Showing papers by "University of Trento published in 1995"

Structural Characterization and High‐Temperature Behavior of Silicon Oxycarbide Glasses Prepared from Sol‐Gel Precursors Containing Si‐H Bonds

Abstract: Silicon oxycarbide glasses have been synthesized by inert atmosphere pyrolysis at 1000°C of gel precursors obtained by cohydrolysis of triethoxysilane, HSi(OEt)3, and methyl-diethoxysilane, HMeSi(OEt)2. The oxycarbide structures have been carefully characterized by means of different techniques such as 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR) and Raman spectroscopies, X-ray diffraction (XRD), and chemical analysis. Experimental results clearly indicate that, depending on the composition of the starting gels, the resulting oxycarbide glass either is formed by a pure oxycarbide phase or contains an extra carbon or silicon phase. By increasing the temperature up to 1500°C, the oxycarbide glasses display compositional and weight stability; however, the amorphous network undergoes structural rearrangements that lead to the precipitation of nano-sized β-SiC crystallites into amorphous silica. Crystallization of metallic silicon is also clearly observed at 1500°C for the samples in which the presence of Si-Si bonds was postulated at 1000°C.

Structural and dynamical properties of superfluid helium: A density-functional approach

Abstract: We present a novel density functional for liquid 4 He, properly accounting for the static response function and the phonon-roton dispersion in the uniform liquid. The functional is used to study both structural and dynamical properties of superfluid helium in various geometries. The equilibrium properties of the free surface, droplets and films at zero temperature are calculated. Our predictions agree closely to the results of ab initio Monte Carlo calculations, when available. The introduction of a phenomenological velocity dependent interaction, which accounts for backflow effects, is discussed. The spectrum of the elementary excitations of the free surface and films is studied. PACS number:67.40

Code assignment for hidden terminal interference avoidance in multihop packet radio networks

Abstract: Hidden terminal interference is caused by the (quasi-) simultaneous transmission of two stations that cannot hear each other, but are both received by the same destination station. This interference lowers the system throughput and increases the average packet delay. Some random access protocols that reduce this interference have been proposed, e.g., BTMA protocol. However, the hidden terminal interference can be totally avoided only by means of code division multiple access (CDMA) schemes. In the paper, the authors investigate the problem of assigning orthogonal codes to stations so as to eliminate the hidden terminal interference. Since the codes share the fixed channel capacity allocated to the network in the design stage, their number must not exceed a given bound. The authors seek assignments that minimize the number of codes used. They show that this problem is NP-complete, and thus computationally intractable, even for very restricted but very realistic network topologies. Then, they present optimal algorithms for further restricted topologies, as well as fast suboptimal centralized and distributed heuristic algorithms. The results of extensive simulation set up to derive the average performance of the proposed heuristics on realistic network topologies are presented. >

Geomorphological signatures of varying climate

Abstract: A LONG-STANDING question in geomorphology1,2 is whether the topography of a particular landscape is in balance with current climate-driven processes, or contains relict signatures of past climates. For the glaciated landscapes of the Northern Hemisphere, the latter obviously applies but the situation is far from clear in regions where climate-driven processes have changed only in intensity, rather than character. We have addressed this question using a mathematical model of landscape evolution3,4 and find that both cases—contemporary balance and relict features—are possible. For a sinusoidal climate fluctuation, we find that all climate states (wet and dry) leave geomorphological signatures only when there is no active uplift. With active uplift and the associated increase in erosion, the topography tracks the current climate and any relict features are likely to reflect only the wettest conditions previously experienced by the landscape. In both cases, the temporal evolution of the landscape in response to cyclic climate forcing is complex, and leads to the unexpected result that valley density is largest during periods dominated by slow downslope movement of sediment, rather than during times of strong fluvial incision, as would be anticipated from steady-state models5–7.

The Economies of Central Asia

Abstract: This is a general introduction to the economies of central Asia, specifically the recently independent countries of Uzbekistan, Kazakhstan, Tajikistan, Kyrgyzstan and Turkmenistan. The book provides a historical and structural analysis of this area of the former Soviet Union, with an emphasis on their economic situation since independence. The first part of the book deals with the five countries' common features, determined by geography and their role in the Soviet division of labour, which left many parts of the region heavily dependent on a cotton monoculture and facing serious environmental problems (notably the shrinking of the Aral Sea and contamination from nuclear testing). The author goes on to deal with the countries as national economies. Finally, he examines common problems facing the countries since they gained independence in late 1991. These last chapters focus on the immediate economic problems of 1992 and 1993 (economic transition and the decision whether or not to remain within the ruble zone), as well as long-term development issues and international economic relations.

