bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of ultrahigh density and ultrasmall coherent β-FeSi2 nanodots on Si (111) substrates using Si and Fe codeposition method
Authors: Nakamura Yoshiaki , Nagadomi Yasushi , Cho Sung-Pyo , Tanaka Nobuo , Ichikawa Masakazu
Published: September 2006

We studied the formation of β-FeSi2 nanodots by codeposition at disilicide stoichiometric deposition rates of Fe and Si on Si (111) substrates covered with ultrathin SiO2 films. Hemispherical β-FeSi2 nanodots with an ultrahigh density (>1012cm−2) and with a narrow size distribution at the average size of ∼5nm diameter were epitaxially grown by the codeposition at 500°C on ultrathin SiO2 films with predeposited Si. High-resolution transmission electron microscope observations showed that the formed nanodots were strained with a main configuration of β-FeSi2 (110)∕Si (111). We propose the formation mechanism that a reaction between the SiO2 films and deposited Si atoms formed voids in the SiO2 films to work as nucleation sites, followed by direct β-FeSi2 formation from Fe and Si without passing through other iron silicide phases.
Tuning donor-acceptor and free-bound transitions in CuInSe2/indium tin oxide heterostructure
Authors: Jayakrishnan R. , Deepa K. G. , Sudha Kartha C. , Vijayakumar K. P.
Published: September 2006

Infrared photoluminescence (PL) emissions centered at 1550 and 1564nm were obtained at 15K from CuInSe2/indium tin oxide heterostructure. PL studies revealed transitions between donor-acceptor pairs and band to acceptor to be the origin of the 1550 and 1564nm emissions, respectively. At low temperatures (15–60K) the 1550nm emission was prominent. Upon increasing the excitation intensity at low temperature the 1564nm emission gained prominence. Increase in temperature also caused the 1564nm emission to gain significance. The possibility of selecting one of the two emissions by adjusting temperature or excitation intensity is discussed.
Stokes experiment in a liquid foam
Authors: Cantat I. , Pitois O.
Published: September 2006

The paper reports on the quasistatic steady flow of a dry liquid foam around a fixed spherical bead, a few times larger than the typical bubble size. The force exerted on the bead is recorded with a precision and a time resolution large enough to show the succession of elastic loading of the foam, separated by sudden force drops. The foam structure is observed by direct light transmission, synchronized with the force measurement, thus allowing us to correlate the plastic events with the force variations. Scaling laws for the force signal as a function of the bubble size are detailed and interpreted with a simple elasto-plastic model. The spatial distribution of the plasticity is strongly localized in the first bubble layers around the bead and the average size of the bubble rearrangements increases with the corresponding force jump amplitude.
Linear stability of the three-dimensional lid-driven cavity
Authors: Non Etienne , Pierre Roger , Gervais Jean-Jacques
Published: September 2006

In this paper we perform a numerical study of the hydrodynamic stability of the infinitely long lid-driven cavity for wavenumbers in the range [0,25]. Using a piecewise linear continuation approach, we have computed for this range of wavenumbers a neutral curve that is longer than those previously computed. We also give some examples of the rich dynamics of the flow beyond the first bifurcation.
Effect of Cr addition on the glass-forming ability, magnetic properties, and corrosion resistance in FeMoGaPCBSi bulk glassy alloys
Authors: Shen Baolong , Akiba Masahiro , Inoue Akihisa
Published: September 2006

The effect of Cr addition on the glass-forming ability (GFA), the magnetic properties, and corrosion resistance in Fe–Mo–Ga–P–C–B–Si glassy alloys was investigated. In addition to a slight increase of supercooled liquid region from 50to55K, the substitution of a small amount of Fe with Cr was found to be effective for approaching alloy to a eutectic point, resulting in an increase in GFA. By copper mold casting, bulk glassy alloy rods with diameters up to 3mm were produced. These glassy alloys exhibit a rather high saturation magnetization of 0.84–1.11T with good soft-magnetic properties, i.e., low coercive force of 2.3–2.9A∕m, and high effective permeability of 13 360–15 960 at 1kHz under a field of 1A∕m. The passive current density of the glassy alloy rod in 3mass% NaCl solution decreased significantly from 1×102to3×10−1A∕m2 with an increase in Cr content, indicating that the addition of Cr is effective in enhancing the corrosion resistance.
Evolution of catalyst particle size during carbon single walled nanotube growth and its effect on the tube characteristics
Authors: Harutyunyan Avetik R. , Tokune Toshio , Mora Elena , Yoo Jung-Woo , Epstein Arthur J.
Published: September 2006

A series of Fe catalysts, with different mean diameters, supported on alumina with different molar ratios, was studied before and after carbon single walled nanotubes growth using magnetic measurements and Raman scattering techniques (laser excitation wavelengths from 1.17to2.54eV) to follow changes on catalyst particle size and composition, as well as the relationship between particle size and diameter of nanotubes grown. In all cases, an increase and redistribution of the particle size after the growth was concluded based on the blocking temperature values and Langevin function analysis. This is explained in terms of agglomeration of particles due to carbon-induced liquefaction accompanied with an increase in the catalyst mobility. For large particles no direct correlation between the catalyst size and the nanotube diameters was observed.
Narrow bandpass optical filters fabricated with one-dimensionally periodic inhomogeneous thin films
Authors: Hawkeye Matthew M. , Brett Michael J.
Published: September 2006

