Showing papers by "Stevens Institute of Technology published in 1994"

Forced Convection in a Porous Channel With Localized Heat Sources

Abstract: A numerical study is performed to analyze steady laminar forced convection in a channel filled with a fluid-saturated porous medium and containing discrete heat sources on the bottom wall. Hydrodynamic and heat transfer results are reported for two configurations: (1) a fully porous channel, and (2) a partially porous channel, which contains porous layers above the heat sources and is nonporous elsewhere. The flow in the porous medium is modeled using the Brinkman-Forchheimer extended Darcy model. Heat transfer rates and pressure drop are evaluated for wide ranges of Darcy and Reynolds numbers. Detailed results of the evolution of the hydrodynamic and thermal boundary layers are also provided

Flow instabilities in capillary flow of concentrated suspensions

Abstract: The development of flow instabilities during the capillary flow of two concentrated suspensions filled with 76.5 and 65.6% by volume solids was investigated. The flow instabilities manifested themselves by the development of concentration gradients as a result of the filtering of the binder, superimposed on the bulk motion of the suspension. The effects of apparent shear rate, capillary diameter and the surface roughness of the particles were investigated. The use of the comparison of the filtration rate with the bulk velocity of the suspension during flow is shown to be promising for the prediction of the apparent shear rate at which filtration-based flow instabilities occur.

Some international evidence regarding the stochastic memory of stock returns

Abstract: The present paper studies international stock indexes of the G-7 countries in the last 40 years. Evidence about the statistical memory of the returns is presented, and only in one country could the existence of long memory be sustained. These results contradict various previous studies that were based on the R/S analysis and consistently claimed the existence of long memory in financial returns. A general ARFIMA model capable of reproducing long- and short-memory properties is directly fitted to the data. The conclusion is then based on the estimated parameters of the model.

Heavy metal removal and recovery by contained liquid membrane permeator

Abstract: Heavy metals like Cu2+, Cr6+, and Hg2+ were removed successfully from wastewater and concentrated in a strip aqueous solution for recycle using the hollowfiber-contained liquid membrane (HFCLM) technique. Using cotransport, Cr6+ and Hg2+ present as anions in the feed solution were transferred individually through a liquid membrane containing tri-n-octylamine in xylene and concentrated in an alkaline solution on the strip side. The removal efficiency of each heavy metal was studied as a function of the aqueous feed flow rate in an HFCLM permeator. Copper present as cation Cu2+ was removed and concentrated by countertransport using LIX84 in n-heptane as the liquid membrane. The efficiency of copper removal was studied as a function of feed copper concentration, feed flow rate, strip flow rate, and area ratio between the feed fiber set and strip fiber set. The strip side flow rate did not affect the Cu2+ transfer rate. A theoretical model presented predicts the copper transport rate from the feed solution to the strip solution in an HFCLM permeator with a variable feed to strip-side membrane area. Both feed aqueous boundary layer and interfacial complexation reaction on the feed side dominate the observed Cu2+ removal process through LIX 84 in n-heptane for feed concentration levels used 90–500 mg/L. It was verified by providing two times larger liquid-liquid interfacial area in the feed aqueous side or in the strip aqueous side. The model can be used to design the membrane area required on the feed and strip fiber sides to remove essentially all of the copper from a given wastewater stream.

Microwave-induced organic reaction enhancement (more) chemistry: Techniques for rapid, safe and inexpensive synthesis

Abstract: Synthetic organic reactions have been conducted under microwave irradiation in open vessels in unaltered domestic microwave ovens. Reaction times vary from a few seconds for sub-milligram reactions to about 15 minutes for reactions carried out on a scale of hundreds of grams. Promising results have been obtained for several condensations, as well as the Bischler-Napieralski reaction, the Wolff-Kishner reduction, free radical dehalogenation reactions, and other standard synthetic operations. Rapid catalytic transfer hydrogenation using ammonium formate as the source of hydrogen has been conducted at about 100-130 °C under microwave irradiation.

