Showing papers by "University of Trento published in 1990"

Expansion of the global error for numerical schemes solving stochastic differential equations

Abstract: Given the solution (Xt ) of a Stochastic Differential System, two situat,ions are considered: computat,ion of Ef(Xt ) by a Monte–Carlo method and, in the ergodic case, integration of a function f w.r.t. the invariant probability law of (Xt ) by simulating a simple t,rajectory. For each case it is proved the expansion of the global approximat,ion error—for a class of discret,isat,ion schemes and of funct,ions f—in powers of the discretisation step size, extending in the fist case a result of Gragg for deterministic O.D.E. Some nn~nerical examples are shown to illust,rate the applicat,ion of extrapolation methods, justified by the foregoing expansion, in order to improve the approximation accuracy

Simultaneous structure and size-strain refinement by the Rietveld method

Abstract: A new procedure for simultaneous refinement of structural and microstructural disorder parameters for polycrystalline materials is proposed. It is based on the Rietveld method combined with Fourier analysis for broadened peaks. Crystallite size and shape and r.m.s. microstrain are regarded as fitting parameters, replacing the well known formula of Caglioti, Paoletti & Ricci [Nucl. Instrum. Methods (1958), 3, 223–228] for the angular dependence of the peak width. In particular, from these microstructural disorder parameters, by inverting the Warren–Averbach procedure [Warren & Averbach (1950). J. Appl. Phys. 21, 595–599; (1952), 23, 1059] for a single peak, it is possible to obtain the parameters of the pseudo-Voigt (pV) functions employed to fit the experimental data. The anisotropy of the crystallite size and microstrain is also taken into account. The method has been tested on three materials with different degrees of crystallization: tetragonal ZrO2 (P42/nmc, a = 3.5961, c = 5.1770 A, Vc = 66.95 A3, Z = 2, Rwp = 0.077, M ≃100 A, 〈∊2〉1/2 ≃ 3 × 10−3); tetragonal Zr0.82Ce0.18O2 (P42/nmc, a = 3.6419, c = 5.2440 A, Vc = 69.556 A3, Z = 2, Rwp = 0.0654, M ≃1000 A, 〈∊2〉1/2 ≃8 × 10−4); α-Al2O3 (R{\bar 3}c, a = 4.7605, c = 12.9956 A, Vc = 255.05 A3, Z = 6, Rwp = 0.0684, M ≃ 1400 A, 〈∊2〉1/2 ≃7 × 10−4).

Density of states and evaporation rate of helium clusters

Abstract: The density of states of4He clusters is calculated on the assumption that only surface vibrations are thermally excited Results for mixed3He-4He and3He clusters are also given The Weisskopf procedure is used to calculate the evaporation rates and the cooling laws of helium clusters at low temperatures

Population models for diseases with no recovery

Abstract: An S → I epidemic model with a general shape of density-dependent mortality and incidence rate is studied The asymptotic behaviour is global convergence to an endemic equilibrium, above a threshold, and to a disease-free equilibrium, below the threshold The effect of vaccination is then examined

Electrical conductivity of tin oxide films prepared by the sol-gel method

Abstract: A preliminary study concerning the electrical conductivity of tin oxide films obtained by a procedure of dipping sol-gel technique onto glass substrate is reported. The tin oxide-sol solution was prepared starting from stannous chloride. Crystallization of amorphous tin oxide was followed by means of XRD and DTA analyses. The morphology of the coatings was observed by SEM. By this method, it has been verified that tin oxide can be easily doped by means of zirconium and titanium oxides, giving films, in which electrical conductivity is strongly increased.

Low-frequency dynamics in superionic borate glasses by coupled Raman and inelastic neutron scattering.

Abstract: We report a study of low-frequency vibrational dynamics and electron-vibration coupling in AgI-doped silver borate glasses, which have been shown to have anomalies which could be connected to microscopic fractality. By using both Raman scattering and time-of-flight neutron-scattering spectroscopies, we were able to determine the vibrational density of states and, for the first time in a fractal system, the frequency dependence of the electron-vibration coupling function C(\ensuremath{\omega}). The results confirm the microscopic fractality of this system. In particular the fracton dimensionality was determined to be 1.4, and a scaling law behavior of C(\ensuremath{\omega}) was confirmed.

