bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatial memory formation induces recruitment of NMDA receptor and PSD‐95 to synaptic lipid rafts
Authors: Delint‐Ramírez Ilse , Salcedo‐Tello Pamela , Bermudez‐Rattoni Federico
Published: June 2008

NMDA receptors (NMDARs) activation in the hippocampus and insular cortex is necessary for spatial memory formation. Recent studies suggest that localization of NMDARs to lipid rafts enhance their signalization, since the kinases that phosphorylate its subunits are present in larger proportion in lipid raft membrane microdomains. We sought to determine the possibility that NMDAR translocation to synaptic lipid rafts occurs during plasticity processes such as memory formation. Our results show that water maze training induces a rapid recruitment of NMDAR subunits (NR1, NR2A, NR2B) and PSD‐95 to synaptic lipid rafts and decrease in the post‐synaptic density plus an increase of NR2B phosphorylation at tyrosine 1472 in the rat insular cortex. In the hippocampus, spatial training induces selective translocation of NR1 and NR2A subunits to lipid rafts. These results suggest that NMDARs translocate from the soluble fraction of post‐synaptic membrane (non‐raft PSD) to synaptic lipid raft during spatial memory formation. The recruitment of NMDA receptors and other proteins to lipid rafts could be an important mechanism for increasing the efficiency of synaptic transmission during synaptic plasticity process.
Antipsychotic drugs increase N‐acetylaspartate and N‐acetylaspartylglutamate in SH‐SY5Y human neuroblastoma cells
Authors: Arun Peethambaran , Madhavarao Chikkathur N. , Moffett John R. , Namboodiri Aryan M. A.
Published: June 2008

N‐Acetylaspartate (NAA) and N‐acetylaspartylglutamate (NAAG) are related neuronal metabolites associated with the diagnosis and treatment of schizophrenia. NAA is a valuable marker of neuronal viability in magnetic resonance spectroscopy, a technique which has consistently shown NAA levels to be modestly decreased in the brains of schizophrenia patients. However, there are conflicting reports on the changes in brain NAA levels after treatment with antipsychotic drugs, which exert their therapeutic effects in part by blocking dopamine D2 receptors. NAAG is reported to be an agonist of the metabotropic glutamate 2/3 receptor, which is linked to neurotransmitter release modulation, including glutamate release. Alterations in NAAG metabolism have been implicated in the development of schizophrenia possibly via dysregulation of glutamate neurotransmission. In the present study we have used high performance liquid chromatography to determine the effects of the antipsychotic drugs haloperidol and clozapine on NAA and NAAG levels in SH‐SY5Y human neuroblastoma cells, a model system used to test the responses of dopaminergic neurons in vitro. The results indicate that the antipsychotic drugs haloperidol and clozapine increase both NAA and NAAG levels in SH‐SY5Y cells in a dose and time dependant manner, providing evidence that NAA and NAAG metabolism in neurons is responsive to antipsychotic drug treatment.
The Effect of Etching Technique on the Retention of Adhesively Cemented Prefabricated Dowels
Authors: Potesta Fiorella L. , Broome James C. , O'Neal Sandra J. , Givan Daniel A. , Ramp Lance C.
Published: June 2008

Purpose: To determine if etching technique influences the bond strength of resin cement to root canal dentin.

Materials and Methods: Fifty‐five extracted teeth were endodontically treated, dowel space prepared, and divided into five groups. Each group was treated with different etchant consistencies: acid gel, semi‐gel, low‐viscosity gel, liquid, and a self‐etching primer. After dowel cementation, four sections were removed from each root and a push‐out test was performed.

Results: Significant effects were found for etching procedure and for location within the root canal. The apical segment produced the lowest bond strength. Self‐etching primer showed the highest bond strength.

Conclusions: The consistency of etchant material influenced the bond strength of a prefabricated dowel in the canal.
Fabricating a Hollow Obturator with Visible Light‐Cured Resin System
Authors: Kocacikli Mustafa , Yalug Suat , Yazicioglu Huseyin , Yilmaz Caner
Published: June 2008

Obturators are routinely used in the rehabilitation of maxillectomy defects. Ideally, obturators should be easily constructed to produce a comfortable and stable prosthesis. This article describes a technique that provides an easy method for the construction of an obturator prosthesis using a visible light‐cured (VLC) resin. As a result, technicians can easily construct obturators in little time; this would, in turn, provide patients with light, comfortable, and tolerable prostheses.
To Stop or Not to Stop the AED?
Authors: Meador Kimford J.
Published: June 2008

