Dipartimento di Ingegneria Enzo Ferrari

Functionalization of silicon surfaces with Nallylurea (CH 2 CH−CNH−CO−NH 2) represents a valuable strategy to obtain covalently bonded Si−C interfaces with amino and/or carbonyl termination. In this work, we studied N-allylurea adsorption on the Si(111)-(7 × 7) surface by combining X-ray and ultraviolet photoemission spectroscopy (XPS and UPS) with high resolution energy loss spectroscopy (HREELS) measurements. XPS core level analysis provides information on the molecular attachment process. Si−C covalent bonding is evidenced by the presence of a C 1s component at 284.8 eV, while interaction through N−Si bonding is proved by the presence of a N 1s component at 397.8 eV. Three different adsorption mechanisms are envisaged: (I) [2 + 2]-like cycloaddition occurring at the rest atom−adatom dimer through cleavage of the vinyl group, (II) Si−N bonding at adatom sites upon cleavage of NH 2 and rearrangement of the ureic group to form an imidol species (−NC−OH), with release of a H atom, and (III) hydrosilylation at adatom sites, through cleavage of the vinyl group and involvement of H atoms provided by reaction II.

We have developed an original method coupling particle induced X ray emission and scanning electron microscope/X ray dispersive analysis in order to characterize the partitioning of Fe, Ca, K, and S between the soluble and the insoluble phases in wind-blown deposits in the Greenland ice. We applied this technique to several sections of the Greenland Ice Core Project ice core. We found that the dominant fraction (from 25 to 100%) of Ca and S deposited in Greenland is soluble, while the proportion of soluble material is lower for K and Fe (between 10 and 80%). For all elements the distribution between soluble and insoluble material varies according to the ice-core depth. The distribution appears to be linked to either meteorological factors, such as temperature or p H of the precipitation (in the case of Ca or S), or the alterability of the mineral assemblages found in the ice (in the case of Fe and K). The fraction of soluble material is therefore linked to the characteristics of source origin (S is predominantly emitted in very soluble forms) and to the dissolution of certain minerals (such as calcium carbonate or Fe-oxides) during transport in the atmosphere. The dynamics of alteration processes is, however, still uncertain, especially for K-containing minerals.

A methodology for the simultaneous detection of several heavy metals in water samples will be described in detail. Targets are chemicaily prepared by preconcentration of the metals. Characteristics of the proton beam and X-ray detection are summarized. The concentration is obtained by using an internal standard in the range ppb-ppm. The method has been tested by calibration with samples of known concentration. The subtraction of background and the data processing are discussed. The error in each concentration is evaluated. Values of the sensitivity are quoted. The method has been extended to marine samples.

The chemical constituents of snow from a 3 m pit sampled at a coastal Antarctic site were characterised by means of PIXE and SEM-EDAX. Oxygen isotope dating revealed that the pit spanned an 8-year period, from 1986 to 1994. Concentrations measured by PIXE ranged from less than 1 ng g À1 of H 2 O for Cu, and Zn to several tens of ng g À1 for Si, S and Cl. The major elements found were Si, S, Al, and Na, which contributed more than 75% of the element loading. Snow layers with elevated Cl concentration were regularly found along the pit. The mineralogy of the insoluble particles determined by SEM-EDAX analyses showed that quartz, plagioclase and clays were the prevalent minerals. The proportion of each element in the insoluble phase was comparable with that previously found in similar studies. Ó 0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 -5 8 3 X ( 9 8 ) 0 0 9 0 1 -X

Acute toxicity of selenium as selenite in Zosterisessor ophiocephalus by ip injection was studied. The 50% lethal dose and 50% lethal time were measured to be 0.29 ppm and 96 h, respectively. Se concentrations in liver, gill, skin and muscle, and Cyt. P450 level, Se-GPx, and Total GPx enzyme activities in liver were also assessed at different doses and times after injection. Starting at 0.3 ppm injected dose, enzyme activities and Se concentration in tissues, but not in muscle, showed significant differences from the control group. A threshold behavior was inferred. Normal conditions of enzyme activities and Se concentration in tissues were restored about 1 wk after injection. Biological elimination half-lives were about 2 d for liver and gill, and 5 d for skin.

DISPERSION MODELS SPRAY (Tinarelli, G. et al., 1998) is a 3D lagrangian stochastic particle dispersion model able to simulate air pollution dispersion and deposition-decay phenomena in non homogenous, non stationary conditions and over complex topography (Thomson, D.J., 1987). The model WinDimula 3.0 (Cagnetti, P. and M.C. Cirillo, 1982; Cirillo, M.C. et al., 1986) is an atmospheric multisource Gaussian steady-state dispersion model of non reagent pollutants generated by point, line and area sources. The dispersion model ISC3 is a steady-state Gaussian model allowing to assess pollutant concentrations from point, area and volume sources.

