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  • 1990-1994  (881)
  • 1970-1974
  • 1994  (881)
  • Engineering General  (702)
  • Conference proceedings  (179)
  • 101
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2119-2126 
    ISSN: 0392-6737
    Keywords: Other low-power superconducting devices ; Superconducting films ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary YBCO thick films were sprayed on amagnetic stainless steel containing chromium with the continuous-detonation spray technique. The presence of chromium requires a buffer layer to be sprayed between the metal and the superconductor. Preliminary results using a nickel-based buffer layer are reported, which show a shielding effectiveness comparable with that of coatings made with the same technique on silver. Preliminary results on multilayers, using YBCO and either silver or CuO as interspacing layer, are reported. The main problem which arises is the lack of full oxygenation of the underneath YBCO layers.
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  • 102
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 737-745 
    ISSN: 0392-6737
    Keywords: Neutron scattering techniques (including small-angle scattering) ; Mixed systems ; liquid helium-3,-4 mixtures ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary An introduction is given to the measurement of atomic momentum distribution by scattering of high-energy neutrons. Examples of measurements on liquid hydrogen, solid lithium, quantum fluids Ne and3He-4He mixtures are given. Future prospects for this new technique are discussed.
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  • 103
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 795-802 
    ISSN: 0392-6737
    Keywords: General studies of phase transitions ; Order-disorder and statistical mechanics of model systems ; Other phase studies of systems ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We consider a spin system with competing interactions isotropic with respect to the axes of a cubic lattice in the presence of an external field. We show that for small values of the external fieldH, the paramagnetic to modulated phase transition is fluctuation-induced first order, while for larger fields, such transition changes to continuous at a tricritical point. Applications for fluids systems are proposed.
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  • 104
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 831-834 
    ISSN: 0392-6737
    Keywords: Renormalization-group, fractal and percolation studies of phase transitions ; Chemical kinetics: Computational modelling: simulation ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We outline a novel method of exploiting distributed computational resources to study problems in soft-matter science. We have applied our method to simulations of polymers, including hydrodynamic effects.
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  • 105
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 849-854 
    ISSN: 0392-6737
    Keywords: Polymers, elastomers, and plastics ; Anelasticity, internal friction, and mechanical resonances ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The elastic modulus and tg δ of poly(2-hydroxyethyl methacrylate) (PHEMA) and its hydrogels have been measured over, the frequency range 0.3–30Hz and the temperature range 120–450 K. The mechanical spectra reveal the presence of a low-temperature γ-relaxation plus a shoulder (β-relaxation) that merges into the α-relaxation. The absorbed water lowers the temperature and rises the hight of the lowest temperature peak (the γ-peak for dry PHEMA) while lowering both temperature and height of the α-peak. This behaviour is interpreted in terms of hydrogen-bonding effects between water and polymer which weaken the intra- and/or interchain interactions. Furthermore, the calorimetric analysis in dry polymer is consistent with the mechanical behaviour in the higher-temeprature region.
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  • 106
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 883-893 
    ISSN: 0392-6737
    Keywords: Diffusion and ionic conduction in liquids ; Studies of specific liquid structures ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Capillary Pumped Loop (CPL) is a thermal management device for advanced spacecrafts. This paper presents the operating principles of a CPL and the design characteristics of the components. Another purpose of this experiment was to build up a transparent loop in order to better determine and observe the internal thermofluid dynamics. The results indicated that the CPL could be in normal operation under various heat modes. Observation also told Non-Condensed Gas (NCG) was moving and collecting to somewhere between the condenser section.
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  • 107
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1223-1230 
    ISSN: 0392-6737
    Keywords: Order-disorder and statistical mechanics of model systems ; Fluctuation phenomena, random processes, and Brownian motion ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary An analytic study of a model of diffusion on random comb-like structures in which a bias field may or not exist along the backbone is presented. First, when no bias is present, the method allows to compute in an exact manner, for any given disordered structure, the asymptotic behaviour at large time of the probability of presence of the particle at its initial site and on the backbone, and of the particle position and dispersion. The expressions of these quantities are shown to coincide asymptotically with those derived in simple «mean-field» treatments. The results for any given sample do not depend on the particular configuration (self-averaging property). When a bias field is present along the backbone, one can also compute directly in an exact manner the asymptotic behaviour at large time of the disorder average of the probability of presence of the particle at its initial site. As for the particle position and dispersion, they can be computed in a periodized system of arbitrary periodN. The corresponding quantities for the random system can then be obtained by taking the limitN→∞. As a result, in both cases the behaviours strongly depend on the distribution of the lengths of the branches. With an exponential distribution transport is normal while anomalous drift and diffusion may take place for a power law distribution (when long branches are present with sufficiently high weights).
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  • 108
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 825-830 
    ISSN: 0392-6737
    Keywords: Polymers, elastomers, and plastics ; Anelasticity, internal friction, and mechanical resonances ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Calorimetric measurements by DSC technique have been made in pure polyethylene glycol (PEG) and oxide (PEO) polymers having a very wide molecular-weight range (from 600 to 4000 000) and in PEO (MW 600 000)-NaSCN complexes. It was found that the melting temperature increases with increasing molecular weight, ranging from 293 K in the polymer with MW=600 to 340 K in that with MW=4000 000. The behaviour of the heats of fusion with increasing molecular weight reflects the trend expected in systems, in which the increase of the main chain length produces a relevant growth of the degree of crystallinity. A distinct maximum of the heat of fusion has been found at about MW=10000, this result being an evidence of the high amount of crystalline regions building up the structure of that system. The dependence of the melting temperature on molecular weight has been nicely accounted for by using the expression of Flory, deduced from the statistical theory of polymers having the most probable molecular-weight distribution. The addition of sodium thiocyanate to PEO modifies the morphology of the host polymer and, for salt concentrations higher than 0.03 molar fraction, gives rise to the formation of a PEO-salt crystalline complex characterized by a high melting temperature.
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  • 109
    ISSN: 0392-6737
    Keywords: Polymers, elastomers, and plastics ; Mechanics and rheology of solids ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Electrical-impedance measurements have been made in the frequency range 5 Hz to10 MHz in pure poly(ethylene oxide) having a molecular weight of 600 000 from 254 K nearly up to the melting point of the crystalline phase (about 330 K). As the temperature approaches the melting point there are large increases in the realε′ and imaginaryε″ parts of the dielectric constant. The frequency dependence ofε′ is characterized by a primary-relaxation process, whose frequency increases with increasing temperature as a consequence of decrease of the average structural relaxation time. There is strong evidence that this low-frequency dispersion rises mainly from the diffusive transport of localised charge carriers rather than a purely orientation relaxation process. In addition the effects of hydrostatic pressure (0–25 Gpa) on the frequency dependences of the realε′ and imaginaryε″ parts of the dielectric constant have been measured in the same temperature range.
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  • 110
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1177-1180 
    ISSN: 0392-6737
    Keywords: Colloids ; Renormalization-group, fractal, and percolation studies of phase transitions ; Fluctuation phenomena, random processes, and Brownian motion ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We have recently reported on a new scaling theory of the rupture of colloid aggregates. The scaling theory is compared with a simulation study of the transient response of sheared 2d aggregates. In this paper we discuss the origins of deviations from the predicted scaling behaviour. We also report some of the steady-state configurations formed post rupture.
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  • 111
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 993-1025 
    ISSN: 0392-6737
    Keywords: Amorphous materials ; glasses ; Liquid-liquid transitions ; Glass transitions ; Molecular dyanamics, conformational changes, and pattern recognition in biomolecules ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Glass formation in nature and materials science is reviewed and the recent recognition of polymorphism within the glassy state, polyamorphism, is discussed. The process by which the glassy state originates during the continuous cooling or viscous slowdown process, is examined and the three canonical characteristics of relaxing liquids are correlated through the fragility. The conversion of strong liquids to fragile liquids by pressure-induced coordination number increases is discussed, and then it is shown that for the same type of system it is possible to have the same conversion accomplished via a first-order transition within the liquid state. The systems in which this can happen are of the same type which exhibit polyamorphism, and the whole phenomenology can be accounted for by a recent simple modification of the van der Waals model for tetrahedrally bonded liquids. The concept of complex amorphous systems which can lose a significant number of degrees of freedom through weak first-order transitions is then used to discuss the relation between native and denatured hydrated proteins, since the latter have much in common with plasticized chain polymer systems. Finally, we close the circle by taking a short-time-scale phenomenon given much attention by protein physicists,viz., the onset of an anomaly in the Debye-Waller factor with increasing temperature, and showing that for a wide variety of liquids, including computer-simulated strong and fragile ionic liquids, this phenomenon is closely correlated with the experimental glass transition temperature. This implies that the latter owes its origin to the onset of strong anharmonicity in certain components of the vibrational density of states (evidently related to the boson peak) which then permits the system to gain access to its configurational degrees of freedom. The more anharmonic these vibrational components, the closer to the Kauzmann temperature will commence the exploration of configuration space and, for a given configurational microstate degeneracy, the more fragile the liquid will be.
