bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure and ordering kinetics of micelles in triblock copolymer solutions in selective solvents
Authors: Bansil R. , Nie H. , Li Y. , Liao G. , Ludwig K. , Steinhart M. , Koňák Č. , Lal J.
Published: February 2003

We have used small‐angle x‐ray scattering (SAXS), and small‐angle neutron scattering (SANS) to study the micelle structure of a polystyrene‐block‐poly(ethene‐co‐butene)‐block‐polystyrene triblock copolymer in dilute ‐ semidilute solutions in solvents selective for either the outer styrene block (dioxane) or for the middle block (heptane or tetradecane). Measurements of equilibrium structure factors showed that micelles were formed in both types of selective solvents. In the case of dioxane, the micelles are isolated whereas in the case of heptane or tetradecane, a bridged micellar structure may be formed at higher copolymer concentrations. In both cases we observed an ordered cubic structure of insoluble domains (micellar cores) at high concentrations (> 8 %). The micellar scattering function was fit to the Percus‐Yevick interacting hard‐sphere model. The temperature dependence of the core radius, the hard‐sphere interaction radius and the volume fraction of hard spheres were obtained. We also used synchrotron‐based time‐resolved SAXS to examine the kinetics of ordering of the micelles on a cubic lattice for many different temperature jumps into the ordered cubic phase starting from the disordered micellar fluid phase in different solvents at different concentrations. The time evolution of the structure changes was determined by fitting the data with Gaussians to describe the structure factor of the ordered Bragg peaks and the Percus‐Yevick structure factor was used to describe the micellar fluid. The time dependence of the peak intensities and widths as well as of the micellar parameters will be presented. The results showing the kinetics of the transformation from the fluid to the ordered phase were analyzed using the Mehl‐Johnson‐Avrami theory of nucleation.
Time‐resolved SAXS studies on crystallization behavior of polyõethylene isophthalate‐co‐ethylene terephthalatešs
Authors: Lee B. , Shin T.J. , Lee S.W. , Lee J.W. , Ree M.
Published: February 2003

Small‐angle X‐ray scattering measurements using synchrotron radiation were carried out for poly(ethylene terephthalate) and poly(ethylene isophthalate‐co‐ethylene terephthalate)s. In addition, differential scanning calorimetric measurements were conducted. Measurements were made both on polymers undergoing isothermal crystallization and during subsequent remelting. The primary and secondary crystallization behaviors are examined. Isophthalate units were found to be excluded from the crystals into amorphous layers during crystallization. No crystal thickening was observed during isothermal crystallization, which may be due to the relatively high chain rigidity. Secondary crystallization, detected predominantly at the later stages of crystallization, causes densification and shrinkage of the amorphous layer. Considering the results, it is proposed that secondary crystallization involves the formation of short‐range molecular order in the amorphous layers of a lamellar stack as well as in the amorphous regions between lamellar stacks. This short‐range‐ordered phase has a lower density than the lamellar crystal formed by primary crystallization.
Neutron reflectivity of polymer‐plasticiser diffusion
Authors: Higgins J.S. , Butler S.A. , Bucknall D.G.
Published: February 2003

Neutron reflectivity (NR) has been widely exploited to look at polymer thin films and in many ways is an ideal technique for studying polymer interfaces and surfaces, providing high‐resolution concentration ‐ depth profiles across the film thickness. Most NR studies to date have concentrated on thin films of amorphous polymers which possess Tg values well above room temperature. These polymers are ideally suited to NR measurements, firstly because they form homogeneously flat films and, secondly, heat‐quench cycles can be used to study time‐dependent processes. This has been used to great effect in NR studies of the initial stages of polymer ‐ polymer interdiffusion or the kinetics of surface segregated layers for instance. One of the biggest drawbacks to this approach is that in polymer systems where one or more of the components has a Tg close to or less than room temperature, the polymers can still move during the measurement time of an NR profile, which typically takes 1‐2 h for a full profile. Therefore, in order to study such systems, we have developed an approach to NR measurements that allows us to investigate diffusion processes in situ. Our new approach allows us to take NR profiles in only 20 s. This paper describes the method of real‐time NR measurements in detail and illustrates the capabilities of the technique with highlights from some of our recent work on the early stages of polymer‐plasticiser interdiffusion.
Structure and dynamics of silica‐filled polymers by SANS and coherent SAXS
Authors: Geissler Erik , Hecht Anne‐Marie , Rochas Cyrille , Horkay Ferenc , Bley Françoise , Livet Frédéric
Published: February 2003

Random crosslinking in elastomers gives birth to local variations in the crosslink density. When the network is swollen in a low‐molecular‐weight solvent, competition between the osmotic pressure and the local elastic constraints transforms these variations into differences in polymer concentration, the range and amplitude of which can be measured by small‐angle X‐ray or neutron scattering (SAXS or SANS). In filled systems, the distribution both of the polymer and of the elastic constraints is modified. By varying the proportion of deuterated solvent in the network, the scattering function of the polymer can be distinguished from that of the filler using SANS. Such measurements yield not only the internal surface area of the filler particles but also the fraction of that surface in contact with the polymer. The recently developed technique of quasi‐elastic SAXS detects slow dynamic processes at wave vectors larger than those accessible with visible light lasers. This technique is used to investigate the dynamics of filler particles in uncrosslinked polymer melts. It is directly shown that the structural reorganization process of the filler following an external mechanical perturbation is diffusion‐controlled.
Light scattering investigation of association phenomena in saline carrageenan solutions
Authors: Cuppo Fabio , Reynaers Harry
Published: February 2003