Consequences of Antiepileptic Drug Withdrawal: A Randomized, Double-Blind Study (Akershus Study). Lossius MI, Hessen E, Mowinckel P, Stavem K, Erikssen J, Gulbrandsen P, Gjerstad L. Epilepsia 2008;49:455–463. OBJECTIVE: Despite side effects associated with the use of antiepileptic drugs (AEDs), withdrawal of AEDs remains controversial, even after prolonged seizure freedom. The main objective of this study was to assess the effects of AED withdrawal on cognitive functions, seizure relapse, health-related quality of life (HRQOL), and EEG results. Additionally, potential predictors for freedom from seizures after AED withdrawal were studied. METHODS: Patients, seizure-free for more than 2 years on AED monotherapy, were recruited for a controlled, prospective, randomized, double-blinded withdrawal study lasting for 12 months, or until seizure relapse. Patients were randomized to AED withdrawal (n = 79) and nonwithdrawal (n = 81) groups. The examination program included clinical neurological examinations, neuropsychological testing, EEG-recordings, cerebral MRI, and assessments of HRQOL. Follow-up data on seizure relapse were also collected beyond the 12-month study period (median 47 months). RESULTS: Seizure relapse at 12 months occurred in 15% of the withdrawal group and 7% of the nonwithdrawal group (RR 2.46; 95% CI: 0.85–7.08; p = 0.095). After withdrawal, seizure relapse rates were 27% after a median of 41 months off medication. A normal result to all 15 neuropsychological tests increased significantly from 11% to 28% postwithdrawal. We found no significant effects of withdrawal on quality of life and EEG. Predictors for remaining seizure-free after AED-withdrawal over 1 year were normal neurological examination and use of carbamazepine prior to withdrawal. CONCLUSION: Seizure-free epilepsy patients on AED monotherapy who taper their medication may improve neuropsychological performance with a relative risk of seizure relapse of 2.46, compared to those continuing therapy.
Activation of autologous leukemia‐specific T cells in acute myeloid leukemia: monocyte‐derived dendritic cells cocultured with leukemic blasts compared with leukemia‐derived dendritic cells
Authors: Draube Andreas , Beyer Marc , Wolf Jürgen
Published: June 2008

In acute myeloid leukemia (AML) blasts can be differentiated into dendritic cell (DC) like cells (AML‐DC). These cells have a mature DC‐like phenotype, are strong stimulators in mixed leukocyte reactions and can be used to generate leukemia‐specific cytotoxic T cells. However, recent reports about naturally existing leukemic DC with immunoregulatory dysfunctions in peripheral blood of AML patients caused concerns about the use of AML‐DC for therapeutic purposes.

Systematic intra‐individual comparisons between AML‐DC and non‐leukemic DC derived from monocytes (MoDC) in AML patients are missing. Thus, we investigated the ability to generate MoDC from peripheral blood of 17 AML patients in first remission and their functional integrity to stimulate leukemia‐specific T cells by simple coculture with leukemic blasts. Phenotypic analysis of AML‐DC and MoDC from the same individual patients revealed that MoDC exhibit a more homogenous mature DC phenotype. Additionally, functional analysis demonstrated the ability of remission MoDC to activate autologous leukemia‐specific T cells in 11 of 12 patients, whereas AML‐DC led to a specific T cell activation in four of eight patients. The presented findings might have impact on the design of further therapeutic studies using autologous antigen‐presenting cells.
Differential expression of microRNAs in hematopoietic cell lineages
Authors: Merkerova Michaela , Belickova Monika , Bruchova Hana
Published: June 2008

Objectives:  MicroRNAs (miRNAs) play key roles in a wide variety of normal and pathological cellular processes. A number of studies identified hematopoietic‐specific miRNAs that are necessary for correct function of blood cells. Out of our microarray data, we chose 13 miRNAs that showed differential expression in peripheral blood cells (miR‐15b, miR‐16, miR‐24, miR‐30c, miR‐106b, miR‐142‐3p, miR‐142‐5p, miR‐150, miR‐155, miR‐181, miR‐223, miR‐342, and miR‐451) and examined their expression in separated hematopoietic cell lineages.

Methods:  Using quantitative real‐time polymerase chain reaction, we measured relative expression of the miRNAs in fractions of reticulocytes, platelets, granulocytes, monocytes, B‐ and T‐lymphocytes as well as in several hematopoietic cell lines.

Results:  We observed that miR‐16 and miR‐142‐3p were highly expressed in all native cell lineages, miR‐451 reached the maximal expression in reticulocytes, miR‐223 in platelets, granulocytes and monocytes, and miR‐150 in B‐ and T‐lymphocytes. Hierarchical clustering analysis grouped the lineage samples according to their origin based on the expression of these miRNAs. To validate discrimination power of the miRNAs, we quantified expression of the 13 miRNAs in several immortalized cell lines. Although the cell lines showed miRNA expression patterns considerably different from those of native cell lineages, clustering analysis distinguished between myeloid, lymphoid and non‐hematopoietic cells.

Conclusions:  In conclusion, the study reports the expression levels of 13 miRNAs in particular blood cell lineages as well as immortalized cell lines. We demonstrate that the expression profiles of these miRNAs may be used for discrimination of the hematopoietic cell lineages.
Control of lipid oxidation during exercise: role of energy state and mitochondrial factors
Authors: Sahlin K. , Harris R. C.
Published: June 2008

Despite considerable progress during recent years our understanding of how lipid oxidation (LOx) is controlled during exercise remains incomplete. This review focuses on the role of mitochondria and energy state in the control of LOx. LOx increases in parallel with increased energy demand up to an exercise intensity of about 50–60% of VO2max after which the contribution of lipid decreases. The switch from lipid to carbohydrate (CHO) is of energetic advantage due to the increased ATP/O2 yield. In the low‐intensity domain (<50%VO2max) a moderate reduction in energy state will stimulate both LOx and CHO oxidation and relative fuel utilization is mainly controlled by substrate availability and the capacity of the metabolic pathways. In the high‐intensity domain (>60%VO2max) there is a pronounced decrease in energy state, which will stimulate glycolysis in excess of the substrate requirements of the oxidative processes. This will lead to acidosis, reduced levels of free Coenzyme A (CoASH) and reduced levels of free carnitine. Acidosis and reduced carnitine may limit the carnitine‐mediated transfer of long‐chain fatty acids (LCFA) into mitochondria and may thus explain the observed reduction in LOx during high‐intensity exercise. Another potential mechanism, suggested in this review, is that Acyl‐CoA synthetase (ACS), an initial step in LCFA catabolism, functions as a regulator of LOx. ACS activity is suggested to be under control of CoASH and energy state. Furthermore, evidence exists that additional control points exist beyond mitochondrial FA influx. The nature and site of this control remain to be investigated.
An objective, rapid and reproducible method for scoring AFLP peak‐height data that minimizes genotyping error
Authors: WHITLOCK R. , HIPPERSON H. , MANNARELLI M. , BUTLIN R. K. , BURKE T.
Published: June 2008

