Showing papers by "Eindhoven University of Technology published in 1995"

The context-tree weighting method: basic properties

Abstract: Describes a sequential universal data compression procedure for binary tree sources that performs the "double mixture." Using a context tree, this method weights in an efficient recursive way the coding distributions corresponding to all bounded memory tree sources, and achieves a desirable coding distribution for tree sources with an unknown model and unknown parameters. Computational and storage complexity of the proposed procedure are both linear in the source sequence length. The authors derive a natural upper bound on the cumulative redundancy of the method for individual sequences. The three terms in this bound can be identified as coding, parameter, and model redundancy, The bound holds for all source sequence lengths, not only for asymptotically large lengths. The analysis that leads to this bound is based on standard techniques and turns out to be extremely simple. The upper bound on the redundancy shows that the proposed context-tree weighting procedure is optimal in the sense that it achieves the Rissanen (1984) lower bound. >

Polystyrene-Dendrimer Amphiphilic Block Copolymers with a Generation-Dependent Aggregation

Abstract: A class of amphiphilic macromolecules has been synthesized by combining well-defined polystyrene (PS) with poly(propylene imine) dendrimers. Five different generations, from PS-dendr-NH(2) up to PS-dendr-(NH(2))(32), were prepared in yields of 70 to 90 percent. Dynamic light scattering, conductivity measurements, and transmission electron microscopy show that in aqueous phases, PS-dendr-(NH(2))(32) forms spherical micelles, PS-dendr-(NH(2))(16) forms micellar rods, and PS-dendr-(NH(2))(8) forms vesicular structures. The lower generations of this class of macromolecules show inverted micellar behavior. The observed effect of amphiphile geometry on aggregation behavior is in qualitative agreement with the theory of Israelachvili et al. The amphiphiles presented here are similar in shape but different in size as compared with traditional surfactants, whereas they are similar in size but different in shape as compared with traditional block copolymers.

Electrochemical codeposition of inert particles in a metallic matrix

Abstract: A survey on electrochemical codeposition of inert particles in a metallic matrix is given. Particles held in suspension in an electroplating bath are codeposited with the metal during electrodeposition. The particles used are inert to the bath and can be of different types, that is, pure metals, ceramics or organic materials. Combining this variety of types of particles with the different electrodeposited metals, electrochemical codeposition enables the production of a large range of composite materials with unique properties. Many experimental factors were found to influence the codeposition process, which led to some understanding of the mechanism. Models to predict the codeposition rate were developed, but were only partly successful.

Concepts in Theoretical Heterogeneous Catalytic Reactivity

Abstract: Introduction A. General The heart of many commercial catalytic processes involves chemistry on transition metal particles and surfaces. The success in designing active surface ensembles, promoters, and selective poisons is inevitably tied to our knowledge of the fundamental principles which control transition metal surface chemistry. One extreme would be the rigorous description and energetic predictions for each elementary reaction step of an entire catalytic cycle from first-principle theoretical methods. While desirable, this has to date been an unattainable goal due to the limitations in both raw computer (CPU) requirements and the accuracy of the available computational methods. Recent advances in both quantum-chemical methods and computational resources, however, are driving this goal closer to reality. Theoretical treatments of adsorbate-surface interactions have rapidly advanced to the stage where detailed understandings of the governing structural and electronic features are readily available. In...

Resonances in ultracold collisions of 6Li, 7Li, and 23Na

Abstract: We investigate the resonance properties of ultracold ground state $^{6}\mathrm{Li}$${+}^{6}$Li, $^{7}\mathrm{Li}$${+}^{7}$Li, and $^{23}\mathrm{Na}$${+}^{23}$Na collisions. The locations of various resonances and their corresponding error bounds due to the uncertainty of the interatomic potentials are presented. Also, the resonance widths are computed using rigorous coupled-channel calculations, as well as a modified version of Feshbach theory valid for strong fields.

Luminescence quenching by OH groups in highly Er-doped phosphate glasses

Abstract: Highly (up to 4 mol% Er2O3) Er-doped phosphate bulk glasses have been prepared by common glass-melting techniques. Afterwards, a heat treatment was performed on the as-melted samples. The photoluminescence lifetime of Er ions for the 4I132–4I152 transition increases substantially, typically from 3 ms up to 7 ms for a sample doped with 2 mol% Er2O3, due to the heat treatment. The increase of the lifetime is ascribed to a decrease in concentration of hydroxyl groups incorporated in the glass, which is confirmed by IR absorption spectroscopic measurements. The photoluminescence peak intensity also increases because of drying by a factor of 3 to 7 depending on glass composition. Based on electric dipole-dipole interaction theory, the luminescence concentration quenching mechanism by hydroxyl groups is modelled. The model predicts that more than half of the hydroxyl groups in the glass is coupled to Er ions. The influence of the glass structure and role of Al3+ on the Er3+ luminescence is studied by infrared spectroscopy.

