bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical simulation of heat transfer and fluid flow in coaxial laser cladding process for direct metal deposition
Authors: Qi Huan , Mazumder Jyotirmoy , Ki Hyungson
Published: August 2006

The coaxial laser cladding process is the heart of direct metal deposition (DMD). Rapid materials processing, such as DMD, is steadily becoming a tool for synthesis of materials, as well as rapid manufacturing. Mathematical models to develop the fundamental understanding of the physical phenomena associated with the coaxial laser cladding process are essential to further develop the science base. A three-dimensional transient model was developed for a coaxial powder injection laser cladding process. Physical phenomena including heat transfer, melting and solidification phase changes, mass addition, and fluid flow in the melt pool, were modeled in a self-consistent manner. Interactions between the laser beam and the coaxial powder flow, including the attenuation of beam intensity and temperature rise of powder particles before reaching the melt pool were modeled with a simple heat balance equation. The level-set method was implemented to track the free surface movement of the melt pool, in a continuous laser cladding process. The governing equations were discretized using the finite volume approach. Temperature and fluid velocity were solved for in a coupled manner. Simulation results such as the melt pool width and length, and the height of solidified cladding track were compared with experimental results and found to be reasonably matched.
Lowering of stimulated emission threshold of zinc oxide by doping with thermally diffused aluminum supplied from sapphire substrate
Authors: Wang Yu-Guang , Ohashi Naoki , Wada Yoshiki , Sakaguchi Isao , Ohgaki Takeshi , Haneda Hajime
Published: August 2006

Undoped zinc oxide (ZnO) thin films were grown on sapphire and zirconia [yttria-stabilized zirconia (YSZ)] substrate to investigate the effect of the annealing on luminescence properties. For the films grown on sapphire substrate, the electron concentration increased to 1.6×1019cm−3 as a result of in-diffusion of aluminum from the substrate during annealing and the threshold pumping power to excite stimulated emission decreased from 530kW∕mm2 (as-deposited film) to 32kW∕mm2 (film annealed at 900°C). In contrast, for ZnO films grown on YSZ substrate, electron concentration was unchanged by annealing and the stimulated emission threshold only slightly decreased after annealing. The current results indicated that electron concentration is an essential parameter for the stimulated emission threshold of ZnO.
Physical effects in laminar microconvection due to variations in incompressible fluid properties
Authors: Mahulikar S. P. , Herwig H.
Published: August 2006

In this investigation we report the identification of laminar microconvection physical effects due to the variation of viscosity and thermal-conductivity of liquid. Viscosity variation significantly distorts the axial velocity profile and varies this distortion along the microflow, thereby inducing radial flow due to flow continuity. The resulting induced radial heat convection can be a significant percentage of the axial heat convection, especially in microconvection. Also, axial conduction is induced due to fluid thermal-conductivity variation along the flow. The effect of distorted axial velocity profile and the induced radial flow on microconvection due to fluid viscosity variation are opposite. However, fluid thermal-conductivity variation along and across the flow have the same effect on microconvection. Thus, a deviation in convection due to thermal-conductivity variation exceeds the deviation due to viscosity variation, although the dimensionless temperature sensitivity of viscosity is higher.
Using capillary forces to determine the geometry of nanocontacts
Authors: Butt Hans-Jürgen , Farshchi-Tabrizi Mahdi , Kappl Michael
Published: August 2006

The capillary force between two fine particles or between the tip of an atomic force microscope (AFM) and a surface depends on the precise geometry of the contact region. In this paper we demonstrate that vice versa from a measurement of the adhesion force versus humidity one can calculate the shape of the AFM tip (or the geometry of the contact between particles). This is verified by adhesion experiments with an AFM.
Lasing mechanisms in organic photonic crystal lasers with two-dimensional distributed feedback
Authors: Forberich K. , Gombert A. , Pereira S. , Crewett J. , Lemmer U. , Diem M. , Busch K.
Published: August 2006

We present a detailed experimental and theoretical investigation of the lasing characteristics of organic photonic crystal lasers. These lasers are based on strongly modulated two-dimensional polymer surface relief structures on which thin films of optically active organic materials have been deposited. We determine the in-plane photonic band structure of the corresponding quasiguided modes within an effective two-dimensional model. In addition, we calculate the total (three-dimensional) losses associated with these modes. This allows us to identify the lasing thresholds for square lattice geometries and to understand the emission pattern.
Output of triple-pulse megahertz burst rate high-voltage induction device
Authors: Chen Sifu , Li Xin , Huang Ziping , Jing Xiaobing , Ding Hengsong , Wang Lei , Yang Xinglin , Wang Minhong , Ma Bing , Shi Jinshui , Zhang Linwen , Ding Bonan , Deng Jianjun
Published: August 2006

Proof-of-principle experiments on the megahertz burst rate high-voltage triple-pulse induction acceleration concept are described and the experimental results are presented. High-voltage triple pulser supplies a triple-pulse burst at pulse rate of 1.25MHz to the acceleration gap of a ferrite induction device after the ferrite cores reset; a train of three 180kV pulses separated by 800ns was obtained in the accelerating gap. Each pulse has a full width at half maximum of 120ns and ±1% flattop of 50ns, with rise time of 40ns, respectively.
Confined breather-type excitation in a quinoidal thiophene after sub-5fs pulse excitation
Authors: Kobayashi Takayoshi , Wang Haibo , Wang Zhuan , Otsubo Tetsuo
Published: August 2006