Thin films with sinusoidally varying refractive index profiles display photonic band gap effects. Intentional deviations from the periodic index profile can be used to tailor the optical properties of the resulting thin film. We present experimental characterization of TiO2 films with periodic index profiles fabricated using a deposition technique known as glancing angle deposition (GLAD). The resulting porous thin films have a microstructure consisting of vertically aligned columns. Sinusoidal porosity gradients, and therefore sinusoidal index profiles, can be introduced in the direction of the substrate normal by fabricating columns with a periodically varying diameter. Local modifications of the index profile are achieved by inserting thin layers of constant porosity into the center of the film, or by discontinuously changing the phase of the sinusoidal gradient. The introduction of these structural defects creates a narrow optical passband inside the larger band gap, and we demonstrate how the properties of this passband can be controlled through modification of the defect parameters. The magnitude of the phase shift constituting the defect is shown to control the location of the resulting passband. By inserting a layer with in-plane birefringence, we show that it is possible to create two separate passbands, one for each polarization, within a single stop band. The results illustrate how nanoscale porosity engineering using GLAD is a precise technique for fabricating one-dimensionally periodic films with a variety of optical characteristics.
Sensitivity enhancement of AlGaAs∕InGaAs∕GaAs quantum well-based Hall device
Authors: Sghaier H. , Bouzaiene L. , Sfaxi L. , Maaref H.
Published: September 2006

We have investigated the consequences of a modified design of modulation doping pseudomorphic AlGaAs∕InGaAs∕GaAs and we propose a quantum well structure for a Hall device with the goal of improving its performances. From self-consistent calculations we find that the electron concentration ns in the interface region is increased. This implies that one can have a wider spacer layer and still have the same ns with the result that the mobility is improved. This result should be valuable for many types of devices. We specifically consider Hall sensors, where it is desirable to have a low electron concentration and a high mobility.
Electric properties of BiFeO3 films deposited on LaNiO3 by sol-gel process
Authors: Liu Zuli , Liu Hongri , Du Guihuan , Zhang Jian , Yao Kailun
Published: September 2006

Sol-gel process was adopted to prepare BiFeO3 films. Pure phase BiFeO3 films were deposited on LaNiO3 coated Si (111) substrates at various annealing temperatures of 450–600°C. The films annealed at 450–600°C are (110) and (1̱10) biaxis preferential oriented. Below 550°C, the remnant polarization increases with the annealed temperature. The film annealed at 550°C has the largest double remnant polarization of 12.8μC∕cm2. For the film annealed at 600°C, small double remnant polarization of 2Pr=4.6μC∕cm2 was observed for its low breakdown electric field. Besides, the electric property is enhanced with the annealing temperature below 550°C and it is deteriorated for the film annealed at 600°C. Large dielectric constant and low leakage conduction were obtained by the improvement of preparation technology.
Si-doped GaN∕AlN quantum dot superlattices for optoelectronics at telecommunication wavelengths
Authors: Guillot F. , Bellet-Amalric E. , Monroy E. , Tchernycheva M. , Nevou L. , Doyennette L. , Julien F. H. , Dang Le Si , Remmele T. , Albrecht M. , Shibata T. , Tanaka M.
Published: September 2006

We report on the controlled growth by molecular beam epitaxy of 20-period Si-doped GaN∕AlN quantum dot (QD) superlattices, in order to tailor their intraband absorption within the 1.3–1.55μm telecommunication spectral range. The QD size can be tuned by modifying the amount of GaN in the QDs, the growth temperature, or the growth interruption time (Ostwald ripening). By adjusting the growth conditions, QDs with height (diameter) within the range of 1–1.5nm (10–40nm), and density between 1011 and 1012cm−2 can be synthesized, fully strained on the AlN pseudosubstrate. To populate the first electronic level, silicon can be incorporated into the QDs without significant perturbation of the QD morphology. All the samples exhibit strong p-polarized intraband absorption at room temperature. The broadening of the absorption peak remains below 150meV and can be as small as ∼80meV. This absorption line is attributed to transition from the s ground level of the QD to the first excited level along the growth axis, pz. The peak energies of both photoluminescence emission and intraband absorption are consistent with the QD structural characteristics, and with their evolution by changing the growth conditions. Tuning of the intraband absorption from 0.740eV (1.68μm)to0.896eV (1.38μm) is demonstrated. Finally, we show that the AlN buffer layer can be replaced by a conductive AlxGa1−xN (x=0.35 and 0.6) ternary alloy without significant modification of the intraband properties of the QD stack.
Classical coupled oscillators model of the rational harmonic mode locked laser
Authors: Li Wei , Chen Gen-Xiang
Published: September 2006

