bluuebunny/crossref_metadata_2025 huggingface dataset:
Large and broadband piezoelectricity in smart polymer-foam space-charge electrets
Authors: Neugschwandtner G. S. , Schwödiauer R. , Vieytes M. , Bauer-Gogonea S. , Bauer S. , Hillenbrand J. , Kressmann R. , Sessler G. M. , Paajanen M. , Lekkala J.
Published: July 2002

Charged closed-cell microporous polypropylene foams are shown to exhibit piezoelectric resonance modes in the dielectric function, coupled with a large anisotropy in the electromechanical and elastic material properties. Strong direct and converse dynamic piezoelectricity with a piezoelectric d33 coefficient of 140 pC/N at 600 kHz is identified. The piezoelectric d33 coefficient exceeds that of the ferroelectric polymer polyvinylidene fluoride by a factor of 5 and compares favorably with ferroelectric ceramics. Applications of similar concepts should provide a broad class of easily fabricated “soft” piezoelectric materials.
Influence of nanocrystallization on the evolution of domain patterns and the magnetoimpedance effect in amorphous Fe73.5Cu1Nb3Si13.5B9 ribbons
Authors: Guo H. Q. , Kronmüller H. , Dragon T. , Cheng Z. H. , Shen B. G.
Published: July 2002

The evolution of the magnetic domain patterns and the giant magnetoimpedance (MI) effect in the Fe73.5Cu1Nb3Si13.5B9 amorphous ribbons are studied as a function of the annealing time ta at 550 °C. It is shown that by annealing at 550 °C for ta from 1 to 150 h, very fine nanocrystalline bcc-FeSi grains are homogeneously formed in the amorphous matrix. Although with increasing ta the grain size remains very fine (⩽11 nm), the coercivity Hc increases rapidly from 0.0014 Oe for ta=3 h to 1.67 Oe for ta=150 h. For the nanocrystalline ribbons with ta=3 h, the domain structure is characterized by a few broad longitudinal together with some broad transverse domain patterns, connected to the minimum coercitive field. With increasing annealing time ta⩽10 h, only transverse domain patterns are observed and the transverse domain width gradually becomes narrow. Considering the different magnetic softness, domain structure, and magnetization process, various types of the MI effect are observed in these nanocryastalline ribbons. A maximum value for the MI ratio ΔZ/Z=[Z(H)−Z(Hmax)]/Z(Hmax), of about 400%, and a maximum sensitivity of the MI ratio, of about 170% Oe−1, are obtained in the nanocrystalline samples annealed at 550 °C for 3 and 10 h, respectively. These maximum MI effects are related to the optimum combination of soft properties with the transverse domain patterns. It is revealed that the magnetic softness together with the domain width of a transverse domain pattern are the key parameters to realize a large MI ratio.
Observation of magnetic gradients in stainless steel with a high-Tc superconducting quantum interference device microscope
Authors: Watanabe Yoshimi , Kang S. H. , Chan J. W. , Morris J. W. , Shaw T. J. , Clarke John
Published: July 2002

Superconducting quantum interference device (SQUID) microscopes may serve as useful nondestructive evaluation (NDE) tools since they can precisely measure the local magnetic field variation that can be related to the characteristics of ferromagnetic materials. To demonstrate this, we have studied magnetic functionally graded materials (FGMs) in the Fe–Cr–Ni alloy system using a high-transition-temperature (HTc) SQUID microscope. The FGMs were either fabricated by inhomogeneous mechanical deformation or by heat treatment in a temperature gradient. The magnetic properties of these materials were measured using the vibrating sample magnetometer technique along the deformation or the temperature gradients. The results from this technique and the microstructural properties from optical imaging are discussed in conjunction with the magnetic field images obtained from the SQUID microscope. By exploring the results, the feasibility and benefit of utilizing SQUID microscopy as a NDE tool are discussed.
Prediction of the melting point of n-alkanes using the molecular dynamics method
Authors: Tsuchiya Y. , Hasegawa H. , Iwatsubo T.
Published: July 2002

We report the melting point of normal alkanes, from molecular dynamics calculations using the NPT ensemble. From the radial distribution function reflecting the arrangement of carbons, the solid–liquid phase transformation was identified in calculating a melting point in good agreement with the measured value. The effectiveness of periodic boundary conditions in the molecular dynamics calculations was also confirmed.
Synthesis of InNxP1−x thin films by N ion implantation
Authors: Yu K. M. , Walukiewicz W. , Wu J. , Beeman J. W. , Ager J. W. , Haller E. E. , Shan W. , Xin H. P. , Tu C. W.
Published: July 2002

Dilute InNxP1−x alloy thin films were synthesized by nitrogen ion implantation into InP using doses corresponding to N mole fraction up to 0.048. In the films with the highest N contents, it was shown using modulated photoreflectance that the fundamental band gap energy was decreased by up to 180 meV. The band gap reduction is similar in magnitude to that observed in epitaxially grown III–NxV1−x alloys. The InNxP1−x layers were thermally stable up to an annealing temperature of 850 °C. Using the recently developed band anticrossing model which relates the band gap reduction to the N content, we estimate that the maximum mole fraction of N achieved in the InNxP1−x alloys is larger than that reported previously for film grown by chemical vapor deposition and exceeds 0.01.
Two-step codeposition process for enhanced C54–TiSi2 formation in the Ti–Si binary system
Authors: Quintero A. , Libera M. , Cabral C. , Lavoie C. , Harper J. M. E.
Published: July 2002