A quinoidal thiophene molecule which is a prototype of the repeated unit of s-trans-cis-polyacetylene was studied by sub-5fs spectroscopy. A breatherlike mode modulation of both the amplitude and frequency of the CC stretching was observed for the first time. It generates two sidebands of 2238 and 700cm−1 with a separation of 769cm−1 corresponding to the modulation frequency. The latter mode is very close to the C–S–C ring deformation, which indicate that this mode mediates coupling between the breather mode and the Cβ–Cβ stretching mode. It was also found that the breather-type excitation has a longer lifetime (2–3ps) than in all-trans-polyacetylene (50fs) due to its confinement.
Using a quantum well heterostructure to study the longitudinal and transverse electric field components of a strongly focused laser beam
Authors: Kihara Rurimo G. , Schardt M. , Quabis S. , Malzer S. , Dotzler Ch. , Winkler A. , Leuchs G. , Döhler G. H. , Driscoll D. , Hanson M. , Gossard A. C. , Pereira S. F.
Published: August 2006

We report a method to measure the electric energy density of longitudinal and transverse electric field components of strongly focused polarized laser beams. We used a quantum well photodetector and exploited the polarization dependent optical transitions of light holes and heavy holes to probe the electric field distribution in the focal region. A comparison of the measured photocurrent spectra for radially and azimuthally polarized beams at the light and heavy hole absorption peaks provides a measure of the amount of the longitudinal electric field component.
In situ magnetic measurements under neutron radiation in Fe metal and low carbon steel
Authors: Takahashi S. , Kikuchi H. , Ara K. , Ebine N. , Kamada Y. , Kobayashi S. , Suzuki M.
Published: August 2006

Magnetic minor hysteresis loops of low carbon steel and Fe metal have been measured during neutron radiation at 563K in a 50MW nuclear reactor. For investigation of nucleation mechanism of copper precipitates and dislocation loops during neutron radiation, special attention was paid to minor-loop coefficients, which are deduced from simple relations between minor-loop parameters and are very sensitive to lattice defects such as dislocations, copper precipitates, and grain boundaries. We found that with increasing neutron fluence, the minor-loop coefficients of low carbon steel sharply increase and show a maximum at the fluence of 1×1019cm−2, followed by a slow decrease. The appearance of the maximum suggests the presence of two mechanisms of internal stress; while copper precipitates and dislocation loops in the matrix make the internal stress increase, those grown in the vicinity of dislocations compensate the internal stress of dislocations so as to minimize the elastic energy. On the other hand, the minor-loop coefficients show a monotonic increase with neutron fluence in Fe metal where dislocation loops play an important role for the brittleness.
Lattice preferred orientation and stress in polycrystalline hcp-Co plastically deformed under high pressure
Authors: Merkel Sébastien , Miyajima Nobuyoshi , Antonangeli Daniele , Fiquet Guillaume , Yagi Takehiko
Published: August 2006

The results of x-ray diffraction data of a polycrystal under nonhydrostatic compression are analyzed for lattice preferred orientation and stress using lattice strain theories with an application to hcp-Co deformed up to 42.6GPa in the diamond anvil cell. We obtain a pure [001] fiber texture that develops primarily between 0 and 15GPa. We also show that for hcp metals the hypothesis of uniform stress across grains and lattice planes cannot be applied. This implies that the effective single crystal elastic moduli deduced from x-ray diffraction under Reuss or geometric averages consistently differ from those measured with other techniques, even after including effects of lattice preferred orientations. These results can be interpreted as an (hkl)-dependent effective differential stress resulting from plastic deformation.
Experiment and modeling of the deposition of ultrananocrystalline diamond films using hot filament chemical vapor deposition and Ar∕CH4∕H2 gas mixtures: A generalized mechanism for ultrananocrystalline diamond growth
Authors: May P. W. , Mankelevich Yu. A.
Published: August 2006

Ar ∕ C H 4 ∕ H 2 gas mixtures have been used to deposit nanocrystalline diamond (NCD) and ultrananocrystalline diamond (UNCD) films using hot filament (HF) chemical vapor deposition. The Ar:H2 concentration was maintained at Ar∕(H2+Ar)=80% while the CH4 concentration was varied over the range CH4∕(H2+CH4)=0.3–6.0. For higher methane concentrations, the filament became coated in a graphitic layer which prevented film growth. For lower CH4 additions, the film morphology depended upon the CH4 concentration, with different gas mixing ratios producing microcrystalline diamond (MCD), NCD, or UNCD films. A two-dimensional computer model was used to calculate the gas phase composition for all these conditions at all positions within the reactor. Using the experimental and calculated data, we show that the observed film morphology can be rationalized using a model based on competition between H atoms, CH3 radicals, and other C1 species reacting with dangling bonds on the surface. The relative concentrations of each of these species close to the growing diamond surface determine the probability of a renucleation event occurring and hence the morphology of the subsequent film. This has been developed into a general mechanism for the deposition of MCD, NCD, and UNCD films from Ar∕CH4∕H2 gas mixtures which is consistent with published experimental observations.
Bouncing dynamics of resistive microswitches with an adhesive tip
Authors: Decuzzi P. , Demelio G. P. , Pascazio G. , Zaza V.
Published: August 2006

