Showing papers by "Instituto Superior Técnico published in 1992"

The evolution of TEOS to silica gel and glass by vibrational spectroscopy

Abstract: Fourier-transform infrared and Raman spectroscopies were simultaneously used in order to study the time evolution of the acid-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS) in ethanol, to form silica xerogel. The transformation to vitreous silica by gradual densification of the gel was also monitored after different heat treatments. The extent of hydrolysis was revealed by the changes in intensity of the IR-active bands of TEOS at 793 cm − (Si O asymmetric stretch), 1102 cm −1 (C O asymmetric stretch), 1168 cm −1 (CH 3 rocking) and 1400 cm −1 (CH 2 wagging) and also the TEOS Raman-active bands corresponding to the strongly polarized Si O symmetric stretch (656 cm −1 ) and the depolarized CH 3 rocking (960 cm −1 ). The intensity of all these bands clearly decreased as the reactions proceeded. Good evidence for the formation of ethanol in the initial steps of hydrolysis and for the subsequent predominance of its evaporation was found in the occurrence of maximum intensities for its infrared and Raman-active bands due to deformation of CH 3 or CH 2 groups (881 cm −1 ) and CH 2 bending (1275 cm −1 ), after a period of time. The intensity of these bands later decreased. The gel to glass conversion was followed through the disappearance of the infrared peaks at 554 cm − ) (Si OH rocking) and 940 cm −1 (Si OH stretching), together with the intensity increase of the peaks at 800 and 1078 cm −1 . The complementary nature of the IR and Raman spectroscopies is of great value for a complete understanding of the TEOS to silica glass transformation.

2 p X-ray absorption of titanium in minerals

Abstract: Ti 2p X-ray absorption spectra for a series of minerals have been measured. Crystal field multiplet calculations can explain the spectral shape. The asymmetry of the e g , peak is shown to be related to distortions of the Ti IV octahedron. It is found, theoretically as well as experimentally, that the absorption spectra are more sensitive to tetragonal distortions than to trigonal distortions. A number of silicate minerals and metamict minerals containing titanium are measured and Ti III has not been observed in any of these minerals. A comparison is made to the 1s X-ray absorption, and the potential of both for the study of minerals is discussed.

Detection of LO modes in glass by infrared reflection spectroscopy at oblique incidence.

Abstract: The occurrence of longitudinal-transverse splittings in the optic-vibrational-mode frequencies of partially ionic glasses has previously been observed in several glass systems and was also theoretically confirmed. The transverse-optic (TO) and longitudinal-optic (LO) components are usually derived from near-normal infrared-specular-reflectivity spectra by Kramers-Kronig analysis, and the TO frequencies often coincide with minima in the transmission spectrum. The approximate positions of the high-frequency LO modes of vitreous silica (v-${\mathrm{SiO}}_{2}$) and two more ionic modified fluoride glasses based on ${\mathrm{ZrF}}_{4}$ (ZBLA glass) and ${\mathrm{ZnF}}_{2}$-${\mathrm{CdF}}_{2}$ (ZnCd glass) have been directly obtained from infrared-external-specular-reflectivity spectra at oblique incidence. Fourier-transform infrared reflectivity spectra were recorded at incidence angles between 10\ifmmode^\circ\else\textdegree\fi{} and 70\ifmmode^\circ\else\textdegree\fi{} off normal, and the results are compared with Kramers-Kronig analysis of the near-normal (10\ifmmode^\circ\else\textdegree\fi{}) incidence data and polarized Raman spectra of the same glasses. The reflection spectra at \ensuremath{\gtrsim}50\ifmmode^\circ\else\textdegree\fi{} off normal contained peaks corresponding very approximately to the LO vibrational modes, while the peaks in the Raman spectra did not generally coincide with either TO or LO frequencies of the glasses. The high-frequency LO modes were experimentally observed at 1286, \ensuremath{\sim}607, and 617 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, for v-${\mathrm{SiO}}_{2}$, ZBLA, and ZnCd glasses, respectively, compared to Kramers-Kronig values calculated at 1266, 625, and \ensuremath{\sim}623 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. Ionicity was found to be an important factor in determining the extent of LO-TO splittings. The original Berreman argument for the case of transmission through thin crystal films may be extended to reflection from bulk glasses.

Tire modeling by finite elements

Abstract: Recent advances in the development of a general three‐dimensional finite element methodology for modeling large deformation steady state behavior of tire structures is presented. The new developments outlined here include the extension of the material modeling capabilities to include viscoelastic materials and a generalization of the formulation of the rolling contact problem to include special nonlinear constraints. These constraints include normal contact load, applied torque, and constant pressure‐volume. Several new test problems and examples of tire analysis are presented.

Mutual diffusivity of a mixture of n-hexane and nitrobenzene near its consolute point

Abstract: It is demonstrated that the Taylor dispersion method can be used to measure the mutual diffusivity of liquid mixtures near a critical mixing point. For this purpose we have measured the mutual diffusivity of a liquid mixture of n-hexane and nitrobenzene at the critical composition at temperatures from 16 K down to 1 K above the critical temperature. The results are in agreement with the theoretically predicted behavior of the diffusivity near a critical point of mixing.

Effects of growth temperature on alginate synthesis and enzymes in Pseudomonas aeruginosa variants.

