Showing papers by "École Polytechnique Fédérale de Lausanne published in 1996"

Hydrophobic, Highly Conductive Ambient-Temperature Molten Salts

Abstract: New, hydrophobic ionic liquids with low melting points (<−30 °C to ambient temperature) have been synthesized and investigated, based on 1,3-dialkyl imidazolium cations and hydrophobic anions. Other imidazolium molten salts with hydrophilic anions and thus water-soluble are also described. The molten salts were characterized by NMR and elemental analysis. Their density, melting point, viscosity, conductivity, refractive index, electrochemical window, thermal stability, and miscibility with water and organic solvents were determined. The influence of the alkyl substituents in 1, 2, 3, and 4(5)-positions on these properties was scrutinized. Viscosities as low as 35 cP (for 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide (bis(triflyl)amide) and trifluoroacetate) and conductivities as high as 9.6 mS/cm were obtained. Photophysical probe studies were carried out to establish more precisely the solvent properties of 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide). The hydrophobi...

Circuit techniques for reducing the effects of op-amp imperfections: autozeroing, correlated double sampling, and chopper stabilization

Abstract: In linear IC's fabricated in a low-voltage CMOS technology, the reduction of the dynamic range due to the dc offset and low frequency noise of the amplifiers becomes increasingly significant. Also, the achievable amplifier gain is often quite low in such a technology, since cascoding may not be a practical circuit option due to the resulting reduction of the output signal swing. In this paper, some old and some new circuit techniques are described for the compensation of the amplifier's most important nonideal effects including the noise (mainly thermal and 1/f noise), the input-referred dc offset voltage as well as the finite gain resulting in a nonideal virtual ground at the input.

Electrochemical and photoelectrochemical investigation of single-crystal anatase

Abstract: Single crystals of TiO2 anatase containing 0.22% of Al and traces of V, Zr, Nb, and La were grown by chemical transport reactions employing TeCl4 as the transporting agent. Electrodes having the (101) face exposed doped by reduction with hydrogen were employed. The electrochemical and photoelectrochemical behavior of a single crystal of anatase were scrutinized for the first time. Properties were compared to those of single-crystal rutile having the (001) face exposed. Impedance analysis established that the flatband potential of anatase (101) is shifted negatively by 0.2 V with regards to that of rutile (001). Interfacial capacitance measurements under forward bias indicate smaller density of surface states on anatase. Photoelectrochemical oxidation of water occurs on both rutile and anatase with incident photon-to-current conversion efficiencies close to unity at λ = 300 nm. From the comparison of Ufb and Eg, it follows that anatase (101) and rutile (001) electrodes differ mainly in the position of the ...

Subpicosecond interfacial charge separation in dye-sensitized nanocrystalline titanium dioxide films

Abstract: We have employed subpicosecond transient absorption spectroscopy to study the rate of electron injection following optical excitation of the ruthenium dye RuII(2,2‘-bipyridyl-4,4‘-dicarboxylate)2(NCS)2 (1) adsorbed onto the surface of nanocrystalline titanium dioxide (TiO2) films. This sensitizer dye is of particular interest as it is the most efficient sensitizer dye reported to date and is receiving considerable attention for applications in photoelectrochemical solar energy conversion. Transient data collected for 1 adsorbed onto TiO2 films were compared with those obtained for control dye-coated ZrO2 films, as the high conduction band edge of ZrO2 prevents electron injection. Adsorption of the dye onto the TiO2 film was found to result in a rapid (<500 ps) quenching of the dye excited-state luminescence. Absorption difference spectra collected for the two dye-coated films were assigned by comparison with the spectroscopy of the dye excited and cation states in solution. These transient absorption data...

The Performance and Stability of Ambient Temperature Molten Salts for Solar Cell Applications

Abstract: Room temperature molten salt systems based on methyl-hexyl-imidazolium iodide (MHImI) have been used to scrutinize the performance characteristics, the stability and the mass-transfer effects in a photoelectrochemical regenerative device, as the latter is influenced and can even be limited by local concentration and mass-transport of the electroactive redox mediator species in the electrolyte phase. These salts appear to afford particular advantages over organic liquids as solvents for solar cell electrolytes. Cell performance showed outstanding stability, with an estimated sensitizer turnover in excess of 50 million. An investigation has been carried out on the physical-electrochemical properties of MHImI and its mixtures with organic solvents such as n-methyl-oxazolidinone, acetonitrile and with other lower viscosity molten salts such as methyl-butyl-imidazolium triflate. The repercussions of these properties on solar cells is described experimentally by the performance of practical application devices. Simulation models of mass transport in the nanocrystalline solar cell help illustrate operational aspects such as concentration profiles, limiting currents, anticipated mass-transfer overpotential as a function of current density, and they help to make projections as to how the properties of molten salt electrolytes can be better exploited toward this practical end.