This work demonstrates the possibility of decreasing the C54–TiSi2 formation temperature during rapid thermal annealing (RTA) by more than 50 °C using a two-step binary Ti–Si codeposition process on Si (100) substrates. This process is based on codepositing a particular double-layer microstructure. The first layer is an amorphous Ti–Si alloy codeposited on Si (100) with a composition close to Ti5Si3. After crystallizing this first layer at temperatures near 600 °C, a second layer is formed by room-temperature codeposition of an amorphous capping layer with a composition close to TiSi2. Analyses by Rutherford backscattering spectrometry and film-thickness measurements by transmission electron microscopy on samples constructed according to this method show a structure of 20 nm TiSi1.3/45 nm Ti3.7Si3/Si. On rapid thermal annealing (3 °C/s to 710 °C), C49–TiSi2 formation occurs at the silicide/silicon interface keeping Ti5Si3 as an intermediate layer, and the capping layer is transformed to C54–TiSi2. This microstructure is fundamentally different from that developed after RTA of Ti/Si bilayers in which C49–TiSi2 forms and subsequently transforms to C54 at temperatures ∼800 °C. The two-step process studied here places hexagonal Ti5Si3 in close contact with the amorphous capping layer. This layer acts as a catalyst for the formation of C54–TiSi2 by decreasing the energy barrier for C54 nucleation. The present experiments also suggest that the transformation from C49 to C54 can be mediated by a layer of Ti5Si3 in much the same fashion as metal-mediated crystallization processes. The enhanced formation of C54–TiSi2 using the two-step silicidation of binary Ti–Si alloys is an attractive alternative to other methods which lower the C54 formation temperature by introducing a third element. Such a third element can produce thermodynamically stable high-resistivity silicides that may decrease device performance.
Vortex circulation control in mesoscopic ring magnets
Authors: Kläui M. , Rothman J. , Lopez-Diaz L. , Vaz C. A. F. , Bland J. A. C. , Cui Z.
Published: July 2002

We present a simple method to control the direction of the circulation of the magnetization in mesoscopic ring magnets, using a uniform magnetic field only. The method is based on the nucleation free switching which occurs when the rings switch from the near-saturated state, referred to as the “onion state,” to the flux-closed vortex state. Two possible onion states, forward or reverse magnetized, are possible for a given direction of the magnetic field. Going from the forward or the backward onion state, both local scanning Kerr microscopy measurements and micromagnetic simulations show that the clockwise or the counterclockwise vortex state, respectively, can be selected due to asymmetric pinning of the two domain walls that are present in the onion state.
Structural characterization of NaOH aqueous solution in the glass and liquid states
Authors: Bruni F. , Ricci M. A. , Soper A. K.
Published: July 2002

Using the technique of hydrogen and deuterium substitution, the structure of water in concentrated NaOH solution (10 M) is explored. It is found that major changes in water structure occur both in the liquid phase at T=300 K and in the glassy phase at T=173 K. In particular the 4.4 Å peak in the OO pair correlation function of pure water, which is normally viewed as indicating tetrahedral short-range coordination in water, is totally absent in the NaOH solution at room temperature, and shows up only as a small feature in the NaOH solution in the glassy state. Corresponding changes occur in the OH and HH correlation functions: The hydrogen bond peak position is shifted from 1.85 Å in pure water to 1.65 Å for both the liquid and glassy NaOH, with a reduced number of hydrogen bonds in the glassy phase. The intramolecular HH distance, 1.5 Å, of the water molecule is unaffected by the presence of the solute, but the positions of the peaks in the HH function at 2.4 and 3.8 Å, due to the orientational correlation between neighboring pure water molecules, are respectively, shifted to 2.15 and 3.5 Å. The above findings indicate that ions in aqueous solutions induce a change in water structure equivalent to the application of high pressures.
Surface spin-glass behavior in La2/3Sr1/3MnO3 nanoparticles
Authors: Zhu T. , Shen B. G. , Sun J. R. , Zhao H. W. , Zhan W. S.
Published: July 2002

The low-temperature magnetic and transport properties of La2/3Sr1/3MnO3 nanoparticles have been investigated. It is found that a surface spin-glass behavior exists in La2/3Sr1/3MnO3 nanoparticles, which undergo a magnetic transition to a frozen state below 45 K. The low-temperature surface spin-glass behavior exists even at the highest field used (H=50 kOe). Moreover, the spin-glass-like transition disappears for particles above 50 nm. In addition, the suppressed low-field magnetoconductivity (LFMC) observed at low temperature for nanosized La2/3Sr1/3MnO3 is obviously lower than the expected upper limit of LFMC, 1/3, for polycrystalline manganites, which is proposed to arise from the higher-order tunneling through the insulating spin-glass-like surface layers.
Analysis of radiation-induced small Cu particle cluster formation in aqueous CuCl2
Authors: Jayanetti Sumedha , Mayanovic Robert A. , Anderson Alan J. , Bassett William A. , Chou I.-Ming
Published: July 2002