Intermolecular association of ι‐ and κ‐carrageenan in aqueous solution has been studied by wide‐angle laser light scattering as a function of salt composition and temperature. For ι‐carragenan, the effect of three different salts (LiCl, NaCl, NaI) on the associative properties was investigated. As to κ‐carrageenan, the association‐inducing effect of Cs+ ions was studied, with particular interest in the time dependence of the aggregation process.
Glass capillaries as primary optics for X‐ray stress analysis
Authors: Gibmeier J. , Scholtes B.
Published: February 2003

For locally resolved X‐ray stress analysis the practical application of industrial glass capillaries was investigated. The capillaries were characterised with respect to their surface roughness and their deviation in alignment. X‐ray interference profiles were measured in order to evaluate the potential gain in primary intensity in comparison to measurements carried out by using conventional pinhole collimators. It can be concluded that industrial sodalime‐glass capillaries manufactured by the redraw‐process can be used as a suitable low‐cost alternative primary aperture in X‐ray stress analysis. For primary beam diameters ≤ Ø1.0 mm an immense gain of intensity was observed that increases with decreasing diameter of the aperture.
Elektronenstrahlbehandlung von PVD‐Hartstoffschichten
Authors: Spies H.‐J. , Friedrich S. , Buchwalder A.
Published: February 2003

Zur Untersuchung der Wirkung einer Elektronenstrahlbehandlung auf die Struktur und die Eigenschaften hartstoffbeschichteter Stähle wurden Schichten des Typs CrNx, (Ti, Cr)N, (Ti, Al)N, Ti(C, N) und Ti(B, N) auf dem vergüteten Stahl C45 abgeschieden. Die Untersuchung des thermischen Verhaltens der Hartstoffschichten erfolgte durch einen gezielt variierten Energieeintrag und Einfrieren der Gefügeänderungen durch eine Selbstabschreckung. Eine parallel zur Schichtbehandlung auftretende unterschiedliche Härtung der Randschicht des Substrates bewirkte eine differenzierte Abstützung der Hartstoffschicht.

Die thermische Stabilität der Hartstoffschichten unterscheidet sich beträchtlich. Die Oberflächenhärte, die Schichthaftung und die Verschleißbeständigkeit des Substrat‐Schicht‐Verbundes wurden in Abhängigkeit vom Energieeintrag erhöht. Die Anwendung von Elektronenstrahltechnologien ermöglicht auf diese Weise die Erzeugung von Stützschichten, die lokal die Beanspruchbarkeit hartstoffbeschichteter Stähle verbessern.
Werkstoffmechanische Bewertung von Laserstrahl‐Schweißnähten in Feinkornbaustählen
Authors: Blauel J. G. , Burget W.
Published: February 2003

An Feinkornbaustählen unterschiedlicher Festigkeitsklassen werden auf der Basis aufeinander abgestimmter experimenteller und numerischer Untersuchungen die Auswirkungen einer werkstoffmechanischen Inhomogenität (mismatch) in fehlerfreien und fehlerbehafteten Laserstrahl‐Schweißverbindungen auf deren Verformungs‐ und Versagensverhalten quantitativ beschrieben und daraus Folgerungen für die Erfüllung des jeweiligen Gebrauchszwecks abgeleitet.
Einfluß trockener Friktionsbeanspruchungen auf Schädigungsverhalten, Restfestigkeit und Oberflächeneigenspannungen von Al2O3
Authors: Erbacher T. , Beck T. , Vöhringer O.
Published: February 2003

Der vorliegende Beitrag zeigt Ergebnisse von Trockenreibversuchen mit der Paarung Al2O3‐Scheibe gegen GG25‐Pellets. Der Einfluß der Friktionsbelastung auf Restfestigkeit, Bruchverhalten und oberflächennahe Eigenspannungen wird dargestellt und diskutiert. Ergebnisse von FEM‐Simulationsrechnungen des Friktionssystems werden vorgestellt. Die Schädigung der Keramikscheibe wird vorwiegend durch den bei der Friktionsbeanspruchung eingebrachten Wärmestrom dQ/dt bestimmt. Ab dQ/dt = 1,3 kJ/s entstehen nach einem Reibzyklus feine Radialrisse entlang der Oberfläche. Der Ort der Rissentstehung und die Ausbreitungsrichtung kann durch die FEM‐Simulation vorhergesagt werden. Bei zunehmender Beanspruchung entstehen mechanisch induzierte tangentiale Brüche im Zentrum des Reibbereichs, welche die primären Radialrisse verbinden, so dass Fragmente entstehen. Die thermisch induzierten, primär entstandenen Radialrisse zeigen einen transkristallinen, die mechanisch induzierten, sekundären Tangentialbrüche einen weitgehend interkristallinen Verlauf. Das Auftreten feiner Radialrisse geht mit einer Abnahme der Restfestigkeit um etwa 20% einher, wobei sich eine bimodale Verteilung der Festigkeiten ergibt. Während eine zyklische Friktionsbeanspruchung das makroskopische Schädigungsbild gegenüber dem Zustand nach einem Reibzyklus nicht erkennbar ändert, führt sie zu einer Homogenisierung der Restfestigkeiten. Die Reibbeanspruchung bewirkt eine Umlagerung der Druckeigenspannungen im Ausgangszustand zu Zugeigenspannungen im belasteten Zustand. In tangentialer Richtung erreichen die Zugeigenspannungen knapp 100 MPa und sind damit schädigungsrelevant.
Simulation der Wärmebehandlung von Stählen am Institut für Werkstoffkunde I
Authors: Franz C. , Schulze V. , Macherauch E. , Löhe D.
Published: February 2003