This paper provides a detailed analysis of the dynamic response of a resistive microswitch. The analysis has been carried out by modeling the microswitch as a cantilever beam, according to the Euler-Bernoulli theory, and considering the damping interaction of the moving beam with the surrounding fluid. Attention has been given to the bouncing of the beam tip on the substrate upon actuation. A general adhesive-repulsive force has been applied at the tip of the beam to model its interaction with the substrate, where the attractive contribution is described by a van der Waals-like term and the repulsive contribution by a classical linear elastic springlike term. The resulting problem has been solved using a second-order-accurate finite difference scheme. It is shown that by tuning the adhesive interaction at the tip/substrate interface the number and amplitude of the bounces can be significantly reduced in favor of the system reliability and performance. Also design maps have been proposed to estimate the actual switching time and bouncing dynamics as a function of the adhesive interaction, applied actuation voltage, and of the geometry of the microdevice. These maps can be useful in a preliminary design of the system.
Polarization effects in the channel of an organic field-effect transistor
Authors: Houili H. , Picon J. D. , Zuppiroli L. , Bussac M. N.
Published: August 2006

We present the results of our calculation of the effects of dynamical coupling of a charge carrier to the electronic polarization and the field-induced lattice displacements at the gate interface of an organic field-effect transistor (OFET). We find that these interactions reduce the effective bandwidth of the charge carrier in the quasi-two-dimensional channel of a pentacene transistor by a factor of 2 from its bulk value when the gate is a high-permittivity dielectric such as (Ta2O5), while this reduction essentially vanishes using a polymer gate insulator. These results point to carrier effective bandwidth as a possible trigger of the dielectric effects on the mobility reported recently in OFETs.
Electric heating effects in nematic liquid crystals
Authors: Yin Y. , Shiyanovskii S. V. , Lavrentovich O. D.
Published: August 2006

Electric heating effects in the nematic liquid crystal change the liquid crystal physical properties and dynamics. We propose a model to quantitatively describe the heating effects caused by dielectric dispersion and ionic conductivity in the nematic liquid crystals upon the application of an ac electric field. The temperature increase of the liquid crystal cell is related to the properties of the liquid crystal such as the imaginary part of the dielectric permittivity, thermal properties of the bounding plates, and the surrounding medium as well as frequency and amplitude of the electric field. To study the temperature dynamics experimentally, we use a small thermocouple inserted directly into the nematic bulk; we assure that the thermocouple does not alter the thermal behavior of the system by comparing the results to those obtained by a noncontact birefringent probing technique recently proposed by Wen and Wu [Appl. Phys. Lett. 86, 231104 (2005)]. We determine how the temperature dynamics and the stationary value of the temperature increase depend on the parameters of the materials and the applied field. We used different surrounding media, from extremely good heat conductors such as aluminum cooling device to extremely poor conductor, Styrofoam; these two provide two limiting cases as compared to typical conditions of nematic cell exploitation in a laboratory or in commercial devices. The experiments confirm the theoretical predictions, namely, that the temperature rise is controlled not only by the heat transfer coefficient of the surrounding medium (as in the previous model) but also by the thickness and the thermal conductivity coefficient of the bounding plates enclosing the nematic layer. The temperature increase strongly depends on the director orientation and can change nonmonotonously with the frequency of the applied field.
Optical properties and electric field enhancement in cholesteric liquid crystal containing different periodicities
Authors: Ozaki Ryotaro , Matsuhisa Yuko , Yoshida Hiroyuki , Yoshino Katsumi , Ozaki Masanori
Published: August 2006

We study a defect mode in a one-dimensional photonic band gap of a cholesteric liquid crystal (CLC) consisting of two helicoidal periodicities. The optical properties of this CLC are analyzed using 4×4 transfer matrix and finite difference time domain (FDTD) methods. In calculated transmission spectra of this CLC, one of the defect modes always appears at the band edge wavelength of the inner CLC having a different helix to that of two sides of CLCs. Furthermore, the electric field analysis of this CLC has also been demonstrated by the FDTD method. At the defect mode wavelength, the electric field enhancement is found to be significant larger than a normal CLC.
Nonlinear spectroscopy of PbS quantum-dot-doped glasses as saturable absorbers for the mode locking of solid-state lasers
Authors: Malyarevich A. M. , Gaponenko M. S. , Yumashev K. V. , Lagatsky A. A. , Sibbett W. , Zhilin A. A. , Lipovskii A. A.
Published: August 2006

The bleaching relaxation characteristics of PbS quantum dots with diameters in the range of 3.8–4.3nm synthesized in silicate and phosphate glasses have been studied under the conditions of femtosecond excitation. It has been observed that increase of pump fluence from 0.1to2.2mJ∕cm2 leads to the appearance of a second (slow) component in the bleaching relaxation kinetics and to the shortening of both the fast (from several tens of picoseconds to several picoseconds) and slow relaxation times. An absorption saturation fluence of ≈15μJ∕cm2 with a nonsaturable absorption coefficient of 2.5–3cm−1 at 1.05μm has been evaluated for the PbS quantum-dot-doped glasses studied. The nature of the nonsaturable absorption and the mechanism of the bleaching relaxation in PbS quantum dots have been discussed.
Inertial effects on the transfer of heat or mass from neutrally buoyant spheres in a steady linear velocity field
Authors: Subramanian Ganesh , Koch Donald L.
Published: August 2006

