bluuebunny/crossref_metadata_2025 huggingface dataset:
A regulatory effect of ENSO on the time‐mean thermal stratification of the equatorial upper ocean
Authors: Sun De‐Zheng , Zhang Tao
Published: April 2006

To investigate the role of ENSO in regulating the time‐mean thermal stratification of the equatorial Pacific, perturbation experiments are conducted in pairs with a coupled model. In one experiment, ENSO is turned off while in the other experiment ENSO is kept on. Perturbations are introduced through either enhancing tropical heating or increasing subtropical cooling. In the absence of ENSO, the time‐mean difference between the warm‐pool SST (Tw) and the characteristic temperature of the equatorial thermocline (Tc) responds sensitively to either enhanced tropical heating or enhanced subtropical cooling. In the presence of ENSO, such a sensitivity to destabilizing forcing disappears. The lack of sensitivity in the response of Tw‐Tc is linked to a stronger ENSO in response to the destabilizing forcing. ENSO in the model acts as a basin‐scale heat “mixer” that enables surface heat to be transported to the depths of the equatorial thermocline. The study raises the question whether models with poor simulations of ENSO can give reliable predictions of the response of the time‐mean climate to global warming.
Crustal images from San Juan, Argentina, obtained using high frequency local event receiver functions
Authors: Calkins Josh A. , Zandt George , Gilbert Hersh J. , Beck Susan L.
Published: April 2006

Images of the crust and upper mantle obtained using teleseismic receiver functions exhibit a weak P‐S conversion from the Moho in Western Argentina, where a segment of the subducting Nazca plate flattens out beneath the overriding South American plate. To better estimate depth to the Moho and search for mid‐crustal impedance contrasts, we calculate high frequency receiver functions using 37 intermediate‐depth local earthquakes. Radial receiver functions from a station located near San Juan, Argentina, show a strong signal from the Moho at 7 seconds corresponding to a depth near 50 km, and conversions from interfaces within the crust at depths of ∼20 and 35 km. The higher frequency results obtained using local events have twice the vertical resolution of the teleseismic results and the combined data sets indicate a gradational increase in S velocity in the lower crust that cannot be detected in the lower frequency teleseismic data alone.
Co‐ and postseismic deformation of the 28 March 2005 Nias Mw 8.7 earthquake from continuous GPS data
Authors: Kreemer Corné , Blewitt Geoffrey , Maerten Frantz
Published: April 2006

Global Positioning System (GPS) measurements registered up to >5m of coseismic displacements during the 28 March 2005 Mw = 8.7 Nias earthquake, Indonesia. The vertical offsets put tight constraints on the northern and southern limit of the rupture. The inferred coseismic slip distribution indicates high slip patches near the epicenter and near the southern extent of the 26 December 2004 Aceh‐Andaman rupture, where aftershocks have been abundant. Six months of postseismic time‐series are better fit with a logarithmic instead of exponential function, suggesting that the postseismic deformation is likely controlled by afterslip. Our inversion model predicts afterslip to be concentrated both up‐ and down‐dip from patches of maximum coseismic slip where aftershocks are sparse. The shallow afterslip adds further evidence that the earthquake probably did not break the surface (with implications for tsunami generation) and instead caused aseismic deformation in shallow parts of the subduction zone after the event.
Search for reliable precursors: A case study of the seismic quiescence of the 2000 western Tottori prefecture earthquake
Authors: Huang Qinghua
Published: April 2006

How to search for reliable precursors is one of the key problems of the study on earthquake forecast. In this paper a method is presented to judge whether or not a precursor is reliable. Before a potential anomaly can be nominated as a reliable precursor, it should pass or be proved by the following tests or analyses: whether or not it is an artificial anomaly, whether or not it correlates with an investigated event, whether or not it is a random anomaly. As a case study, I have investigated the reliability of seismic quiescence for an M = 7.3 earthquake, which occurred in the western region of Tottori prefecture, Japan, on 6 October 2000. The earthquake data (1975–2000) of the Japan Meteorological Agency (JMA) were used in this study. After analyzing the completeness of the catalog and removing aftershocks, a statistical method, which is called the RTL (region‐time‐length) algorithm and takes into account the information of magnitude, occurrence time and location of earthquakes, was applied to the above preprocessed earthquake data. A clear seismic quiescence anomaly was detected before the 2000 western Tottori prefecture earthquake. Close investigation indicated that the above anomaly is not due to artifacts of the selections of model parameters. After quantifying the anomaly and making statistical analysis over the whole investigated spatiotemporal domain, reasonable correlation between the above anomaly and the main shock in 2000 was recognized. The stochastic test using synthetic randomized catalogs showed the above anomaly is unlikely due to chance. Thus the above seismic quiescence anomaly before the 2000 main shock should be a significant and reliable precursor and the research method revealed in this study can enhance the reliability of precursors.
Effect of Marangoni and Coriolis forces on multicritical points in a Faraday experiment
Authors: Mondal Gour Chandra , Kumar Krishna
Published: April 2006