Die Simulation von Fertigungsprozessen ist ein entscheidendes Werkzeug im Rahmen des Simultaneous Engineering. Ziel ist es, durch Simulation der kompletten Fertigungskette Entwicklungszeiten zu verkürzen und Verfahren zu optimieren. Der Bereich der Wärmebehandlung bietet dem Einsatz von Simulationswerkzeugen ein breites Anwendungsgebiet. Die Bauteilgeometrie, die Werkstoffzusammensetzung, das Wärmebehandlungsverfahren und dessen Ablauf sowie der Ausgangszustand mit den herstell‐ und fertigungsbedingten Einflüssen auf das Bauteil sind miteinander in komplexer Weise verknüpft und beeinflussen das Maß‐ und Formänderungsverhalten der Bauteile während der Wärmebehandlung. Die Berechnung von Gefüge‐ und Härteverteilungen kann konkrete Hinweise auf geeignete Abschreckbedingungen, Anlagentechnik oder Chargierbedingungen geben, und berechnete Eigenspannungen sowie Maß‐ und Formänderungen können bei der Entwicklung und Konstruktion der Bauteile berücksichtigt werden. Am Institut für Werkstoffkunde I der Universität Karlruhe (TH) wird seit fast 30 Jahren auf diesem Gebiet sowohl numerisch als auch experimentell geforscht.
Röntgenographische Spannungsanalyse in einzelnen Kristalliten – Mess‐ und Auswerteroutinen sowie Einflussgrößen
Authors: Turki J. , Löhe D.
Published: February 2003

Für die röntgenographische Spannungsanalyse innerhalb einzelner Kristalle von grobkörnigen Werkstoffen kann das Einkristallverfahren eingesetzt werden. In dieser Arbeit werden sowohl die theoretischen Grundlagen als auch Messalgorithmen für dieses Verfahren präsentiert. An einem Kristallit einer Eisen‐Silizium‐Modellegierung wurden Eigenspannungsmessungen mit verschiedenen Wellenlängen und an verschiedenen Gitterebenen durchgeführt. Es wird gezeigt, dass Mo‐Kα‐Strahlung für röntgenographische Spannungsanalysen in einzelnen Körnern des untersuchten Werkstoffzustandes sehr gut geeignet ist.
Foamability of Particle Reinforced Aluminum Melt
Authors: Babcsán N. , Leitlmeier D. , Degischer H. P.
Published: February 2003

The foaming of aluminum melt of LKR‐Austria is investigated. The foaming was performed in a furnace chamber at adiabatic conditions to keep the foam in liquid state. The foams were taken out from the chamber after 1, 10 and 100 min isothermal holding time and cooled in air or quenched in water. The final cellular structure depends on the following parameters: particle (composition, shape and size, volume fraction), gas (composition and purity), the particle‐surface interactions, matrix alloy composition and temperature of foaming.

Metallographical examinations are carried out after freezing the foamed melt by the help of Scanning Electron Microscopy. The cell wall thickness, the particle distribution and the existing phases are determined. Transmission Electron Microscopy and Auger Electron Spectroscopy are used to analyze the surface skin of the cells and to determine the oxide layer thickness depending on the different foaming gases.

An equipment called High Temperature Maximum Bubble Pressure Tensiometer (HTMBPT) was built to measure the apparent surface tension of the particle‐stabilized melt. Semi quantitative description of the experimental phenomena are presented.
Residual Stress Distributions in Glass/Metal‐Joints produced by Ultrasonic Torsion Welding
Authors: Kuckert H. , Born Ch. , Wagner G. , Eifler D.
Published: February 2003

The development and industrial application of efficient methods to join glass with metals which combine the individual advantages of both material groups are a great technological challenge. One research field of the Institute of Materials Science and Engineering is the production of glass/metal joints by means of ultrasound. Industrial applications are for example the sealing of glass vessels, fixtures in the vacuum technique or lens mounts. For this reason an industrial ultrasonic torsion welding system normally used for metal weldings was modified to be suitable for the demands of high sensitive glass/metal‐joints. With the developed welding system helium‐tight joints of glass and metals can be realized. In comparison to the conventional welding techniques [1] for glass like diffusion welding or adhesive bonding, ultrasonic torsion welding is characterized by very short welding times (< 1s) as well as low welding temperatures (< 450°C). Further advantages of this joining technique are the high automation potential and the environmental compatibility. Furthermore this technique can be applied under normal or specific atmospherical conditions. In spite of the low joining temperatures thermal residual stresses occur during the cooling of the joints due to the different coefficients of thermal expansion of the used materials [2]. In the present paper the measurement and calculation of the temperature distribution and the development of thermal residual stresses are described.
Entwicklung des dynamischen Abschreckens in Hochdruck‐Gasabschreckanlagen
Authors: Heuer V. , Löser K.
Published: February 2003

Es wurde ein neuer Abschrecksensor zum Einsatz in Hochdruck‐Gasabschreckkammern entwickelt. Dieser QC3‐Sensor (Quench Control in Cold Chambers) misst die Abkühlkurve eines vorgeheizten Messkopfs. Der Sensor kann zur Prozessdokumentation, zur Prozesskontrolle und zur Prozesssteuerung eingesetzt werden.

