Showing papers by "Rensselaer Polytechnic Institute published in 1997"

Nanometre-size tubes of carbon

Abstract: We review the present state of understanding of the structure, growth and properties of nanometre-size tubes of carbon. Two different types of carbon nanotubes, namely single-shell nanotubes made of single layers of graphene cylinders and multishell nanotubes made of concentric cylinders of graphene layers have now become available. The subtle structure parameters such as helicity in the carbon network and the nanometre diameters give the nanotubes a rich variety in physical properties. Recent experimental progress on the measurements of properties using electron-energy loss spectroscopy, Raman spectroscopy, electron-spin resonance, electrical conductance, mechanical stiffness and theoretical predictions on electronic and mechanical properties of nanotubes will be discussed. In addition to synthesis techniques, methods to purify and make aligned arrays of nanotubes will be described. Different approaches for fabricating composite structures using nanotubes as moulds and templates and their future implications in materials science will be evaluated. Finally, promising areas of future applications, for example as tiny field-emitting devices, micro-electrodes, nanoprobes and hydrogen storage material will be outlined.

Free-space electro-optics sampling of mid-infrared pulses

Abstract: We report on the coherent detection of ultra-broadband mid-infrared electromagnetic pulses using a 30-μm-thick ZnTe electro-optic sensor. The detected frequency spectrum exceeds 37 THz, extending from microwave to the mid-infrared. The frequency response can be further improved by reducing the sensor thickness to 10 μm.

An embedded wavelet video coder using three-dimensional set partitioning in hierarchical trees (SPIHT)

Abstract: The SPIHT (set partitioning in hierarchical trees) algorithm by Said and Pearlman (see IEEE Trans. on Circuits and Systems for Video Technology, no.6, p.243-250, 1996) is known to have produced some of the best results in still image coding. It is a fully embedded wavelet coding algorithm with precise rate control and low complexity. We present an application of the SPIHT algorithm to video sequences, using three-dimensional (3D) wavelet decompositions and 3D spatio-temporal dependence trees. A full 3D-SPIHT encoder/decoder is implemented in software and is compared against MPEG-2 in parallel simulations. Although there is no motion estimation or compensation in the 3D SPIHT, it performs measurably and visually better than MPEG-2, which employs complicated motion estimation and compensation.

Multilevel interconnections for ULSI and GSI era

Abstract: A high performance interconnection network on a chip is essential to match the ever improving performance of the semiconductor devices they interconnect. This issue reviews the need, some fundamental background justifying the need, approaches one can take in trying to satisfy the need, and associated issues related to the concept of multilevel interconnection (MLI) technology for the ULSI/GSI era of the silicon integrated circuits, with emphasis on materials and the processes that will lead to an acceptable MLI scheme. Thus besides discussing the MLI concepts and implementation issues, properties of metals and their alloys (especially those of Cu and Al), diffusion barrier/adhesion promoter, interlayer dielectrics, deposition and etching of mateials, planarization issues, reliability issues, and new materials concepts are presented. Emphasis is obviously placed on the presently focused research on replacing aluminum with copper based interconnections. The author's viewpoint on gradual changes in replacing current materials with newer mateials as they become available is also presented. A synergism between the materials sets for on-chip and off-chip interconnections has been pointed out.

Diffusion of tetravalent cations in zircon

Abstract: Diffusion rates for the three tetravalent cations U, Th and Hf have been measured in synthetic zircon. Diffusant sources included oxide powders and ground pre-synthesized silicates. Rutherford backscattering spectrometry (RBS) was used to measure depth profiles. Over the temperature range 1400–1650 °C, the following Arrhenius relations were obtained (diffusion coefficients in m2sec−1): log D Th = (1.936 ± 0.9820) + (− 792 ± 34 kJ mol−1 /2.303 RT) log D U = (0.212 ± 2.440) + (− 726 ± 83 kJ mol−1 /2.303 RT) log D Hf = (3.206 ± 1.592) + (− 812 ± 54 kJ mol−1 /2.303 RT) The data show a systematic increase in diffusivity with decreasing ionic radius (i.e., faster diffusion rates for Hf than for U or Th), a trend also observed in our earlier study of rare earth diffusion in zircon. Diffusive fractionation may be a factor in the Lu-Hf system given the much slower diffusion rates of tetravalent cations when compared with the trivalent rare earths. The very slow diffusion rates measured for these tetravalent cations suggest that they are essentially immobile under most geologic conditions, permitting the preservation of fine-scale chemical zoning and isotopic signatures of inherited cores.