Local Excitation, Scattering, and Interference of Surface Plasmons.

Abstract: The optical probe of a scanning near-field optical microscope is shown to act as a point source of surface plasmon (SP) polaritons on gold and silver films. Plasmon excitation manifests itself by emission of light in the direction of the SP resonance angle, originating from an area with the shape of a dipole radiation pattern whose extension is given by the SP decay length. Interaction with selected, individual surface inhomogeneities gives rise to characteristic modifications of the emitted radiation, which provide detailed information about SP scattering, reflection, and interference phenomena.

Nanocapillarity and Chemistry in Carbon Nanotubes

Abstract: Open carbon nanotubes were filled with molten silver nitrate by capillary forces. Only those tubes with inner diameters of 4 nanometers or more were filled, suggesting a capillarity size dependence as a result of the lowering of the nanotube-salt interface energy with increasing curvature of the nanotube walls. Nanotube cavities should also be less chemically reactive than graphite and may serve as nanosize test tubes. This property has been illustrated by monitoring the decomposition of silver nitrate within nanotubes in situ in an electron microscope, which produced chains of silver nanobeads separated by high-pressure gas pockets.

Simple distributed fiber sensor based on Brillouin gain spectrum analysis

Abstract: A novel configuration of a distributed fiber sensor by Brillouin gain analysis has been developed for temperature and strain monitoring. It uses a single laser source, and the required light signals are all generated with an electro-optic modulator, resulting in high stability and excellent reliability of the measuring setup. Measurement of the induced strain in a wound fiber is presented as a demonstration of the system performance.

A comparison of selection schemes used in evolutionary algorithms

Abstract: Evolutionary algorithms are a common probabilistic optimization method based on the model of natural evolution. One important operator in these algorithms is the selection scheme, for which in this paper a new description model, based on fitness distributions, is introduced. With this, a mathematical analysis of tournament selection, truncation selection, ranking selection, and exponential ranking selection is carried out that allows an exact prediction of the fitness values after selection. The correspondence of binary tournament selection and ranking selection in the expected fitness distribution is proved. Furthermore, several properties of selection schemes are derived (selection intensity, selection variance, loss of diversity), and the three selection schemes are compared using these properties.

Evolution of homing navigation in a real mobile robot

Abstract: In this paper we describe the evolution of a discrete-time recurrent neural network to control a real mobile robot. In all our experiments the evolutionary procedure is carried out entirely on the physical robot without human intervention. We show that the autonomous development of a set of behaviors for locating a battery charger and periodically returning to it can be achieved by lifting constraints in the design of the robot/environment interactions that were employed in a preliminary experiment. The emergent homing behavior is based on the autonomous development of an internal neural topographic map (which is not pre-designed) that allows the robot to choose the appropriate trajectory as function of location and remaining energy.

Specific Adsorption of Nitroaromatic Explosives and Pesticides to Clay Minerals

Abstract: It is demonstrated that nitroaromatic compounds (NACs) may adsorb specifically and reversibly to natural clay minerals in aqueous suspension. Adsorption of NACs to clays is high when the exchangeable cations at the clays include K+ or NH4+ but is negligibly small for homoionic Na+-, Ca2+-, Mg2+-, and Al3+-clays. Highest adsorption coefficients (Kd values up to 60 000 L kg-1) are found for polynitroaromatic compounds including some important contaminants such as explosives (e.g., trinitrotoluene, trinitrobenzene, dinitrotoluidines) and dinitrophenol herbicides (e.g., DNOC, DINOSEB). Nonaromatic nitro compounds (e.g., RDX) generally exhibit very low Kd values. The specific adsorption of NACs can be rationalized by electron donor−acceptor (EDA) complex formation with oxygens present at the external siloxane surface(s) of clay minerals. Kd values of a given NAC and clay mineral can be estimated from known Kd values of other NACs, even when measured at other clay minerals. The affinity and the adsorption capac...