Is it possible to derive a reliable estimate of human visceral and subcutaneous abdominal adipose tissue from simple anthropometric measurements

Abstract: The aim of the study was to generate equations predicting visceral (VAT) and subcutaneous (SAT) abdominal adipose tissue (AT) from simple anthropometric measurements. Magnetic resonance imaging (MRI) was used to measure VAT and SAT cross-sectional areas at the level of L4 in 49 subjects (19 men and 30 women) with a large range of age and body mass index (BMI). BMI, waist and hip circumferences, waist to hip ratio (WHR), subscapular and paraumbilical skinfolds (i.e., "simple" anthropometric measurements), total body fat content by the isotope-dilution method, and abdominal sagittal diameter by MRI (i.e., "nonsimple" anthropometric measurements) were also measured. Equations to estimate VAT and SAT from age and simple anthropometric measurements (i.e., excluding total body fat and abdominal sagittal diameter) were developed. These equations were then used in 24 subjects (nine men and 15 women) to cross-validate them. The best regression equations, including waist circumference in men and waist circumference and age in women, explained 56% and 68% of VAT variability, respectively. The corresponding standard error of the estimate (SEE) in men was approximately 40% and in women approximately 37% of the mean value of VAT measured by MRI. The best regression equations developed to predict SAT had a higher explained variability (approximately 87% in both men and women) and a lower SEE (< 20% of the mean values of SAT measured by MRI). In men, the equation included BMI and hip circumference, and in women, BMI and age. The inclusion of a higher number of simple anthropometric parameters in the predictive models neither significantly increased the explained variability of VAT or SAT nor significantly decreased the SEE of VAT or SAT. Also, inclusion in the multiple regression analysis of total body fat content and abdominal sagittal diameter did not improve prediction. In the cross-validation study, differences between predicted and observed values of VAT were large, with a tendency to overestimation in both men and women. In contrast, differences between predicted and observed values of SAT were small. We suggest that SAT but not VAT can be estimated from age and simple anthropometric measurements. Direct methods (MRI, computed tomography [CT], or other options) should be used for assessment of VAT.

Training neural nets with the reactive tabu search

Abstract: In this paper the task of training subsymbolic systems is considered as a combinatorial optimization problem and solved with the heuristic scheme of the reactive tabu search (RTS). An iterative optimization process based on a "modified local search" component is complemented with a meta-strategy to realize a discrete dynamical system that discourages limit cycles and the confinement of the search trajectory in a limited portion of the search space. The possible cycles are discouraged by prohibiting (i.e., making tabu) the execution of moves that reverse the ones applied in the most recent part of the search. The prohibition period is adapted in an automated way. The confinement is avoided and a proper exploration is obtained by activating a diversification strategy when too many configurations are repeated excessively often. The RTS method is applicable to nondifferentiable functions, is robust with respect to the random initialization, and effective in continuing the search after local minima. Three tests of the technique on feedforward and feedback systems are presented. >

Raman scattering from small spherical particles.

Abstract: Raman coupling coefficients are calculated for the acoustic vibrations of a small dielectric sphere The mean components of the strain tensor are calculated for the symmetric and quadrupolar Raman-active vibrational modes, within a continuum approximation which considers a vibrating homogeneous sphere The Raman coupling coefficient depends on the crystalline structure and on the microscopic scattering mechanism For cubic Bravais lattices and for a dipole-induced dipole scattering mechanism, the coupling coefficient of the symmetric vibrations vanishes The Raman intensity of the inner modes is found to be small with respect to that of surface modes The scaling law, which gives the Raman coupling coefficient as a function of the particle size, has been derived The coupling of the sphere with a surrounding elastic medium has been considered and found to cause shift and broadening of the lines This effect can alter significantly the estimated mean value and distribution of particle sizes