On quantum stochastic differential equations with unbounded coefficients

Abstract: We prove an existence, uniqueness and unitarity theorem for quantum stochastic differential equations with unbounded coefficients which satisfy an analyticity condition on a common dense invariant domain. This result, applied to the quantum harmonic oscillator, gives a rigorous meaning to a large class of stochastic differential equations that have been considered formally in quantum probability.

On a Consensus Measure in a Group MCDM Problem

Abstract: A method for consensus measuring in a group decision problem is presented for the multiple criteria case. The decision process is supposed to be carried out according to Saaty’s Analytic Hierarchy Process, and hence using pairwise comparison among the alternatives. Using a suitable distance between the experts’ judgements, a scale transformation is proposed which allows a fuzzy interpretation of the problem and the definition of a consensus measure by means of fuzzy tools as linguistic quantifiers. Sufficient conditions on the expert’s judgements are finally presented, which guarantee any a priori fixed consensus level to be reached.

Surface interaction potentials in elasticity

Abstract: The problem of modelling body-environment interactions is a rather formidable one: neither is there a wealth of inspiring examples nor is the corpus of knowledge in mathematical analysis such as to allow indiscriminate generalization of the few well understood cases.

Energetic ion beam analysis in the earth sciences

Abstract: Analytical techniques using energetic ion beams are now being used to address important problems of water–rock interaction and mantle dynamics. Their unique capability is to probe the outermost few micrometres of minerals and glasses, providing multi-element depth profiles of this very important surface layer.

Optimal patterns of growth and reproduction for perennial plants with persisting or not persisting vegetative parts

Abstract: Optimal allocation of energy to growth, reproduction and storage was considered for perennial plants differing in the proportion of vegetative structures persisting over winter and/or in the amount of resources which can be relocated to storage before abscission of some organs. It was found that for every mortality level there exists a critical proportion of persistent organs. Below this critical value it is optimal to grow without reproduction for the first years until a characteristic size is reached; afterwards, that size is maintained year after year and all extra resources are devoted to reproduction. Some storage is also necessary to maintain constant size. If the proportion of retained vegetative mass is above the critical value, the optimal strategy is gradual growth to an asymptotic size, with growth and reproduction occurring in several years following maturation. In this case real storage occurs only until maturation is reached, then storage is realized only by energy relocation from the vegetative body. Although the optimal solution changes abruptly qualitatively at a given proportion of resources saved from year to year, further growth of this proportion above the critical level brings about a greater difference between size reached at maturity and final size. The predictions of the model seem to follow the pattern of nature qualitatively.

Study of the diamond-matrix interface in hot-pressed cobalt-based tools

Abstract: The interface between the constituents of a tool material produced by hot pressing of diamond particles and a metallic powder was studied. Morphological, structural and chemical analyses by scanning electron microscopy, X-ray diffraction and Auger electron spectroscopy were performed in order to characterize the interaction between diamond and cobalt powder during the consolidation. They revealed that graphite, cobalt carbide and a CoC solid solution do form and are linked to the action of strong bonding between the constituents, but that diamond deterioration is also induced. To avoid graphitization of the diamond, and to improve the technological properties of the composite material, low percentages of tin were added to the metallic matrix; this proved to be effective.

Numerical study of Raman scattering from fractals.

Abstract: Raman scattering from square and cubic site-percolation lattices has been calculated and a linear scalling for the Raman coupling coefficient is found. Our results show the fluctuation origin of Raman scattering, thereby invalidating the models used so far which are based on scaling properties of the local strain and on the existence of a smooth vibrational wave function

The application of age, period and cohort models to predict Swiss cancer mortality.