Influence of a lossy ground on lightning-induced voltages on overhead lines

Abstract: A comprehensive study on the effect of a lossy ground on the induced voltages on overhead power lines by a nearby lightning strike is presented The ground conductivity plays a role in both the evaluation of the lightning radiated fields and of the line parameters To be calculated by means of a rigorous theory, both fields and line constants need important computation time, which, for the problem of interest, is still prohibitive The aim of this paper is to discuss and analyze the various simplified approaches and techniques that have been proposed for the calculation of the fields and the line constants when the ground cannot be assumed as a perfectly conducting plane Regarding the radiated electromagnetic field, it is shown that the horizontal electric field, the component which is most affected by the ground finite conductivity, can be calculated in an accurate way using the Cooray-Rubinstein simplified formula The presence of an imperfectly conducting ground is included in the coupling equations by means of two additional terms: the longitudinal ground impedance and the transverse ground admittance, which are both frequency-dependent The latter can generally be neglected for typical overhead lines, due to its small contribution to the overall transverse admittance of the line Regarding the ground impedance, a comparison between several simplified expressions used in the literature is presented and the validity limits of these expressions are established It is also shown that for typical overhead lines the wire impedance can be neglected as regard to the ground impedance

Simulating facial surgery using finite element models

Abstract: This paper describes a prototype system for surgical planning and prediction of human facial shape after craniofacial and maxillofacial surgery for patients with facial deformities. For this purpose it combines, unifies, and extends various methods from geometric modeling, finite element analysis, and image processing to render highly realistic 3D images of the post surgical situation. The basic concept of the system is to join advanced geometric modeling and animation systems such as Alias with a special purpose finite element model of the human face developed under AVS. In contrast to existing facial models we acquire facial surface and soft tissue data both from photogrammetric and CT scans of the individual. After initial data preprocessing, reconstruction, and registration, a finite element model of the facial surface and soft tissue is provided which is based on triangular finite elements. Stiffness parameters of the soft tissue are computed using segmentations of the underlying CT data. All interactive procedures such as bone and soft tissue repositioning are performed under the guidance of the modeling system which feeds the processed geometry into the FEM solver. The resulting shape is generated from minimizing the global energy of the surface under the presence of external forces. Photorealistic pictures are obtained from rendering the facial surface with the advanced animation system on which this prototype is built. Although we do not claim any of the presented algorithms themselves to be new, the synthesis of several methods offers a new facial model quality. Our concept is a significant extension to existing ones and, due to its versatility, can be employed in different applications such as facial animation, facial reconstruction, or the simulation of aging. We illustrate features of our system with some examples from the Visible Human Data Set.TM CR Descriptors: I.3.5 [Computational Geometry and Object Modeling]: Physically Based Modeling; I.3.7 [Three-Dimensional Graphics and Realism]; I.4.6 [Segmentation]: Edge and Feature Detection Pixel Classification; I.6.3 [Applications]; Additional

Nanocrystalline TiO2 (Anatase) Electrodes: Surface Morphology, Adsorption, and Electrochemical Properties

Abstract: Films of sintered anatase nanoparticles are promising electrode materials for solar cells, electrochromics, and lithium batteries. For the last application, the anatase phase is essential, since the Li{sup +} insertion into other TiO{sub 2} polymorphs (rutile, brookite) is mostly considered to be negligible. Ten different nanotextured TiO{sub 2} films were prepared from six commercial and four laboratory synthesized precursors. Krypton adsorption isotherms on the films and apparent powders indicate the effect of sintering and agglomeration. Electrochemical characterization of the films was aimed at double-layer charging and Li{sup +} insertion. The relations between the film`s morphology, adsorption properties, and electrochemical behavior in the accumulation regime are discussed.

Soliton mode-locking with saturable absorbers

Abstract: We investigate ultrashort pulse generation based on the fundamental soliton generation that is stabilized by a saturable absorber. The case of an absorber with a recovery time much longer than the pulsewidth of the generated soliton is investigated in detail. Based on soliton perturbation theory we derive equations for the soliton variables and the continuum generated in a mode-locked laser. Analytic criteria for the transition from stable to unstable soliton generation are derived. The results demonstrate the possibility of ultrashort pulse generation by a slow saturable absorber only. The theoretical results are compared with experiments. We generate pulses as short as 13 fs using only semiconductor saturable absorbers.

Perceptual quality measure using a spatiotemporal model of the human visual system

Abstract: This paper addresses the problem of quality estimation of digitally coded video sequences. The topic is of great interest since many products in digital video are about to be released and it is thus important to have robust methodologies for testing and performance evaluation of such devices. The inherent problem is that human vision has to be taken into account in order to assess the quality of a sequence with a good correlation with human judgment. It is well known that the commonly used metric, the signal-to-noise ratio is not correlated with human vision. A metric for the assessment of video coding quality is presented. It is based on a multi- channel model of human spatio-temporal vision that has been parameterized for video coding applications by psychophysical experiments. The visual mechanisms of vision are simulated by a spatio-temporal filter bank. The decomposition is then used to account for phenomena as contrast sensitivity and masking. Once the amount of distortions actually perceived is known, quality estimation can be assessed at various levels. The described metric is able to rate the overall quality of the decoded video sequence as well as the rendition of important features of the sequence such as contours or textures.