A coupled modes method is proposed to simulate the active mode locked laser. In this method, the electric fields of optical modes in the laser cavity are treated as free classical oscillators. The optical modulator provides the coupling among them. It is found that the eigenvalue of this coupled system is corresponding to the threshold optical gain, and the eigenfunction is corresponding to the optical field of each mode. The simulation results agree with the Master equation and experiments. Especially the model can be applied to the rational harmonic mode locking case by introducing the concept of ghost modes. The multitrip photons form a set of ghost modes or very weak oscillators. These ghost oscillators play the crucial rule working as the bridge to transfer energy and couple the real cavity modes/oscillators together. The model demonstrates both time domain pulse repetition rate multiplication and the frequency domain mode distribution. The proposed method will help understand the physics of rational harmonic mode locking mechanism and assist the design of devices such as optical pulse generator and multiwavelength laser. Therefore it should have great application in the optical signal generation and processing.
Formation mechanism of silicon carbide nanotubes with special morphology
Authors: Pei L. Z. , Tang Y. H. , Zhao X. Q. , Chen Y. W. , Guo C.
Published: September 2006

SiC nanotubes have been synthesized under hydrothermal conditions. Research results show that SiC nanotubes consist of hollow inner pore, crystalline SiC wall layers, and thin amorphous silica sheath. Besides most abundant normal nanotubes, bamboo-shaped SiC nanotubes and elliptically hollow SiC nanospheres were also observed. The different morphologies of the SiC nanotubes are closely relative to the diameter of the SiC nanotubes. The confinement effect of other SiC nanotubes may play an important effect retarding the growth of SiC nanotubes to form the hollow SiC nanospheres. Defect inducing growth is proposed to explain the formation of bamboo-shaped SiC nanotubes.
The electrical conductivity of microcellular metals
Authors: Goodall Russell , Weber Ludger , Mortensen Andreas
Published: September 2006

The electrical conductivity of metallic foams varies strongly with porosity. Data are presented for replicated open-pore microcellular metal over a wide range of relative density. These data, together with data from the literature for different foam structures, are compared with previously suggested models for the conductivity of porous materials. We show that clear differences exist between the behavior of foams according to their different structural types and that simple models exist to capture these differences.
A quasi-two-dimensional depth-dependent mobility model suitable for device simulation for Coulombic scattering due to interface trapped charges
Authors: Potbhare Siddharth , Goldsman Neil , Pennington Gary , Lelis Aivars , McGarrity James M.
Published: September 2006

The silicon carbide (SiC)–silicon dioxide (SiO2) interface in SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) has shown the presence of a very large number of trap states. These traps become filled during inversion causing a lowering of conduction charge in the inversion layer, and increases Coulombic scattering of mobile charges. Owing to the large number of occupied interface traps, Coulomb interaction is likely to be an important scattering mechanism for SiC MOSFET device operation, resulting in very low surface mobilities. We have developed a first principles physics-based Coulomb scattering mobility model to understand this phenomenon and to study its effect on mobility in SiC devices. This type of Coulombic scattering is a quasi-two-dimensional phenomenon. Mobile charges located closer to the interface are scattered at a higher rate than those located far away from the interface. Screening of the traps and fixed oxide charges by the inversion layer mobile charges causes a decrease in the scattering rate. Also, at higher temperatures, due to a reduction in occupied trap density, and increasing energy of mobile charges, Coulombic scattering is greatly reduced. Our mobility model incorporates and accounts for all these effects. We have implemented this physics-based Coulomb scattering mobility model into a device simulator and have obtained agreement with experimental current-voltage characteristics.
Pressure calibration of diamond anvil Raman gauge to 310GPa
Authors: Akahama Yuichi , Kawamura Haruki
Published: September 2006

In order to develop an optical method for pressure determination in the multimegabar region, the first-order Raman spectra of diamond anvils were investigated at pressures up to 310GPa. The high-frequency edge of the Raman band, which corresponds to the Raman shift of the anvil culet due to the normal stress, was calibrated against the sample pressure derived from the equation of state of Pt. The obtained pressure dependence of the edge frequency demonstrates the reliability of this diamond anvil Raman gauge. Up to the maximum pressure of this study, the relation between Raman frequency and normal stress at the diamond anvil culet is formally similar to the equation of state of a hydrostatically compressed isotropic elastic body having a bulk modulus of K0=547(11)GPa and a pressure derivative of the bulk modulus K0′=3.75(20).
Second harmonic generation in random composites of particles with core-shell structure
Authors: Xu C. , Hui P. M.
Published: September 2006

We study the effective second harmonic generation (SHG) coefficient in a random composite consisting of particles with a core-shell structure embedded in a linear dielectric host. The material making up the core of the particles is assumed to be nonconducting, but with a nonlinear susceptibility for SHG. The coating material is assumed to be linear and metallic. An expression for the effective SHG coefficient is obtained, in terms of various local field factors. The effective SHG response per unit volume of nonlinear material is found to be greatly enhanced at certain frequencies. For coated particles, the core-shell structure allows for tuning of the resonance through the choice of material parameters and∕or the ratio of the core to shell volume fraction.
Coupling of a vacuum-ultraviolet-radiation source to a processing system
Authors: Chatterton J. D. , Upadhyaya G. S. , Shohet J. L. , Lauer J. L. , Bathke R. D. , Kukkady K.
Published: September 2006

