Showing papers by "Instituto Superior Técnico published in 1990"

Characterization of silica gels by infrared reflection spectroscopy

Abstract: The FTIR specular reflection spectra of porous silica gels, either dried at room temperature or subjected to partial densification at 400°C, are significantly different from the spectrum of bulk vitreous silica. Namely, the high frequency SiOSi stretching vibrational peak near 1100 cm −1 exhibits shifts in the gels of up to 40 cm −1 towards lower frequencies, whereas the relative intensity of the companion high frequency shoulder simultaneously increases. The origin and possible structural meaning of the observed changes were investigated by Kramers-Kronig analysis of the experimental near-normal infrared reflectivity spectra of a series of silica xerogels, porous Vycor and bulk vitreous silica. Major differences were found for the energy loss function Im(−1/ϵ ∗ ), the peaks of which have been associated with the LO components of the dominant vibrational modes, and the largest red shifts corresponded to the highest frequency mode. Simple calculations show that the observed behavior of the infrared spectra could in part be due to a certain degree of strain in the SiOSi bridging bonds at the surface of the gel pores.

Acceleration Techniques for the Backpropagation Algorithm

Abstract: Like other gradient descent techniques, backpropagation converges slowly, even for medium sized network problems. This fact results from the usually large dimension of the weight space and from the particular shape of the error surface in each iteration point. Oscillation between the sides of deep and narrow valleys, for example, is a well known case where gradient descent provides poor convergence rates.

Multifractal properties of snapshot attractors of random maps.

Abstract: We consider qualitative and quantitative properties of ``snapshot attractors'' of random maps. By a random map we mean that the parameters that occur in the map vary randomly from iteration to iteration according to some probability distribution. By a ``snapshot attractor'' we mean the measure resulting from many iterations of a cloud of initial conditions viewed at a single instant (i.e., iteration). In this paper we investigate the multifractal properties of these snapshot attractors. In particular, we use the Lyapunov number partition function method to calculate the spectra of generalized dimensions and of scaling indices for these attractors; special attention is devoted to the numerical implementation of the method and the evaluation of statistical errors due to the finite number of sample orbits. This work was motivated by problems in the convection of particles by chaotic fluid flows.

A Categorial Theory of Objects as Observed Processes

Abstract: The semantic domain for object-oriented languages and systems outlined in this paper is an amalgamation of two approaches: the objects-as-sheaves approach of the second author published nearly two decades ago, and the more recent ideas of the first and third authors on objects as processes endowed with observation. The basic insight is that objects in the latter sense correspond to object morphisms in the former sense. After an informal introduction into both approaches, we first elaborate on the sheaf model, using the term "behaviour" for objects in this sense, and avoiding concepts from topology. Behaviours and behaviour morphisms are shown to form a complete category where parallel composition is reflected by limits. Objects are defined to be behaviour morphisms, giving rise to a cocomplete category of objects where colimits reflect object aggregation. Object morphisms reflect different forms of inheritance, and also object reification (implementing objects over objects) is conveniently expressed in this framework.

Experimental dynamic analysis of cracked free-free beams

Abstract: The use of modal-analysis techniques as nondestructive methods for the detection of cracks in beams is a current field of interest for various researchers. However, an extensive search of the published literature shows that there is very little information on experimental results, thus making it almost impossible for theoretical researchers to use published experimental data to validate their models. The authors, who are currently working in the field of crack detection through modal analysis, performed an extensive experimental dynamic analysis of free-free beams with the objective of validating theoretical techniques, under development, for the prediction of the location and depth of cracks in straight beams. This paper describes the first part of the authors' work. The experimental techniques used are described and results obtained for various locations and depths of cracks in a straight free-free beam are presented.