Amplified fragment length polymorphism (AFLP) fingerprint data are now commonly collected using DNA sequencers. AFLP genotypes are still often scored by eye from such data — a time‐consuming, error‐prone and subjective process. We present a semi‐automated method of genotyping sequencer‐collected AFLPs at predefined fragment locations (loci) within the fingerprint. Our method uses thresholds of AFLP‐polymerase chain reaction‐product fluorescence intensity (peak height) in order to: (i) exclude AFLP loci that are likely to contribute high rates of error to data sets, and (ii) determine the AFLP phenotype (fragment presence or absence) at the retained loci. Error rate analysis is an integral part of this process and is used to determine optimal thresholds that minimize genotyping error, while maximizing the numbers of retained loci. We show that application of this method to a large AFLP data set allows genotype calls that are rapid, objective and repeatable, facilitating the extraction of reliable genotype data for molecular ecological studies.
Development of polymorphic microsatellite markers for the livebearing fish Poecilia parae
Authors: NATER ALEXANDER , KRÜTZEN MICHAEL , LINDHOLM ANNA K.
Published: June 2008

We developed 16 novel polymorphic tetranucleotide microsatellite markers for Poecilia parae, a livebearing fish used in evolutionary studies because of its Y‐linked colour and size polymorphism. A set of 199 clones was sequenced out of an enriched genomic library, and we achieved an enrichment efficiency of nearly 80%. Primers were designed for 16 pure repeats, and 59 P. parae were screened for polymorphism. Cross‐amplification was tested on Poecilia picta and Poecilia reticulata, the guppy. The new microsatellite markers showed an exceptionally high allelic diversity and low stutter formation, proving their suitability for a broad range of applications in these species.
Seventy new microsatellites for the pied flycatcher, Ficedula hypoleuca and amplification in other passerine birds
Authors: LEDER E. H. , KARAISKOU N. , PRIMMER C. R.
Published: June 2008

The pied flycatcher (Ficedula hypoleuca) is a small migratory passerine bird commonly distributed across Europe which has been the focus of considerable ecological and evolutionary research. Here, we present details of 70 microsatellite markers for the species adding to the six which are currently available. Sixty‐six markers were also polymorphic in the closely related collared flycatcher (Ficedula albicollis), while 54 were polymorphic in another related passerine, the bluethroat (Luscinia svecica), and 12 were polymorphic in the more distantly related Siberian jay (Perisoreus infaustus).
Hydrodynamic Coefficients of Vertical Cylinders of Arbitrary Section
Authors: Isaacson M. , Mathai T.
Published: June 2008

The calculation of added masses and damping coefficients of a large surface-piercing vertical cylinder of arbitrary section extending to the seabed and undergoing harmonic oscillations is described. The linear radiation problem in three dimensions is reduced to a series of two-dimensional problems in the horizontal plane by the use of appropriate eigenfunctions that represent the variation of the velocity potential in the vertical direction. Each of these is solved by a numerical approach based on the method of integral equations. Comparisons are made with an analytic solution available for the case of a circular cylinder. Results are also provided for square cylinders, and the application to typical offshore structures subject to base motions is discussed.
Estimation of Drag and Inertia Coefficients From Random Wave Data
Authors: Isaacson M. , Baldwin J. , Niwinski C.
Published: June 2008

A summary is given of available methods of estimating the drag and inertia coefficients used in the Morison equation from random wave and wave force data. The methods provide coefficients that are either constant or variable for a given wave record. Numerical random wave and wave force records have been generated and are used to provide a comparison of five different methods.
Stabilization of Moored Vessels Using a Second-Order Volterra Filter and Feedforward Compensator
Authors: Cho Y. S. , Kim S. B. , Powers E. J. , Miksad R. W. , Fischer F. J.
Published: June 2008

Modeling and forecasting of the sway of a moored vessel subjected to random waves were studied recently using quadratic digital filtering techniques. Moreover, it was observed that the future response of the vessel can be predicted with relatively high accuracy. This paper describes a control scheme, which utilizes a quadratic digital filter, to stabilize the low-frequency drift oscillation (LFDO) of moored vessels by a process, whose transfer function may be unknown or time-varying, and an appropriately chosen feedforward compensator. The feedforward compensator generates a control signal that counteracts the LFDO. The predictive filter coefficients are determined from experimental time series data of random sea excitation and the associated sway response of a scaled (1:48) moored barge. The parameters of the process are estimated using a recursive least squares (RLS) method. Simulation results show that the sway of the controlled vessel is reduced considerably, compared with that of a uncontrolled vessel.
Failure Analysis of Impacting Ice Floes
Authors: Bhat S. U. , Choi S. K. , Wierzbicki T. , Karr D. G.
Published: June 2008

