Showing papers by "University of Trento published in 1991"

Substantive and procedural uncertainty

Abstract: Different sources of uncertainty are analysed and a representation of decision-making in principle consistent with behavioural evidence is proposed. The endogenous emergence of “innovations”, in the forms of unexpected events and novel behaviours is also examined.

Dynamic mechanical and calorimetric analysis of compression-molded PLLA of different molecular weights : effect of thermal treatments

Abstract: The effect of thermal treatment on compression-molded poly(L-lactic acid) (PLLA) has been investigated by means of viscosimetric molecular weight () determination, differential scanning calorimetry (DSC), and dynamic mechanical thermal analysis (DMTA). Starting from two initial molecular weight PLLAs, amorphous samples with different have been obtained due to degradation occurring during the molding. The crystallization capability of the materials after different thermal treatments has been determined as a function of the molecular weight, and their dynamic mechanical properties have been measured. Initially fully amorphous PLLA matrices attained very high degrees of crystallinity (up to 90%) following different annealing processes. Concomitantly, PLLA degrades due to thermal cleavage of the chains. This is an unavoidable effect that must be taken into consideration when defining the material processing and annealing conditions. Crystallization phenomena occurring in the material during the treatment are clearly documented by DMTA.

6061 aluminium alloy-SiC particulate composite: a comparison between aging behavior in T4 and T6 treatments

Abstract: Kinetics of natural aging and precipitation hardening (T4 and T6 tempers) were compared in 6061 aluminium alloy (AA) and 6061 AA−14vol.%SiC particle composite. Aging processes have been studied by hardness measurements and differential scanning calorimetry (DSC). The latter technique is used as a screening tool to follow the aging sequence; in fact, the DSC thermogram depicts characteristic features of different heat treatments undergone by samples. DSC curves for both 6061 AA and 6061AA-SiCp, after solution treatment, display the same aging path; on the contrary, the two materials differ in solution temperature, which is about 30 °C higher in the case of the composite. The precipitation hardening (at 180°C) occurs faster in the composite than in 6061 alloy. The latter presents a broad hardness peak ranging between 4 and 10 h of aging while the composite shows a sharp peak after about 4 h. Probably this behaviour is due to the high dislocation density close to the metal matrix-SiC particle interface. In these strongly deformed zones nucleation processes are accelerated. The aging kinetics at room temperature for the composite is slower than for the 6061 AA. In fact the two materials show the same aging rate when solution treatment in the case of the composite is complete. On the contrary, the composite ages more slowly than 6061 AA. Hence the high dislocation density at the interface, between composite constituents, does not affect T4 treatment.

Uncertainty principle, quantum fluctuations, and broken symmetries

Abstract: We discuss a natural generalization of the uncertainty inequality holding in the case of non-hermitian operators. The inequality is employed to derive useful constraints on the behaviour of quantum fluctuations in problems with continuous group symmetries. Applications to Bose superfluids, antiferromagnets and crystals at zero temperature are discussed. We provide, in particular, a simple and direct proof of the absence of long range order at zero temperature in the1D case. A new inequality involving the spectral function at finite temperature is finally derived.

Generalized coarea formula and fractal sets

Abstract: For any domain ω of R N (N≥1), the classGC (ω) of functionals Λ:L 1 (ω) → [0, +∞] fulfilling the followinggeneralized coarea formula is introduced: The main properties of this class of functionals are here investigated.\gL r and $$\tilde \Lambda _r $$ also allow us to construct two new definitions offractional dimension for set boundaries. Applications to models ofsurface tension effects in two-phase systems are then outlined. In particular,\gL r and $$\tilde \Lambda _r $$ allow us to represent very irregular phase interfaces.

Temperature-dependence of the inp band-gap from a photoluminescence study

Abstract: A set of parameters for the temperature dependence of the direct band gap of InP has been determined by fitting the excitonic recombination energy in the photoluminescence (PL) spectra between 2 and 250 K. We have used high-quality InP samples grown by chemical beam epitaxy. The superfluid-helium temperature PL spectra are characterized by strong excitonic transitions (the excited state n = 2 of the exciton is observed too) from which we have determined the binding energy of the exciton (E(b) = 5.3 meV) and the band-gap energy (E(g) = 1.4239 eV at 1.3 K). The variable-temperature PL spectra are characterized by excitonic transitions for temperatures as high as 250 K. The expression E(g)(T) = 1.4539 - 0.0359(1 + 2/ {exp[209/T (K)] - 1}) eV, which extrapolates to E(g) = 1.347 eV at 300 K, is proposed for the temperature dependence of the InP band gap over the temperature range 0-300 K.