A hollow capillary array is examined as a coupling window between an electron cyclotron resonance plasma vacuum ultraviolet (vuv) source and a separate processing chamber. The transmission of vuv through the capillary array as a function of wavelength is measured and shown to agree with theoretical calculations. A silicon wafer with a dielectric surface is then placed in the processing chamber and exposed to vuv, both with and without the capillary array. A Kelvin probe is used to measure the surface charge induced on the wafer by photoemission in both cases, which confirms the previously measured transmission values. The results show that a capillary array can efficiently couple vuv radiation from a source to a processing chamber without significant modification in the spectrum and its resulting effects on a material.
High resolution x-ray diffraction analysis of InGaAs∕InP superlattices
Authors: Cornet D. M. , LaPierre R. R. , Comedi D. , Pusep Y. A.
Published: September 2006

The interfacial properties of lattice-matched InGaAs∕InP superlattice (SL) structures grown by gas source molecular beam epitaxy were investigated by high resolution x-ray diffraction (HRXRD). SLs with various periods were grown to determine the contributions of the interface layers to the structural properties of the SLs. The HRXRD curves exhibited a number of features indicative of interfacial layers, including weak even-order satellite peaks, and a zero-order diffraction peak that shifted toward lower diffraction angles with decreasing SL period. A detailed structural model is proposed to explain these observations, consisting of strained InAsP and InGaAsP monolayers due to the group-V gas switching and atomic exchange at the SL interfaces.
Incident light polarization dependence of terahertz emission spectrum of crystalline 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate
Authors: Kuroyanagi Kazuyoshi , Yanagi Kazuhiro , Sugita Atsushi , Hashimoto Hideki , Takahashi Hironori , Aoshima Shin-ichiro , Tsuchiya Yutaka
Published: September 2006

In this work we have measured the dependence of terahertz emission via optical rectification from the 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate crystal as a function of incident light polarization. Many structures, which varied depending on the polarization of the incident pulse, were observed in the frequency spectra. The observed structures can be explained by incorporating the refractive index dispersion, the response function, and the birefringence properties of the crystal into the mathematical model used to reconstruct the experimental data. Our results show that careful attention must be paid to these three properties when enhancing nonlinear susceptibility during the fabrication of efficient terahertz emitters.
Spark light radiation coupled with the field electron emission from carbon nanotube forests
Authors: Zakhidov Al. A. , Nanjundaswamy R. , Zhang M. , Lee S. B. , Obraztsov A. N. , Cunningham A. , Zakhidov A. A.
Published: September 2006

The phenomenon of field emission accompanied by light emission (LE) from oriented multiwalled carbon nanotube arrays was investigated. Transient (i.e., sparklike) in nature, the LE spectral profile consisted atomic carbon and iron transitions superimposed on a visible and near infrared continuum. The exact blend of line and continuum contributions varied with the magnitude and nature [direct current (dc) or pulsed regime] of the applied voltage. Adopting a blackbody description of the continuum radiation yielded a local temperature between 2500K for pulsed excitation and 3100K for dc voltage excitation. A mechanism involving the evaporation of Fe catalyst clusters is proposed to explain the differences in the emission profiles and inferred blackbody temperatures for the two voltage regimes.
Scouring of granular beds by jet-driven axisymmetric turbulent cauldrons
Authors: Bombardelli Fabián A. , Gioia G.
Published: September 2006

We study a sustained, jet-driven, axisymmetric turbulent cauldron that scours a pothole in a cohesionless granular bed. We focus on the energetics of the turbulent cauldron and use dimensional analysis and similarity methods to derive (up to a multiplicative constant) a formula for the equilibrium depth of the pothole. To that end, we assume that the power of the jet is stationary and that under equilibrium conditions no air or granular material from the bed is entrained in the cauldron. The resulting formula contains a single similarity exponent, which we show can be determined via the phenomenological theory of turbulence. Our method of analysis may prove useful in developing a theoretical understanding of mine burial, bridge pier-induced erosion, and other applications in which a localized turbulent flow interacts with a granular bed.
Optical-fiber interferometer for velocity measurements with picosecond resolution
Authors: Weng Jidong , Tan Hua , Wang Xiang , Ma Yun , Hu Shaolou , Wang Xiaosong
Published: September 2006

The conventional Doppler laser-interference velocimeters are made up of traditional optical elements such as lenses and mirrors and will generally restrict its applications in multipoint velocity measurements. By transfering the light from multimode optical fiber to single-mode optical fiber and using the currently available conventional telecommunications elements, the authors have constructed a velocimeter called all-fiber displacement interferometer system for any reflector. The unique interferometer system is only made up of fibers or fiber-coupled components. The viability of this technique is demonstrated by measuring the velocity of an interface moving at velocity of 2133m∕s with 50ps time resolution. In addition, the concept of optical-fiber mode conversion would provide a way to develop various optical-fiber sensors.
Numerical and experimental characterization of 4H-silicon carbide lateral metal-oxide-semiconductor field-effect transistor
Authors: Potbhare Siddharth , Goldsman Neil , Pennington Gary , Lelis Aivars , McGarrity James M.
Published: September 2006