Mit Hilfe des Sensors kann das neu entwickelte “dynamische Abschrecken” präzise gesteuert werden. Unter dynamischem Abschrecken versteht man eine Gasabschreckung mit zeitlich variabler Abschreckintensität. Sowohl an ringförmigen Probekörpern als auch an Zahnrädern aus 16MnCr5 wurde nachgewiesen, dass das dynamische Abschrecken ein enormes Potential zur Minimierung der während der Einsatzhärtung auftretenden Maß‐ und Formänderungen besitzt. Insbesondere die Streuungen der Maß‐ und Formänderungen können signifikant verringert werden, wenn während der Abschreckung die Abschreckintensität gezielt abgesenkt wird.
Einfluss unterschiedlicher Oberflächenzustände vor dem Plasmanitrieren auf Eigenschaften und Zerspanungsverhalten des Schnellarbeitsstahls S 6‐5‐2
Authors: da Silva Rocha A. , Strohaecker T. , Hirsch T.
Published: February 2003

In der vorliegenden Arbeit wurde der Einfluss unterschiedlicher Oberflächenvorbehandlungen auf die nach dem Plasmanitrieren von Schnellarbeitsstahl S 6‐5‐2 vorliegenden Randschichtzustände untersucht. Das Plasmanitrieren von geschliffenen und sandgestrahlten Proben und Zentrierbohrern wurde bei Temperaturen von 400°C und 500°C mit den beiden Gaszusammensetzungen 5 Vol.% N2 und 76 Vol.% N2 in Wasserstoff durchgeführt. Die Randschichten vor und nach dem Plasmanitrieren wurden durch die Mikrostruktur, die Oberflächentopographie, die chemische Zusammensetzung, die Phasenzusammensetzung und die Eigenspannungen charakterisiert. Die Zentrierbohrer wurden Zerspanungsversuchen unterzogen, bei denen die Vorschubkräfte und Drehmomente aufgezeichnet wurden. Aus den Untersuchungen ergab sich ein deutlicher Einfluss der Oberflächenzustandes vor dem Plasmanitrieren auf die Eigenspannungen und die Werkstoffzustände der Nitrierschicht. Dünnere Diffusionsschichtdicken bei geschliffenen und gestrahlten Zuständen werden auf die hohen Druckeigenspannungen des Ausgangszustands und eine eigenspannungsbedingte Behinderung der Diffusion des Stickstoffs bzw. Kohlenstoffs zurückgeführt. In den Zerspanungsversuchen zeigten sandgestrahlte und plasmanitrierte Werkzeuge das beste Verschleißverhalten. Die kleineren Stickstoffgehalte in der Plasmanitrieratmosphäre und die damit nicht vorhandene Verbindungsschicht resultierten in den geringsten Verschleißbeträgen der Zentrierbohrer während die hohen Stickstoffgehalte eine Verminderung der benötigten Drehmomente und Vorschubkräfte ergaben.
Verallgemeinerungsfähige Merkmale und Besonderheiten des Sprühkompaktierens
Authors: Bauckhage K. , Schulz A. , Uhlenwinkel V.
Published: February 2003

Dadurch, daß beim Sprühkompaktieren die schnelle Erstarrung und Abkühlung des Materials das Ausmaß von Entmischungen eindämmen, können Makroseigerungen unterbunden und Anreicherungen an den Korngrenzen deutlich verringert werden. Ferner wird das Risiko der Lunker‐ und Warmrißbildung durch den Impuls des Massetransportes während des Kompaktiervorganges stark reduziert. Dies bedeutet, daß solche Materialien, die bspw. beim Gießen zu starker Seigerung neigen und besonders anfällig für die Lunkerbildung und/oder Warmrißbildung sind sowie bei Umformverfahren zur Zeilenbildung von Carbiden, Nitriden oder Sulfiden neigen, mit Hilfe des Sprühkompaktierens im Freiformverfahren mit großen Abmaßen chemisch homogen und fehlerfrei herstellbar werden. Ein besonderes Merkmal sprühkompaktierter Deposits ist ihr feines Gefüge und ihre vergleichsweise hohe Duktilität. Die Besonderheiten des neuartigen Urformverfahrens werden anhand von Beispielen erläutert.
Schubspannungsbomben in Faserverbunden
Authors: Mattheck C. , Tesari I.
Published: February 2003

Auch bei optimaler äußerer Bauteilgestalt mit gleichförmiger Spannungsverteilung können lokale Schubspannungsmaxima zur Entstehung von Schubrissen führen, insbesondere wenn der maximale Schub entlang des Faserverlaufes verläuft.

Bäume können einmal angelegte Fasern nicht an neu auftretende Schubmaxima anpassen und sind daher eher schubspannungsgefährdet als der Knochen, der seine Innenarchitektur auch nachträglich noch an Laständerungen anpassen kann.

Auch die Umlenkung des Kraftflusses um Kerben kann lokale Schubmaxima bewirken, die gleichsam als Schubkerbspannungen zu betrachten sind, der Kerbe aber nicht direkt anliegen müssen.
Electrical Resistivity and Phase Transformation in Steels
Authors: Mohanty O. N. , Bhagat A. N.
Published: February 2003

Changes in electrical resistance accompanying transformations in steels with magnetic change (e.g. γ → martensite/bainite) and without magnetic change (e.g. γ → α, above Curie temperature) have been examined; the former class affects the resistivity the latter does not. Next, while the efficacy of electrical resistivity measurement in capturing the well‐known features of austenite stabilization (e.g. over – ageing, reversibility, and influence of prior martensite amount and so on) in high carbon steels has been reported in an earlier publication, new features (e.g. increase in resistance ‐increase at very low temperatures, change in temperature co‐efficient of resistivity in the stabilized material etc.) are highlighted here. Finally, the work shows that a quantitative estimate of precipitation in the copper bearing, age‐hardenable HSLA‐100 steel during tempering can be done by continuous electrical resistivity measurement. These data also allow an in‐depth kinetic analysis using the Johnson‐Mehl‐Avrami equation.
A study on the convergence of least‐squares meshfree method under inaccurate integration
Authors: Park Sang‐Hoon , Kwon Kie‐Chan , Youn Sung‐Kie
Published: February 2003