Pulsed second-harmonic generation in nonlinear, one-dimensional, periodic structures

Abstract: We present a numerical study of second-harmonic (SH) generation in a one-dimensional, generic, photonic band-gap material that is doped with a nonlinear ${\ensuremath{\chi}}^{(2)}$ medium. We show that a 20-period, 12-\ensuremath{\mu}m structure can generate short SH pulses (similar in duration to pump pulses) whose energy and power levels may be 2--3 orders of magnitude larger than the energy and power levels produced by an equivalent length of a phase-matched, bulk medium. This phenomenon comes about as a result of the combination of high electromagnetic mode density of states, low group velocity, and spatial phase locking of the fields near the photonic band edge. The structure is designed so that the pump pulse is tuned near the first-order photonic band edge, and the SH signal is generated near the band edge of the second-order gap. This maximizes the density of available field modes for both the pump and SH field. Our results show that the ${\ensuremath{\chi}}^{(2)}$ response is effectively enhanced by several orders of magnitude. Therefore, mm- or cm-long, quasi-phase-matched devices could be replaced by these simple layered structures of only a few micrometers in length. This has important applications to high-energy lasers, Raman-type sources, and frequency up- and down-conversion schemes.

The relationship between the distribution of electronic states and the optical absorption spectrum of an amorphous semiconductor: An empirical analysis

Abstract: An elementary empirical model for the distribution of electronic states of an amorphous semiconductor is presented. Using this model, we determine the functional form of the optical absorption spectrum, focusing our analysis on the joint density of states function, which dominates the absorption spectrum over the range of photon energies we consider. Applying our optical absorption results, we then determine how the empirical measures commonly used to characterize the absorption edge of an amorphous semiconductor, such as the Tauc gap and the absorption tail breadth, are related to the parameters that characterize the underlying distribution of electronic states. We, thus, provide the experimentalist with a quantitative means of interpreting the physical significance of their optical absorption data.

The Evolution of Truly Social Cognition: The Core Configurations Model

Abstract: This article introduces a vocabulary suitable for evolutionary analyses in the human cognitive, social, and behavioral sciences. The vocabulary carves a middle way between advocates and critics of evolutionary perspectives by substituting the concept of repeated assembly for nature-nurture dualism. A model of core configurations-based on human morphology and ecology in human evolutionary history-is presented, and I argue that these configurations of face-to-face groups are the selective context for uniquely human mental systems. Hence, human cognition is "truly social,"" specialized for group living. The relevance of the core configuration model is illustrated with respect to two areas of interest to social psychologists: the self and social identity, and distributed cognition and shared reality. A final section illustrates the integrative power of the core configuration model with a brief comparison of the social and cognitive tasks faced by scientists and foragers.

Neutrophil transendothelial migration is independent of tight junctions and occurs preferentially at tricellular corners.

Abstract: Since macromolecular permeability between endothelial cells is regulated by tight junctions (zonula occludens), we wished to determine whether they also regulate neutrophil transendothelial migration. HUVEC monolayers, a commonly used model for studying leukocyte transmigration, were characterized using electric cell substrate impedance sensing and transmission electron microscopy. We show that culture medium containing endothelial cell growth supplement (50 microg/ml) was sufficient and necessary for the development of endothelial tight junctions. The frequency with which tight junctions were observed by transmission electron microscopy was further increased (twofold) by culturing HUVEC monolayers in a 1:1 mixture of endothelial medium and astrocyte-conditioned medium. These astrocyte-conditioned HUVEC monolayers showed a >1.5-fold increase in transcellular electrical resistance. The extent of neutrophil migration across IL-1-treated (10 U/ml for 4 h) HUVEC monolayers was the same whether tight junctions were present or absent, and the molecular requirements for neutrophil transmigration (CD18 and intercellular adhesion molecule-1) were unaffected by culturing in astrocyte-conditioned medium. Immunostaining for proteins associated with the intercellular junctional domain (occludin, ZO-1, cadherin, beta-catenin, gamma-catenin, and platelet-endothelial cell adhesion molecule-1) was localized to the endothelial borders, regardless of the culture conditions. Discontinuities were observed in the border staining for occludin, ZO-1, cadherin, and beta-catenin at the tricellular corner where the borders of three endothelial cells intersected. Significantly, 75% of neutrophil migration across IL-1-treated HUVEC monolayers occurred at tricellular corners. It appears that neutrophils preferentially migrate around endothelial tight junctions by crossing at tricellular corners rather than passing through the tight junctions that lie between two endothelial cells.

Microbes and metals

Abstract: Many base metals and a few precious metals as well as some metalloids can be enzymatically or non-enzymatically concentrated and dispersed by microbes in their environment. Some of these activities are commercially exploited or have a potential for it. This article summarizes these activities and the commercial or potentially commercial use of some of them.