Abstract: Summary: Spontaneous variation of the level of alginate synthesis in Pseudomonas aeruginosa was associated with changes in the activity of all four enzymes leading to synthesis of GDP-mannuronic acid, the activated precursor for polymerization. For the high-alginate-producing variant 8821M, alginate yield and properties, as well as the levels of alginate enzymes, were dependent on growth temperature. In contrast, levels of alginate and enzymes in the mucoid parent strain 8821 were very low and near temperature-independent. The difference in the specific activity of GDP-mannose dehydrogenase (GMD), encoded by the algDgene, between the two strains was associated with the alginate biosynthetic ability and with the degree of activation of the algD promoter, measured using the algD-xylE transcription fusion on plasmid pVD2X. Maximal activity of the four enzymes was observed in strain 8821M grown at 30 °, a temperature below the optimum for growth (35 °). The effect of temperature on GMD activity could not be explained by the regulation of the algD promoter by temperature, since expression of pVDZX appeared to be more active at 35 °, when the decrease of pVD2X copy number with increasing temperature was taken into account. The involvement of enzymes that catalyse steps downstream from the formation of the activated precursor should also be considered, as suggested by differences in the molecular mass of alginates synthesized by the two strains at various temperatures. Acetyl content of alginates increased as temperature decreased and strain 8821M produced the highest levels of acetylated polymers. The degree of acetylation appeared to be related to growth rate and could reflect acetyl-CoA availability.

Biological treatment of phenolic wastes: Comparison between free and immobilized cell systems

Abstract: Phenol degradation by free and immobilized cells ofFusarium flocciferum was studied in a chemostat at steady-state conditions. For the free cell system the dilution rates varied from 0.02 to 0.13h−1, with a total phenol removal up to 0.08h−1. Wash-out seemed to set in at 0.11h−1. The immobilized cells showed virtually complete phenol utilization at 1g/l, over a period of four months. At D=0.2h−1 and above 1g/l phenol, the complete phenol removal is not achieved: a progressive increase in the outlet concentration was observed attaining a value of 284mg/l at 1.5g/l.

Shape design sensitivity analysis of nonlinear structural systems

Abstract: A unified approach is presented for shape design sensitivity analysis of nonlinear structural systems that include trusses and beams. Both geometric and material nonlinearities are considered. Design variables that specify the shape of components of built-up structures are treated, using the continuum equilibrium equations and the material derivative concept. To best utilize the basic character of the finite element method, shape design sensitivity information is expressed as domain integrals. For numerical evaluation of shape design sensitivity expressions, two alternative methods are presented: the adjoint variable and direct differentiation methods. Advantages and disadvantages of each method are discussed. Using the domain formulation of shape design sensitivity analysis, and the adjoint variable and direct differentiation methods, design sensitivity expressions are derived in the continuous setting in terms of shape design variations. A numerical method to implement the shape design sensitivity analysis, using established finite element codes, is discussed. Unlike conventional methods, the current approach does not require differentiation of finite element stiffness and mass matrices.

Selection of controller parameters using genetic algorithms

Abstract: In this paper the problem of setting the initial values for tho parameters of a class of controllers is approached using genetic algorithms. The controller parameters, computed off-line via the optimization of a quadratic error performance index, can be used to initialize the controller incorporated in the real plant. During the on-line control stage and to increase the overall performance, other tuning mechanisms can be considered [5].

Meromorphic Factorization, Partial Index Estimates and Elastodynamic Diffraction Problems1)

Abstract: This work is motivated by some elliptic boundary and transmission problems in mathematical physics, in particular by elastodynamic wave propagation. The analytical solution of the boundary pseudodifferential equations requires a generalized factorization of the lifted Fourier symbol which is a non-rational matrix-function. In the factorization procedure poles and increasing terms appear, and cause enormous practical and theoretical problems due to the possible occurance of partial indices different from zero. The paper presents an approach which avoids those difficulties by use of a factorization of the symbol matrix into meromorphic factors. An operator theoretic interpretation yields resolvents up to finite dimensional operators, whose ranks are closely related to partial indices, order of the algebraic increase at infinity, and the multiplicities of the poles in the factors.

Continuity conditions for finite element analysis of solids

Abstract: In conventional finite element formulations the concept of node—a point where one of the shape functions is unitary and all others are nil—is used to advantage as it simplifies the definition of interelement continuity conditions. This constraint on the definition of the shape functions hinders the formulation of elements with complex shapes and, in particular, of equilibrium elements. In the approach presented herein linearly independent functions are defined within each element irrespectively of the location of the nodes. Interelement continuity conditions are imposed ‘a posteriori’, as in hybrid elements. The derivation of the element matrices is based upon the equations expressing equilibrium, compatibility and the constitutive relations without explicitly using variational principles. This results in a wider choice of available funciiuns ami in an easier way to formulate equilibrium elements and/or to use conforming or non-conforming elements. As the approach used is independent of the choice of basic functions and of the shape of the elements, it is perfectly general. It allows the parallel analysis of kinematically and statically admissible formulations, as proposed by Fraeijs de Veubeke.1 As the interelement continuity conditions are imposed ‘a posteriori’ new variables are used to express this condition.

Nongyrotropy as a source of instability and mode coupling

Abstract: Nongyrotropic particle populations can bring about linear mode coupling in homogeneous media among the three eigen-modes of parallel propagation in gyrotropic magnetoplasmas. These interactions stimulate, in general, wave activity that does not occur in corresponding (random gyrophase) gyrotropic ambients. Solutions of the dispersion equation illustrate that simple introduction of gyrophase organization can (1) excite electrostatic (and electromagnetic) perturbations in media whose free energy sources are solely electromagnetic, and (3) drive hybrid (both electrostatic and electromagnetic) wave growth in thoroughly stable Maxwellian plasmas.