In the authors' previous work, it has been shown through numerical examples that the least‐squares meshfree method (LSMFM) is highly robust to the integration errors while the Galerkin meshfree method is very sensitive to them. A mathematical study on the convergence of the solution of LSMFM under inaccurate integration is presented. New measures are introduced to take into account the integration errors in the error estimates. It is shown that, in LSMFM, solution errors are bounded by approximation errors even when integration is not accurate. Copyright © 2003 John Wiley & Sons, Ltd.
Simulation of high velocity concrete fragmentation using SPH/MLSPH
Authors: Rabczuk T. , Eibl J.
Published: February 2003

The simulation of concrete fragmentation under explosive loading by a meshfree Lagrangian method, the smooth particle hydrodynamics method (SPH) is described. Two improvements regarding the completeness of the SPH‐method are examined, first a normalization developed by Johnson and Beissel (NSPH) and second a moving least square (MLS) approach as modified by Scheffer (MLSPH). The SPH‐Code is implemented in FORTRAN 90 and parallelized with MPI. A macroscopic constitutive law with isotropic damage for fracture and fragmentation for concrete is implemented in the SPH‐Code. It is shown that the SPH‐method is able to simulate the fracture and fragmentation of concrete slabs under contact detonation. The numerical results from the different SPH‐methods are compared with the data from tests. The good agreement between calculation and experiment suggests that the SPH‐program can predict the correct maximum pressure as well as the damage of the concrete slabs. Finally the fragment distributions of the tests and the numerical calculations are compared. Copyright © 2003 John Wiley & Sons, Ltd.
Point collocation methods using the fast moving least‐square reproducing kernel approximation
Authors: Kim Do Wan , Kim Yongsik
Published: February 2003

A pseudo‐spectral point collocation meshfree method is proposed. We apply a scheme of approximating derivatives based on the moving least‐square reproducing kernel approximations. Using approximated derivatives, we propose a new point collocation method. Unlike other meshfree methods that require direct calculation of derivatives for shape functions, with the proposed scheme, approximated derivatives are obtained in the process of calculating the shape function itself without further cost. Moreover, the scheme does not require the regularity of the window function, which ensures the regularity of shape functions. In this paper, we show the reproducing property and the convergence of interpolation for approximated derivatives of shape functions. As numerical examples of the proposed scheme, Poisson and Stokes problems are considered in various situations including the case of randomly generated node sets. In short, the proposed scheme is efficient and accurate even for complicated geometry such as the flow past a cylinder. Copyright © 2003 John Wiley & Sons, Ltd.
Recovering lower bounds of the error by postprocessing implicit residual a posteriori error estimates
Authors: Díez Pedro , Parés Núria , Huerta Antonio
Published: February 2003

Classical residual type error estimators approximate the error flux around the elements and yield upper bounds of the exact (or reference) error. Lower bounds of the error are also needed in goal oriented adaptivity and for bounds on functional outputs. This work introduces a simple and cheap strategy to recover a lower bound estimate from standard upper bound estimates. This lower bound may also be used to assess the effectivity of the former estimate and to improve it. Copyright © 2003 John Wiley & Sons, Ltd.
Earthquake‐induced hydrodynamic pressures on a 3D rigid dam–reservoir system using DRBEM and a radiation matrix
Authors: Fahjan Yasin M. , Börekçi Osman S. , Erdik Mustafa
Published: February 2003

The dual reciprocity method is applied to determine the hydrodynamic pressure distribution in a three‐dimensional dam–reservoir system subjected to earthquake excitation. The reservoir domain is idealized as a finite region of irregular geometry adjacent to the dam followed by an infinite domain of uniform cross‐section in the upstream direction. The reservoir hydrodynamic pressure response is governed by the Helmholtz equation subject to free surface, dam–reservoir interface, absorbing bottom/banks and radiation boundary conditions. A three‐dimensional (3D) dual reciprocity model is developed to determine the hydrodynamic pressure in the finite reservoir domain. A radiation matrix is developed and introduced to relate the hydrodynamic pressure and its normal derivative on the interface between the finite and infinite domains. The three‐dimensional radiation model used is developed by applying a two‐dimensional dual reciprocity formulation along the interface of the finite and infinite reservoirs together with a continuum solution in the upstream direction of the infinite domain. The model is compared for the hydrodynamic response of a three‐dimensional rectangular reservoir and found to be in excellent agreement with results obtained from a model based on the analytical formulation existing in the literature. Copyright © 2003 John Wiley & Sons, Ltd.
A domain decomposition preconditioner for hermite collocation problems
Authors: Mateescu Gabriel , Ribbens Calvin J. , Watson Layne T.
Published: February 2003

We propose a preconditioning method for linear systems of equations arising from piecewise Hermite bicubic collocation applied to two‐dimensional elliptic PDEs with mixed boundary conditions. We construct an efficient, parallel preconditioner for the GMRES method. The main contribution of the article is a novel interface preconditioner derived in the framework of substructuring and employing a local Hermite collocation discretization for the interface subproblems based on a hybrid fine‐coarse mesh. Interface equations based on this mesh depend only weakly on unknowns associated with subdomains. The effectiveness of the proposed method is highlighted by numerical experiments that cover a variety of problems. © 2003 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 19: 135–151, 2003
Semilinear parabolic problem with nonstandard boundary conditions: Error estimates
Authors: Slodička Marián
Published: February 2003

We study a semilinear parabolic partial differential equation of second order in a bounded domain Ω ⊂ ℝN, with nonstandard boundary conditions (BCs) on a part Γnon of the boundary ∂Ω. Here, neither the solution nor the flux are prescribed pointwise. Instead, the total flux through Γnon is given, and the solution along Γnon has to follow a prescribed shape function, apart from an additive (unknown) space‐constant α(t). We prove the well‐posedness of the problem, provide a numerical method for the recovery of the unknown boundary data, and establish the error estimates. © 2003 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 19: 167–191, 2003
On a finite element method for unsteady compressible viscous flows
Authors: Liu Biyue
Published: February 2003