Failure analysis of an ice floe impacting on a fixed arctic offshore structure has been carried out. The ice floe has been idealized to be an edge-loaded, inertia-driven, thin circular disk made of an elastic-brittle material obeying the Drucker-Prager failure envelope. The location of failure initiation and the associated load have been calculated. Fracture mechanics of global splitting of the floe by crack propagation has been presented. An appropriate limit analysis solution of the problem has also been developed. A comparison of the various results developed in the paper leads to some interesting conclusions.
The Effect of Voltage Polarity on Ice Accretions on Short String Insulators
Authors: Farzaneh M. , LaForte J.-L.
Published: June 2008

The present study investigated the effect of voltage polarity on ice deposits accumulated on a short-string of porcelain insulators from 85 μm supercooled droplets in a cold room maintained at a temperature of −12°C. The air velocity and the liquid water content were 3.3 m/s and 2.0 g/m3, respectively. Under these growth conditions, the type of ice accretion was hard rime with a density of 0.87 g/cm3 and the highest probability of flashover occurrence. With an applied voltage of 15 kV per insulator unit, corresponding to the operational voltage in service, there were no major changes in weight, density or the texture of the ice grown on the surface of the insulators.
An Experimental Investigation of Forces Induced on Cylinders by Random Oscillatory Flow
Authors: Longoria R. G. , Beaman J. J. , Miksad R. W.
Published: June 2008

Results are presented from experiments investigating hydrodynamic forces induced on fixed circular cylinders by random oscillatory flows. The oscillatory flow was generated in a water tunnel and the inline and transverse forces induced on the cylinders were measured simultaneously. Analysis of the measured forces shows that significant differences exist when comparing drag and inertia coefficients to those measured under sinusoidal flow conditions, particularly in the inertia/ drag regime of the Keulegan-Carpenter number. The comparison of results for random versus sinusoidal flow conditions generated in the same experimental apparatus quantifies the difference in the physical mechanisms, specifically the vortex shedding and pairing, which influence the induced forces. A study is also made of the dependence of measured force power spectral densities on the statistical Keulegan-Carpenter number, Reynold’s number and on the spectral width of the flow velocity auto-power spectrum.
Finite Depth and Tank Wall Effects Upon First and Second-Order Forces
Authors: Hearn G. E. , Liou S. Y.
Published: June 2008

This paper presents a hybrid method of solution of the radiation and diffraction fluid-structure interaction problems based upon Rankine source distributions and eigenfunction expansion matching techniques. Using direct pressure integration of the first-order solutions, the second-order drift forces are calculated in “open” water and “confined” water situations with and without forward speed effects included. The method has been developed to provide an alternative way of calculating low-frequency damping coefficients.
Buckling of Short, Thin-Walled Cylinders Under Combined Loading
Authors: Goltermann P.
Published: June 2008

Short cylindrical shells are often used in offshore structures. Such cylinders are loaded by axial compression as well as hydrostatical pressure. The load-carrying capacity is for practical purposes determined for each of the two load cases separately. The determination of the load-carrying capacity for a combined loading is then based on a combination of those two load-carrying capacities. This combination differs from code to code and has a significant influence on the load-carrying capacity. This paper presents a rational way of estimating the capacity by using simple, well-known theories. The elastic, critical stress (fe) of a perfect cylinder is estimated according to the classic shell theory for the two load cases, and the respective knock-down factors (α) are calculated according to a code or according to Koiter’s classic stability theory. This leads to an estimate of the ratio between actual stress and the elastic, critical stress (fe·α) of the imperfect cylinder in the two load cases. The membrane stresses and the bending stresses due to the oval imperfection of the cylinder are calculated according to the plate theory, in which the stiffness is reduced corresponding to those ratios. The capacity is defined as the load level at which a point yields according to von Mises’ yield condition. The method is easily applicable for practical purposes and has the advantage that it estimates the capacity at the actual geometry, yield stress, imperfection level and load combination, and thus enables a better estimation. The paper shows that the interaction curves depend severely on the geometry, the level of imperfection, and the size of the yield stress.
3-D Inelastic Behavior of Tubular Members Supported on Nonlinear Rotational Springs
Authors: Abbassian F. , Lau T. B. , Zintilis G. M.
Published: June 2008

This paper considers a theoretical analysis of the inelastic response of tubular brace members of offshore jackets which are supported on nonlinear rotational end springs and subjected to axial load and generally applied end bending moments. The present analysis is based on the concept of 3-D plastic hinge formation. The method assumes that plasticity is confined to discrete plastic hinges while the rest of the member remains elastic. This allows a rapid nonlinear analysis of the axial behavior of the member by a sequence of elastic solutions of the classical beam-column equations. It is shown that the model is able to simulate closely and efficiently the 3-D inelastic behavior of brace members under the foregoing support conditions.
Viscosity Effects on the Radiation Hydrodynamics of Horizontal Cylinders
Authors: Yeung R. W. , Wu C.-F.
Published: June 2008

The problem of a body oscillating in a viscous fluid with a free surface is examined. The Navier-Stokes equations and boundary conditions are linearized using the assumption of small body-motion to wavelength ratio. Generation and diffusion of vorticity, but not its convection, are accounted for. Rotational and irrotational Green functions for a divergent and a vorticity source are presented, with the effects of viscosity represented by a frequency Reynolds number Rσ = g2/νσ3. Numerical solutions for a pair of coupled integral equations are obtained for flows about a submerged cylinder, circular or square. Viscosity-modified added-mass and damping coefficients are developed as functions of frequency. It is found that as Rσ approaches infinity, inviscid-fluid results can be recovered. However, viscous effects are important in the low-frequency range, particularly when Rσ is smaller than O(104).
Dispersion Characterization of Nanoclay in Molded Epoxy Disks by Combined Image Analysis and Wavelength Dispersive Spectrometry
Authors: Aktas Levent , Hamidi Youssef K. , Altan M. Cengiz
Published: June 2008