Modelling a Public-Key Infrastructure

Abstract: A global public-key infrastructure (PKI), components of which are emerging in the near future, is a prerequisite for security in distributed systems and for electronic commerce The purpose of this paper is to propose an approach to modelling and reasoning about a PKI from a user Alice's point of view Her view, from which she draws conclusions about the authenticity of other entities' public keys and possibly about the trustworthiness of other entities, consists of statements about which public keys she believes to be authentic and which entities she believes to be trustworthy, as well as a collection of certificates and recommendations obtained or retrieved from the PKI The model takes into account recommendations for the trustworthiness of entities Furthermore, it includes confidence values for statements and can exploit arbitrary certification structures containing multiple intersecting certification paths to achieve a higher confidence value than for any single certification path Confidence values are measured on a continuous scale between 0 and 1 and, in contrast to previous work in this area, are interpreted as probabilities in a well-defined random experiment

Nucleation of Magnetization Reversal in Individual Nanosized Nickel Wires.

Abstract: The mechanisms of magnetization reversal in small magnetic particles have been much discussed in the last decades and prompted intense research activities, motivated in particular by applications in magnetic recording technology [1]. However, experiments were performed, in general, on large assemblies of particles, and the dispersion of morphologies, compositions, orientations, and separations of the magnetic entities limited the interpretation of the results. Furthermore, interactions between particles were difficult to take into account. Single particle studies were possible only in few cases [2]. Recently, insights into the magnetic properties of individual and isolated particles were obtained with the help of near field magnetic force microscopy [3], electron Lorentz microscopy or holography [4], and micro-SQUID (superconducting quantum interference device) magnetometry [5]. It is now possible to make a clear link between experiments performed on nanometer-sized single objects (particles, wires, etc.) and the numerical calculations based on the Brown micromagnetic equations [6]. We report the first study of isolated nanoscale wires with diameters smaller than 100 nm, for which singledomain states could be expected. The cylindrical geometry, with its large shape anisotropy, is well suited for comparison with theory. We obtained unique insight into the process of magnetization reversal by measuring histograms of the switching field as a function of the orientation of the wires in the applied field, their diameter, and the temperature. Furthermore, we measured the probability of switching as a function of the applied field and the temperature. Our studies reveal that the magnetization reversal proceeds by a distortion of the magnetization followed by a nucleation and a propagation process. The observed behavior illustrates the fundamental importance of the study of single, isolated magnetic particles in comparing models and experiments. We developed planar microbridge dc SQUID [7], made of Nb (thickness 20 nm), which were shown to be able to detect 10 4 mB [8]. The SQUID is made of a thin (20 nm) Nb layer in order to prevent flux trapping. The experimental setup allows measurements of hysteresis loops in magnetic fields of up to 0.5 T and temperatures below 6 K, with a time resolution of 100 ms. Ni wires were produced by electrochemically filling the pores of commercially available nanoporous tracketched polycarbonate membranes of thicknesses of 6 to 10 mm [9]. The pore size was chosen in the range of 30 to 100 nm [10]. In order to place one wire on the SQUID detector, we dissolved the membrane in chloroform and put a drop on a chip of some hundreds of SQUID’s. Magnetization measurements were performed on SQUID’s with a single isolated wire. Scanning electron microscopy (SEM) (Fig. 1) was used to determine the position and morphology of the wire. The surface roughness was around 5 nm, corresponding to our SEM resolution.

Transport properties, thermodynamic properties, and electronic structure of SrRuO3.

Abstract: ${\mathrm{SrRuO}}_{3}$ is a metallic ferromagnet. Its electrical resistivity is reported for temperatures up to 1000 K, its Hall coefficient for temperatures up to 300 K, and its specific heat for temperatures up to 230 K. The energy bands have been calculated by self-consistent spin-density functional theory, which finds a ferromagnetic ordered moment of 1.45${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ per Ru atom. The measured linear specific heat coefficient \ensuremath{\gamma} is 30 mJ/mol, which exceeds the theoretical value by a factor of 3.7. A transport mean free path at room temperature of \ensuremath{\approxeq}10 \AA{} is found. The resistivity increases nearly linearly with temperature to 1000 K in spite of such a short mean free path that resistivity saturation would be expected. The Hall coefficient is small and positive above the Curie temperature, and exhibits both a low-field and a high-field anomalous behavior below the Curie temperature. \textcopyright{} 1996 The American Physical Society.