Combined simulation and experimental analyses are performed to characterize the 4H-silicon carbide (SiC) lateral metal-oxide-semiconductor field-effect transistor (MOSFET). Using a quasi-two-dimensional depth dependent Coulomb mobility model for scattering due to interface and oxide charge, along with existing models for other scattering mechanisms, and an in-house drift diffusion device simulator tailored for SiC MOSFETs, we have extracted values for interface trap density of states for 4H-SiC MOSFETs. Characterization shows that the interface trapped charge in 4H-SiC MOSFETs is responsible for mobility degradation and reduction in mobile inversion charge, and therefore reduced current. Its effect on mobility degradation decreases at higher gate voltages due to increased screening. Our results show that at high gate voltages, surface roughness plays the major role in surface mobility degradation in 4H-SiC MOSFETs. Results indicate that due to high Coulomb scattering near the interface, current density is maximum a few nanometers away from the surface. The model indicates overall mobility values of approximately 20cm2∕Vs at the interface, and increasing to approximately 250cm2∕Vs near the bottom of the inversion layer. Simulations predict that tenfold reduction in interface and fixed oxide charge density would give rise to very favorable device characteristics.
Transition radiation in metal-metal multilayer nanostructures as a medical source of hard x-ray radiation
Authors: Pokrovsky A. L. , Kaplan A. E. , Shkolnikov P. L.
Published: September 2006

We show that a periodic metal-metal multilayer nanostructure can serve as an efficient source of hard x-ray transition radiation. Our research effort is aimed at developing an x-ray source for medical applications, which is based on using low-energy relativistic electrons. The approach toward choosing radiator-spacer couples for the generation of hard x-ray resonant transition radiation by few-MeV electrons traversing solid multilayer structures for the energies of interest to medicine (30–50keV) changes dramatically compared with that for soft x-ray radiation. We show that one of the main factors in achieving the required resonant line is the absence of the contrast of the refractive indices between the spacer and the radiator at the far wings of the radiation line; for that purpose, the optimal spacer, as a rule, should have a higher atomic number than the radiator. Having experimental goals in mind, we have considered also the unwanted effects due to bremsstrahlung radiation, absorption and scattering of radiated photons, detector-related issues, and inhibited coherence of transition radiation due to random deviation of spacing between the layers. Choosing as a model example a Mo–Ag radiator-spacer pair of materials, we demonstrate that the x-ray transition radiation line can be well resolved with the use of spatial and frequency filtering.
Nature of the hemodynamic forces exerted on vascular endothelial cells or leukocytes adhering to the surface of blood vessels
Authors: Wang Yechun , Dimitrakopoulos P.
Published: September 2006

The hemodynamic forces on endothelial cells or leukocytes attached to the inner surface of blood vessels affect the physiological behavior of these systems via mechano-transduction or receptor-ligand binding. The present study investigates the relative importance and nature of the two components of the hemodynamic force, i.e., the shear and normal force, on a cell and its vicinity. We consider a wide range of blood vessels (from capillaries to arteries) and cell’s spreading angles. Based on computational investigation and analysis, our study demonstrates that the normal force contributes significantly to the total force on the cell; the influence is much more pronounced in small vessels. We also show that the spreading of the cell on the vessel surface should not be discounted, especially in small vessels. Our results are also applicable to the fluid forces on other protuberances of biological nature attached to vascular vessels (e.g., cancer cells and biofilm) as well as to the fluid forces over protuberances in microfluidics and porous media.
Device for measuring water content of wood using piezoelectric elements
Authors: Taniwaki Mitsuru , Sakurai Naoki
Published: September 2006

A device for measuring water content of wood was developed. The measurement method is based on a vibration technique. The device consists of two piezoelectric elements: one for exciting a wood sample and another for receiving the resultant vibration. Use of piezoelectric elements allows the design of a smaller and more cost-effective device than the previously tested system using a laser Doppler vibrometer and a vibration shaker. Two cylindrical wood samples of Cercidiphyllum japonicum with different water contents were used for testing. Linear relationships were obtained between the water content of wood samples and the inverse of their resonance frequencies.
Reconstruction of quasimonochromatic images for multispectral x-ray imaging with a pinhole array and a flat Bragg mirror
Authors: Izumi N. , Barbee T. W. , Koch J. A. , Mancini R. C. , Welser L. A.
Published: September 2006

We have developed a software package for reconstruction of quasimonochromatic images from a multiple monochromatic x-ray imager for inertial confinement fusion implosions. The instrument consists of a pinhole array, a multilayer Bragg mirror, and an image detector. The pinhole array projects hundreds of images onto the detector after reflection off the multilayer Bragg mirror, which introduces spectral dispersion along the reflection axis. The quasimonochromatic images of line emissions and continuum emissions can be used for measurement of temperature and density maps of implosion plasmas. In this article, we describe a computer-aided processing technique for systematic reconstruction of quasimonochromatic images from raw data. This technique provides flexible spectral bandwidth selection and allows systematic subtraction of continuum emission from line emission images.
Absolute measurement of the reference surface profile of a phase shifting interferometer
Authors: Lim Jun , Rah Seungyu
Published: September 2006