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  • 112
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1199-1204 
    ISSN: 0392-6737
    Keywords: Magnetic liquids ; Pattern selection ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Phase separation was found in an eicosane-based ferrofluid in zero applied field. Upon applying a magnetic field, the condensed phase breaks up to form a lattice of periodic columns. The periodicity λ scales with the layer thicknessL. Two scaling regimes were found. When thickness is small, where no branching is observed, λ ∞L 1/2. In the large-thickness regime, a tree type of structure develops at the tail of each column, and the columns are separated by side bands while the scaling relation crosses over to λ ∞L 2/3.
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  • 113
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1237-1241 
    ISSN: 0392-6737
    Keywords: Birefringence (including stress birefringence, flow birefringence, etc. ; Fluctuation phenomena, random processes, and Brownian motion ; Extinction, absorption, scattering, and transmission coefficients, emissivity ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We study the slow aggregation of aqueous suspensions of optically isotropic microspheres by means of transient electric birefringence. Aggregating clusters exhibit large Kerr constants, and consequently we provide direct experimental evidence that the clusters do posses form anisotropy. From the birefringence relaxation time we derive information on the average cluster size and on the size distribution, and determine their evolution during the aggregation process. Static and dynamic light scattering data confirm the validity of the transient electric birefringence results.
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  • 114
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1339-1356 
    ISSN: 0392-6737
    Keywords: General, theoretical, and mathematical biophysics (including logic of biosystems, quantum biology, and relevant aspects of thermodynamics, information theory, cybernetics, and bionics) ; Statistical mechanics ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We present evidence supporting the idea that the DNA sequence in genes containingnoncoding regions is correlated, and that the correlation is remarkably long range-indeed, base pairsthousands of base pairs distant are correlated. We do not find such a long-range correlation in the coding regions of the gene; we utilize this fact to build aCoding Sequence Finder algorithm, which uses statistical ideas to locate the coding regions of an unknown DNA sequence. We resolve the problem of the «non-stationarity» feature of the sequence of base pairs (that the relative concentration of purines and pyrimidines changes in different regions of the mosaic-like chain) by describing a new algorithm calledDetrended Fluctuation Analysis (DFA). We address the claim of Voss that there is no difference in the statistical properties of coding and noncoding regions of DNA by systematically applying the DFA algorithm, as well as standard FFT analysis, to every DNA sequence (33 301 coding and 29 453 non-coding) in the entire GenBank database. We describe a simple model to account for the presence of long-range power law correlations (and the systematic variation of the scaling exponent α with evolution) which is based upon a generalization of the classic Lévy walk. Finally, we describe briefly some recent work showing that thenoncoding sequences have certain statistical features in common with natural languages. Specifically, we adapt to DNA the Zipf approach to analyzing linguistic texts, and the Shannon approach to quantifying the «redundancy» of a linguistic text in terms of a measurable entropy function. We suggest that noncoding regions in eukaryotes may display a smaller entropy and larger redundancy than coding regions for plants and invertebrates, further supporting the possibility that noncoding regions of DNA may carry biological information.
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  • 115
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1391-1400 
    ISSN: 0392-6737
    Keywords: Physical chemistry of solutions of biological macromolecules ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Glucosidic surfactants play an important role in living matter. An interesting class of them are the gangliosides which have a double-tail hydrophobic part, like phospholipids, and a ramified oligosaccharide chain as polar head. The large hydrophobic moiety ensures very low CMC values, of the order of 10−8 M. Phospholipids have a small head group compared with their hydrophobic part. Therefore they form mainly bilayer-type structures in water. Gangliosides, instead, may form either micelles or bilayers depending on the length and conformation of the oligosaccharide chain. For long chains, micelles are formed with a large aggregation number and a long lifetime (many hours), while vesicles are found for short sugar chains. The presence of a large head group makes vesicle mechanical properties rather peculiar. For instance, the ganglioside GM3, which has the longest sugar chain compatible with bilayer-type structures, forms vesicles spontaneously in solution without supply of external energy. This means that the bending rigidity of the GM3 bilayer is very low, allowing very large thermal undulations. GM3 vesicles, made of a single surfactant, are then found to be in thermodynamic equilibrium with a small amount of large bilayer and multilamellar structures. Frustration of the single-component vesicle system is released if a second amphiphile, of the same type of the ganglioside GM3 but with a larger head group, is added. In this case, spontaneous-curvature readjustements of the two monolayers via demixing can give rise to a spontaneous curvature of the bilayer, energetically favouring vesicles over other structures. It is, in fact, observed experimentally that by adding increasing proportions of GM1 the large aggregates gradually diminish in number, until a pure vesicle solution is obtained for a GM1 to GM3 ratio of 35 to 65.
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  • 116
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1565-1571 
    ISSN: 0392-6737
    Keywords: Spectra, photodissociation, and photoionization of biomolecules, bioluminescence ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Experimental research plays an important role to point out non-invasive diagnostic techniques to show eventual pathologies of the breast. The aim of the present investigation is to study the specific response of complex liquids as solutions originating from normal and neoplastic mammary tissue in the interaction with e.m. waves, in order to discover all the eventual frequencies at which e.m. radiation can be selectively absorbed by neoplastic mammary tissue, chromophores to eventually use these elements in diagnosis of these neoplastic diseases. Characteristic chromophores absorption has been enhanced in spectra of samples drawn from neoplastic tissues. We did this by carrying out a spectrophotometric examination in the IR, VIS and UV range of solutions of the various fractions of healthy and neoplastic mammary tissue, obtained after preparation including differentiated ultracentrifugation of the nuclear, mitochondrial and lysosomial fractions.
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  • 117
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1595-1599 
    ISSN: 0392-6737
    Keywords: Thermodynamics of surfaces and interfaces ; Disperse systems ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We present a model for a polydisperse ensemble of two-dimensional droplets wich accounts for the effects of arbitrarily large distortions of the droplet shape. Interactions within a droplet include bending rigidity and spontaneous curvature. Interactions between droplets are omitted. Even at high temperatures, the effects of the shape fluctuations on the droplet size distribution remain small, as they are dominated by the contributions from the mixing entropy. In contrast, shape fluctuations lead to a pronounced peak in thermodynamic quantities like specific heat. This peak occurs at temperatures where thermal excitations become of the order of the bending energies of the droplet surface. The fluctuation-dominated regime extends to temperatures far lower than expected from a mean-field calculation.
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  • 118
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1627-1633 
    ISSN: 0392-6737
    Keywords: Brillouin and Rayleigh scattering ; Liquids ; Emulsions and suspensions ; Order-disorder and statistical mechanics of model systems ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We have performed extensive small-angle light scattering (SALS) measurements on a three-component microemulsion (AOT/decane/water) as a function of the dispersed phase concentration and the temperature. All samples have a water/AOT molar fractionw=40.8. Such a system presents a very complex phase diagram with many structural configurations. With the SALS technique, we have been able to observe all the phase separation lines. In particular we give details on the system structure on the percolation phenomenon and on the bicontinuous phase recently observed. In particular we show that the percolation is driven by a long-scale aggregation between microemulsion droplets.
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  • 119
    ISSN: 0392-6737
    Keywords: Thin films ; growth, structure, epitaxy, and nonelectronic properties ; Superconducting materials ; Superconducting films ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Metal-Organic Deposition (MOD) technique has been used for the first time, in the spray version, to obtain thin and thick films of BSCCO on substrates like MgO and Ag and, in the dipping version, to deposit YBCO thick films directly on a thin Ag wire. Diffraction data show that BSCCO films are textured and oriented with their normal axis along the [001] direction. Electron microprobe data confirm a strong presence of (2212) modification of BSCCO. The YBCO/Ag wire is very uniform, as confirmed by SEM micrographs, and shows a transition temperature around 90 K. Thick films and platelets of YBCO have been also prepared by metalorganic solution spraying and by using a commercial microwave oven in the last annealing stage, reducing the duration of this step from several hours to about 65 minutes. YBCO samples show good stoichiometry, a very smooth surface morphology and transition temperatures, as measured by a.c. susceptometry at low magnetic fields, which are around 90 K. Films, deposited on MgO, are oriented with their growth axis along the [005] direction and, if deposited on SrTiO3, along the [006] direction. Physical, chemical, structural and morphological properties of these HTSC samples obtained by MOD are presented and discussed, with the conclusion that this technique, which is simple, flexible, suitable for large surface areas, for ribbons and for wires, has achieved a good stage of maturity.
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  • 120
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1809-1814 
    ISSN: 0392-6737
    Keywords: Superconducting perovskites and related structures ; Anharmonic lattice modes ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Starting from the recent experiments that have revealed large-amplitude anharmonic vibrations in the cuprates, the possibility of formation of bipolarons has been investigated. A bipolaron model has been proposed, which tightly relies on the structural features of the cuprates,i.e. their layered structure.
