bluuebunny/crossref_metadata_2025 huggingface dataset:
Simultaneous activation of gamma and theta network oscillations in rat hippocampal slice cultures
Authors: Fischer Yacov , Wittner Lucia , Freund Tamas F. , Gähwiler Beat H.
Published: July 2002

Hippocampal activity in vivo is characterized by concurrent oscillations at theta (4–15 Hz) and gamma (20–80 Hz) frequencies. Here we show that cholinergic receptor activation (methacholine 10–20 nm) in hippocampal slice cultures induces an oscillatory mode of activity, in which the intrinsic network oscillator (located in the CA3 area) expresses simultaneous theta and gamma network oscillations. Pyramidal cells display synaptic theta oscillations, characterized by cycles consisting of population EPSP‐IPSP sequences that are dominated by population IPSPs. These rhythmic IPSPs most probably result from theta‐modulated spiking activity of several interneurons. At the same time, the majority of interneurons consistently display synaptic gamma oscillations. These oscillatory cycles consist of fast depolarizing rhythmic events that are likely to reflect excitatory input from CA3 pyramidal cells. Interneurons comprising this functional group were identified morphologically. They include four known types of interneurons (basket, O‐LM, bistratified and str. lucidum‐specific cells) and one new type of CA3 interneuron (multi‐subfield cell). The oscillatory activity of these interneurons is only weakly correlated between neighbouring cells, and in about half of these (44 %) is modulated by depolarizing theta rhythmicity. The overall characteristics of acetylcholine‐induced oscillations in slice cultures closely resemble the rhythmicity observed in hippocampal field and single cell recordings in vivo. Both rhythmicities depend on intrinsic synaptic interactions, and are expressed by different cell types. The fact that these oscillations persist in a network lacking extra‐hippocampal connections emphasizes the importance of intrinsic mechanisms in determining this form of hippocampal activity.
Histamine promotes excitability in bovine adrenal chromaffin cells by inhibiting an M‐current
Authors: Wallace Damian J. , Chen Chen , Marley Philip D.
Published: July 2002

The current study has investigated the electrophysiological responses evoked by histamine in bovine adrenal chromaffin cells using perforated‐patch techniques. Histamine caused a transient hyperpolarization followed by a sustained depolarization of 7.2 ± 1.4 mV associated with an increase in spontaneous action potential frequency. The hyperpolarization was abolished after depleting intracellular Ca2+ stores with thapsigargin (100 nm), and was reduced by 40 % with apamin (100 nm). Membrane resistance increased by about 60 % during the histamine‐induced depolarization suggesting inhibition of a K+ channel. An inward current relaxation, typical of an M‐current, was observed in response to negative voltage steps from a holding potential of −30 mV. This current reversed at −81.6 ± 1.8 mV and was abolished by the M‐channel inhibitor linopirdine (100 μm). During application of histamine, the amplitude of M‐currents recorded at a time corresponding with the sustained depolarization was reduced by 40 %. No inward current rectification was observed in the range −150 to −70 mV, and glibenclamide (10 μm) had no effect on either resting membrane potential or the response to histamine. The results show that an M‐current is present in bovine chromaffin cells and that this current is inhibited during sustained application of histamine, resulting in membrane depolarization and increased discharge of action potentials. These results demonstrate for the first time a possible mechanism coupling histamine receptors to activation of voltage‐operated Ca2+ channels in these cells.
A fundamental oscillatory state of isolated rodent hippocampus
Authors: Wu Chiping , Shen Hui , Luk Wah Ping , Zhang Liang
Published: July 2002

Population neuronal rhythms of various frequencies are observed in the rodent hippocampus during distinct behavioural states. However, the question of whether the hippocampus exhibits properties of spontaneous rhythms and population synchrony in isolation has not been definitively answered. To address this, we developed a novel preparation for studying neuronal rhythms in a relatively large hippocampal tissue in vitro. We isolated the whole hippocampus from mice up to 28 days postnatal age, removing the dentate gyrus while preserving the functional CA3‐to‐CA1 connections. Placing the hippocampal isolate in a perfusion chamber for electrophysiological assessment extracellular recordings from the CA1 revealed rhythmic field potential of 0.5 to ≤ 4 Hz that occurred spontaneously and propagated along the ventro‐dorsal hippocampal axis. We provide convergent evidence, via measurements of extracellular pH and K+, recordings of synaptic and intracellular activities and morphological assessments, verifying that these rhythms were not the consequence of hypoxia. Data obtained via simultaneous extracellular and patch clamp recordings suggest that the spontaneous rhythms represent a summation of GABAergic IPSPs originating from pyramidal neurons, which result from synchronous discharges of GABAergic inhibitory interneurons. Similar spontaneous field rhythms were also observed in the hippocampal isolate prepared from young gerbils and rats. Based on these data, we postulate that the spontaneous rhythms represent a fundamental oscillatory state of the hippocampal circuitry isolated from extra‐hippocampal inputs.
Combining Leakage and Orifice Flows in a Hydraulic Servovalve Model1
Authors: Eryilmaz Bora , Wilson Bruce H.
Published: July 2002