Single-rail handshake circuits

Abstract: Single-rail handshake circuits are introduced as a cost effective implementation of asynchronous circuits. Compared to double-rail implementations, the circuits are smaller, faster, and more energy-efficient. Furthermore, in contrast to common belief, all four phases of the four-phase handshake protocol can be productive. An important selling point for single-rail circuits is that they can be implemented in any (generic) standard-cell library. This facilitates technology migration and makes asynchronous circuits a potential technology of choice for low-power applications.

Methodologies for information systems investment evaluation at the proposal stage : a comparative review

Abstract: The evaluation of information systems (IS) investments has been a recognized problem area for the last three decades, but has recently gained renewed interest of both management and academics. IS investments constitute a large and increasing portion of the capital expenditures of many organizations. However, it is difficult to evaluate the contribution of an IS investment to the goals pursued. Consequently, there is a great call for methods and techniques that can be of help in evaluating IS investment at the proposal stage. The contribution of the paper to the problem area is twofold. First, the different concepts which are used in evaluation are discussed and more narrowly defined. When speaking about IS investments, concepts are used that originate from different disciplines. In many cases there is not much agreement on the precise meaning of the different concepts used. However, a common language is a prerequisite for the successful communication between the different organizational stakeholders in evaluation. In addition to this, the paper reviews the current methods and puts them into a frame of reference. All too often new methods and guidelines for investment evaluation are introduced, without building on the extensive body of knowledge that is already incorporated in the available methods. Four basic approaches are discerned: the financial approach, the multi-criteria approach, the ratio approach and the portfolio approach. These approaches are subsequently compared on a number of characteristics on the basis of methods that serve as examples for the different approaches. The paper concludes with suggestions on how to improve evaluation practice and recommendations for future research.

Axiomatizing probabilistic processes: ACP with generative probabilities

Abstract: This paper is concerned with finding complete axiomatizations of probabilistic processes. We examine this problem within the context of the process algebra ACP and obtain as our end-result the axiom system prACP I −- , a probabilistic version of ACP which can be used to reason algebraically about the reliability and performance of concurrent systems. Our goal was to introduce probability into ACP in as simple a fashion as possible. Optimally, ACP should be the homomorphic image of the probabilistic version in which the probabilities are forgotten.

A new method for parameterization of phase shift and backscattering amplitude

Abstract: Parameterization of phase and backscattering amplitude with cubic splines is described. Using the cubic spline, the analytical partial derivatives of the plane wave EXAFS function can be calcalated. The use of analytical partial derivatives decreases the CPU time needed for a refinement by over 60% for a three shell system compared to a refinement with partial derivaties calculated with the finite difference method.

DARWIN: CMOS opamp Synthesis by Means of a Genetic Algorithm

Abstract: DARWIN is a tool that is able to synthesize CMOS opamps, on the basis of a genetic algorithm. A randomly generated initial set of opamps evolves to a set in which the topologies as well as the transistor sizes of the opamps are adapted to the required performance specifications. Several design examples illustrate the behavior of DARWIN.

Stereomutation in optically active regioregular polythiophenes

Abstract: It is shown that stereomutation in thin films of optically active head-to-tail poly[3-(2-[(S)-2-methylbutoxy]ethyl)thiophene] can be observed and modulated via the thermal history of the material. The use of these materials for reversible optical recording is discussed

Applications of magnetoelectrolysis

Abstract: A broad overview of research on the effects of imposed magnetic fields on electrolytic processes is given. As well as modelling of mass transfer in magnetoelectrolytic cells, the effect of magnetic fields on reaction kinetics is discussed. Interactions of an imposed magnetic field with cathodic crystallization and anodic dissolution behaviour of metals are also treated. These topics are described from a practical point of view.

Negative ions in a radio-frequency oxygen plasma.

Abstract: A systematic study of a low pressure (5--200 mTorr) oxygen discharge is presented. Measurements of the electron and negative ion densities in the 13.56 MHz capacitively coupled plasma are performed by means of a microwave resonance technique in combination with photodetachment. A kinetic model is developed, incorporating volume and wall reactions of ions as well as neutral species. It is shown, by matching the experimental results with the model, that the dominant ion is ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ whereas the ${\mathrm{O}}_{2}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{O}}_{3}^{\mathrm{\ensuremath{-}}}$ densities reach 10 to 20 % of the total negative ion density. The ratio of negative ion to electron density varies between 5 and 10 and decreases with pressure and rf power. The total negative ion density is about 5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$, it increases with gas flow, is independent of the rf input power, and has a maximum at a pressure of 30 mTorr. The agreement between the measurements and the model is within the experimental error for a wide range of conditions. Deviations are explained by changes in the gas and wall temperatures. From the known ion density the effective ionization rate has been determined and related to the electron temperature. The ionization temperature obtained in this way varies between 2 and 4 eV and decreases with increasing gas pressure and power, as expected for the bulk electron temperature in this type of discharge.

