Showing papers by "Eindhoven University of Technology published in 2002"

Exploiting the 60 GHz band for local wireless multimedia access: prospects and future directions

Abstract: This article addresses basic issues regarding the design and development of wireless access and wireless LAN systems that will operate in the 60 GHz band as part of the fourth-generation (4G) system. The 60 GHz band is of much interest since this is the band in which a massive amount of spectral space (5 GHz) has been allocated worldwide for dense wireless local communications. The article gives an overview of 60 GHz channel characteristics and puts them in their true perspective. In addition, we discuss how to achieve the exploitation of the abundant bandwidth resource for all kinds of short-range communications. The main tenor is that an overall system architecture should be worked out that provides industry with plenty of scope for product differentiation. This architecture should feature affordability, scalability, modularity, extendibility, and interoperability. In addition, user convenience and easy and efficient network deployment are important prerequisites for market success. This article discusses these features and indicates a number of key research topics.

YAWL: yet another workflow language

Abstract: Based on a rigorous analysis of existing workflow management systems and workflow languages, a new workflow language is proposed: yet another workflow language (YAWL). To identify the differences between the various languages, we have collected a fairly complete set of workflow patterns. Based on these patterns we have evaluated several workflow products and detected considerable differences in their ability to capture control flows for non-trivial workflow processes. Languages based on Petri nets perform better when it comes to state-based workflow patterns. However, some patterns (e.g. involving multiple instances, complex synchronisations or non-local withdrawals) are not easy to map onto (high-level) Petri nets. This inspired us to develop a new language by taking Petri nets as a starting point and adding mechanisms to allow for a more direct and intuitive support of the workflow patterns identified. This paper motivates the need for such a language, specifies the semantics of the language, and shows that soundness can be verified in a compositional way. Although YAWL is intended as a complete workflow language, the focus of this paper is limited to the control-flow perspective.

Multi-scale constitutive modelling of heterogeneous materials with a gradient-enhanced computational homogenization scheme

Abstract: A gradient-enhanced computational homogenization procedure, that allows for the modelling of microstructural size effects, is proposed within a general non-linear framework. In this approach the macroscopic deformation gradient tensor and its gradient are imposed on a microstructural representative volume element (RVE). This enables us to incorporate the microstructural size and to account for non-uniform macroscopic deformation fields within the microstructural cell. Every microstructural constituent is modelled as a classical continuum and the RVE problem is formulated in terms of standard equilibrium and boundary conditions. From the solution of the microstructural boundary value problem, the macroscopic stress tensor and the higher-order stress tensor are derived based on an extension of the Hill-Mandel condition. This automatically delivers the microstructurally based constitutive response of the higher-order macro continuum and deals with the microstructural size in a natural way. Several examples illustrate the approach, particularly the microstructural size effects.

Vicious and virtuous cycles in ERP implementation : a case study of interrelations between critical success factors

Abstract: ERP implementations are complex undertakings. Recent research has provided us with plausible critical success factors (CSFs) for such implementations. This article describes how one list of CSFs (Somers & Nelson, 2001) was used to analyse and explain project performance in one ERP implementation in the aviation industry. In this particular case, poor project performance led to a serious project crisis but this situation was turned around into a success. The list of CSFs employed was found to be helpful and appropriate in explaining both the initial failure and the eventual success of the implementation. CSFs in this case appeared to be highly correlated, ie changes in any one of them would influence most of the others as well. The reversal in performance after the project crisis was caused by substantial changes in attitudes with most of the stakeholders involved, such as top management, project management, project champion and software vendor.

Macromolecules containing bipyridine and terpyridine metal complexes: towards metallosupramolecular polymers.

Abstract: The ability of a broad range of N-heterocycles to act as very effective and stable complexation agents for several transition metal ions, such as cobalt(II), copper(II), nickel(II), and ruthenium(II), has long been known in analytical chemistry. This behavior was later utilized in supramolecular chemistry for the construction of highly sophisticated architectures, such as helicates, racks, and grids. The discovery of macromolecules by Staudinger in 1922 opened up avenues towards sophisticated materials with properties hitherto completely unknown. In the last few decades, the combination of macromolecular and supramolecular chemistry has been attempted by developing metal-complexing and metal-containing polymers for a wide variety of applications that range from filtration to catalysis. The stability of the polymer-metal complex is a fundamental requirement for such applications. In this respect, the use of bi- and terpyridines as chelating ligands is highly promising, since these molecules are known to form highly stable complexes with interesting physical properties with transition-metal ions. A large number of different structures have been designed for many different applications, but polymers based on the application of coordinative forces have been prepared in a few cases only. Furthermore, the synthetic procedures applied frequently resulted in low yields. During the last few years, strong efforts have been made in the direction of self-assembling and supramolecular polymers as novel materials with "intelligent" and tunable properties. In this review, an overview of this active area at the interface of supramolecular and macromolecular chemistry is given.

