bluuebunny/crossref_metadata_2025 huggingface dataset:
Stress-related problems in silicon technology
Authors: Hu S. M.
Published: July 2002

The silicon integrated-circuits chip is built by contiguously embedding, butting, and overlaying structural elements of a large variety of materials of different elastic and thermal properties. Stress develops in the thermal cycling of the chip. Furthermore, many structural elements such as CVD (chemical vapor deposition) silicon nitride, silicon dioxide, polycrystalline silicon, etc., by virtue of their formation processes, exhibit intrinsic stresses. Large localized stresses are induced in the silicon substrate near the edges and corners of such structural elements. Oxidation of nonplanar silicon surfaces produces another kind of stress that can be very damaging, especially at low oxidation temperatures. Mismatch of atomic sizes between dopants and the silicon, and heteroepitaxy produce another class of strain that can lead to the formation of misfit dislocations. Here we review the achievements to date in understanding and modeling these diverse stress problems.
Electrical and optical properties of boron-doped ZnO thin films for solar cells grown by metalorganic chemical vapor deposition
Authors: Wenas Wilson W. , Yamada Akira , Takahashi Kiyoshi , Yoshino Masahiro , Konagai Makoto
Published: July 2002

The highly conductive and textured ZnO films have been grown by metalorganic chemical vapor deposition using diethylzinc and H2O as reactant gases. The B2H6 gas has also been successfully used as an n-type dopant gas to obtain highly conductive ZnO with a sheet resistivity for 2-μm-thick film as low as 10 Ω/⧠ at the very low temperature of 150 °C. It was found that the crystal orientation and grain structure change with B2H6 flow rate. The decreasing of the film transmittance due to the free-carrier absorption in the wavelength region above 1000 nm was observed and it seems that the impurity scattering was the dominant interaction during this process. The shift of the absorption edge due to band filling was also observed as the B2H6 flow rate was increased.
Optimization of growth conditions of vapor deposited Mo/Si multilayers
Authors: Stearns Mary Beth , Chang Chung-Hee , Stearns Daniel G.
Published: July 2002

The dependence of the layer structure and interfaces of e-beam deposited Mo/Si multilayers (ML) on growth conditions has been studied. The substrate temperature was varied over a range of 300–600 K at deposition rates of 1 and 3 Å/s. The structure of the ML was determined using small-angle x-ray scattering, large-angle x-ray scattering, and cross-sectional high-resolution electron microscopy. The variation of the normal incidence reflectivity was measured as a function of soft x-ray wavelength using synchrotron radiation. We found that the lateral extent of the Mo crystallites and the thickness and roughness of interlayers are a sensitive function of the substrate temperature. The morphology shows a lesser dependence on the deposition rate. The formation of the Mo-on-Si interlayer is partially thermally activated with an activation energy of ∼0.2 eV. We propose that the rate limiting mechanism is surface diffusion during growth. At the optimal deposition conditions, of a substrate temperature of ∼525 K and a deposition rate of 1 Å/s, the ML structure was found to have a smoothness and maximum normal incidence reflectivity comparable to the best sputtered Mo/Si films.
Electroluminescence and electrical transport in poly(3-octylthiophene) diodes
Authors: Braun D. , Gustafsson G. , McBranch D. , Heeger A. J.
Published: July 2002

We report visible light emission from diodes made from poly(3-octylthiophene). Use of a soluble derivative of polythiophene allows fabrication of the light emitting diodes by casting the polymer film from solution with no subsequent processing or heat treatment required. The devices emit dim red-orange light with relatively low external quantum efficiencies, below 2.5×10−5 photons per electron at room temperature. Electrical characterization reveals diode behavior with rectification ratios greater than 102. The temperature dependence indicates that tunneling phenomena dominate the charge injection.
A systematic approach to the measurement of ideality factor, series resistance, and barrier height for Schottky diodes
Authors: Lee T. C. , Fung S. , Beling C. D. , Au H. L.
Published: July 2002

A nongraphical approach is proposed for measuring and evaluating the ideality n factor and the series resistance of a Schottky diode. The approach involves the use of an auxiliary function and a computer-fitting routine. This technique has been found to be both accurate and reliable. The validity of this has also been confirmed by way of I-V measurements using both commercially available and laboratory-prepared Schottky diodes.
Manipulation of van der Waals forces to improve image resolution in atomic-force microscopy
Authors: Hutter Jeffrey L. , Bechhoefer John
Published: July 2002

Although the atomic force microscope (AFM) resembles superficially the scanning tunneling microscope (STM), its imaging resolution is in general much coarser. For the AFM, long-range interactions—most notably the van der Waals force—imply that image resolution is set by the macroscopic tip radius rather than by a single atom, as with the STM. Experimentally, we show that van der Waals forces can be measured using an AFM. By immersing tip and sample in an appropriate fluid, we can effectively eliminate the van der Waals force, leading to a marked improvement in AFM image quality.
Giant magnetoresistance studies in (Fe,Co)-Ag films
Authors: Tsoukatos A. , Wan H. , Hadjipanayis G. C. , Unruh K. M. , Li Z. G.
Published: July 2002