Microscale inertia is found to break the degenerate closed-streamline configuration that occurs in a shearing flow past a neutrally buoyant torque-free spherical particle in the inertialess limit. The broken symmetry at small but finite Re allows heat or mass to be convected away in an efficient manner in sharp contrast to the inertialess diffusion-limited scenario. Inertial forces scale with the particle Reynolds number, defined as Re=γ̇a2∕ν, where a is the radius of the particle, γ̇ is the characteristic magnitude of the velocity gradient, and ν is the kinematic viscosity of the suspending fluid. The dimensionless heat or mass transfer rate is then given by Nu=C(RePe)1∕3+O(1) when Re≪1 and RePe≫1, the constant C being a function of the flow in the vicinity of the particle. Here, Nu is the Nusselt number defined as Q∕(4πkaΔF), where Q is the dimensional heat/mass flux, k the appropriate transport coefficient, and ΔF the driving force viz. the temperature or concentration difference between the particle and the ambient fluid; for pure diffusion, Nu=1. The Peclét number (Pe) is a dimensionless measure of the relative dominance of the convective and diffusive transfer mechanisms. It is shown that C equals 0.325(1+λ)2∕3 for a two-dimensional linear flow, where λ measures the relative magnitudes of extension and vorticity. For simple shear (λ=0), knowledge of the inertial velocity field to O(Re3∕2) enables one to determine the next term in the asymptotic expansion for Nu; one finds Nu=(RePe)1∕3(0.325−0.0414Re1∕2)+O(1) in the limit 1≫Re≫Pe−2∕5. It is argued that the convective enhancement at finite Re via symmetry-breaking streamline bifurcations will occur in generic shearing flows with nonlinear velocity profiles; the degenerate Stokes streamline pattern around a neutrally buoyant torque-free particle in a quadratic flow serves to reinforce this assertion. The above mechanism represents a possible means for heat or mass transfer enhancement from the dispersed phase in multiphase systems. Implications for particles in turbulent flows are also discussed.
Possible spatial ordering of particles in electron chains over liquid helium
Authors: Nikolaenko V. A. , Kovdrya Yu. Z. , Mukharskii Yu.
Published: August 2006

A quasi-one-dimensional electron system of high quality and uniformity over liquid helium (quasi-one-dimensional nanochannels) is obtained with the use of glass optical fibers. It is found that the character of the temperature dependence of the conductance depends on the linear electron density in the channels, nl. At low nl (∼103cm−1) the conductance in the gas-scattering region increases exponentially with decreasing temperature and is described satisfactorily by the theory. At T<1.1K a weak decrease of σ with decreasing temperature is observed. At large nl a substantial decrease of the conductance at low temperatures is found, due either to spatial ordering of the particles in the one-electron chain or to the influence of flaws on the carrier mobility in the quasi-one-dimensional conducting nanochannels.
Study of locally frozen magnetic field in a high-Tc superconducting ceramic
Authors: Bondarenko S. I. , Shablo A. A. , Koverya V. P.
Published: August 2006

The properties of a locally frozen (in a region of diameter 0.5mm) magnetic field in a YBa2Cu3O7−x slab 0.5μm thick are investigated as a function of the value of the excitation field, the regime of freezing, and the transport current through the sample. The first regime is cooling of the ceramic to 77K in the excitation field with a subsequent turning off of the excitation field, and the second regime is cooling in the Earth’s magnetic field with a subsequent turning on and off of the excitation field. At an excitation field up to 2000A∕m in these regimes two different types of macroscopic current vortex structures, which generate the frozen field, are formed. The local critical field of excitation when the vortex structure is formed in the second regime exceeds the uniform perpendicular critical field of the slab by a factor of 10 and equals 1700A∕m. On the other hand, the vortex structure of the first type can be formed by practically any weak excitation field, including fields smaller than the critical field of the vortex structure of the second type. In a representation of the ceramic as a Josephson medium, physical models of the two types of vortex structures are proposed which correspond most fully to the results of experiments. The displacement of the vortex structure of the first type upon the passage of transport current through the slab, as a result of the action of the Lorentz force on that structure, is registered. This makes it possible to calculate the pinning force Fp and to estimate the value of the viscosity η for the motion of such a vortex structure in the ceramic: Fp=6×10−8N, η=6×10−5kg∕s.
Electromagnetic excitation of sound in iron borate
Authors: Khizhny V. I. , Tarakanov V. V. , Koroljuk A. P. , Khizhnaya T. M.
Published: August 2006

The observation of linear electromagnetic generation of sound of longitudinal (at a frequency of ∼230MHz) and transverse (∼180MHz) polarization in an iron borate slab is reported. The sound is excited by a magnetic field h∼ with an amplitude of up to 1Oe and a direction parallel to the (111) basal plane of the slab. The linear generation of sound of longitudinal polarization is observed for the first time. The inverse effect—generation of a field h∼ by sound relayed into the iron borate from an external source—is also observed. The field dependence of the signal in an external bias field H lying in the basal plane of the sample is also investigated. A piezomagnetic mechanism of excitation of longitudinal sound by a Bloch domain wall is discussed qualitatively.
Self-consistent field approximation in the theory of hopping transport in disordered systems
Authors: Fateev M. P.
Published: August 2006