Non-axisymmetric subcritical bifurcations in viscoelastic Taylor–Couette flow

Abstract: Recently, an account of the linear and nonlinear analysis of the viscoelastic Taylor–Couette flow between independently rotating cylinders against axisymmetric disturbances was presented (Avgousti & Beris 1993 a ). However, more recent linear stability analysis has shown that for the range of geometric and kinematic parameters studied and for high enough values of flow elasticity, the critical instabilities are caused by non-axisymmetric, time-dependent disturbances (Avgousti & Beris 1993 b ). In this work, we calculate the bifurcating families corresponding to each one of the two possible non-axisymmetric patterns emerging at the point of criticality, namely the spirals and ribbons and determine their stability. It is shown that for a narrow gap size, for upper convected Maxwell and Oldroyd-B fluids, at least one of the non-axisymmetric families bifurcates subcritically. This result, in conjunction with the theoretical analysis of Hopf bifurcation in presence of symmetries (Golubitsky et al . 1988), implies that neither of the bifurcating families is stable. Consequently, there is a finite transition corresponding to infinitesimal changes of the flow parameters in the vicinity of the Hopf bifurcation point. Although a change in the ratio of the Deborah and Reynolds numbers has not been found to qualitatively affect this behaviour, calculations with a wider gap size have shown that both bifurcating families become supercritical. There, a Ginzburg–Landau analysis shows that the ribbons are the stable pattern. This behaviour is qualitatively similar to that seen for the newtonian fluid, but for counterrotating cylinders, albeit there, spirals have been found to be stable (Golubitsky & Langford 1988).

Excited-state absorption in Pr3+:Y3Al5O12.

Abstract: The optical-absorption transitions originating from the lowest 4f5d excited state of ${\mathrm{Pr}}^{3+}$ in ${\mathrm{Y}}_{3}$${\mathrm{Al}}_{5}$${\mathrm{O}}_{12}$ have been studied. The experimental arrangement uses a 7-ns 266-nm light pulse to populate the lowest 4f5d state, and a spatially overlapped, time-sequenced pulse from a frequency-tunable dye laser to induce transitions from that excited state. The excited-state-absorption (ESA) spectrum covering the 900--300-nm probe wavelength range is characterized by two broad peaks centered at 700 and 350 nm on a slowly rising background. The peak ESA cross section at 355 nm is (1\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ ${\mathrm{cm}}^{2}$. The position, shape, and strength of the spectrum suggest that the terminal states of the ESA transitions are the second 4f5d state of ${\mathrm{Pr}}^{3+}$ and the conduction band of the host lattice. The ESA measurements have been extended to transitions from the $^{3}$${\mathit{P}}_{0}$ and $^{1}$${\mathit{D}}_{2}$ excited states of the 4${\mathit{f}}^{2}$ configuration of ${\mathrm{Pr}}^{3+}$ as well. The ESA cross sections at a probe wavelength of 532 nm are (8\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ ${\mathrm{cm}}^{2}$ and (5.7\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ ${\mathrm{cm}}^{2}$ for transitions from the $^{3}$${\mathit{P}}_{0}$ and $^{1}$${\mathit{D}}_{2}$ excited states, respectively. The terminal state of ESA transitions from these two levels is the lowest 4f5d state. The strong ESA completely inhibits the potential laser action based on emission from these states in this crystal.

Optical nonlinearities of tea studied by Z-scan and four-wave mixing techniques

Abstract: Optical nonlinearities of black-tea extract dissolved in water are studied using the Z-scan and the four-wave mixing techniques with 7-ns, 532-nm pulses from a Q-switched Nd:YAG laser. The magnitude of the third-order nonlinear optical susceptibility of tea in water is comparable with that of carbon disulfide. The nonlinearity is identified to be of thermal origin.

Kinetic-Studies and Process Analysis of the Reactive Compatibilization of Nylon-6 Polypropylene Blends

Abstract: This article describes a study on the reactive compatibilization of nylon 6 (N6) and polypropylene (PP) blends through a functionalized PP. A graft copolymer was formed in-situ by reacting an acid modified PP with N6 during blend compounding. The compatibilization reactions are studied in detail. Kinetics were estimated by means of experiments in a batch mixer. Three time constants were estimated, corresponding to (a) reactions, (b) melting of polymers, and (c) melt mixing. The effects of temperature and rotor speed on the reaction kinetics were also measured. There was a substantial increase in initial reaction rate, as the rotor speed was increased. Increasing the temperature did not significantly affect the reaction rate. Process parameters important for such a reactive compatibilization process were identified by statistically designed experiments in a co-roatating intermeshing twin-screw extruder. Screw speed, presence of venting, and sequence of feeding were shown to have a noticeable effect on the reactive compatibilization process during continuous compounding.

