1,720,978 research outputs found
Hydrogen divacancy diffusion: A new perspective on H migration in MgH2 materials for energy storage
Full text linkThe formation and diffusion of pairs of hydrogen vacancies (divacancies) in magnesium hydride is modeled using density functional theory. Compared to the commonly studied case of single hydrogen vacancies, it is found that divacancies are energetically favored over two isolated vacancies. Also, as a function of the diffusion axis considered, the calculated diffusion barriers of divacancies are either smaller or of comparable magnitude with respect to the diffusion barriers of a single vacancy. These findings shed new light on hydrogen transport in MgH2, which is of crucial importance to understand the kinetics of hydrogen take-up and release in this storage material.Fil: German, Estefania. The Abdus Salam; Italia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: Gebauer, Verónica. The Abdus Salam; Itali
Improvement of Hydrogen Vacancy Diffusion Kinetics in MgH2 by Niobium- and Zirconium-Doping for Hydrogen Storage Applications
Full text linkTransition metal (TM) catalytic dopants are broadly used in hydrogen storage materials to increase H2 desorption and absorption kinetics. We have studied H vacancy formation energy in pure, Nb- or Zr- doped bulk magnesium hydride using density functional theory based calculations. The preferential dopant location was determined by means of occupation energy analysis. Both TM species prefer substitutional locations, Zr being more stable than Nb. Five different sites for H vacancy formation have been considered, all of them near the dopant. The vacancy formation energy decreases, especially in the interstitial Zr system in comparison with a pure one (from 1.35 to 0.51 eV). Concerning diffusion, we consider four paths in the pure and doped systems: the doping with TMs diminishes the activation energy barriers improving the diffusion kinetics, being more considerable for Zr. Effects of a possible spin polarization induced in the system by TM atoms and H vacancies generation have been considered as well.Fil: German, Estefania. The Abdus Salam. International Centre For Theoretical Physics; Italia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: Gebauer, Ralph. The Abdus Salam. International Centre For Theoretical Physics; Itali
Why are MoS2 monolayers not a good catalyst for the oxygen evolution reaction?
Full text linkWe use density functional theory based calculations to study the energetics of the oxygen evolution reaction on a monolayer of MoS2. This material, a prototypical example of a layered transition metal dichalcogenide, is intensely studied in the context of many important catalytical applications, in particular for the hydrogen evolution reaction. The second half-reaction of the water-splitting process, the oxygen evolution reaction, is almost never considered on this material, due to its low activity. Based on our calculations, we explain this experimentally observed poor catalytic activity for the oxygen evolution by the weak binding of two key reaction intermediates (hydroxyl and hydroperoxyl) to the substrate. We explore substitutional doping with oxygen and phosphorous as means to facilitate the oxygen evolution on MoS2 layers. The oxygen substitution slightly increases the reaction´s overpotential, but does not significantly change the energetics. The doping with phosphorous, on the other hand, is not a promising way to promote the oxygen evolution on MoS2 layers. We also explore the role of the edges of MoS2 layers. We find that while the adsorption energies of reaction intermediates are strongly influenced by the presence of an edge, the final reaction overpotential remains nearly the same as on a pristine monolayer, meaning that the presence of edges is not favoring the OER.Fil: German, Estefania. Universidad de Valladolid; España. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: Gebauer, Ralph. The Abdus Salam. International Centre for Theoretical Physics; Itali
Ruthenium decorated single walled carbon nanotube for molecular hydrogen storage: A first-principle study
No full textMolecular hydrogen storage on Ruthenium (Ru) decorated single-walled carbon nanotube (SWCNT) has been studied by using spin-polarized density functional theory (DFT). When a Ru atom is adsorbed on SWCNT, the Bader analysis reveals that Ru transfers a charge of 0.44e to SWCNT. Accordingly, Ru acts as adsorption center for H2 molecules; thus, it can hold up to four H2 molecules with an adsorption energy (Eads) of 0.93 eV/H2. A uniform addition of Ru atoms on SWCNT shows that this nanomaterial can adsorb up to five Ru without clustering. Each Ru atom of 5Ru-decorated SWCNT system can bind up to four H2 molecules involving an Eads of 0.83 eV/H2. After H2 molecules adsorption, Ru atoms shifted from a near hollow site to a bridge site. Moreover, Ru-decorated systems reduce their magnetic moment when the number of H2 molecules increase from 2 mB to 0 mB.Fil: Verdinelli, Valeria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; Argentina. Universidad Nacional del Sur. Departamento de Química; ArgentinaFil: Juan, Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: German, Estefania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; Argentina. Universidad de Valladolid; Españ
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Comparison of standard DFT and Hubbard-DFT methods in structural and electronic properties of TiO 2 polymorphs and H-titanate ultrathin sheets for DSSC application
No full textThe structural and electronic properties of several TiO 2 polymorphs and hydrogen titanate surfaces are modeled and studied by density functional theory (DFT). By implementing the Hubbard parameter “U” in our calculations more realistic results of electronic properties are obtained, paying with a small deviation in geometric optimization. Lattice parameters difference is found to be less than 6.2%, as well as some changes in surface energy are found, but the reactivity tendency of surfaces is maintained. Calculated work function is less energetic for faces (001) for anatase, (101) for rutile and (001) for TiO 2 -B.Fil: German, Estefania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: Faccio, Ricardo. Centro Nanomat-detema; UruguayFil: Mombrú, Alvaro W.. Centro Nanomat-detema; Urugua
A DFT + U study on structural, electronic, vibrational and thermodynamic properties of TiO 2 polymorphs and hydrogen titanate: tuning the Hubbard ‘ U -term’
Full text linkStructural, electronic, vibrational and thermodynamic properties have been tested when Hubbard parameter U is implemented in density functional theory calculations for TiO2 polymorphs: anatase, rutile, TiO2?B and for hydrogen titanate (H2Ti3O7) bulk. Optimum U parameter values were found for each system, balancing geometric changes and electronic properties, namely, U = 4 eV for anatase and TiO2?B, U = 5 eV for rutile and hydrogen titanate. Although the addition of this parameter improves the prediction of electronic properties, with no significant structural changes, we found that it would not be adequate for predicting vibrational properties.Fil: German, Estefania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; Argentina. Universidad de la República; UruguayFil: Faccio Sgiorovello, Ricardo Juan. Universidad de la República; UruguayFil: Mombrú, Alvaro W. Universidad de la República; Urugua
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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