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  • 121
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1709-1714 
    ISSN: 0392-6737
    Keywords: Superconducting materials ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary A new technique, specifically designed for the preparation of preferentially oriented HTSC (YBCO and BSCCO) films on metallic substrates has been developed. The method, named cryoelectrophoresis, is essentially based on the transport by electric fields (of the order of 1 kV/cm) of charged superconducting powders kept in a cryogenic fluid (liquid nitrogen). We show that, in these conditions, it is possible to deposit the HTSC grains with a desired orientation upon the application of an appropriate magnetic field. So far conventional electrophoretic oriented deposition was made using the weak normal-state paramagnetic moment, which required the application of magnetic fields of about 9 T. In the method that we introduced, the more intense diamagnetic moment due to the superconducting state yields comparable orientation in a small field, of about 90 G. The orientation degree is pointed out through XRD measurements, from which the fraction of grains with a particular orientation with respect to the substrate is inferred: we show that, in a film deposited with an orienting magnetic field of 90 G the number of grains with the (a, b)-planes parallel to the substrate is 2–5 times greater than in samples obtained without magnetic field and an average misorientation angle of about 25 degrees is estimated.
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  • 122
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1743-1753 
    ISSN: 0392-6737
    Keywords: Superconducting materials (excluding high-T c cuprates) ; Nuclear magnetic resonance and relaxation ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary An overview on some works recently carried out by means of pulsed NMR-NQR experiments is given. The choice of the arguments is aimed at providing a flash on the topics and phenomena most tipically studied with the afore-mentioned approach: from the antiferromagnets (AF) precursors of high-temperature superconductors upon charge and spin doping (such as CuO-doped Li, La2CuO4-doped Sr and Zn) to the doping regime leading to superconductivity in LASCO and YBCO with the related issues involving the comparison with the Fermi-liquid behaviour, the possible opening of a pseudo-spin-gap aboveT c and the anisotropy and field effect ratios belowT c, the latter also in relation to thes-wave ord-wave orbital coupling. Finally, the topic of the motions of the flux lines in YBCO is addressed, in particular by discussing some experiments based on spin-echo dephasing due to thermal excitations and in the presence of d.c. currents and pulsed magnetic fields.
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  • 123
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1769-1776 
    ISSN: 0392-6737
    Keywords: Insulator-superconductor transitions ; unconventional mechanisms of high-T c superconductivity ; Exchange and superexchange interactions ; Narrow-band systems ; heavy-fermion metals ; intermediate-valence solids ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We provide a microscopic modelling of the strong variation of the effective exchange interactionJ eff between Cu2+ spins as a function of hole doping δ in La2-δSrδCuO4, as derived from complementary Raman scattering and paramagnetic susceptibility data.
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  • 124
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1803-1807 
    ISSN: 0392-6737
    Keywords: Response to electromagnetic fields ; nuclear magnetic resonance ; ultrasonic attenuation ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary An accurate determination of the optical conductivity σ(ν) of high-T c materials at low frequencies (ν〈100 cm−1) is still an open problem. In the present paper we report new far-infrared data showing that the LaAlO3 crystal, on which high-quality YBCO films can be deposited, is well transparent at low frequencies. Test measurements have also been performed by employing the ENEA compact FEL facility; preliminary reslts indicate the possibility to obtain high-accuracy spectroscopic data in the mm-wave region. On the basis of these results, we have in program mm-wave and far-infrared transmittance measurements on thin YBCO films on LaAlO3, in order to get new information on the low-frequency behaviour of σ(ν).
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  • 125
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1659-1664 
    ISSN: 0392-6737
    Keywords: Superconducting materials ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The formation of Hg-based HTSC materials was investigated through the development of non-conventional techniques. The former was based on a singlestep synthesis at temperatures significantly lower than usually reported. The basic idea was to exploit the low vapuur pressure of HgO at low temperatures (≈600°C) and in oxygen atmosphere to favour the solid-state reaction between HgO and precursors. Recently, a novel alternative approach essentially based on the oxygen and mercury vapours reactivity for the synthesis of the superconducting phases has been successfully developed. Basically, solid precursors (powders, pellet or thick-film forms) are processed in a two-temperature zone reactor which allowed the independent control of precursors and HgO source temperatures, the partial pressure of HgO and the total pressure during the reaction. In the latter approach, a very low mercury vapour partial pressure (intrinsically below 1–2 atm) was demonstrated to be effective for synthesizing the superconducting phase. The respective merits of both these alternative techniques will be discussed in terms of structural, morphological and magnetic measurements. The low reaction pressure common to both the techniques simplifies largely and makes safer the preparation of these materials, especially in view of applications.
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  • 126
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1961-1965 
    ISSN: 0392-6737
    Keywords: Superconducting films ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We realized BSCCO thin films withT c(R=0)=88 K using the Molecular Beam Epitaxy (MBE) technique. The stoichiometric control is reached by a multilayer deposition technique in which Ca−Cu−Sr−Bi are sequentially deposited using the two effusive cells and the two e-guns present in the system. EDS and X-ray analyses are used to cross-check the obtained results, after theex situ in air annealing. The recent acquisition of an atomic oxygen source will allow in the close future the completelyin situ realization of BSCCO thin films.
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  • 127
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    Keywords: Proximity effects, tunneling phenomena and Josephson effects ; Superconducting perovskites and related structures ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We discuss the influence of the intrinsic surface depression of the order parameter on the temperature dependence of the Josephson critical currentI c(T) and, tentatively, on the quasi-particle tunnelling conductance in a superconductor with a very short coherence length. The comparison with the experiments shows that the presence of a surface-depressed pair potential can explain the large deviations of theI c(T) from the ideal BCS behaviour that we recently observed in Bi2Sr2CaCu2O8+x Josephson break junctions and could mimic the presence of a broadening in the Superconductor-Insulator-Normal tunnelling conductance of the same high-T c superconductor.
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  • 128
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2011-2017 
    ISSN: 0392-6737
    Keywords: Superconducting films ; Infrared and Raman spectra and scattering ; Y-based compounds ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We have used a Raman microprobe technique for the characterisation and the diagnostic of YBCO superconducting thin films deposited by Pulsed-Laser Ablation (PLA) on MgO insulating substrates. Using polarisation analysis associated with sample rotations we developed a method for films orientation determination without any request for absolute calibration of the Raman spectra. The use of a bidimensional multichannel detector (OMA 4) allowed an overall detection time of 40 minutes. Each spectrum (shift range from 100 cm−1 to 700 cm−1) takes about 600 seconds. The results of this detection were used to determine the oxygen content, from the position of the Raman mode at 500 cm−1 Homogeneity was checked with the spatial resolution allowed by the dimension of the focused laser beam (10 μm).
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  • 129
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    Keywords: Superconducting films ; Conference proceedings
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    Topics: Physics
    Notes: Summary We present a structural analysis of YBCO superconducting thin films fabricatedin situ by Inverted Cylindrical Magnetron (ICM) sputtering on commercial SrTiO3 single-crystal and bicrystal substrates. A detailed structural characterization of the superconductor films was performed by using single-crystal X-ray diffractometry confirming that YBCO films have a strongc-axis orientation of the grains with a small mosaic spread. In the films grown on bicrystal substrates we observed a strong correlation with the lattice structure of the substrate. In addition, a surface analysis of the region across the grain boundary edge has been performed by using scanning electron microscopy.
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  • 130
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2053-2059 
    ISSN: 0392-6737
    Keywords: Proximity effects tunneling phenomena, and Josephson effects ; Amorphous, highly disordered, and granular superconductors ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Subharmonic Shapiro Steps (SSS) are usually observed in 2D arrays of superconducting junctions when their dynamical states are perturbed by factors like disorder, frustration, current inhomogeneities and so on. In order to clarify the conditions under which SSS can be generated we decided to investigate arrays of very simple shape like the ladder ones. In particular we have studied the influence of the frustration (i.e. of an external magnetic field) on the time evolution of the vortex configuration and the commensurability of the latter with the geometry and the dimension of the array. Ladders, parallel or perpendicular to the bias current, turn out to be systems where the properties of the square arrays can be suitably tested.
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  • 131
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1649-1657 
    ISSN: 0392-6737
    Keywords: Superconductivity ; Theory ; Insulator-superconductor transitions ; unconventional mechanisms of high-T c superconductivity ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We define a chain-cluster Fermi-linearized Hubbard model, in which the hopping between adjacent 1-d chains is described in terms of an order parameter (valued in a suitable Grassmann field) describing the (transverse)intra-bond correlation, whereas theintra-chain hopping is treated rigorously. The free energy of the 2-d Hubbard model, where neighbouring chains are connected by hopping processes, can be straightforwardly reconducted to that—evaluated self-consistently—of the 1-d model. The self-consistency equations for the one-dimensional chain, when solved explicitly using the exact Lieb-Wu-Takahashi equations, lead to the appearance of a phase transition, for finite values of the Coulomb repulsion strengthU (U〉U c , whereU c is a critical finite value ofU), which can be interpreted as evidence for a mott-Hubbard-typemetal-insulator transition.
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  • 132
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2079-2086 
    ISSN: 0392-6737
    Keywords: Other low-power superconducting devices ; Superconducting films ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The main specifications for the use of high-T c superconductors in the realization of extended surfaces to be used for the fabrication of electromagnetic shields are illustrated. In particular, the differences are stressed between the specifications required by this application and those required by other large-scale applications, namely electrotechnical machinery and magnetic levitation. From these premises, the approach to the fabrication technology of shields is briefly discussed, which also shows important differences with respect to other applications. The analysis is supported by examples of the solutions which are being developed and adopted in the world and in Torino.
