1,721,067 research outputs found
Thermo-chemical and thermo-physical properties of stishovite: An ab-initio all-electron investigation
No full textAb-initio thermo-chemical and thermo-physical properties of stishovite, the high pressure polymorph of SiO2, were computed with the hybrid B3LYP density functional method. First, investigation of static properties through a symmetry preserving relaxation procedure was carried out and then, on the equilibrium structure, harmonic vibrational modes were computed at the long-wavelength limit corresponding to the center of the Brillouin zone (k -> 0). While optic modes are the eigenvectors of the Hessian matrix, acoustic modes at Gamma point are obtained by solving the non-zero components of the strain matrix. Acoustic branches were then assumed to follow a sine wave dispersion when traveling within the Brillouin zone (Kieffer model). The quasi-harmonic mode-gamma analysis of volume effects
on vibrational frequencies allowed computation of all vibrationally dependent thermodynamic properties (isochoric and isobaric heat capacity, thermal expansion, thermal derivative of the bulk modulus, thermal correction to internal energy, enthalpy and Gibbs free energy, thermal pressure, elastic and calorimetric
Debye temperature) with sufficient accuracy. Moreover, the conformation of the strain tensor allowed satisfactory reproduction of known thermo-physical properties, viz. transverse and longitudinal wave velocities, shear modulus, Young's modulus and Poisson's rati
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
Elucidating the interaction of CO2 in the giant metal–organic framework MIL-100 through large-scale periodic ab initio modeling
Full text linkIn this work, we have tackled the challenging task of the ab initio modeling of a gigantic metal–organic framework (MOF). The structural features of MIL-100(MIII) with different metals (i.e., MIII = Al, Sc, Cr, Fe) and the interaction of CO2 with the unsaturated coordination metal sites were investigated by means of large-scale quantum mechanical calculations with hybrid Hartree–Fock/density functional theory methods augmented with an atom–atom pairwise dispersion correction in a fully periodic approach. In doing this, we took advantage of the high scalability of the massively parallel version of the CRYSTAL code. Overall results are in good agreement with the experiment, in particular for the predicted structures, whereas some discrepancies have been found for adsorption energies in the case of MIL-100(Cr) and MIL-100(Sc). We argue that this is due to a deviation of the ideal model structure of MIL-100 adopted in this work from the actual synthesized material. This is also supported by additional calculations on a MOF with similar adsorption sites and computed data from the literature. Above all, we demonstrate that ab initio modeling of the structure and properties of MOFs can now be performed on very large and complex frameworks
Inelastic Neutron Scattering Investigation of MgCl2 Nanoparticle-Based Ziegler–Natta Catalysts for Olefin Polymerization
Full text linkThe effect of nanosize and structural disorder on the MgCl2 support of Ziegler-Natta catalysts has been investigated in terms of induced changes to their vibrational spectroscopic fingerprint. In particular, the inelastic neutron scattering (INS) technique was used, which allowed sampling of the whole lattice dynamics of the support. The experimental INS spectra of several ball-milled, polycrystalline, samples of MgCl2 were collected for the first time and were compared to simulated spectra from quantum-mechanical density functional theory calculations. Theoretical calculations were performed on a variety of MgCl2 structural models: (i) ordered and disordered bulk (3D); (ii) low-dimensional structures such as surfaces (2D) and nanorods (1D); and nanoclusters (0D). This allowed us to link specific features of the spectra to specific changes in the atomic structure and dynamics of the catalyst support. In particular, the effect of translational symmetry breaking and rotational disorder is discussed. Furthermore, the present data suggest that the ball-milling process mostly leads to the formation of bulk-like crystallites rather than nanoparticles. This work ultimately highlights the combined use of INS measurements and quantum-mechanical simulations as an effective approach for the atomistic characterization of defective (nano)materials
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
Application of metal-organic frameworks and covalent organic frameworks as (photo)active material in hybrid photovoltaic technologies
No full textMetal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are two innovative classes of porous coordination polymers. MOFs are three-dimensional materials made up of secondary building blocks comprised of metal ions/clusters and organic ligands whereas COFs are 2D or 3D highly porous organic solids made up by light elements (i.e., H, B, C, N, O). Both MOFs and COFs, being highly conjugated scaffolds, are very promising as photoactive materials for applications in photocatalysis and artificial photosynthesis because of their tunable electronic properties, high surface area, remarkable light and thermal stability, easy and relative low-cost synthesis, and structural versatility. These properties make them perfectly suitable for photovoltaic application: throughout this review, we summarize recent advances in the employment of both MOFs and COFs in emerging photovoltaics, namely dye-sensitized solar cells (DSSCs) organic photovoltaic (OPV) and perovskite solar cells (PSCs). MOFs are successfully implemented in DSSCs as photoanodic material or solid-state sensitizers and in PSCs mainly as hole or electron transporting materials. An innovative paradigm, in which the porous conductive polymer acts as standing-alone sensitized photoanode, is exploited too. Conversely, COFs are mostly implemented as photoactive material or as hole transporting material in PSCs
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