Chemical characterization of Si-Al-C-O precursor and its pyrolysis

Abstract: Polycarbosilane was modified by reaction with an aluminum alkoxide to get a precursor for Si-Al-C-O ceramics. The precursor was essentially characterized by magic-angle spinning nuclear magnetic resonance (29Si, 27Al, and 13C) and appeared as a dispersion of Al(OH)3-based particles in a polycarbosilane chain matrix. After the material was heat-treated under argon at 1500°C, X-ray diffraction showed that it crystallized mainly as SiC 2H. The presence of this rather unusual polytype for polycarbosilane-derived ceramics seemed to be related to the presence of aluminum atoms.

Computer simulation of plasma-sprayed coatings. I, Coating deposition model

Abstract: This paper develops a model for the deposition process of a thermal barrier plasma-sprayed coating. This takes experimental data for the plasma temperature and enthalpy content and the velocity, temperature and size distributions of the particles. The model uses a set of physically based rules to construct the coating. Simulations obtained using this model are compared with available experimental data. The results from the model are used in the next (companion) paper to calculate the effective elastic and thermal properties of the coating and hence to calculate the stresses within the coating for some simple configurations.

Sum rules for electron nucleus scattering

Abstract: The interaction of electrons with nuclei is well understood and sufficiently weak to allow the separation of nuclear structure aspects from those connected with the reaction itself. Nevertheless the complexity of the nuclear spectrum remains as a stumbling block for a simple interpretation of the large mass of experimental data. Sum rules, which wash out the details of nuclear excited states, allow one to remain with the basic features of the nucleus. The rather long history of electronuclear sum rules has proved the power of the method, while the progress made in the many-body problem, together with the high accuracy of electron scattering data have transformed the sum rule techniques into stringent tests for nuclear theories and experiments.

Asymptotic behavior of the Landau--Lifshitz model of ferromagnetism

Abstract: According to the classical theory of Weiss, Landau, and Lifshitz, on a microscopic scale a ferromagnetic body is magnetically saturated (i.e., |M| =ź: constant) and consists of regions in which the magnetization is uniform, separated by thin transition layers. Any stationary configuration corresponds to a minimum point of an energy functional in which a small parameterź is present. The asymptotic behavior asź ź 0 is studied here. It is easy to see that any sequence of minimizers contains a subsequenceMźj that converges to a fieldM. By means of a Γ-limit procedure it is shown that this fieldM is a minimizer of a new functional containing a term proportional to the area of the surfaces separating different domains of uniform magnetization. TheC1,ź-regularity of these surfaces, forź < 1/2, is also proved under suitable assumptions for the external magnetic field.

Theory and Application of Steklov-Poincaré Operators for Boundary-Value Problems

Abstract: This paper deals with interface operators in boundary value problems: how to define them, which is their meaning in both mathematical and physical sense, how to use them to derive numerical approximations based on domain decomposition approaches. When matching partial differential equations set in adjacent subregions of a domain Ω of ℝn, the interface operators ensure the fulfillement of transmission conditions between the different solutions. From the mathematical side, they make it possible to reduce the overall boundary value problem into a subproblem depending solely on the trace of the solution upon the interface. Once the solution of such a problem is available, the original solution can be reconstructed through the solution of independent boundary value problems within each subregion. The above independency feature is very likely behind the increasing interest for the use in the recent years of interface operators in scientific computing. Indeed, the subdomain approach yields a problem to be solved for the interface gridvalues only, then a family of reduced problems are left to solve simultaneously, hopefully by a multiprocessor architecture.

On the existence of solutions to a problem in multidimensional segmentation

Abstract: We prove the existence of a minimizer for a multidimensional variational problem related to the Mumford-Shah approach to computer vision.

Optimal Design for Two-Dimensional Structures Made of Composite Materials

Abstract: The paper deals with the problem of finding the optimal orientation of orthotropic properties for an elastic body, subjected to a plane state of stress, in order to maximize the stiffness of the body itself

Compact p -Kähler manifolds

Abstract: In this paper we study compact complex non-Kahler manifolds, in particular the holomorphically parallelizable ones, by means of their degree of p-Kahlerianity, which can also be characterized by the absence of a certain class of holomorphic k-forms or of a certain class of currents on the manifold. We give several examples to emphasize the structure and the behaviour of this kind of non-Kahler manifolds.