Synchrotron x-ray radiation is being used extensively as a structure probe to investigate the coordination environment and thus gain insight into the ion–water and ion–ion interactions in aqueous solutions. However, under favorable conditions, there may be instances where the incident x-ray beam can induce oxidation and/or reduction in the solution, thus altering its chemistry. Successive x-ray absorption fine structure spectra, measured in the fluorescence mode from a 55 ppm Cu in CuCl2 aqueous solution, show the formation of copper clusters and their growth as a function of time of irradiation. Initially, the clusters have a nearest neighbor distance of 2.48±0.02 Å which, with increase in time of irradiation, increases to 2.55±0.01 Å, indicating that the clusters approach the lattice dimensions of bulk copper. Similarly, the Debye–Waller factor of the copper clusters is found to increase by ∼50%–55% over the range of time of irradiation. Analysis of spectra measured in the intermediate time period shows signal contributions from a mixture of clusters that can be represented by a mixture of a small cluster (5–10 Å across) and bulk copper. The nearest neighbor coordination number is found to increase in a manner consistent with the decrease in the surface to volume ratio as the average cluster size approaches its bulk dimensions. The initiation of cluster growth occurs through agglomeration of copper atoms that possibly react to form dimers upon reaction. The copper ions in the solution are reduced to the metallic state by reacting with hydrated electrons produced as a result of radiolysis of water by the incident x-ray beam.
Charge transport through a single tetracene grain boundary
Authors: Schön Jan Hendrik , Kloc Christian
Published: July 2002

The hole transport through a single grain boundary is investigated in the temperature range from 4 to 300 K using a tetracene bicrystal. The carrier concentration is varied by the field effect. The results can be explained using the grain-boundary-trapping model. A potential barrier is formed at the grain boundary due to charged traps at the grain boundary. The barrier height depends significantly on the carrier density within the grain. At low temperatures, tunneling through the grain boundary dominates over thermionic emission over the barrier.
Thermodynamic properties and phase equilibria of branched chain fluids using first- and second-order Wertheim’s thermodynamic perturbation theory
Authors: Blas Felipe J. , Vega Lourdes F.
Published: July 2002

We present an extension of the statistical associating fluid theory (SAFT) for branched chain molecules using Wertheim’s first- and second-order thermodynamic perturbation theory with a hard-sphere reference fluid (SAFT-B). Molecules are formed by hard spherical sites which are tangentially bonded. Linear chains are described as freely jointed monomeric units, whereas branched molecules are modeled as chains with a different number of articulation points, each of them formed by three arms. In order to calculate the vapor–liquid equilibria of the system, we have considered attractive interactions between the segments forming the chain at the mean-field level of van der Waals. The Helmholtz free energy due to the formation of the chain is explicitly separated into two contributions, one accounting for the formation of the articulation tetramer, and a second one due to the formation of the chain arms. The first term is described by the second-order perturbation theory of Phan et al. [J. Chem. Phys. 99, 5326 (1993)], which has been proven to predict the thermodynamic properties of linear chain fluids in a similar manner to Wertheim’s approach. The formation of the chain arms is calculated at Wertheim’s first-order perturbation level. The theory is used to study the effect of the chain architecture on the thermodynamic properties and phase equilibria of chain molecules. The equation predicts the general trends of the compressibility factor and vapor–liquid coexistence curve of the system with the branching degree, in qualitative agreement with molecular simulation results for similar models. Finally, SAFT-B is applied to predict the critical properties of selected light alkanes in order to assess the accuracy of the theory. Experimental trends of the critical temperature of branched alkanes are qualitatively captured by this simple theory.
Shock-induced α–ω transition in titanium
Authors: Greeff C. W. , Trinkle D. R. , Albers R. C.
Published: July 2002

Equilibrium free energies for the α and ω phases of Ti are constructed. The result is a consistent picture of the ambient pressure, static high pressure, and shock data, as well as first-principles electronic structure calculations. The Hugoniot consists of three segments: a metastable α-phase region, a transition region, and an ω-phase branch. All the Hugoniot data are consistent with a transition occurring at ∼12 GPa. An early identification [R. G. McQueen et al., in High Velocity Impact Phenomena, edited by R. Kinslow (Academic, New York, 1970)] of a phase transition at 17.5 GPa appears to have been an artifact. The shock Hugoniot extends further into the metastable region than static data, indicating the existence of a relaxation process occurring on a time scale intermediate between those of the static and dynamic measurements.
Epitaxial growth of Pr2O3 on Si(111) and the observation of a hexagonal to cubic phase transition during postgrowth N2 annealing
Authors: Liu J. P. , Zaumseil P. , Bugiel E. , Osten H. J.
Published: July 2002

We demonstrate the epitaxial growth of hexagonal Pr2O3(001) on Si(111), with x-ray ω-scan full width at half maximum values as low as 0.06°, which is comparable with Si substrates. We find that a phase transition takes place during the anneal of the as-grown films in N2 below the growth temperature. The annealed films display a cubic structure isomorphic to manganese oxide, (111) oriented but 180° rotated about the Si(111) surface normal. The phase transition can be due to nitrogen incorporation.
Guanine tautomerism revealed by UV–UV and IR–UV hole burning spectroscopy
Authors: Nir E. , Janzen Ch. , Imhof P. , Kleinermanns K. , de Vries M. S.
Published: July 2002