Stability of the free surface of thin sheets of a metallic liquid on a vertically vibrating hot plate, in the presence of a uniform and small rigid body rotation about the vertical axis, is investigated. The inhomogeneity in the surface tension due to a uniform thermal gradient across the liquid sheet prefers subharmonic response, while the rigid body rotation prefers harmonic response at the fluid surface. The competition results in Marangoni and Coriolis forces acting as fine-tuning parameters in the selection of wave numbers corresponding to different instability tongues for subharmonic and harmonic responses of the fluid surface. Solutions corresponding to various pairs of tongues may be induced in a thin layer of metallic liquid at the onset of parametrically forced surface waves. These give rise to multicritical points involving standing waves of two or more different wave numbers. Bicritical points may involve both the solutions oscillating subharmonically, harmonically, or one oscillating subharmonically and the other harmonically with respect to the vertical forcing frequency. Two tricritical points involving different types of solutions are also possible in a thin layer of mercury. The effect of variation of the Galileo number on critical acceleration and wave number in very low Prandtl number liquids is also presented.
Wetting hysteresis of a dry patch left inside a flowing film
Authors: Rio E. , Limat L.
Published: April 2006

We investigate the influence of wetting hysteresis on the shape and stability of dry patches. Such patches are generated in a film flowing over an inclined plane in a situation of partial wetting. We performed experiments on silicon oil flowing over a glass plate coated with fluoropolymers in which direct visualizations are combined with refraction of a laser sheet to probe contact angle distribution around the patches. The shape evolution depends on the history of the flow, and is different for progressive increase or decrease of flow rate. For increasing flow rates, the shape is in agreement with a simple model based upon a balance between gravity and capillarity, and in which the contact angle is supposed to be uniform. Laser measurements indicate that its value coincides with the static advancing contact angle. For a decreasing flow rate the situation is much more complicated: the shape remains qualitatively that predicted by the model, but quantitatively, the curvature of the patch boundary is not reproducible. Laser measurements suggest that this is linked to wetting heterogeneities (wetting defects) that impose nonuniform contact angle distributions. Finally, the stability is explored in terms of two critical flow rates (expansion or advection of the patches).
Photodissociation of the BrO radical using velocity map ion imaging: Excited state dynamics and accurate D(BrO) evaluation
Authors: Kim Hahkjoon , Dooley Kristin S. , Johnson Elizabeth R. , North Simon W.
Published: April 2006

We have studied the photodissociation dynamics of expansion-cooled BrO radical both above (278–281.5nm) and below (355nm) the AΠ3∕22 state threshold using velocity map ion imaging. A recently developed late-mixing flash pyrolytic reactor source was utilized to generate an intense BrO radical molecular beam. The relative electronic product branching ratios at 355nm and from 278to281.5nm were determined. We have investigated the excited state dynamics based on both the product branching and the photofragment angular distributions. We find that above the O(D21) threshold the contribution of the direct excitation to states other than the AΠ3∕22 state and the role of curve crossing is considerably larger in BrO compared to that observed for ClO, in agreement with recent theoretical studies. The measurement of low velocity photofragments resulting from photodissociation just above the O(D21) threshold provides an accurate and direct determination of the AΠ3∕22 state dissociation threshold of 35418±35cm−1, leading to a ground state bond energy of D00(BrO)=55.9±0.1kcal∕mol.
Photoemission electron microscopy study of remanent magnetic domain states in ferromagnetic wedge films deposited on substrates with micrometer-sized square plateaus
Authors: Heyderman L. J. , Czekaj S. , Nolting F. , Müller E. , Fischer P. , Gasser Ph. , López-Díaz L.
Published: April 2006

We present a detailed study of the remanent magnetic domain configurations in demagnetized polycrystalline ferromagnetic thin film wedges of cobalt and Permalloy deposited on prepatterned silicon substrates with micrometer-sized square plateaus, which have a height of 125 nm, using photoemission electron microscopy. We have observed the continuous evolution of the magnetic domain states in the square ferromagnetic elements on top of the plateaus as a function of film thickness. At high film thickness we observe the Landau state, which is the expected lowest energy state, but at lower thickness we see a variety of metastable states which are trapped as a result of local pinning. In a small thickness range below 10 nm, the square elements contain 360° walls and small domains which are likely to be a result of local effects such as magnetocrystalline anisotropy and edge roughness. We are able to simultaneously observe the development of the magnetic domains in the continuous polycrystalline film surrounding the plateaus and, rather than the expected large domains, we observe at intermediate film thickness a significant modification of the domain configuration to small domains. Here the roughness of the silicon substrate surrounding the plateaus, which is due to the reactive ion etching process used to prepare the prepatterned substrates, gives rise to local stray fields in the ferromagnetic film which play an important role in determining the resulting domain structure.
Two-scale modeling in porous media: Relative permeability predictions
Authors: Markicevic B. , Djilali N.
Published: April 2006

We present a numerical analysis of fluid flow through a porous medium with two distinct characteristic scales. The system considered is a monodisperse matrix with porosity ϕ and permeability Kpm with an embedded second phase, characterized by a phase content or saturation s and phase length scales Lϕ and Ls. Both two- and three-dimensional simulations are performed to compute the mobile fluid phase relative permeability kr,m and its dependence on s and Kpm. The relative permeability is found to vary as a power law of saturation, with a quasilinear behavior for low permeability, and increasing values of the exponent as Kpm increases. For media with low permeability, the linearity of kr,m is attributed to the drag force, whereas for high Kpm, the decrease of kr,m with s is due primarily to viscous forces. An analytical model for kr,m is also presented to aid the interpretation and to corroborate the simulation results. In the second part, in order to elucidate the role of the length scales on kr,m, simulations explicitly resolving both porous media and second-phase scales are performed. The relative permeability is found to drop rapidly when both scales are of the same order (Ls≈Lϕ) or when Ls<Lϕ. Three regimes (Darcy, Brinkman, Stokes) are consequently identified based on the length scales.
Characterization of blinking dynamics in quantum dot ensembles using image correlation spectroscopy
Authors: Bachir Alexia I. , Durisic Nela , Hebert Benedict , Grütter Peter , Wiseman Paul W.
Published: April 2006