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  • 133
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2103-2106 
    ISSN: 0392-6737
    Keywords: Magnetization curves, Meissner effect, penetration depth ; Critical currents ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary In this work we report on the anisotropic physical properties of silver-sheathed Bi-2223 tapes fabricated by means of hot extrusion and repeated pressing and sintering processes. The obtained Bi-2223/Ag short tapes, having critical current densitiesJ c of 20–30 kA/cm2 at 77 K, 0 T, were measured in external magnetic fields up to 0.5T applied in two different orientations (i.e. μ0H‖(a,b)-planes and μ0H ⊥(a,b)-planes). The magnetic characterizations were performed in a wide range of temperatures and magnetic fields to study the first magnetization curve of tapes evaluating the lower critical fields μ0Hc1⊥ab and ⊥0Hc1#x2016;ab and their dependences on temperature. TheJ c values at different fields in the temperature range 4.6–90 K, calculated from the magnetization data by the critical state model, are also presented.
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  • 134
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    Keywords: Superconducting films ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Epitaxial (001)-oriented ultrathin YBCO films of different thicknesses are deposited by Inverted Cylindrical Magnetron Sputtering (ICMS) on (100) MgO single-crystal substrates. The mean films stoichiometry is determined by Rutherford Backscattering (RBS). Auger Electron Spectroscopy (AES), X-ray Photoemission Spectroscopy (XPS) and Scanning Electron Microscopy (SEM) are employed in order to analyse film growth, and identify spurious phases present in the samples.
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  • 135
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1827-1833 
    ISSN: 0392-6737
    Keywords: Flux pinning, flux motion, fluxon-defect interactions ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We discuss the resistivity, the Hall, the Seebeck and the Nerst effect in the mixed state taking the motion of Abrikosov and Josephson vortices into account. Measurements on a Bi(2223) tape point out how the contribution of Josephson vortices to the resistivity prevails at low temperatures while the thermomagnetic effects are, in a first approximation, due to the motion of Abrikosov vortices.
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  • 136
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1863-1869 
    ISSN: 0392-6737
    Keywords: Thermodynamic properties ; thermal conductivity ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Superconducting powders with nominal composition Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06Ox were prepared by two different routes: by solid-state reaction of the respective oxides and carbonates and by pyrolysis of organic precursors. Highly dense superconducting specimens characterized by textured microstructure were prepared by hot pressing. The thermal-expansion coefficients of different typologies of samples were measured by dilatometric tests in both directions parallel and perpendicular to the pressure application direction. Structural rearrangements occurring during thermal treatments were studied. As regards mechanical characterization, microhardness, flexural strength and fracture toughness were evaluated and related to the microstructure anisotropic degree of the sample.
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  • 137
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1895-1901 
    ISSN: 0392-6737
    Keywords: Response to electromagnetic fields ; nuclear magnetic resonance ; ultrasonic attenuation ; Mixed state ; H c2 ; surface sheath ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We report a set of experimental results on second- and third-harmonic generation at microwave frequencies in YBa2Cu3O7 single crystals nearT c. The investigation of the harmonic signals as a function of the intensity and orientation of the d.c. magnetic field yields information on the upper critical field and crystal anisotropy.
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  • 138
    ISSN: 0392-6737
    Keywords: Magnetization curves, Meissner effect, penetration depth ; Insulator-superconductor transitions ; unconventional mechanisms of high-T c superconductivity ; Superconducting films ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Accurate measurements of the magnetic penetration depth λ ofn-doped (Nd1.85Ce0.15CuO4−δ) andp-doped (YBa2Cu3O7−δ) high-temperature superconductors using a microscopic resonator are reported. The temperature dependence λ(T) in Nd1.85Ce0.15CuO4−δ films shows that the electrodynamics of this cuprate is well described in the framework of the BCS theory, with a single-valued finite-gap ratio 2Δ 0/K B T c≈3.1 throughout the entire temperature range belowT c. In contrast, measurements of λ(T) in YBa2Cu3O7−δ cannot be fitted to a BCS temperature dependence derived from a single energy gap. In the low-temperature region, a quadratic behaviour of the penetration depth is observed, instead of the usual exponential law. These peculiar features are discussed in the light of recent theoretical models predicting a different symmetry state for the high-T c superconductors.
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  • 139
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1947-1951 
    ISSN: 0392-6737
    Keywords: Flux pinning, flux motion, fluxon-defect interactions ; Critical currents ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The effect of the oxygen content on the superconducting properties of BSCCO-2212 epitaxial films has been investigated by means of magnetic measurements. The oxygen content was changed by oxidizing and reducing thermal treatments. With decreasing the oxygen contentT c increases (T c=77 K for annealing in oxygen;T c=84 K in air,T c=95 K in argon), butJ c decreases. Moreover, the results indicate that the position of the irreversibility line and the value ofH 3D–2D crossover field are ruled by the oxygen content through the associated changes in the anisotropy factor γ.
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  • 140
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1665-1670 
    ISSN: 0392-6737
    Keywords: Superconducting magnets and magnetic levitation devices ; superconducting cables and conductors ; Superconducting perovskites and related structures ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The role of silver in catalysing and promoting the textured growth of 2223 phase in Ag-sheathed (Powder In Tube) tapes, starting from a precursor «bi-component» powder, constituted by Bi-2212 and other mixed oxides, was investigated in detail by means of DTA and X-ray analyses at the different stages of the annealing of the tapes. By monitoring the reaction it was possible to show how the presence of silver modifies the thermal behaviour of the reactants. The reduction of the temperature at which a liquid phase is formed is believed to be the key mechanism through which the presence of silver enhances the overall conversion to 2223 phase. This temperature decrease was shown (for the first time, to our knowledge) to be active only as long as some precursor (unreacted) powder was present, and not on fully reacted 2223 phase. Further investigation on 2212 phase and the other mixed oxides, separately analysed by DTA, clarified that only the thermal behaviour of the former was modified by the presence of silver. An explanation based on oxygen unbalancing in the material possibly induced, at the annealing temperature, by the presence of silver, is suggested: as a consequence of the very high solubility of oxygen in silver at high temperature, O2 would be «pumped off» the superconducting 2212 phase, with the result of lowering its melting point.
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  • 141
    ISSN: 0392-6737
    Keywords: Superconducting films ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Superconducting thin films of the TlBaCaCuO family have been prepared through a combined approach of MOCVD and thallium vapour diffusion. A three-step process has been adopted involving i) the synthesis of Ba−Ca−Cu−O films by MOCVD using «second-generation» barium and calcium sources, ii) the hydrolysis of the fluoride phases by annealing in water vapour-saturated oxygen and iii) the annealing in the presence of a thallium source. Reproducible results, in terms of phase formation, are discussed depending on the different conditions of the preannealing process. The superconducting films consist predominantly of single-phase Tl2Ba2Ca2Cu3O x (2223). Preferential orientation of the crystallitec-axes perpendicular to the substrate surface has been observed. The films exhibit superconducting onset temperatures of 115–120 K with zero resistance higher than 100 K.
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  • 142
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1967-1972 
    ISSN: 0392-6737
    Keywords: Proximity effects, tunneling phenomena and Josephson effects ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Bicrystal grain boundary junctions of YBCO have been fabricated by using the Inverted Cylindrical Magnetron d.c. sputtering technique. The main characteristics of the junction behaviour are reported. Some aspects of the experimental results are discussed in the framework of existing models developed for describing weak-link structures.
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  • 143
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2073-2077 
    ISSN: 0392-6737
    Keywords: Superconducting magnets and magnetic levitation devices ; superconducting cables and conductors ; Critical currents ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Monofilamentary Bi(2223) tapes withJ c(77 K, 0 tesla) up to 30 000 A/cm2 have been prepared by cold rolling using the powder-in-tube method. An optimization of the precursor powders has led to a higher phase purity after the reaction heat treatment. The deformation process has been optimized in order to increase the oxide density and to reduce sausaging effects on the oxide thickness. The transport properties of these tapes have been studied in a wide range of temperature (4.2K-T c) and magnetic fields (up to 28 tesla). The critical-current values at 77 K fields of 0.5 T and 1 T parallel to the tape surface are 10 000 A/cm2 and 5400 A/cm2, respectively. At 4.2 K theJ c value decreases from 1.6·105 A/cm2 at 0 T to 6·104 A/cm2 at 15 T. At fields higher than 15 tesla a very low field dependence ofJ c has been found, regardless of the tape orientation. Transport properties have also been studied by cutting small sections of the tape in order to investigate the local critical-current distribution. It has been found that, even in rolled tapes of good quality (J c (77 K, 0 T)〉20000 A/cm2), theJ c distribution is homogeneous: the critical current density increases gradually from the centre of the tape to the sides, the latter exhibiting much higherJ c (46000 A/cm2) than in the centre (18000 A/cm2).
