Showing papers by "École Polytechnique Fédérale de Lausanne published in 1987"

Green's-function approach to linear response in solids.

Abstract: We present a new scheme to study the linear response of crystals which combines the advantages of the dielectric-matrix and supercell (``direct'') approaches yet avoids many of their drawbacks. The numerical complexity of the algorithm is of the same order as that of a self-consistent calculation for the unperturbed system. The method is not restricted to local perturbations as the dielectric-matrix one nor to short wavelengths as the direct one. As an application, we calculate the long-wavelength optical phonons in Si and GaAs, both transverse and longitudinal, using norm-conserving pseudopotentials, and without any use of supercells.

Using tabu search techniques for graph coloring

Abstract: Tabu search techniques are used for moving step by step towards the minimum value of a function. A tabu list of forbidden movements is updated during the iterations to avoid cycling and being trapped in local minima. Such techniques are adapted to graph coloring problems. We show that they provide almost optimal colorings of graphs having up to 1000 nodes and their efficiency is shown to be significantly superior to the famous simulated annealing.

A 4-MHz CMOS Continuous-Time Filter with On-Chip Automatic Tuning

Abstract: This paper presents a 3rd order low-pass continuous-time filter with 4 MHz cut-off frequency, integrated in a 3 ?m CMOS process. The design approach is based on the direct simulation of a doubly-terminated LC ladder using capacitors and fully-balanced, current-controlled transconductance amplifiers with extended linear range. PLL techniques, involving a 8.5 MHz controlled oscillator that matches a specific part of the filter, are used to realize on-chip automatic tuning. The complete circuit features 71 dB dynamic range and consumes only 16 mW from a single 5 V supply.

Metastable Pitting of Stainless Steel

Abstract: Current transients resulting from metastabie pitting events on stainless steel at potentials below the pitting potential are studied. Metastable pits are covered during growth and exhibit a constant current density. Rupture of the cover during metastable growth leads to repassivation of the pit. If a salt film precipitates on the pit surface before the cover ruptures, pit growth is stabilized. The kinetic factors hindering the replacement of a passive film by a salt film are discussed.

An automatic phase picker for local and teleseismic events

Abstract: An automatic detection algorithm has been developed which is capable of time P -phase arrivals of both local and teleseismic earthquakes, but rejects noise bursts and transient events. For each signal trace, the envelope function is calculated and passed through a nonlinear amplifier. The resulting signal is then subjected to a statistical analysis to yield arrival time, first motion, and a measure of reliability to be placed on the P -arrival pick. An incorporated dynamic threshold lets the algorithm become very sensitive; thus, even weak signals are timed precisely. During an extended performance evaluation on a data set comprising 789 P phases of local events and 1857 P phases of teleseismic events picked by an analyst, the automatic picker selected 66 per cent of the local phases and 90 per cent of the teleseismic phases. The accuracy of the automatic picks was “ideal” (i.e., could not be improved by the analyst) for 60 per cent of the local events and 63 per cent of the teleseismic events.

Charge injection in analog MOS switches

Abstract: Charge injection in MOS analog switches, also called pass transistors or transmission gates, is approached by using the continuity equation. Experimental results show the negligible influence of substrate current which leads to a unidimensional model. An easy-to-handle simplified model is deduced and its predictions compared to the injection obtained by measurements. It is shown that this model, which can be used to implement various strategies to reduce charge injection, is valid in any realistic situation.

