bluuebunny/crossref_metadata_2025 huggingface dataset:
Pressure effects on Al89La6Ni5 amorphous alloy crystallization
Authors: Zhuang Y. X. , Jiang J. Z. , Zhou T. J. , Rasmussen H. , Gerward L. , Mezouar M. , Crichton W. , Inoue A.
Published: July 2002

The pressure effect on the crystallization of the Al89La6Ni5 amorphous alloy has been investigated by in situ high-pressure and high-temperature x-ray powder diffraction using synchrotron radiation. The amorphous alloy crystallizes in two steps in the pressure range studied (0–4 GPa). The first process, corresponding to simultaneous precipitation of fcc-Al crystals and the metastable bcc-(AlNi)11La3-like phase, is governed by a eutectic reaction. The second process corresponds to the transformation of a residual amorphous alloy into fcc-Al, Al11La3, Al3Ni, and as yet unidentified phase(s). The applied pressure strongly affects the crystallization processes of the amorphous alloy. Both temperatures first decrease with pressure in the pressure range of 0–1 GPa and then increase with pressure up to 4 GPa. The results are discussed with reference to competing processes between the thermodynamic potential barrier and the diffusion activation energy under pressure.
Material transport regimes and mechanisms for growth of molecular organic thin films using low-pressure organic vapor phase deposition
Authors: Shtein Max , Gossenberger Herman F. , Benziger Jay B. , Forrest Stephen R.
Published: July 2002

We determine the physical mechanisms controlling the growth of amorphous organic thin films by the process of low-pressure organic vapor phase deposition (LP-OVPD). In LP-OVPD, multiple host and dopant molecular sources are introduced into a hot wall reactor via several injection barrels using an inert carrier gas, allowing for controlled film growth rates exceeding 10 Å/s. The temperature and carrier flow rate for each source can be independently regulated, allowing considerable control over dopant concentration, deposition rate, and thickness uniformity of the thin films. The rate of film deposition is limited either by the rate of condensation on the substrate or by the rate of supply from the source. The source-limited regime can be further classified into equilibrium or kinetically limited evaporation, coupled to convection- or diffusion-limited deposition. Models are developed to relate the rate of film growth to source and substrate temperature, and carrier gas flow rate. These models characterize and predict the performance of the LP-OVPD system used to grow high performance organic light emitting devices.
Microstructure and growth mode at early growth stage of laser-ablated epitaxial Pb(Zr0.52Ti0.48)O3 films on a SrTiO3 substrate
Authors: Goh W. C. , Xu S. Y. , Wang S. J. , Ong C. K.
Published: July 2002

The surface morphology and microstructure of laser-ablated Pb(Zr0.52Ti0.48)O3 (PZT) films on a (100) SrTiO3 (STO) substrate at early growth stage are characterized by means of atomic-force microscope, x-ray diffraction, and high-resolution transmission electron microscopy analysis. The (100) STO surface is found to be very favorable for epitaxial growth of (001) PZT films, which undergo a three-dimensional island growth mode. We observed a two-layer structure at the film thickness around 40–50 nm when small nuclei/grains merge into large grains. With further increase of film thickness, a column-like growth mode dominates the film crystalline structure, which results in an almost independent in-plain grain size of 100–150 nm with increasing film thickness and a limited film roughness. A very sharp interface between the PZT thin film and STO substrate is observed. The PZT film shows a perfect stacking lattice at a thickness of around 20 nm and above, indicating that the misalignment due to the interface stress and defects is healed after stacking about 50 ML of the film. These results have shed some light on the growth mechanism of epitaxial PZT film on YBCO or other bottom layers for microelectromechanical systems application.
Ground and excited states of HNC, NH, and NH2 transients: Ab initio geometries, electronic structures, and molecular properties
Authors: Pd. Rajendra , Chandra P.
Published: July 2002

Geometrical and vibrational characterization of NH2(X̃ 2B1,Ã 2A1,B̃ 2B2), NH(X̃ 3Σ−,ã 1Δ, b̃ 1Σ+,Ã 3Π,c̃ 1Π,d̃ 1Σ+), HNC(X̃ 1Σ+) and hitherto experimentally unreported HNC(ã 3A′,b̃ 3A″) systems is reported by taking different sets of active electrons/orbitals in complete active space self consistent field (CASSCF) calculations. Employing CASSCF optimized geometries, the excitation energies, electric field gradients, and dipole moments for these systems have been computed at complete active space second-order perturbation (CASPT2) and multireference singles and doubles configuration interaction (MRD-CI) levels of theory. Computed field gradients are very similar at CASSCF and CASPT2 levels, but differ significantly with those obtained at MRD-CI level. The outcome of present numerical experiment is that the inclusion of excitations higher than singles and doubles and use of a very large CI space employing iterative natural orbitals is necessary to account for the anisotropy of the molecular charge distribution around any nucleus in a molecule.
Dyadic Green’s function calculations on a layered dielectric/ferrite structure
Authors: How Hoton , Zuo Xu , Vittoria Carmine
Published: July 2002

Dyadic Green’s function techniques have been applied to solve the transmission properties of a microstrip line fabricated on top of a single-crystal Y-type hexaferrite substrate. Current potentials are used to construct the Galerkin elements to facilitate solution accuracy even in the FMR region. Transmission coefficients of a microstrip line fabricated on a Y-type hexaferrite substrate were thus calculated, which compared reasonably well with measurements.
High frequency high field permeability of patterned Ni80Fe20 and Ni45Fe55 thin films
Authors: Hsu Yimin , Fontana Robert , Williams Mason , Kasiraj Prakash , Lee Eric , McCord Jeffery
Published: July 2002