Experiments indicate that at small servovalve spool displacements, leakage flow between the valve spool and body dominates the orifice flow through the valve. In precision positioning applications, where the servovalve operates within the null region, this flow, if ignored, may severely degrade the performance of a conventional servohydraulic design. We have developed an improved servovalve model that combines both leakage and orifice flow. The model was developed by reviewing experimental leakage flow data and identifying a simple mathematical form that (1) made physical sense and (2) replicated experimental data. When combined with orifice relations, the model extends the accuracy and region of applicability of existing servovalve models. Furthermore, the combined model is easily parameterized using available manufacturer data. [S0022-0434(00)00403-2]
Thin-Film Flow at Moderate Reynolds Number
Authors: Ruschak Kenneth J. , Weinstein Steven J.
Published: July 2002

Viscous, laminar, gravitationally-driven flow of a thin film over a round-crested weir is analyzed for moderate Reynolds numbers. A previous analysis of this flow utilized a momentum integral approach with a semiparabolic velocity profile to obtain an equation for the film thickness (Ruschak, K. J., and Weinstein, S. J., 1999, “Viscous Thin-Film Flow Over a Round-Crested Weir,” ASME J. Fluids Eng., 121, pp. 673–677). In this work, a viscous boundary layer is introduced in the manner of Haugen (Haugen, R., 1968, “Laminar Flow Around a Vertical Wall,” ASME J. Appl. Mech. 35, pp. 631–633). As in the previous analysis of Ruschak and Weinstein, the approximate equations have a critical point that provides an internal boundary condition for a bounded solution. The complication of a boundary layer is found to have little effect on the thickness profile while introducing a weak singularity at its beginning. The thickness of the boundary layer grows rapidly, and there is little cumulative effect of the increased wall friction. Regardless of whether a boundary layer is incorporated, the approximate free-surface profiles are close to profiles from finite-element solutions of the Navier-Stokes equation. Similar results are obtained for the related problem of developing flow on a vertical wall (Cerro, R. L., and Whitaker, S., 1971, “Entrance Region Flows With a Free Surface: the Falling Liquid Film,” Chem. Eng. Sci., 26, pp. 785–798). Less accurate results are obtained for decelerating flow on a horizontal wall (Watson, E. J., 1964, “The Radial Spread of a Liquid Jet Over a Horizontal Plane,” J. Fluid Mech. 20, pp. 481–499) where the flow is not gravitationally driven. [S0098-2202(00)01904-0]
Use of Graded Finite Elements to Model the Behavior of Nonhomogeneous Materials
Authors: Santare M. H. , Lambros J.
Published: July 2002

A finite element with a spatially varying material property field is formulated and compared to a conventional, homogeneous element for solving boundary value problems involving continuously nonhomogeneous materials. The particular element studied is a two-dimensional plane stress element with linear interpolation and an exponential material property gradient. However, the main results are applicable to other types of elements and property gradients. Exact solutions for a finite rectangular plate subjected to either uniform displacement or traction either perpendicular or parallel to the property gradient are used as the basis for comparison. The results show that for identical meshes with equal number of degrees-of-freedom, the graded elements give more accurate local stress values than conventional elements in some boundary value problems, while in other problems the reverse is true. [S0021-8936(00)01504-X]
A Model for Simulating the Performance of a Pavement Heating System as a Supplemental Heat Rejecter With Closed-Loop Ground-Source Heat Pump Systems
Authors: Chiasson Andrew D. , Spitler Jeffrey D. , Rees Simon J. , Smith Marvin D.
Published: July 2002

The thermal loads of commercial and institutional buildings are generally cooling-dominated. When such buildings use ground source heat pump systems (GSHP) they reject more heat to the ground-loop heat exchanger than they extract over the annual cycle. In these situations, supplemental heat rejecters can be used to reduce the required size of the ground-loop heat exchanger, thereby reducing the first cost of the system. This paper describes the development, validation, and use of a design and simulation tool for modeling the performance of a hydronic pavement heating system as a supplemental heat rejecter in ground-source heat pump systems. The model uses a finite difference method to solve the transient two-dimensional heat conduction equation and has been formulated for use in component based system simulation. Full-scale experiments were conducted concurrently with the development of the model, the results of which have been used for validation purposes. An example application simulation is presented to demonstrate the use of the model as well as the viability of the use of pavement heating systems as supplemental heat rejecters in GSHP systems. [S0199-6231(00)00404-4]
Mist/Steam Heat Transfer in Confined Slot Jet Impingement
Authors: Li X. , Gaddis J. L. , Wang T.
Published: July 2002

Internal mist/steam blade cooling technology has been considered for the future generation of Advanced Turbine Systems (ATS). Fine water droplets of about 5 μm were carried by steam through a single slot jet onto a heated target surface in a confined channel. Experiments covered Reynolds numbers from 7500 to 25,000 and heat fluxes from 3 to 21 kW/m2. The experimental results indicate that the cooling is enhanced significantly near the stagnation point by the mist, decreasing to a negligible level at a distance of six jet widths from the stagnation region. Up to 200 percent heat transfer enhancement at the stagnation point was achieved by injecting only ∼1.5 percent of mist. The investigation has focused on the effects of wall temperature, mist concentration, and Reynolds number.
Experiments and Numerical Simulations on Thin Metallic Plates Subjected to an Explosion
Authors: Woznica Krzysztof , Pennetier Olivier , Renard Je´ro^me
Published: July 2002

