Publications

The divertor of a fusion reactor like DEMO requires materials able to withstand high heat fluxes and neutron irradiation for several years. For the water cooling concept of this essential part of the reactor, the most likely plasma facing material will be W, while the heatsink material considered is CuCrZr or an improved version of such a Cu-based alloy. To realize W-Cu alloy joints able to withstand thousands of thermal cycles can be difficult due to the difference between the thermal expansion coefficients of these materials. In this work we investigate the possibility to realize such joints by using W-Cu functional gradient materials (FGMs) produced from nanometric and micrometric metallic powders mixtures and consolidated by spark plasma sintering at about 900 degrees C. Morphological and thermal properties investigations, performed for typical compositions, shows that the best results are obtained using powders with micrometric dimensions. A resulting 1 mm thick, 3 layers W-Cu FGM produced by this simple method shows a remarkable almost constant thermal conductivity value of 200 W m(-1) K-1, from room temperature up to 1000 degrees C.

Thermoluminescent measurements between 40 and 300 C revealed two traps in the irradiated Alq(3) powder synthetized from Al(OH)(3), at 0.8 and 1.1 eV below the conduction band. These two traps have been assigned with the Alq(3) facial and meridional isomers and the trapping process stand for a long time after the switching off the irradiation source. The spectra recorded an hour after switching recover 60% from the photoluminescence and also decreasing the TL signals. A theoretical model was proposed in order to explain the trapping and detrapping processes in both meridional and facial Alq(3) isomers based on the XPS measurements for the HOMO, LUMO, ionization potential and optical bandgap, completed with the DFT calculated parameters and similar experimental results concerning the evidences of the triplet states in Alq(3) compound.

This paper reports the influence of the charge carrier mobility on the electroluminescent properties of a dual-emitter organometallic compound dispersed in two conjugated organic small-molecule host materials and embedded in organic light-emitting devices (OLEDs). The electroluminescent processes in OLEDs are strongly influenced by the host-guest interaction. The charge carrier mobility in the host material plays an important role in the electroluminescent processes but also depends on the triplet-triplet interaction with the organometallic compound. The low charge carrier mobility in 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) host material reduces the electroluminescent processes, but they are slightly enhanced by the triplet-triplet exothermic charge transfer. The higher charge carrier mobility in the case of N, N'-bis(3-methylphenyl)-N, N'-diphenylbenzidine (TPD) host material influences the electroluminescent processes by the endothermic energy transfer at room temperature, which facilitates the triplet-triplet harvesting in the host-guest system. The excitation is transferred to the guest molecules by triplet-triplet interaction as a Dexter transfer, which occurs by endothermic transfer from the triplet exciton in the host to the triplet exciton in the guest.

DEMO fusion reactor divertor is expected to extract a heat flux of about 10 MW/m(2). One of the most promising concept design for it is the W-monoblock, which should be connected to a CuCrZr or an advanced Cu ODS alloy pipe passing through the W component. Since the optimum operating temperature windows for W and existing Cu alloys are far away from overlapping, a suited interface is needed to keep the adjacent materials as close as Possible to their respective temperature operating windows. The interface material should therefore have a low enough thermal conductivity to act as a thermal barrier and a thermal expansion coefficient suited to protect the W-pipe joint from stresses induced by the different thermo-mechanical properties of W and Cu-alloys. As interface materials we have considered Cu-ZrO2 composites produced by powder metallurgy route. Such materials can be realized in an unexpected large compositional range (up to at least 90% ZrO2 volume concentration) and be easily further joined to both W and Cu-alloys by an electrical field assisted technology. We analyse their microstructure and thermo-physical properties both as single materials and included in W-thermal barrier-CuCrZr 3-layers systems in comparison to those of previously produced Cu-ased composites and commercially available Cu foams.

The main goal of this paper is to put on solid mathematical grounds the so-called non-equilibrium Green's function transport formalism for open systems. In particular, we derive the Jauho-Meir-Wingreen formula for the time-dependent current through an interacting sample coupled to non-interacting leads. Our proof is non-perturbative and uses neither complex-time Keldysh contours nor Langreth rules of 'analytic continuation.' We also discuss other technical identities (Langreth, Keldysh) involving various many-body Green's functions. Finally, we study the Dyson equation for the advanced/retarded interacting Green's function and we rigorously construct its (irreducible) self-energy, using the theory of Volterra operators.

Samples of MgB2 with relative density above 95% were obtained by spark plasma sintering (SPS) at 1150 A degrees C for a heating time of 100 A degrees C/min and a maximum pressure of 95 MPa. Dwell time was of 1, 4, 7, 10, and 20 min. Samples show that dwell time has a low influence on superconducting characteristics. However, small differences were observed and they are discussed from the viewpoint of density, structural, microstructural, critical current density, irreversibility field, and pinning type. The dominant contribution for all investigated samples is given by point and delta T (c) pinning.

