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Morales M.,University of Santiago de Compostela | Pecharroman C.,CSIC - Institute of Materials Science | Mata-Osoro G.,Ecole Nationale Superieure de Chimie de Paris | Diaz L.A.,Nanomaterials and Nanotechnology Research Center | Garzon J.A.,University of Santiago de Compostela
Journal of Instrumentation | Year: 2013

Development of new electrodes is a key element for the improvement of high rate Resistive Plate Chambers (RPC). In the particular case of resistive electrodes, the fabrication of these elements is a challenging problem from a material science point of view. The combination of resistivity, permittivity and stability requirements is really hard to satisfy for any known material. Only a few materials have been found to be suitable for fabrication of resistive plates. In this work, we have carried out electrical characterizations of some of the materials that are currently used in RPCs or are solid candidates to be used as resistive plates in high rate RPCs. As a result, we have found strong evidences that ion conduction processes under moderate to high electric fields are able to drift high amounts of charge which allow to understand the degradation nature of the involved processes. © 2013 IOP Publishing Ltd and Sissa Medialab srl.

Rahman G.,Quaid-i-Azam University | Din N.U.,Quaid-i-Azam University | Garcia-Suarez V.M.,University of Oviedo | Garcia-Suarez V.M.,Nanomaterials and Nanotechnology Research Center | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

The magnetism and electronic structure of Li-doped SnO2 are investigated using first-principles LDA/LDA+U calculations. We find that Li induces magnetism in SnO2 when doped at the Sn site but becomes nonmagnetic when doped at the O and interstitial sites. The calculated formation energies show that Li prefers the Sn site as compared with the O site, in agreement with previous experimental works. The interaction of Li with native defects (Sn VSn and O VO vacancies) is also studied, and we find that Li not only behaves as a spin polarizer, but also a vacancy stabilizer, i.e., Li significantly reduces the defect formation energies of the native defects and helps the stabilization of magnetic oxygen vacancies. The electronic densities of states reveals that these systems, where the Fermi level touches the conduction (valence) band, are nonmagnetic (magnetic). © 2013 American Physical Society.

Rahman G.,Quaid-i-Azam University | Morbec J.M.,University of Chicago | Ferradas R.,University of Oviedo | Ferradas R.,Nanomaterials and Nanotechnology Research Center | And 3 more authors.
Journal of Magnetism and Magnetic Materials | Year: 2016

The electronic and magnetic structures of cubic BaFeO3 (BFO) in the ferromagnetic (FM) and antiferromagnetic (AFM) states are studied using density functional theory (DFT) with the local spin density approximation (LSDA) and the generalized gradient approximation (GGA), with and without a Coulomb U term. Our LSDA/GGA and LSDA+U/GGA+U results show that cubic BFO has a FM ground state, in agreement with recent experimental studies. Two types of distortions, denoted as D1 and D2, are considered. The source of the distortion in the D1 (D2) case is the displacement of the oxygen (iron) atoms from their equilibrium positions. FM to ferrimagnetic (FIM) and FM to AFM magnetic phase transitions are found in the D1 and D2 distortions, respectively. Larger strains are required for the FM-AFM transition as compared to the FM-FIM. DFT+U calculations also show that the magnetic moments dramatically decrease at large strains due to strong overlapping between the iron and oxygen atoms. The origin of these transitions is discussed in terms of a competition between double exchange and superexchange interactions. From these results it is possible to conclude that oxygen and iron displacements are responsible for the magnetic phase transitions and the reduction of the magnetic moments in BFO. © 2015 Published by Elsevier B.V.

Borrell A.,Nanomaterials and Nanotechnology Research Center | Fernandez A.,Fundacion ITMA | Torrecillas R.,Nanomaterials and Nanotechnology Research Center
International Journal of Materials Research | Year: 2010

Graphitic materials obtained at low temperatures are interesting for a wide range of industrial applications including bipolar plates. In this work, graphite based nanocomposites have been obtained starting from carbon nanofibers and a mixture of carbon nanofibers with 20 vol.% of alumina nanopowders. High density carbon components were obtained by using Spark Plasma Sintering at temperatures as low as 1500-1800°C for this kind of materials. The effect of spark plasma sintering parameters on the final density, and the mechanical and electrical properties of resulting nanocomposites have been investigated. Pure carbon nanofibers with around 90% of theoretical density and fracture strength of 60 MPa have been obtained at temperatures as low as 1500°C applying a pressure of 80 MPa during sintering. It has been proved that attrition milling is a suitable method for preparing homogeneous mixtures of carbon nanofibers and alumina powders. © Carl Hanser Verlag GmbH & Co. KG.

Kostyrko T.,Adam Mickiewicz University | Garcia-Suarez V.M.,University of Oviedo | Garcia-Suarez V.M.,Nanomaterials and Nanotechnology Research Center | Wawrzyniak-Adamczewska M.,Adam Mickiewicz University | And 2 more authors.
Applied Surface Science | Year: 2016

A metallic carbon nanotube with point-like defects under influence of a local potential due to a point charge probe is theoretically studied. A combination of density functional theory and the Landauer-Büttiker formalism is used to compute the electronic conductance in the zero-voltage limit. From a collection of the results obtained by varying the probe position around different defects the conductance maps are created. The analysis of the conductance maps allows us to formulate conditions under which several point-like defects (the Stone-Wales defect, a simple carbon vacancy, hydrogen-passivated vacancies) can be distinguished and identified in experiments with the help of scanning probe microscopy. © 2015 Elsevier B.V. All rights reserved.

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