Giant magnetoresistance (GMR) values were measured in thin films of Fe and Co in Ag. The best GMR results were observed in Ag-rich specimens, with maxima of 25% (30 K) and 14% (20 K) observed in Co20Ag80 and Fe25Ag75, respectively. Magnetic data indicate a spin glasslike behavior in the as-deposited Ag-rich films. The as-made samples have a nanostructure, with a face centered cubic structure. Annealing of the samples over the temperature range of 200 °C to 700 °C led to grain growth and subsequent phase separation of the constituent metals. A summary of the magnetic and electrical transport properties is presented, in relation to the crystal structure and microstructure of the TM-Ag films (TM=Fe,Co).
Self-consistent simulation of quantum wires in periodic heterojunction structures
Authors: Kerkhoven Thomas , Raschke Michael W. , Ravaioli Umberto
Published: July 2002

We simulate the low temperature confinement of electrons in quantum wires formed at the corners of periodic saw-tooth structures consisting of layers of AlxGa1−xAs and GaAs materials. A very efficient self-consistent procedure for the solution of Schrödinger’s and Poisson’s equations, including shifted periodic boundary conditions, is used to obtain the wave functions and energy levels in the quantum well. Self-consistency is very important to obtain an accurate result; in particular, the precise details of the potential at the corners must be resolved. The formation of a quantum wire at the corners of the GaAs layer is found to be very sensitive to the physical parameters of the neighboring AlxGa1−xAs layers. We find that to avoid sizeable electron population in the GaAs region between the corners, asymmetric doping is necessary, which normally results in electron confinement only at every other corner.
Theoretical calculation of impact ionization rate in SiO2
Authors: Mizuno H. , Morifuji M. , Taniguchi K. , Hamaguchi C.
Published: July 2002

Impact ionization rate in SiO2 was numerically calculated using both pseudo-wave functions and energy band structure based on a self-consistent pseudopotential method. To avoid numerical complexity due to amorphous structure, SiO2 was assumed to be a crystalline α-quartz. The calculated impact ionization rate shows a strong wave vector anisotropy near a threshold energy regime, primary electrons existing at Γ point yield the strongest impact ionization rate. It was found that calculated results are not expressed by a Keldysh formula since SiO2 has complex band structure (e.g., indirect transition gap and nonparabolic bands). The magnitude of the theoretical impact ionization rate was very close to the experimental results recently reported by E. Cartier and F.R. McFeely [Phys. Rev. B 44, 10689 (1991)]. Detailed theoretical study clearly demonstrates that the average energy of secondary generated carriers depends linearly on the energy of primary electrons.
Ruby spectral band-profile analysis for temperature sensing
Authors: Huang Tzer-Hsiang , Hsu Chia-Chen , Kuo C.-T. , Lu P. , Tse W.-S. , Wang D. P. , Chou T. C. , Fuh A. Y. G.
Published: July 2002

The spectral band profiles of the Ē→4A2g and 2Ā→4A2g transitions of Cr3+ in Al2O3, i.e., the commonly known ruby 6943 Å (R1) and 6929 Å (R2) emission lines, in the temperature range 10–300 K were recorded and deconvoluted numerically with excellent accuracy into a Lorentzian (homogeneous) component and a Gaussian component. The Gaussian width was taken as arising from sample inhomogeneity and spectrometer slit function, and as expected was found to be independent of temperature. The Lorentzian width was found to vary with temperature in accordance with the two-phonon relaxation processes reported previously, plus a thermal broadening process. While the former processes were quenched completely as temperature was lowered to about 90 K, the latter process, describable with the Debye–Waller factor, still caused a Lorentzian width of about 0.2 cm−1 (for both R1 and R2 lines) which then leveled out quickly to about 0.1 cm−1 as the temperature was lowered further. For temperatures above 100 K, the positions of the R1 and R2 lines were found to have the same temperature dependence as that reported previously. The use of the temperature dependencies of Lorentzian width and line position for temperature sensing was discussed; such an instrument could indeed be made portable because of the simple optical and electronic systems used in our experiments.
Electroluminescence of pure poly(N-vinylcarbazole) and its blends with a multiblock copolymer
Authors: Hu B. , Yang Z. , Karasz F. E.
Published: July 2002

Electroluminescence (EL) in pure poly(N-vinylcarbazole) (PVK) has been observed in a simple single active layer EL device with the structure of ITO/PVK/Al (or Ca). The emitted light is violet with a maximum intensity at 426 nm; a part of the spectrum lies within the UV region. Blends of PVK with a conjugated-nonconjugated multiblock copolymer (CNMBC) previously described are also electroluminescent. In these blends a new emission peak in the EL spectra appeared and can be attributed to the emission from an exciplex formed by the two polymers. As the composition of the aforementioned blend changes from a PVK-poor to a PVK-rich ratio, the emitted light changes from green to blue. Further, blends containing 97 wt. % PVK and 3 wt. % CNMBC yielded an EL spectrum with a single emission peak different in location from either that of pure PVK or the pure CNMBC; the latter is blue with a maximum intensity at 476 nm. The EL output of the blends can be clearly seen in a brightly lit room at ambient temperature. An electron transport layer consisting of butyl-2-(4-biphenyl)-5-(4-tert-butylphenyl-1,3,4-oxadiazole) (PBD) in PMMA on the cathodic side of the EL device greatly increased the brightness of the emitted light to 200 cd/m2 in the case of a PVK-rich blend, but it had less effect on a PVK-poor blend or in pure PVK and had little effect on the pure form of the CNMBC.
Crystalline quality and residual stresses in diamond layers by Raman and x-ray diffraction analyses
Authors: Rats D. , Bimbault L. , Vandenbulcke L. , Herbin R. , Badawi K. F.
Published: July 2002