Quantum dots (QDs) are being increasingly applied as luminescent labels in optical studies for biophysical and cell biological applications due to their unique spectroscopic properties. However, their fluorescence “blinking” characteristics that follow power law statistics make it difficult to use QDs in some quantitative biophysical applications. We present image correlation spectroscopy (ICS) in combination with total internal reflection fluorescence microscopy as a tool to characterize blinking dynamics in QDs. We show that the rate of decay of the ICS measured ensemble correlation function reflects variation in blinking dynamics and can be used to distinguish different blinking distribution regimes. To test and confirm our hypothesis, we also analyze image time series simulations of ensembles of point emitters with set blinking statistics. We show that optimization of the temporal sampling and the number of QDs sampled is essential for detecting changes in blinking dynamics with ICS. We propose that this experimental characterization of the QD blinking statistics can actually serve as a sensitive reporter for certain quantitative biological applications.
Imaging stability and average tip-sample force in tapping mode atomic force microscopy
Authors: Kowalewski Tomasz , Legleiter Justin
Published: April 2006

In tapping mode atomic force microscopy (AFM), a cantilever is driven near its resonance frequency and intermittently strikes the sample while raster scanned across a surface. The oscillation amplitude is monitored via a feedback loop to construct topography maps of surfaces at the nanoscale. This paper deals with two major limits on scanning rates when operating in air: (1) the slow transient response of the cantilever and (2) instabilities associated with systems with high quality factors (Q). Due to the slow transient response, the AFM has difficulty in instantly responding to steps along the surface, resulting in the need for slower scan rates and higher gains to more accurately track the surface. However, the use of higher gains leads to more pronounced instabilities associated with high Q systems. By driving the cantilever well below its resonance frequency, stability of the system is greatly improved, resulting in better feature tracking and allowing for scanning at higher speeds with larger gains. Also, the impact of the cantilever spring constant and sample modulus on the response time was explored at different operating frequencies. The experimental results were further verified using numerical simulations of a tapping mode AFM experiment, in which a well-defined step was scanned and tracked via a feedback loop equipped with an integral gain. These simulations helped to elucidate the physics behind this improvement and the tip/sample forces associated with imaging far below resonance.
Inertial migration of neutrally buoyant particles in a square duct: An investigation of multiple equilibrium positions
Authors: Chun B. , Ladd A. J. C.
Published: April 2006

Inertial migration of neutrally buoyant particles in a square duct has been investigated by numerical simulation in the range of Reynolds numbers from 100 to 1000. Particles migrate to one of a small number of equilibrium positions in the cross-sectional plane, located near a corner or at the center of an edge. In dilute suspensions, trains of particles are formed along the axis of the flow, near the planar equilibrium positions of single particles. At high Reynolds numbers (Re⩾750), we observe particles in an inner region near the center of the duct. We present numerical evidence that closely spaced pairs of particles can migrate to the center at high Reynolds number.
Large magnetically induced strains in Ni50Mn28.7Ga21.3 driven with collinear field and stress
Authors: Malla A. , Dapino M. J. , Lograsso T. A. , Schlagel D. L.
Published: April 2006

Despite the huge magnetic-field-induced strain (MFIS) of up to 9.5% exhibited by certain Ni–Mn–Ga alloys, their usefulness in applications is severely hindered by the electromagnet device needed for driving the alloys with a magnetic field and orthogonal loading stress. In this paper we present macroscopic measurements obtained from a single crystal of Ni50Mn28.7Ga21.3 which demonstrate a large reversible MFIS of −4100ppm when the alloy is driven with quasistatic magnetic fields and fixed compressive stresses applied collinearly along the [001] axis. This collinear configuration marks a fundamental difference with prior research in the field and points to the existence in this alloy of stable bias or residual stresses—likely associated with pinning sites in the alloy—which provide the energy necessary to restore the original variant state when the field is reversed. We present macroscopic magnetomechanical measurements which show a decrease of the MFIS with increasing stress loading and a stiffening of the alloy with increasing dc fields. The latter behavior is phenomenologically similar to the ΔE effect in magnetostrictive materials. The large reversible MFIS and tunable stiffness properties exhibited by this alloy could enable practical Ni–Mn–Ga actuators for high-deflection, low-force applications which due to being driven by a solenoid transducer are more compact, energy efficient, and faster than their electromagnet counterpart. A thermodynamic model is presented which qualitatively characterizes the decay in MFIS with increasing compressive external load and provides a starting point for the characterization, design, and control of the proposed Ni–Mn–Ga devices.
Photoluminescence of wurtzite ZnO under hydrostatic pressure
Authors: Chen S. J. , Liu Y. C. , Shao C. L. , Xu C. S. , Liu Y. X. , Wang L. , Liu B. B. , Zou G. T.
Published: April 2006