The state of nanoclay dispersion in a molded epoxy disk and its effects on the thermomechanical properties of the resulting nanocomposite are analyzed. A commercially available nanoclay, Cloisite® 25A, is mechanically mixed at 2wt% with EPON 815C epoxy resin. The epoxy∕clay compound is then mixed with EPI-CURE 3282 curing agent by a custom made molding setup and injected into a disk shaped mold cavity. Upon completion of curing, nanoclay dispersion is quantified on a sample cut along the radius of the composite disk. Dispersion of nanoclay clusters larger than 1.5μm are analyzed by digital image processing of scanning electron micrographs taken radially along the sample, whereas dispersion at smaller scales is quantified by compositional analysis of clay via wavelength dispersive spectrometry (WDS). Digital images of the microstructure indicate that amount of nanoclay clusters that are larger than 1.5μm remain approximately constant along the radius. However, size analysis of nanoclay clusters revealed that they are broken down into finer clusters along the radius, possibly due to the high shear deformation induced through the thickness during mold filling. Compositional analysis by WDS signified that approximately 0.4wt% of the nanoclay is dispersed to particles smaller than 1.5μm, which are not visible in micrographs. Tensile and three-point bending tests are conducted on additional samples cut from the molded disks. Except for slight reduction in flexural strength, up to 9.5% increase in tensile strength, stiffness, and flexural modulus are observed. Glass transition temperature is determined under oscillatory torsion and observed to increase by 4.5% by the addition of nanoclay.
Flow and Mold Filling Modeling and Simulation to Enhance Resin Transfer Molding Processes
Authors: Mosella C. J. , Montecinos J. P. , Ramos-Grez J. A.
Published: June 2008

Among the multiple stages of the resin transfer molding (RTM) processes, flow and mold filling of injected resin correspond to the most complex and crucial stage. During the latter, air bubble agglomeration must be avoided and complete wetting of fibers must be achieved in order to ensure the maximum quality of the parts at the lowest possible manufacturing time. Focusing on these manufacturing issues, a mathematical model and a numerical resolution are presented to predict the resin flow throughout the fiber reinforcement inside the mold cavity. The methodology employs conventional finite element techniques for solving the flow problem through a porous medium governed by Darcy’s law and mass conservation. Simultaneously, a state of the art numerical scheme known as the discontinuous Galerkin method is implemented to determine the location and shape of the advancing flow fronts ruled by a hyperbolic transport equation. These two schemes are implemented to work with a two-dimensional domain, handling diverse geometries with multiple injection and ventilation ports. The results for key process parameters, such as filling time and position of the advancing flow fronts, show a good agreement with results from analytical solutions for particular cases and from empirical data. When several simulated results are taken into account in the design process of RTM cavities, the overall process could be enhanced.
Properties of Friction-Stir Welded Aluminum Alloys 6111 and 5083
Authors: Gan W. , Okamoto K. , Hirano S. , Chung K. , Kim C. , Wagoner R. H.
Published: June 2008

Friction-stir welding (FSW) promises joints with low porosity, fine microstructures, minimum phase transformation, and low oxidation compared with conventional welding techniques. It is capable of joining combinations of alloys not amenable to conventional welding. Certain combinations of FSW parameters were used to create FSWs of aluminum alloys 5083-H18 and 6111-T4, and the physical weld defects were measured. The mechanical behavior of FSW welds made under the most favorable choice of parameters was determined using tensile tests and hardness measurements and was correlated to the microstructures of the weld and base material. Stir zones (SZs) in the 5083 specimens were much softer than the strain-hardened base materials. SZs in the 6111 material are approximately as hard as the base material. Natural aging of 6111 FSW specimens occurred in some parts of the heat-affected zone and produced hardening for up to 12weeks after welding. Annealing of 5083 FSW specimens produced abnormal grain growth (AGG) for welds produced under certain welding conditions and in certain parts of the weld zone. AGG is more severe for low-heat conditions, i.e., higher tool travel speed but lower rotational speed. The conditions for most favorable FSW are presented, as well as the expected microstructures and mechanical properties, along with the weld conditions that promote AGG.
Detection and Monitoring of Corrosion in Structural Carbon Steels Using Electromagnetic Sensors
Authors: Rumiche F. , Indacochea J. E. , Wang M. L.
Published: June 2008

An electromagnetic sensor was assessed as a possible instrument for nondestructive detection and monitoring of corrosion in structural carbon steels. In this study, the magnetic response of three structural carbon steel rods (AISI 1018, AISI 1045, and AISI 1045-High Mn), was evaluated in the as-received (uncorroded) and corroded conditions. Initially, the material was systematically machined out from each steel rod, followed by the magnetic evaluation of each specimen. Other set of metal rods were exposed to uniform corrosion and later examined by the electromagnetic sensor. Correlations have been established between the degree of mass loss and magnetic response of the test specimen. Based on the results, it can be said that the electromagnetic sensor has the potential to be used as a reliable nondestructive tool to detect corrosion at early stages based on the variation in magnetic properties. A metallurgical analysis of all test rods was also undertaken, which showed that microstructures have an important effect of the magnetic properties of the steels.
Constitutive Modeling and Finite Element Analysis of the Formability
of TRIP Steels
Authors: Serri J. , Cherkaoui M.
Published: June 2008