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  • 144
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 2107-2111 
    ISSN: 0392-6737
    Keywords: Y-based compounds ; Granular, melt-textured and amorphous superconductors ; powders ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The influence of a presintering treatment on Y-Ba-Cu-O prepared by a modified MPMG process was investigated. The sinterization, reducing the material porosity, was found to enhance the homogeneity of the Y2BaCuO5 (Y-211) particles distribution during the partial melting at 1100°C. As a consequence, the melttextured YBa2Cu3O7−x (Y-123) grows more uniformly producing oriented domain up to 3cm in size. Microstructural and diffractometrical analyses were performed on samples quenched at different stages during the thermal treatment.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 675-687 
    ISSN: 0392-6737
    Keywords: Polymers, elastomers, and plastics ; Polymer reactions and polymerization ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We discuss some recent theoretical studies of the kinetics of the collapse transition in homopolymers. An isolated polymer is modelled using computer simulation, and a time-dependent mean-field theory. The mean-field theory is analysed analytically for early stages, and for short polymers the equations are studied numerically. The results of simulation and theory are compared yielding, we argue, a consistent physical picture. Quantitative comparisons are not yet given, but seem relatively promising.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 689-696 
    ISSN: 0392-6737
    Keywords: Gels and sols ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We present results of static and dynamic light scattering study of the critical phenomena in semi-dilute solutions and gels of poly(methyl methacrylate) (PMMA) prepared by radical copolymerization of methyl methacrylate and ethylene dimethacrylate in the θ-solvent (4-heptanone). The correlation length ξ and the extrapolated zero-angle scattering intensityI c,0 of the solutions with degree of cross-linkingf c≤1% diverge at the pseudospinodal temperature with exponents ofv=0.5 and γ=1, respectively, characteristic of mean-field behaviour. The values ofv and y for samples withf c≥1.5% are about 0.6 and 1.2, respectively, indicating that the phase transition in solutions of branched PMMA molecules and PMMA gels can be classified to the 3D Ising model. The pseudospinodal temperaturesT s, for semi-dilute solutions of branched molecules and gels were found as a function of the degree of cross-linking,f c. The time autocorrelation functions consist of two contributions of which the faster one is attributed to the cooperative motions of the polymer matrix, while the broad slow mode is due to the dynamics of the partly penetrating polydisperse clusters and can be interpreted in terms of a simple cluster diffusion model.
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  • 147
    ISSN: 0392-6737
    Keywords: Polymers, elastomers, and plastics ; Neutron scattering techniques (including small-angle scattering) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The present paper reports the results of compressibility and incoherent quasi-elastic neutron scattering measurements performed on polymeric systems and on their aqueous solutions. From the compressibility data, the temperature evolution of the polymer hydration number can be derived. On the other hand, neutron data show that the translational diffusion coefficientD T turns out to be higher in the case of ethylene glycol + water, with respect to that of pure, suggesting that the water molecules act as a «structure breaker» of the intermolecular connectivity existing into the pure. Furthermore the dynamical properties of the H2O molecules in the presence of poly(ethylene glycol) deeply differ from those in the bulk, and show that we are in the presence of entangled water.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 843-848 
    ISSN: 0392-6737
    Keywords: Polymers, elastomers, and plastics ; Nonequilibrium thermodynamics, irreversible processes ; Physics of crystal growth ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Differential scanning calorimetric measurements in the early stage of isothermal crystal growth of polyethylene oxide are analysed in the light of irreversible thermodynamics. An accurate evaluation of the equilibrium melting temperature is done by fitting the thermograms obtained at different undercoolings and referring to the activation energy values already known from the literature.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 873-877 
    ISSN: 0392-6737
    Keywords: Physical chemistry of solutions of biological macromolecules ; Order-disorder and statistical mechanics of model systems ; Multicritical points ; Macromolecules and polymer molecules: Constitution (chains and sequences) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary A two-letter random copolymer with attraction between similar monomers and repulsion between different ones is investigated using the replica method. This type of interactions favors microphase separation (MPS) in a compact state of a polymer or in a melt. Frustrations between interactions and polymeric bonds may lead to freezing transition in a phase where only a few conformations dominate and replica symmetry is broken. Our analysis reveals that stiff polymers have a frozen phase and do not undergo transition to a phase with microdomain structure. In flexible polymers the microphase transition may occur before freezing. A complete phase diagram showing the interplay between the two phase transitions is constructed for the two-letter random copolymer.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 901-910 
    ISSN: 0392-6737
    Keywords: Brillouin and Rayleigh scattering ; Visible and ultraviolet spectra: Liquids ; Molecular liquids ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary An analysis of Rayleigh-wing spectra on two isomers of propanol (C3H7OH) and on two of their Br-based isomorphous systems (C3H7Br) is presented. The performed measurements, over a wide temperature range, allow us to identify two different processes characterizing the response of both the alcohols. A comparison between the rotational dynamics of the linear normal-propanol (1-propanol) and the more sterically hindered iso-propanol (2-propanol) is performed. At the same time, the role played by the H-bond in the associative properties of hydrogenous systems is investigated by comparing the dynamical response of the two alcohols with that of 1-bromopropane and 2-bromopropane, in which the O−H «active» sites are replaced by «inert» bromine atoms.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 933-937 
    ISSN: 0392-6737
    Keywords: Phase equilibria, phase transitions, and critical points of specific substances ; Dielectric loss and relaxation ; Amorphous materials ; glasses ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Dielectric and ultrasonic measurements have been performed in the temperature and in the frequency domain to investigate the α (or primary) relaxation in the supercooled state of a «fragile» molecular liquid. This relaxation is strictly connected with the phenomenon of the glass transition and therefore an understanding of the former is a prerequisite for the explanation of the latter. The dynamic susceptibilities relative to these probes have been analysed with the Cole-Davidson function employing (where required) a Vogel-Fulcher-Tamman law to relate the main relaxation time to temperature. The results obtained are consistent with the dynamic behaviour of liquids of high fragility.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 747-755 
    ISSN: 0392-6737
    Keywords: Neutron scattering techniques (including small-angle scattering) ; Polymers, elastomers, and plastics ; Fluctuation phenomena, random processes and Brownian motion ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary A-B block copolymers in a selective solvent—good for the B-species and bad for the A-species—form micellar aggregates with a compact A-core with a corona (brush) of B «hairs» reaching into the solvent. Whereas polystyrene(PS)-polyisoprene(PI) in decane forms spherical micelles with a PS core of about 10 nm radius, polyethylene(PE)-polyethylenepropylene(PEP) forms micellar platelets, the shape of which is goverend by the habitus of PE crystallites forming the core. These planar aggregates have large (several hundred nanometers) lateral extension and a core thickness in the range of 10 nm. Both systems are model systems for polymer brushes, either on a spherical surface or planar. Neutron spin-echo experiments allow for the investigation of the dynamics of the brushes which reflects their viscoelastic properties. Results of neutron small-angle and spin-echo investigations are reported. The brush dynamics is explained using a model based on an idea of de Gennes describing the brush properties in terms of scaling relations for osmotic pressure and viscosity of a semi-dilute solution with inhomogeneous density.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 771-781 
    ISSN: 0392-6737
    Keywords: Neutron scattering techniques (including small-angle scattering) ; Gels and sols ; Brillouin and Rayleigh scattering ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The statical and dynamical properties of lecithin/H2O/cyclohexane cylindrical reverse micelles are investigated as a function of lecithin volume fraction, ϕ, and temperature at fixed water/lecithin ratio,w 0. The viscosity data are well fitted by the Cates model when the breathing mode of micelles is taken into consideration, overlapping with the breaking and reforming mechanisms. We present some results from Brillouin-scattering experiment, performed across the sol-gel transition. In order to explain the experimentally observed ϕ-dependence of the hyperacoustic parameters, a mechanical model was developed from which the ϕ-dependence of the micelle size distribution was obtained. From a comparison with the viscosity data the entanglement length was estimated. Furthermore some new results from an incoherent quasi-elastic neutron scattering experiment are presented. The whole body of the experimental results suggests for a sol-gel transition triggered by topologic phenomena. When the lecithin volume fraction increases, the kinetic equilibrium between the breaking and reforming mechanisms of the micelles shifts the mean micelle length towards higher values and the entanglement of the micelles becomes highly favourable. The obtained results are discussed and compared with other findings in literature.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 783-788 
    ISSN: 0392-6737
    Keywords: Free radicals ; Amorphous materials ; glasses ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Linear and non-linear Electron Spin Resonance spectroscopies have been designed to study the decay of the orientation correlations on different time scales. The reorientation of radicals («spin probes») dissolved in host phases (in the present case a polymeric liquid crystal (PLC)) may be investigated in the time window 10−11–10−5 s. Evidence of the non-exponential regression of fluctuations is given. The temperature dependence of the correlation time of the spin probe orientation is discussed in the case of semi-crystalline PLCs and contrasted with the case of amorphous PLCs.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1181-1191 
    ISSN: 0392-6737
    Keywords: Viscosity and diffusion: experimental ; Optical system design ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We describe the design and operation of an improved two-colour dynamic light scattering (TCDLS) equipment which effectively suppresses multiple scattering and allows the study of the Brownian dynamics of optically turbid samples. The technique operates by cross correlating scattered light of two different colours. With the appropriate scattering geometry, only singly scattered light contributes to the time-dependent part of the measured intensity cross-correlation function thus allowing straightforward interpretation of the data. To test the performance of the instrument we have studied several dilute colloidal suspensions, which, although dilute enough that free diffusion of the particles can be assumed, ranged from quite transparent to distinctly turbid. We have confirmed that the measured cross-correlation functions accurately reflect only single scattering even in the presence of strong multiple scattering. As a first application of the technique, we have performed light scattering measurements on concentrated suspensions of PMMA particles in cis-decalin.
