the third and important reaction of the fatty acid elongation cycle. The crystal structure of PfFabZ is available in hexameric (active) and dimeric (inactive) forms. However, PfFabZ has not been crystallized with any bound inhibitors until now. We have designed a new condition to crystallize PfFabZ with its inhibitors bound in the active site, and determined the crystal structures of four of these complexes. This is the first report on any FabZ enzyme with active site inhibitors that interact directly with the catalytic residues. Inhibitor binding not only stabilized the substrate binding loop but also revealed that the substrate binding tunnel has an overall shape of ``U''. In the crystal structures, residue Phe169 located in the middle of the tunnel was found to be in two different conformations, open and closed. Thus, Phe169, merely by changing its side chain conformation, appears to be controlling the length of the tunnel to make it suitable for accommodating longer substrates. The volume of the substrate binding tunnel is determined by the sequence as well as by the conformation of the substrate binding loop region and varies between organisms for accommodating fatty acids of different chain lengths. This report on the crystal structures of the complexes of PfFabZ provides the structural basis of the inhibitory mechanism of the enzyme that could be used to improve the potency of inhibitors against an important component of fatty acid synthesis common to many infectious organisms. (C) 2011 Elsevier Inc. All rights reserved.

Dynamic and thermodynamic anomalies of water at low temperatures: from bulk water to reverse micelles and DNA hydration layer

Liquid water is known to exhibit remarkable thermodynamic and dynamic anomalies, ranging from solvation properties in supercritical state to an apparent divergence of the linear response functions at a low temperature. Anomalies in various dynamic properties of water have also been observed in the hydration layer of proteins, DNA grooves and inside the nanocavity, such as reverse micelles and nanotubes. Here we report studies on the molecular origin of these anomalies in supercooled water, in the grooves of DNA double helix and reverse micelles. The anomalies have been discussed in terms of growing correlation length and intermittent population fluctuation of 4- and 5-coordinated species. We establish correlation between thermodynamic response functions and mean squared species number fluctuation. Lifetime analysis of 4- and 5-coordinated species reveals interesting differences between the role of the two species in supercooled and constrained water. The nature and manifestations of the apparent and much discussed liquid-liquid transition under confinement are found to be markedly different from that in the bulk. We find an interesting `faster than bulk' relaxation in reverse micelles which we attribute to frustration effects created by competition between the correlations imposed by surface interactions and that imposed by hydrogen bond network of water.

Current‐limiting action of mixed‐phase BaTiO3 ceramic semiconductors

Stable and highly reproducible current‐limiting characteristics are observed for polycrystalline ceramics prepared by sintering mixtures of coarse‐grained, donor‐doped BaTiO3 (tetragonal) as the major phase and ultrafine, undoped cubic perovskite such as BaSnO3, BaZrO 3, SrTiO3, or BaTiO3 (cubic). The linear current‐voltage (I‐V) relation changes over to current limiting as the field strength increases, when thermal equilibrium is attained. The grain‐boundary layers with low donor and high Sn, Zr, or Sr have depleted charge carrier density as compared to that in the grain bulk. The voltage drop at the grain‐boundary layers diminishes the temperature gradient between the interior and surface regions.