A major problem for diamond coating applications is that diamond films tend to exhibit poor adherence on many substrates and typically disbond at thicknesses of the order of a few micrometers principally because of residual stresses. Residual stresses are composed of thermal mismatch stresses and intrinsic tensile film stresses induced during film growth. Diamond films were deposited in a classical tubular microwave plasma reactor from hydrocarbon–hydrogen–oxygen gas mixtures. The stress level was investigated as a function of the gaseous composition (especially oxygen concentration) and deposition temperature (700–900 °C). Thermal stress was directly calculated from Hooke’s law using a biaxial Young’s modulus value of 1230 GPa for polycrystalline diamond. Total diamond film stress was directly determined by the radius of curvature method and by x-ray diffraction using the sin2 ψ method. The microdistorsions and the size of the coherently diffracting domains have been determined from the broadening of the diffraction peak. When coupled also with a Raman study, these investigations allow discussion of the origin of intrinsic stress.
The effect of annealing and implantation temperature on the structure of C ion-beam-irradiated glassy carbon
Authors: McCulloch D. G. , Prawer S.
Published: July 2002

Raman spectroscopy has been used to investigate the effects of dynamic and postimplantation annealing on glassy carbon implanted with 50 keV C ions to a dose of 5×1016 ions/cm2. The postimplantation annealing of damage in the ion-beam modified material was found to occur in two stages as a function of postimplantation annealing temperature Ta. These occur for 500<Ta<800 K and Ta≳1300 K and correspond to the thermal energy required to activate C interstitials and vacancies, respectively. Once mobile these defects diffuse through the implanted layer, reducing bond angle disorder which leads to an increase in graphitic order as interstitial-vacancy recombination occurs. The effects of the ion-beam irradiation on the final structure of glassy carbon were found to be a strong function of the temperature of the sample during the irradiation, Ti. This dependence is interpreted in terms of dynamic annealing and radiation-enhanced diffusion. Three temperature regimes were found to be important. For Ti<300 K defect motion during irradiation is suppressed. For 300<Ti<600 K, the mobility of C interstitials during irradiation results in dynamic annealing which prevents amorphization, with the result that the ion irradiation creates a highly disordered, but essentially graphitically bonded carbon. For Ti≳600 K, vacancy mobilities are sufficiently high such that most ion-beam-induced defects are dynamically annealed and, for Ti≳800 K the unimplanted glassy carbon microstructure is retained following the ion-beam irradiation. Finally, activation energies for interstitial and vacancy mobilities were determined and found to compare favorably with those found in other forms of carbon.
Defect-enhanced electron field emission from chemical vapor deposited diamond
Authors: Zhu W. , Kochanski G. P. , Jin S. , Seibles L.
Published: July 2002

Diamond samples with varying defect densities have been synthesized by chemical vapor deposition, and their field emission characteristics have been investigated. Vacuum electron field emission measurements indicate that the threshold electric field required to generate sufficient emission current densities for flat panel display applications (≳10 mA/cm2) can be significantly reduced when the diamond is grown so as to contain a substantial number of structural defects. The defective diamond has a Raman spectrum with a broadened peak at 1332 cm−1 with a full width at half maximum (FWHM) of 7–11 cm−1. We establish a strong correlation between the field required for emission and the FWHM of the diamond peak. The threshold fields are typically less than 50 V/μm and can reach as low as 30 V/μm for diamond with a FWHM greater than 8.5 cm−1. It is believed that the defects create additional energy bands within the band gap of diamond and thus contribute electrons for emission at low electric fields.
Study of interface roughness dependence of electron mobility in Si inversion layers using the Monte Carlo method
Authors: Yamakawa Shinya , Ueno Hiroaki , Taniguchi Kenji , Hamaguchi Chihiro , Miyatsuji Kazuo , Masaki Kazuo , Ravaioli Umberto
Published: July 2002

The electron mobility in the inversion layer of a metal–oxide semiconductor field effect transistor formed on the (100) silicon surface is calculated by using a Monte Carlo approach which takes into account size quantization, acoustic phonon scattering, intervalley phonon scattering and surface roughness scattering. Degeneracy is also considered because it is important at higher normal effective fields (high gate voltages). The main emphasis is placed on the influence of the specific autocovariance function, used to describe the surface roughness, on the electron mobility. Here we compare the mobilities obtained using exponential and Gaussian autocovariance functions. It is found that the electron mobility calculated with roughness scattering rates based on the exponential function shows good agreement with experiments. The effect of the degeneracy and screening on the roughness scattering is also discussed.
Effect of surface free energy of underlayer materials on crystal growth of Co polycrystalline films
Authors: Kitakami O. , Okamoto S. , Shimada Y.
Published: July 2002