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  • 156
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1231-1236 
    ISSN: 0392-6737
    Keywords: Disperse systems ; Fluctuation phenomena, random processes, and Brownian motion ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The scaling of the ratio of the hydrodynamic radius to the radius of gyration of fractal aggregates is probed by numerical simulations for several different algorithms. In common with previous work it is found that this ratio shows a significant and long-lasting finite-size effect for particle-cluster aggregates, but a much less significant finite-size effect for hierarchical cluster-cluster aggregates. Simple theoretical models which reproduce the behaviour are discussed.
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  • 157
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 653-660 
    ISSN: 0392-6737
    Keywords: Macromolecular and polymer solutions (solubility, swelling, etc.) ; polymer melts ; Critical point phenomena ; Liquid-liquid transitions ; Monte Carlo and statistical methods ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The unmixing transition of both symmetrical polymer blends AB (i.e. chain lengthsN A=N B=N) and asymmetrical ones (N B/N A=2,3) is studied by large-scale Monte Carlo simulations of the bond fluctuation model. Combination of semi-grand-canonical simulation techniques, «histogram reweighting» and finitesize scaling allows an accurate location of the coexistence curve in the critical region. The variation of the critical temperature with chain length (N) is studied and compared to theoretical predictions. For the symmetrical case, use of chain lengths up toN=512 allows a rough estimation of crossover scaling functions for the crossover from Ising to mean-field exponents. The order-disorder transitions in melts of both symmetric (compositionf=N A/(N A+N B)=1/2) and asymmetric (f=3/4) block copolymers is studied for very short chains (16≤N≤60). The interplay between structure and chain configuration is emphasized. Qualitative evidence for «dumbell formation» of chains and vacancy enrichment in A-B-interfaces and near hard walls is presented.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1053-1063 
    ISSN: 0392-6737
    Keywords: Amorphous materials ; glasses ; Theory of diffusion and ionic conduction in solids ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We discuss the dynamic structure model recently introduced by Bunde, Maass and Ingram to account for the anomalies of ionic transport in glassy ionic conductors. The model is based on the experimental evidence that ions in glass maintain their distinct local environments. Key features include a site memory effect that introduces vacancies appropriate to each kind of mobile ion, and a mismatch energy that emerges whenever an ion attempts to enter a different kind of site, the combination of both leads to the formation of fluctuating percolation pathways. The connectivity of these pathways determines the ion mobility in the glassy network. The exploration of this model by numerical methods leads i) to a power law relationship between ionic conductivity and cation content (now confirmed in the literature) and ii) to the elucidation of many facets of the mixed alkali effect. It is suggested that the model could form the basis for a comprehensive theory of vitreous electrolytes.
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  • 159
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1285-1289 
    ISSN: 0392-6737
    Keywords: Amorphous materials, glasses ; Free radicals ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We study the reorientation process of a paramagnetic radical dissolved in supercooledo-terphenyl around and belowT g via the Electron Spin Resonance spectroscopy. The experimental results are at variance with the hypothesis of diffusive reorientation of the radical. Instead, the ESR lineshapes are fairly well reproduced by assuming that the radical reorientates via large angular jumps with a width Δϕ=80° −5° +10° . The results are compared with previous2H NMR studies.
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  • 160
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1311-1324 
    ISSN: 0392-6737
    Keywords: Hydrodynamic stability ; Theory and models of chaotic systems ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Some results obtained introducing two-unit generalized complex numbers to determine Mandelbrot and Julia sets for quadratic maps are presented. We also highlight some analogies and differences between classic complex numbers and one of the proposed extensions.
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  • 161
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1325-1338 
    ISSN: 0392-6737
    Keywords: Free-electron lasers ; Integral equations ; Functional analysis ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We discuss some mathematical and computational problems relevant to the solutions of both linear and non-linear integral equations arising in radiation transport. By means of a functional-operator approach, analytic solutions for the FEL (Free Electron Laser) integral equations are found in terms of Bessel-Clifford functions. As for applications of this method to non-linear equations, the same technique allows us to obtain an efficient yet simple algorithm for the numerical solution of the Ambartsumian-ChandrasekharH-equation corresponding to a Neumann-series expansion.
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  • 162
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1217-1221 
    ISSN: 0392-6737
    Keywords: General theory of resonances and relaxations ; Fluctuation phenomena, random processes, and Brownian motion ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The effect of non-exponential relaxation on the lineshape of the linear Electron Spin Resonance spectroscopy is discussed. In the simple case of a paramagnetic system withS=1/2 it is found that the lineshape is mainly affected by the tails of the correlation function.
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  • 163
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1243-1246 
    ISSN: 0392-6737
    Keywords: Granular materials: aggregation characteristics (e.g., grain size, particle size distribution, porosity) ; Order-disorder and statistical mechanics of model systems ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We present a simple phenomenological model, based on mass conservation arguments, in order to describe aggregation in dense solutions. It has been previously shown that fordiffusion-limited processes scattered intensity distributions exhibit a peak atq≠0 which grows in time and moves to smaller and smallerq vectors. According to the model, each aggregate is surrounded by a depletion region whose size depends on the aggregation kinetics. The form factor for such a cluster should satisfy local mass conservation and hence exhibit a pronounced depression atq=0. the initial stages of aggregation, when the clusters are far apart, are accounted on the basis of a form factor alone. The model shows good agreement with the data and also explains the experimental evidence that the peak does not appear inreaction-limited conditions.
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  • 164
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1091-1101 
    ISSN: 0392-6737
    Keywords: Colloids ; Brillouin and Rayleigh scattering ; General theory of equations of state and phase equilibria ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Equilibrium sedimentation profiles of concentrated suspensions of charged optically anisotropic colloids are accurately determined by performing depolarized light scattering measurements. From the data we derive the osmotic pressure πvs. the particle volume fraction Φ. The π(Φ) values obtained with strongly screened colloids are in excellent agreement with the predicted hard-sphere equation of state. The data suggest that, besides gravitation, there is an additional force acting on the particles, probably due to the build-up of an electrostatic sedimentation potential.
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  • 165
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1119-1125 
    ISSN: 0392-6737
    Keywords: Dielectric loss and relaxation ; Fluctuation phenomena, random processes, and Brownian motion ; Turbulent diffusion ; Chaos ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We investigate transport in complex fluids including glassy materials and turbulent flows. We focus on regimes where fractal hierarchical scaling exists in time, space or both simultaneously.
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  • 166
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1159-1169 
    ISSN: 0392-6737
    Keywords: Order-disorder and statistical mechanics of model systems ; Fluctuation phenomena, random processes, and Brownian motion ; Colloids ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary In this paper, we discuss the evolution of the scattered intensityI(q) during irreversible diffusion-limited cluster-cluster aggregation. We analyse twodimensional simulations and interpret the results within the framework of a recently proposed theoretical approach. The theory describes the correlation among different clusters which develops during the irreversible aggregation process. The model is based on two coupled differential equations, controlling the growth of the average cluster mass and the time dependence of the probability of finding pairs of clusters as a function of their distance.
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  • 167
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1193-1197 
    ISSN: 0392-6737
    Keywords: Colloids ; Statistical mechanics ; Linear viscoelasticity ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary A nonequilibrium fluid lattice model of concentrated colloidal dispersions is presented to predict the effects of the microstructure, particle interactions, volume fraction (ϕ), frequency (ω), and the longest viscoelastic relaxation time on the complex shear viscosity. In addition to the pair interactions between colloidal particles, the many-body interactions between the particles and the equilibrium microstructure have to be included in the analysis. As ϕ approaches a critical valueφ c, the fluidity of concentrated dispersions slows down drastically. This percolation thresholdφ c scales ad (AP)−0.5, whereA andP are related to the repulsive interparticle potential and microstructure, respectively.
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  • 168
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1253-1258 
    ISSN: 0392-6737
    Keywords: Liquid crystals ; Turbulence, fluid dynamics ; Diffusion and ionic conduction in liquids ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We report on the diffusive motion of dielectric testing particles floating in a turbulent nematic fluid under the effect of an external oscillating electric field. The particle is viewed as a tracer of the underlying turbulence. We calculate the diffusion coefficients and the probability distribution of the particle displacements, thus showing that the motion of the particle is a classical Brownian diffusion.