Parallel local search

Abstract: We present a survey of parallel local search algorithms in which we review the concepts that can be used to incorporate parallelism into local search For this purpose we distinguish between single-walk and multiple-walk parallel local search and between asynchronous and synchronous parallelism Within the class of single-walk algorithms we differentiate between multiple-step and single-step parallelism To describe parallel local search we introduce the concepts of hyper neighborhood structures and distributed neighborhood structures Furthermore, we present templates that capture most of the parallel local search algorithms proposed in the literature Finally, we discuss some complexity issues related to parallel local search

Rotating stall and surge control: a survey

Abstract: The paper presents an analysis of the current state of the art in the control of aero- or hydrodynamic instabilities in turbomachines. It describes the flow phenomena associated with rotating stall and surge, discusses methods devised to prevent these instabilities occuring, but concentrates mainly on the active control (stabilization) of the unstable flows. It appears that lately significant progress has been made in this area. It seems to foster to a more mature state, although several problems deserve further consideration. The consequences of this state of the art for several interested parties, researchers, developers, manufacturers, and users, are stipulated.

A congruence theorem for structured operational semantics with predicates and negative premises

Abstract: We proposed a syntactic format, the panth format, for structured operational semantics in which besides ordinary transitions also predicates, negated predicates, and negative transitions may occur such that if the rules are stratifiable, strong bisimulation equivalence is a congruence for all the operators that can be defined within the panth format. To show that this format is useful we took some examples from the literature satisfying the panth format but no formats proposed by others. The examples touch upon issues such as priorities, termination, convergence, discrete time, recursion, (infinitary) Hennessy-Milner logic, and universal quantification.

On gradient-enhanced damage and plasticity models for failure in quasi-brittle and frictional materials

Abstract: Gradient-enhanced damage and plasticity approaches are reviewed with regard to their ability to model localization phenomena in quasi-brittle and frictional materials. Emphasis is put on the algorithmic aspects. For the purpose of carrying out large-scale finite element simulations efficient numerical treatments are outlined for gradient-enhanced damage and gradient-enhanced plasticity models. For the latter class of models a full dispersion analysis is presented at the end of the paper. In this analysis the fundamental role of dispersion in setting the width of localization bands is highlighted.

Chapter 3 Progress in spinel ferrite research

Abstract: Publisher Summary This chapter discusses the progress in spinel ferrite research. A large number of oxides with a metal–oxygen ratio of 0.75 as composition is known to crystallize into the spinel structure. Among these oxides, magnetite Fe 3 O 4 is an important compound, from which the spinel ferrites can be derived by partial substitution of the iron ions by other cations. Between most of the spinel oxides solid solutions can be formed, which means that a great variety of spinel oxides is possible. The simple spinel ferrites are those ferrites in which x = 0 and the bivalent metal ion is Mg or Cd or one of the bivalent transition metal elements Mn, Fe, Co, Ni, Cu, and Zn. Partial solid solutions of the simple ferrites are also possible with compounds that either do not exist or that do not have the spinel structure. A great variety in the chemical composition of spinel ferrites is possible. The diversity in composition results into a large range of physical properties that permits the tuning of the properties for specific applications and makes spinel ferrites of particular interest.

An investigation on the reaction mechanism for the partial oxidation of methane to synthesis gas over platinum

Abstract: The partial oxidation of methane to synthesis gas has been investigated by admitting pulses of pure methane, pure oxygen and mixtures of methane and oxygen to platinum sponge at temperatures ranging from 973 to 1073 K. On reduced platinum the decomposition of methane results in the formation of surface carbon and hydrogen. No deposition of carbon occurs during the interaction of methane with a partly oxidised catalyst. Oxygen is present in three different forms under the conditions studied: platinum oxide, dissolved oxygen and chemisorbed oxygen species. Carbon monoxide and hydrogen are produced directly from methane via oxygen present as platinum oxide. Activation of methane involving dissolved oxygen provides a parallel route to carbon dioxide and water. Both platinum oxide and chemisorbed oxygen species are involved in the oxidation of carbon monoxide and hydrogen. In the presence of both methane and dioxygen at a stoichiometric feed ratio the dominant pathways are the direct formation of CO and H2 followed by their consecutive oxidation. A Mars-van Krevelen redox cycle is postulated for the partial oxidation of methane: the oxidation of methane is accompanied by the reduction of platinum oxide, which is reoxidised by incorporation of dioxygen into the catalyst.