Harmonic coding at 4.8 kb/s

Abstract: A harmonic coder designed for operation at 4.8 kb/s is presented. Sinusoidal modeling of speech is reviewed and the use of harmonically related frequencies is extended to unvoiced and transition regions. The estimation of the fundamental frequency with emphasis on the fine tuning of the pitch estimates is discussed. The structure of the 4.8-kbit/s harmonic coder is described, and a detailed discussion of phase quantization issues is presented. Experimental results are presented which reveal a reverberant character in many speech utterances and difficulties in some classes of sounds (plosives, liquids, and voiced fricatives). These distortions are mostly produced by the quantization of phase, but they are also explained by the difficulties of the sinusoidal analysis/synthesis in transition regions when the frame length is large. >

Metal Complexes with Macrocyclic Ligands. Part XXXI. Protonation studies and complexation properties of tetraazamacrocyclic methylenephosphonates with earth-alkali ions†

Abstract: The three ligands 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylenephosphonic acid) (1), 1,4,7,11-tetraazacyclotridecane-1,4,7,11-tetrakis(methylenephosphonic acid) (2), and 1,4,8,11-tetraazacyclotetradecane-1,4,8,1 1-tetrakis(methylenephosphonic acid) (3) have been synthesized by condensation of the corresponding macrocycles with formaldehyde and phosphorous acid. The protonation and stability constants with the earth-alkali ions have been determined at 25° and I = 0.1 M (Me4)N(NO3) by potentiometric titrations. Because of the high values of the first two protonation constants, 1H-NMR measurements were necessary to determine them. Titrations in different supporting electrolytes (NaNO3, KNO3, RbNO3, CsNO3, and Me4N(NO3)) show that their choice is of paramount importance, as the above ligands can form complexes with alkali-metal ions. The potentiometric results for the earth-alkali ions show that beside mononuclear complexes of different degrees of protonation ([MLHn], n = 0–4), also binuclear species are formed ([M2LHm], m = 0–2). It is interesting that 1 with the smallest macrocyclic ring has the greatest tendency to form binuclear complexes, which are so stable that they partially prevent the formation of the corresponding mononuclear species. For [ML], [MLH], [M2L], and [M2LH], the stability sequence is Mg2+ Sr2+ > Ba2+, whereas for [MLH2], [MLH3], and [MLH4], the stability steadily decreases from Mg2+ to Ba2+.

Detection of LO mode in v-SiO2 by infrared diffuse reflectance spectroscopy

Abstract: It has been found that the high frequency longitudinal optical (LO) mode in vitreous SiO 2 can be observed in the diffuse reflectance spectra of bulk glass or powder compacts. The sensitivity to this mode arises because of the off-normal components of reflectance which are collected in the diffuse reflectance spectra. The frequency and intensity of the mode are only approximately determined, however, and Kramers-Kronig transformation of specular reflectivity data is required to calculate precisely the mode location and intensity. The significance of this finding is that, in contrast to the use of infrared reflectance or Raman spectroscopy, where polished bulk specimens or single crystals are required, the present approach permits the determination of LO modes with with powder.

Highly nonlinear magnetic pulses at Comet Giacobini-Zinner

Abstract: Nonlinear solitary magnetic pulses have been detected near the outbound bow shock/wave of Comet Giacobini-Zinner. The pulses have peak field-to-background compression ratios ranging from 2.3 to 7.0 and full-width half-maximum durations from 12 to 72s, comparable to the H2O group ion gyroperiod. Because all of the solitary waves have the same spacecraft-frame polarizations, it is suggested that the waves may have phase speeds greater than the solar wind speed. The role that the waves may play with the bow shock is discussed briefly.

Three-dimensional finite element contact algorithm for metal forming

Abstract: This work presents a three-dimensional rigid plastic finite element formulation. The workpiece is discretized with eight node hexahedral isoparametric elements. Friction is included in the formulation by means of a shear stress depending on the relative velocity between the workpiece and the tool. Special attention is given to the contact problems, and a three-dimensional contact algorithm based on a discretization of the tool surface with triangular elements is presented. Finally, some selected examples are solved, in order to show the capabilities of the formulation.

Improving the Learning Speed in Topological Maps of Patterns

Abstract: Topological Maps of Patterns are a very powerful neural network paradigm. However, the learning method originally proposed by T. Kohonen for these maps, is a very time-consuming process. In this paper we propose a method of improving the learning speed, by starting the map with very few units and increasing that number progressively until the map reaches its final size. When the number of units increases, the locations of the new units are interpolated from the locations of the old units. The use of this method dramatically reduces the time needed for the “unfolding” phase and also yields some improvements in the asymptotic convergence phase. The improvements observed in this second phase can vary from marginal improvements for small sized networks, to very significant improvements for large networks.

Purification of Chromobacterium viscosum lipases using reversed micelles

Abstract: Two upases produced by Chromobacterium viscosum were isolated and purified by liquid-liquid extraction using organic solvents containing reversed micelles. One lipase (lipase A) was purified from the original crude enzyme preparation by 4,3-fold with a recovery of 91% and the other (lipase B) by 3,7-fold with a recovery of 76%.