High frequency high field permeability of patterned Ni80Fe20 and Ni45Fe55 thin films was measured from lithographically defined toroids. Measurement frequency is up to 500 MHz and excitation field up to 5 Oe. Permeability rolloff is calculated from the inductance data. In this experiment, patterned widths range from 30 down to 0.5 μm. Our data suggest that the rotational permeability measured at low field excitation is sufficient to predict high frequency characteristics. The rotational permeability decreases with decreasing patterned width. The 0.5 μm wide permalloy device has μ of 80 and the Ni45Fe55 50.
Structure, magnetic properties, and coercivity mechanism of nanocomposite SmCo5/α-Fe magnets prepared by mechanical milling
Authors: Zhang Jian , Zhang Shao-ying , Zhang Hong-wei , Shen Bao-gen
Published: July 2002

Nanocomposite SmCo5+x wt %α-Fe (x=0, 20, 30, and 35) powders were prepared by mechanical milling and subsequent annealing. X-ray analyses show that hard phases can be 1:7, 1:5, or 2:7 phase with the increase of α-Fe content in as-milled powders annealed at 550 °C for 30 min. The high remanence and maximum energy product (BH)max were obtained by this method. The single-phase behavior of some powders was discussed according to the exchange spring model of Kneller and Hawig. For as-milled SmCo5+x wt %α-Fe (x=0, 20, 30, and 35) powders annealed at 550 °C for 30 min, analyses of their initial magnetization curves and the coercivities of minor hysteresis loops in dependence on the applied field reveal that coercivities of these powders are all controlled mainly by domain wall pinning. The addition of α-Fe can not change the coercivity mechanism of powders, although it affects strongly their magnetic properties.
Study with density functional theory method on methane dehydro-aromatization over Mo/HZSM-5 catalysts I: Optimization of active Mo species bonded to ZSM-5 zeolite
Authors: Zhou Danhong , Ma Ding , Liu Xianchun , Bao Xinhe
Published: July 2002

A study of the structure of active Mo species on HZSM-5 zeolite was carried out by applying the density functional theory (DFT). The interaction of the MoO2+ unit with the zeolite framework had been verified by an optimization procedure based on the DFT. The result of calculation reveals a dicoordinated bridging geometry in which the molybdenum coordinated simultaneously to both of the two lattice oxygen atoms, leading to a quadrangular geometry with an O(24)–Al(12)–O(20) bridge structure. The Mo ions having O(24)–Al(12)–O(20) bridges exist in identical planes, with equal bond distances of 1.72 Å to both the O(24) and O(20) atoms and an interatomic distance of 2.86 Å to lattice Al. The molybdenum has a tetrahedral geometry and a formal charge of +5. This kind of active Mo center possesses a typical C2v symmetry. Approximation vibrational analysis indicates that the vibrations of Mo–O (framework oxygen) bonds occur at the frequencies between 580 and 928 cm−1, while the vibrations of the Mo–O (extra-framework oxygen) bonds appear between 956 and 975 cm−1. Population analysis confirmed that the bonds between extra-framework oxygen atoms and the central Mo atom are of a typical covalent double bond in nature, whereas the coupling between the lattice oxygen atoms and the central Mo atom shows a sizable ionic contribution. Theoretical predictions from the present model of the Mo/HZSM-5 active centers agree well with our experimental results from magic angle spinning nuclear magnetic resonance (MAS NMR), electron spin resonance (ESR), and Fourier-transform infrared (FTIR) experiments.
Paraxial WKB description of short wavelength eigenmodes in a tokamak
Authors: Pereverzev G. V.
Published: July 2002

A novel asymptotic approach for solving the eigenvalue problem in the short-wavelength limit is developed. This approach, called the paraxial WKB method, is presented and applied to the analysis of gradient-driven instabilities in a tokamak. In some respects, the paraxial WKB method is more general than the ballooning representations usually used for this problem. For instance, the paraxial WKB method can describe drift-ballooning instabilities in the presence of sheared plasma rotation when the ballooning representation fails. Being different from other techniques, the paraxial WKB method not only sheds new light on the physics of drift-ballooning instabilities, but also provides a description for short-wave instabilities of other types. Other possible extensions of the paraxial WKB technique, such as the application to three-dimensional systems like stellarators, are briefly discussed. Finally, advantages and disadvantages of the paraxial WKB method in comparison with the ballooning representation are considered.
Two-wire thermocouples: A nonlinear state estimation approach to temperature reconstruction
Authors: O’Reilly Paul G. , Kee Robert J. , Fleck Robert , McEntee Paul T.
Published: July 2002

The problem of measuring high frequency variations in temperature is described, and the need for some form of reconstruction introduced. One method of reconstructing temperature measurements is to use the signals from two thermocouples of differing diameter. Two existing methods for processing such measurements and reconstructing the higher frequency components are described. These are compared to a novel reconstruction algorithm based on a nonlinear extended Kalman filter. The performance of this filter is found to compare favorably, in a number of ways, with the existing techniques, and it is suggested that such a technique would be viable for the online reconstruction of temperatures in real time.
Simulation of polyethylene oxide: Improved structure using better models for hydrogen and flexible walls
Authors: Halley J. W. , Duan Yuhua , Nielsen B. , Redfern Paul C. , Curtiss Larry A.
Published: July 2002

