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Luna C.R.,UNCentro and ANPCYT | MacChi C.E.,CONICET | Juan A.,National University of the South | Somoza A.,IFIMAT
International Journal of Hydrogen Energy | Year: 2010

The magnesium hydride stability and bonding have been studied using density functional theory (DFT). To this aim, calculations on the electronic structure were performed. We also modeled the bulk hydride with a Nb atom as a substitutional impurity. Furthermore, both systems were modeled containing different types of vacancies (Mg, H or H-Mg complex). The crystal orbital overlap population for both the metal-metal and metal-hydrogen bonds was also computed. The influence of vacancy-like defects was studied through the calculation of the positron lifetimes in defected MgH2 and defected MgH2-Nb. For the pure hydride, the results show an increment in the atom bonds in correlation with an increase of the positron localization reflected in a rise of the positron lifetimes. On the other hand, in all considered cases for Mg or/and H vacancies, the presence of Nb reduces the hydride bond about 36%. This decrease in the hydride stability was associated with a decrease in the probability of the positron localization and a consequently reduction of the positron lifetimes. © 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

MacChi C.,CONICET | Maurizio C.,University of Padua | Checchetto R.,University of Trento | Mariazzi S.,University of Trento | And 7 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

The structural evolution of nanostructured Nb-doped magnesium film samples and its correlation with the change of the H 2 desorption kinetics after successive H 2 sorption cycles at 623 K was investigated by different techniques. The variation of the dispersed Nb fraction and the Nb clusterization was followed by extended x-ray absorption fine structure (EXAFS), while the progressive Mg nanostructuring was monitored by x-ray diffraction. The presence of vacancylike defects and their evolution was studied using positron annihilation lifetime spectroscopy and Doppler broadening spectroscopies. It was found that, with successive H 2 sorption cycles: (i) the H 2 desorption kinetics progressively becomes slower until stationary conditions are reached and (ii) the Nb dopant atoms, dispersed in the nanocrystalline Mg layers, aggregate, forming nanoclusters. Our results show that the progressive Nb aggregation drives the H 2 desorption kinetics. EXAFS analysis show that fast desorption kinetics is due to the presence of small (∼1 nm) Nb aggregates rather than Nb atoms dispersed into the Mg matrix. With cycling, the Nb aggregates progressively grow, forming larger bcc Nb nanoclusters and the H 2 desorption kinetics becomes slower. In the as-deposited Nb-doped Mg samples, analysis of the positron data reveals the presence of intragranular vacancylike defects and of vacancy clusters which are inferred to be mainly located at the grain boundaries of the nanocrystalline Mg layers. With H 2 cycling: (i) a decrease of the atomic fraction of the intragranular vacancylike defects after the first two sorption cycles was observed, and (ii) an increase of the atomic fraction of vacancy clusters at grain boundaries and the appearance of vacancylike defects located at the interface between the Nb aggregates and the Mg matrix was probed. It was also found that the kinetics follows a nucleation and growth mechanism and, under stationary conditions, the Mg nucleation is controlled by vacancy-decorated bcc Nb nanoclusters rather than by vacancy clusters, as in undoped Mg samples. © 2012 American Physical Society.

Mansilla M.A.,IFIMAT | Mansilla M.A.,CONICET | Rodriguez Garraza A.L.,LPyMC | Silva L.,IFIMAT | And 8 more authors.
Polymer Testing | Year: 2013

An investigation of the influence of the cure time on the glass transition temperature and nanostructure for two isomers of polybutadiene rubbers, CB25 (97% cis) and CB55 (38% cis), vulcanized at 433 K is presented. The samples were prepared with sulphur and TBBS (N-t-butyl-2-benzothiazole sulfenamide), and several crosslink densities were achieved at the different selected cure times. The studies on the samples were carried out using differential scanning calorimetry and positron annihilation lifetime spectroscopy. The variation of the glass transition temperature with the degree of cure and its relation with the crosslink density is analyzed for each type of isomer. A correlation between the free nanohole volume and the crosslink density is also analyzed. From swelling tests, the absorbed solvent at room temperature is discussed in terms of the obtained free nanohole volumes, showing a correspondence between these parameters. In the frame of the obtained results, the cis to trans isomerisation of the elastomer with high cis content during the vulcanization is discussed. © 2013 Elsevier Ltd. All rights reserved.

MacChi C.,CONICET | Burgi J.,CONICET | Garcia Molleja J.,CONICET | Mariazzi S.,University of Trento | And 5 more authors.
EPJ Applied Physics | Year: 2014

It is well-known that the characteristics of aluminum nitride thin films mainly depend on their morphologies, the quality of the film-substrate interfaces and the open volume defects. A study of the depth profiling and morphological characterization of AlN thin films deposited on two types of Si substrates is presented. Thin films of thicknesses between 200 and 400 nm were deposited during two deposition times using a reactive sputter magnetron. These films were characterized by means of X-ray diffraction and imaging techniques (SEM and TEM). To analyze the composition of the films, energy dispersive X-ray spectroscopy was applied. Positron annihilation spectroscopy, specifically Doppler broadening spectroscopy, was used to gather information on the depth profiling of open volume defects inside the films and the AlN films-Si substrate interfaces. The results are interpreted in terms of the structural changes induced in the films as a consequence of changes in the deposition time (i.e., thicknesses) and of the orientation of the substrates. © 2014 EDP Sciences.

Mansilla M.A.,University of Buenos Aires | Mansilla M.A.,CONICET | Silva L.,CONICET | Salgueiro W.,IFIMAT | And 4 more authors.
Journal of Applied Polymer Science | Year: 2012

A study of the thermal behavior of cured elastomeric blends of natural rubber (NR) and styrene butadiene rubber (SBR) prepared by solution blending in toluene is presented. Binary blends with different compositions of NR/SBR were produced using a conventional cure system based on sulfur and TBBS (n-t-butyl-2-benzothiazole sulfonamide as accelerator. The compounds were vulcanized at 433 K up to an optimum time of cure determined by rheometric tests. From swelling tests, the crosslink densities of the compounds were obtained and compared with those obtained in similar blends prepared by mechanical mixing. The results were analyzed in terms of the disentangling of the chain structures of the SBR and NR phases and the achieved cure state of the blend. Using differential scanning calorimetry, the glass transition temperature T g of each blend was measured. In most compounds, the value of T g corresponding to each phase of the blend was determined, but in some blends a single value of T g was obtained. The variation of T g with the composition and cure level in each phase was analyzed. On the other hand, a physical mixture of two equal parts of NR and SBR vulcanized was measured and the results were compared to those of the NR50/SBR50 cured blend. Besides, to analyze the influence of the network structure, pure NR and SBR unvulcanized samples were measured. On the basis of all the obtained results, the influence of the interphase formed in the blend between SBR and NR phases is discussed. Copyright © 2011 Wiley Periodicals, Inc.

Luna C.R.,CONICET | MacChi C.,CONICET | Juan A.,CONICET | Somoza A.,IFIMAT
Journal of Physics: Conference Series | Year: 2013

First results of a systematic study on the vacancy clustering process in Al, Cu, Mg and Nb are presented. To this aim first principle calculation of positron lifetimes and positron-electron momentum distributions were performed. We test the reliability of the computational scheme used by comparing some of the calculated results with experimental ones.

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