The problem of the hopping conductivity of a system with randomly distributed impurity centers at low temperatures is considered on the basis of a linearized balance equation. Diagram techniques are used to find a self-consistent expression for the configuration-averaged Green’s function describing charge transport in a disordered system with Fermi correlations taken into account. It is shown that the approximation chosen corresponds to the well-known coherent potential approximation. An expression for the hopping conductivity as a function of temperature and frequency for a power-law density of states is found in the low-frequency limit. The results are in good agreement with the percolation approach and, in the static limit, lead to Mott's law.
Hydrogen resistive double-side (Pb0.72La0.28)Ti0.93O3 buffered Pb(Zr0.52Ti0.48)O3 thin films
Authors: Lee Eun Sun , Li Dong Hua , Chung Hyun Woo , Lee Sang Yeol
Published: August 2006

The hydrogen annealing effect and the fatigue resistance of the Pb(Zr0.52Ti0.48)O3 (PZT) thin films with double-side (Pb0.72La0.28)Ti0.93O3 (PLT) buffer layers were investigated. With the PLT buffer layers, the films showed better hydrogen resistance than the PZT films because of the enhancement of the structural properties of the double-side PLT buffered PZT films. This structure also exhibited good fatigue endurance after 109 switching cycles even without oxide electrodes, mainly because the accumulated charges were compensated at the interface between the PLT buffer layers and the electrode. Moreover, the fatigue endurance was maintained even after the hydrogen annealing, so we could confirm the reliability of the structure after the fabrication process.
Influences of ac electric field on the spatial distribution of carbon nanotubes formed between electrodes
Authors: Peng Ning , Zhang Qing , Li Jingqi , Liu Ningyi
Published: August 2006

Influences of ac electric field on the spatial distribution of single wall carbon nanotubes (SWCNTs) between two adjacent electrodes have been studied. The SWCNTs are found to be well aligned with the electric field direction and the number density of the SWCNTs attached to the electrodes is increased with the magnitude of the electric field. Induced ac dielectrophoresis force and torque on the SWCNTs are analyzed. It is suggested that the SWCNTs rotate to align with the external field direction almost instantaneously once the electric field is applied. In contrast, the translational motion along the field gradient takes a much longer time. Our results show that it is possible to separate metallic and semiconducting SWCNTs by frequency tuning. Taking the influences of frequency and viscosity into consideration, we simulate the distributions of SWCNTs between different electrode structures. Both theoretical and experimental results show that perpendicular electrodes have better control over the SWCNT’s location and direction than parallel electrodes.
Thermal deflections in multilayer microstructures and athermalization
Authors: Torun Hamdi , Urey Hakan
Published: August 2006

Exact and approximate analytical solutions are developed for calculating the thermally induced deformation of three-layer cantilever structures. The solution is derived from the closed-form solutions for multilayer films. Thermal deformation and athermalization conditions are derived using dimensionless parameters for film to substrate thickness ratios for three-layer structures. The analytical solution for a narrow beam is applied to a scan mirror plate suspended with two torsional flexures. The results agreed well with finite element method simulations and experiments. Tests are performed using a bulk-micromachined silicon microelectromechanical system scanner that has a thin gold (Au) coil layer on one side and an aluminum (Al) mirror layer on the other side. Useful figures using film-to-substrate thickness ratios and the material independent normalized parameters are introduced for easy thermal deformation computations and performance trades for three-layer structures.
Effects of annealing on pentacene field-effect transistors using polyimide gate dielectric layers
Authors: Sekitani Tsuyoshi , Someya Takao , Sakurai Takayasu
Published: August 2006

We report systematic studies on the effects of annealing on pentacene field-effect transistors (FETs). The FETs are fabricated on plastic films with polyimide gate dielectric layers, encapsulated with poly-chloro-para-xylylene (parylene) passivation layers, and annealed in a nitrogen environment at different substrate temperatures. The annealed FETs are functional at a measurement temperature of 160°C and exhibit no degradations in the transistor performance after being subjected to a number of heat cycles between room temperature and 160°C. The annealed FETs exhibit a change of less than 5% in the source-drain currents even after the application of dc voltage biases of VDS=VGS=−40V for 41h. When the FETs are annealed at 140°C for 12h in a nitrogen environment, the mobility increases from 0.52to0.56cm2∕Vs and the on/off ratio also improves to 106.
Dielectric relaxation in gigahertz region and phase transition of BaTiO3-based ceramics
Authors: Li Jianyong , Kakemoto Hirofumi , Wada Satoshi , Tsurumi Takaaki , Kawaji Hitoshi
Published: August 2006