Plasma needle: a non-destructive atmospheric plasma source for fine surface treatment of (bio)materials

Abstract: A non-thermal plasma source (`plasma needle') generated under atmospheric pressure by means of radio-frequency excitation has been characterized. Plasma appears as a small (sub-mm) glow at the tip of a metal pin. It operates in helium, argon, nitrogen and mixtures of He with air. Electrical measurements show that plasma needle operates at relatively low voltages (200–500 V peak-to-peak) and the power consumption ranges from tens of milliwatts to at most a few watts. Electron-excitation, vibrational and rotational temperatures have been determined using optical emission spectroscopy. Excitation and vibration temperatures are close to each other, in the range 0.2–0.3 eV, rotational gas temperature is at most a few hundred K. At lowest power input the source has the highest excitation temperature while the gas remains at room temperature. We have demonstrated the non-aggressive nature of the plasma: it can be applied on organic materials, also in watery environment, without causing thermal/electric damage to the surface. Plasma needle will be used in the study of plasma interactions with living cells and tissues. At later stages, this research aims at performing fine, high-precision plasma surgery, like removal of (cancer) cells or cleaning of dental cavities.

Active shape model segmentation with optimal features

Abstract: An active shape model segmentation scheme is presented that is steered by optimal local features, contrary to normalized first order derivative profiles, as in the original formulation [Cootes and Taylor, 1995, 1999, and 2001]. A nonlinear kNN-classifier is used, instead of the linear Mahalanobis distance, to find optimal displacements for landmarks. For each of the landmarks that describe the shape, at each resolution level taken into account during the segmentation optimization procedure, a distinct set of optimal features is determined. The selection of features is automatic, using the training images and sequential feature forward and backward selection. The new approach is tested on synthetic data and in four medical segmentation tasks: segmenting the right and left lung fields in a database of 230 chest radiographs, and segmenting the cerebellum and corpus callosum in a database of 90 slices from MRI brain images. In all cases, the new method produces significantly better results in terms of an overlap error measure (p<0.001 using a paired T-test) than the original active shape model scheme.

Knowledge Spillovers in Europe: A Patent Citations Analysis

Abstract: This paper addresses the pattern of knowledge flows as indicated by patent citations between European regions. Our findings support the hypothesis that there are important barriers to knowledge flows in Europe. Patent citations occur more often between regions which belong to the same country and which are in geographical proximity. Furthermore, patent citations are industry specific and occur most often between regions that are specialised in industrial sectors with specific technological linkages between them. Patent citations are also more frequent when the citing region belongs to the same linguistic group as the cited region.

A low-bandgap semiconducting polymer for photovoltaic devices and infrared emitting diodes

Abstract: A novel low-bandgap conjugated polymer (PTPTB, E-g = similar to1.6 eV), consisting of alternating electron-rich N-dodecyl-2,5-bis(2'-thienyl)pyrrole (TPT) and electron-deficient 2,1,3-benzothiadiazole (B) units, is introduced for thin-film optoelectronic devices working in the near infrared (NIR). Bulk heterojunction photovoltaic cells from solid-state composite films of PTPTB with the soluble fullerene derivative [6,6]-phenyl C-61 butyric acid methyl ester (PCBM) as an active layer shows promising power conversion efficiencies up to 1% under AM1.5 illumination. Furthermore, electroluminescent devices (light-emitting diodes) from thin films of pristine PTPTB show near infrared emission peaking at 800 nm with a turn on voltage below 4 V. The electroluminescence can be significantly enhanced by sensitization of this material with a wide bandgap material such as the poly(p-phenylene vinylene) derivative MDMO-PPV.

All-optical probe of coherent spin waves.