Spiders in natural pest control: A review

Abstract: In spite of the large number of studies about the ecology of spiders carried out in the last two decades in different types of ecosystems all over the world, the significance of these animals as natural control agents is still largely unknown. In this paper the literature about that subject is reviewed. Totally 300 scientific papers, published between 1920 and 1984, are cited here. Several European and American studies have provided evidence, that in undisturbed grassland ecosystems and forest ecosystems spiders can play an important ecological role as predators of insects and other invertebrates. Also in orchards, not treated with pesticides, that are to a certain degree comparable with forest ecosystems, spiders can be abundant predators. In contrast to that, the opinion about the predatory importance of spiders inhabiting cultivated fields is controversial. The results of some European studies indicate, that the foliage-dwelling spiders of cultivated fields, because of their low population densities, are of minor importance as predators of insects. Other European studies show, that the ground-dwelling spiders of cultivated fields are concerning their abundance a dominant predator group, those significance as control agents still is largely unknown up to the present. In rice fields (swamp ecosystems) in Asia, receiving little or no pesticides, as well as in European and American swamp ecosystems, spiders may be an important predator group. In houses in South Africa spiders were used successfully as biological control agents against flies. Zusammenfassung Spinnen in der naturlichen und biologischen Schadlingsbekampfung: Ein Literaturuberblick Weltweit wurden in den letzten 20 Jahren in verschiedenen Okosystem-Typen zahlreiche Studien uber die okologische Bedeutung der Spinnen als Pradatoren durchgefuhrt. Trotz der grosen Zahl durchgefuhrter spinnenokologischer Studien ist die Funktion der Spinnen als Pradatoren von Insekten und anderen Invertebraten heute noch weitgehend ungeklart. In vorliegender Arbeit wird die Literatur uber dieses Thema (insgesamt sind 300 wissenschaftliche Arbeiten, die zwischen 1920 und 1984 uber dieses Thema publiziert wurden, aufgefuhrt) analysiert. Mehrere europaische und amerikanische Studien haben gezeigt, das Spinnen in unbewirtschafteten Grasland-Okosystemen und Forst-Okosystemen eine grose okologische Bedeutung als Pradatoren von Insekten haben konnen. Auch in ungespritzten Obstanlagen, die man bis zu einem gewissen Grad mit Forst-Okosystemen vergleichen kann, stellen Spinnen manchmal eine haufige Pradatorengruppe dar. Im Gegensatz dazu ist die okologische Bedeutung der in Kulturfeldern lebenden Spinnen als Insektenvertilger noch weitgehend umstritten. Einige europaische Studien deuten darauf hin, das den in der Vegetationsschicht von Kulturfeldern lebenden Spinnen ihrer niedrigen Populationsdichten wegen wahrscheinlich keine grose Bedeutung als Pradatoren von Insekten zukommt. Aus anderen europaischen Studien geht hervor, das die epigaischen Spinnen der Kulturfelder bezuglich ihrer Abundanz eine dominante Pradatorengruppe darstellen, deren Funktion als Kleinrauber allerdings noch weitgehend unbekannt ist. In asiatischen Reisfeldern, die nicht oder wenig mit Pestiziden behandelt wurden, konnen Spinnen eine wichtige Bedeutung als Pradatoren von Schadlingen haben. Reisfelder sind Sumpf Okosysteme; auch in europaischen und amerikanischen Sumpfokosystemen konnen Spinnen eine abundante Pradatorengruppe darstellen. In Sudafrika wurden Spinnen erfolgreich bei der biologischen Bekampfung von Fliegen in Hausern eingesetzt.

A CMOS chopper amplifier

Abstract: A highly sensitive CMOS chopper amplifier for low-frequency applications is described. It is realized with a second-order low-pass selective amplifier using a continuous-time filtering technique. The circuit has been integrated in a 3-/spl mu/m p-well CMOS technology. The chopper amplifier DC grain is 38 dB with a 200-Hz bandwidth. The equivalent input noise is 63 nV//spl radic/Hz and free from 1/f noise. The input offset is below 5 /spl mu/V for a tuning error less than 1%. The amplifier consumes only 34 /spl mu/W.

Parameters determining the strength and toughness of particulate filled epoxide resins

Abstract: The effects of such parameters as the filler volume fraction, particle size, aspect ratio, modulus and strength of filler, resin-filler adhesion and toughness of the matrix on the stiffness, strength and toughness of particulate filled epoxide resins have been evaluated. The mechanisms of deformation and rupture in these multiphase materials are discussed, illustrated byin situ mechanical tests in the scanning electron microscope.

The Derivation of Constitutive Relations from the Free Energy and the Dissipation Function

Abstract: Publisher Summary This chapter examines the derivation of constitutive relations from the free energy and the dissipation function. Continuum mechanics allows one to establish constitutive relations, deducing them from a single pair of scalar functions characterizing the material. The simplest materials dealt with in continuum mechanics are elastic. More general processes and those taking place in more general materials are irreversible and require more constitutive relations, connecting the dissipative forces with the velocities. The orthogonality condition and the equivalent extremum principles have been established for velocities in the form of vectors or symmetric tensors. It is found that if the deformation of an elastic body is neither isothermal nor adiabatic, the strain tensor has to be supplemented by the additional independent state variable. The connection between stress and elastic strain is given by the generalized Hooke's law and connects the stress with the plastic strain and its time rate. It is found that orthogonality in velocity space, which is essentially responsible for the results, does not necessarily imply orthogonality in force space.

Vertical transport of heavy metals by settling particles in Lake Zurich

Abstract: Transport into sediments of the trace elements Cu, Zn, Pb, Cd, Cr, and Sr by settling particles was investigated in Lake Zurich; the concentrations of these elements in settling particles collected in sediment traps and in the water column were determined at different times of year. Correlations of trace element concentrations with the various main components of the settling particles (biological material, calcium carbonate, manganese and iron oxides, silicate minerals) and examination of seasonal variations both of these concentrations in particles and of particle fluxes to the sediments show that biological material is an important carrier phase (especially for Cu and Zn). Iron and manganese oxides also contribute to trace element transport; calcium carbonate is inefficient as a carrier material.