Photoluminescence (PL) spectra of single-crystal ZnO bulk under hydrostatic pressure are studied using the diamond-anvil-cell technique at room temperature. The PL spectrum of ZnO single crystal taken at atmospheric pressure was dominated by a strong near-band-edge exciton emission. The emission line was found to shift towards higher energy with increasing applied pressure. By examining the pressure-dependent PL spectral features, the pressure coefficient of the direct Γ band gap of the wurtzite ZnO was determined. The hydrostatic deformation potential of the band gap has also been deduced from the experimental results.
Binding energy of negative trions in a CdTe quantum well at high magnetic fields
Authors: Redliński P.
Published: April 2006

We present the results of numerical calculations of electronic states of an exciton (X) and of a trion (X−) in a CdTe quantum well for magnetic fields B up to 150T. Using variational method we estimate the binding energy of the free X− in the singlet state (z component of angular momentum Lz=0), as well the binding energy of the free X− in the triplet state (Lz=−ℏ). Recent experiments have shown that even up to B=44T the binding energy of the negative trion in a singlet state is greater than in the triplet state. We show, however, that the binding energies of these two states should cross one another at very high magnetic fields. Specifically, using material parameters typical for these experiments, we show that the crossing of these levels will take place at field Bc≈65T. The binding energy of free X− in the triplet state exceeding that of the singlet state for fields B>Bc.
Quantifying transport within a porous medium over a hierarchy of length scales
Authors: Holland D. J. , Scheven U. M. , Middelberg A. P. J. , Gladden L. F.
Published: April 2006

Magnetic resonance techniques are used to probe transport within a porous medium over length scales of microns to centimeters. In particular, the apparent discrepancy between estimates of dispersion within porous media determined by pulsed field gradient magnetic resonance techniques and a conventional elution analysis is addressed. The model porous medium considered is a packed bed of height and internal diameter 22.5 and 16.8mm, respectively, packed with highly porous cross-linked dextran particles approximately 50μm in diameter. Experiments were performed for Peclet numbers in the range 1<Pe<31. First, a nonspatially resolved displacement encoding Alternating Pulsed Field Gradient Stimulated Echo Nuclear Magnetic Resonance (APGSTE NMR) measurement was used to yield estimates of bed porosity (0.898±0.004), mobile phase volume fraction (0.29±0.02), intraparticle diffusion coefficient [(2.8±0.2)×10−10m2s−1], and characteristic time, Te, for exchange between the intra- and interparticle pore space (∼300ms). The value of porosity was in excellent agreement with that obtained by elution analysis. However, values of the axial dispersion coefficient obtained using the two approaches did not agree well. For example, at Pe=1.1, the dispersion coefficients measured by APGTSE NMR and elution analysis were (1.6±0.1)×10−9m2s−1 and (1.8±0.2)×10−8m2s−1, respectively. These results suggest that whilst the micro-/mesolength scale properties of the porous medium are well characterized using the APGSTE NMR measurement, the technique is unable to probe the millimeter length scales in the bed over which heterogeneities in the flow may exist and therefore contribute significantly to the macroscopic dispersion characteristic of the bed, as determined by elution analysis. This is confirmed by demonstrating that the contribution of mechanical mixing to dispersion within the porous medium extends to the longest time scales studied (>6Te). To identify the dominant influences on the macroscopic dispersion characteristics of the porous medium, magnetic resonance flow velocity images within the packed bed were acquired. Numerical reconstructions of the residence time distribution of the fluid within the bed using these data yielded a value of the dispersion coefficient of (0.8±0.4)×10−8m2s−1, in far better agreement with the elution analysis, thereby demonstrating that it is the millimeter-scale heterogeneity in the flow field within the bed that is the dominant contribution to the macroscopic dispersion. Extension of the model to incorporate the effect of maldistribution of the input pulse further improves agreement with the elution analysis.
On the optical anisotropy of poly(ethylene terephthalate) and poly(ethylene naphthalate) polymeric films by spectroscopic ellipsometry from visible-far ultraviolet to infrared spectral regions
Authors: Laskarakis A. , Logothetidis S.
Published: April 2006

The optical anisotropy of biaxially stretched poly(ethylene terephthalate) and poly(ethylene naphthalate) films has been investigated, by visible-far ultraviolet to IR spectroscopic ellipsometry (SE). The consistency of the SE spectra analysis results between the two energy regions, in a full rotational scan of the angle θ between the plane of incidence and the stretching direction, justified the approximation of the films as uniaxial materials with their optic axis parallel to the surface.
Three-dimensional self-assembly of structures using the pressure due to a ferrofluid in a magnetic field gradient
Authors: Feinstein Efraim , Prentiss Mara
Published: April 2006

We developed an inexpensive and simple system for three-dimensional self-assembly of micron-sized nonmagnetic particles into millimeter-scale structures using the differential pressure exerted by ferrofluids in the presence of magnetic field gradients. We demonstrate it by assembling separate individual 5, 10, and 21μm diam polystyrene beads into millimeter-sized spherical and ellipsoidal structures. The system can also self-organize its smaller components by volume and provide compressive forces of hundreds of piconewtons on millimeter-scale structures. Extensions of this method have assembled multicellular systems.
Effect of poling on dielectric anomalies at phase transitions for lead magnesium niobate-lead titanate crystals in the morphotropic phase boundary region
Authors: Sehirlioglu Alp , Payne David A. , Han Pengdi
Published: April 2006