Logics of Modal Terms for Systems Specification

Abstract: The use of modal qualification on terms is advocated for a more intuitive account of the description of the effects of events on objects such as program variables or database attributes, and also for an easier verification of the intended temporal integrity constraints. We develop two logics of modal terms focusing on positional and temporal qualification, and show by means of an example how they can be used to support the description and prescription of actions, as well as to reason about the properties of the specified systems.

Thermophysical Properties of Alkali Metal Vapours, Part I — Theoretical Calculation of the Properties of Monatomic Systems

Abstract: Theoretical calculations of the second virial coefficients, self-diffusion, viscosity and thermal conductivity of monatomic alkali metal vapours, lithium, sodium, potassium, cesium and rubidium are presented. The calculations have been carried out using reliable analytical two body interaction potential energy functions for the ground singlet 1Σ+g and the excited triplet 3Σ+g molecular states of the alkali dimers. The results are compared with previously calculated values. — The interpretation of experimental data on the thermophysical properties of alkali metal vapours is complicated by the formation of dimers. It follows that theoretical calculations for the monatomic species should thus provide an independent information on the zero density limits for the experimental properties. Full utilization of this principle is tried in Part II of this paper where an assessment of the available experimental data is attempted.

A CCITT compatible coding algorithm for digital recording of moving images

Abstract: This paper describes the work carried out in CSELT with the aim of providing a sensible solution to the problem of recording moving images on digital storage media. The starting point for the proposed algorithm has been the CCITT H.261 Reference Model, future standard for synchronous transmissions at p × 64 kbits/s ( p = 1,…,30). The algorithm provides all the facilities required for recording purposes.

Solvent Induced Intramolecular Electron Exchange Kinetics in 1,3‐Isodisubstituted Aromatic Radical Anions II. Activation Parameters

Abstract: From the alternant line broadening effect in the ESR-spectra of the radical anions of 1,3-dinitrobenzene and benzene-1,3-dicarbaldehyde in various protic and aprotic solvents the rates of intramolecular electron exchange were determined in the temperature range 260–340 K. Using Marcus theory and the Rips-Jortner approach for the uniform adiabatic limit of electron transfer it is shown that the amount of transferred charge ze0 decreases with increasing temperature. This result is explained on the basis of the effect of temperature on the specific asymmetric solvation of the anions. For the 1,3-dinitrobenzene radical anion in protic solvents the charge transfer number z changes slightly from 0.96 to 0.90 within 250–340 K and in aprotic solvents from 0.55 to 0.32 within 280–340 K. For benzene-1,3-dicarbaldehyde radical anion in protic solvents z changes from 0.40 to 0.29 within 260–320 K. From the measured activation parameters for the intramolecular electron exchange the values for the inner sphere reorganization energies λi were estimated, using values for the outer sphere reorganization λ0 as obtained by applying an ellipsoidal cavity model to the measured solvent dependence of the rates. For the 1,3-dinitrobenzene radical anion λ0 increases with the degree of solvent association (from 18.4 to 106.4 kJ/mol), whereas λi varies much less (82.4–116.0 kJ/mol) for all protic and aprotic solvents investigated. From the difference in λ0 for protic and aprotic solvents, the value for the energy of solvation of the anions by hydrogen bonding is obtained as ≈ 10 kJ/mol.

Generalized factorization for a class of non-rational 2×2 matrix functions

Abstract: In this paper the generalized factorization for a class of 2×2 piecewise continuous matrix functions on ℝ is studied. Using a space transformation the problem is reduced to the generalized factorization of a scalar piecewise continuous function on a contour in the complex plane. Both canonical and non-canonical generalized factorization of the original matrix function are studied.

Water-gas shift reaction over chromia-promoted magnetite. Use of temperature-programmed desorption and chemical trapping in the study of the reaction mechanism

Abstract: The mechanism of WGSR has been studied on an Fe−Cr catalyst. Although after reaction (from 250 to 400°C) formate species have been characterized on the Fe−Cr catalyst, by chemical trapping, TPD studies after reaction and after HCOOH adsorption have shown that formates are not the main intermediates. The disproportionation of CO or the regenerative mechanism seems to be more probable.