Synthesis and characterization of SiZrCO ceramics from polymer precursors

Abstract: Polyzirconocarbosilane has been prepared from polycarbosilane and zirconium alkoxide. The structure of this Zr-modified polymer as well as its conversion process into SiC/ZrC crystalline material has been investigated, using mainly MAS-NMR and XRD techniques. This work is part of a more comprehensive study on the pyrolysis process of polycarbosilanes modified with metallic alkoxide M(OR) n (M = Ti, Zr, Al). The crystallization behavior has been compared with results already found for SiTiCO and SiAlCO systems and will be discussed.

Rogiolenyne A, B, and C: The First Branched Marine C15 Acetogenins. Isolation from the Red Seaweed Laurencia microcladia or the Sponge Spongia zimocca of II Rogiolo

Abstract: It is shown here that the red seaweed Laurencia microcladia, collected off the torrent 1l Rogiolo, south of Livorno, contains rogiolenyne A (=(−)-(1R*, 2S*,3R*,5S,7S*)-2-(bromomethyl)-5-[(Z)-1-chlorohex-3-en-5-ynyl]-3-ethyl-4,8-dioxabicyclo [5.1.0] octane; (−)-1) while the sponge Spongia zimocca, which grows in the same small area, contains rogiolenyne B (=(−)-(2R*,3R*,4R*,5R*,7S*)-3-(bromomethyl)-5-chloro-7-[(Z)-1-chlorohex-3-en-5-ynyl]-2-ethyloxepan-4-ol; (−)-4a) and its acetate, rogiolenyne C((−)-4b). These structures, which are based on extensive NMR and MS data and on chemical transformation, are the first examples of branched marine C 15 acetogenins. Biogenesis of (−)-1 in L. microcladia is thought to involve C(12) extrusion form a C 15 linear tetraen-1-yne precursor via H + -induced cyclopropane ring closure, followed by Br + -induced cyclopropane ring opening, aided by C-O − attack (Scheme 2). It is also proposed that transfer of (−)-1 to S. zimocca is followed by epoxide ring opening by Cl − to give (−)-4a and acetylation to give (−)-4b

A Survey of Fuzzy Optimization and Mathematical Programming

Abstract: A brief survey of various concepts, problem classes, issues, etc related to fuzzy optimization and fuzzy mathematical programming is provided Emphasis is on various approaches to fuzzy linear programming as the most important from both the practical point of view and the purpose and scope fo the volume

Synthesis and Characterization of β’-SiAlON Ceramics from Organosilicon Polymers

Abstract: A polycarbosilane has been modified with aluminum alkoxide to obtain a new preceramic compound. The pyrolysis in NH3 flow of this polyaluminocarbosilane leads to the formation of an amorphous Si-Al-O-N phase. The nitridation process has been followed by IR and 29Si MAS-NMR spectroscopies. A fine-grained β-SiAION ceramic is obtained by firing the amorphous phase at 1500°C. The crystallization process has been studied by XRD, 29Si MAS-NMR, and TEM techniques.

Variations in Human Atrial Flutter Cycle Length Induced by Ventricular Beats: Evidence of a Reentrant Circuit with a Partially Excitable Gap

Abstract: Variations in Atrial Flutter Cycle Length. Introduction: The purpose of this investigation was to study the mechanisms responsible for small variations in atrial flutter cycle lengths. Methods and Results: In a study group of 11 patients with common atrial flutter, atrial electrograms were recorded from an intraesophageal lead together with a surface lead (V1). Upon the onset of the QRS complex, atrial flutter intervals consistently increased by an average of 1.8% (SD± 0.9; P <0.01) and subsequently decreased by 2.1% (SD ± 0.8; P <0.01) before returning to the average flutter rate. Carotid sinus massage, which temporally prevented ventricular activation, markedly reduced the variations in atrial flutter intervals. Ventricular pacing at different rates clearly demonstrated that the pattern in atrial flutter intervals was coupled to the moment of ventricular contraction. The hypothesis was formulated that these periodic variations in atrial flutter intervals following a ventricular contraction were caused by the influence of stretch of the atrial myocardium on the conduction properties of a circulating impulse in the atrium. The secondary decrease in flutter rate could be explained if a partial excitable gap is assumed between head and tail of the circus movement. This hypothesis was tested in a simulation study, which revealed that the alternation in intervals as found in patients could only be reproduced if the excitable gap in the circus movement was partially excitable. Conclusion: In conclusion, the analysis of variations in atrial flutter cycle lengths points to a mechanism of circus movement with a partially excitable gap in common atrial flutter.