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  • 169
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1277-1284 
    ISSN: 0392-6737
    Keywords: Vibrational states in disordered systems ; Heat capacities of solids ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The specific heats of (R2O3) x (P2O5)1−x glasses containing high concentrations of La3+ and Y3+ ions have been measured between 1.5K and 30K. It is shown that, in addition to the usual Debye contribution, there is an excess specific heat arising from localized vibrational states which has been discussed in terms of two distinct models. The first predicts a maximum in the temperature dependence of the excess specific heat associated with the crossover frequency from phonon to fracton behaviour. The phonon-fracton density of states used to fit the excess specific heat gives rise to model parameters having the same magnitudes as those found previously for other glasses including samarium phosphates. The second model, formulated on the basis of soft vibrations in glasses, predicts a minimum in the excess specific heat, which is also observed.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1305-1309 
    ISSN: 0392-6737
    Keywords: Systems displaying chaotic behaviour ; Theory and models of chaotic systems ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The finiteness of observational and computational precision suggests that not only the initial condition (as usually assumed) but also the evolution law of dynamical systems is affected by unavoidable uncertainties. The consequences are explored by suitably generalizing the concept of the Lyapunov exponent. The relation between the results obtained and the shadowing problem is discussed as well as some implications concerning the Kolmogorov entropy.
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  • 171
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 949-962 
    ISSN: 0392-6737
    Keywords: General studies of phase transitions ; Transitions in liquid crystals ; Colloids ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The effect of non-adsorbing, flexible polymer on the isotropic-nematic transition in dispersions of rod-like colloids is investigated. A widening of the biphasic gap is observed, in combination with a marked polymer partitioning between the coexisting phases. Under certain conditions, areas of isotropic-isotropic-nematic or isotropic-nematic-nematic three-phase coexistence appear in the phase diagram of rod-polymer mixtures.
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  • 172
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 963-979 
    ISSN: 0392-6737
    Keywords: Colloids ; Statistical theories of liquid structure ; Time-dependent properties ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Charge-stabilized suspensions are characterized by the strong electrostatic interactions between the particles so that rather dilute systems may exhibit strong correlation resulting in a well-developed short-range order. This microstructure, quantitatively described by the pair distribution functiong(r), is rather different from that of (uncharged) hard spheres. It is shown how this difference affects the «hydrodynamic function»H(k), which appears in the expression for the first cumulant Γ(k)=k 2 D eff(k)=k 2 H(k)/S(k) of the dynamic autocorrelation function. Without hydrodynamic interaction,H(k)=D 0, which is the free-diffusion coefficient. Using pairwise additive hydrodynamic interaction and the lowest-order many-body theory of hydrodynamic interaction, it is found thatH(k) can deviate considerably fromD 0 even for systems of volume fractions ϕ as low as 10−3. These effects are more pronounced for collective diffusion than for self-diffusion. SinceH(k=0) is closely related to the sedimentation velocity, we have studied this quantity as a function of volume fraction. It is found that (H(0)/D 0) −1 scales asφ 1/3 at low ϕ in salt-free suspensions.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1357-1366 
    ISSN: 0392-6737
    Keywords: Fluid surfaces and fluid-fluid interfaces ; Neutron scattering techniques (including small-angle scattering) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Water-octane-C10E4 microemulsions in both the isotropic and lamellar phases were measured using small-angle neutron scattering. Varying the scattering length density of both the water and oil through hydrogen-deuterium substitution enabled us to isolate the scattering contributions of the surfactant monolayers and the water-surfactant and oil-surfactant interfaces. Analyses of the resulting scattering patterns allow us to directly determine the small mean curvature of the surfactant film as a function of temperature and correlate this quantity with the overall phase behavior. A simulation using a Gaussian random field yields the three-dimensional structure of the bicontinuous microemulsion having a surfactant monolayer with zero mean curvature.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1419-1431 
    ISSN: 0392-6737
    Keywords: Phase equilibria, phase transitions, and critical points of specific substances ; Optical properties of bulk materials ; Statistical theory of liquid structure ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The three-component ionic microemulsion system consisting of AOT/water/decane shows an unusual phase behavior in the vicinity of room temperature. The phase diagram in the temperature-volume fraction (of the dispersed phase) plane exhibits a lower consolute critical point at about 40 degrees centigrades and 10% volume fraction. A percolation line, starting from the vicinity of the critical point, cuts across the plane, extending to high volume fraction side at progressively lower temperatures. In this paper we review the evidence that allows to interpret the phase behavior of our system in terms of interacting spherical droplets. We also investigate the dynamics of droplets, below and approaching the critical point by dynamic light scattering. The first cumulant and time evolution of the droplet density correlation function can be quantitatively calculated by assuming the existence of polydispersed fractal clusters formed by the microemulsion droplets due to attraction. The relaxation phenomena observed in an extensive set of measurements of electrical conductivity and permittivity close to percolation is also reviewed and interpreted through the same cluster-forming mechanism, which reproduces the most relevant features of the frequency-dependent complex dielectric constant of this system.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1457-1470 
    ISSN: 0392-6737
    Keywords: Active transport processes across biological membranes cellular and subcellular transmembrane electrochemical potentials intracellular electron transport and energy transduction ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The question is asked on the physiological role of the universal molecular biological function activity of lipid transmembrane translocation in living-cell membranes. The elastic response of closed bilayers to phospholipid translocation is theoretically found to be scale dependent, leading to vesiculation for plasma membrane characteristic-size bilayers (R m≈10 μm), and to dilation strain for endocytic-vesicles characteristic-size bilayers (R m≈0.1 μm). Both vesiculation and dilation effects are experimentally proved. On the basis of this scaledependent behaviour, it is argued that transmembrane pumping activity could control both the vesiculation and the fusion processin vivo, namely the exo-endocytic cellular-membrane traffic process itself.
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  • 176
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    Keywords: Surface energy (surface tension, interface tension, angle of contact, etc.) ; Phase transitions and critical phenomena ; Constant-composition solid-solid phase transformations: polymorphic ; massive, and order-disorder ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary 1,2-sn-diglyceride monolayers exhibit unique and complex phase transitions as a function of surface pressure. The dynamical response of the layer on expanding the film has been investigated by computer simulations, (π-A) isotherms and grazing-incidence X-ray diffraction. Good agreement is found between the simulations and experiments. The Langmuir film undergoes two phase transitions occurring at 38.3 and 39.8 Å2/molecule. The transition at low surface density is associated with a tilt of ≈14° in the direction close to nearest neighbour. The first transition is unique for the diglyceride molecules and has not been observed for other amphiphilic molecules. It is driven by a competition of hydrophobic/hydrophilic and intra/intermolecular forces and can be pictured as a «seesaw» mechanism. Due to the close packing of the chains at high surface pressure, thesn-1 ester group aligns with the alkyl chain, and only thesn-2 ester group is favoured to lay at the aqueous interface. Hydrophilic forces attract thesn-1 ester group, but its motion towards the aqueous subphase is hindered by intra- and intermolecular chain interactions. On expansion, the intermolecular interaction decreases, and at the first transition, the intramolecular interaction between the two chains is strong enough to cause a «swelling» of the molecules. The diffraction pattern determined in the different mesophases reveals that the layer maintains its hexagonal structure up to the second phase. Concomitant with the tilt, the structure relaxed from a hexagonal to a distorted hexagonal lattice.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1523-1528 
    ISSN: 0392-6737
    Keywords: Molecular biophysics ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The planarity and electron delocalization of the 2, 2′:6′, 2″-terpyridine (terpy) moiety in salts of the type [Pt(terpy)Me]X leads to extensive stacking interactions in aqueous solution. UV/VIS,1H NMR and resonance light scattering spectra provide evidence for the tendency of the complex to form large aggregates even under low ionic strength conditions. The interaction of the cationic complex with calf thymus DNA was investigated by spectroscopic techniques and mobility assays. At highr f ratios the complex seems to form extended aggregates on the surface of the nucleic acid but at lowerr f evidence was obtained for intercalation.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1559-1564 
    ISSN: 0392-6737
    Keywords: Spectra, photodissociation, and photoionizzation of biomolecules, bioluminiscence ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary In this paper the authors intend to illustrate the results of optical properties of human blood as suspension of cells in aqueous solutions of electrolytes and non-electrolytes, which are highly complex liquid systems susceptible to important phase transitions. They carried out normal and derivative spectrophotometric measurements of non-pathologic diluted blood and serum samples to show optical absorption peak positions of haematological chromophores including haemoglobin and fibrinogen. The authors think that optical behaviour of the chromophores may be influenced by interaction with different environmental situations relative to coagulated and uncoagulated samples. They continue these studies with particular reference to the nature of the solute-induced modifications.
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    Il nuovo cimento della Società Italiana di Fisica 16 (1994), S. 1613-1618 
    ISSN: 0392-6737
    Keywords: Surface energy ; thermodynamic properties ; Permittivity ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary In this paper we present conductivity measurements on different concentrations of C12E6 non-ionic surfactant solutions as a function of temperature and buffer ionic strength. The experimental data were analysed by means of Looyenga equation. The isothermal slopes of the linearized Looyenga equation depend on the ionic strength, showing saturation effect. Our results indicate that one of the basic hypotheses of the mixture equations is not fulfilled. Adsorption of light ions at micellar interface is necessary to justify the experimental data. Then for non-ionic surfactant solutions the ion adsorption prevents any explicit computation of the micellar hydrated volume.
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    Communications in Numerical Methods in Engineering 10 (1994) 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
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    Communications in Numerical Methods in Engineering 10 (1994), S. 1-9 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: An iterative method is introduced for computing second-order partial derivatives (sensitivities) of eigenvalues and eigenvectors of matrices which depend on a number of real design parameters. Numerical tests confirm the viability of the method and support our theoretical analysis. Alternative methods are reviewed briefly and compared with the one proposed here.