Crystallographic texture of polycrystalline Co films prepared by sputtering on a variety of underlayers has been investigated. The texture of polycrystalline Co films greatly depends on underlayer material, and this dependence is well explained by taking into account the surface free energy difference between the underlayer material (γu) and Co (γCo). When γu is larger than γCo, Co films can be grown with a well-defined crystallographic texture of hcp-Co(001), which is the lowest surface energy crystal plane in order to minimize the total surface energy. In contrast, if γu is much smaller than γCo, no remarkable underlayer effects can be observed in the crystallographic structure of Co films.
Thermionic emission and tunneling in InGaAs/GaAs quantum well infrared detectors
Authors: Karunasiri Gamani
Published: July 2002

Thermionic emission and tunneling current measurements have been performed on a high In composition (x=0.3) pseudomorphic InxGa1−xAs/GaAs multiple quantum well infrared detector grown on GaAs substrate. It was found that at low temperatures (<40 K), the leakage current of the detector dominates by sequential tunneling through the ground states of quantum wells while at high temperatures (≳40 K) the data is in good agreement with the thermionic emission over the quantum well barriers. This indicates that the layers have good electrical quality, suggesting a low density of dislocations. The estimated barrier height relative to the ground state using the thermionic emission analysis of 101 meV is in good agreement with the measured cut-off energy of 98 meV of the photoresponse. The conduction band offset between In0.3Ga0.7As and GaAs is estimated to be about 200 meV.
Spall fracture properties of aluminum and magnesium at high temperatures
Authors: Kanel G. I. , Razorenov S. V. , Bogatch A. , Utkin A. V. , Fortov V. E. , Grady D. E.
Published: July 2002

Measurements of the dynamic tensile strength of aluminum and magnesium have been carried out by investigations of the spall phenomena over a wide range of temperatures, shock-wave intensities, and load durations. Free-surface velocity profiles were recorded with VISAR and used to provide the spall strength measurements. The initial temperature of samples was varied from room temperature to near the melting point. The peak compressive pressure in the shock waves was varied from 5 to 50 GPa for aluminum and from 2 to 10 GPa for magnesium. The load duration was varied by more than one order of magnitude. The free-surface velocity measurements showed a precipitous drop in the spall strength of preheated samples as temperatures approached the melting point. No significant influence of the peak pressure on the spall strength was observed. The strain-rate dependencies of the spall strength could be represented as power functions with a power index of 0.060 for aluminum and 0.072 for magnesium. Unexpectedly large amplitudes for the Hugoniot elastic limit of both aluminum and magnesium were observed at temperatures approaching the melting point.
Diagnostics of the magnetized low-pressure hydrogen plasma jet: Molecular regime
Authors: Qing Zhou , Otorbaev D. K. , Brussaard G. J. H. , van de Sanden M. C. M. , Schram D. C.
Published: July 2002

Optical emission and absorption spectroscopy and double Langmuir probe diagnostics have been applied to measure the plasma parameters of an expanding magnetized hydrogen plasma jet. The rotational temperature of the excited state H2(d2Πu) has been determined by analyzing the intensity distribution of the spectral lines of the Fulcher-α system of H2. The gas temperature in the plasma, which is twice the value of the rotational temperature is equal to ≂ 520 K. Several clear indications of presence of the ‘‘hot’’ electrons have been observed in the plasma: (1) Langmuir probe measurements (Te≂1.4 eV), (2) appearance of the Fulcher-α system of H2 (excitation potential ΔE=13.87 eV), (3) low rotational temperature (T*rot≂260 K) of the excited H2(d3Πu) molecules, (4) local excitation in the plasma of Ar I(ΔE=15.45 eV), and Ar II(ΔE=19.68 eV) spectral lines, (5) local excitation in the plasma of He I(ΔE=23.07 eV and ΔE=24.04 eV) spectral lines. Optical actinometry has been applied to measure the absolute density of hydrogen atoms and hydrogen dissociation degree in the plasma. The measured absolute density of hydrogen atoms are in the (1–1.4)×1020 m−3 range, and the corresponding dissociation degree of the hydrogen plasma is in the range of 8%–13%.
Inverse melting of metastable Nb–Cr solid solutions
Authors: Michaelsen C. , Sinkler W. , Pfullmann Th. , Bormann R.
Published: July 2002

Metastable body-centered-cubic (bcc) Nb–Cr solid solutions were produced by mechanical alloying over a wide concentration range, and investigated by x-ray diffraction, differential thermal analysis, and transmission electron microscopy. For comparison, metastable bcc and amorphous Nb–Cr alloys were fabricated by sputtering. Upon annealing, the solid solutions prepared by mechanical alloying undergo a transformation to an amorphous phase. The vitrification is polymorphous for a composition Nb45Cr55 whereas two-phase mixtures of amorphous and bcc phases are formed for other concentrations, indicating that metastable equilibria between these two phases develop upon heat treatment. These results, combined with a detailed analysis of the thermodynamic functions of the system, reveal that the amorphous phase has a lower enthalpy and entropy than the bcc phase near equiatomic composition. In consequence, the Gibbs-energy curves of bcc and liquid/amorphous Nb45Cr55 phases versus temperature exhibit two points of intersection, i.e., two melting points, with the amorphous phase being thermodynamically more stable than the bcc phase at low temperatures. Inverse melting is thus thermodynamically possible in the Nb–Cr system, so that the metastable crystalline phase would melt upon cooling.
Schottky contact and the thermal stability of Ni on n-type GaN
Authors: Guo J. D. , Pan F. M. , Feng M. S. , Guo R. J. , Chou P. F. , Chang C. Y.
Published: July 2002