In this article we present an analysis of a finite element method for solving two‐dimensional unsteady compressible Navier‐Stokes equations. Under the time‐stepping size restriction Δt ≤ Ch, we prove the existence and uniqueness of the numerical solution and obtain an a prior error estimate uniform in time. © 2003 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 19: 152–166, 2003
A modified alternating‐direction finite volume method for modeling secondary hydrocarbon migration and accumulation processes
Authors: Wang Hong , Zhao Weidong
Published: February 2003

We present a finite volume method for the coupled systems of nonlinear partial differential equations arising in secondary hydrocarbon migration and accumulation processes in basin modeling. We propose a modified alternating direction implicit splitting acceleration technique to solve the scheme in an effective and robust manner. We apply the scheme to simulate model problems with clear physical features and to secondary migration and accumulation problems in real petroleum reservoirs. The numerical experiments show that the scheme is very robust and generates stable and physically reasonable numerical results, even in the extremely long time period that ranges from millions of years to tens of millions of years. This clearly shows the strong potential of the method developed. © 2003 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 19: 254–270, 2003
An upwind finite volume scheme and its maximum‐principle‐preserving ADI splitting for unsteady‐state advection‐diffusion equations
Authors: Wang Hong , Zhao Weidong
Published: February 2003

We develop an upwind finite volume (UFV) scheme for unsteady‐state advection‐diffusion partial differential equations (PDEs) in multiple space dimensions. We apply an alternating direction implicit (ADI) splitting technique to accelerate the solution process of the numerical scheme. We investigate and analyze the reason why the conventional ADI splitting does not satisfy maximum principle in the context of advection‐diffusion PDEs. Based on the analysis, we propose a new ADI splitting of the upwind finite volume scheme, the alternating‐direction implicit, upwind finite volume (ADFV) scheme. We prove that both UFV and ADFV schemes satisfy maximum principle and are unconditionally stable. We also derive their error estimates. Numerical results are presented to observe the performance of these schemes. © 2003 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 19: 211–226, 2003
Adaptive numerical solution of thick plates using first‐order shear deformation theory. Part II: Adaptivity
Authors: Ludwig M. J. , Shaw Simon , Whiteman J. R.
Published: February 2003

This is the second in a pair of articles concerned with the adaptive finite element solution of Riessner‐Mindlin thick plates modeled using first‐order shear deformation theory. This article is concerned with enhancing the a posteriori energy‐error estimators developed in Part I in order to accomodate transition elements in the finite element mesh. The resulting estimators are then used in an adaptive finite element model employing transition elements and the subsequent results discussed and compared with those in Part I. A major part of the article is devoted to identifying a novel patch assembly node algorithm for using the ZZ recovery‐type estimator with transition elements. © 2003 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 19: 227–253, 2003.
Thermo‐ and galvanomagnetic measurements of semiconductors at ultrahigh pressure
Authors: Shchennikov Vladimir V. , Ovsyannikov Sergey V.
Published: February 2003

The thermoelectric power, magnetoresistance, and thermomagnetic effects were measured for Te and Se micro‐samples in the vicinity of semiconductor–metal phase transitions at high pressure. From longitudinal and transverse Nernst‐Ettingshausen effects the scattering parameterrof holes was estimated and a decrease of the effective mass of holes was found during the closing of semiconductor gap. After the high pressure treatment an inversion of the sign of parameterrcorresponding to a change of scattering mechanism was observed for the Te sample due to an increase of the density of lattice defects.
THz lasing due to resonant acceptor states in strained p‐Ge and SiGe quantum‐well structures
Authors: Kagan M. S. , Altukhov I. V. , Chirkova E. G. , Sinis V. P. , Troeger R. T. , Ray S. K. , Kolodzey J.
Published: February 2003

An intense THz emission was observed from strained SiGe/Si quantum‐well structures under a strong pulsed electric field. The p‐type structures were MBE‐grown on n‐type Si substrates and δ‐doped with boron. Lines with wavelengths near 100 microns were observed in the emission spectrum. The modal structure in the spectrum gave evidence for the stimulated nature of the emission. The origin of the THz emission was attributed to intra‐center optical transitions between resonant and localized boron levels similar to that in compressed p‐Ge.
Hydrostatic pressure coefficients of the valence band maximum in Ge, InSb, InAs, and GaAs
Authors: Daunov M. I. , Kamilov I. K. , Gabibov S. F. , Magomedov A. B.
Published: February 2003

The Hall coefficient R and specific resistance ρ have been measured in p‐InSb:Cr, with a hole concentration of about 1012 cm–3 at T = 77 K, as a function of pressure up to P = 1.5 GPa at T = 77 and 300 K. The deep acceptor level is found to be located at the distance εA2 = 0.11 eV from the top of the valence band. The pressure coefficient of the valence band maximum ∂εV/∂P was determined relative to the vacuum in semiconductors Ge, InSb, InAs, and GaAs.
Electronic and structural study of β‐FeSi2 under high pressure
Authors: Mori Y. , Ikai T. , Teranishi R. , Takarabe K.
Published: February 2003

High‐pressure X‐ray and optical studies of β‐FeSi2 were carried out up to 60.7 GPa and 5 GPa, respectively. The bulk modulus was determined to be (243.46 ± 7.14) GPa by the Birch‐Murnaghan equation. A new phase appeared at about 20 GPa. The pressure coefficient of the direct bandgap was determined to be +15.9 meV/GPa by the high‐pressure optical absorption experiment. Consequently the deformation potential for the direct bandgap was determined to be –3.87 eV. The deformation potential for the direct gap at the Y point was obtained to be –2.4 eV using a FLAPW (full‐potential linearized augmented plane wave) method.
The effect of pressure on the radiative efficiency of InAs based light emitting diodes
Authors: Choulis S. A. , Andreev A. , Merrick M. , Jin S. , Clarke D. G. , Murdin B. N. , Adams A. R. , Krier A. , Sherstnev V. V.
Published: February 2003

