1,721,015 research outputs found
What can we learn by squeezing a liquid?
No full textRelaxation times tau(T,v) for different temperatures, T, and specific volumes, v, collapse to a master curve vs Tu(gamma), with gamma a material constant. The isochoric fragility, m(V), is also a material constant, inversely correlated with gamma. From these experimental facts, we obtain a three-parameter function that accurately fits tau(T,v) data for several glass-formers over the supercooled regime, without any divergence of tau below T(g). Although the values of the three parameters depend on the material, only gamma significantly varies; thus, by normalizing material-specific quantities related to gamma, a universal power law for the dynamics is obtained
A market-consistent framework for the fair evaluation of insurance contracts under Solvency II
Full text linkThe entry into force of the Solvency II regulatory regime is pushing insurance companies in engaging into market consistence evaluation of their balance sheet, mainly with reference to financial options and guarantees embedded in life with-profit funds. The robustness of these valuations is crucial for insurance companies in order to produce sound estimates and good risk management strategies, in particular, for liability-driven products such as with-profit saving and pension funds. This paper introduces a Monte Carlo simulation approach for evaluation of insurance assets and liabilities, which is more suitable for risk management of liability-driven products than common approaches generally adopted by insurance companies, in particular, with respect to the assessment of valuation risk
Ecological approach in selecting extensive green roof plants: A data set of Mediterranean plants
No full textThe need of planning more sustainable cities leads to a wider use of extensive green roofs (EGRs) as they provide significant advantages to the urban environment (e.g., energy conservation and increase of biodiversity). In Central and North Europe, as well as in North America and Asia, EGRs are generally included in new building designs, whereas they are still uncommon in Mediterranean countries. The adaptations of many Mediterranean plants to drought stress and their floristic diversity constitute, however, positive elements in finding solutions for them. This research proposes a methodological approach to select wild species for EGRs based on ecological characteristics (using natural ecosystems as templates for green roof design). An extensive bibliographic search on plants proposed for EGRs in Mediterranean countries has led to the creation of a wide database. Other plants were selected considering their synecological, structural, and autoecological characteristics. All the data were integrated in a comprehensive database of 138 taxa potentially suitable for setting EGRs according to their syntaxonomical classification and their ecological behavior (fitting both the Mediterranean and EGR environmental conditions). The selected taxa could enlarge the pool of species for EGRs in Mediterranean cities, increasing urban biodiversity
Effect of the isobaric and isothermal reductions in excess and configurational entropies on glass-forming dynamics
No full textDielectric relaxation times tau, over a broad range of temperatures and pressures for simple molecular glass-forming systems reveal good agreement with the pressure-extended Adam-Gibbs equation derived from the Adam Gibbs model, relating directly the dynamics to the thermodynamics through the configurational entropy S-c, assuming it to be proportional to the excess entropy S-exc of the melt with respect to the crystal. In the present study by making use of both calorimetric and expansivity measurements it was possible to make an accurate determination of the pressure and temperature dependences of S-exc over the whole investigated range, and the expected proportionality between Sc and Sexc was experimentally checked. Results show that different proportionality factors g(P) and g(T) exist between S-c and S-exc in isothermal and isobaric conditions and the ratio g(T)/g(P) of these is nearly 0.7 for all the systems
Do theories of the glass transition, in which the structural relaxation time does not define the dispersion of the structural relaxation, need revision?