The semiphysics constitutive model developed by Cherkaoui et al. (2006, “From Micro to Macroscopic Description of Martensitic Transformation in Steels: A Viscoplastic Model,” Philos. Mag., in press;) has been implemented in the user’s material subroutine of the finite element code ABAQUS∕EXPLICIT (2006, Version 6.6 Manuals, Dassault Systemes) to predict the thermomechanical behavior of unstable transformation induced plasticity (TRIP) steel sheets under conditions of forming, which are essentially composite materials with evolving volume fractions of the individual phases. These steels undergoing α′ martensitic phase transformation exhibit an additional inelastic strain resulting from the phase transformation itself and from the plastic accommodation in parent (austenite) and product (martensite) phases due to different sources of internal stresses. This inelastic strain known as the TRIP strain enhances ductility at an appropriate strength level due to the typical properties of the martensite. A numerical analysis of the effects of the martensitic phase transformation on formability is performed. A validation of the stress-strain behavior and the volume fraction of the martensite are carried out in the case of multiaxial paths at various temperatures. The effects of the stress state and of the kinetics of the martensite phase transformation are analyzed in the case of the cup drawing test. Finally, the numerical predictions are compared to experimental tests on type AISI304 austenitic stainless steels and TRIP800 multiphase industrial steels.
Reliability-Based Design With the Mixture of Random and Interval Variables
Authors: Du Xiaoping , Sudjianto Agus
Published: June 2008

In Reliability-Based Design (RBD), uncertainties usually imply for randomness. Nondeterministic variables are assumed to follow certain probability distributions. However, in real engineering applications, some of distributions may not be precisely known or uncertainties associated with some uncertain variables are not from randomness. These nondeterministic variables are only known within intervals. In this paper, a method of RBD with the mixture of random variables with distributions and uncertain variables with intervals is proposed. The reliability is considered under the condition of the worst combination of interval variables. In comparison with traditional RBD, the computational demand of RBD with the mixture of random and interval variables increases dramatically. To alleviate the computational burden, a sequential single-loop procedure is developed to replace the computationally expensive double-loop procedure when the worst case scenario is applied directly. With the proposed method, the RBD is conducted within a series of cycles of deterministic optimization and reliability analysis. The optimization model in each cycle is built based on the Most Probable Point (MPP) and the worst case combination obtained in the reliability analysis in previous cycle. Since the optimization is decoupled from the reliability analysis, the computational amount for MPP search is decreased to the minimum extent. The proposed method is demonstrated with a structural design example.
Molecular beam epitaxy III-nitride growth for polarization sensitive devices based on M-plane films with in situ real time analysis by spectroscopic ellipsometry
Authors: Boney C. , Misra P. , Pillai R. , Starikov D. , Bensaoula A.
Published: June 2008

Nonpolar oriented III nitrides have recently attracted considerable interest, owing to the absence of spontaneous and piezoelectric polarization and resulting electrostatic fields in heterostructures, which can be detrimental to the optoelectronic device efficiency. In this article, the authors present a study on the growth of M-plane GaN epilayers on LiAlO2 substrates under Ga rich conditions and Ga stable conditions by plasma-assisted molecular beam epitaxy coupled with in situ spectroscopic ellipsometry. Previous studies on M-plane materials have suggested that optimum growth conditions with respect to surface morphology are those giving rise to a Ga trilayer coverage at low growth temperatures. By studying Ga adsorption/desorption in vacuum and in the presence of active nitrogen, we find that the total coverage for M-plane material is distinctly different for these two conditions. Under vacuum, the stable Ga coverage was 0.48nm, while under N plasma, a thicker Ga stable coverage of 0.7nm was determined. Using GaN layers grown at these conditions as buffer layers, M-plane InGaN layers were grown and processed into photodetectors exhibiting a polarization sensitive response.
Fabrication and characterization of metal-oxide-semiconductor GaAs capacitors on Ge∕Si1−xGex∕Si substrates with Al2O3 gate dielectric
Authors: Shahrjerdi D. , Nuntawong N. , Balakrishnan G. , Garcia-Gutierrez D. I. , Khoshakhlagh A. , Tutuc E. , Huffaker D. , Lee J. C. , Banerjee S. K.
Published: June 2008

In this article, we have studied fabrication and characterization of GaAs metal-oxide-semiconductor (MOS) capacitors with Al2O3 gate dielectric. 300nm thick GaAs layers were grown epitaxially on Ge∕Si1−xGex∕Si substrates. Cross-sectional transmission electron microscopy (TEM) confirmed a threading dislocation density of ∼107∕cm2 in the GaAs layer. In addition, it was observed that threading dislocations were mainly confined within the first ∼50nm of the GaAs layer, adjacent to the Ge film. Interfacial self-cleaning attribute of GaAs upon atomic layer deposition of Al2O3 was confirmed by x-ray photoelectron spectroscopy (XPS) analysis. However, the Al2O3∕GaAs interface properties were remarkably improved by GaAs native removal in dilute HF (1%) followed by sulfur treatment in (NH4)2S, substantiated by probing electrical characteristics of the MOS capacitors and cross-sectional TEM analysis. Thermodynamic properties of Al2O3/sulfide-treated GaAs interface was also studied by monitoring the C-V characteristics of GaAs MOS capacitors implying excellent thermal stability of the Al2O3∕GaAs interface.
Low-strain InAs∕InGaAs∕GaAs quantum dots-in-a-well infrared photodetector
Authors: Shenoi R. V. , Attaluri R. S. , Siroya A. , Shao J. , Sharma Y. D. , Stintz A. , Vandervelde T. E. , Krishna S.
Published: June 2008