By chance and prestige: legal transplants in Russia and Eastern Europe

Abstract: GIANMARIA AJANi is Professor of Private Comparative Law, University of Trento, Faculty of Law. Director of the Department of Law, University of Trento. I wish to thank James Gordley, Professor of Law at the University of California, Berkeley, and Ugo Mattei, Professor of Law at the Universities of Trento and California at Hastings, for their helpful comments. Research for this paper has been done at the University of California, Berkeley and San Diego, and at the University of Leiden, in The Netherlands. 1. This article deals with the fact of a widespread borrowing of legal models in post-Soviet States and Central and Eastern Europe; it also raises some questions on the effects of that borrowing. In order not to lose the focus of this study, I have not entered the terminological debate on the meaning of current notions used to explain legal change. "Legal transplants," "circulation," "influence," "borrowing," are used here as words that refer to the same phenomenon: a wide supply of scholarly and statutory legal models to post-socialist legislators. This is not to say that I do not consider the difference between a set of more general terms (such as borrowing, or influence) that indicate the process of legal change, and narrower concepts (such as legal transplants, or reception), that refer to the result of a circulation. A second note is related to the aptitude of the new models to match the needs of post-socialist societies. Here I am simply casting some doubts on the consistency between the contents of new models supplied by foreign institutions and the needs of the post-socialist economies. Further research on the implementation of the borrowing, as well as on the role of legal professions in the application of new solutions, will help us to understand the capacity and the significance of legal reforms in post-Soviet states and Central and Eastern Europe. On terminology see Wise, "The Transplant of Legal Patterns," 38 Am. J. Comp. L. 1 (1990 Supp.). Following Wolfgang Wiegand: "It is possible to differentiate between transfer, transplantation, importation and reception. A closer observation, however, reveals that such terminology does not adequately describe or explain the effective procedure of reception," "The Reception of American Law in Europe," 39 Am. J. Comp. L. 229, at 236, fi. 14 (1991). Alan Watson has noticed that: "Actually, receptions and transplants come in all shapes and sizes. One might think also of an imposed reception, solicited imposition, penetration, infiltration, crypto-reception, inoculation and so on, and it would be perfectly possible to distinguish these and classify them systematically. [Watson suggests that there is] no point in elaborating a detailed classification of borrowing until individual instances have been examined to see what they reveal." Legal Transplants. An Approach to Comparative Law at 30 (1974).

Enhanced optical properties in porous silicon microcavities

Abstract: We report the experimental investigation of the optical properties of porous silicon embedded in a planar microcavity structure in which both the active layer and the two Bragg reflectors are fabricated by electrochemical processing of a p-type porous silicon wafer. By tuning the cavity resonance energy around the maximum of the porous silicon emission we have observed photoluminescence linewidths as narrow as 18--25 meV and an intensity enhancement of more than one order of magnitude. The experimental results are clarified by theoretical calculations performed with the standard transfer-matrix approach in the framework of a porous silicon quantum-box model.

Failure Mechanisms of a Zirconia-8 wt% Yttria Thermal Barrier Coating

Abstract: Isothermal and cyclic heat treatments of a plasma-sprayed zirconia-8 wt% yttria thermal barrier coating on a nickel superalloy substrate highlighted coating failure mechanisms. A reaction layer formed at the bond coat/ceramic interface, and failure of the coating initiated in the reaction layer due to opening thermal expansion mismatch stresses associated with a nonplanar interface. Crack propagation occurred in the ceramic in all cases, due to the high fracture energy of the interface. The relatively low fracture energy of the coating, in planes parallel to the interface, is the ultimate factor limiting coating lifetime.

Routines, Hierarchies of Problems, Procedural Behaviour: Some Evidence from Experiments

Abstract: A laboratory experiment was performed as replication of the original one created by M. Cohen and P. Bacdayan at Michigan University. It consists in a twopersons card game played by a large number of pairs, whose actions are stored in a computer’s memory. In order to achieve the final goal each player must discover his sub-goals, and must coordinate his action with the partner’s one. The game therefore involves the division of knowledge and cooperation among players, and gives rise to the emergence of organizational routines. It is suggested that the organizational routines, i.e. the sequences of patterned actions which lead to the realization of the final goal, cannot be fully memorized because of their variety and number. It is shown that players do not possess all the knowledge needed by an hypothetical supervisor to play the best strategy: they generally explore only a limited part of the space of the potential rules, and therefore learn and memorize a simple, bounded set of "personal" meta-rules. These meta-rules, also called "production rules" in standard Cognitive Science’s language, are of the form . Each "Condition" can concern either the game configurations or the partner’s action. In the former case the identification of an appropriate "Action" depends on the sub-goals exploration. In the latter it depends on the recognition (or discovery) of interaction rules ; in this eventuality the production rule embodies a dynamic - and possibly cooperative - reaction to the partner’s action. Organizational procedures (routines) therefore emerge as the outcome of a distributed process generated by "personal" production rules . These routines, as in von Hayek’s view, "can be understood as if it were made according to a single plan, although nobody has planned it." (Hayek, 1980, p. 54). Empirical evidence is provided to support the above statements.