The vibronic spectrum of laser desorbed and jet cooled guanine consists of bands from three different tautomers of guanine as revealed by UV–UV and IR–UV double resonance spectroscopy. 1-methylguanine, in which the Keto–Enol tautomerism is blocked, shows hole burning spectra from the 9H-and 7H-Keto form. A comparison of the vibronic pattern of the different tautomers demonstrates that the vibronic spectrum built on the redmost guanine band at 32 870 cm−1 (electronic origin 0) can be traced back to the 9H-Enol tautomer, while the spectra built on the origins at 0+404 cm−1 and 0+1044 cm−1 stem from the two Keto tautomers. The IR–UV double resonance spectra of the OH-and NH-stretch vibrations of the different tautomers support this assignment. The UV and IR spectra can be partly assigned by comparison with ab initio calculated vibrational frequencies and with the help of deuteration experiments.
Development of a thermoelectric power generation system using reciprocating flow combustion in a porous FeSi2 element
Authors: Katsuki Futoshi , Tomida Toshiro , Nakatani Hiroko , Katoh Masahiro , Takata Akihiro
Published: July 2002

A thermoelectric power generation system using reciprocating flow combustion in a porous thermoelectric conversion element has been developed and examined in its performance. Mn- (n-type) and Co- (p-type) doped FeSi2 powders were molded into the cylindrical element via a spark plasma sintering process, in which Mn- and Co-doped parts were separated by a thin insulator sheet exclusive of a terminus. The porous element consisted of two semicylindrical p/n couples, arranged electrically in series but thermally in parallel. A thermopower of 1.0–1.2 mV/K at 295–624 K and an apparent internal resistivity of 1.6×10−1 Ω cm at 556–624 K have been obtained for the element. A power generation system was then made using a pair of the elements, which were arranged lengthwise in a cylindrical combustion chamber. A reciprocatory flow of dilute fuel gas was introduced into the element, and it was ignited between the element. A steep temperature gradient of about 200 K/cm was formed lengthwise in both elements. The energy density has reached as much as 7 kW/m3 (excluding combustion chamber and mounting clamp) by the reciprocating flow combustion with a dilute fuel gas which may not even be normally flammable.
Low-resistance and high-transparency Ni/indium tin oxide ohmic contacts to p-type GaN
Authors: Horng Ray-Hua , Wuu Dong-Sing , Lien Yi-Chung , Lan Wen-How
Published: July 2002

The characteristics of Ni/indium tin oxide (ITO) ohmic contacts to p-type GaN (∼2×1017 cm−3) have been studied. The Ni/ITO (10 nm/250 nm) layers were prepared by thermal evaporation and rf magnetron sputtering, respectively. Although the as-deposited Ni/ITO contacts present rectified behavior, the linear current–voltage characteristics can be obtained. The contact resistance can be reduced significantly for the ITO/Ni/p-GaN samples after suitable rapid thermal process. The contact property of ITO/Ni/p-GaN shows lowest specific contact resistivity of 8.6×10−4 Ω cm2 and high transparency (above 80% for 450–550 nm) as the sample annealed at 600 °C in air. Possible mechanisms for the observed low contact resistance and high transparency will be discussed. The present process is compatible with the fabrication for the high-efficient GaN light-emitting devices.
Sound-excited flow and acoustic nonlinearity at an orifice
Authors: Jing Xiaodong , Sun Xiaofeng
Published: July 2002

In this paper a potential model is developed to study the sound-excited flow and the acoustic nonlinearity at an orifice. It is assumed that the vortices shed from the orifice edge are simply pushed to move by the fluid slug coming out of the orifice. The fluid slug is either cylindrical or in the shape of an inviscid steady jet through the orifice. The strength of the shed vortices is determined by applying the Kutta condition at the orifice edge. For a sinusoidal applied sound pressure, the fluctuating orifice flow is computed, and some basic flow features such as the velocity distortion in the orifice and the asymmetry of velocity between the inflow and outflow half cycle at positions away from the orifice are described. The nonlinear acoustic resistance as well as reactance of the orifice is also obtained from the average flow velocity through the orifice. The present theory has been compared with the existing studies for the acoustic nonlinearity of an orifice. For the purpose of further validation an experiment is carried out. The theoretical predictions agree fairly well with the experimental data despite the simplifications having been made in this model. It is thereby concluded that the nonlinear acoustic behavior of an orifice depends mainly on the vortex shedding rate at the orifice edge and the convection speed of the shed vortex in the vicinity of the orifice rather than the fine details of the shed vortices.
Morphological self-organization by dewetting in thin films on chemically patterned substrates
Authors: Kargupta Kajari , Sharma Ashutosh
Published: July 2002

The spontaneous pattern formation and morphological transitions in thin liquid films on chemically heterogeneous, periodic patterned surfaces are studied based on nonlinear simulations. Conditions are identified for the creation of desired mesostructures in soft materials by spontaneous dewetting on patterned substrates. On a surface consisting of alternating less and more wettable stripes, dewetting is suppressed on some less wettable stripes when their spacing is below a characteristic length scale of instability (λh), which is smaller than the spinodal length scale of instability. Ideal templating, i.e., replication of the substrate surface energy pattern in the thin film morphology occurs only when (a) the periodicity of substrate pattern is greater than λh, (b) width of the less wettable stripe is within a range bounded by a lower critical length, below which no heterogeneous rupture occurs, and an upper transition length above which complex morphological features bearing little resemblance to the substrate pattern are formed, (c) the contact line eventually rests close to the stripe boundary, and (d) the liquid cylinders that form on the more wettable stripes remain stable. Conditions (a) and (b) ensure the onset of dewetting at the center of every less wettable stripe, and conditions (c) and (d) ensure full coverage of every more wettable site by liquid cylinders. The above conditions for ideal templating on a periodic heterogeneous substrate can be engineered by modulating the stripe-periodicity and width, film thickness and the wettability gradient across the stripe boundary. For example, ideal templating on a given substrate always occurs at an intermediate film thickness, since dewetting is suppressed at some less wettable stripes at higher thickness and multiple dewetting sites form on less wettable stripes at lower thickness.
Photoinduced permanent gratings inside bulk azodye-doped polymers by the coherent field of a femtosecond laser
Authors: Si Jinhai , Qiu Jianrong , Zhai Jianfeng , Shen Yuquan , Hirao Kazuyuki
Published: July 2002