Dielectric measurements are reported as a function of temperature for phase transformations in the lead magnesium niobate-lead titanate system (PMN-PT). Data are given for single crystal specimens in the morphotropic phase boundary (MPB) region. Transition temperatures were determined from dielectric loss data. The characteristics for both poled and unpoled crystals are compared. Values of dielectric constant were found to increase after poling at room temperature, and an anomaly was induced at the lower-temperature transition. Details are reported for these properties depending upon composition within the MPB region. At room temperature, the poled crystals had high values for dielectric constant (e.g., 5000–15 000), piezoelectric coefficient (e.g., 1000–4000pC∕N), and electromechanical coupling factor (e.g., >0.9). The temperature dependence of dielectric properties is treated in terms of competing phases at transformations, with a change from continuous to discontinuous behavior with increasing PT content towards the MPB. The effect of poling on the induction of the tetragonal (T) phase, with a lowering of the low temperature to tetragonal (LT→T) phase transformation temperature, is discussed. Piezoelectric crystals with the highest depoling temperature were farthest away from the MPB in the MPB region. This information should be useful for the application of PMN-PT piezoelectric crystals.
Self-excited instability occurring during the nanoparticle formation in an Ar–SiH4 low pressure radio frequency plasma
Authors: Cavarroc M. , Jouanny M. C. , Radouane K. , Mikikian M. , Boufendi L.
Published: April 2006

An experimental investigation of an instability occurring during dust nanoparticle formation is presented in this paper. The present study has been performed in radio frequency low pressure plasma in an argon-silane mixture. The formation and growth of nanoparticles is followed, thanks to the analysis of the amplitude of the third harmonics (40.68MHz) of the discharge current and the self-bias voltage (Vdc). In some cases, at the end of the accumulation phase of the nanocrystallites an instability occurs. It seems to be an attachment induced ionization instability as observed in electronegative plasmas. A detailed study of the influence of different operating conditions (injected power, gas temperature, and silane flow rate) on this instability behavior and frequencies is presented. The paper concludes by examining a very particular case of the instability.
Sapphire: A kinking nonlinear elastic solid
Authors: Basu S. , Barsoum M. W. , Kalidindi S. R.
Published: April 2006

Kinking nonlinear elastic (KNE) solids are a recently identified large class of solids that deform fully reversibly by the formation of dislocation-based kink bands [Barsoum et al. Phys. Rev. Lett. 92, 255508 (2004)]. We further conjectured that a high c∕a ratio—that ensures that only basal slip is operative—is a sufficient condition for a solid to be KNE. The c∕a ratio of sapphire is 2.73 and thus, if our conjecture is correct, it should be a KNE solid. Herein by repeatedly loading—up to 30 times—the same location of sapphire single crystals of two orientations—A and C—with a 1μm radius spherical nanoindenter, followed by atomic force microscopy, we showed that sapphire is indeed a KNE solid. After pop-ins of the order of 100nm, the repeated loadings give rise to fully reversible, reproducible hysteresis loops wherein the energy dissipated per unit volume per cycle Wd is of the order of 0.5GJ∕m3. Wd is due to the back and fro motion of the dislocations making up the incipient kink bands that are fully reversible. The results presented here strongly suggest that—like in graphite and mica—kink bands play a more critical role in the room temperature constrained deformation of sapphire than had hitherto been appreciated. Our interpretation is also in agreement with, and can explain most, recent nanoindentation results on sapphire.
Weakly nonlinear shock propagation in slowly varying one-dimensional flows
Authors: Prasad Dilip
Published: April 2006

A theoretical model for the linear and nonlinear evolution of acoustic perturbations in nonuniform one-dimensional isentropic flows is developed. On the assumption that the length scale of the mean flow greatly exceeds that of the imposed disturbances, it is shown that the propagation of linear waves can be described in a closed form that is consistent with the conservation of acoustic power. This permits the application of a “nonlinearization” procedure (Landau [J. Phys. USSR 9, 496 (1945)],Whitham [Proc. R. Soc. London, Ser. A 201, 89 (1950)]), wherein the linear functional forms are assumed to hold for weakly nonlinear simple-wave disturbances, and the effects of amplitude on the wave speed are included to leading order. When nonlinear steepening results in the formation of shocks, they are fitted into the solution using the Rankine-Hugoniot relations. The method is applicable to arbitrary waveforms and, in the present study, the evolution of hump-like and periodic disturbances through nonuniform flows is examined. Focusing on the latter class of disturbances, the parametric range of applicability of the asymptotic theory is assessed by comparing the model predictions with numerical solutions of the Euler equations for different excitation frequencies and amplitudes. The usefulness of the present approach is illustrated by its application to periodic shocks propagating through accelerating and diffusive mean flows.
A multiscale model for dilute turbulent gas-particle flows based on the equilibration of energy concept
Authors: Xu Ying , Subramaniam Shankar
Published: April 2006