Micromechanical models for graded composite materials

Abstract: Elastic response of selected plane-array models of graded composite microstructures is examined under both uniform and linearly varying boundary tractions and displacements, by means of detailed finite element studies of large domains containing up to several thousand inclusions. Models consisting of piecewise homogeneous layers with equivalent elastic properties estimated by Mori-Tanaka and selfconsistent methods are also analysed under similar boundary conditions. Comparisons of the overall and local fields predicted by the discrete and homogenized models are made using a C/SiC composite system with very different Young's moduli of the phases, and relatively steep composition gradients. The conclusions reached from these comparisons suggest that in those parts of the graded microstructure which have a well-defined continuous matrix and discontinuous second phase, the overall properties and local fields are predicted by Mori-Tanaka estimates. On the other hand, the response of graded materials with a skeletal microstructure in a wide transition zone between clearly defined matrix phases is better approximated by the self-consistent estimates. Certain exceptions are noted for loading by overall transverse shear stress. The results suggest that the averaging methods originally developed for statistically homogeneous aggregates may be selectively applied, with a reasonable degree of confidence, to aggregates with composition gradients, subjected to both uniform and nonuniform overall loads.

Preisach modeling of piezoceramic and shape memory alloy hysteresis

Abstract: Smart materials such as piezoceramics, magnetostrictive materials, and shape memory alloys exhibit hysteresis, and the larger the input signal the larger the effect. Hysteresis can lead to unwanted harmonics, inaccuracy in open loop control, and instability in closed loop control. The Preisach independent domain hysteresis model has been shown to capture the major features of hysteresis arising in ferromagnetic materials. Noting the similarity between the microscopic domain kinematics that generate static hysteresis effects in ferromagnetics, piezoceramics, and shape memory alloys (SMAs), we apply the Preisach model for the hysteresis in piezoceramic and shape memory alloy materials. This paper reviews the basic properties of the Preisach model, discusses control-theoretic issues such as identification, simulation, and inversion, and presents experimental results for piezoceramic sheet actuators bonded to a flexible aluminum beam, and a Nitinol SMA wire muscle that applies a bending force to the end of a beam.

Analysis and modeling of the primary cilium bending response to fluid shear.

Abstract: Since a nonmotile, primary (9 + 0) cilium projects from most mammalian kidney epithelial cells into the tubule lumen, where it is exposed to fluid motion, the present study examined primary cilium response to fluid shear stress. The reversible, large-angle bending of the primary cilium upon exposure to fluid shear forces (10(-11)-10(-10) N.m2 = 10(-8)-10(-7) dyn/cm) was characterized in vitro using videomicroscopic side views of PtK1 cells, and the cilium was then mathematically modeled as a cantilevered beam. The flexural rigidity of the primary cilium was calculated to be 3.1 +/- 0.8 x 10(-23) N.m2 with a corrected quadruple integration approach and 1.4-1.6 x 10(-23) N.m2 with the "heavy elastica" theory. Comparison of theoretical profiles to the experimental bending responses of cilia established the validity of the "heavy elastica" model; this model, in turn, was used to predict primary cilium bending behavior under representative conditions in the rat nephron. The results of the study are consistent with the hypothesis that primary cilia serve a mechanosensory function in kidney epithelial cells.

Introspective sorting and selection algorithms

Abstract: Quicksort is the preferred in-place sorting algorithm in many contexts, since its average computing time on uniformly distributed inputs is Θ(N log N), and it is in fact faster than most other sorting algorithms on most inputs. Its drawback is that its worst-case time bound is Θ(N2). Previous attempts to protect against the worst case by improving the way quicksort chooses pivot elements for partitioning have increased the average computing time too much – one might as well use heapsort, which has a Θ(N log N) worst-case time bound, but is on the average 2–5 times slower than quicksort. A similar dilemma exists with selection algorithms (for finding the i-th largest element) based on partitioning. This paper describes a simple solution to this dilemma: limit the depth of partitioning, and for subproblems that exceed the limit switch to another algorithm with a better worst-case bound. Using heapsort as the ‘stopper’ yields a sorting algorithm that is just as fast as quicksort in the average case, but also has an Θ(N log N) worst case time bound. For selection, a hybrid of Hoare's FIND algorithm, which is linear on average but quadratic in the worst case, and the Blum–Floyd–Pratt–Rivest–Tarjan algorithm is as fast as Hoare's algorithm in practice, yet has a linear worst-case time bound. Also discussed are issues of implementing the new algorithms as generic algorithms, and accurately measuring their performance in the framework of the C+:+ Standard Template Library. ©1997 by John Wiley & Sons, Ltd.