Abstract: In order to project trends in mortality from 11 major cancer sites in Switzerland to the end of the current century, a log-linear Poisson age/period/cohort model with arbitrary constraints on the parameters was used, fitted to the observed rates for the period 1950–84. One projection was based on the assumption of a total absence of change in the effect of period, the second was based on a linear extrapolation of the logarithms of the seven known periods, and the third was related to a series of a priori external epidemiological hypotheses, whenever available. For instance, coefficients below unity were used for lung and other tobacco-related neoplasms in men, since some decline in exposure to tobacco carcinogens was observed among Swiss men, and above unity for women since the prevalence of smoking has risen among successive generations of women. Although the method has limitations and uncertainties, several qualitative indications could be derived from this exercise. For instance, the various models suggest that the age-standardized mortality from oral cancer in men will probably increase up to the end of the century, even under the optimistic assumption of an appreciable decline in smoking, while cancer of the oesophagus is likely to level-off around current values, as other tobacco-related neoplasms, prostate cancer in men, and breast cancer in women will probably do. Some steady decline is predicted by various models fitted to the incidence of stomach and intestinal cancer in both sexes, and to ovarian cancer. Lung cancer will continue to rise in women but will stop rising in men, and it will possibly fall if the hypothesis of a decline in exposure to tobacco carcinogens proves correct. Although any prediction has, by definition, substantial difficulties and uncertainties, projections of cancer mortality in the near future are based on a substantial amount of information already available, and may offer valuable information for epidemiological inferences and health planning purposes.

Surface engineering and chemical characterization in ion-nitrided titanium and titanium alloys

Abstract: The chemical and physical characteristics of ion-nitrided surface layers, obtained on α-β titanium alloys, are examined and correlated both with the working conditions adopted in the ion-nitriding process and with the alloy chemical composition. Besides the influence of the working parameters on the morphology and on the microstructures of the ion-nitrided surface layers, mainly the alloy element distributions both in surface coatings and in the substrate are analysed for five α-β titanium alloys of industrial use, and for titanium c,p. as reference, ionnitrided at various treatment temperatures. The nitriding process forms, on titanium alloy parts, high-hardness surface layers consisting of TiN (δ phase) and Ti2N (ɛ phase) nitrides and an interstitial solid solution of nitrogen in the close-packed hexagonal lattice of titanium (α phase). The presence and the extent of these phases as well as the ion-nitrided layer morphology are essentially determined by the alloy chemical composition and the working parameters. In particular a low-temperature treatment produces an extended nitrogen diffusion in the matrix beneath a thin continuous nitrided layer, while a high-temperature treatment produces prevalently a continuous nitrided surface layer. The alloy element distribution appears differentiated in the various phases and may be correlated with the different affinity of these elements with nitrogen.

Short range order in AgI:Ag2O:B2O3 glasses: results from EXAFS and related techniques

Abstract: The structure of the (AgI) x (Ag 2 O n B 2 O 3 ) 1−x superionic conducting glasses is being studied by X-ray absorption spectroscopy (EXAFS and related techniques). This paper reviews the results obtained up to date. The information obtained concerns the local environment of silver and iodine atoms and allows us to place constraints on the structural models consistent with the conductivity properties of the glasses.

Metal–GaAs interaction and contact degradation in microwave MESFETs

Abstract: This work reports and critically reviews failure mechanisms induced by metal-GaAs interaction and contact degradation in low and medium power GaAs MESFETs in the framework of a comprehensive reliability evaluation test plan, performed mainly on commercially purchased devices manufactured by different technologies. The results show that, at least as regards contact degradation phenomena, these technologies have reached sufficient maturity, and significant reliability levels have been achieved even for the most severe application and environments. Devices coming from some suppliers still suffer from reliability problems. such as ‘sinking’ of Au-based gate metallization into the active channel. Al electromigration, Al/GaAs interdiffusion enhanced by high contacts electromigration, surface metal migration and short circuiting of closely spaced electrodes on GaAs with a non-suitable surface preparation and/or passivation. All these failure mechanisms have been identified by means of suitable microanalytical techniques, correlated with device electrical degradation and thoroughly discussed in this paper by comparison with results previously reported in the technical literature.