The Schottky barrier height of Ni on n-GaN has been measured to be 0.56 and 0.66 eV by capacitance–voltage (C–V) and current–density–temperature (J–T) methods, respectively. Gallium nickel (Ga4Ni3) is formed as Ni is deposited on the GaN film, which affects the barrier height markedly. The thermal stability of Ni on GaN is also investigated by annealing these specimens at various temperatures. Specimen annealing at temperatures above 200 °C leads to the formation of nickel nitrides Ni3N and Ni4N at the interface of Ni and GaN. These interfacial compounds change the measured barrier height to 1.0 and 0.8 eV by C–V and J–T methods, respectively. Comparisons of Schottky characteristics of Ni with those of Pt, Pd, Au, and Ti are also discussed.
Temperature dependence of quantized states in an In0.86Ga0.14As0.3P0.7/InP quantum well heterostructure
Authors: Li C. F. , Lin D. Y. , Huang Y. S. , Chen Y. F. , Tiong K. K.
Published: July 2002

Piezoreflectance (PzR) and contactless electroreflectance (CER) measurements of an In0.86Ga0.14As0.3P0.7/InP quantum well heterostructure as a function of temperature in the range of 20–300 K have been carried out. A careful analysis of the PzR and CER spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state and the nth heavy (light)-hole band state. The parameters that describe the temperature dependence of EmnH(L) are evaluated. A detailed study of the temperature variation of excitonic transition energies indicates that the main influence of temperature on quantized transitions is through the temperature dependence of the band gap of the constituent material in the well. The temperature dependence of the linewidth of 11H exciton is evaluated and compared with that of the bulk material.
Influence of Fe in giant magnetoresistance ratio and magnetic properties of La0.7Ca0.3Mn1−xFexO3 perovskite type compounds
Authors: Righi L. , Gorria P. , Insausti M. , Gutiérrez J. , Barandiarán J. M.
Published: July 2002

In this work polycrystalline perovskite with composition La0.7Ca0.3Fe0.05Mn0.95O3 has been produced by standard ceramic method. A considerable change occurs in magnetic, transport and magnetoresistance properties with respect to the classic composition without Fe. The introduction of a different metal, Fe, in Mn-O layer causes a decrease of about 50 K in the value of TC. In the same way, a decrease of about 10–15% in the average magnetic moment measured at 1 T is also observed. On the other hand, the introduction of Fe does not cause any appreciable change in the value of the lattice parameter. This new compound presents 60% of giant magnetoresistance ratio at 200 K. Magnetization vs. temperature measurements in zero field cooling and field cooling show clear differences at applied fields below 80 kA m−1. Such a behavior, also observed in La0.7Ca0.3MnO3 sample, is not recognizable as a simple ferromagnetic one. A qualitative discussion of the effect in the magnetic and transport properties of these compounds, caused by the substitution of some Mn atoms by Fe is given.
Magnetic studies of the two-dimensional, S=1/2 Heisenberg antiferromagnets (5CAP)2CuCl4 and (5MAP)2CuCl4
Authors: Hammar Philip R. , Dender Daniel C. , Reich Daniel H. , Albrecht A. S. , Landee C. P.
Published: July 2002

The magnetic materials (5CAP)2CuCl4 and (5MAP)2CuCl4 form effective square planar lattices of CuCl42− anions. The Cu2+ ions have weak antiferromagnetic interactions in the plane through Cl–Cl contacts. These planes are well separated by the 5MAP and 5CAP organic groups. High temperature susceptibility measurements give J=0.57 K for (5CAP)2CuCl4 and J=0.38 K for (5MAP)2CuCl4. χ(T,H=0) for these materials has a rounded maximum at Tpeak=1.1 K for (5CAP)2CuCl4 and Tpeak=0.62 K for (5MAP)2CuCl4. A kink in χ(T,H=0) for the (5MAP)2CuCl4 suggests a phase transition to three-dimensional long range order at TC=0.44 K. Measurements of χ(T,H) show that the saturation field HC for the transition to a fieldinduced saturated state is 3.78 T for (5CAP)2CuCl4 and 2.17 T for (5MAP)2CuCl4. Thus these materials are excellent model systems in which to explore the full H–T phase diagram of the two-dimensional S=1/2 Heisenberg antiferromagnet.
Constructal tree networks for heat transfer
Authors: Ledezma G. A. , Bejan A. , Errera M. R.
Published: July 2002

This paper addresses the fundamental problem of how to connect a heat generating volume to a point heat sink by using a finite amount of high-conductivity material that can be distributed through the volume. The problem is one of optimizing the access (or minimizing the thermal resistance) between a finite-size volume and one point. The solution is constructed by covering the volume with a sequence of building blocks, which proceeds toward larger sizes (assemblies), hence, the “constructal” name for this approach. Optimized numerically at each stage are geometric features such as the overall shape of the building block, its number of constituents, and the internal distribution of high-conductivity inserts. It is shown that in the optimal design, the high-conductivity material has a distribution with the shape of a tree. Every aspect of the tree architecture is deterministic: the shapes of the largest assembly and all its constituents, the number of branches at each level of assembly, the relative position of building blocks in each assembly, and the relative thicknesses of successive branches. The finer, innermost details of the tree architecture (e.g., the branching angle) have a negligible effect on the overall thermal resistance. The main conclusion is that the structure, working mechanism, and minimal resistance of the tree network can be obtained deterministically, and that the constrained optimization of access routes accounts for the macroscopic structure in nature.
Bias voltage dependent field-emission energy distribution analysis of wide band-gap field emitters
Authors: Schlesser R. , McClure M. T. , McCarson B. L. , Sitar Z.
Published: July 2002