Modeling of particles impacting on a rigid substrate under plasma spraying conditions

Abstract: Finite-element methods have been applied for the spreading process of a ceramic liquid droplet impacting on a flat cold surface under plasma spraying conditions. The goals of the present investigation are to predict the geometrical form of the splat as a function of process parameters, such as initial temperature and velocity, and to follow the thermal field developing in the droplet up to solidification. A nonlinear finite-element procedure has been extended to model the complex physical phenomena involved in the impact process. The dynamic motion of the viscous melt in the drops as constrained by elastic surface tensions and in interaction with the developing contact with the target has been coupled to transient thermal phenomena to account for the solidification of the material. A model is used to study the impact of spherical particles of liquid ceramic of given temperature and velocity on a flat, cool rigid surface. The deformation of the splat geometry as well as the evolution of the thermal field within the splat are followed up to the final state and require adaptive discretization techniques. The proposed model can be used to correlate flattening degrees with the initial process parameters.

O futuro do welfare state na nova ordem mundial

Abstract: Os welfare states enfrentam um trade-off, que parece ser universal, entre a criacao de empregos e a igualdade. Apesar da globalizacao ser um componente do dilema, um estudo comparativo de tipos diferentes de welfare state mostra que os arranjos institucionais e as estrategias nacionais de provisao de bem-estar permanecem como variaveis-chave para avaliar o panorama atual e as perspectivas do welfare state no seculo XXI.

Local disorder in crystalline and amorphous germanium.

Abstract: Anharmonic contributions to the pair potential of near-neighbor interactions of crystalline and amorphous germanium have been determined by temperature-dependent x-ray absorption fine structure measurements. The measurements have been carried out at the K edge of Ge in the temperature range 77\char21{}450 K and analyzed by the cumulant method. In c-Ge the temperature dependence of the first four cumulants has been determined for the first three coordination shells. The radial distribution function of the nearest-neighbor atoms has been found Gaussian in the examined temperature range; the second and third shell distance distributions show a symmetric but non-Gaussian behavior even below the Debye temperature. Cumulants have been related to the force constants of the effective pair potential. The harmonic contribution to thermal disorder has been extracted from the second cumulant for each coordination shell and compared with the mean square relative displacements calculated by harmonic models of lattice dynamics; experimental results are in good agreement with theoretical calculations. In amorphous germanium the first shell radial distribution function is asymmetric even at the lowest temperatures. Static and dynamic effects to the local disorder have been separately determined and compared with the results of previous studies.

Non-uniform sediment transport: dynamics of the active layer

Abstract: In the paper die concept of active (or mixing) layer, on which almost all the approaches of non-uniform sediment transport in rivers are based, is reviewed. Accounting for bed level fluctuations, mass balance equations are treated in the same way as Reynolds decomposition for turbulent motion: instantaneous variables are decomposed in the average, over a large number of realizations, and in the fluctuating pan. The resulting equations are correct from a theoretical point of view and have precise physical meanings. The theoretical findings are compared to some experimental data by Ribberink [1987].

An analytical solution for the transient response of saturated linear elastic porous media

Abstract: The motions of fluid and solid phases in saturated porous media are coupled by inertial, viscous and mechanical interactions as described by Biot's equations. A one-dimensional exact analytical solution of the Biot's equations for the completely general solution of the transient problem in saturated, linear, elastic, porous media is presented. The problem is solved by using the Fourier series. The transient response of porous media is shown for typical material properties of a natural granular deposit and for different degrees of viscous coupling. The analytical results show the mechanics of dispersive wave propagation in saturated porous media and they should provide a useful comparison term for the existing numerical solution methods.