The spontaneous emission of 3.3 μm light emitting diodes (LEDs) and lasers based on InAs alloys were studied as a function of hydrostatic pressure. An increase in light output with increasing pressure has been observed, with a general comparison of the performances of type I and type II structures given in terms of the radiative and non‐radiative processes involved. The experimental results provide evidence that the so‐called CHSH Auger process is insignificant in these devices. However, Auger recombination mechanisms are still shown to dominate the response of these type II LEDs in contrast to type I lasers where competing radiative processes appear more significant.
A personal view of the physics of high pressure studies of solids
Authors: Cohen Marvin L.
Published: February 2003

I will discuss some background and motivation for studies of high pressure properties of solids. My description will reflect my personal view as a theorist who feels strongly that condensed matter physics is a very important field of science and that high pressure studies have contributed greatly to this field.
Anomalous pressure dependence of acoustic phonons of AgGaSe2 investigated by inelastic neutron scattering to 4.3 GPa
Authors: Klotz S. , Derollez P. , Fouret R. , Braden M. , Hennion B. , Hubert Ch. , Meducin F. , Łażewski J. , Gonzalez J.
Published: February 2003

The pressure dependence of the transverse acoustic (TA) phonons along [100] and [001] and the low‐frequency optical Λ3 modes of AgGaSe2 has been measured at pressures to 4.3 GPa by inelastic neutron scattering. We find an exceptionally strong frequency decrease of the TA modes with Grüneisen parameters as low as –10 for wave vectors close to the zone centre. The associated elastic coefficient C44 decreases by ∼50% between 0 and 2.6 GPa. The softening also affects the optical modes to a less extent. These lattice dynamical anomalies seem to be related to a continuous structural phase transition located at ∼2.6 GPa. Single crystal X‐ray diffraction results are consistent with a monoclinic lattice of the high pressure phase which can be regarded as a slightly distorted chalcopyrite structure, similar to AgGaS2.
High pressure structural phase transitions in IV–VI semiconductors
Authors: Ahuja Rajeev
Published: February 2003

We have studied high pressure structural phase transitions in lead chalcogenides by means of first principles total energy calculations within the generalized gradient approximation (GGA) using the full potential linear‐muffin‐tin‐orbital (FPLMTO) method. These compounds are stable in the NaCl‐type structure at ambient conditions and transform to the CsCl‐type of structure under high pressure through an intermediate phase. The experimental data suggests that this intermediate phase could be either the TlI‐type phase or the GeS‐type phase. However this data was not enough to solve the full structural refinement. So we have performed fully relaxed total energy calculations to get the full structure of this intermediate phase. Our calculated transition pressures are in very good agreement with experiment.
The effect of anisotropic intermolecular interactions on the pressure response of polymeric fullerenes
Authors: Arvanitidis J. , Assimopoulos S. , Papagelis K. , Ves S. , Prassides K. , Iwasa Y. , Kourouklis G.A.
Published: February 2003

High pressure Raman scattering was used to probe the pressure response of the two‐dimensional tetragonal and rhombohedral C60 polymers. The phonon frequencies as a function of pressure for the various Raman modes of the polymeric phases show a slope change near ∼4 GPa. As pressure increases above 4 GPa, the pressure coefficients for the majority of the modes exhibit reversible change to lower values. These changes may be attributed to a possible structural modification of the polymers from their initial state, which is characterized by highly anisotropic intermolecular interactions (strong in‐plane covalent bonds and weak out‐of‐plane van der Waals interactions), to a more isotropic one upon application of the external pressure. This behavior is a common theme of other fullerene materials displaying bonding anisotropy.
Thermoactivated conductivity in p‐GaAs/Al0.5Ga0.5As below 5 K under combined influence of illumination and uniaxial stress
Authors: Kraak W. , Minina N. Ya. , Ilievsky A. A. , Sorensen C. B. , Berman I. V.
Published: February 2003

Strongly thermoactivated negative photoconductivity in p‐GaAs/Al0.5Ga0.5As doped with Be is observed under the combined influence of illumination by a red light emitting diode and uniaxial compression at liquid helium temperatures. The effect can be described in a model with deep donor‐like traps at the heterointerface below the Fermi level, if a barrier between the ground and excited donor‐like states EB = 6 meV is introduced. The barrier does not depend on uniaxial stress that governs the strong resistivity on temperature dependence mainly by the reduction of 2D hole concentration and 2D hole mobility both in dark and under illumination.
Reliable non‐linear elastic constants
Authors: Dunstan D. J. , Bosher S. H. B.
Published: February 2003

The values given in the literature for the third‐order elastic constants cIJK of the standard semiconductors have long been suspect. We have applied modern statistical analysis to the raw data from which the databook cIJK were obtained, and we give revised values with reliable error estimates for Si, Ge and GaAs. The databook cIJK are appropriate for an expansion of the elastic energy of a body as a Taylor series in Lagrangian strain ηIJ, but there are formidable mathematical difficulties in using Lagrangian strain in the analysis of physical problems. To facilitate such analysis, we introduce effective second‐order elastic constants for materials under finite strain. From these, strain, stress and pressure derivatives are readily calculated. The pressure derivatives are given for Si, Ge and GaAs. Using the stress derivatives, the relationships between physical observables such as acoustic velocities and the databook cIJK can be obtained by simple algebra.
High‐pressure photoluminescence study of the electronic structure of InP/GaP quantum dots
Authors: Kristukat C. , Goñi A.R. , Hatami F. , Dreßler S. , Masselink W.T. , Thomsen C.
Published: February 2003