Proactive network fault detection

Abstract: The increasing role of communication networks in today's society results in a demand for higher levels of network availability and reliability. At the same time, fault management is becoming more difficult due to the dynamic nature and heterogeneity of networks. We propose an intelligent monitoring system using adaptive statistical techniques. The system continually learns the normal behavior of the network and detects deviations from the norm. Within the monitoring system, the measurements are segmented, and features extracted from the segments are used to describe the normal behavior of the measurement variables. This information is combined in the structure of a Bayesian network. The proposed system is thereby able to detect unknown or unseen faults. Experimental results on real network data demonstrate that the proposed system can detect abnormal behavior before a fault actually occurs.

Heat Transfer Enhancement—The Encouragement and Accommodation of High Heat Fluxes

Abstract: This review considers the many techniques that have been developed to enhance convective heat transfer. After introducing the techniques, the applications to most of the modes of heat transfer (single-phase forced convection, including compound techniques, pool boiling, convective boiling/evaporation, vapor-space condensation, and convective condensation) are described. Comments are offered regarding commercial introduction of this technology and the generations of heat transfer technology : advanced enhancement represents third-generation heat transfer tehnology.

The dynamics of a swirling flow in a pipe and transition to axisymmetric vortex breakdown

Abstract: This paper provides a new study of the axisymmetric vortex breakdown phenomenon. Our approach is based on a thorough investigation of the axisymmetric unsteady Euler equations which describe the dynamics of a swirling flow in a finite-length constant-area pipe. We study the stability characteristics as well as the time-asymptotic behaviour of the flow as it relates to the steady-state solutions. The results are established through a rigorous mathematical analysis and provide a solid theoretical understanding of the dynamics of an axisymmetric swirling flow. The stability and steady-state analyses suggest a consistent explanation of the mechanism leading to the axisymmetric vortex breakdown phenomenon in high-Reynolds-number swirling flows in a pipe. It is an evolution from an initial columnar swirling flow to another relatively stable equilibrium state which represents a flow around a separation zone. This evolution is the result of the loss of stability of the base columnar state when the swirl ratio of the incoming flow is near or above the critical level.

Trajectory tracking control of a car-trailer system

Abstract: This paper presents the experimental results of the tracking control of a car-trailer system. The proposed scheme involves three steps: 1) generate a path off-line using a path space iterative algorithm; 2) linearize the kinematic model about a trajectory which is constructed using the path; and 3) apply a time-varying linear quadratic regulator to track the trajectory. Experiments presented include parallel parking a car, docking a tractor-trailer vehicle, and parallel parking a double tractor-trailer vehicle.

The Sweet Smell of... Helping: Effects of Pleasant Ambient Fragrance on Prosocial Behavior in Shopping Malls

Abstract: In a preliminary study, passersby in a large shopping mall were significantly more likely to help a same-sex accomplice (by retrieving a dropped pen or providing change for a dollar) when these helping opportunities took place in the presence of pleasant ambient odors (e.g., baking cookies, roasting coffee) than in the absence of such odors. Participants also reported significantly higher levels of positive affect in the presence of pleasant odors. In a second study, the order in which passersby were exposed to a helping opportunity and rated their current mood was systematically varied. Results similar to those of the first study were obtained; order of task had no effect on either mood or helping, but helping was significantly greater in the presence of pleasant fragrances than in their absence. In addition, there was some evidence that fragrance-induced increments in helping were mediated by increments in positive affect.

Accommodation and presbyopia in the human eye. Changes in the anterior segment and crystalline lens with focus.

Abstract: PURPOSE To characterize changes in the sagittal dimensions of the human crystalline lens and anterior segment as a function of accommodation, to determine the potential age dependence of these changes, and to evaluate these changes in relation to the development of presbyopia. METHODS Scheimpflug slit-lamp photography, as well as a variety of standard ophthalmologic methods, was used to collect information about lens and anterior segment sagital dimensions in a population of 82 adults with refractive error < or = magnitude of 2.0 diopters and at least 0.25 diopter of accommodation for subjects 18 to 70 years of age. Data were analyzed statistically for dependence on accommodation, age, and age dependence of accommodative rate. RESULTS The rate of change per diopter of accommodation for each measured variable within the lens is independent of age for the entire adult age range. With increasing accommodation, the lens becomes thicker and the anterior chamber shallower along the polar axis. This increase in sagittal lens thickness is entirely because of an increase in the thickness of the lens nucleus. Because the anterior and posterior halves of the nucleus increase in thickness at approximately the same rate with accommodation, the increase in lens thickness results from equal changes in the lengths of the anterior and posterior portions. CONCLUSIONS Because changes along the sagittal axis of the anterior segment with accommodation are independent of age, any explanation of presbyopia that relies on simple changes in the rates of lens thickening and anterior chamber shallowing with age does not hold. In light of other age-related changes in the anterior segment and lens (e.g., increased sharpness of lens curvature, increased lens sagittal thickness, decreased anterior chamber depth), it appears that compensatory mechanisms to preserve far vision with age also preserve the rate of change per diopter of sagittal spacings.