Ultra-short pulsed laser deposition of gallium arsenide: a comprehensive study

A Gallium Arsenide target has been ablated by using a frequency-doubled Nd:glass laser with a pulse duration of 250 fs and thin films have been deposited in vacuum. The plasma produced by the ablation process and the deposited films have been studied by several different techniques, including optical emission spectroscopy, ICCD fast imaging and electron microscopies, X-ray diffraction, Raman spectroscopy, respectively. The data evidence that the films, which composition shows an excess of Ga, are formed by the coalescence of a large number of nanoparticles. These results, even if the plasma does not evidence the presence of nanoparticles, seem to indicate that the ablation-deposition mechanism is the same found for the majority of the other systems deposited by ultra-short pulse lasers.     

Mit der analytischen Charakterisierung des Safrans

Ohne Zusammenfassung     

Untersuchung von Milch und Milchprodukten

Ohne Zusammenfassung     

A New Efficient Heuristic for the Minimum Set Covering Problem

We solve approximately the minimum set covering problem by developing a new heuristic, which is essentially based on the flow algorithm originally developed by Ford and Fulkerson. We perform a comparative study of the performances (concerning solution qualities and execution times) of the flow algorithm as well as of the natural greedy heuristic for set covering originally studied by Johnson and Lovász.     

Femtosecond pulsed laser ablation deposition of tantalum carbide

In this work a frequency-doubled Nd:glass laser with a pulse duration of 250 fs has been used to ablate a TaC target and to deposit thin films on silicon. The results have been compared with those previously obtained by nanosecond pulsed laser deposition and evidence of large differences in the plasma characteristics has been revealed. In particular, in the femtosecond and nanosecond plumes the energy and the velocity of neutral and ionized particles are very different. The features of femtosecond ablation include the delayed emission from the target of large and slow particles. The characteristics of the femtosecond plasma are clearly related to the morphology and composition of the deposited films and the results show a nanostructure consisting of a large number of spherical particles, with a mean diameter of about 50 nm, with a stoichiometry corresponding to Ta₂C. To explain these features, an ablation-deposition mechanism, related to the ejection of hot particles from the target, is proposed.     

fs/ns dual-pulse LIBS analytic survey for copper-based alloys

The quantitative analytic capability of a fs/ns dual-pulse Laser-Induced Breakdown Spectroscopy technique, based on the orthogonal reheating of a fs-laser ablation plume by a ns-laser pulse, is presented. In this work, it is shown how the effect played by the delay times between the two laser beams can vary the analytical response of this dual-pulse LIBS configuration. In order to address this task, the Sn, Pb and Zn calibration curves of five certified copper-based samples have been investigated. These calibration curves have been obtained, in air at atmospheric pressure, by integrating the emission data collected in two different inter-pulse delay zones, one in the delay interval of 1-41 μs, the other within the range of 46-196 μs. For drawing the species calibration curves, the emission intensities of the considered Pb(I), Sn(I) and Zn(I) electronic transitions have been normalized with a non-resonant Cu(I) emission line. The experimental results have shown that, by varying the inter-pulse delay between the two laser beams, complementary analytical results can be induced. By considering at once all data acquired within the inter-pulse delay time of 1-196 μs, this hypothesis has been strengthened. The calibration curves obtained in this way are characterized by excellent linear regression coefficients (0.988-0.999) despite of the large Sn, Pb and Zn compositional variation of the targets employed. The results presented reveal, for the first time, that, by taking into account the role played by the inter-pulse delay time between the two laser beams, the fs/ns dual-pulse LIBS configuration here used can be improved and provide very good opportunities for performing quantitative analysis of copper-based alloys.     

On the optimal level of a small country's strategic petroleum reserve

Decisions relating to a country's strategic petroleum reserve must take into account the level of risk inherent in its petroleum imports, the cost resulting from any shortfall in the import level, the cost of storage, and finally the effects of stockpiling transactions on the sensitive spot oil markets. Of course, small countries need not take into account their effect on the global market, a fact that drastically simplifies their decision problem. We present such a simple decision model for a small country's petroleum reserve which in addition to the above factors take into account the uncertainty of the country's refining capacity. A complete analytical treatment is feasible for this model, and a specific numerical example is presented for the case of Greece.     

Enhancement of proton mobility and mitigation of methanol crossover in sPEEK fuel cells by an organically modified titania nanofiller

An organically functionalized titania, TiO₂-RSO₃H, was evaluated as filler in sulfonated polyetheretherketone (sPEEK)-based composite membranes for application in high temperature direct methanol fuel cells. The presence of propylsulfonic acid groups covalently bound onto the TiO₂ surface and the nanometric nature of the additive were analyzed by Raman spectroscopy and transmission electron microscopy, respectively. The properties of the sPEEK/TiO₂-RSO₃H composite membranes were compared with those of the pure sPEEK membranes and those of the sPEEK/TiO₂ composite membranes containing pristine titania nanoparticles at same filler content. Water and methanol transport properties were investigated by NMR methods, including relaxation times and self-diffusion coefficients as function of temperature (up to 130 °C), and pressure (from 0 up to 2 kbar). The incorporation of the nanoadditivies in the sPEEK polymer demonstrates considerable effects on the morphology and stiffness of the membranes, as well as on the transport properties and barrier effect to the methanol crossover. In particular, the functionalization by propylsulfonic acid groups promotes a higher reticulation between the polymeric chains, increasing the tortuosity of the methanol diffusional paths, so reducing the molecular diffusion, while the proton mobility increases being favored by the Grotthus-type mechanism. Conductivity measurements point out that the filler surface functionalization avoids the reduction of the overall proton conduction of the electrolyte due to the embedding of the low-conducting TiO₂. Finally, remarkable improvements were found when using the sPEEK/TiO₂-RSO₃H composite membrane as electrolyte in a DMFC, in terms of reduced methanol crossover and higher current and power density delivered.     

Cytotoxic bromoditerpenes from the red alga Sphaerococcus coronopifolius

Three new brominated diterpenes (1, 2, 8), along with six previously reported metabolites (3-7, 9), were isolated from the organic extract of Sphaerococcus coronopifolius, collected in Palaiokastritsa bay at the west coasts of Corfu Island. The structures of the new natural products, as well as their relative stereochemistry, were established by means of spectral data analyses, including 2D experiments. The absolute stereochemistry of 2 and 4 as well as the structure revision of the previously reported metabolite 3 were established by X-ray crystallographic analyses. The cytotoxicity of the isolated metabolites was evaluated against the NSCLC-N6-L16 and A549 human lung cancer cell lines.