Foraminifera, unicellular organisms that are widespread throughout marine ecosystems, build Ca-carbonate shells that may incorporate trace metals present in the ocean waters because of natural or anthropogenic supply. In this study, we focussed on the trace element Zn, which is abundant in both contaminated and clean waters. We used X-ray and electron spectromicroscopy to investigate the Zn coordinative environment in individual shells of two species of benthic foraminifera, Elphidium aculeatum and Quinqueloculina seminula, that were sampled from a heavy-metal polluted area of Sardinia, Italy. These species synthesise the Ca-carbonate in extracellular and intracellular spaces, respectively, which implies some diversity in their physiologies and cation transport processes, and they can adapt and survive in metal-polluted environments. Our analyses of X-ray micro-fluorescence (mu-XRF) maps and Zn-K mu-X-ray absorption near-edge spectroscopy (XANES) reveal that although 50% of Zn occurs as a Ca substitute in calcite or as a tetracoordinate oxygen-adsorbed ion, detectable amounts can also be found in other Zn-independent mineral phases, particularly hydrozincite, whose formation is due to foraminiferal cellular processes. Other Zn phases, sphalerite (attributed to early diagenetic process) and Zn-phosphate, were recognised. Moreover, we found distinct differences in the Zn concentration, distribution and chemical speciation at the micro- and nano-metric scales of the investigated E. aculeatum and Q. seminula species. In the calcite needles of Q. seminula, Zn is uniformly distributed and recognised in a disordered local environment, which suggests that it is incorporated in the calcite phase during the intracellular calcification process. These findings offer insight into Zn incorporation in foraminifera and its potential application in biomonitoring and environmental studies.

Be-C-W mixed materials with variable atomic ratios were exposed to high power (TW) laser induced filamentation plasma in air in normal conditions and in deuterium at a reduced pressure of 20 Torr. Morphological and structural investigations were performed on the irradiated zones for both ambient conditions. The presence of low-pressure deuterium increased the overall ablation rate for all samples. From the elemental concentration point of view, the increase of the carbon percentage has led to an increase in the ablation rate. An increase of the tungsten percentage had the opposite effect. From structural spectroscopic investigations using XPS, Raman and FT-IR of the irradiated and non-irradiated sample surfaces, we conclude that deuterium-induced enhancement of the ablation process could be explained by preferential amorphous carbon removal, possibly by forming deuterated hydrocarbons which further evaporated, weakening the layer structure.

A new bisimidazolium salt with dodecyl sulfate as counterion has been designed and prepared. This salt shows a SmA phase that is stable at room temperature. The new ionic liquid crystal (ILC) was characterized by H-1 NMR, C-13 NMR and IR spectroscopy. Its liquid crystalline properties were analyzed by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD) studies. The dielectric spectra of the ILC doped with different concentrations of carbon nanotubes (CNT) were recorded over a wide frequency and temperature range of 10(-1) to 10(7) Hz and 293-338 K, respectively. The values of the activation energy were found in the range of 0.46-0.61 eV; the characteristic time was obtained by fitting the spectra of the dielectric loss with the Havriliak-Negami functions. As a result of doping the ILC with CNT, the electric conductivity increases significantly. Ionic conductivity is dominant and it was indirectly observed through the electrode polarization (EP) effect. The very high dielectric permittivity values and the decrease of the electric conductivity at low frequencies confirm the presence of EP.

Trilayer memory capacitors with Ge nanocrystals (NCs) floating gate in HfO2 were obtained by magnetron sputtering deposition on p-type Si substrate followed by rapid thermal annealing at relatively low temperature of 600 degrees C. The frequency dispersion of capacitance and resistance was measured in accumulation regime of Al/HfO2 gate oxide/Ge NCs in HfO2 floating gate/HfO2 tunnel oxide/SiOx/p-Si/Al memory capacitors. For simulation of the frequency dispersion a complex circuit model was used considering an equivalent parallel RC circuit for each layer of the trilayer structure. A series resistance due to metallic contacts and Si substrate was necessary to be included in the model. A very good fit to the experimental data was obtained and the parameters of each layer in the memory capacitor, i.e. capacitances and resistances were determined and in turn the intrinsic material parameters, i.e. dielectric constants and resistivities of layers were evaluated. The results are very important for the study and optimization of the hysteresis behaviour of floating gate memories based on NCs embedded in oxide. (C) 2017 Published by Elsevier B.V.