Permanent holographic gratings were fabricated in bulk azodye-doped polymethylmethacrylate by the coherent field of a femtosecond laser. Optical microscopy and atomic force microscopy observations revealed that the gratings consist of two parts, surface relief gratings on both surfaces and refractive index modulated volume gratings in the interior of the polymers. The diffraction efficiency of the first-order Bragg for the gratings was estimated to be 90%.
Hydrodynamic stability of a suspension in cylindrical Couette flow
Authors: Ali Mohamed E. , Mitra Deepanjan , Schwille John A. , Lueptow Richard M.
Published: July 2002

A linear stability analysis was carried out for a dilute suspension of rigid spherical particles in cylindrical Couette flow. The perturbation equations for both the continuous fluid phase and the discontinuous particle phase were decomposed into normal modes resulting in an eigenvalue problem that was solved numerically. At a given radius ratio, the theoretical critical Taylor number at which Taylor vortices first appear decreases as the particle concentration increases. Increasing the ratio of particle density to fluid density above one decreases the stability. However, using an effective Taylor number based on the suspension density and viscosity largely accounts for this effect. The axial wave number is the same for a suspension as it is for a pure fluid. Experiments using neutrally buoyant particles in a Taylor–Couette apparatus show that the flow is more stable as the particle concentration increases. The reason that the theory does not fully capture the physics of the flow should be addressed in future research.
Production of nitrogen acceptors in ZnO by thermal annealing
Authors: Garces N. Y. , Giles N. C. , Halliburton L. E. , Cantwell G. , Eason D. B. , Reynolds D. C. , Look D. C.
Published: July 2002

Nitrogen acceptors are formed when undoped single crystals of zinc oxide (ZnO) grown by the chemical-vapor transport method are annealed in air or nitrogen atmosphere at temperatures between 600 and 900 °C. After an anneal, an induced near-edge absorption band causes the crystals to appear yellow. Also, the concentration of neutral shallow donors, as monitored by electron paramagnetic resonance (EPR), is significantly reduced. A photoinduced EPR signal due to neutral nitrogen acceptors is observed when the annealed crystals are exposed to laser light (e.g., 364, 442, 458, or 514 nm) at low temperature. The nitrogens are initially in the nonparamagnetic singly ionized state (N−) in an annealed crystal, because of the large number of shallow donors, and the light converts a portion of them to the paramagnetic neutral acceptor state (N0).
Photomagnetism and structure in cobalt ferrite nanoparticles
Authors: Giri A. K. , Kirkpatrick E. M. , Moongkhamklang P. , Majetich S. A. , Harris V. G.
Published: July 2002

Light-induced changes in the coercivity ΔHc are reported for cobalt ferrite nanoparticles. These changes arise from charge transfer initiated by optical absorption near 2 eV. An insulator-to-metal transition at 170 K is an upper limit for significant ΔHc. The larger ΔHc for smaller (17 nm) particles was correlated with a distortion about the Co2+ ions at B sites, which increases the absorption probability.
Nonresonant detection of terahertz radiation in field effect transistors
Authors: Knap W. , Kachorovskii V. , Deng Y. , Rumyantsev S. , Lü J.-Q. , Gaska R. , Shur M. S. , Simin G. , Hu X. , Khan M. Asif , Saylor C. A. , Brunel L. C.
Published: July 2002

We present an experimental and theoretical study of nonresonant detection of subterahertz radiation in GaAs/AlGaAs and GaN/AlGaN heterostructure field effect transistors. The experiments were performed in a wide range of temperatures (8–300 K) and for frequencies ranging from 100 to 600 GHz. The photoresponse measured as a function of the gate voltage exhibited a maximum near the threshold voltage. The results were interpreted using a theoretical model that shows that the maximum in photoresponse can be explained by the combined effect of exponential decrease of the electron density and the gate leakage current.
Relaxation dynamics in plastic crystals
Authors: Brand R. , Lunkenheimer P. , Loidl A.
Published: July 2002

We report dielectric data on six different plastic crystalline materials, namely 1-cyanoadamantane, adamantanone, pentachloronitrobenzene, cyclo-hexanol, ethanol, and meta-carborane, covering a frequency range of up to 14 decades and up to 20 GHz. Information on phase transitions, the α-relaxation, and relaxation processes beyond the α-relaxation are provided. The α-relaxation shows clear non-Debye behavior and varying degrees of deviation from thermally activated behavior. Our results reveal a generally rather low fragility of plastic crystals. In some of the investigated materials evidence for Johari–Goldstein type β-relaxations is obtained. In addition, the question of the so-called excess wing of the α-relaxation peak is addressed in detail. In all cases, it is either absent or can be ascribed to a β-relaxation submerged under the α-peak. Overall, the present work provides a broad database on the dielectric behavior of plastic crystals, and may be taken as a review of the dynamic phenomena occurring in these materials, many of them being observed also in structural glass formers.
Solid-state soft actuator exhibiting large electromechanical effect
Authors: Shahinpoor Mohsen , Kim Kwang J.
Published: July 2002