We describe calculations of the structure of amorphous polyethylene oxide using a previously reported model, but with better treatment of hydrogen positions and in a code which allows relaxation of stresses in the polymerized sample by Rahman-Parrinello techniques. We also report the effects of two different intermolecular force field potentials and find that our earlier, empirical force field produces better agreement with experimental neutron scattering results than a force field derived from ab initio electronic structure calculations.
Quantitative in situ nanoindentation in an electron microscope
Authors: Minor A. M. , Morris J. W. , Stach E. A.
Published: July 2002

We report the development of a method for quantitative, in situ nanoindentation in an electron microscope and its application to study the onset of deformation during the nanoindentation of aluminum films. The force–displacement curve developed shows the characteristic “staircase” instability at the onset of plastic deformation. This instability corresponds to the first appearance of dislocations in a previously defect-free grain. Plastic deformation proceeds through the formation and propagation of prismatic loops punched into the material, and half loops that emanate from the sample surface. These results represent the first real time observations of the discrete microstructural events that occur during nanoindentation.
Rayleigh–Bénard convection in liquid metal layers under the influence of a vertical magnetic field
Authors: Burr Ulrich , Müller Ulrich
Published: July 2002

The influence of a vertical magnetic field on the integral heat transfer and the temporal dynamics of liquid metal Rayleigh–Bénard convection is studied in an experiment using a small Prandtl number (Pr≈0.02) sodium potassium alloy Na22Kr78 as a test fluid. The test section is a rectangular box of large aspect ratio 20 : 10 : 1 that covers a parameter range of Rayleigh numbers, 103<Ra<105, and Chandrasekhar numbers, 0<Q<1.44×104. The integral heat transfer across the layer is evaluated from the measured temperatures at the upper and the lower boundary and the applied heat flux. Local, time-dependent temperatures are obtained from a four-element temperature probe placed in the middle of the liquid metal layer. The noncoplanar arrangement of the thermocouples enables the evaluation of the time-dependent temperature gradient vector that allows us to estimate the local isotropy properties of the time-dependent flow. From the damping effect of Joule dissipation, the convective heat transport decreases monotonically with increasing Chandrasekhar numbers. Fluctuations of the temperature field are damped significantly by the magnetic field. However, this effect is selective with respect to frequency. Long period fluctuations are strongly damped whereas short period fluctuations are less damped or may even be amplified. The observations show that significant convective heat transport is practically always associated with time-dependent flow. The fluctuating part of the local temperature gradient confirms the horizontal isotropy of the velocity field; no predominant orientation of time-dependent flow structures is established either with or without a magnetic field.
Pulsed-laser Doppler flowmetry provides basis for deep perfusion probing
Authors: Kolkman Roy G. M. , Hondebrink Erwin , Bolt René A. , Steenbergen Wiendelt , de Mul Frits F. M.
Published: July 2002

A setup for pulsed-laser Doppler flowmetry (LDF) measurements has been built and tested. Measurements were carried out comparing continuous-wave and pulsed LDF. With pulsed LDF a higher peak power can be injected into the tissue without exceeding the safety limits. This enables a much larger spacing between the locations of illumination and detection. Thus, the penetration depth, and thus the measurement volume, can be enlarged using the pulsed-LDF method. This method will allow, e.g., monitoring of the cerebral perfusion.
Oriented growth of β-SiC on diamond crystals at high pressure
Authors: Voronin G. A. , Pantea C. , Zerda T. W. , Ejsmont K.
Published: July 2002

Interaction between diamond crystals and liquid silicon at pressures of 2 and 9 GPa and various temperatures was studied by Raman spectroscopy, x-ray single crystal diffractometry, and scanning electron microscopy. The mechanism of growth of silicon carbide (SiC) film on diamond crystals depended on the magnitude of applied pressure. At low pressures, in the graphite stable region, only disoriented, fine grain β-SiC crystals were formed inside silicon surrounding diamond crystals. At higher pressures corresponding to the diamond stable region, oriented growth of β-SiC film on both {111} and {100} faces of diamond crystals was observed, even at high growth rates.
The structure of a poly(ethylene oxide) melt from neutron scattering and molecular dynamics simulations
Authors: Annis B. K. , Borodin Oleg , Smith Grant D. , Benmore C. J. , Soper A. K. , Londono J. D.
Published: July 2002

We have determined the static structure factor for a poly(ethylene oxide) (PEO) melt from neutron scattering and atomistic molecular dynamics simulations. The experimental total structure factors for protonated and deuterated samples were found to be in good agreement with simulation. The calculated partial structure functions facilitate the interpretation of features in the total radial distribution function in the 5–15 Å range and comparison is made with previous work on poly(ethylene). Using hydrogen–deuterium isotope substitution methods, the intermolecular H–H pair distribution function was determined. Unlike a previous comparison for polyethylene, only qualitative agreement between experiment and simulation for the intermolecular H–H pair distribution function was obtained.
Determination of the glass transition and nucleation temperatures in Ge2Sb2Te5 sputtered films
Authors: Morales-Sánchez E. , Prokhorov E. F. , Mendoza-Galván A. , González-Hernández J.
Published: July 2002