We have studied the origin of field emission from wide band-gap semiconductors by a combination of voltage dependent field-emission energy distribution and I–V measurements. For this purpose, tip-shaped molybdenum emitters were coated with 100–1000 nm thick layers of nominally undoped diamond and cubic boron nitride (c-BN) powders. Electron energy spectra revealed that significant band bending occurred due to field penetration into wide band-gap materials. Voltage drops on the order of several volts were measured across the coatings, for applied voltages on the order of 1 kV, and a cathode–gate distance of 500 μm. These voltage drops showed a linear dependence with the applied bias voltage for well-annealed diamond coatings and a strongly nonlinear behavior for unannealed diamond and c-BN coatings. In general, annealing of diamond coated Mo tips led to improved emission current stability and lower “turn-on” voltages due to the removal of oxide and the formation of conductive carbide layers between the metal and semiconductor. From the extrapolation of the linear behavior to the flat-band condition, we concluded that the emission from diamond, as well as c-BN, originated from the conduction-band minimum. Nonlinear behavior was attributed to positive space-charge accumulation at the field-emitting surface, which arose due to electron depletion by field emission from wide band-gap material.
Parametric interactions in the organic salt 4-N,N-dimethylamino-4′-N′- methyl-stilbazolium tosylate at telecommunication wavelengths
Authors: Meier U. , Bösch M. , Bosshard Ch. , Pan F. , Günter P.
Published: July 2002

We show that the organic salt 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate, originally developed for electro-optic applications, is also a very interesting material for phase-matched parametric interactions such as frequency-doubling and optic parametric oscillation in the near infrared. These favorable properties are due to the large off-diagonal element d26 which gives measured effective phase-matchable nonlinear optical coefficients of deff=31±5 and 15±3 pm/V at the telecommunication wavelengths of λ=1313 and 1535 nm, respectively.
Temperature analysis of induction motors using a hybrid thermal model with distributed heat sources
Authors: Mukhopadhyay S. C. , Pal S. K.
Published: July 2002

The article presents a hybrid thermal model for the accurate estimation of temperature distribution of induction motors. The developed model is a combination of lumped and distributed thermal parameters which are obtained from motor dimensions and other constants such as material density, specific heats, thermal conductivity, etc. The model is especially suited for the derating of induction motors operating under distorted and unbalanced supply condition. The model have been applied to a small (2hp, 415 V, 3-phase) cage rotor induction motor. The performance of the model is confirmed by experimental temperature data from the body and the conductor inside the slots of the motor.
Electrostriction of polarizable materials: Comparison of models with experimental data
Authors: Shkel Yuri M. , Klingenberg Daniel J.
Published: July 2002

A microscopic model is employed to relate the dielectric tensor to the microstructure of isotropic amorphous or cubic crystalline materials. Electrostriction is characterized by dielectric parameters that describe how the dielectric tensor varies with deformation and the resulting change in the microstructure. These dielectric parameters are obtained for small deformations, and expressed in terms of measurable, macroscopic properties. Unlike previous approaches, we avoid employing the Lorentz cavity approximation to determine the local polarization. Dipole sums are renormalized by treating the dielectric constant of the undeformed material as a known quantity. Predictions agree with our experimental data for different block copolymer films. Further progress in understanding electrostriction and developing improved models requires measuring the deformation dependence of the dielectric tensor for various materials under various conditions (field strength and frequency, deformation frequency, temperature, etc.).
Universal power optimized work for reciprocating internally reversible Stirling-like heat engine cycles with regeneration and linear external heat transfer
Authors: Blank David A.
Published: July 2002

When bounded by two infinite thermal reservoirs, the theory of irreversible thermodynamics for reciprocating externally irreversible cycles yields to an optimum efficiency at maximum power output of η=1−(TL/TH)0.5 for internally reversible Stirling-like cycles using regeneration and linear heat transfer modes is in contrast to the upper limit for Stirling cycles of η=1−(TL/TH) obtained from classical thermodynamics. This optimum behavior is, however, only based on cycle temperature bounds. For reciprocating cycles one must go a step further and minimize cycle time. While executing this new step for finite thermal reservoirs, it was discovered that, for the general family of reciprocating Stirling-like cycles, the finite-time optimum work output (Wopt) at maximum power is less than (and in the limit of ideal regeneration, infinite reservoirs and of no internal irreversibility, is equal to) exactly one-half of the work of the externally reversible cycle operating at maximum thermal efficiency (Carnot work, Wrev) between the same temperature limits (i.e., Wopt⩽12Wrev). To accomplish this the analysis goes beyond earlier works to use time symmetry to better optimize overall cycle power. Because this procedure results in the concurrent employment of the first and second laws of thermodynamics, it ensures optimal allocation of thermal conductances at the hot and cold ends while simultaneously achieving both minimization of internal entropy generation and maximization of specific cycle work for a given set of operating temperatures. Based on linear heat transfer laws, this expression for optimum work is shown to be independent of heat conductances. Finally, the analysis establishes that the maximum power attainable for a Stirling-like reciprocating cycle operating between two temperature bounds is always less than (and in the limit of power optimized Carnot conditions, equal to) one-half of that obtained for the continuous counterpart of the same cycle operating between the same temperature bounds.
Grain size dependence of the current–voltage characteristics and critical current density in the self-field approximation
Authors: Kiliç A. , Kiliç K. , Senoussi S.
Published: July 2002