The radiocarbon chronology of the neolithic and copper age of northern italy

Abstract: Summary. Until twenty years ago the chronology of the Neolithic and Copper Age settlement of Northern Italy was almost exclusively based on the stratigraphical sequence of the Arene Candide cave in Liguria. The research carried out since the sixties has strongly increased our knowledge of the earliest farming communities and the first copper using people who inhabited the country between the end of the seventh and the beginning of the fourth millennium BP. This article considers the available evidence for this period which is now supported by a good set of radiocarbon dates.

Isolation of the Cembranoid Preverecynarmin Alongside Some Briaranes, the Verecynarmins, from Both the Nudibranch Mollusc Armina maculata and the Octocoral Veretillum cynomorium of the East Pyrenean Mediterranean Sea†

Abstract: It is shown that the nudibranch mollusc Armina maculata and its prey, the pennatulacean octocoral Veretillum cynomorium, of east Pyrenean waters contain a novel cembranoid, preverecynarmin ((+)-8), besides the known cembranoid cembrene C (9) and three further briarane diterpenoids (verecynarmin E ((−)-5), F ((−)-6), and G ((−)-7)). A biogenetic scheme is proposed.

A novel glyceryl ester (glyceryl dendryphiellate A), a trinor‐eremophilane (dendryphiellin A1), and eremophilanes (dendryphiellin E1 and E2) from the marine deuteromycete Dendryphiella salina (SUTHERLAND) PUGH et NICOT

Abstract: Continuing studies of the global extracts from cultures of the marine deuteromycete Dendryphiella salina have led to the isolation of novel compounds that add to the scarce list of marine fungal metabolites. Besides (22E)-ergosta-4.6,8(14),22-tetraen-3-one which, though known from basidiomycetes, was unknown in the sea, they are an unusual glyceryl ester, i.e. glycer-1-yl dendryphiellale A (= (+)-(2R)-2,3-dihydroxyprop-l-yl (6S,2E,4E)6-methylocta-2,4-dienoate; (+)-1), a trinor-eremophilane, i.e. dendryphiellin A1 ( = (+)-(3R*,4E,6E)-7-{[(1R*,2S*,7R*,8aR*)-1,2,6,7,8,8a-hexahydro-7-hydroxy-1,8a-dimethyl-6-oxonaphthalen-2yl]oxycarbonyl}-3-methylhepta-4,6-dienoic acid; (+)-11), and two eremophilanes, i.e. dendryphiellin El ( = (+)-(1R*, 2S*, 7S*,8aR*)-1,2,6,7,8,8a-hexahydro-1,8a-dimethyl-7-(1-methylethenyl)-6-oxonaphthalen-2-yl(6S,2E,4E)-6-methyl-octa-2,4-dienoate; (+)-13) and dendryphiellin E2 ( = (+)-(1R*, 2S*, 8aR*)-1,2,6,7,8,8a-hexahydro-7-isopropyl-idene-1,8a-dimethyl-6-oxonaphthalen-2-yl (6S,2E,4E)-6-Methylocta-2,4-dienoate; (+)-14). Absolute configurations have been established for (+)-1via total synthesis and for the acid portion of (+)-13 and (+)-14via transesterification in NaOMe/MeOH which gave in both cases melhyl dendryphiellate A ((+)-16) of known configuration and the free alcoholic moiety of (+)-14, i.e. (+)-17.

The pyrolysis process of a polytitanocarbosilane into SiC/TiC ceramics: An XPS study

Abstract: The pyrolysis process of polytitanocarbosilane, precursor for SiC/TiC ceramics, has been followed step by step using x-ray photoclectron spectroscopy. This study shows that Ti–O bonds, present in the precursor, are stable up to 700 °C. Above this temperature, Ti–C bonds, precursor for TiC network, start to form. From a detailed analysis of Ti(2p) and Si(2p) peaks, the formation of intermediate species such as SiCxOy and TiCxOy has been detected.