Submitted to a sudden and a transient loading, a metallic structure quickly evolves toward viscoplastic behavior, including high strain rates. After generalities about the specific loading and the description of the experimental setup, a numerical simulation based on the F.E.M. using an explicit scheme is presented. Three different constitutive laws are discussed: the Perzyna, the Bodner-Partom, and that of Chaboche. The identification of constitutive parameters is made using unidirectional tests on the studied material (aluminum alloy 5754). A comparison between numerical simulations and experiments shows that the method is relevant.
Solar Thermal Splitting of Zinc Oxide: A Review of Some of the Rate Controlling Factors
Authors: Le´de´ Jacques , Elorza-Ricart Enrique , Ferrer Monique
Published: July 2002

It is possible to use concentrated solar energy for producing zinc by direct high temperature dissociation of ZnO. The purpose of the present paper is to show that the overall efficiency of such a process as well as the quality of the products may be controlled by several chemical and physical factors such as heat and mass transfer efficiencies, hydrodynamics and reactor design. The results of three complementary experimental approaches are reported. They are interpreted in connection with simplified models in order to describe the controlling processes occurring in the high temperature and quench sections of the reactor. One of the conclusions is that zinc can be recovered by condensation on the cold walls of a heat exchanger and/or under the form of small dispersed particles. Finally, it is shown that the reactor design must be optimized inside the perspective of a given future use of zinc.
Imbalance Estimation for Speed-Varying Rigid Rotors Using Time-Varying Observer
Authors: Zhou Shiyu , Shi Jianjun
Published: July 2002

Rigid rotor dynamic model is widely used to model rotating machinery. In this paper, a speed-varying transient rigid rotor model is developed in the state space form. The states of this model are augmented to include imbalance forces and moments. A time-varying observer can then be designed for the augmented system by using canonical transformation. After obtaining an estimation of the imbalance forces and moments as the states of the augmented system, the estimated imbalance can be directly calculated. This estimation method can be used in the active vibration control or active balancing schemes for a rigid rotor.
Aseismic Vibration Control of Flexible Rotors Using Active Magnetic Bearing
Authors: Matsushita Osami , Imashima Toshio , Hisanaga Yoshitaka , Okubo Hiroki
Published: July 2002

The wide application of active magnetic bearing (AMB) requires an aseismic evaluation with respect to AMB rotor vibrations caused by actual earthquakes. A flexible rotor supported by AMB is selected for this purpose. A shaking simulation obtained using the quasi-modal model and the actual Kobe earthquake was completed. A corresponding test rotor was excited by seismic waves and the resulting vibration was measured for the vibration evaluation. In order to reduce the response severity against earthquakes, we propose an additional feed forward control method which is proportional to the signal detected by the accelerometers attached to the bearing housings. Since this additional control can cancel rotor vibration generated by the earthquakes, AMB rotor vibrations are successfully suppressed at a low level.
On the Theoretical Prediction of Fuel Droplet Size Distribution in Nonreactive Diesel Sprays
Authors: Cao Jianming
Published: July 2002

Droplet size distribution function and mean diameter formulas are derived using information theory. The effects of fuel droplet evaporation and coalescence within combustion chamber on the droplet size are emphasized in nonreactive diesel sprays. The size distribution function expressions at various spray axial cross sections are also formulated. The computations are compared with experimental data and KIVA-II code. A good agreement is obtained between numerical and experimental results. Droplet size distribution and mean diameter at various locations from injector exit and at various temperature conditions are predicted. The decreases of droplet number and variations of mean diameter are computed at downstream and higher temperature.
Combustion Characteristics of Heavy Liquid Fuels in a Bubbling Fluidized Bed
Authors: Anthony E. J. , Lu D. Y. , Zhang J. Q.
Published: July 2002

Liquid fuels such as bitumen, tars, and pitches are byproducts of heavy oil upgrading processes, and are usually contaminated with high sulphur and sometimes heavy metals contents as well. Fluidized bed combustion (FBC) appears to be a promising technology for the combustion of such fuels due to its inherent fuel flexibility and low emissions characteristics. The combustion of three liquid fuels, i.e., no. 6 oil, bitumen and pitch was investigated in a pilot-scale bubbling FBC unit. An efficient liquid fuel feeding system was developed and a bubbling FBC was successfully used to combust all three liquid fuels. The proportion of fuel escaping in the form of unburnt hydrocarbons in the flue gas was less than 0.4 percent and combustion efficiencies higher than 98.5 percent were achieved. However, combustion of liquid fuels tended to occur in the freeboard and, therefore, good mixing of the fuels in the bed was critical in achieving satisfactory combustion performance.
Interactive Virtual Tools for Manipulating NURBS Surfaces in a Virtual Environment
Authors: Perles Brian P. , Vance Judy M.
Published: July 2002