In the self-field approximation, the influence of the grain radius on the current–voltage characteristics and associated critical current density Jc are investigated by considering the conventional flux flow motion of vortices. It is shown that the direct summation theory of Campbell et al. could be adopted to the granular systems [A. M. Campbell, J. E. Evetts, and D. Dew-Hughes, Philos. Mag. 10, 333 (1964)]. We found that Jc is enhanced as the size of the grains is decreased. The case of the critical current density being greater for smaller grains, which is consistent with both transport and low-field magnetization measurements is discussed by the weak links between the superconducting grains and also with the percolation model.
Fe
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Mössbauer effect study of Nd–(Fe, Cr)–B permanent magnetic materials
Authors: Manivel Raja M. , Narayanasamy A.
Published: July 2002

The alloys Nd16Fe76−xCrxB8, with x=0, 3, 5, and 7, were prepared by induction melting technique. The x-ray powder diffractograms show that all the samples crystallized in tetragonal Nd2Fe14B phase along with NdFe4B4 as a secondary phase. The lattice parameters are found to decrease with Cr concentration. The Curie temperature, determined by the differential scanning calorimetry technique, decreases with Cr concentration at the rate of 7.4 K/at. % Cr. The Mössbauer study clearly shows that Cr has a strong preference for the j2 site; the weighted average hyperfine magnetic field decreases by 40 kOe when an Fe atom in the Nd2Fe14B formula unit is replaced by a Cr atom. Cr substitution decreases the transferred hyperfine magnetic field at the Fe sites and also the Fe-magnetic moment.
Low density artificial microwave magnetic composites
Authors: Acher O. , Adenot A. L. , Lubrano F. , Duverger F.
Published: July 2002

The high frequency permeability of composites made of hollow conducting planes or tubes is derived. It is shown that such composites may exhibit permeability levels significantly different from unity, even for nonmagnetic metal volume fractions lower than 1%. This behavior is related to the skin effect, but it can occur for metal shell thicknesses much lower than the skin depth. The influence of the shape of the hollow inclusion on the permeability is reported. The maximum of the imaginary permeability can be tuned over a wide frequency range by adjusting the conductivity of the metal and the characteristic dimensions of the inclusions. The properties of composites made of hollow ferromagnetic planes are also investigated using experimental values obtained from materials made of ferromagnetic laminations.
Magnetic field enhancement of water vaporization
Authors: Nakagawa Jun , Hirota Noriyuki , Kitazawa Koichi , Shoda Makoto
Published: July 2002

The water vaporization rate, an essential process for all the biological processes, was found to be significantly influenced under static magnetic fields up to 8 T in air and oxygen. The magnitude of the effect depended on the field–field gradient product B⋅dB/dx rather than on B itself. Under forced flow conditions of the atmosphere, both enhancement and suppression of the vaporization rate were observed depending upon the direction of the gas flow relative to the field gradient. A mechanism is proposed to explain the results in a systematic manner based on the assumption of the creation of magnetic wind driven by the gradient susceptibility distribution caused by water content distribution in the atmosphere. It is discussed that this magneto enhancement of vaporization may be the indirect cause of frequently reported field effects on living organisms.
Intensity of a source of atomic hydrogen based on a hot capillary
Authors: Tschersich K. G.
Published: July 2002

A source of atomic hydrogen incorporating a hot capillary was fabricated and investigated. The absolute and angularly resolved intensity of atomic hydrogen is derived from three experimentally determined quantities: (1) the flow rate of the hydrogen feed gas, (2) the degree of dissociation, and (3) the angular distribution of the emitted hydrogen atoms. The measurement of quantities (1) and (2) is reported here, quantity (3) has been investigated previously. The intensity of atomic hydrogen depends on the temperature of the capillary and the gas flow rate. At a temperature of 2600 K and a flow rate of 8×10−3mbar l/s the center-line intensity is about 4×1017atoms/sr s. This is about four times the center-line intensity from a thin-walled aperture subjected to the same total flux. The enhancement is due to beam formation by the capillary and is even more pronounced at lower flow rates.
Turbostratic boron carbonitride films produced by bias-assisted hot filament chemical vapor deposition
Authors: Yu J. , Wang E. G. , Ahn J. , Yoon S. F. , Zhang Q. , Cui J. , Yu M. B.
Published: July 2002

Boron carbonitride (BCN) films with various compositions have been prepared by bias-assisted hot filament chemical vapor deposition. The three elements of B, C, and N are chemically bonded with each other and an atomic-level BCN hybrid has been formed in the films. The deposited films are composed of turbostratic structural regions ranging from a few to a few tens of nanometers. Besides, there exist some amorphous domains in the films. Boron atoms have been confirmed to be incorporated into the films with a concentration up to 70 at. %. The interplanar spacing of 3.49 Å is found to be independent of the film composition in this range. These films show a blueshift in photoluminescence peak with increasing B content. These findings show that the electronic structure of BCN compounds can be controlled by changing compositions and the BCN compounds are blue-light emitting materials.
Thermal convection control by gradient magnetic field
Authors: Uetake Hiromichi , Hirota Noriyuki , Nakagawa Jun , Ikezoe Yasuhiro , Kitazawa Koichi
Published: July 2002