The electronic subband structure of self‐assembled InP/GaP quantum dots (QDs) has been investigated by means of photoluminescence (PL) measurements as a function of hydrostatic pressure up to 8 GPa and temperature. At ambient pressure the PL emission of the sample arises from direct optical transitions between the lowest electron and hole Γ‐point states confined in the QDs. At a very low pressure of about 0.15 GPa, the Γ–X conduction band crossover occurs, after which the PL emission of the dots becomes roughly 20 times weaker in intensity and its energy exhibits the slight redshift typical of indirect recombination processes from the conduction band X valleys. Our results indicate a type‐I band alignment for the strained InP/GaP dot structure at low pressure and yield a value of 300 ± 30 meV for the valence‐band offset.
Correlation between lasing properties and band alignment of edge emitting lasers with (Ga,In)(N,As)/Ga(N,As) active regions
Authors: Grüning H. , Klar P. J. , Heimbrodt W. , Weiser G. , Koch J. , Nau S. , Stolz W. , Fehse R. , Adams A. R. , Ramakrishnan A. , Ebbinghaus G.
Published: February 2003

We have investigated the optical properties of edge‐emitting laser structures containing three Ga0.7In0.3N0.005As0.995 quantum wells embedded in GaNxAs1–x barriers grown by metal‐organic vapour‐phase epitaxy. In a series of three samples the nitrogen content x of the barrier was varied from 0% to 3%. We studied the optical transitionsand the threshold of these laser structures by using pressure‐dependent photomodulated reflectance (PR) up to 20 kbar at room temperature. Additionally we measured the pressure dependence of the lasing energy and the threshold current of the corresponding laser structures as function of hydrostatic pressure. Due to the large red‐shift of the Ga(N,As) band gap with increasing N of about 150 meV per percent N, the variation of x leads to a considerable change of the carrier confinementof holes and, in particularly, of electrons. The strong increase of the threshold current of the laser device with pressure suggests a swiftly increasing threshold carrier density due to the increasing effective mass and non‐parabolicity.an enlarged carrier density at threshold Comparing the pressure dependence of the lasing energy and the conduction band edge furthersupports this conclusion.
Pressure dependence of optical phonons in ZnCdSe alloys
Authors: Camacho J. , Loa I. , Cantarero A. , Syassen K. , Hernández‐Calderón I. , González L.
Published: February 2003

We report on resonant Raman scattering measurements under hydrostatic pressure and at low temperature on two ZnxCd1–xSe alloy films, which were grown by different epitaxial techniques on GaAs. This ternary alloy exhibits an intermediate (I) mode between the typical transverse (TO) and longitudinal optical (LO) modes of the zinc‐blende structure. The shift of the vibrational frequencies of the three modes with pressure is analyzed. The Grüneisen parameters of these three optical modes are determined. Moreover, overtones of the LO modes as well as TO + nLO and I + nLO zone‐center phonon combinations are observed.
Ab initio study of BAs under pressure
Authors: Herrera‐Cabrera M. J. , Rodríguez‐Hernández P. , Muñoz A.
Published: February 2003

We use an ab initio total energy pseudopotential technique within the Density Functional Theory in the local‐density approximation to determine the vibrational normal modes (phonons) defined within the first Brillouin Zone of BAs. We also study the effects of hydrostatic pressure on these phonons and the mode Grüneisen parameters using the response‐function formalism for the computation. There is a very good agreement between the transversal optical phonon frequency at the Γ point calculated on this way and that with the frozen phonon technique. We also study the electronic band structure, the band‐gap pressure dependence and the generalized stability criteria.
Theoretical study of ZnS under high pressure
Authors: López‐Solano J. , Mujica A. , Rodríguez‐Hernández P. , Muñoz A.
Published: February 2003

The structural phase transformations under pressure of ZnS are studied within the density functional formalism, using the local density approximation and pseudopotential scheme. We show results in agreement with the well‐established zinc‐blende and NaCl phases present at low pressures, and also confirm a recently proposed Cmcm structure found experimentally by Desgreniers et al. [Phys. Rev. B 61, 8726 (2000)] to be more stable than NaCl at pressures higher than 65 GPa.
First‐principles study of phonon‐mode softening under pressure: the case of GaN and AlN
Authors: Wagner J.‐M. , Bechstedt F.
Published: February 2003

The origin of the mode softening under pressure of the low‐frequency E2 phonon of GaN and AlN is investigated by first‐principles density‐functional calculations. Using the eigenvectors of these modes as obtained from diagonalizing a dynamical matrix which is calculated using density‐functional perturbation theory, a frozen‐phonon calculation shows that the softening is mainly caused by an increased destabilization due to the Ewald energy, which is nearly counterbalanced by the other contributions to the total energy in the case of AlN, but only partially so for GaN.
Wavelength dependence of the modal refractive index in 1.3 μm InGaAsP, AlGaInAs and GaInNAs lasers using high pressure
Authors: Jin S. R. , Sweeney S. J. , Adams A. R. , Higashi T. , Riechert H. , Thijs P. J. A.
Published: February 2003

We report the variation of the modal refractive index at threshold with lasing wavelength in 1.3 μm InGaAsP, AlGaInAs and GaInNAs multiple quantum well lasers by measuring mode spectra under pressure up to 15 kbar. It is shown that the decrease of index with lasing wavelength is about 4% on average in the pressure range studied. We also find that the modal refractive index reduces with increasing injection current and is very sensitive to the injection current around laser threshold.