DN-Edit is a virtual environment developed to allow the manipulation of non-uniform rational b-spline (NURBS) surfaces using virtual shaping tools. NURBS have become the industry standard for representation of free-form curves and surfaces. The contribution of the work presented here is in the development of shaping tools which are used to operate directly on the NURBS data and change the shape of the surfaces in a virtual environment. These shaping tools allow surface manipulations to be made using methods that match the shaping of real malleable objects. The virtual shaping tools are three-dimensional shapes that are controlled through a six degree-of-freedom tracking system that converts user hand motions into computer input. The NURBS surface updates itself in real time due to the effect of the tools on the surface. The new shape of the surface is dependent on the position and orientation of the shaping tool relative to the surface. Constraint-based surface manipulation is used to obtain multiple point direct manipulation of NURBS surfaces. In addition, computational methods to allow the user to have direct control of the first derivative of the surface over an area are implemented. This application has been developed using the C2 software libraries and Iowa State University’s C2 surround screen virtual environment.
Optimization of Turbine Disk Profiles by Metamorphic Development
Authors: Liu Jing-Sheng , Parks Geoffrey T. , Clarkson P. John
Published: July 2002

A novel topology/shape optimization method, Metamorphic Development, is applied to an axisymmetric thermo-elasticity design problem. Based on solid modeling and finite element analysis, optimal profiles of minimum mass turbine disks are sought by growing and degenerating simple initial structures subject to both response and geometric constraints. Radial stress, axial stress, hoop stress and von Mises stress are analyzed throughout the optimization and a constraint is imposed on von Mises stress everywhere in the disk. The optimal structures are developed metamorphically in specified infinite design domains using both quadrilateral and triangular axisymmetric finite elements. Comparisons are made of the results obtained for different optimization scenarios: (a) with and without thermal loading; (b) with and without centrifugal body forces; (c) with and without a fit pressure on the inner surface of the hub; and (d) operating at different rotational speeds.
Axial and Secondary Flow Study in a 90 Deg Bifurcation Under Pulsating Conditions Using PIV
Authors: Nikolaidis N. M. , Mathioulakis D. S.
Published: July 2002

Both axial and secondary flow were experimentally studied in a 90-degree bifurcation of square tubes, under pulsating conditions and equal branch flow rates. The examined peak Reynolds number (Re) was in the range 330–830, and the Womersley parameter (α) between 5 and 15. In the extension of the mother tube during deceleration, axial flow separated from the wall opposite of the 90-degree branch. The formed shear layer rolled up, for higher values of Re and low α, generating discrete vortices. However, for the highest value of α, the shear layer was limited close to the wall, without rolling up, for all the examined Re. During acceleration, all recirculation zones were washed out and the flow became attached before flow peak. In the 90-degree branch during acceleration, a vortex was formed at the bifurcation apex and moved streamwise towards the center of the cross-section where it degenerated. The secondary flow of this branch had the following characteristics: during deceleration two Dean vortices were located close to the outer bend side of the cross-section, and their strength was progressively reduced. A little before flow peak any Dean vortices of the previous phase were destroyed and two new ones were born. Maximum secondary velocities were on the order of 20% higher than the bulk velocity of the branch.
Sliding of a Mass on an Inclined Driven Plane With Randomly Varying Coefficient of Friction
Authors: Vielsack P. , Spiess H.
Published: July 2002

Investigated are sliding motions of a rigid body on a harmonically driven inclined plane. Coulomb’s law with a random coefficient of friction is assumed. The mean sliding velocity in a steady state of deterministic motions is taken as a measure to compare deterministic with stochastic behavior. Not only do the random parameters influence the deviation in the results but strongly influence the typical features of the different motions themselves. [S0021-8936(00)01701-3]
Vortex Shedding as a Mechanism for Free Emboli Formation in Mechanical Heart Valves
Authors: Bluestein Danny , Rambod Edmond , Gharib Morteza
Published: July 2002

The high incidence of thromboembolic complications of mechanical heart valves (MHV) limits their success as permanent implants. The thrombogenicity of all MHV is primarily due to platelet activation by contact with foreign surfaces and by nonphysiological flow patterns. The latter include elevated flow stresses and regions of recirculation of blood that are induced by valve design characteristics. A numerical simulation of unsteady turbulent flow through a bileaflet MHV was conducted, using the Wilcox k–ω turbulence model for internal low-Reynolds-number flows, and compared to quantitative flow visualization performed in a pulse duplicator system using Digital Particle Image Velocimetry (DPIV). The wake of the valve leaflet during the deceleration phase revealed an intricate pattern of interacting shed vortices. Particle paths showed that platelets that were exposed to the highest flow stresses around the leaflets were entrapped within the shed vortices. Potentially activated, such platelets may tend to aggregate and form free emboli. Once formed, such free emboli would be convected downstream by the shed vortices, increasing the risk of systemic emboli. [S0148-0731(00)01202-4]
Use of Miniature Specimens for Creep-Crack-Growth Testing
Authors: Jaske Carl E. , Viswanathan R.
Published: July 2002