A new technique to control thermal convection has been demonstrated by utilizing a gradient magnetic field. When the center of the superconducting magnet was fixed below the center of the heating region, the thermal convection flow was accelerated. In contrast, the convection flow was suppressed when the field center was fixed above the heating center, and under certain conditions, a reverse direction of flow was observed. The control of the flow was made possible by the experimental procedure, in which a downward flow was induced when the magnetic field was applied prior to the heating, and an upward flow was observed under the reverse procedure. These phenomena can be understood by taking account of the balance between the thermal driving force and the magnetic force acting on the air. A detailed analysis was made by examination of the temperature distribution in the observed system. The results suggest the possibility of using a gradient magnetic field for controlling thermal convection without any mechanical driving parts.
Optical analogue of a type II semiconductor heterostructure
Authors: Dragoman D.
Published: July 2002

A succession of directional couplers, which differ only through their refractive index distribution, is shown to be the optical analog of type II semiconductor heterostructures. The design of such a dielectric structure with the same transmission characteristic of the electric field propagating through it as the electron wave function in a type II heterostructure is discussed. An example of the design procedure is also given.
All-electron and valence-electron calculations on AgH, Ag2, and AgO
Authors: Hay P. Jeffrey , Martin Richard L.
Published: July 2002

Valence electron calculations have been carried out on the ground states of AgH, Ag2, and AgO and the excited 2∑+ state of AgO using the relativistic effective-core potential (ECP) technique. Nonrelativistic all-electron and valence-electron calculations have also been performed on AgH and AgO. These results are compared to previous theoretical treatments and to existing experimental data. The previously undiscovered 2∑+ state of AgO is predicted to lie 1.29 eV above the 2∏ ground state.
C+(2P)+H2(D2,HD)→CH+(CD+)+H(D). I. Reaction cross sections and kinetic isotope effects from threshold to 15 eV c.m.
Authors: Ervin Kent M. , Armentrout P. B.
Published: July 2002

Total cross sections for the reactions of carbon(1+) ions with isotopic molecular hydrogen (H2, D2, and HD) to form methyliumylidene (CH+ and CD+) have been measured using guided ion beam techniques. Cross sections are reported as a function of the translational energy of the reactants from the reaction threshold up to 15 eV c.m. The true cross sections are shown to rise sharply from thresholds given by the thermochemical endothermicities. Inter- and intramolecular isotope effects in the threshold region can be attributed to the different endothermicities due to vibrational zero-point energies. At higher energies, an unexpected intermolecular isotope effect is found. Thermal reaction rates (300 K) derived from the data are 1.2×10−16 for H2, 2.3×10−17 for D2, and 1.2×10−16 for HD (17% CH+, 83% CD+), all in units of cm3 s−1.
Monte Carlo study of a model of diffusion-controlled reactions
Authors: Beenakker C. W. J. , Ross John
Published: July 2002

Diffusion-controlled reactions between solute particles and immobile spherical sinks are studied, using the Monte Carlo method to perform averages over sink configurations. The average steady-state solute concentration profile c̄(r) in a locally perturbed solution is determined for sink volume fractions φ≤0.3, by numerically solving the diffusion equation in the monopolar+dipolar approximation of diffusive couplings between the sinks. At low volume fractions the analytical result c̄(r)∝r−1 exp(−r/λ), with the screening length λ∝φ−1/2, is recovered, whereas for φ≳0.1 significant deviations from this functional form are found. The Monte Carlo method is shown to be most accurate and efficient in the region 10−3≲φ≲10−1 in which (a) a system of only 25 sinks suffices, and (b) the monopolar approximation alone is sufficiently accurate. In this regime the reaction rate coefficient calculated numerically is found to be in good agreement with previous analytical theories.
The collapse transition of semiflexible polymers. A Monte Carlo simulation of a model system
Authors: Kolinski Andrzej , Skolnick Jeffrey , Yaris Robert
Published: July 2002

Monte Carlo simulations have been performed on a diamond lattice model of semiflexible polymers for a range of flexibilities and a range of chain lengths from 50 to 800 segments. The model includes both repulsive (excluded volume) and attractive segment–segment interactions. It is shown that the polymers group into two classes, ‘‘flexible’’ and ‘‘stiff.’’ The flexible polymers exhibit decreasing chain dimensions as the temperature decreases with a gradual collapse from a loose random coil, high temperature state to a dense random coil, low temperature state. The stiffer polymers, on the other hand, exhibit increasing chain dimensions with decreasing temperature until at a critical temperature there is a sudden collapse to an ordered high density, low temperature state. This difference is due to the relative strength of the segment–segment attractive interactions compared to the energetic preference for a trans conformational state over a gauche state. When the attractive interaction is relatively strong (flexible case) the polymer starts to collapse before rotational degrees of freedom freeze out, leading to a disordered dense state. When the attractive interaction is relatively weak (stiff case) the polymer starts to freeze out rotational degrees of freedom before it finally collapses to a highly ordered dense state.