Monte-Carlo simulations of the recombination dynamics in porous silicon

Abstract: A simple lattice model describing the recombination dynamics in visible light emitting porous Silicon is presented. In the model, each occupied lattice site represents a Si crystal of nanometer size. The disordered structure of porous Silicon is modeled by modified random percolation networks in two and three dimensions. Both correlated (excitons) and uncorrelated electron-hole pairs have been studied. Radiative and non-radiative processes as well as hopping between nearest neighbor occupied sites are taken into account. By means of extensive Monte-Carlo simulations, we show that the recombination dynamics in porous Silicon is due to a dispersive diffusion of excitons in a disordered arrangement of interconnected Si quantum dots. The simulated luminescence decay for the excitons shows a stretched exponential lineshape while for uncorrelated electron-hole pairs a power law decay is suggested. Our results successfully account for the recombination dynamics recently observed in the experiments. The present model is a prototype for a larger class of models describing diffusion of particles in a complex disordered system.

Excess of low-energy excitations in glasses

Abstract: Low-frequency Raman spectra of metaphosphate glasses (${\mathrm{Sm}}_{2}$${\mathrm{O}}_{3}$${)}_{\mathit{x}}$(${\mathrm{P}}_{2}$${\mathrm{O}}_{5}$${)}_{1\mathrm{\ensuremath{-}}\mathit{x}}$, (${\mathrm{Eu}}_{2}$${\mathrm{O}}_{3}$${)}_{\mathit{x}}$(${\mathrm{P}}_{2}$${\mathrm{O}}_{5}$${)}_{1\mathrm{\ensuremath{-}}\mathit{x}}$, (${\mathrm{Gd}}_{2}$${\mathrm{O}}_{3}$${)}_{\mathit{x}}$(${\mathrm{P}}_{2}$${\mathrm{O}}_{5}$${)}_{1\mathrm{\ensuremath{-}}\mathit{x}}$, and ${\mathrm{GeO}}_{2}$ have been measured in a wide temperature range from T\ensuremath{\approxeq}10 K to the respective glass transition temperatures T\ensuremath{\approxeq}1000 K. Analysis of the Raman data in comparison with complementary low-temperature specific heat, and ultrasonic measurements reveals that the temperature and frequency dependences of the quasielastic scattering and boson peak, as well as anomalous low-temperature specific heat, are in qualitative agreement with the predictions of soft potential model (SPM). In the case of the spectral density of ``excess'' soft modes, which are assumed to coexist and interact with ordinary phonons, the agreement can even be made quantitative under the restricted assumptions that the low-frequency Raman scattering reflects the total (phonon + excess) density of states, and the photon-vibration coupling constant is approximately linearly dependent upon frequency. However neither of these assumptions are compatible with recent formulations of SPM.

Uncertain Boundaries: The Social and Political Construction of European Economies

Abstract: Preface Acknowledgements Introduction: the shifting boundaries between market, politics and society Part I. The Rise and Decline of the Political Regulation of the Economy: 1. The Keynesian welfare state and its crisis 2. Unstable concertation 3. Organised interests and public policies 4. An anomalous case? State, economy and organised interests in Italy Part II. The Micro-Social Regulation of Economic Adjustment: 5. The crisis of political exchange and the growth of micro-concertation 6. The search for flexibility 7. The problem of consensus in production 8. An emblematic case: industrial adjustment and micro-concertation in Italy Conclusion: the uncertain boundaries between macro and micro - the production of collective goods in the European economies Notes References Index.

Novel Naamidine‐Type Alkaloids and Mixed‐Ligand Zinc(II) Complexes from a Calcareous Sponge, Leucetta sp., of the Coral Sea

Abstract: As compounds from a calcareous sponge, Leucetta sp., of the Coral Sea, we isolated a series of novel naamidine-type alkaloids, 6–10, which are oxidized at a single benzylic position. We also report on the first marine, mixed-ligand metal complex 5 and on the first natural metal complexes 3 and 4 derived from classical naamidines, i.e., 1 and 2. The latter are also present in free form in the sponge.

Local search with memory: benchmarking RTS

Abstract: The purpose of this work is that of presenting a version of the Reactive Tabu Search method (RTS) that is suitable for constrained problems, and that of testing RTS on a series of constrained and unconstrained Combinatorial Optimization tasks. The benchmark suite consists of many instances of the N-K model and of the Multiknapsack problem with various sizes and difficulties, defined with portable random number generators. The performance of RTS is compared with that of Repeated Local Minima Search, Simulated Annealing, Genetic Algorithms, and Neural Networks. In addition, the effects of differenthashing schemes and of the presence of a simple “aspiration” criterion in the RTS algorithm are investigated.