Ba ( Zr 0.25 Ti 0.75 ) O 3 (BZT) and (Ba0.6Sr0.4)TiO3 (BST) ceramics were prepared as typical samples of lead-free relaxors and ferroelectrics with the diffuse phase transition, respectively. A suitable measurement technique using microplanar electrode and electromagnetic field analysis was developed to measure the microwave dielectric properties for materials with high permittivity and high dielectric loss. Frequency dependence of complex permittivity of the two ceramics was measured up to 3GHz as a function of temperature. The dielectric dispersion of both ceramics was well explained by the stretched exponential model of the decay function in dielectric relaxation. In BST, a phase transition was observed at the permittivity peak temperature in the results of x-ray diffraction analysis, specific heat measurement, and the analysis of dielectric relaxation. The relaxation frequency of dipole fluctuation in BST was much higher than that of BZT. This high relaxation frequency enables us to use BST for dielectrics with high permittivity and low loss factors in microwave region. The continuous change from normal ferroelectrics to relaxors in BaTiO3-based ceramics was discussed.
Optical luminescence from alkyl-passivated Si nanocrystals under vacuum ultraviolet excitation: Origin and temperature dependence of the blue and orange emissions
Authors: Chao Y. , Houlton A. , Horrocks B. R. , Hunt M. R. C. , Poolton N. R. J. , Yang J. , Šiller L.
Published: August 2006

The origin and stability of luminescence are critical issues for Si nanocrystals which are intended for use as biological probes. The optical luminescence of alkyl-monolayer-passivated silicon nanocrystals was studied under excitation with vacuum ultraviolet photons (5.1–23eV). Blue and orange emission bands were observed simultaneously, but the blue band only appeared at low temperatures (<175K) and with high excitation energies (>8.7eV). At 8K, the peak wavelengths of the emission bands were 430±2nm (blue) and 600±2nm (orange). The orange and blue emissions originate from unoxidized and oxidized Si atoms, respectively.
Young’s modulus, Poisson’s ratio, and residual stress and strain in (111)-oriented scandium nitride thin films on silicon
Authors: Moram M. A. , Barber Z. H. , Humphreys C. J. , Joyce T. B. , Chalker P. R.
Published: August 2006

Epitaxial scandium nitride films (225nm thick) were grown on silicon by molecular beam epitaxy, using ammonia as a reactive nitrogen source. The main crystallographic orientation of ScN with respect to Si is (111)ScN‖(111)Si and [1–10]ScN‖[0–11]Si; however, some twinning is also present in the films. The films displayed a columnar morphology with rough surfaces, due to low adatom mobility during growth. The strain-free lattice parameter of ScN films grown under optimized conditions was found to be 4.5047±0.0005Å, as determined using high-resolution x-ray diffraction (HRXRD). In-plane and out-of-plane strains were subsequently evaluated using HRXRD and were used to determine the Poisson ratio of ScN along the ⟨111⟩ direction, which is found to be 0.188±0.005. Wafer curvature measurements were made and combined with the strain information to determine the average Young’s modulus of the films, which is found to be 270±25GPa. Residual film stresses ranged from −1to1GPa (depending on film growth temperature and film thickness) due to competition between the tensile stress (induced by the differential thermal contraction between the ScN film and the Si substrate) and intrinsic compressive stresses generated during growth.
Compound nanostructures formed by metal nanoparticles dispersed on nanodendrites grown on insulator substrates
Authors: Xie Guoqiang , Song Minghui , Furuya Kazuo , Louzguine Dmitri V. , Inoue Akihisa
Published: August 2006

A composite nanostructure consisting of Pt nanoparticle/W nanodendrite was fabricated on an insulator Al2O3 substrate using an electron-beam-induced deposition process combined with an ion sputtering method. W nanodendrites with the tips of 3nm were grown self-standing at the edge of the Al2O3 substrate. The observed morphology is attributed to a growth mechanism involving an electrical charge-up on the substrate surface, movement of charges, and their accumulation on the convex surface of the substrate as well as at the tips of the deposits. Pt nanoparticles with an equilibrium face-centered-cubic structure were uniformly distributed on the W nanodendrites. These composite nanostructures have potential for application in the fields of catalysis and nanodevices.
Two-compartment radial diffusive exchange analysis of the NMR lineshape of Xe129 dissolved in a perfluorooctyl bromide emulsion
Authors: Gherase Mihai R. , Wallace Julia C. , Cross Albert R. , Santyr Giles E.
Published: August 2006

Hyperpolarized Xe129 (xenon) gas dissolved in a perfluorooctyl bromide (PFOB) emulsion stabilized with egg yolk phospholipid (EYP) is a possible contrast agent for quantitative blood flow measurements using magnetic resonance imaging. The NMR line shape of xenon dissolved in PFOB emulsion depends strongly on the exchange of spins between PFOB and water. The exchange in this system depends on three factors: the geometrical factors (i.e., droplet size and surrounding water volume), the permeability of the EYP monolayer surrounding the droplet, and the diffusion coefficients of xenon in the two media. A theoretical model which predicts the line shape of xenon in the emulsion based on the Bloch-Torrey equations is presented. Fitting the full width at half maximum (FWHM) of the theoretical line shapes with the FWHM of the experimental spectra obtained from emulsions with different water dilutions allows estimation of the volume-weighted average diameter of the PFOB droplets (3.5±0.8)μm and the permeability of the EYP membrane surrounding the droplet (58±14)μm∕s.
An experimental study of drop-on-demand drop formation
Authors: Dong Hongming , Carr Wallace W. , Morris Jeffrey F.
Published: August 2006