No full textUpon decreasing temperature or increasing pressure, a noncrystallizing liquid will vitrify; that is, the structural relaxation time, becomes so long that the system cannot attain an equilibrium configuration in the available time. Theories, including the well-known free volume and configurational entropy models, explain the glass transition by invoking a single quantity that governs the structural relaxation time. The dispersion of the structural relaxation (i.e., the structural relaxation function) is either not addressed or is derived as a parallel consequence (or afterthought) and thus is independent of tau(alpha). In these models the time dependence of the relaxation bears no fundamental relationship to the value of tau(alpha) or other dynamic properties. Such approaches appear to be incompatible with a general experimental fact recently discovered in glass-formers: for a given material at a fixed value of T, the dispersion is constant, independent of thermodynamic conditions (T and P); that is, the shape of the a-relaxation function depends only on the relaxation time. If derived independently of tau(alpha), it is an unlikely result that the dispersion of the structural relaxation would be uniquely defined by tau(alpha)
Reply to "Comment on 'Correlation between configurational entropy and structural relaxation time in glass-forming liquids' "
No full textIn this Reply we show that contrary to the statements of Goldstein, the empirical function we proposed [D. Prevosto, M. Lucchesi, S. Capaccioli, R. Casalini, and P. A. Rolla, Phys. Rev. B 67, 174202 (2003)] for the pressure and temperature dependencies of the configurational entropy is mathematically and physically valid. Specifically we demonstrate that the condition of equal second derivatives, necessary for the configurational entropy to be a function of state, has physically reasonable solutions without the necessity of imposing implausible constraints
Intermittent-contact local dielectric spectroscopy of nanostructured interfaces
No full textLocal dielectric spectroscopy (LDS) is a scanning probe method, based on dynamic-mode atomic force microscopy (AFM), to discriminate dielectric properties at surfaces with nanometer-scale lateral resolution. Until now a sub-10 nm resolution for LDS has not been documented, that would give access to the length scale of fundamental physical phenomena such as the cooperativity length related to structural arrest in glass formers (2-3 nm). In this work, LDS performed by a peculiar variant of intermittent-contact mode of AFM, named constant-excitation frequency modulation, was introduced and extensively explored in order to assess its best resolution capability. Dependence of resolution and contrast of dielectric imaging and spectroscopy on operation parameters like probe oscillation amplitude and free amplitude, the resulting frequency shift, and probe/surface distance-regulation feedback gain, were explored. By using thin films of a diblock copolymer of polystyrene (PS) and polymethylmethacrylate (PMMA), exhibiting phase separation on the nanometer scale, lateral resolution of at least 3 nm was demonstrated in both dielectric imaging and localized spectroscopy, by operating with optimized parameters. The interface within lamellar PS/PMMA was mapped, with a best width in the range between 1 and 3 nm. Changes of characteristic time of the secondary (β) relaxation process of PMMA could be tracked across the interface with PS
Lateral resolution of electrostatic force microscopy for mapping of dielectric interfaces in ambient conditions
Full text linkThe attainable lateral resolution of electrostatic force microscopy (EFM) in an ambient air environment on dielectric materials was characterized on a reference sample comprised of two distinct, immiscible glassy polymers cut in a cross-section by ultramicrotomy. Such a sample can be modeled as two semi-infinite dielectrics with a sharp interface, presenting a quasi-ideal, sharp dielectric contrast. Electric polarizability line profiles across the interface were obtained, in both lift-mode and feedback-regulated dynamic mode EFM, as a function of probe/surface separation, for different cases of oscillation amplitudes. We find that the results do not match predictions for dielectric samples, but comply well or are even better than predicted for conductive interfaces. A resolution down to 3 nm can be obtained by operating in feedback-regulated EFM realized by adopting constant-excitation frequency-modulation mode. This suggests resolution is ruled by the closest approach distance rather than by average separation, even with probe oscillation amplitudes as high as 10 nm. For better comparison with theoretical predictions, effective probe radii and cone aperture angles were derived from approach curves, by also taking into account the finite oscillation amplitude of the probe, by exploiting a data reduction procedure previously devised for the derivation of interatomic potentials
Pressure dependence of structural relaxation times in terms of the Adam-Gibbs model
No full textA new equation describing the behavior of the structural relaxation lime, tau (T,P), as a function of both pressure and temperature, is discussed. This equation has been derived from the Adam-Gibbs theory by writing the configurational entropy, S-c, in terms of the excess thermal heat capacity and of the molar thermal expansion. Consequently, the parameters introduced in the expression are directly related to specific physical properties of the material, such as the thermal expansion coefficient alpha and the isothermal bulk modulus K-o. At a fixed pressure, for low pressures, the found equation reduces to a Vogel-Fulcher-Tammann equation of tau versus temperature with the fragility parameter independent from pressure. The equation for tau (T,P) was successfully tested directly by fitting the dielectric relaxation time data for two isothermal and one isobaric measurements on diglycidyl ether of bisphenol-A, carried out in previous experiments. The parameters estimated by the best fit were in reasonable agreement with the Values determined from the known physical properties of the material. Finally, the expression for the change versus pressure of the temperatures at which the same value of tau (max) is obtained (e.g., the change versus pressure of the glass transition temperature) agrees with several expressions previously proposed in the literature to provide a phenomenological description of the observed phenomena
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