The objective of this study is to improve Eulerian-Eulerian models of particle-laden turbulent flow. We begin by understanding the behavior of two existing models—one proposed by Simonin [von Kármán Institute of Fluid Dynamics Lecture Series, 1996], and the other by Ahmadi [Int. J. Multiphase Flow 16, 323 (1990)]—in the limiting case of statistically homogeneous particle-laden turbulent flow. The decay of particle-phase and fluid-phase turbulent kinetic energy (TKE) is compared with direct numerical simulation results. Even this simple flow poses a significant challenge to current models, which have difficulty reproducing important physical phenomena such as the variation of turbulent kinetic energy decay with increasing particle Stokes number. The model for the interphase TKE transfer time scale is identified as one source of this difficulty. A new model for the interphase transfer time scale is proposed that accounts for the interaction of particles with a range of fluid turbulence scales. A new multiphase turbulence model—the equilibration of energy model (EEM)—is proposed, which incorporates this multiscale interphase transfer time scale. The model for Reynolds stress in both fluid and particle phases is derived in this work. The new EEM model is validated in decaying homogeneous particle-laden turbulence, and in particle-laden homogeneous shear flow. The particle and fluid TKE evolution predicted by the EEM model correctly reproduce the trends with important nondimensional parameters, such as particle Stokes number.
Response of unconfined vortex breakdown to axial pulsing
Authors: Khalil Sammy , Hourigan Kerry , Thompson Mark C.
Published: April 2006

The response of unconfined swirling jets undergoing vortex breakdown to axial pulsing is explored experimentally. In particular, the combination of particle visualization and particle image velocimetry has enabled the effect of axial pulsing on the development of shear-layer flow structures and vortex breakdown to be quantified, as a function of both pulsing frequency and amplitude. A range of Reynolds and swirl numbers is considered. A key result is that application of low-level forcing at the natural frequency intensifies the shear-layer vortices considerably and results in a substantial relative downstream shift of the mean breakdown position.
An electron counting modification to potentials for covalently bonded surfaces
Authors: Zhou X. W. , Murdick D. A. , Wadley H. N. G.
Published: April 2006

The surface structure of covalently bonded semiconductor materials undergoes reconstructions that are driven by electron redistribution between dangling and interatom bonds. Conventional interatomic potentials account for neither this electron redistribution nor its effects upon the atomic structure of surfaces. We have utilized an electron counting analysis to develop a surface interatomic potential that captures many of the effects of electron redistribution upon the surface structures of covalently bonded materials. The contributions from this potential decrease rapidly to zero beneath a surface. As a result, this surface potential can be added to many interatomic potentials for covalent materials without affecting its predictions of bulk properties such as cohesive energy, lattice parameters, and elastic constants. We demonstrate the approach by combining the surface potential with a recently proposed bond order potential and use it in a molecular statics simulation of the atomic reconstruction of a well studied (001) GaAs surface. Many of the experimentally observed surface reconstructions are well predicted by the surface modified potential.
Dielectric function and electric properties of germanium thin films prepared by gold mediated crystallization
Authors: Giangregorio Maria M. , Losurdo Maria , Ambrico Marianna , Capezzuto Pio , Bruno Giovanni , Tapfer Leander
Published: April 2006

Crystallization of hydrogenated amorphous germanium (a-Ge:H) thin films deposited by plasma enhanced chemical vapor deposition using the GeH4 and H2 precursors has been investigated. A comparative analysis of the kinetics of the thermal crystallization by annealing to 650°C and of the gold-mediated crystallization (Au-MMC) is carried out. The impact of the Au-MMC on the microcrystalline Ge film microstructure and electrical properties is discussed. The Au thin layer results in a more dense and ordered structure with lower roughness of the microcrystalline Ge films. In order to describe the Ge crystallization kinetics, the dielectric functions of a-Ge:H and microcrystalline germanium μc-Ge have also been determined by spectroscopic ellipsometry in the range of 0.75−6.0eV and parametrized using the Tauc-Lorentz dispersion equation.
Preparation and properties of the trivalent-ion doped tungsten bronze LaxWO3
Authors: Kasl C. , Hoch M. J. R.
Published: April 2006

The production and characterization of lanthanum-doped tungsten oxide, LaxWO3, sintered samples are presented for 0.05⩽x⩽0.23. The x-ray diffraction results show that the cubic perovskite phase is stable for x in the range of 0.086–0.21. Outside of this range additional phases are present. Resistivity measurements made between 64 and 300K provide information on the electronic properties. Room temperature Hall effect results show that the trivalent La ions donate their valence electrons into the WO3 conduction band. The samples have been prepared for an investigation of the metal-insulator transition in this tungsten bronze system.
Effects of dc-dielectrophoretic force on particle trajectories in microchannels
Authors: Hyoung Kang Kwan , Xuan Xiangchun , Kang Yuejun , Li Dongqing
Published: April 2006