Abstract: A novel, all-optical method to excite and detect spin waves in magnetic materials is presented. By exploiting the temperature dependence of the magnetic anisotropy, an ultrashort laser pulse is efficiently converted in a picosecond "anisotropy field" pulse that triggers a coherent precession of the magnetization. Recording the temporal evolution of the precessing spins by a time-delayed probe-pulse provides a quantitative method to study locally the magnetic anisotropy, as well as switching and damping phenomena in micromagnetic structures. Applications to nickel and permalloy ( Ni80Fe20) films are discussed, particularly showing the possibility to explore standing spin waves in thin films.

Sorptive sample preparation – a review

Abstract: Most sample-enrichment procedures currently available rely on adsorption of the analytes of interest by a suitable adsorbent material. Although good performance can be obtained for many practical problems, in some cases the applicability of adsorptive sample preparation falls short, particularly for the enrichment of polar and/or high-molecular-weight compounds, especially in combination with thermal desorption. Because of the very strong retention of adsorbent materials, undesired effects such as incomplete desorption and artifact formation are observed. Polar solutes are easily adsorbed but readily undergo surface-catalyzed reactions and on desorption yield compounds different than those originally sampled. High-molecular-weight compounds cannot be desorbed because of extremely strong interactions with the adsorbent and their low volatility. To overcome some of these problems sample-preparation techniques based on polydimethylsiloxane sorption have been developed over the past 15 years. In contrast with adsorptive trapping, sorption is based on dissolution of the analytes in a liquid polymeric material. This is a much more inert means of solute retention which overcomes some of the limitations encountered when working with adsorbents. In this contribution, the basic principles of sorption, the different instrumentation used, and applications of the technique will be reviewed. The review covers the sorptive sample-preparation techniques, open-tubular trapping (OTT), solid-phase microextraction (SPME), gum-phase extraction (GPE), equilibrium gum-phase extraction (EGPE), and stir-bar-sorptive extraction (SBSE). Because of the nature of sorptive sample-preparation techniques, which perform particularly well in combination with thermal desorption, this review focuses strongly on gas chromatography as the means of chemical analysis.

C3-Symmetrical Supramolecular Architectures: Fibers and Organic Gels from Discotic Trisamides and Trisureas

Abstract: Hydrogen bonded C(3)-symmetrical molecules that associate into supramolecular stacks are described. Structural mutation on these molecules has been performed to elucidate the contribution of the different secondary interactions (hydrogen bonding, pi-pi stacking) to the self-assembly of the disks into chiral stacks. Twelve C(3)-symmetrical molecules have been investigated, six of which contain three central amide functionalities (1a-f) and six of which contain three central urea groups (2a-f). Peripheral groups of the disks are "small", "medium", or "large", half of them being achiral and the other half being chiral, to enable investigation of the supramolecular architectures with CD spectroscopy. In all of the cases, elongated, helical stacks are formed in apolar solution, except for the "medium" amide disks 1c/d. The elongated stacks of the C(3)-symmetrical disks form gels, which are visualized by AFM and SANS, and this confirms the directionality of the interactions. For the "large" urea disk, 2f, fibers with a length of up to 2 microm are observed. Temperature dependent and "sergeants-and-soldiers" CD measurements reveal that the urea stacks are much more rigid than the corresponding amide ones. In case of the "medium" urea disks, 2c/d, a true rigid rod, is formed. Where amide disks immediately reach their thermodynamic equilibrium, kinetic factors seem to govern urea aggregation. In a number of experiments aimed at reversibility with the urea stacks, hysteresis is observed, implying that these urea disks initially form a poorly defined stack, which subsequently transforms slowly into a well-defined, chiral architecture.

Postoperative Cognitive Dysfunction in Middle-aged Patients

Abstract: Background: Postoperative cognitive dysfunction (POCD) after noncardiac surgery is strongly associated with increasing age in elderly patients; middle-aged patients (aged 40-60 yr) may be expected to have a lower incidence, although subjective complaints are frequent. Metbods: The authors compared the changes in neuropsychological test results at 1 week and 3 months in patients aged 40-60 yr, using a battery of neuropsychological tests, with those of age-matched control subjects using Z-score analysis. They assessed risk factors and associations of POCD with measures of subjective cognitive function, depression, and activities of daily living. Results: At 7 days, cognitive dysfunction as defined was present in 19.2% (confidence interval [CI], 15.7-23.1) of the patients and in 4.0% (CI, 1.6-8.0) of control subjects (P < 0.001). After 3 months, the incidence was 6.2% (CI, 4.1-8.9) in patients and 4.1% (CI, 1.7-8.4) in control subjects (not significant). POCD at 7 days was associated with supplementary epidural analgesia and reported avoidance of alcohol consumption. At 3 months, 29% of patients had subjective symptoms of POCD, and this finding was associated with depression. Early POCD was associated with reports of lower activity scores at 3 months. Conclusions: Postoperative cognitive dysfunction occurs frequently but resolves by 3 months after surgery. It may be associated with decreased activity during this period. Subjective report overestimates the incidence of POCD. Patients may be helped by recognition that the problem is genuine and reassured that it is likely to be transient.