To absolutely determine the roughness of a highly polished surface, the profile of the reference surface must be known accurately. In this study, we report on the measurement of a liquid surface layer to precisely determine the surface profile of the reference surface. This method uses the fact that time averaging of the sinusoidal capillary waves of the liquid surface makes a perfectly smooth surface. Using this technique, we measured the surface profile of a Mirau objective’s reference surface, which had a rms roughness of 1.5Å. Note that this method has a simple and inexpensive procedure.
Marangoni flows during drying of colloidal films
Authors: Yiantsios Stergios G. , Higgins Brian G.
Published: September 2006

In this study, we consider the drying of a thin film that contains a stable dispersion of colloidal particles so that a coating of these particles is formed after the liquid is driven off by evaporation. For sufficiently thin films, we show that evaporative cooling can drive a Marangoni flow that results in surface deformation of the drying film. A thin-film approximation is used to describe the velocity and temperature fields, and the particle transport equation with convective terms retained is used to describe the concentration field. A coupled finite difference/spectral element scheme is implemented numerically to solve the particle transport equation, where high accuracy is required to resolve sharp gradients within the film and to ensure particle conservation during drying. The model employed is capable of describing the evolution of film thickness and concentration field up to the time when maximum packing is nearly reached at some point in the domain. Three types of film structures are observed, all characterized by a final nonuniform thickness. In the first type, observed at low Peclet numbers, the maximum concentration is reached at the thinnest points in the film, which surround elevations with lower particle concentrations. This mode of instability suggests that dried coatings will have pronounced nonuniformities, resulting in the formation of craters or pinholes. In the second type, observed at high Peclet numbers, a closely packed skin of nonuniform thickness is formed, with low concentration fluid remaining beneath the elevations. In the final stages of drying one would expect capillary pressure to pull particles in the underlying fluid toward the skin, thus creating voids under a seemingly homogeneously applied coating. Finally, still at relatively large particle Peclet numbers and when the destabilizing Marangoni stresses are sufficiently strong, floating lumps of closely packed particles may form in the vicinity of film elevations.
The effect of transfer printing on pentacene thin-film crystal structure
Authors: Shao Y. , Solin S. A. , Hines D. R. , Williams E. D.
Published: September 2006

The thermal deposition and transfer printing method had been used to produce pentacene thin films on SiO2∕Si and plastic substrates poly(methyl methacrylate) (PMMA) and poly(vinyl pyridine), respectively. X-ray diffraction patterns of pentacene thin films showed reflections associated with highly ordered polycrystalline films and a coexistence of two polymorph phases classified by their d spacing, d(001): 14.4 and 15.4Å. The dependence of the c-axis correlation length and the phase fraction on the film thickness and printing temperature were measured. A transition from the 15.4Å phase towards 14.4Å phase was also observed with increasing film thickness. An increase in the c-axis correlation length of approximately 12%–16% was observed for pentacene (Pn) films transfer printed onto a PMMA coated poly(ethylene terephthalate) substrate at 100–120°C as compared to as-grown Pn films on SiO2∕Si substrates. The transfer printing method is shown to be attractive for the fabrication of pentacene thin-film transistors on flexible substrates partly because of the resulting persistence in the quality of the pentacene film.
Laminar flow in a microchannel with superhydrophobic walls exhibiting transverse ribs
Authors: Davies J. , Maynes D. , Webb B. W. , Woolford B.
Published: September 2006

One approach recently proposed for reducing the frictional resistance to liquid flow in microchannels is the patterning of microribs and cavities on the channel walls. When treated with a hydrophobic coating, the liquid flowing in the microchannel wets only the surfaces of the ribs, and does not penetrate the cavities, provided the pressure is not too high. The net result is a reduction in the surface contact area between channel walls and the flowing liquid. For microribs and cavities that are aligned normal to the channel axis (principal flow direction), these micropatterns form a repeating, periodic structure. This paper presents results of a study exploring the momentum transport in a parallel-plate microchannel with such microengineered walls. The investigation explored the entire laminar flow Reynolds number range and characterized the influence of the vapor cavity depth on the overall flow field. The liquid-vapor interface (meniscus) in the cavity regions is treated as flat in the numerical analysis and two conditions are explored with regard to the cavity region: (1) The liquid flow at the liquid-vapor interface is treated as shear-free (vanishing viscosity in the vapor region), and (2) the liquid flow in the microchannel core and the vapor flow within the cavity are coupled by matching the velocity and shear stress at the interface. Regions of slip and no-slip behavior exist and the velocity field shows distinct variations from classical laminar flow in a parallel-plate channel. The local streamwise velocity profiles, interfacial velocity distributions, and maximum interfacial velocities are presented for a number of scenarios and provide a sound understanding of the local flow physics. The predictions and accompanying measurements reveal that significant reductions in the frictional pressure drop (enhancement in effective fluid slip at the channel walls) can be achieved relative to the classical smooth-channel Stokes flow. Reductions in the friction factor and enhancements in the fluid slip are greater as the cavity-to-rib length ratio is increased (increasing shear-free fraction) and as the channel hydraulic diameter is decreased. The results also show that the slip length and average friction factor–Reynolds number product exhibit a flow Reynolds dependence. Furthermore, the predictions reveal the global impact of the vapor cavity depth on the overall frictional resistance.
Viscoelastic flow in rotating curved pipes
Authors: Chen Yitung , Chen Huajun , Zhang Jinsuo , Zhang Benzhao
Published: September 2006

