Traumatismo encéfalocraneano por agresión con objeto contundente sólido

Fil: Cutroni, Francisco. Universidad Nacional de Cuyo. Facultad de Ciencias MédicasFil: Santamarina, M.. Universidad Nacional de Cuyo. Facultad de Ciencias Médica

Universal behavior of internal friction in glasses below T : anharmonicity vs relaxation

Comparison of the internal friction at hypersonic frequencies between a few K and the glass transition temperature Tg for various glasses brings out general features. At low temperature, internal friction is only weakly dependent on the material. At high temperature but still below Tg the internal friction for strong glasses shows a T-independent plateau in a very wide domain of temperature; in contrast, for fragile glass, a nearly linear variation of internal friction with T is observed. Anharmonicity appears dominant over thermally activated relaxational processes at high temperature.Comment: accepted in Physical Review

On the correlation between fragility and stretching in glassforming liquids

We study the pressure and temperature dependences of the dielectric relaxation of two molecular glassforming liquids, dibutyl phtalate and m-toluidine. We focus on two characteristics of the slowing down of relaxation, the fragility associated with the temperature dependence and the stretching characterizing the relaxation function. We combine our data with data from the literature to revisit the proposed correlation between these two quantities. We do this in light of constraints that we suggest to put on the search for empirical correlations among properties of glassformers. In particular, argue that a meaningful correlation is to be looked for between stretching and isochoric fragility, as both seem to be constant under isochronic conditions and thereby reflect the intrinsic effect of temperature

Lie Transformation Method for Dynamical System having Chaotic Behavior

The paper persents recent developments in a singular perturbation method, known as the "Lie transformation method" for the analysis of nonlinear dynamical systems having chaotic behavior. A general approximate solution for a system of first-order differential equations having algebraic nonlinearities is introduced. Past applications to simple dynamical nonlinear models have shown that this method yields highly accurate solutions of the systems. In the present paper the capability of this method is extended to the analysis of dynamical systems having chaotic behavior: indeed, the presence of "small divisors" in the general expression of the solution suggests a modification of the method that is necessary in order to analyze nonlinear systems having chaotic behavior (indeed, even non-simple-harmonic behavior). For the case of Hamiltonian systems this is consistent with the KAM (Kolmogorov-Arnold-Moser) theory, which gives the limits of integrability for such systems; in contrast to the KAM theory, the present formulation is not limited to conservative systems. Applications to a classic aeroelastic problem (panel flutter) are also included. © 1995 Kluwer Academic Publishers

Lie Transform Method for the Analysis of Dynamical Systems

sez. Meccanica General

Dinamica rilassamentale di liquidi fragili

By means of ultrasonic and dielectric relaxation spectroscopy an experimental investigation on the $\alpha$-relaxation process in fragile molecular liquids has been performed. The structural relaxation of glass-formers around the glass transition point is a complex process whose microscopic description constitutes an unsolved problem in condensed matter physics. On the other hand, being this relaxation intimately relatcd to the liquid-glass transition, its explanation represents an essential prerequisite for the interpretation of this last phenomenon. For the analitical representation of the dynamic susceptibilities involved the Cole-Davidson function has been employed while the Vogel-Fulcher-Tamman (VFT) law has been taken into account to describe the temperature dependence of the mean relaxation time. A substantial accord with the dynamical behaviour of very fragile liquids has been found

Proprietà di trasporto dei vetri ionici: rilevanza del moto di hopping

The paper investigates the dielectric properties of the (Ag_2SO_4)_x(AgPO_3)_1_-_x and (Ag_2SO)_x(AgPO_3)_1_-_x glasses by impedance measurements at low frequencies (l Hz to l0 MHz)and at high frequencies (8.2 GHz to l8 GHz) by waveguide measurements. The frequency dependence of a.c. conductivity has been interpreted in terms of translational and reorientational ionic hopping motion. The activation energies values extracted from the temperature dependence of fixed frequency a.c. conductivity agree with the values of the acoustic attenuation