The dynamics of drop-on-demand (DOD) drop formation have been studied experimentally using an imaging system with an interframe time of 1μs and a spatial resolution of 0.81μm∕pixel. Using a piezoelectrical actuated inkjet printhead with the nozzle orifice diameter of 53μm, experiments were conducted over a range of viscosities (1.0–5.0cP) and surface tensions (35–73mN∕m). The effects of the driving signal, which controls the piezoelectric transducer that produces the pressure pulse to drive the liquid from the reservoir through the orifice, have been examined along with those of liquid properties. The main stages of DOD drop formation, including ejection and stretching of liquid, pinch-off of liquid thread from the nozzle exit, contraction of liquid thread, breakup of liquid thread into primary drop and satellites, and recombination of primary drop and satellites, are analyzed based on the experimental results. The breakup time of liquid threads was found to be dependent mainly on the capillary time based on the length scale of the nozzle orifice and the growth rate of the most unstable disturbance normalized by the inverse of the capillary time. However, a well-designed waveform of driving signal can initiate an abrupt pinch-off of the liquid thread from the nozzle exit. During the contraction of the liquid thread after it has pinched off from the nozzle, two modes of breakup were observed: end-pinching where the liquid thread pinches off from an almost spherical head, and multiple breakup due to capillary waves. The effects of liquid and system parameters on the formation and recombination of the primary drop and satellites were investigated. Based on experimental observations, a necessary condition for the recombination of the primary drop and satellite and the limit for liquid thread length without breakup during contraction are proposed. The primary drop size increases slightly with increasing surface tension and/or decreasing viscosity. The driving voltage to the piezoelectric transducer mainly determines whether satellite formation will occur and the size of satellites, and it has insignificant effect on primary drop size.
Preparation and characterization of LaMnO3 thin films grown by pulsed laser deposition
Authors: Aruta C. , Angeloni M. , Balestrino G. , Boggio N. G. , Medaglia P. G. , Tebano A. , Davidson B. , Baldini M. , Di Castro D. , Postorino P. , Dore P. , Sidorenko A. , Allodi G. , De Renzi R.
Published: August 2006

We have grown LaMnO3 thin films on (001) LaAlO3 substrates by pulsed laser deposition. X-ray diffraction confirms that the films are only slightly relaxed and are oriented “square on square” relative to the substrate. The measured Raman spectra closely resemble that observed in bulk LaMnO3, which indicates no relevant distortions of the MnO6 octahedra induced by the epitaxial strain. Therefore, no detectable changes in the lattice dynamics occurred in our LaMnO3 strained films relative to the bulk case. Mn55 nuclear magnetic resonance identifies the presence of localized Mn4+ states. Superconducting quantum interference device magnetization measures TN=131(3)K and a saturation moment μ=1.09μB∕Mn, revealing a small concentration of Mn4+ and placing our films within the antiferromagnetic insulating phase.
Below-band-gap excitation of a terahertz surface plasmon-polariton
Authors: Bakunov M. I. , Maslov A. V. , Bodrov S. B.
Published: August 2006

We analyze the excitation of a surface plasmon-polariton at terahertz frequencies by an optical pulse with tilted intensity front incident on a semiconductor surface. We show that one can achieve an order of magnitude increase in the generated terahertz power by using an optical pulse with frequencies below the band gap compared to the above-band-gap excitation.
ZnO layers deposited by the ion layer gas reaction on Cu(In,Ga)(S,Se)2 thin film solar cell absorbers: Morphology, growth mechanism, and composition
Authors: Bär M. , Reichardt J. , Sieber I. , Grimm A. , Kötschau I. , Lauermann I. , Sokoll S. , Lux-Steiner M. C. , Fischer Ch.-H. , Niesen T. P.
Published: August 2006

Cu ( In , Ga ) ( S , Se ) 2 (CIGSSe) based solar cells with a ZnO window extension layer (WEL) deposited by the ion layer gas reaction (ILGAR) reach competitive efficiencies compared to corresponding references with CdS buffer and lead to a simplified device structure. The WEL replaces not only the CdS buffer, but also the undoped part of the usually applied rf-sputtered ZnO window bilayer. Since the performance of corresponding solar cell devices depends strongly on the ILGAR process parameters (number of deposition cycles and process temperature), respective ILGAR-ZnO∕CIGSSe test structures were investigated by means of scanning electron microscopy and x-ray photoelectron spectroscopy. Thereby, the growth mechanism of ILGAR-ZnO on CIGSSe absorbers and its morphology was investigated. In addition, the surface composition was determined, showing that ILGAR-ZnO layers contain a certain amount of metastable hydroxide. Due to the systematic variation of the ILGAR process parameters it could be demonstrated that it is possible to directly tune the hydroxide content in the ILGAR-ZnO layers.
Calculation and measurement of bianisotropy in a split ring resonator metamaterial
Authors: Smith David R. , Gollub Jonah , Mock Jack J. , Padilla Willie J. , Schurig David
Published: August 2006