Amorphous Ge2Sb2Te5 films were prepared by rf sputtering from the bulk alloy. Their electrical and optical properties were analyzed using impedance and optical transmission measurements as a function of temperature. From this analysis, it is found that the glass transition temperature of the as-prepared amorphous films is of about 100 °C. This result is confirmed by calorimetric measurements. Impedance and optical measurements in films measured at temperatures between the glass transition and the crystallization temperatures show the appearance of the nucleation centers. Using models for two-phase materials, the electrical parameters of the crystalline and amorphous phases were estimated and related with structural parameters of the samples.
A theoretical study of liquid alcohols using averaged solvent electrostatic potentials obtained from molecular dynamics simulations: Methanol, ethanol and propanol
Authors: Martı́n M. E. , Sánchez M. L. , Olivares del Valle F. J. , Aguilar M. A.
Published: July 2002

We applied a quantum mechanics/molecular mechanics method that makes use of the mean field approximation to study the polarization of several alcohols in the liquid phase. The method is based on the calculation of the averaged solvent electrostatic potential from molecular dynamics data. Because of the reduced number of quantum calculations that our approximation involves, it permits the use of flexible basis sets, the consideration of the electron correlation and the solvent and solute polarization. We found that the molecules studied undergo strong polarization when they pass from the gas to the liquid phase. From this point of view, the polarization methanol displays a behavior lightly different from ethanol and propanol. The vaporization energies are very well reproduced especially when the correlation energy is included. The differences with the experimental values are less than 3% in the three systems studied. Finally, we consider the effect on the thermodynamics and the structure of the solution of the choice of the Lennard-Jones parameters.
A globally smooth ab initio potential surface of the 1 A′ state for the reaction S(1D)+H2
Authors: Ho Tak-San , Hollebeek Timothy , Rabitz Herschel , Der Chao Sheng , Skodje Rex T. , Zyubin Alexander S. , Mebel Alexander M.
Published: July 2002

A procedure based on the reproducing kernel Hilbert space (RKHS) interpolation method has been implemented to produce a globally smooth potential energy surface (PES) for the 1 A′ state of the S(1D)+H2 reaction from a set of accurate ab initio data, calculated at the multireference configuration interaction level with augmented polarized quadruple-zeta basis sets and arranged on a three-dimensional regular full grid in the Jacobi coordinates. The procedure includes removing a small number of questionable ab initio data points, implementing a recently developed technique for efficiently handling a partially filled grid, and adopting a sequence of regularizations for attaining additional smoothness. The resulting RKHS PES is analytic, first-order differentiable, and fast to evaluate. Quasiclassical trajectory calculations have been performed and compared with the results based on a recent hybrid PES obtained from a combination of the RKHS interpolation in the entrance channel and Murrell–Carter (MC)-type fitting in the exit channel from the same set of ab initio data. Comparisons with recent experimental measurements show improvement of the present RKHS PES over the existing hybrid RKHS-MC PES. The results demonstrate that the entrance channel Jacobi coordinates can still be a good candidate in sampling the full configuration space for reactive systems involving three atoms.
Low-voltage organic electroluminescent devices using pin structures
Authors: Huang Jingsong , Pfeiffer Martin , Werner Ansgar , Blochwitz Jan , Leo Karl , Liu Shiyong
Published: July 2002

We have realized a small-molecule organic light-emitting diode where the intrinsic emitter layer is sandwiched by n- and p-doped transport layers with appropriate blocking layers. The diodes based on this pin concept have exponential forward characteristics up to comparatively high current densities. The diodes reach high brightness at very low operating voltage: for instance, 1000 cd/m2 at a voltage of 2.9 V. Despite the highly doped transport layers, the devices reach very high efficiency for the given emitter system up to high brightness.
Experimental and theoretical confirmation of Bloch-mode light propagation in planar photonic crystal waveguides
Authors: Lončar Marko , Nedeljković Dušan , Pearsall Thomas P. , Vučković Jelena , Scherer Axel , Kuchinsky Sergey , Allan Douglas C.
Published: July 2002

The dispersion diagram of the leaky modes in the planar photonic crystal waveguide is experimentally obtained for the wavelengths from 1440 to 1590 nm. A small stop band, around wavelength 1500 nm, is detected. The experimentally obtained results are in very good agreement with our three-dimensional finite difference time domain calculations. Propagation losses of the leaky modes are estimated and we have found that they decrease as we approach the ministop band.
Evaluation of grain boundary trap states in polycrystalline–silicon thin-film transistors by mobility and capacitance measurements
Authors: Ikeda H.
Published: July 2002

A method for evaluating localized grain boundary (GB) trap states in polycrystalline–silicon thin-film transistors (poly-Si TFTs) is proposed. Field effects in poly-Si TFTs are analyzed by decomposition of the two-dimensional Poisson’s equation. This analysis gives the theoretical basis for evaluating the localized GB trap state density. The trap states are evaluated with the GB barrier potential given by the activation energy of the effective mobility and with the surface potential calculated from the capacitance–voltage characteristics in poly-Si TFTs. This evaluating method was applied to several different types of poly-Si TFTs fabricated with excimer laser annealing crystallization processes. This method has succeeded experimentally in eliminating the error effects related to oxide–semiconductor interface trap states and in clarifying that the GB barrier potential has a peak in its gate bias dependence. This method has also elucidated that the GB trap state density has almost the same value independent of the grain size when the hydrogenation efficiency in poly-Si is in the same level between TFTs. The good agreement of the simulated and the measured drain currents confirm the validity of this method.
Convective flows in rapidly rotating spheres and their dynamo action
Authors: Busse F. H.
Published: July 2002

