A Thermodynamically Consistent Damage Model for Advanced Composites

A continuum damage model for the prediction of damage onset and structural collapse of structures manufactured in fiber-reinforced plastic laminates is proposed. The principal damage mechanisms occurring in the longitudinal and transverse directions of a ply are represented by a damage tensor that is fixed in space. Crack closure under load reversal effects are taken into account using damage variables established as a function of the sign of the components of the stress tensor. Damage activation functions based on the LaRC04 failure criteria are used to predict the different damage mechanisms occurring at the ply level. The constitutive damage model is implemented in a finite element code. The objectivity of the numerical model is assured by regularizing the dissipated energy at a material point using Bazant’s Crack Band Model. To verify the accuracy of the approach, analyses ofcoupon specimens were performed, and the numerical predictions were compared with experimental data

Optical response of Cu3Ge thin films

We report an investigation on the optical properties of Cu3Ge thin films displaying very high conductivity, with thickness ranging from 200 to 2000 Å, deposited on Ge substrates. Reflectance, transmittance, and ellipsometric spectroscopy measurements were performed at room temperature in the 0.01-6.0, 0.01-0.6, and 1.4-5.0 eV energy range, respectively. The complex dielectric function, the optical conductivity, the energy-loss function, and the effective charge density were obtained over the whole spectral range. The low-energy free-carrier response was well fitted by using the classical Drude-Lorentz dielectric function. A simple two-band model allowed the resulting optical parameters to be interpreted coherently with those previously obtained from transport measurements, hence yielding the densities and the effective masses of electrons and holes.

Modifications in the Si valence band after ion-beam-induced oxidation

The changes undergone by the Si surface after oxygen bombardment have special interest for acquiring a good understanding of the Si+-ion emission during secondary ion mass spectrometry (SIMS) analysis. For this reason a detailed investigation on the stoichiometry of the builtup surface oxides has been carried out using in situ x-ray photoemission spectroscopy (XPS). The XPS analysis of the Si 2p core level indicates a strong presence of suboxide chemical states when bombarding at angles of incidence larger than 30°. In this work a special emphasis on the analysis and interpretation of the valence band region was made. Since the surface stoichiometry or degree of oxidation varies with the angle of incidence, the respective valence band structures also differ. A comparison with experimentally measured and theoretically derived Si valence band and SiO2 valence band suggests that the new valence bands are formed by a combination of these two. This arises from the fact that Si¿Si bonds are present on the Si¿suboxide molecules, and therefore the corresponding 3p-3p Si-like subband, which extends towards the Si Fermi level, forms the top of the respective new valence bands. Small variations in intensity and energy position for this subband have drastic implications on the intensity of the Si+-ion emission during sputtering in SIMS measurements. A model combining chemically enhanced emission and resonant tunneling effects is suggested for the variations observed in ion emission during O+2 bombardment for Si targets.

Model for the emission of Si+ ions during oxygen bombardment of Si(100) surfaces

The variation in the emission of Si+ ions from ion-beam-induced oxidized silicon surfaces has been studied. The stoichiometry and the electronic structure of the altered layer has been characterized using x-ray photoelectron spectroscopy (XPS). The XPS analysis of the Si 2p core level indicates the strong presence of suboxide chemical states when bombarding at angles of incidence larger than 30 °. Since the surface stoichiometry or degree of oxidation varies with the angle of incidence, the corresponding valence-band structures also differ among each other. A comparison between experimental measurements and theoretically calculated Si and SiO2 valence bands indicates that the valence bands for the altered layers are formed by a combination of those two. Since Si-Si bonds are present in the suboxide molecules, the top of the respective new valence bands are formed by the corresponding 3p-3p Si-like subbands, which extend up to the Si Fermi level. The changes in stoichiometry and electronic structure have been correlated with the emission of Si+ ions from these surfaces. From the results a general model for the Si+ ion emission is proposed combining the resonant tunneling and local-bond-breaking models.

Continuum Double Exchange Model

We present a continuum model for doped manganites which consist of two species of quantum spin-1 / 2 fermions interacting with classical spin fields. The phase structure at zero temperature turns out to be considerably rich: antiferromagnetic insulator, antiferromagnetic two band conducting, canted two band conducting, canted one band conducting, and ferromagnetic one band conducting phases are identified, all of them being stable against phase separation. There are also regions in the phase diagram where phase separation occurs

Surface structure and stability of PdZn and PtZn alloys: Density-functional slab model studies

Geometric parameters of binary (1:1) PdZn and PtZn alloys with CuAu-L10 structure were calculated with a density functional method. Based on the total energies, the alloys are predicted to feature equal formation energies. Calculated surface energies of PdZn and PtZn alloys show that (111) and (100) surfaces exposing stoichiometric layers are more stable than (001) and (110) surfaces comprising alternating Pd (Pt) and Zn layers. The surface energy values of alloys lie between the surface energies of the individual components, but they differ from their composition weighted averages. Compared with the pure metals, the valence d-band widths and the Pd or Pt partial densities of states at the Fermi level are dramatically reduced in PdZn and PtZn alloys. The local valence d-band density of states of Pd and Pt in the alloys resemble that of metallic Cu, suggesting that a similar catalytic performance of these systems can be related to this similarity in the local electronic structures.

Effect of radar rainfall time resolution on the predictive capability of a distributed hydrologic model

The performance of a hydrologic model depends on the rainfall input data, both spatially and temporally. As the spatial distribution of rainfall exerts a great influence on both runoff volumes and peak flows, the use of a distributed hydrologic model can improve the results in the case of convective rainfall in a basin where the storm area is smaller than the basin area. The aim of this study was to perform a sensitivity analysis of the rainfall time resolution on the results of a distributed hydrologic model in a flash-flood prone basin. Within such a catchment, floods are produced by heavy rainfall events with a large convective component. A second objective of the current paper is the proposal of a methodology that improves the radar rainfall estimation at a higher spatial and temporal resolution. Composite radar data from a network of three C-band radars with 6-min temporal and 2 × 2 km2 spatial resolution were used to feed the RIBS distributed hydrological model. A modification of the Window Probability Matching Method (gauge-adjustment method) was applied to four cases of heavy rainfall to improve the observed rainfall sub-estimation by computing new Z/R relationships for both convective and stratiform reflectivities. An advection correction technique based on the cross-correlation between two consecutive images was introduced to obtain several time resolutions from 1 min to 30 min. The RIBS hydrologic model was calibrated using a probabilistic approach based on a multiobjective methodology for each time resolution. A sensitivity analysis of rainfall time resolution was conducted to find the resolution that best represents the hydrological basin behaviour.