The authors report the design, growth, fabrication, and characterization of a low-strain quantum dots-in-a-well (DWELL) infrared photodetector. This novel DWELL design minimizes the inclusion of the lattice-mismatched indium-containing compounds while maximizing the absorption cross section by enabling larger active region volume. The improved structure uses an In0.15Ga0.85As∕GaAs double well structure with Al0.10Ga0.90As as the barrier. Each layer in the active region was optimized for device performance. Detector structures grown using molecular beam epitaxy were processed and characterized. This new design offers high responsivity of 3.9A∕W at a bias of 2.2V and a detectivity of 3×109 Jones at a bias of 2.2V for a wavelength of 8.9μm. These detectors offer significant improvement in the responsivity while retaining the long wave infrared spectral properties of the InAs∕In0.15Ga0.85As∕GaAs DWELL. These detectors if coupled with improved noise characteristics could enable higher temperature operation of DWELL detectors, thus reducing the dependence on cooling equipment.
Designing high electron mobility transistor heterostructures with quantum dots for efficient, number-resolving photon detection
Authors: Rowe M. A. , Gansen E. J. , Greene M. B. , Rosenberg D. , Harvey T. E. , Su M. Y. , Hadfield R. H. , Nam S. W. , Mirin R. P.
Published: June 2008

We describe the design of the epitaxial layers for an efficient, photon-number-determining detector that utilizes a layer of self-assembled quantum dots as an optically addressable gate in a field-effect transistor. Our design features a dedicated absorption layer where photoexcited holes are produced and directed with tailored electric fields to the quantum dot layer. A barrier layer ensures that the quantum dot layer is located at a two-dimensional potential minimum of the structure for the efficient collection of holes. Using quantum dots as charge traps allows us to contain the photoexcited holes in a well-defined plane. We derive an equation for a uniform size of the photon signal based on this precise geometry. Finally, we show corroborating data with well-resolved signals corresponding to different numbers of photons.
Manufacturing of laser diodes grown by molecular beam epitaxy for coarse wavelength division multiplexing systems
Authors: Anselm K. A. , Hwang W. Y. , Ren H. W. , Zhang D. , Um J.
Published: June 2008

Molecular beam epitaxy (MBE) was used to grow ridge-waveguide distributed feedback (DFB) lasers for uncooled coarse wavelength division multiplexing applications. The lasers covered a wavelength range from 1270to1610nm and a temperature range of at least −20to85°C. The MBE growth includes atomic hydrogen cleaning of exposed InGaAsP surfaces for regrowth over a DFB grating. The devices show good performance over the whole wavelength range, with 85°C median threshold currents from 25to40mA depending on the wavelength. Accelerated lifetime testing indicates minimal degradation with TO packaged devices showing less than a 1% increase in operating current after 4000h at 85°C.
Effects of bismuth on wide-depletion-width GaInNAs solar cells
Authors: Ptak A. J. , France R. , Jiang C.-S. , Reedy R. C.
Published: June 2008

GaInNAs solar cells could be useful in next-generation multijunction solar cells if issues surrounding low photocurrents and photovoltages are surmounted. Wide-depletion-width devices generate significant photocurrent using a p-i-n structure grown by molecular beam epitaxy, but these depletion widths are only realized in a region of parameter space that leads to rough surface morphologies. Here, bismuth is explored as a surfactant for the growth of GaInNAs solar cells. Very low fluxes of Bi are effective at maintaining smooth surfaces, even at high growth temperatures and In contents. However, Bi also increases the net donor concentration in these materials, manifested in our n-on-p device structures as a pn-junction that moves deeper into the base layer with increasing Bi fluxes. Quantum efficiency modeling and scanning kelvin probe microscopy measurements confirm the type conversion of the base layer from p type to n type. Bi incorporation in GaAsBi samples shows signs of surface segregation, leading to a finite buildup time, and this effect may lead to slow changes in the electrical properties of the GaInNAs(Bi) devices. Bi also appears to create a defect level, although this defect level is not deleterious enough to increase the dark current in the devices.
Quantitative mobility spectrum analysis of carriers in GaSb/InAs/GaSb superlattice
Authors: Chandrasekhar Rao T. V. , Antoszewski J. , Rodriguez J. B. , Plis E. , Krishna S. , Faraone L.
Published: June 2008

The authors have investigated electrical transport in a type II GaSb/InAs superlattice grown on GaSb using “quantitative mobility spectrum analysis.” Their results indicate that the superlattice contributes a lone electron specie with an ambient temperature mobility of ∼104 cm2/V s. Variable temperature studies in the range 50–300 K show that the carrier is associated with an activation energy of 0.27 eV, which is very close to the superlattice band gap.
Real-time mass spectroscopy of reflected fluxes during molecular beam epitaxy growth of HgCdTe
Authors: Sewell R. H. , Dell J. M. , Faraone L.
Published: June 2008