The present understanding of convection in rotating spherical fluid shells is reviewed and some recent results on coherent structures and magnetic field generation are considered in detail. The constraints exerted by the Coriolis force leads to unusual properties not found in nonrotating systems, such as vacillations, localized convection, and chaotic relaxation oscillations. The central role played by the differential rotation generated by the Reynolds stress of convection in the case of Prandtl numbers of the order unity or less is emphasized. Magnetic fields generated through the dynamo process offer new degrees of freedom. Through the braking of the differential rotation the Lorentz force contributes to an enhanced heat transport.
Application of a Drude model to the binding of excess electrons to water clusters
Authors: Wang F. , Jordan K. D.
Published: July 2002

A one-electron model potential approach for calculating the binding energies of an excess electron interacting with water clusters is presented. Drude oscillators are employed on the water monomers to describe electron–water polarization and dispersion interactions. It is demonstrated that the resulting model gives electron binding energies very close to the predictions of ab initio CCSD(T) calculations. Dispersion interactions and high-order renormalization effects are found to make large contributions to the electron binding energies.
Resonant detection of subterahertz radiation by plasma waves in a submicron field-effect transistor
Authors: Knap W. , Deng Y. , Rumyantsev S. , Lü J.-Q. , Shur M. S. , Saylor C. A. , Brunel L. C.
Published: July 2002

The resonant detection of subterahertz radiation by two-dimensional electron plasma confined in a submicron gate GaAs/AlGaAs field-effect transistor is demonstrated. The results show that the critical parameter that governs the sensitivity of the resonant detection is ωτ, where ω is the radiation frequency and τ is the momentum scattering time. By lowering the temperature and hence increasing τ and increasing the detection frequency ω, we reached ωτ∼1 and observed resonant detection of 600 GHz radiation in a 0.15 μm gate length GaAs field-effect transistor. The evolution of the observed photoresponse signal with temperature and frequency is reproduced well within the framework of a theoretical model.
Hyperbolic lines and the stratospheric polar vortex
Authors: Koh Tieh-Yong , Legras Bernard
Published: July 2002

The necessary and sufficient conditions for Lagrangian hyperbolicity recently derived in the literature are reviewed in the light of older concepts of effective local rotation in strain coordinates. In particular, we introduce the simple interpretation of the necessary condition as a constraint on the local angular displacement in strain coordinates. These mathematically rigorous conditions are applied to the winter stratospheric circulation of the southern hemisphere, using analyzed wind data from the European Center for Medium-Range Weather Forecasts. Our results demonstrate that the sufficient condition is too strong and the necessary condition is too weak, so that both conditions fail to identify hyperbolic lines in the stratosphere. However a phenomenological, nonrigorous, criterion based on the necessary condition reveals the hyperbolic structure of the flow. Another (still nonrigorous) alternative is the finite-size Lyapunov exponent (FSLE) which is shown to produce good candidates for hyperbolic lines. In addition, we also tested the sufficient condition for Lagrangian ellipticity and found that it is too weak to detect elliptic coherent structures (ECS) in the stratosphere, of which the polar vortex is an obvious candidate. Yet, the FSLE method reveals a clear ECS-like barrier to mixing along the polar vortex edge. Further theoretical advancement is needed to explain the apparent success of nonrigorous methods, such as the FSLE approach, so as to achieve a sound kinematic understanding of chaotic mixing in the winter stratosphere and other geophysical flows.
Dye-doped polymers for blue organic diode lasers
Authors: Dı́az-Garcı́a Marı́a A. , Fernández De Ávila Susana , Kuzyk Mark G.
Published: July 2002

Amplified spontaneous emission (ASE) in optically pumped polymeric films containing the luminescent and hole-transporting organic molecule N,N′-Bis(3-methylphenyl)-N,N′diphenylbenzidine (TPD) is demonstrated. ASE is observed in polymer films of polystyrene and poly(N-vinylcarbazole) doped up to 20% with TPD. From these results, we conclude that TPD-based films are candidates for blue-emitting organic diode lasers.
Solar thermophotovoltaics: An assessment
Authors: Spirkl W. , Ries H.
Published: July 2002

A general model of a solar thermophotovoltaic device is discussed both for improving the efficiency of one-band-gap photovoltaic cells by matching the photon energy to the band gap and for concentrating diffuse radiation. First we assume ideal components to calculate theoretical maximum efficiency. It corresponds to that of a perfect selective absorber in conjunction with a Carnot-engine ranging from 53% for 1 sun to 85% for the highest possible irradiance of 5×104 suns. The improvement over an ideal one-gap device is roughly a factor of 2. Consideration of available materials shows that any improvement in efficiency can be expected only for high irradiance of 1000 suns. The sensitive parameter is the selectivity of the absorber-emitter. Concentration of diffuse solar radiation is not feasible. Perspectives appear not much better than for existing technologies such as photovoltaics or solar tower applications.
Pulsed excimer (KrF) laser melting of amorphous and crystalline silicon layers
Authors: Narayan J. , White C. W. , Aziz M. J. , Stritzker B. , Walthuis A.
Published: July 2002