Role of Crystalline Defects in Electrocatalysis: Mechanism and Kinetics of CO Adlayer Oxidation on Stepped Platinum Electrodes

Abstract: The kinetics of the electrochemical oxidation of a CO adlayer on Pt[n(111)×(111)] electrodes in 0.5 M H2SO4 has been studied using chronoamperometry. The objective is to elucidate the effect of the crystalline defects on the rate of the reaction by using a series of stepped surfaces. The reaction kinetics of the main oxidative process can be modeled using the mean-field approximation for the Langmuir−Hinshelwood mechanism, implying fast diffusion of adsorbed CO on the Pt[n(111)×(111)] surfaces under electrochemical conditions. The apparent rate constant for the electrochemical CO oxidation, determined by a fitting of the experimental data with the mean-field model, is found to be proportional to the step fraction (1/n) for the surfaces with n > 5, proving steps to be the active sites for the CO adlayer oxidation. An apparent intrinsic rate constant is determined. The potential dependence of the apparent rate constants is found to be structure insensitive with a Tafel slope of ca. 80 mV/dec, suggesting the...

Interisotope Determination of Ultracold Rubidium Interactions from Three High-Precision Experiments

Abstract: Combining the measured binding energies of four of the most weakly bound rovibrational levels of the ${}^{87}{\mathrm{Rb}}_{2}$ molecule with results of two other recent high-precision experiments, we obtain exceptionally strong constraints on the atomic interaction parameters in a highly model independent analysis. The comparison of ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ data, where the two isotopes are related by a mass scaling procedure, plays a crucial role. We predict scattering lengths, clock shifts, and Feshbach resonances with an unprecedented level of accuracy. Two of the Feshbach resonances occur at easily accessible magnetic fields in mixed-spin channels. One is related to a $d$-wave shape resonance.

Image based flow visualization

Abstract: A new method for the visualization of two-dimensional fluid flow is presented. The method is based on the advection and decay of dye. These processes are simulated by defining each frame of a flow animation as a blend between a warped version of the previous image and a number of background images. For the latter a sequence of filtered white noise images is used: filtered in time and space to remove high frequency components. Because all steps are done using images, the method is named Image Based Flow Visualization (IBFV). With IBFV a wide variety of visualization techniques can be emulated. Flow can be visualized as moving textures with line integral convolution and spot noise. Arrow plots, streamlines, particles, and topological images can be generated by adding extra dye to the image. Unsteady flows, defined on arbitrary meshes, can be handled. IBFV achieves a high performance by using standard features of graphics hardware. Typically fifty frames per second are generated using standard graphics cards on PCs. Finally, IBFV is easy to understand, analyse, and implement.

Spectroscopic Studies of Photoexcitations in Regioregular and Regiorandom Polythiophene Films

Abstract: Using a variety of optical probe techniques we studied the steady state and transient dynamics of charged and neutral photoexcitations in thin films of poly-3-alkyl thiophene with regioregular order, which forms self-assembled lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chain-like morphology. In regiorandom polythiophene films we found that intrachain excitons with correlated photoinduced absorption and stimulated emission bands are the primary photoexcitations; they give rise to a moderately strong photoluminescence band, and long-lived triplet excitons and intrachain charged polarons. In regioregular polythiophene films, on the contrary we found that the primary photoexcitations are excitons with much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and a very weak photoluminescence band. The long-lived photoexcitations in regioregular polythiophene films are interchain excitons and delocalized polarons (DP) within the lamellae, with very small relaxation energy. The characteristic properties of the DP species are thoroughly investigated as a function of the alkyl side group of the polymer backbone, film deposition conditions and solvents used, as well as at high hydrostatic pressure. The quantum interference between the low energy absorption band of the DP species and a series of photoinduced infrared active vibrations, which give rise to antiresonances that are superimposed on the electronic absorption band is studied and explained by a Fano-type interference mechanism, using the amplitude mode model.