The interaction between molecular beams present in the growth of HgCdTe was studied using reflection mass spectroscopy (REMS). The incident Hg flux was found to strongly influence the amount of Te and Cd reflected from the growth surface, and also, the incident Te flux influences the amount of Cd reflected from the surface. Based on these observations, it appears that the CdTe growth rate is influenced by the amount of excess Te and/or Hg available at the growth surface. A technique to control accurately the layer mole fraction by measuring the ratio of Cd to Cd+Te fluxes during growth has been investigated. The results of the postgrowth layer characterization by secondary ion mass spectroscopy and infrared transmission indicate a strong correlation between the REMS ratio and the mole fraction of the resulting layer. This technique has successfully been used to correct for long term and short term drifts in effusion cell output.
Tailoring detection wavelength of InGaAs quantum wire infrared photodetector
Authors: Tsai C. L. , Cheng K. Y. , Chou S. T. , Lin S. Y. , Xu C. , Hsieh K. C.
Published: June 2008

The authors report on tailoring detection wavelengths of InGaAs quantum wire infrared photodetectors, using different numbers of short-period superlattice (SPS) pairs to vary the thickness of quantum wires (QWRs) along the growth direction, prepared by the strain-induced lateral-layer ordering process. The QWR characteristics are maintained for QWRs using either In0.52Al0.24Ga0.24As or In0.52Al0.48As barriers and for thin QWR layers made of six SPS pairs. Sharp photoresponses at 6.3 and 8.4 μm are observed for quantum wire infrared photodetectors with ten and six SPS pairs, respectively.
High-power, narrow-ridge, mid-infrared interband cascade lasers
Authors: Canedy C. L. , Kim C. S. , Kim M. , Larrabee D. C. , Nolde J. A. , Bewley W. W. , Vurgaftman I. , Meyer J. R.
Published: June 2008

Two ten-stage interband cascade laser structures were grown by solid-source molecular beam epitaxy. The wafers were processed into both broad-area ridges for threshold characterization and Au-electroplated narrow ridges for high-temperature cw operation. Pulsed threshold current densities in the broad-area devices were as low as 3.8 A/cm2 at 78 K and 590 A/cm2 at 300 K. An 11-μm-wide ridge exhibited a new maximum cw operating temperature of 288 K at λ=4.1 μm. An even longer-wavelength device emitted at 5.1 μm with a maximum cw operating temperature of 229 K.
High electron mobility InAs0.8Sb0.2 grown on InP substrates by gas source molecular beam epitaxy
Authors: Liao Chichih , Wu Bing-Ruey , Hsieh K. C. , Cheng K. Y.
Published: June 2008

High-quality InAs0.8Sb0.2 lattice matched to AlSb has been successfully grown on semi-insulating InP substrates by gas source molecular beam epitaxy using an AlSb/AlAs0.5Sb0.5 buffer layer structure. Due to the inserted AlAs0.5Sb0.5 layer between AlSb and InP, a two-dimensional growth mode is always maintained during the growth. Despite the large lattice mismatch between the InAs0.8Sb0.2 epilayer and InP substrate, the room temperature electron mobility of a 2000 Å thick InAs0.8Sb0.2 has reached ∼15 000 cm2/V s. Compared with InAs0.8Sb0.2 grown on GaAs using a single AlSb buffer layer, this method requires a thinner (≤1 μm) buffer layer structure.
Oxide scalability in Al2O3∕Ga2O3(Gd2O3)∕In0.20Ga0.80As∕GaAs heterostructures
Authors: Shiu K. H. , Chiang C. H. , Lee Y. J. , Lee W. C. , Chang P. , Tung L. T. , Hong M. , Kwo J. , Tsai W.
Published: June 2008

The scalability of molecular beam epitaxy grown Ga2O3(Gd2O3)∕In0.2Ga0.8As∕GaAs with in situ Al2O3 capping layers has been studied, in which the InGaAs surface Fermi level has been unpinned. The electrical and structural properties were improved with rapid thermal annealing to high temperatures of 800°C under N2 flow. As Ga2O3(Gd2O3) is scaled down to 8.5nm, the dielectric constant maintained at 14–15, similar to those of thicker oxides, resulting in an equivalent oxide thickness of 2.3nm. A low gate oxide leakage current density of 10−9A∕cm2 at ∣VG−Vfb∣=1, small flatband voltage shift (ΔVfb), low interfacial density of states (Dit) of (1–3)×1011cm−2eV−1 have been achieved.
Molecular beam epitaxy growth of AlGaAs on the (631)-oriented GaAs substrates
Authors: Méndez-García V. H. , Ramírez-Elías M. G. , Gorbatchev A. , Cruz-Hernández E. , Rojas-Ramírez J. S. , Martínez-Velis I. , Zamora-Peredo L. , López-López M.
Published: June 2008

The authors report the molecular beam epitaxy growth and characterization of AlGaAs∕GaAs(631) heterostructures grown at different As4 molecular beam equivalent pressures. The reflection high-energy electron diffraction patterns taken along the [−120] azimuth showed that the twofold reconstruction commonly observed during the GaAs-buffer layer growth is preserved during the AlGaAs deposition. The 10K photoluminiscence (PL) characterization of the samples showed transitions related to the AlGaAs band edge, the incorporation of impurities, and deep centers. The temperature dependence of the band to band PL transition was fitted with the models developed by Varshni, Pässler, and Viña. The intensity of the PL spectra drastically decreases as the As4 pressure is increased. Photoreflectance (PR) spectroscopy also showed the best crystal quality for the sample grown at low As pressure. The authors obtained the built-in internal electric field strength and the band-gap energy from an analysis of the PR spectra close to the GaAs band edge employing the Franz-Keldysh model.