A method of controlling the particle trajectory in a microchannel is demonstrated. The method utilizes the dc-dielectrophoretic (dc-DEP) force created around an insulating hurdle in a microchannel under an applied dc electric field. This method does not require a complicated electrode array which is commonly used in the conventional ac-DEP system. The “proof-of-principle” experiments were carried out using a straight microchannel with a rectangle-shaped hurdle in the middle. The experiments showed that the trajectories of the micron-sized particles can be controlled by the DEP force under electric-field strengths of 5–20kV∕m. To compare with the experimental results, the particle motion was simulated using the Lagrangian tracking method, taking into consideration of the electrophoretic force, the dielectrophoretic force, and the dielectric interaction between the particle and the channel wall. The numerical simulation based on the finite-element method showed a reasonable agreement with the experimental data.
Co
2
+
Ti
4
+
substituted Z-type barium ferrite with enhanced imaginary permeability and resonance frequency
Authors: Li Z. W. , Guoqing Lin , Chen Linfeng , Yuping Wu , Ong C. K.
Published: April 2006

Co 2 + Ti 4 + substitution for Fe3+ in Co2Z (Ba3Co2Fe24O41) has been prepared. The crystal structure and static and high-frequency magnetic properties have been studied for Ba3Co2+xTixFe24−2xO41 and the corresponding ferrite/polymer composites. As compared to the general Co2Z ferrite/polymer composite, the CoTi substituted ferrite/polymer composite with x=1.0 has a high natural resonance frequency (4.5GHz), due to its large out-of-plane anisotropy fields Hθ. Furthermore, the maximum imaginary permeability μmax″ is increased by about 50%. The increase is attributed to a decreased damping coefficient, based on the curve-fitted results to the complex permeability spectra. The composites are good electromagnetic attenuation materials with low reflectivity and broad bandwidth at microwave frequencies.
Structural dependence of some physical properties of the Ni2MnGe Heusler alloy films
Authors: Oksenenko V. A. , Trofimova L. N. , Petrov Yu. N. , Kudryavtsev Y. V. , Dubowik J. , Lee Y. P.
Published: April 2006

Structural ordering of the flash-evaporated Ni2MnGe Heusler alloy films has been studied by magneto-optical (MO), optical, and magnetic methods. The properties of the films have been compared with the corresponding properties of the bulk Ni2MnGe samples with L21 and tetragonally distorted L21 structures, respectively. Depending on a substrate temperature (Ts) and/or postdeposition annealing temperature (Ta) the films have been amorphous (Ts=150K), disordered (Ta<650K), or well ordered with L21 structure (Ts>720K). The ordered films have exhibited optical and MO properties close to those of the bulk Ni2MnGe, while the amorphous films showed no ferromagnetic ordering. The optical and MO properties of Ni2MnGe with L21 structure have been proved almost the same as those of Ni2MnGa and Ni2MnIn. Changes in the optical properties of Ni2MnGe alloy films have been discussed in terms of Ni2MnGe band structure.
Solid-state formation of titanium carbide and molybdenum carbide as contacts for carbon-containing semiconductors
Authors: Leroy W. P. , Detavernier C. , Van Meirhaeghe R. L. , Kellock A. J. , Lavoie C.
Published: April 2006

Metal carbides are good candidates to contact carbon-based semiconductors (SiC, diamond, and carbon nanotubes). Here, we report on an in situ study of carbide formation during the solid-state reaction between thin Ti or Mo films and C substrates. Titanium carbide (TiC) was previously reported as a contact material to diamond and carbon nanotubes. However, the present study shows two disadvantages for the solid-state reaction of Ti and C. First, because Ti reacts readily with oxygen, a capping layer should be included to enable carbide formation. Second, the TiC phase can exist over a wide range of composition (about 10%, i.e., from Ti0.5C0.5 to Ti0.6C0.4), leading to significant variations in the properties of the material formed. The study of the Mo–C system suggests that molybdenum carbide (Mo2C) is a promising alternative, since the phase shows a lower resistivity (about 45% lower than for TiC), the carbide forms below 900°C, and its formation is less sensitive to oxidation as compared with the Ti–C system. The measured resistivity for Mo2C is ρ=59μΩcm, and from kinetic studies an activation energy for Mo2C formation of Ea=3.15±0.15eV was obtained.
Low temperature melting of copper nanorod arrays
Authors: Karabacak Tansel , DeLuca James S. , Wang Pei-I , Ten Eyck Gregory A. , Ye Dexian , Wang Gwo-Ching , Lu Toh-Ming
Published: April 2006

We report the melting of nanorod arrays of copper at temperatures much lower than its bulk melting point (1083°C). The Cu nanorods were produced by an oblique angle sputter deposition technique through a physical self-assembly mechanism due to the shadowing effect. The as-deposited nanorods were ∼2300nm in length, ∼100nm in diameter, and separated from each other with gaps varying between ∼10 and ∼30nm. The melting process was investigated through the analysis of scanning electron microscopy, transmission electron microscopy, and x-ray diffraction measurements. Start of premelting (or surface melting) has been observed to occur at annealing temperature ∼400°C under vacuum (10−6Torr). As the temperature was raised the arrays of Cu nanorods started to coalesce and formed a dense continuous film with a (111) texture at ∼550°C. The results of this work may be useful for low temperature soldering applications.
A portable thermoelectric-power-generating module composed of oxide devices
Authors: Funahashi R. , Mikami M. , Mihara T. , Urata S. , Ando N.
Published: April 2006