This letter deals with the discovery of large low-voltage electromechanical actuation effects in an electroactive polymer made with poly(ethylene oxide) and poly(ethylene glycol). The experimental observations and robust performance of the solid-state polymer actuators in the form of a strip, which is suitably surface electroded and cation doped, are reported herein. Recent laboratory discovery shows that such solid-state actuators are polymeric materials capable of exhibiting large motion actuation capabilities (>1% bending strain) in a low electric field imposed across the strip (<10 V/mm) with considerable stress (>10 MPa) and fast responses (>10 Hz). Moreover, a stable operation over ten millions of cycles in air is achieved with nearly no performance degradation.
Protein detection by optical shift of a resonant microcavity
Authors: Vollmer F. , Braun D. , Libchaber A. , Khoshsima M. , Teraoka I. , Arnold S.
Published: July 2002

We present an optical biosensor with unprecedented sensitivity for detection of unlabeled molecules. Our device uses optical resonances in a dielectric microparticle (whispering gallery modes) as the physical transducing mechanism. The resonances are excited by evanescent coupling to an eroded optical fiber and detected as dips in the light intensity transmitted through the fiber at different wavelengths. Binding of proteins on the microparticle surface is measured from a shift in resonance wavelength. We demonstrate the sensitivity of our device by measuring adsorption of bovine serum albumin and we show its use as a biosensor by detecting streptavidin binding to biotin.
Noise sustained waves in subexcitable media: From chemical waves to brain waves
Authors: Jung P. , Cornell-Bell A. , Moss F. , Kadar S. , Wang J. , Showalter K.
Published: July 2002

We discuss a novel type of spatiotemporal pattern that can be observed in subexcitable media when coupled to a thermal environment. These patterns have been recently observed in several different types of systems: a subexcitable photosensitive Belousov–Zhabotinsky reaction, hippocampal slices of rat brains, and astrocyte syncytium. In this paper, we introduce the basic concepts of subexcitable media, describe recent experimental observations in chemistry and neurophysiology, and put these observation into context with computer simulations.
Physical interpretation of eddy current losses in ferromagnetic materials. I. Theoretical considerations
Authors: Bertotti G.
Published: July 2002

The basic physical mechanism responsible for the general behavior of eddy current losses versus magnetizing frequency fm is recognized in the competition between the external field and local internal fields, due to magnetostatic, coercive, and eddy current effects. The concept of magnetic object, corresponding to a group of neighboring walls evolving in a highly correlated fashion, is introduced in order to take into proper account the role of short-range internal correlation fields. With a random spatial distribution of magnetic objects, the dynamic loss is essentially a function of the number ñ of magnetic objects which are simultaneously active at each value of fm. It is shown that the dynamic balance between applied field and local eddy current counterfields leads, with increasing fm, to a progressive increase of ñ governed by the simple linear law ñ=ñ0+Hexc/V0, where Hexc represents the part of the applied field in excess of the hysteresis and classical contributions. Under these conditions, a general law of losses is derived, which turns out to be in excellent agreement with several experimental results concerning different iron-based ferromagnetic alloys. On the basis of the general validity of the obtained loss equation, a bidimensional map of dynamic losses is defined and constructed, in which the parameters ñ0 and V0 are used as orthogonal coordinates of representative points characterizing the loss behavior of different materials. This map provides the proper basis for a general classification of dynamic losses, which might also be useful from the applicative point of view.
Plasma potentials of 13.56-MHz rf argon glow discharges in a planar system
Authors: Köhler K. , Coburn J. W. , Horne D. E. , Kay E. , Keller J. H.
Published: July 2002

The plasma potential of 13.56-MHz low-pressure argon glow discharges has been measured for various modes of applying the rf power in a geometrically asymmetric planar system. The plasma potential is determined from the energy distribution of positive ions incident on the grounded electrode. The voltages on the excitation electrode (target electrode) are carefully measured and the capacitive sheath approximation is used to relate these measured voltages to the measured plasma potential. This approximation is successful in most of the situations encountered in this low-pressure (20 mTorr) relatively low-power density regime. The effects of superimposing dc voltages on the excitation electrode are discussed.
Electrolyte thickness dependence of the electrochromic behavior of ‘‘a-WO3’’ films
Authors: Ashrit P. V. , Pelletier Denis J. , Girouard Fernand E. , Truong Vo-Van
Published: July 2002

The electrochromic behavior of all-solid-state device, Au/a-WO3/LiF/Au has been studied as a function of the thickness of LiF as electrolyte. Thickness-dependent coloration and response time have been observed. The electrochromic coloration takes place at different stages, and a significant change in the transmittance of the tandem over a wide wavelength range (500–1000 nm) occurs. Hence, the performance of this configuration suggests the possible application of the tandem in all-solid-state ‘‘switchable windows’’.
Flux formulation of hyperbolic heat conduction
Authors: Frankel J. I. , Vick Brian , Özisik M. N.
Published: July 2002