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    Communications in Numerical Methods in Engineering 10 (1994), S. 11-19 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: In the formulation of the semi-Loof element the rotation of the tangent plane is derived from the interpolation of the transverse displacement, while the rotation of the normal is interpolated separately by another set of shape functions. The geometric stiffness matrix can be formulated by use of either of the two rotation representations. It is demonstrated that the use of the tangent plane representation in the geometric stiffness matrix is far superior to the common form at present.
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    Communications in Numerical Methods in Engineering 10 (1994), S. 21-32 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: An efficient solution to boundary-value problems may be based on the application of a suitably truncated T-complete set of Trefftz functions over individual subdomains and on linking the fields by a least-squares procedure. Although it yields a symmetric system of linear equations, this approach as originally presented by Zielinski and Zienkiewicz is not suited for implementation into FE codes. The present paper presents two equivalent formulations, which take respectively the form of the finite (FE) and non-conventional boundary-element (BE) approach. Both allow the resulting simultaneous equations to be assembled following the standard direct stiffness methods and can readily be implemented into existing FE codes.As in the conventional p-method, the accuracy may be controlled within large limits without increasing the number of elements. The present approach allows substantial saving in computer time in comparison with the so-called hybrid-Trefftz (HT) elements, though the assumed displacement fields are identical. The practical efficiency of the new T-element approach is assessed on the problem of stress concentration in a symmetrically compressed perforated panel.
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    Communications in Numerical Methods in Engineering 10 (1994), S. 167-178 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A sixth-order polynomial shape function is developed for BIE analysis. The function is applied over only three-noded elements, but with the support for the function extending over adjacent elements. This avoids the oscillations near the ends of the range which otherwise are characteristic of high-order polynomial interpolation. Various test problems are explored, and it is shown that results as accurate as those from conventional quadratic elements are obtained with larger nodal spacings, and thus giving the potential for significant reductions in matrix storage requirements and solution times.
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  • 185
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    Communications in Numerical Methods in Engineering 10 (1994), S. 155-166 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A finite element formulation based on superposition is proposed for a lifting aerofoil in incompressible potential flow. An accuracy improvement technique for the singularity at the aerofoil trailing edge is discussed. It is shown that the quarter-node quadrilateral element can be readily employed to simulate this singularity. It is also demonstrated that the circulation in the flow field can be easily represented by a single constraint equation rather than introducing an artificial cut line in the mesh. The influence of finite-element mesh size on solution accuracy and the correct form of the boundary condition have been investigated as well. Numerical examples are given for both steady and quasisteady Joukowski aerofoils of various thicknesses and at a range of incidences. In all test cases, good agreement is observed between the analytical solution and the numerical result.
    Additional Material: 6 Ill.
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  • 186
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    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
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  • 187
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    Communications in Numerical Methods in Engineering 10 (1994), S. 183-194 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: An 8-node element (HMITC) for 3D non-linear analysis of solids is presented. The new element is based on an element developed by Wilson and Ibrahimbegovic, that incorporates incompatible modes, and on the method of mixed interpolation of tonsorial components. The HMITC element does not contain spurious zero energy modes and satisfies Irons' Patch Test. The numerical experimentation indicates that the HMITC has good performance even with distorted meshes.
    Additional Material: 6 Ill.
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  • 188
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    Communications in Numerical Methods in Engineering 10 (1994), S. 203-215 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: In this paper we consider an explicit finite-element method, with elements adaptively oriented in space-time, for the solution of one-dimensional conservation laws, extending previous work dealing with linear convection-diffusion and incompressible flow. In particular we consider Burgers' equation and the compressible Euler equations.
    Additional Material: 8 Ill.
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  • 189
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    Communications in Numerical Methods in Engineering 10 (1994), S. 195-201 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A numerical method, based on neural-network-based functions, for solving partial differential equations is reported in the paper. Using a ‘universal approximator’ based on a neural network and point collocation, the numerical problem of solving the partial differential equation is transformed to an unconstrained minimization problem. The method is extremely easy to implement and is suitable for obtaining an approximate solution in a short period of time. The technique is illustrated with the aid of two numerical examples.
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  • 190
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    Communications in Numerical Methods in Engineering 10 (1994), S. 217-225 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: An efficient algorithm for consideration of axial and cyclic symmetry in the boundary-element method is presented. The appropriate transformation relationships was derived. The algorithm is then implemented in a boundary element program for the analysis of 2D elastostatic problems. Through analysis of typical problems the validity of the algorithm and its implementation is verified. A high level of accuracy and substantial reduction in computer time and storage was achieved.
    Additional Material: 5 Ill.
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  • 191
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    Communications in Numerical Methods in Engineering 10 (1994), S. 227-235 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The diffusion of oxygen into an absorbing medium as an example of an implicit moving boundary problem has been dealt with by a number of authors using various numerical techniques, and, where appropriate, approximate analytical expressions. To evaluate the time for complete absorption, extrapolation is usually employed. An unconditionally stable explicit numerical scheme that avoids the limitations of such methods is presented and tested herein. Unlike existing schemes this method is fully numerical; it avoids the large array size, generally required for existing methods, by using a variable-length time step. The time for complete absorption emerges from the final step in the normal computing procedure with no recourse to extrapolation. Furthermore, owing to the implicit condition prevailing at the moving boundary, no iterations are needed to evaluate the time step required for the moving boundary to move a single space increment. The numerical results obtained compare very favourably with those due to earlier authors.
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  • 192
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    Communications in Numerical Methods in Engineering 10 (1994), S. 237-248 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: In the paper we have developed a new formulation for solution of structural-acoustic coupling problems by boundary elements using the multiple reciprocity method. It is assumed that the structure is composed of plate components and is excited by the external or the internal noise source. The efficiency of the proposed formulation becomes especially remarkable if the boundary-value problem is to be solved repeatedly for different values of frequency. The accuracy of the numerical computations has been compared with the analytical solution in a test example.
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  • 193
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    Communications in Numerical Methods in Engineering 10 (1994), S. 257-265 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: Techniques for stress- and strain-controlled in situ homogenization of inelastic periodic composites are presented. The results of homogenization computations on a specific elastoplastic composite solid are then employed to validate the form of an orthotropic elastoplasticity model with a tensorial kinematic hardening law.
    Additional Material: 3 Ill.
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  • 194
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    Communications in Numerical Methods in Engineering 10 (1994), S. 249-255 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: An algorithm for evaluation of friction in general contact-impact interfaces is described. The algorithm is based on an explicit finite-element method. Coulomb's friction law is assumed. The defence node algorithm is used such that the sticking condition can be imposed with the Lagrange multiplier method even in explicit dynamic analysis. The algorithm is supposed to be applicable in general situations, including large deformations of the contact-impact bodies and large relative sliding between the contact-impact boundaries. Numerical results are presented to demonstrate the performance of the algorithm.
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  • 195
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    Communications in Numerical Methods in Engineering 10 (1994), S. 275-277 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
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  • 196
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    Communications in Numerical Methods in Engineering 10 (1994) 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
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  • 197
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    Communications in Numerical Methods in Engineering 10 (1994), S. 267-273 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A generalization of the r(1-m)/m strain singularity of higher-order isoparametric elements is presented. It is shown that, by variable placement of the side nodes between their original and singular positions, the point of singularity sensed by the element can be controlled. The transition elements have a strain singularity outside their domain. The singular and non-singular elements are elements are special cases of the general mapping. The transition elements, together with the singular isoparametric elements, can be used for solving crack problems.
    Additional Material: 4 Ill.
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  • 198
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    Communications in Numerical Methods in Engineering 10 (1994), S. 279-290 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: In the design of members with flaws, it is necessary to keep the stress intensity factor K of any sharp crack below the fracture toughness Kcr of the materials. Stress-intensity factor equations for the more common basic specimen geometries and various loading conditions are available in the literature. The application of these equations to complex structures involves geometric problems such as the identification of the outline of each member and the sizing of the equivalent specimen for each flaw. The paper gives a response to such difficulties.
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  • 199
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    Communications in Numerical Methods in Engineering 10 (1994), S. 303-312 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A simple, yet storage-effective ‘linear’ programming code is given. The assumption of non-negative variables is bypassed without increasing the size of the problem. Furthermore, the objective is allowed to be summed over not just linear, but also concave, functions. A specific truss topology optimization example is shown.
    Additional Material: 1 Ill.
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  • 200
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    Communications in Numerical Methods in Engineering 10 (1994), S. 291-296 
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: We propose in this paper a nine-point, fourth-order difference method for the numerical solution of the quasilinear Poisson equation \documentclass{article}\pagestyle{empty}\begin{document}$$ Au_{zz} + \frac{1}{r}u_r + Bu_{rr} = f\left({r,z,u,u_r,u_z} \right) $$\end{document} with appropriate boundary conditions. The method is based on five evaluations of f. The numerical results of four problems obtained using this method are listed. The results demonstrate the fourth-order accuracy of the method.
    Additional Material: 4 Tab.
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