High power density is a strong point of thermoelectric generation. Exploitation of this salient characteristic would make thermoelectric modules promising candidates for mobile power applications. Here we show how power can be generated using a small thermoelectric module composed of 140 pairs of oxide thermoelectric unicouples. The module weighs 19.8g and its dimensions are 53mm long, 32mm wide, and 5.0mm thick. The hot-pressed thermoelectric oxide bulk materials used were connected with a Ag paste, incorporating oxide powder, and Ag electrodes. The module’s open circuit voltage increases with increasing hot-side temperature (TH) and reaches 4.5V at a TH of 1072K in air. No deterioration in output power was seen when power generation was carried out ten times at a TH of 723K with intermediate cooling to room temperature. The module was successfully used to charge a lithium-ion battery in a mobile phone.
Optical and electrical properties of single-ion transport light-emitting electrochemical cells
Authors: Kong F. , Wu X. L. , Zheng Y. , Ou C. G. , Yuan R. K. , Yang C. Z. , Chu Paul K.
Published: April 2006

We present electrical and optical characteristics of single-ion transport light-emitting electrochemical cells (SLECs) based on a blend film of poly(p-phenylene vinylene) derivative with ion-conducting side groups and ionized polyurethane-poly(ethylene glycol). The turn-on voltage of the SLECs is lower than 4V. The response time of the SLECs is less than 10ms, which is approximately two orders of magnitude faster than that of conventional bi-ionic transport light-emitting electrochemical cells. The quick response can be explained on the basis of both good compatibility between the two polymers and weak scattering behavior of the counter ions. In addition, according to the transient current and electroluminescence characteristics, we reveal that the electrical field is close to zero in the inner of the SLEC under the condition of steady state.
Characterization of a channel spark discharge and the generated electron beam
Authors: Krasik Ya. E. , Gleizer S. , Chirko K. , Gleizer J. Z. , Felsteiner J. , Bernshtam V. , Matacotta F. C.
Published: April 2006

We report on an experimental study of a channel spark discharge (CSD) and the generated electron beam. The CSD was operated at a discharge voltage Ud⩽30kV and a discharge current Id⩽3.5kA. The CSD system consists of a glass tube placed between a hollow cathode and a grounded anode electrode. The parameters of the CSD operation, the potential distribution along the glass tube, and the generated beam were studied by electrical, optical, and spectroscopic diagnostics in the Ar gas pressure range of P=0.005–2Pa. At P⩾0.5Pa, electrons with energy ∼eUd appeared prior to the start of the main CSD with a current amplitude ⩽10−4Id. These high-energy electrons are responsible for the initiation of the CSD inside the glass tube. The generation of the electron beam which was composed of low-energy electrons with a current amplitude up to 3kA occurred during a fast fall in the discharge voltage. Decreasing the Ar gas pressure below 0.1Pa allows one to increase significantly the beam duration and the part of the high-energy electrons in the beam, and to increase the efficiency of the electron beam generation up to 75%. The high-current CSD inside the glass tube is accompanied by the formation of a plasma consisting of protons, ArII, ArIII, CII-IV, and OII-IV ions. The plasma electron density and temperature were found to be in the ranges of 1015–1016cm−3 and 10-15eV, respectively.
Cell pattern of thermal convection induced by internal heating
Authors: Ichikawa Hiroki , Kurita Kei , Yamagishi Yasuko , Yanagisawa Takatoshi
Published: April 2006

A numerical simulation of thermal convection induced by internal heating in a three-dimensional thin flat box is performed. The upper boundary is held at a constant temperature and the lower boundary is thermally insulated with a uniform internal heating in a fluid layer. We observe hexagonal cells with downwelling at their center in moderate Rayleigh number (RaH≲30RaHc). As the Rayleigh number increases, the cell size increases. We also observe a pattern transition: sheet-like downwellings gradually develop around the columnar downwelling with increasing Rayleigh number. At ∼30RaHc, hexagonal cells with sheet downwelling at their center (spoke pattern) is observed. These sheets can be seen only near the upper boundary. At 30RaHc∼70RaHc, the cells experience successive transitions to the pattern which are dominated by sinking sheet.
Laser oscillation from hexagonal crystals of a thiophene∕phenylene co-oligomer
Authors: Shimizu Kouki , Mori Yuji , Hotta Shu
Published: April 2006

The characteristics of the unusually narrowed emission from hexagon BP1T crystals were investigated by photopumping them with a line laser beam. The wavelengths and strengths of the emission lines sensitively depended on the position and the angle of the crystal. The threshold of the excitation fluence decreases inversely proportional to the crystal length. We have concluded that the unusually narrowed emission is the laser oscillation by laser cavity resonance within a waveguide. The shape and size of the excitation laser beam play an important role in the laser oscillation.
Desiccation kinetics of biopreservation solutions in microchannels
Authors: Aksan Alptekin , Irimia Daniel , He Xiaoming , Toner Mehmet
Published: April 2006

A microfluidic device was utilized to measure the viscosity gradients formed in carbohydrate solutions of biological significance during desiccation and skin formation. A complementary numerical model employed the free volume theory to predict the concentration-dependent diffusion coefficients and viscosity gradients in concentrated solutions. It was established that the glassy skin formation at the gas-liquid interface played a key role in water entrapment and the formation and persistence of very steep concentration and viscosity gradients in the desiccating solutions. The results of this study highlighted an important phenomenon that should be accounted for during isothermal drying of glass-forming solutions: solutions with high glass transition temperatures, inevitably, dry heterogeneously. In the final product, there are significant spatial variations in water and solute content affecting the storage stability.