Optical absorption spectrum of trivalent cerium in Y 2 O 3 , Ba 2 GdTaO 6 , ThO 2 , and related compounds

Abstract: The optical absorption coefficient at room temperature has been measured for cerium-doped Y2O3 and Y1.4Gd0.6O3 samples from 1500 to 45000 wave numbers. Diffuse reflectivity has been measured on cerium-doped powders of Y2O3, Lu2O3, Sc2O3, Ba2GdTaO6, and ThO2 over the same range. A number of 4f − 4f and 4f − 5d transitions of trivalent Ce have been seen, and some experimental values for their oscillator strengths have been determined. A tentative assignment of particular transitions to either C2 or C3i sites in the Y2O3 structure has been made. Some of the transitions have also been seen by use of Raman scattering. Several phonon replicas of the 4f − 4f transitions have been identified. The charge-transfer band of tetravalent cerium in Y1.4Gd0.6O3 has been identified.

Noimisothermal model of single screw extrusion of generalized newtonian fluids

Abstract: A nonisothermal model of Ike single screw extrusion processing of generalized Newtonian fluids is presented. Various temperature dependent forms of a generalized Newtonian fluid constitutive equation representing the Herschel-Bulkley fluid and its simplifications, including Bingham plastic, power law of Ostwald-de Waele, and Newtonian fluids, are applicable. The model includes the generally ignored transverse convection terms of the equation of energy. The importance of keeping the transverse convection terms in the analysis is demonstrated by applying the model and comparing findings to experimental results involving the transverse flow temperature distributions in single screw extruders, available in the literature. The numerical instabilities, arising principally from the convection terms, generally encountered in high-Peclet-number extrusion flows, could be eliminated by the use of the streamline upwind / Petrov-Galerkin formulation. The model is sufficiently general to accommodate Navier's w...

Electrical characterization of an ultrahigh concentration boron delta‐doping layer

Abstract: We report a boron δ‐doping layer in crystalline silicon with an electrically active concentration of 1×1022 cm−3 and a mobility of ∼20 cm2/V s This structure was fabricated by low‐temperature molecular‐beam epitaxy with boron confined to 3 monolayers in the silicon growth direction Complete electrical activation is observed, showing metallic conduction down to 4 K This two‐dimensional doped layer, incorporated into the crystal lattice, represents a volume concentration exceeding the solid solubility of boron in silicon by two orders of magnitude These high‐concentration structures fill an unexplored region of the mobility versus concentration curve

An adaptive Chebyshev iterative method for nonsymmetric linear systems based on modified moments

Abstract: Large, sparse nonsymmetric systems of linear equations with a matrix whose eigenvalues lie in the right half plane may be solved by an iterative method based on Chebyshev polynomials for an interval in the complex plane. Knowledge of the convex hull of the spectrum of the matrix is required in order to choose parameters upon which the iteration depends. Adaptive Chebyshev algorithms, in which these parameters are determined by using eigenvalue estimates computed by the power method or modifications thereof, have been described by Manteuffel [1978]. This paper presents adaptive Chebyshev iterative methods, in which eigenvalue estimates are computed from modified moments determined during the iterations. The computation of eigenvalue estimates from modified moments requires less computer storage than when eigenvalue estimates are computed by a power method and yields faster convergence for many problems.

Asymmetric induction in [2+2]cycloaddition of chlorosulfonyl isocyanate to 1,2-O-isopropylidene-3-O-vinyl-glycofuranoses

Abstract: The asymmetric [2+2]cycloaddition of chlorosulfonyl isocyanate to 1,2- O -isopropylidene-3- O -vinyl-glycofuranoses is presented. Bulky substituent at the C-4 carbon atom promotes excellent stereoselectivity affording (R) configuration at the C-4′ carbon atom of the azetidinone ring. Intramolecular cyclization in compounds 9 or 12 provides diastereomeric cephem 20 and 21 .

Peroxide modification of a multicomponent polymer blend with potential applications in recycling

Abstract: A mixture of seven immiscible “virgin” plastics, namely low and high density polyethylene (LDPE and HDPE), polypropylene (PP), polyvinyl chloride (PVC), crystal and high impact polystyrene (PS and HIPS), and polyethylene terephthalate (PET), in proportions representative of those currently found in post-consumer plastics containers, was compounded in a corotating twin-screw extruder. The mechanical properties of the blend were improved through the addition of relatively high concentrations of a dialkyl peroxide which was also found to drastically affect the blend morphology. The results are interpreted in terms of improved dispersive mixing favored by the similarity in the rheological behavior of the modified blend components, improved physical properties of certain blend components upon peroxide modification, and, possibly, enhanced intefacial adhesion. © 1994 John Wiley & Sons, Inc.