Fully developed viscoelastic flows in rotating curved pipes with circular cross section are investigated theoretically and numerically employing the Oldroyd-B fluid model. Based on Dean’s approximation, a perturbation solution up to the secondary order is obtained. The governing equations are also solved numerically by the finite volume method. The theoretical and numerical solutions agree with each other very well. The results indicate that the rotation, as well as the curvature and elasticity, plays an important role in affecting the friction factor, the secondary flow pattern and intensity. The co-rotation enhances effects of curvature and elasticity on the secondary flow. For the counter-rotation, there is a critical rotational number RΩ′, which can make the effect of rotation counteract the effect of curvature and elasticity. Complicated flow behaviors are found at this value. For the relative creeping flow, RΩ′ can be estimated according to the expression RΩ′=−4Weδ. Effects of curvature and elasticity at different rotational numbers on both relative creeping flow and inertial flow are also analyzed and discussed.
Application of wavelet multiresolution analysis to the study of self-similarity and intermittency of plasma turbulence
Authors: Xu G. S. , Wan B. N. , Zhang W.
Published: September 2006

Multiresolution property of wavelet analysis makes it a powerful tool in describing turbulent signals. Plasma turbulence similar to its fluid turbulence counterpart shows a degree of self-similarity, intermittency, and multifractal. The multiresolution analysis technique based on a biorthogonal wavelet was applied to the Langmuir probe data to study the self-similarity and the intermittency of plasma fluctuations. The data were collected in the plasma edge region of the HT-7 superconducting tokamak. It is found that plasma turbulence is composed of multiscale eddies; these eddies are similar in structure between different scales; the coexistence of multiscale eddies results in the self-similarity in plasma fluctuations. Large-scale turbulence eddies intermittently burst out; the superposition of these large-scale eddies with those small ones is found responsible for the intermittency behavior. Success of utilizing wavelet multiresolution analysis in the HT-7 tokamak suggests it is a promising technique in the understanding of the turbulence structures.
Mass and heat fluxes for a binary granular mixture at low density
Authors: Garzó Vicente , Montanero José María , Dufty James W.
Published: September 2006

The Navier–Stokes order hydrodynamic equations for a low-density granular mixture obtained previously from the Chapman–Enskog solution to the Boltzmann equation are considered further. The six transport coefficients associated with mass and heat flux in a binary mixture are given as functions of the mass ratio, size ratio, composition, and coefficients of restitution. Their quantitative variation across this parameter set is demonstrated using low-order Sonine polynomial approximations to solve the exact integral equations. The results are also used to quantify the violation of the Onsager reciprocal relations for a granular mixture. Finally, the stability of the homogeneous cooling state is discussed.
Electrical properties of sputtered-indium tin oxide film contacts on n-type GaN
Authors: Hwang J. D. , Lin C. C. , Chen W. L.
Published: September 2006

A transparent indium tin oxide (ITO) Ohmic contact on n-type gallium nitride (GaN) (dopant concentration of 2×1017cm−3) having a specific contact resistance of 4.2×10−6Ωcm2 was obtained. In this study, ITO film deposition method was implemented by sputtering. We found that the barrier height, 0.68eV, between ITO and n-type GaN is the same for both evaporated- and sputtered-ITO films. However, the 0.68eV in barrier height renders the evaporated-ITO∕n-GaN Schottky contact. This behavior is different from that of our sputtered-ITO∕n-GaN, i.e., Ohmic contact. During sputtering, oxygen atoms on the GaN surface were significantly removed, thereby resulting in an improvement in contact resistance. Moreover, a large number of nitrogen (N) vacancies, caused by sputtering, were produced near the GaN surface. These N vacancies acted as donors for electrons, thus affecting a heavily doped n-type formed at the subsurface below the sputtered ITO∕n-GaN. Both oxygen removal and heavy doping near the GaN surface, caused by N vacancies, in turn led to a reduction in contact resistivity as a result of electrons tunneling across the depletion layer from the ITO to the n-type GaN. All explanations are given by Auger analysis and x-ray photoelectron spectroscopy.
Hybrid synthetic jets as the nonzero-net-mass-flux synthetic jets
Authors: Trávníček Zdeněk , Vít Tomáš , Tesař Václav
Published: September 2006

The “hybrid synthetic jet” combines the zero-net-mass-flux synthetic jet and fluidic pumping through a valveless pump. No fluid is supplied from an external source (blower or compressor). Hot-wire experiments on a demonstration model confirm that the hybrid synthetic jet exhibits a higher extrusion volume flow rate than the ordinary synthetic jet. For the tested configuration, the increase is by 25% at the 1.30 ratio of the extrusion and suction volume fluxes.
On the power required to control the circular cylinder wake by rotary oscillations
Authors: Bergmann Michel , Cordier Laurent , Brancher Jean-Pierre
Published: September 2006