The development of the general flux formulation for heat conduction based on the modified Fourier’s law is presented. This new formulation produces a hyperbolic vector equation in heat flux which is more convenient to use for analysis in situations involving specified flux conditions than the standard temperature formulation. The recovery of the temperature distribution is obtained through integration of the energy conservation law with respect to time. The Green’s function approach is utilized to develop a general solution for hyperbolic heat conduction in a finite medium. The utility of the flux formulation and the unusual nature of heat conduction based on the hyperbolic formulation are demonstrated by developing analytical expressions for the heat flux and temperature distributions in a finite slab exposed to a pulsed surface heat flux.
Dielectric properties of fine-grained barium titanate ceramics
Authors: Arlt G. , Hennings D. , de With G.
Published: July 2002

Dielectric properties, lattice- and microstructure of ceramic BaTiO3 showing grain sizes of 0.3–100 μm were studied. At grain sizes <10 μm the width of ferroelectric 90° domains decreases proportionally to the square root of the grain diameter. The decreasing width of the domains can be theoretically explained by the equilibrium of elastic field energy and domain wall energy. The smaller the grains, the more the dielectric and the elastic constants are determined by the contribution of 90° domain walls. The permittivity below the Curie point shows a pronounced maximum εr ≊5000 at grain sizes 0.8–1 μm. At grain sizes <0.7 μm the permittivity strongly decreases and the lattice gradually changes from tetragonal to pseudocubic.
Electrical conductivity and thermoelectric power measurements of some lithium–titanium ferrites
Authors: Manjula R. , Murthy V. R. K. , Sobhanadri J.
Published: July 2002

The electrical conductivity and thermoelectric power of some lithium–titanium ferrites with small amounts of manganese and zinc have been studied as a function of temperature in the range of 300–550 K and are reported here. The conductivity variation shows two different regions with a large variation in the activation energies. The possible mechanisms with respect to ionic conduction and electron hopping are discussed with the support of thermoelectric power measurements. Lattice parameters are also calculated from x-ray diffractograms for confirming the single-phase nature of the ferrites.
Dipole moments of conducting particle chains
Authors: Jones T. B.
Published: July 2002

The method of images is used to calculate the effective dipole and quadrupole moments of short chains consisting of identical spherical conducting particles. For the case where the particles are not in physical contact, the electrical constraint imposed on the particles in the chain strongly influences the results. The dipole moments for chains of electrically isolated particles are very sensitive to spacing. If chains of particles are levitated dielectrophoretically, this sensitivity of the effective moment to spacing may be exploited in order to characterize conducting particles with insulating surface layers.
Application of the physics of plasma sheaths to the modeling of rf plasma reactors
Authors: Metze A. , Ernie D. W. , Oskam H. J.
Published: July 2002

An equivalent circuit model is presented for a planar rf plasma reactor. The physical properties of the plasma sheath adjacent to the electrodes are incorporated in the model. The sheath capacitances and the conduction currents through the sheaths are time varying and have a highly nonlinear dependence on the potentials across the plasma sheaths. The model shows that the waveforms of the voltage differences across the sheaths are highly nonsinusoidal and agree with reported measurements.
Calculation of transmission tunneling current across arbitrary potential barriers
Authors: Ando Yuji , Itoh Tomohiro
Published: July 2002

This paper presents a simple method for accurately calculating quantum mechanical transmission probability and current across arbitrary potential barriers by using the multistep potential approximation. This method is applicable to various potential barriers and wells, including continuous variations of potential energy and electron effective mass. Various potential barrier structures and a hot-electron transistor are analyzed to show the feasibility of this method.
Inelastic neutron scattering studies on 1D near-Heisenberg antiferromagnets: A test of the Haldane conjecture
Authors: Steiner M. , Kakurai K. , Kjems J. K. , Petitgrand D. , Pynn R.
Published: July 2002

Our studies of the spin dynamics for two near-Heisenberg antiferromagnets (AFMs) revealed: In CsNiCl3 (S=1) at low T≪TN we confirm the conclusions given earlier, but disagreement with the spin-wave calculations used is found for the eigenvectors. For T>TN, the most important result is the observation that all 〈SαSα〉 (α=x,y,z) are identical, demanding another interpretation for the gap observed than the easy axis one. In CsVCl3 (S= (3)/(2) ) we could not observe any inconsistency in the microscopic parameters determined above and below TN. Although there is a striking coincidence with Haldane’s conjecture [unexplained gap for S=1 (CsNiCl3), no such gap for S= (3)/(2) (CsVCl3)], it is not clear whether this can be considered a proof for Haldane’s conjecture.
Two-dimensional studies of streamers in gases
Authors: Dhali S. K. , Williams P. F.
Published: July 2002

We present the results of two-dimensional computer simulations of streamer initiation and propagation in atmospheric pressure N2. The simulation algorithm makes use of flux-corrected transport techniques and was used as a tool to study the solutions of the transport equations under conditions suitable for streamers, for which realistic analytic solutions are not known. We present and discuss conclusions about streamer transport based on the results of these studies. Finally, we present a novel method of checking on the numerical accuracy with which the algorithm solves the transport equations.
Simple model for thin ferromagnetic films exchange coupled to an antiferromagnetic substrate
Authors: Mauri D. , Siegmann H. C. , Bagus P. S. , Kay E.
Published: July 2002