We have investigated depth of melting as a function of pulse energy density in amorphous and crystalline silicon layers. The melting threshold for KrF laser pulses (λ=0.249 μm, τ=24×10−9 s) in amorphous (7660-Å-thick) and crystalline silicon layers were determined to be 0.16±0.02 and 0.75±0.05 J cm−2, respectively. The formation of fine- and large-polycrystalline regions was clearly identified in the amorphous silicon layers for energy densities below that needed for complete annealing. The role of explosive recrystallization in the formation of the fine polycrystalline region is discussed.
Comparative electron spectroscopic studies of surface segregation on SiC(0001) and SiC(0001̄)
Authors: Muehlhoff L. , Choyke W. J. , Bozack M. J. , Yates John T.
Published: July 2002

Electron spectroscopic comparison of the C-rich SiC(0001̄) and Si-rich SiC(0001) surfaces after cleaning and disordering by Ar+ ion sputtering and subsequent annealing is reported. The chemical behavior of the two disordered surfaces differs significantly. Three distinct temperature regions with different carbon surface segregation kinetics are discernible on SiC(0001̄). On SiC(0001) only one temperature region for C-segregation is observed. Below 900 K, no spectroscopic differences between the two crystal surfaces are observed. Between 900 and 1300 K, both faces are terminated by a surface graphite layer and the C-rich face shows an additional carbon surface segregation process. Above 1300 K, the C-terminated surface graphitizes at a higher rate than the Si-terminated surface. Massive graphitization on both surfaces above 1300 K is attributed to Si(g) sublimation from the SiC surfaces. The results demonstrate that extensive surface disordering of polar SiC faces does not destroy the memory for the polarity of the original crystal insofar as high-temperature surface chemistry is concerned.
The characteristics of optical waveguides fabricated in Y- and Z-cut LiNbO3 by He+ implantation
Authors: Weiss B. L. , Flint J. L.
Published: July 2002

The effects of the orientation of the LiNbO3 samples on the characteristics of planar waveguides fabricated by He+ implantation are presented. They show that the number of extraordinary modes is the same in Y- and Z-cut samples whereas the Z-cut samples support more ordinary modes than do Y-cut samples. Also the attenuation of each mode is lower in Z-cut material, showing that Z-cut material is more suitable for the fabrication of low loss waveguides.
Thin-film palladium and silver alloys and layers for metal-insulator-semiconductor sensors
Authors: Hughes R. C. , Schubert W. K. , Zipperian T. E. , Rodriguez J. L. , Plut T. A.
Published: July 2002

The addition of Ag to Pd in the gate metal of a metal-insulator-semiconductor gas sensing diode can improve the performance and change the selectivity of the sensors for a variety of reactions. Data on the response of diodes with 12 different ratios of Ag to Pd in alloys and layers of Pd and Ag to hydrogen and other gases are reported. Diodes with as much as 32% Ag respond very well to H2 gas and the films are much more durable to high hydrogen exposure than pure Pd films. Improvements in the rate of response and aging behavior are found for certain Ag combinations; others give poorer performance. The presence of Ag on the surface changes the catalytic activity in some cases and examples of H2 mixed with O2 and/or NO2, propylene oxide, ethylene, and formic acid are given. Such selectivity forms the basis for miniature chemical sensor arrays which could analyze complex gas mixtures.
The Hugoniot and shock sensitivity of a plastic-bonded TATB explosive PBX 9502
Authors: Dick J. J. , Forest C. A. , Ramsay J. B. , Seitz W. L.
Published: July 2002

The plane shock response of PBX 9502 was measured from 0.5 to 25 GPa. The explosive is 95-wt. % triamino-trinitrobenzene powder coated with Kel-F 800 plastic; porosity was 2.6%. Shock initiation sensitivity, the Hugoniot, and dynamic yield behavior were studied. Experiments done were explosively driven wedge tests and particle velocity history measurements using electromagnetic gauges at a light-gas gun. A new analysis of the wedge test was implemented. Based on wedge test results, an approximation for the particle velocity at the shock front is constructed, which is used in functional composition with the Hugoniot to calculate the shock path of each experiment. Optimization on the parameters of the approximation and the Hugoniot simultaneously ensures agreement of the Hugoniot with the overall kinematics of the shock growth as well as the particle velocity/shock velocity data. The individual shock paths are used to examine the single-curve buildup assumption; the assumption is not valid for PBX 9502. A ramping elastic precursor of 0.073 GPa strength was observed. The yield and weak shock behavior are interpreted in terms of porosity, crystalline anisotropy, and residual strain in the pressed, plastic-bonded explosive.
Material properties determining the resistance of ceramics to high velocity penetration
Authors: Sternberg Joseph
Published: July 2002

The relationships between target material properties and the target strength term in the analytic representation of impact is examined. For ductile materials hardness is closely related to the magnitude of the strength term. It is shown that the key parameters correlating microhardness measurements in ceramics are similar to those for ductile materials. However, the strength terms that have been measured in ballistic tests are much lower than the values that would be predicted on the basis of the indentation measurements. It is found that the penetration resistance depends on the fracture toughness, where the ratio of the measured target strength term to the hardness increases with the fracture toughness of the target.
Figure of merit for thermoelectrics
Authors: Mahan G. D.
Published: July 2002

Theoretical calculations are presented for the figure of merit Z in thermoelectrics. The maximum values of Z are obtained in semiconductors which are doped so that the chemical potential is near the band edge. The highest Z is related to the B parameter of Chasmar and Stratton [J. Electron. Control 7, 52 (1959)].
Loss of epitaxy during diamond film growth on ordered Ni(100)
Authors: Belton D. N. , Schmieg S. J.
Published: July 2002