Measuring the condition of high-temperature components after many years of service is an important part of remaining life assessment. Tests of miniature specimens have been utilized for this purpose because they require that only small samples be removed from the components, which minimizes or eliminates the need for costly repairs. Miniature-specimen techniques have been developed and validated for creep-rupture testing but not for creep-crack-growth testing. The objective of this research was to develop and validate a procedure for measuring creep-crack-growth behavior using miniature specimens. Based on a past study of Type 316 stainless steel, the single-edge-notch-tension (SENT) specimen configuration was chosen for the current work. SENT specimens were designed and prepared from both base metal and weldments of 2-1/4Cr-1Mo and 1-1/4Cr-1/2Mo steels. The base metal was ASME SA 387, Grade P22 and Grade P11 plate, respectively. The 2-1/4Cr-1Mo steel weldment was a seam-weld hot reheat steam pipe that had been in service for 106,000 hours, while the 1-1/4Cr-1/2Mo steel weldment was from a pipe that had been weld repaired after 244,200 hours of service. SENT specimens with test sections of 10×5×1.5 mm, 15×7.5×1.5 mm, and 20×10×1.5 mm were evaluated. Tests were conducted under constant weight loading or constant load-line displacement loading. Specimens were heated to 538°C using a standard laboratory furnace. Crack length was measured using the DC electric potential drop method. The test results were analyzed to produce creep-crack-growth rate da/dt as a function of the C* integral and compared with data from tests of half-size (1/2T-CT) and standard full-size (1T-CT) compact-tension specimens. For both base metals, the 1/2T-CT and three sizes of SENT specimens gave results that agreed well with those of 1T-CT specimens. For the 2-1/4Cr-1Mo steel weldments, SENT specimens produced good results. However, SENT specimens of the 1-1/4Cr-1/2Mo steel weldments exhibited cracking blunting and no creep crack growth. The reasons for the differences in behavior are discussed and guidance for application of miniature specimens to creep-crack-growth testing is presented. [S0094-4289(00)01403-1]
Pioneering Turbojet Developments of Dr. Hans Von Ohain—From the HeS 1 to the HeS 011
Authors: Meher-Homji Cyrus B. , Prisell Erik
Published: July 2002

On March 13, 1998, Dr. Hans Joachim Pabst von Ohain, co-inventor of the turbojet, passed away at the age of 86. As a young doctoral student, von Ohain conceived of and built a demonstrator turbojet engine. He was hired by the Heinkel Aircraft Company in 1936 and under intense time pressure imposed by Ernst Heinkel, designed the world’s first flight turbojet engine. This paper traces the technical antecedents leading to historic jet-powered flight made on August 27, 1939 by a Heinkel He 178 aircraft powered by von Ohain’s HeS 3B turbojet. During his tenure at Heinkel and thereafter at the Heinkel-Hirth Company, he was responsible for a series of turbojet engines culminating in the advanced second generation HeS 011 with a thrust of 2860 lbs. This paper is a tribute to an outstanding scientist who made possible the turbojet revolution and who will forever be remembered as the inventor of the world’s first flight turbojet. [S0742-4795(00)02102-5]
Two-Step Approach of Stress Classification and Primary Structure Method
Authors: Lu Ming-Wan , Chen Yong , Li Jian-Guo
Published: July 2002

A key problem in engineering applications of “design by analysis” approach is how to decompose a total stress field obtained by the finite element analysis into different stress categories defined in the ASME Code III and VIII-2. In this paper, we suggest a two-step approach (TSA) of stress classification and a primary structure method (PSM) for identification of primary stress. Together with the equivalent linearization method (ELM), the stress classification problem is well solved. Some important concepts and ideas discussed by Lu and Li [Lu, M. W., and Li, J. G., 1986, ASME PVP-Vol. 109, pp. 33–37; Lu, M. W., and Li, J. G., 1996, ASME PVP-Vol. 340, pp. 357–363] are introduced. They are self-limiting stress, multi-possibility of stress decomposition, classification of constraints, and primary structures. For identification of peak stress, a modified statement of its characteristic and a “1/4 thickness criterion” are given. [S0094-9930(00)00201-8]
Three-Dimensional Stress Criteria—Summary of the PVRC Project
Authors: Hollinger Greg , Hechmer John
Published: July 2002

This paper summarizes work done by the Pressure Vessel Research Council (PVRC) on three-dimensional stress criteria, using primarily elastic two and three-dimensional analytical techniques, finite element analysis. The focus of the work was to recommend guidelines on evaluation of elastic stresses relative to the ASME Boiler and Pressure Vessel Code (Code) defined failure modes as they relate to stress limits. The guidelines are developed such that they may be used to update and expand the procedures for evaluating the required stress limits in Code Section VIII, Division 2 and Code Section III, Subsection NB. This paper summarizes the recommendation using ten guidelines. The project addresses eleven example problems, nine of which include finite element analyses. These example problems validate the recommendations. The detailed information is published in the Welding Research Council Bulletin 429, “3-D Stress Criteria Guidelines for Applications.” [S0094-9930(00)01601-2]
Characterization of the Surface of γ-Fe2O3 Powder by XPS
Authors: Mansour A. N. , Brizzolara Robert A.
Published: July 2002

We report x-ray photoemission spectra of the surface of iron (III) oxide (γ-Fe2O3) particles. The research grade high purity (99.0%) (γ-Fe2O3) sample was commercially obtained from Alfa/AESAR. The XPS spectra were measured with the Physical Electroncs Model 5400 x-ray photoelectron spectrometer using unmonochromatized Mg Kα x-rays at two pass energy settings corresponding to analyzer resolutions of 1.34 and 0.54 eV. We present the survey spectrum (binding energy range of 0–1100 eV) measured at an analyzer energy resolution of 1.34 eV. Multiplexes of the C 1s, O 1s and 2s, and Fe 2p and 3p photoemission lines, valence band region as well as the Fe LVV Auger line were measured at an analyzer energy resolution of 0.54 eV. The XPS data indicate that the surface of (γ-Fe2O3) powdered material consists mainly of its bulk chemistry with small quantities of FeOOH, oxidized carbon, and hydrocarbon as contaminants.
Abatement of C2F6 in rf and microwave plasma reactors
Authors: Vitale Steven A. , Sawin Herbert H.
Published: July 2002