The static and dynamic behavior of a compressed circular cylindrical shell having geometric imperfections is analyzed. The analysis is mainly performed by means of the DonnellÕs nonlinear shallow-shell theory. However, the refined Sanders shell theory is also used for comparison. A suitable expansion of the radial displacement, able to describe both buckling and dynamic behaviors is developed; the effect of geometric imperfections is accounted for by means of a modal representation. The response of the shell subjected to a sinusoidal axial excitation at its ends, giving rise to a parametric excitation, is considered. The effect of imperfections on the critical value of the dynamic load, that causes the loss of stability of the system, is analyzed. Interesting nonlinear dynamic phenomena are observed: direct resonance with softening behavior and parametric instability with period doubling response.

The non-linear dynamics and stability of simply supported, circular cylindrical shells containing inviscid, incompressible fluid flow is analyzed. Geometric non-linearities of the shell are considered by using the Donnell's non-linear shallow shell theory. A viscous ...

The stability of circular cylindrical shells with supported ends in compressible, inviscid axial flow is investigated. Nonlinearities due to finite-amplitude shell motion are considered by using Donnell's nonlinear shallow-shell theory; the effect of viscous structural damping is taken into account. Two different in-plane constraints are applied at the shell edges: zero axial force and zero axial displacement; the other boundary conditions are those for simply supported shells. Linear potential flow theory is applied to describe the fluid-structure interaction. Both annular and unbounded external flow are considered by using two different sets of boundary conditions for the flow beyond the shell length: (i) a flexible wall of infinite extent in the longitudinal direction, and (ii) rigid extensions of the shell (baffles). The system is discretized by the Galerkin method and is investigated by using a model involving seven degrees-of-freedom, allowing for traveling-wave response of the shell and shell axisymmetric contraction. Results for both annular and unbounded external flow show that the system loses stability by divergence through strongly subcritical bifurcations. Jumps to bifurcated states can occur well before the onset of instability predicted by linear theory, showing that a linear study of shell stability is not sufficient for engineering applications.

The aeroelastic stability of simply supported, circular cylindrical shells in supersonic flow is investigated by using both linear aerodynamics (first-order piston theory) and nonlinear aerodynamics (third-order piston theory). Geometric nonlinearities, due to finite amplitude shell deformations, are considered by using the Donnell's nonlinear shallow-shell theory, and the effect of viscous structural damping is taken into account. The system is discretized by Galerkin method and is investigated by using a model involving up to 22 degrees-of-freedom, allowing for travelling-wave flutter around the shell and axisymmetric contraction of the shell. Asymmetric and axisymmetric geometric imperfections of circular cylindrical shells are taken into account. Numerical calculations are carried out for a very thin circular shell at fixed Mach number 3 tested at the NASA Ames Research Center. Results show that the system loses stability by travelling-wave flutter around the shell through supercritical bifurcation. Nonsimple harmonic motion is observed for sufficiently high post-critical dynamic pressure. A very good agreement between theoretical and existing experimental data has been found for the onset of flutter, flutter amplitude, and frequency. Results show that onset of flutter is very sensible to small initial imperfections of the shells. The influence of pressure differential across the shell skin has also been deeply investigated. The present study gives, for the first time, results in agreement with experimental data obtained at the NASA Ames Research Center more than three decades ago.

The nonlinear free and forced vibrations of a simply supported, circular cylindrical shell in contact with an incompressible and inviscid, quiescent and dense fluid are investigated. Donnell's shallow-shell theory is used, so that moderately large vibrations are analysed. The boundary conditions on radial displacement and the continuity of circumferential displacement are exactly satisfied, while axial constraint is satisfied on the average. The problem is reduced to a system of ordinary differential equations by means of the Galerkin method. The mode shape is expanded by using three degrees of freedom; in particular, two asymmetric modes (driven and companion modes), plus an axisymmetric mode are employed. The time dependence of each term of the expansion is general and the axisymmetric mode is obtained from a series involving all axisymmetric linear modes. Different tangential constraints can be imposed at the shell ends. Effects of both internal and external dense fluid are studied. Internally, the shell is considered completely filled, while externally, an unbounded fluid domain is considered around the shell in the radial direction. The solution is obtained both numerically and by the Method of Normal Forms. Numerical results are obtained for both free and forced vibrations of empty and water-filled shells.

THE RECENT LETTER by Dowell (1998) touched on several aspects of the literature and the physics of nonlinear vibrations of circular cylindrical shells, clarifying some of the issues while attempting to reconcile some hitherto contradictory findings, and at the same time outlining some of the unresolved issues and potentially profitable directions for further research.