Filament assisted diamond film growth was studied on a clean and highly oriented Ni(100) substrate using x-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (EELS), x-ray excited Auger electron spectroscopy (XAES), and low-energy electron diffraction (LEED) to examine the sample at selected intervals in the growth process. The sample was transferred between the growth chamber and the ultrahigh vacuum analytical chamber without exposure to air. Scanning electron microscopy (SEM) was also used to characterize the sample; however, this required exposing the sample to air. Prior to growth the Ni(100) crystal was cleaned and ordered with a combination of Ar-ion bombardment, oxygen/hydrogen treatments, and annealing. After 2 min growth, microcrystalline graphite islands formed on top of a c(2×2) carbon layer. Later, LEED showed a graphite surface with very poor azimuthal orientation. This surface developed into a disordered nondiamond surface that we assign as glassy carbon. Next, EELS and XAES showed new features due to diamond. Growth was stopped when a continuous diamond film was formed. SEM confirmed the presence of well-faceted diamond crystals on a disordered and highly three-dimensional surface. These experiments show that the graphite and glassy carbon layers, formed prior to diamond, are poorly oriented with respect to the Ni(100) surface. Since these intermediate layers are not in registry with the Ni(100) surface, subsequent diamond growth is not epitaxial.
Anisotropic perpendicular axis magnetostriction in twinned TbxDy1−xFe1.95
Authors: Teter J. P. , Wun-Fogle M. , Clark A. E. , Mahoney K.
Published: July 2002

The longitudinal magnetostriction (Δl/l) for twinned TbxDy1−xFe1.95 material, prepared by the free-standing float-zone method, has been previously measured and found to be very large (2000×10−6) at room temperature. The magnetostrictions for the [111] and [11̄0] crystallographic axes perpendicular to the applied stress and magnetic-field [112̄] direction are presented as functions of temperature, applied stress, and applied magnetic field. The temperature range is ±60 °C centered about the anisotropy compensation temperature of Terfenol-D (+10 °C). The stress ranges from 2 to 32 MPa and the magnetic field to ±2000 Oe. The temperature dependence of the perpendicular axes magnetostriction is similar to that of the conventional magnetostriction measured parallel to the [112̄] growth axis. Relative values for the saturation magnetostriction exhibit a large anisotropy in the perpendicular direction, ranging from −117% for the [111] to +19% for the [11̄0] direction. This ratio stays constant as a function of temperature above the compensation temperature for a given prestress above 8 MPa. The largest absolute value of magnetostriction (2260×10−6) occurs in the [111] direction at 10 °C at a prestress pressure of 12 MPa. Above this temperature the magnetostriction falls at a rate of −8.5×10−6/°C. The volume magnetostriction is shown to be small and highly sample dependent. All data is consistent with the parent-twin magnetization model and also the magnetostriction values along nonprincipal axes.
Magnetic and mechanical properties of rotary forged aluminum compacted Nd-Fe-B magnets
Authors: Brett R. L. , Rowlinson N. , Ashraf M. M. , Harris I. R. , Bowen P.
Published: July 2002

Magnetic and mechanical properties have been studied and compared for rotary forged aluminum compacts containing MQI and MQIII (isotropic and anisotropic melt-spun Nd-Fe-B, respectively) in the as-forged condition, and after subsequent heat treatment both above and below the Curie point. A maximum coercivity of 1165 kA m−1 and a maximum energy product of 84 kJ m−3 have been measured for compacts containing MQI material and 10% aluminum by volume. Compacts containing MQIII material possess superior fracture strengths to those containing MQI material in both bend tests (30 compared with 17 MPa) and uniaxial compression tests (185 compared with 67 MPa), and this is attributed primarily to more uniform compaction due to the morphology of the MQIII particles. Energy dispersive spectrography and fractography have indicated failure of MQIII compacts along interfaces between the aluminum and magnetic material, whereas MQI compacts fail through areas of incomplete compaction. In addition, heat treatment in vacuum subsequent to forging is shown to strengthen compacts containing both MQI and MQIII material substantially with only minor reductions measured in magnetic properties.
Current melt-wave model for transitioning solid armature
Authors: Parks Paul B.
Published: July 2002

A model is developed to describe the evolution of a solid armature to the point where metallic contact with the rails is lost. The idea is that a current/melt wave begins at the rear of the armature, where current is concentrated by velocity skin effect, and propagates forward along the rail/armature interface as molten armature material is lost. When the melt wave reaches the front of the armature, the transition to plasma brush contact occurs. A calculation with the model simulating an earlier solid armature experiment at General Atomics shows close agreement with the measured transition velocity, as inferred from the increase in muzzle voltage observed during the shot.
The initial susceptibility of ferrites: A quantitative theory
Authors: Bouchaud J. P. , Zerah P. G.
Published: July 2002

In this contribution a firm foundation to the observed complex permeability spectrum of ferrites (such as NiZn or MnZn spinels) in the gyromagnetic regime (100 MHz, 10 GHz) is provided. Many attributes of the initial permeability of uniaxial ferrites are not correctly reproduced by a straightforward application of the usual Landau–Lifschitz–Gilbert equation. Among those one can cite the observed, non-Lorentzian shape of μ′ and μ″, and the permanence of losses (μ″(ω)>0) up to the frequency ωm=γ4πMs, above which they disappear (‘‘low field loss’’). The problem is recast here in the general framework of effective-medium theory which provides a firm interpretation of the observed complex permeability spectrum of ferrites (such as NiZn or MnZn spinels) in the gyromagnetic regime (100 MHz, 10 GHz).
A transmission electron microscopy study of low-temperature reaction at the Co-Si interface
Authors: Ruterana P. , Houdy P. , Boher P.
Published: July 2002