Experimental results in a low temperature, low power density rf plasma reactor show that C2F6 can be decomposed by plasma reaction with oxygen, but CF4, an undesirable byproduct, is produced. Previously, it had been shown in a high temperature, high power density microwave reactor that C2F6 can be decomposed without CF4 formation. Calculations show that low temperature neutral species kinetics favor the formation of CF4 over COF2, while the opposite is true at high temperature. Further, a high degree of feedstock dissociation such as that observed in high density plasma reactors is predicted to lead to very little CF4 formation. Calculations show that adding hydrogen, water, or hydrocarbons to the C2F6+O2 mixture should reduce the power necessary for abatement, and will reduce CF4 formation. Hydrogen is predicted to getter atomic fluorine to form thermodynamically stable HF, and thus prevent perfluorocompound reformation.
Growth and characterization of Au clusters on alkanethiol self-assembled monolayers
Authors: Wang Bing , Xiao Xudong , Sheng Ping
Published: July 2002

We have prepared nanometer-sized gold clusters on alkanethiol [CH3(CH2)n−1SH, abbreviated as Cn] self-assembled monolayers (SAMs) on Au(111) substrates by thermal evaporation. Typically, the Au clusters are one atomic layer thick and plate-like, with a diameter of ∼2–10 nm. While most of the Au clusters are found at the SAM/Au(111) interface, a fraction of the clusters reside on top of the SAMs as evidenced by the existence of the Coulomb blockade and Coulomb staircase in the scanning tunneling spectroscopy. Using scanning tunneling microscopy under ambient conditions, we also investigated the stability of the gold clusters at the SAM/Au(111) interface. For a given nominal coverage of Au, the gold clusters on C7 are larger and diffuse faster than those on C10 and C18.
Characteristics of InAs quantum dots on GaAs/InP with different InAs coverage
Authors: Wang Xinqiang , Du Guotong , Jin Zhi , Li Mingtao , Yin Jingzhi , Li Zhengting , Liu Shiyong , Yang Shuren
Published: July 2002

In this article, a thin tensile GaAs interlayer was used to get narrower size distribution and regular arrangement of InAs quantum dots (QDs) on InP substrate by low-pressure metalorganic chemical vapor deposition. The comparison results of the photoluminescence spectrum and the atomic force microscopy image show better properties after using GaAs interlayer. Also investigated were the surface behaviors of InAs QDs with different InAs coverage on GaAs/InP in order to reveal the detailed information of InAs QDs.
Scanning tunneling microscopy study of organometallic molecules adsorbed on a GaAs(001)-2×4 surface
Authors: Kuramochi Hiromi , Cui Jie , Iwai Hidekazu , Ozeki Masashi
Published: July 2002

The interaction of tertiarybutylarsine (TBAs), triisobutylgallium (TiBGa) and trisdimethylaminoarsenic (TDMAAs) with the GaAs(001)-2×4 surface is studied by scanning tunneling microscope. No adsorbates were observed after TBAs exposure at room temperature. In the case of TiBGa and TDMAAs, adsorbates were observed on the As dimer rows. They were adsorbed randomly at low coverage and formed a 4×4 structure at a full monolayer. Weak interaction between molecules and the GaAs surface is expected because the 2×4 structure of the clean surface was kept under the adsorbates.
Bilayer, nanoimprint lithography
Authors: Faircloth Brian , Rohrs Henry , Tiberio Richard , Ruoff Rodney , Krchnavek Robert R.
Published: July 2002

Nanoimprint lithography has been shown to be a viable means of patterning polymer films in the sub-100 nm range. In this work, we demonstrate the use of a bilayer resist to facilitate the metal liftoff step in imprinter fabrication. The bilayer resist technology exhibits more uniform patterns and fewer missing features than similar metal nanoparticle arrays fabricated with single layer resist. The bilayer resist relies upon the differential solubility between poly(methyl methacrylate) and poly(methyl methacrylate methacrylic acid copolymer). Evidence is presented that shows the technique has a resolution of better than 10 nm.
Actuation and internal friction of torsional nanomechanical silicon resonators
Authors: Olkhovets A. , Evoy S. , Carr D. W. , Parpia J. M. , Craighead H. G.
Published: July 2002

We report on the actuation and mechanical properties of silicon resonators with nanometer-scale supporting rods operating in the 3–20 MHz range. The symmetrically designed paddles can be excited both in their flexural and torsional modes of motion. Fabrication imperfections as small as 10–20 nm provide enough asymmetry to allow such torsional excitation. We also report on internal friction studies in these systems. Thin Al overlayers contribute to the room temperature internal losses, as quality factor drops from 3300 to 380 for 160 Å thick film. A temperature dependence of internal friction has a broad peak in the T=160–190 K range, and attributed to the Debye relaxation and thermally activated friction mechanisms. Analysis shows that the peak shifts to higher temperatures with increasing resonator frequency.
Inhibition of Electromigration Damage in Thin Films
Authors: Rosenberg R.
Published: July 2002