The exchange field Hex transferred from a thick antiferromagnetic substrate to a thin exchange coupled ferromagnetic film is shown to reach a limiting value no matter how large the exchange coupling is. The limit is due to domain-wall formation in the antiferromagnet. Numerical results based on a simple model for the interface are presented and compared to experimental results.
High-efficiency Au/CdTe photovoltaic cells
Authors: Raychaudhuri Pranab K.
Published: July 2002

Au/CdTe photovoltaic cells have been prepared on modified surfaces of single-crystal substrates with AM2 efficiencies of about 15%. The modification effected by heat treatment in air is found to decrease the reverse saturation current and increase the width of the depletion layer. As a result, the open-circuit voltage, the short-circuit current, and the fill factor are all significantly enhanced. The maximum values of cell parameters obtained in different cells suggest that significantly higher efficiency is feasible.
Microstructure of hot-pressed and die-upset NdFeB magnets
Authors: Mishra Raja K.
Published: July 2002

Transmission electron microscopy was used to establish densification, alignment, and magnetization reversal mechanisms in hot-pressed and die-upset NdFeB magnets. Microstructures of these materials reveal two principal phases: Nd2Fe14B grains and a grain boundary phase of approximate composition Nd7Fe3. The grains in the hot-pressed material are polygonal and isotropic, while those in the die-upset material are flat platelets. The alignment of die-upset magnets arises exclusively from the fact that hot deformation produces platelets of Nd2Fe14B grains with their c axes parallel to the stress axis. It is argued that yielding plays an important role in the densification of the hot-pressed samples, and diffusion slip is critical to the alignment. The grain boundary phase appears as a crystalline or noncrystalline phase in the hot-pressed samples but has an fcc structure in the die-upset samples. Magnetic domain walls are shown to be pinned at the grain boundary phase in both cases. A Nd-rich spongy phase is observed to form at the boundaries where two ribbons fuse, but has no significance for the magnetic properties.
Gases released by surface flashover of insulators
Authors: Miller H. Craig , Ney Robert J.
Published: July 2002

The gases released by surface flashovers on alumina ceramics, glass, and quartz, using 20-μs voltage pulses, were investigated using an ion-pumped metal vacuum system. Appreciable quantities of gas were measured (approximately 10−6 Torr liter or 1013 molecules). The composition of this gas differed significantly from the background gas. The dominant species in the surface flashover gases were CO2, CO, and H2; a modest amount of CH4 was observed, along with lesser quantities of N2 and H2O. The dominant gas in the system background was nitrogen, accompanied by (in decreasing amounts) H2, H2O, CO/CO2, Ar, CH4, and He. We concluded that the gas released by flashovers on the surface of the insulators was not adsorbed system background gas, but was gas adsorbed during the handling and processing of the insulator.
An examination of double positioning boundaries and interface misfit in beta-SiC films on alpha-SiC substrates
Authors: Kong H. S. , Jiang B. L. , Glass J. T. , Rozgonyi G. A. , More K. L.
Published: July 2002

Beta-SiC thin films epitaxially grown on 6H-SiC (0001) substrates were examined via scanning-reflection x-ray topography (XRT) and x-ray rocking curve analysis. The lattice misfit between the (111) plane of the β-SiC epitaxial layer and the (0001) plane of the 6H-SiC substrate was determined to be 7.6×10−4. Boundaries separating regions of the β-SiC film rotated from each other by 60° were observed and identified by XRT as double positioning boundaries (DPBs), which are a special type of twin boundary. The XRT maps showed that the mosaic structure examined via optical microscopy was also caused by the DPBs. Finally, many stacking faults were generated at the boundaries as revealed by plan-view transmission electron microscopy, indicating the high internal energy of these DPBs. The formation mechanisms of the DPBs is also discussed.
Characterization of device parameters in high-temperature metal-oxide-semiconductor field-effect transistors in β-SiC thin films
Authors: Palmour J. W. , Kong H. S. , Davis R. F.
Published: July 2002

Both inversion- and depletion-mode n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) have been fabricated on β-SiC thin films grown by chemical-vapor deposition. The inversion-mode devices were made on in situ doped (Al) p-type β-SiC(100) thin films grown on Si(100) substrates. The depletion-mode MOSFETs were made on n-type β-SiC(111) thin films grown on the Si(0001) face of a 6H α-SiC substrates. Stable saturation and low subthreshold currents were achieved at drain-source voltages exceeding 5 and 25 V for the inversion-mode and depletion-mode devices, respectively. The transconductance increased with temperature up to 673 K for the short-gate-length devices, of either mode, and then decreased with further increases in temperature. It is proposed that the transconductances and threshold voltages for the inversion-mode devices are greatly affected by minority-carrier injection from the source. Stable transistor action was observed for both types of devices at temperatures up to 823 K, with the depletion-mode devices operating very well up to 923 K.
A constitutive model for strain rates from 10−4 to 106 s−1
Authors: Steinberg D. J. , Lund C. M.
Published: July 2002

We have developed an addition to the Steinberg–Guinan high strain-rate constitutive model that extends its validity to strain rates as low as 10−4 s−1. With this new model, we have successfully reproduced a number of rate-dependent, shock-induced phenomena in tantalum, such as precursor on reshock, precursor decay, and shock smearing. We have also successfully calculated a plate-impact experiment at a loading stress of 230 GPa as well as extensive data for yield strength versus strain rate at room temperature and yield strength versus temperature at a strain rate of 10−4 s−1.