An efficient preparation method, which provides wedge-shaped cross-section transmission electron microscopy samples, has been developed. It was then used to investigate the structure of as-deposited cobalt multilayers on silicon substrates by rf plasma sputtering. It was found that an extended reaction takes place between Co and Si probably during the deposition. The cobalt atoms react with the silicon substrate to form an amorphous silicide layer. When the deposited layer is <3 nm thick, it entirely reacts with the substrate and can form an amorphous silicide as large as 5 nm. Above 4–5 nm thickness, growth of Co crystallites comes in competition with the formation of the amorphous silicide and limits it to 2 nm. The composition of this amorphous silicide is estimated to be Co2Si. In Co/C multilayers, the reactivity between the two materials is negligible, and the coalescence thickness of cobalt is 2–3 nm. At 2 nm, the cobalt layers are noncontinuous and very rough, whereas at 3 nm the critical thickness for crystalline nuclei coalescence has already been reached. The cobalt layers are then polycrystalline and have a reasonable roughness.
Current distribution of an electric arc at the surface of plasma torch electrodes
Authors: Szente R. N. , Drouet M. G. , Munz R. J.
Published: July 2002

Current distributions of the arc foot on the surface of the electrodes in a geometry simulating a plasma torch are reported. Most of the experiments were to determine the arc current distribution at the cathode. The results were obtained using a novel technique and represent experiments using different plasma gases (Ar, He, Ar+0.3% CO, He+0.4% CO, He+0.4% N2, N2, CO) and operating conditions (magnetic field and arc velocity). It is shown that all three, the surface composition due to contamination, the transverse magnetic field used to move the arc, and the arc velocity have a strong influence on the current distribution. Higher erosion rates were found for higher current densities of the arc foot.
Planarization of SiO2 films using reactive ion beam in plasma enhanced chemical vapor deposition
Authors: Huo D. T. C. , Yan M. F. , Chang C. P. , Foo P. D.
Published: July 2002

We have developed a substrate biased plasma enhanced chemical vapor deposition technique to fill high aspect ratio submicron gap in first metal contact layer. In this technique, active ion deposition from tetraethylorthosilicate and oxygen and ion bombardment from oxygen and argon ions occurred concurrently to fill 0.5 μm height/0.5 μm space aluminum patterns on silicon wafer without forming soft spots and keyholes. The effects of argon ions to the modification of surface topography are discussed. The deposited films have a low residual stress (< −3 × 109 dyne/cm2) and wet etching rate. The properties of the deposited films have been evaluated by infrared spectroscopy and triangular voltage sweeping technique.
Metal bonded Sm2Fe17N3−δ magnets
Authors: Otani Yoshichika , Moukarika A. , Sun Hong , Coey J. M. D. , Devlin E. , Harris I. R.
Published: July 2002

Sm2Fe17N3−δ is a promising permanent magnet material but disproportionation above 650 °C means that only low-temperature processing can be considered. Sintering micron-size Sm2Fe17N3−δ powder with the soft, low-melting metals Zn, Bi, Sn, and Al has been investigated. Good results are obtained with zinc. A coercivity of 0.6 T and a maximum energy product of 84 kJ/m3 (10.5 MGOe) have been obtained. Zn7Fe3 is formed as a secondary phase.
X-ray photoemission spectra of reactively sputtered TiN
Authors: Delfino M. , Fair J. A. , Hodul D.
Published: July 2002

X-ray photoemission spectra of reactively sputtered TiN1.0 films are recorded without interference from adsorbed contaminants or ion sputter cleaning damage. In this way, the transition from hcp TiN0.3 to fcc TiN1.0 is characterized by a discontinuity in film stoichiometry, Ti 2p splitting energy, and Ti 2p3/2 binding energy as a function of the Ar/N2 ratio during sputtering. The line shapes of the N 1s and 2s transitions experience only an asymmetric broadening on forming fcc TiN. The core-level N 1s transition of fcc TiN is modeled as two components peaks with binding energies at 396.8 and 396.0 eV. Similarly, the valence band N 2s transition has corresponding component peaks at 16.8 and 16.2 eV. These high and low binding energy pairs are interpreted as on-site Ns and interstitial site Ni populations of nitrogen in a fcc TiN lattice, respectively. The ratio of N/Ti is 1.0 and the Ns/Ni ratio is approximately 6. Both ratios are independent of the composition of the sputtering gas mixture and the substrate temperature once fcc TiN is formed. The core level Ti 2p transition in fcc TiN is characteristic of a single Ti3+ oxidation state with a line shape that is also insensitive to the gas composition and the substrate temperature during sputtering.
Optimization of injection current and feedback phase of an optically self-locked laser diode
Authors: Ohshima Shin-ichi , Schnatz Harald
Published: July 2002

The injection current and feedback phase of an optically self-locked laser diode system are controlled simultaneously to the optimum condition by modulating the injection current and using phase sensitive detection. This simple method requires no special devices and techniques. A small modulation index, less than 0.2, was achieved and this method is expected to be applicable in many fields.