Mechanisms proposed to describe electromigration damage in thin film stripes are based on the observations that mass transport in films is controlled mainly by grain boundary diffusion and that irregularities in the boundary network give rise to divergencies in the mass flux. Two such irregularities, for example, are the triple point configuration (three grain junction) and mixed grain size. Calculation of the vacancy supersaturation level at these sites gives a value of about unity, which can allow condensation into voids at heterogeneous nucleating surfaces, but not within pure boundaries. In lieu of void nucleation, vacancies diffuse to the surface where grain boundary grooving takes place, eventually resulting in a hole. Grooving can be restricted by inhibiting surface diffusion, as is shown in the case of silver stripes with a superimposed 100-Å chromium film. Inhibition of boundary electromigration is achieved by either producing a textured film or by adding solute. In the textured film, the boundaries are mostly of tilt type, providing ledges and dislocations perpendicular to the field, thus lowering sensitivity to electromigration forces. A model is presented for solute effects that assumes segregation to boundary ledges and restriction of the matrix atom diffusion flux. Migration of the solute, which is rapid, results in local depletion and higher solvent flux rates in the solute depleted regions. This leads to mass depletion of the solvent. Loss in solute is limited somewhat by the presence of second phase particles which, by dissolving, maintain an equilibrium boundary concentration. Addition of two solutes which form a complex at boundary diffusion sites should tend to restrict depletion of either solute.
An Analytical System for Secondary Ion Mass Spectrometry in Ultra High Vacuum
Authors: Huber W. K. , Selhofer H. , Benninghoven A.
Published: July 2002

A system for routine secondary ion mass spectrometry (sputter ion mass spectrometry) under uhv conditions has been developed. Positive as well as negative ions released from metals, semiconductors, and insulators can be measured. The system can be used for monolayer analysis (with low primary intensities) and for profile measurements in thin films. Electron induced desorption investigations are possible with the same arrangement. Means are provided to combine the secondary ion analysis with Auger spectrometry. A multitarget array allows up to 12 samples to be measured. The analytical system consists of a primary ion gun, a charge neutralizing electron source and a specially modified quadrupole mass spectrometer. Fast pulse counting technique is used for the ion detection. The uhv system is pumped by a turbomolecular pump, a liquid nitrogen cooled trap and a titanium sublimation pump. Pressures in the 10−11 Torr range can be obtained. In a number of typical examples the versatility of the instrument is demonstrated.
Improved M50 Aircraft Bearing Steel through Advanced Vacuum Melting Processes
Authors: Schlatter R. , Stroup J. P.
Published: July 2002

A brief review of the vacuum metallurgical processes used for the production of high performance aircraft bearing steels is presented. Primary vacuum induction melting of M50 bearing steel in the Therm-I-Vac furnace and subsequent vacuum arc remelting has brought about a two- to threefold improvement in rolling contact fatigue life which appears to be related to precise composition control and very low gas and inclusion contents. Extensive vacuum-carbon deoxidation by an active boil and resultant more stable vacuum arc remelting conditions have been found to be important factors in producing a superior quality consumable ingot with a uniform structure free of macro-segregations. Property improvements in regard to product consistency, structural uniformity, and microcleanliness as well as the evaluation of bearing materials by rolling contact fatigue testing are discussed. It is believed that the significantly better rolling contact fatigue lives achieved by an optimized TIV+VAR practice are basically due to marked reductions of (1) interstitial gas content (H, O, N), (2) nonmetallic (S) and metallic impurities, (3) total inclusion content.
The Use of Scanning Auger Microscopy in Molecular Beam Epitaxy of GaAs and GaP
Authors: Arthur J. R.
Published: July 2002

The morphology of epitaxial films of Groups III–V compounds grown by molecular beam epitaxy has been found to be strongly dependent on the presence of surface impurities, notably carbon. In order to obtain a two-dimensional analysis of substrate surface purity, scanning Auger micrographs were obtained using a modified hemispherical retarding grid analyzer. The surface images were obtained rapidly (<1 min/frame) with a sensitivity in some cases of better than 1% monolayer. The resolution was low (0.1 mm); however, surface areas as large as 5×5 mm could be scanned in a single image, a feature which greatly facilitated substrate evaluation. In some cases crystals showed pronounced spatial variations in surface purity. The technique has also been used to measure surface diffusion rates of Te, As, P, and Cs on GaAs with the result that in most cases the diffusion rates were quite low at temperature up to 600 °C.
Directed assembly of carbon nanotube electronic circuits by selective area chemical vapor deposition on prepatterned catalyst electrode structures
Authors: Wei Y. Y. , Fan X. , Eres Gyula
Published: July 2002

Electron-beam lithography was used to pattern Fe thin film electrode structures on silicon dioxide covered Si wafers. The Fe film also serves as a catalyst in subsequent chemical vapor deposition of carbon nanotubes. Chemical vapor deposition was performed in a stainless steel chamber using acetylene at a partial pressure of 100 mTorr and a substrate temperature of 660 °C. Transmission electron microscopy images reveal that the carbon nanotubes grown by this method are multiwalled. The carbon nanotubes selectively grow only on the catalyst film and eventually bridge the intentionally designed micron size electrode gap forming carbon nanotube electronic circuits. The resulting devices were characterized by electronic transport measurements in a temperature range from room temperature down to 2 K.
Nanofabrication techniques for lasers with two-dimensional photonic crystal mirrors
Authors: Moosburger J. , Happ Th. , Kamp M. , Forchel A.
Published: July 2002