Compatibilization of refined commingled post‐consumer plastics

Abstract: In attempts to improve the properties of post-consumer commingled plastics waste a variety of additives that could potentially act as compatibilizers/impact modifiers were evaluated. The feedstock was representative of a model curbside collection program and contained a variety of polymers, mostly polyethylenes with PET, PP, PVC, etc., in smaller amounts. The ground mixed plastics were first compounded and melt filtered in a counter-rotating nonintermeshing twin-screw extruder and then combined with different amounts of additives in a corotating intermeshing twin-screw extruder. Additives included unmodified and maleated polyolefin elastomers and styrene/olefin block copolymers. Blends were analyzed for thermal and mechanical properties, and processability. The most effective modifier in terms of impact strength improvement was a styrenic block copolymer with very similar rheological characteristics to the commingled plastics matrix. The experimental observations were interpreted by considering the complex morphological features of the injection molded multicomponent, multiphase systems. © 1994 John Wiley & Sons, Inc.

Single screw extrusion of viscoplastic fluids subject to different slip coefficients at screw and barrel surfaces

Abstract: Commonly encountered viscoplastic fluids including concentrated suspensions of polymeric and ceramic composites, foams, gels, concrete, food products, and energetic compounds exhibit wall slip during their flow and processing. For some viscoplastics fluids, especially highly filled suspensions, wall slip may dominate the flow and deformation and hence the processing behavior of the suspension. The wall slip velocity is generally a function of the wall shear stress and temperature. Various factors including the materials of construction i.e., chemical nature and the roughness of the wall surface affect the wall slip behavior of viscoplastic fluids. In this study an analytical model of the extrusion of viscoplastic fluids under isothermal and fully-developed conditions in shallow channels is developed. The model accommodates the use of different slip coefficients at barrel and screw surfaces. It thus permits the investigation of effects of introducing different materials of construction for the barrel and screw surfaces and development of design expressions.

Toughness, minimum degree, and the existence of 2-factors

Abstract: Degree conditions on the vertices of a t-tough graph G(1 ≦ t ≦ 2) that ensure the existence of a 2-factor in G are presented. These conditions are asymptotically best possible for every t ϵ [1, 3/2] and for infinitely many t ϵ [3/2, 2].

UV-Visible-IR Electroluminescence from Si and Ge Nanocrystals in a Wider Bandgap Matrix

Abstract: The demonstration of photoluminescence (PL) and electroluminescence (EL) in nanostructures of Si or Ge, such as those found in porous silicon, has significantly improved the prospects of all Si based photonic devices. While the physical mechanisms at work are still a subject of much study, it is clear that the luminescence is associated with the formation of nanometer or “quantum” sized particles. Further, it is clear that prototype NanoCrystal Displays (NCDs) and communication devices are being fabricated in these material systems. We report here on the electroluminescent properties of nanometer sized particles in an SiO2 host matrix, which were fabricated by LPCVD techniques. The films have demonstrated reproducible emission from well below 400 nm to well above 800 nm. We believe that dispersion effects of the nanocrystals can account for “white” light emission. The films have been characterized using PL, Raman, XRD, TEM, and SIMS. The nanocrystals are primarily in the 2-7 nm range although larger crystal clusters are also observed. The development of stable and efficient Si or Ge nanocrystalline EL based devices could find applications in lamps/LEDs, photonic integrated circuits, and displays.

Magneto-optical absorption in a one-dimensional array of narrow antiwires

Abstract: We present a self-consistent field theory for the infrared absorption coefficient of an array of narrow antiwires in a perpendicular external magnetic field. A detailed study is made of the way in which the collective mode changes from a cyclotron mode when the confining potential is weak to tunneling coupled modes for intermediate antiwire potential strength and then to edge and one-dimensional lattice magnetoplasmon modes for strong potentials. Our numerical calculations show that at low magnetic fields, there is appreciable electron tunneling between quantum wires. However, as the magnetic field is increased, the electron tunneling is suppressed. The suppression of electron tunneling between wires is observed when the tunneling coupled modes emerge into cyclotron modes in the strong magnetic field regime. The edge mode excitation energy oscillates as a function of the electron density. These oscillations corespond to soft or hard potential walls for which the electron states are extended and localized, respectively.