Low-Temperature Processing of ZrB2-ZrC Composites by Reactive Hot Pressing

Dense ZrB₂-ZrC and ZrB₂-ZrC _x∼0.67 composites have been produced by reactive hot pressing (RHP) of stoichiometric and nonstoichiometric mixtures of Zr and B₄C powders at 40 MPa and temperatures up to 1600 °C for 30 minutes. The role of Ni addition on reaction kinetics and densification of the composites has been studied. Composites of ∼97 pct relative density (RD) have been produced with the stoichiometric mixture at 1600 °C, while the composite with ∼99 pct RD has been obtained with excess Zr at 1200 °C, suggesting the formation of carbon deficient ZrC _x that significantly aids densification by plastic flow and vacancy diffusion mechanism. Stoichiometric and nonstoichiometric composites have a hardness of ∼20 GPa. The grain sizes of ZrB₂ and ZrC _x∼0.67 are ∼0.6 and 0.4 μm, respectively, which are finer than those reported in the literature. This article is based on a presentation given in the symposium entitled “Materials Behavior: Far from Equilibrium” as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred during December 15–16, 2006 in Mumbai, India.     

Direct observation of mode selective electron-phonon coupling in suspended carbon nanotubes

Raman spectra of individual pristine suspended single-walled carbon nanotubes are observed under high electrical bias. The LO and TO modes of the G band behave differently with respect to voltage bias, indicating preferential electron-phonon coupling and nonequilibrium phonon populations, which cause negative differential conductance in suspended devices. By correlating the electron resistivity to the optically measured phonon population, the data are fit using a Landauer model to determine the key scattering parameters.     

Survivable Network Design: The State of the Art

In the past few years the telecommunications industry has undergone significant changes. We are rapidly evolving to a state where audio/visual and data traffic is all provided on the same networks. Telecommunication companies are investing billions of dollars in the design and maintenance of telecommunication networks to provide the users with the better quality of service that they have begun to expect. Due to their high capacity, fiber optic cables have become the medium of choice in the deployment of such new networks worldwide. Such high capacities encourage telecommunication providers to create networks that are substantially more sparse than previous copper based networks. Unfortunately, with sparsity comes vulnerability to failure. Given the dependence on the varied services offered by the modern networks, the magnitudes of the investments involved and the costs of disasters it is only logical that researchers look at problems in survivable network design as an interesting research question. There have been a number of papers that have addressed these and other related issues. In this paper we try to classify the area of survivable network design and provide a classification scheme for the same.     

The fine structure of bicomponent polyester fibers

The application of alkaline hydrolysis to study the change in the fine structure of bicomponent polyester fibers as their surface is removed progressively was explored. The samples were prepared with a poly(butylene terephthalate) (PBT) sheath and a poly(ethylene terephthalate) (PET) core. The reagent used to hydrolyze the PBT was 1M NaOH in 75/25 methanol to water since it appeared to react topochemically with the fiber. The solution reacted more rapidly with PET than with PBT. Thus, when necessary to retard the weight loss of the bicomponent fibers, after a 2‐h hydrolysis with this reagent to remove PBT, it was replaced with aqueous 1M NaOH solution containing 0.1% cetrimmonium bromide. Unlike homofil PET or PBT fibers, where alkaline attack appeared to be confined to the surface and left the residue relatively smooth, the bicomponent fiber was attacked unevenly, and penetration to the PET core occurred before all the PBT at the surface was removed. Nevertheless, most of the reaction was confined initially to the PBT sheath. The tenacity and extension at break of the PBT–PET fiber passed through a maximum as hydrolysis progressed. The fall in tenacity at high weight losses is ascribed to increasing surface defects in the fiber surface. After removal of the PBT by the hydrolysis, the birefringence of the residue became progressively higher. The synergistic effect of the PBT sheath on the properties of the PET core and the possible causes of the nonuniform hydrolysis at the PBT surface are discussed. An equation is proposed that includes an interaction parameter, which can be utilized to determine which property is affected most by the hydrolysis of a bicomponent fiber. In this instance, it appears from the parameters that the order is strength > extension at break ≈ birefringence. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1163–1173, 1999     

Experimental Investigation and optimization of Process Parameters for Shear Strength of Compound Cast Bimetallic Joints

Joining of A356 alloy and magnesium was carried out by vacuum assisted sand mold compound casting process. Microstructure at the joint interface was studied by using optical microscope, scanning electron microscope, energy dispersive X-ray spectroscopy and X-ray diffractometer. Characterization indicated that a relatively uniform joint interface was obtained. The joint interface was composed of three distinct layers containing Mg₂Al₃ on aluminum side, Mg₁₇Al₁₂?+?δ eutectic structure on magnesium side and Mg₁₇Al₁₂ as middle layer. As a result of interaction between silicon, present in A356 with magnesium, Mg₂Si compound was formed. Push out test was conducted on electronics universal testing machine to measure the shear strength across the joint interface. The important process parameters (grit size of sand paper, insert temperature, pouring temperature and vacuum pressure) were optimized to maximize the shear strength. Optimization was carried out by using response surface methodology, desirability analysis and genetic algorithm (GA) techniques. It was observed that the shear strength increased by 14.21, 8.60 and 4.80% with genetic algorithm, desirability analysis and regression model respectively. GA reported the optimal value of shear strength.     