The fabrication of photonic crystals (PC) involves a periodic modulation of the refractive index on nanometer scales thus requiring state-of-the-art patterning and dry etch technologies. In this article, we compare three different mask techniques for the fabrication of ridge waveguide lasers with two-dimensional triangular PC mirrors. After defining the hole pattern in 500 nm polymethylmetacrylate by 100 kV e-beam lithography the structure is transferred to the mask layer. The first mask layer is a metal mask consisting of Ti/Cr/Ti. The second one is formed by wet chemical oxidation of a buried 60 nm AlAs layer and the third one uses a single SiO2 layer in order to transfer the hole pattern into the semiconductor. The periods of the PC, with an average air filling factor of 45%, were chosen in the range of 160–400 nm in order to probe the region where high reflectivity of the PC is expected. The observed laser performance is used to deduce information about the quality of the fabricated PCs. The advantages of each technique are discussed, leading to conclusions for future techniques for the fabrication process of photonic crystals.
Analytic study of gratings patterned by evanescent near field optical lithography
Authors: McNab S. J. , Blaikie R. J. , Alkaisi M. M.
Published: July 2002

Simulation results which investigate the near field of conducting gratings are presented to show some of the major issues affecting evanescent near field optical lithography (ENFOL), namely ultimate resolution, depth of field, exposure variations due to edge enhancements, and resonant diffraction. Ultimate resolution down to 10 nm for grating structures is predicted, independent of illumination wavelength. The depth of field reduces approximately linearly as the feature size reduces in the evanescent regime. Variations in mask profile were investigated by modeling the radii of curvature of mask conductors from 1–10 nm. Strict mask profile control is shown to be important to avoid exposure variations due to the increasing zeroth transmitted order with increasing radii. A diffraction resonance occurs when the grating pitch matches the wavelength for a transverse magnetic excited grating. The cut off of the ±1 diffracted orders coincides with a plasmon resonance and a strong, frequency doubled interference pattern is produced. To avoid such resonant conditions, standard ENFOL requires a low coherence source and/or strongly absorbing resists. However, this near field interference offers the possibility of frequency-doubled interferometric replication of quasiperiodic structures, with strong intensity enhancement at the expense of reduced depth of field. Overall, the key to successful evanescent lithography is restricting the lithography to a depth in which high contrast is available with good process latitude due to the presence of sufficient numbers of diffracted orders of sufficient strength.
Characterization of the surface morphology and electronic properties of microwave enhanced chemical vapor deposited diamond films
Authors: Fitzgerald A. G. , Fan Y. , John P. , Troupe C. E. , Wilson J. I. R.
Published: July 2002

The surface morphology, electronic structure and atomic bonding configurations of chemical vapor deposition (CVD) diamond films prepared at different stages of the deposition process and subjected to different postdeposition surface treatments have been studied by scanning probe microscopy (SPM), scanning tunneling spectroscopy (STS), and x-ray photoelectron spectroscopy (XPS) surface analysis techniques. SPM image observations show that (a) in the biasing nucleation process, diamond crystallites grow in a three-dimensional manner and the nucleation density reaches 109–1010/cm2; (b) both as-deposited and boron ion implanted films exhibit a hillock morphology on (100) crystal faces; (c) atomic flatness can be achieved on crystal faces by hydrogen plasma etching. STS analysis indicates that (i) the films obtained after an initial biasing nucleation process show a metallic tunneling behavior; (ii) both as-deposited and hydrogen plasma etched CVD diamond films possess typical p-type semiconductor surface electronic properties; (iii) when the as-deposited diamond films are subjected to boron implantation or argon ion etching, the surface electronic properties change from p-type semiconducting behavior to metallic behavior. XPS analysis confirmed that the surfaces for both as-deposited and hydrogen plasma etched diamond films have a tetrahedral atomic bonding configuration. However, the surfaces of boron ion implanted and argon ion etched diamond films exhibited an amorphous carbon-like feature which can be attributed to the surface damage caused by ion bombardment.
Effect of photoenhanced minority carriers in metal-oxide-semiconductor capacitor studied by scanning capacitance microscopy
Authors: Shin S. , Kye J.-I. , Pi U. H. , Khim Z. G. , Hong J. W. , Park Sang-il , Yoon S.
Published: July 2002

A scanning capacitance microscope was used to study the photoenhanced minority-carrier contribution to the capacitance of the metal-oxide-semiconductor (MOS) capacitor at high frequencies. When a light is induced over the semiconductor surface, electron-hole pairs are generated and recombined. This steady-state generation-recombination process yields the temporary source of minority carriers, and the inversion layer underneath the oxide layer can respond to very fast-varying ac bias. We measured the differential capacitance (dC/dV) of the MOS capacitor under various light intensities, and observed a peak at the inversion region where the amplitude increased as the irradiation intensity increased. By integrating dC/dV with respect to V, we obtained C–V curves in which the capacitance of the depletion region recovered its value up to that of the accumulation region as the light intensity increased. We also observed that the C–V curves shifted in one direction under irradiation which we believe is due to the surface photovoltaic effect.