Random telegraph signals and low-frequency voltage noise in Y-Ba-Cu-O thin films

Abstract: Excess low‐frequency noise extending to MHz frequencies was observed in dc current biased granular high‐Tc thin films. At particular bias conditions random telegraph signal produced by a single, fast two‐level fluctuator dominated the noise properties of the sample. Lifetimes of the low‐ and high‐voltage states of the fluctuating system were found to be exponentially distributed. Power spectra of the excess noise signal could be well fitted with a single Lorentzian contribution. Duty cycle dependence of the random telegraph signal on bias conditions was used to get an insight into physical mechanism causing the fluctuations. Charge trapping events in the intergranular intrinsic Josephson junctions and trapped flux hopping were identified as possible alternative sources of the observed noise.

Modification of lysine residues of Staphylococcus aureus α-toxin: Effects on its channel-forming properties

Abstract: Staphylococcus aureaus α-toxin opens an ion channel in planar phospholipid bilayers which is selective for anions over cations, supposedly because of the presence of positively charged groups along the ion pathway. To remove some positive charges of this protein toxin, we chemically modified part of its lysine residues either with diethylpyrocarbonate, followed by histidine regeneration with hydroxylamine, or with trinitrobenzenesulfonic acid. The extent of chemical modification can be followed accurately by native polyacrylamide gel electrophoresis and isoelectric focusing. Ethoxyformilation of two to three lysine residues per toxin monomer does not impair hemolysis of rabbit red blood cells nor formation of pores in model membranes. It reduces the conductance and the anion selectivity of the channel and changes the shape of its current-voltage characteristic. This indicates that positively charged lysine residues are actually important in determining the electrical properties of the pore. Ethoxyformilation of channels preassembled in planar bilayers produces the same changes as modification of toxin monomers before channel formation. Furthermore, it can be performed by adding diethylpyrocarbonate on either side of the bilayer. This suggests that the lysine residues relevant for the electrical properties of the pore are located inside its lumen where they can be reached by diethylpyrocarbonate diffusing from either entrance of the channel.

Disorder-induced light scattering in solids: Microscopic theory and applications to some model systems

Abstract: A microscopic approach to evaluate the disorder-induced light-scattering spectra in solids is presented. A quite general theory is developed and then specialized to some model systems that can represent an extended class of real systems. The general features of the disorder-induced spectra of real systems can be predicted, once the microscpic disorder and the mechanism responsible for the induced polarizability are correctly taken into account.

Collective excitations in deformed alkali metal clusters

Abstract: A theoretical study of collective excitations in deformed metal clusters is presented. Sum rules are used to study the splittings of the dipole surface plasma resonance originating from the cluster deformation. The vibrating potential model is developed and used to predict the occurrence of a low lying collective mode of orbital magnetic nature.

A Novel, Degraded Polyketidic Lactone, Leptosphaerolide, and Its Likely Diketone Precursor, Leptosphaerodione. Isolation from Cultures of the Marine Ascomycete Leptosphaeria oraemaris (LINDER)†

Abstract: Acetone extraction of cultures of the marine ascomycete Leptosphaeria oraemaris (LINDER) on cornmeal disk gave the novel polyketide derivative leptosphaerolide ( = (+)-7-[(1E)-l,3-dimethylpent-1-enyl]-10-hydroxy-3-methoxybenzo[1,2-b:5,4-c′]dipyran-2(9H)-one; (4+)-8) besides the o-dihydroquinone 3-[(1E)-1,3-dimethylpent-1-euyl]-8,10-dihydroxy-7-methoxy-8-(2-oxopropyl)-1H-naphtho[2,3-c]pyran-9(8H)-one (1) as a 10:9 mixture of epimers. retro-Aldol reaction of 1 gave leptosphaerodione ( = (−)-3-[(1E)-1,3-dimethylpent-1-enyl]-10-hydroxy-7-methoxy-1H-naphtho[2,3-c]pyran-8,9(8H)-dione; (−)-6) which was also present in small amounts in the extracts and which gave 1 on reaction with acetone. It is thus likely that 1 is an artefact of the extraction by acetone. Biogenetically (+)-8 might derive from (−)-6 via an unusual oxidation with loss of CO2.