Scheduling sensor activity for information coverage of discrete targets in sensor networks

In this paper, we study the problem of scheduling sensor activity to cover a set of targets with known locations such that all targets can be monitored all the time and the network can operate as long as possible. A solution to this scheduling problem is to partition all sensors into some sensor covers such that each cover can monitor all targets and the covers are activated sequentially. In this paper, we propose to provide information coverage instead of the conventional sensing disk coverage for target. The notion of information coverage is based on estimation theory to exploit the collaborative nature of geographically distributed sensors. Due to the use of information coverage, a target that is not within the sensing disk of any single sensor can still be considered to be monitored (information covered) by the cooperation of more than one sensor. This change of the problem settings complicates the solutions compared to that by using a disk coverage model. We first define the target information coverage (TIC) problem and prove its NP‐completeness. We then propose a heuristic to approximately solve our problem. Simulation results show that our heuristic is better than an existing algorithm and is close to the upper bound when only the sensing disk coverage model is used. Furthermore, simulation results also show that the network lifetime can be significantly improved by using the notion of information coverage compared with that by using the conventional definition of sensing disk coverage. Copyright © 2008 John Wiley & Sons, Ltd.     

Simple online recognition of optical data strings based on conservative optical logic

Optical packet switching relies on the ability of a system to recognize header information on an optical signal. Unless the headers are very short with large Hamming distances, optical correlation fails and optical logic becomes attractive because it can handle long headers with Hamming distances as low as 1. Unfortunately, the only optical logic gates fast enough to keep up with current communication speeds involve semiconductor optical amplifiers and do not lend themselves to the incorporation of large numbers of elements for header recognition and would consume a lot of power as well. The ideal system would operate at any bandwidth with no power consumption. We describe how to design and build such a system by using passive optical logic. This too leads to practical problems that we discuss. We show theoretically various ways to use optical interferometric logic for reliable recognition of long data streams such as headers in optical communication. In addition, we demonstrate one particularly simple experimental approach using interferometric coinc gates.     

Multiplexed cancer biomarker detection using quartz-based photonic crystal surfaces

A photonic crystal (PC) surface is demonstrated as a high-sensitivity platform for detection of a panel of 21 cancer biomarker antigens using a sandwich enzyme-linked immunosorbent assay (ELISA) microarray format. A quartz-based PC structure fabricated by nanoimprint lithography, selected for its low autofluorescence, supports two independent optical resonances that simultaneously enable enhancement of fluorescence detection of biomarkers and label-free quantification of the density of antibody capture spots. A detection instrument is demonstrated that supports fluorescence and label-free imaging modalities, with the ability to optimize the fluorescence enhancement factor on a pixel-by-pixel basis throughout the microarray using an angle-scanning approach for the excitation laser that automatically compensates for variability in surface chemistry density and capture spot density. Measurements show that the angle-scanning illumination approach reduces the coefficient of variation of replicate assays by 20-99% compared to ordinary fluorescence microscopy, thus supporting reduction in limits of detectable biomarker concentration. Using the PC resonance, biomarkers in mixed samples were detectable at the lowest concentrations tested (2.1-41 pg/mL), resulting in a three-log range of quantitative detection.     

Tracking mesenchymal stem cells with iron oxide nanoparticle loaded poly(lactide-co-glycolide) microparticles

Monitoring the location, distribution and long-term engraftment of administered cells is critical for demonstrating the success of a cell therapy. Among available imaging-based cell tracking tools, magnetic resonance imaging (MRI) is advantageous due to its noninvasiveness, deep penetration, and high spatial resolution. While tracking cells in preclinical models via internalized MRI contrast agents (iron oxide nanoparticles, IO-NPs) is a widely used method, IO-NPs suffer from low iron content per particle, low uptake in nonphagocytotic cell types (e.g., mesenchymal stem cells, MSCs), weak negative contrast, and decreased MRI signal due to cell proliferation and cellular exocytosis. Herein, we demonstrate that internalization of IO-NP (10 nm) loaded biodegradable poly(lactide-co-glycolide) microparticles (IO/PLGA-MPs, 0.4-3 μm) in MSCs enhances MR parameters such as the r(2) relaxivity (5-fold), residence time inside the cells (3-fold) and R(2) signal (2-fold) compared to IO-NPs alone. Intriguingly, in vitro and in vivo experiments demonstrate that internalization of IO/PLGA-MPs in MSCs does not compromise inherent cell properties such as viability, proliferation, migration and their ability to home to sites of inflammation.     

Preparation and characterization of spray-dried mucoadhesive microspheres of aceclofenac

Objective: Microencapsulation of the anti-inflammatory drug aceclofenac (ACE) was investigated as a means of controlling drug release and minimizing or eliminating local side effects. Method: Microspheres were prepared by a spray-drying technique using solutions of ACE and three polymers, namely, carbopol, chitosan, and polycarbophil, in different weight ratios. Results: The spray-dried mucoadhesive microspheres were characterized in terms of shape (scanning electron microscope), size (6.60–8.40 μm), production yield (34.10–55.62%), and encapsulation efficiency (58.14–90.57%). In vitro release studies were performed in phosphate buffer (pH 6.8) up to 10 hours. The spray-drying process of solutions of ACE with polymeric blends can give prolonged drug release. The in vitro release data were well fit into Higuchi and Korsmeyer–Peppas model and followed Fickian diffusion mechanism. In vivo data showed that the administration of ACE in polymeric microspheres prevented the gastric side effects. Conclusion: The formulations here described can be proposed for the oral administration of nonsteroidal anti-inflammatory drugs with minimal side effects on gastric mucosa.