Anaerobic dechlorination of tetrachloromethane and 1,2-dichloroethane to degradable products by pure cultures of Desulfobacterium sp. and Methanobacterium sp.

Abstract: Methanobacterium thermoautotrophicum grew autotrophically and Desulfobacterium autotrophicum grew heterotrophically in the presence of one of several highly chlorinated aliphatic hydrocarbons in the low μM range. M. thermoautotrophicum transformed a portion of 1,2-dichloroethane, and ethene was identified as the product. D. autotrophicum also displayed substrate specificity; it reduced tetrachloromethane quantitatively to trichloromethane and dichloromethane, and converted 1,1,1-trichloroethane to 1,1-dichloroethane.

Formation of oligosaccharides during enzymatic lactose: Part I: State of art.

Abstract: Enzymatic lactose hydrolysis by beta-galactosidase (lactase) was investigated with respect to the formation of oligosaccharides. An analysis of the formation of oligosaccharides and their control is important in the development of technical applications for enzymatic lactose hydrolysis. The available literature data on transfer reactions of lactase were reviewed, compared, and presented in a concise tabular form. Mechanisms and possible ways of modelling enzymatic lactose hydrolysis, including formation of oligosaccharides, are presented.

Specificity of Bacillus thuringiensis delta-endotoxin

Abstract: The insecticidal activity of the delta-endotoxins of 14 Bacillus thuringiensis strains belonging to 12 subspecies was determined against Pieris brassicae, Heliothis virescens, and Spodoptera littoralis. Larvae of P. brassicae were highly susceptible to purified crystals of strains of B. thuringiensis subsp. thuringiensis and B. thuringiensis subsp. morrisoni, whereas H. virescens responded best to B. thuringiensis subsp. kenyae and B. thuringiensis subsp. kurstaki. The crystals of the B. thuringiensis subsp. entomocidus strain were the most potent against S. littoralis. It was shown that the solubility of the crystals within the gut of the three insect species is a first important step in the mode of action. Predissolution of the crystals especially enhanced the insecticidal activity against H. virescens. When in vitro-activated toxins were applied, the relative potency range varied greatly from one insect species to another. It can be concluded that at least three factors influence the potency of B. thuringiensis delta-endotoxins: the strain-related origin of the toxin, the degree of solubility of the crystals in the gut juice, and the intrinsic susceptibility of the insect to the toxin.

A 16-bit low-voltage CMOS A/D converter

Abstract: A/D converters used in telemetry, instrumentation, and measurements require high accuracy, excellent linearity, and negligible DC offset, but need not be fast. A simple and robust instrumentation A/D converter, fabricated in a low-voltage 4-/spl mu/m CMOS technology, is described. The measured overall accuracy was 16 bits. Using a digital compensation for parasitic effects, both offset and nonlinearity were below 12 /spl mu/V. With analog compensation, the offset was 60 /spl mu/V and the nonlinearity below 15 /spl mu/V. These results indicate that even higher accuracy can be achieved using higher voltage technology.

Equiaxed dendrite growth in alloys at small supercooling

Abstract: A dimensionless model for growth of equiaxed dendrites in alloys under small Peclet number conditions is presented. The behavior of this model is compared with that of Karma and Langer and to experiments of Chopra. Good agreement between both models and with experiments is found. It is shown that the maximum in growth rate,viz., minimum in tip radius with composition, depends strongly on the ratio of heat-to-solute diffusivity and on the solute distribution coefficient.

Four classes of perfectly orderable graphs

Abstract: A graph is called “perfectly orderable” if its vertices can be ordered in such a way that, for each induced subgraph F, a certain “greedy” coloring heuristic delivers an optimal coloring of F. No polynomial-time algorithm to recognize these graphs is known. We present four classes of perfectly orderable graphs: Welsh–Powell perfect graphs, Matula perfect graphs, graphs of Dilworth number at most three, and unions of two threshold graphs. Graphs in each of the first three classes are recognizable in a polynomial time. In every graph that belongs to one of the first two classes, we can find a largest clique and an optimal coloring in a linear time.

Improved Fourier and Hartley transform algorithms: Application to cyclic convolution of real data

Abstract: This paper highlights the possible tradeoffs between arithmetic and structural complexity when computing cyclic convolution of real data in the transform domain. Both Fourier and Hartley-based schemes are first explained in their usual form and then improved, either from the structural point of view or in the number of operations involved. Namely, we first present an algorithm for the in-place computation of the discrete Fourier transform on real data: a decimation-in-time split-radix algorithm, more compact than the previously published one. Second, we present a new fast Hartley transform algorithm with a reduced number of operations. A more regular convolution scheme based on FFT's is also proposed. Finally, we show that Hartley transforms belong to a larger class of algorithms characterized by their "generalized" convolution property.