In this Brief Communication, we determine an approximate relation that gives the mean time power required to control the wake flow downstream from a circular cylinder. The control law is the sinusoidal tangential velocity imposed on whole or part of the cylinder surface. The mean control power thus depends on four parameters: the amplitude and the Strouhal number of forcing, the control angle that defines the controlled upstream part of the cylinder, and the Reynolds number. This relation indicates that the control power grows like the square of the forcing amplitude, like the square root of the forcing Strouhal number, linearly with the control angle and varies like the inverse of the square root of the Reynolds number. We show that the values obtained with this approximate relation are in very good agreement with the corresponding values given numerically. Finally, the energetic efficiency of the control is discussed. We claimed that the most energetically efficient control law corresponds a priori to low forcing amplitudes applied to a restricted upstream part of the cylinder for relatively high values of the Reynolds number.
A three-dimensional spectral boundary element algorithm for interfacial dynamics in Stokes flow
Authors: Wang Yechun , Dimitrakopoulos P.
Published: September 2006

In the present study we describe a novel three-dimensional spectral boundary element algorithm for interfacial dynamics in Stokes flow and/or gravity. The main attraction of this approach is that it exploits all the benefits of the spectral methods (i.e., exponential convergence and numerical stability) with the versatility of the finite element method. In addition, it is not affected by the disadvantage of the spectral methods used in volume discretization to create denser systems. Our algorithm also exploits all the benefits of the boundary element techniques, i.e., a reduction of the problem dimensionality and great parallel scalability. To achieve continuity of the interfacial geometry and its derivatives at the edges of the spectral elements during the droplet deformation, a suitable interfacial smoothing is developed based on a Hermitian-like interpolation. An adaptive mesh reconstructing procedure based on the relevant lengths of the spectral elements is also described. In addition, we consider the inertialess motion of a buoyant droplet left to rise (or sediment) near a vertical solid wall and compare our numerical results with analytical predictions. In our study we emphasize the need for computational studies for the accurate determination of droplet migration near solid walls.
Phase behavior of dipolar fluids from a modified statistical associating fluid theory for potentials of variable range
Authors: Zhao Honggang , McCabe Clare
Published: September 2006

A statistical associating fluid theory for potentials of variable range to model dipolar fluids is presented. The new theory, termed the SAFT-VR+D equation (the statistical associating fluid theory for potentials of variable range plus dipole), explicitly accounts for dipolar interactions and their effect on the structure of the fluid. This is achieved through the use of the generalized mean spherical approximation (GMSA) to describe a reference fluid of dipolar square-well segments. Isothermal-isobaric and Gibbs ensemble Monte Carlo simulations have been performed in order to test the new theoretical approach. Predictions for the thermodynamic properties and phase behavior of dipolar square-well monomer and chain fluids, in which one or more segments are dipolar, are considered and compared with new computer simulation data. The results show that the equation of state in general provides a good description of the phase behavior of dipolar monomer and chain fluids. Some deviations are seen between the simulation data and theoretical predictions for monomer fluids with large dipole moments and for molecules composed of segments with dipoles in different orientations. This is addressed through the replacement of the GMSA by the linearized exponential approximation.
Observing the superparaelectric limit of relaxor (Na1∕2Bi1∕2)0.9Ba0.1TiO3 nanocrystals
Authors: Tiruvalam Ramchandra , Kundu Animesh , Soukhojak Andrey , Jesse Stephen , Kalinin Sergei V.
Published: September 2006

Applications of ferroelectric materials for nonvolatile memory, data storage devices, nanosensors, and nanoactuators necessitate fundamental studies of ferroelectric behavior, including the presence of switchable polarization and switching behavior, on the nanoscale. Here the authors investigated the switching properties of (Na1∕2Bi1∕2)0.9Ba0.1TiO3 nanocrystals prepared by a mist deposition technique using piezoresponse force microscopy (PFM) and spectroscopy. By using stiff cantilevers, reliable PFM data have been obtained and local electromechanical response was measured. The transition from ferroelectric to superparaelectric behavior in these nanocrystals was observed at sizes of ∼10nm. The effects of particle shape on the PFM imaging are also discussed.
Thermal radiation and thickness fluctuations in freely suspended liquid films
Authors: Zhang Jie , Wu X. L. , Rashidnia Nasser
Published: September 2006

Thermal convection in a vertically suspended soap film subjected to a vertical temperature gradient is marked by intense density fluctuations δρ2, uncommon to laboratory Rayleigh-Bénard convection (RBC). Such large fluctuations result from stratification in the film under the influence of gravity. Herein we present the first direct measurement of two-dimensional density fluctuations in a free-standing soap film using a single-point infrared detector. The radiation densitometer is nonintrusive and responds instantaneously to local density variations. Measurements of the power spectrum Γ(f) of δρ2 are carried out using a variety of sample geometries of the aspect ratio of unity. In all cases, Γ(f) scales with the frequency as f−1.4±0.1 in the low frequency regime, which is in good agreement with the Bolgiano’s theoretical prediction for a stably stratified fluid. Combining thermal imaging and particle tracking, we also show that it is feasible to measure the full-field density flux j(x,y)=ρ2(x,y)v(x,y) in the film. Despite that individual snapshots of j(x,y) appear random, the time-averaged flux forms a close loop similar to the large-scale circulation commonly seen in RBC, when a sufficiently large temperature gradient is present.