A medium that exhibits artificial magnetism can be formed by assembling an array of split ring resonators (SRRs)—planar conducting elements that exhibit a resonant response to electromagnetic radiation. The SRR exhibits a large magnetic dipole moment when excited by a magnetic field directed along its axis. However, the SRR also exhibits an electric response that can be quite large depending on the symmetry of the SRR and the orientation of the SRR with respect to the electric component of the field. So, while the SRR medium can be considered as having a predominantly magnetic response for certain orientations with respect to the incident wave, it is generally the case that the SRR exhibits magnetoelectric coupling, and hence a medium of SRRs arranged so as to break mirror symmetry about one of the axes will exhibit bianisotropy. We present here a method of directly calculating the magnetoelectric coupling terms using averages over the fields computed from full-wave finite-element based numerical simulations. We confirm the predicted bianisotropy of a fabricated SRR medium by measuring the cross polarization of a microwave beam transmitted through the sample. We also demonstrate that the magnetoelectric coupling that gives rise to the bianisotropic response is suppressed by symmetrizing the SRR composite structure and provide measurements comparing the cross polarization of the symmetric and asymmetric structures.
Experimental evaluation of curved polymer waveguides with air trenches and offsets
Authors: Howley Brie , Wang Xiaolong , Chen Ray T. , Chen Yihong
Published: August 2006

Excess waveguide bend loss can be minimized through the use of offsets and air trenches. Offsets, used for reducing the junction loss between straight and curved waveguides, and air trenches, which prevent bend radiation loss, were simulated by a three-dimensional, semivectorial beam propagation method. Low loss polymer waveguide bend structures, employing both offsets and trenches, were fabricated. A reduction of the 180° bend insertion loss from 17.7to3.0dB with a bending radius (BR) of 1.5mm is experimentally confirmed at λ=1.55μm. BR ranging from 5to0.5mm are evaluated with decent match when compared with simulation results. The polarization dependent loss is BR dependent with a maximum value of 0.4dB when the BR is reduced to 0.5mm. The experimental results confirm that the joint use of air trenches and junction offsets is effective in reducing the bend radii of low index contrast polymer waveguides in planar lightwave circuits.
Effect of ambient pressure on resistance and resistance fluctuations in single-wall carbon nanotube devices
Authors: Vijayaraghavan A. , Kar S. , Rumyantsev S. , Khanna A. , Soldano C. , Pala N. , Vajtai R. , Kanzaki K. , Kobayashi Y. , Nalamasu O. , Shur M. S. , Ajayan P. M.
Published: August 2006

We report low-frequency resistance fluctuation (noise) measurements in semiconducting and metallic Ti∕Au-contacted single-wall carbon nanotube devices. In both types of devices, the noise power spectra has a “1∕f” dependence, and is proportional to the squared current. Semiconducting devices were found to have three orders of magnitude higher noise levels compared to the metallic ones. In vacuum, the resistance increases but noise decreases by over an order of magnitude for both metallic and semiconducting devices. The resistance and noise levels recover to their original values when the samples are brought back to atmospheric pressure. Both noise and resistance change simultaneously when the chamber is evacuated. However, when the chamber is brought back to atmospheric pressure, the noise level takes several tens of hours longer to recover.
Properties of lanthanum oxide thin films deposited by cyclic chemical vapor deposition using tris(isopropyl-cyclopentadienyl)lanthanum precursor
Authors: No Sang Yong , Eom Dail , Hwang Cheol Seong , Kim Hyeong Joon
Published: August 2006

Lanthanum oxide thin films were deposited at deposition temperatures ranging from 170to370°C using alternate injection of tris(isopropyl-cyclopentadienyl)lanthanum and various reactants (H2O, O2, and NH3). It was possible to deposit films containing a carbon impurity concentration <1at.%. The residual carbon could be further reduced by postdeposition annealing. X-ray photoelectron spectroscopy (XPS) showed that after rapid thermal annealing (RTA) at 870°C the films transformed to silicate which increased the band gap of the films (7.0±0.15eV). With changing the RTA ambient from O2 to N2, a binding energy shift of the substrate’s Si peaks by 0.9eV was observed in the XPS spectra relative to the signals from the oxide layer. The shift was attributed to the electric field in the oxide film caused by the fixed charge distribution. A flat band voltage shift indicating the change of the fixed charge was also observed according to the RTA ambient. From Pt∕La-oxide∕p-Si(100) structures, the dielectric constant and the interface trap density were estimated to be ∼16 and ∼4×1011eV−1cm−2, respectively. An equivalent oxide thickness of 0.9nm was obtained after RTA.
Eigenvalue problems and their application to the wavelet method of chaotic control
Authors: Juang Jonq , Li Chin-Lung
Published: August 2006

Controlling chaos via wavelet transform was recently proposed by Wei, Zhan, and Lai [Phys. Rev. Lett. 89, 284103 (2002)]. It was reported there that by modifying a tiny fraction of the wavelet subspace of a coupling matrix, the transverse stability of the synchronous manifold of a coupled chaotic system could be dramatically enhanced. The stability of chaotic synchronization is actually controlled by the second largest eigenvalue λ1(α,β) of the (wavelet) transformed coupling matrix C(α,β) for each α and β. Here β is a mixed boundary constant and α is a scalar factor. In particular, β=1 (respectively, 0) gives the nearest neighbor coupling with periodic (respectively, Neumann) boundary conditions. The first, rigorous work to understand the eigenvalues of C(α,1) was provided by Shieh et al. [J. Math. Phys. (to be published)]. The purpose of this paper is twofold. First, we apply a different approach to obtain the explicit formulas for the eigenvalues of C(α,1) and C(α,0). This, in turn, yields some new information concerning λ1(α,1). Second, we shed some light on the question whether the wavelet method works for general coupling schemes. In particular, we show that the wavelet method is also good for the nearest neighbor coupling with Neumann boundary conditions.