1,720,961 research outputs found
DFT investigation of structural and vibrational properties of type B and mixed A-B carbonated hydroxylapatite
No full textIn nature, hydroxylapatite [Ca-10(PO4)(6)(OH)(2)] is mostly present with various stoichiometric defects. The most abundant is the carbonate ion that can occupy different crystallographic sites (namely A and B types), however, its effects on the apatite structure is still an object of debate. Type A carbonated apatite was quantum mechanically simulated in a previous study, here we extend the simulation to bulk structural and vibrational features of Na-bearing type B and mixed type A-B carbonated hydroxylapatite [Ca10-xNax(PO4)(6-x)(CO3)(x+y)(OH)(2(1-y)), space group P1]. The simulation has been performed by ab initio density functional methods. The geometry of the models (lattice parameters and internal coordinates) have been fully optimized exploring different positions of the sodium ion in the apatite unit cell. The results, in agreement with XRD data, suggest that in each crystallographic cell in the biological mineral there is at least one calcium ion substitution or vacancy per cell. The carbonate ion presence in the apatite structure is in good agreement with biological/chemical data. Furthermore, there is also a very good agreement with FTIR data reported in literature.In nature, hydroxylapatite [Ca10(PO4) 6(OH)2] is mostly present with various stoichiometric defects. The most abundant is the carbonate ion that can occupy different crystallographic sites (namely A and B types), however, its effects on the apatite structure is still an object of debate. Type A carbonated apatite was quantum mechanically simulated in a previous study, here we extend the simulation to bulk structural and vibrational features of Na-bearing type B and mixed type A-B carbonated hydroxylapatite [Ca10-xNa x(PO4)6-x(CO3)x+y(OH) 2(1-y), space group P1]. The simulation has been performed by ab initio density functional methods. The geometry of the models (lattice parameters and internal coordinates) have been fully optimized exploring different positions of the sodium ion in the apatite unit cell. The results, in agreement with XRD data, suggest that in each crystallographic cell in the biological mineral there is at least one calcium ion substitution or vacancy per cell. The carbonate ion presence in the apatite structure is in good agreement with biological/chemical data. Furthermore, there is also a very good agreement with FTIR data reported in literature. © 1997 - 2014 Mineralogical Society of America. All rights reserved
Periodic ab initio bulk investigation of hydroxylapatite and type A carbonated apatite with both pseudopotential and all-electron basis sets for calcium atoms
Full text linkApatite minerals draw the attention of many researchers not only in mineralogy, but also in biology, biochemistry, and medicine because hydroxylapatite [Ca-10(PO4)(6)(OH)(2)] is the main component of the mineral phase of mammalian bones. However, in nature this mineral is mostly present with various stoichiometric defects. The carbonate ion is found commonly in its structure where it can occupy different crystallographic sites; however, its configurational energy and relative orientation in the apatite lattice is still debated. In this work, bulk structural features of hexagonal hydroxylapatite (space group P6(3)) and type A carbonated apatite [Ca-10(PO4)(6)(CO3), space group P1] have been modeled by density function method using the hybrid B3LYP functional and an all-electron polarized double-zeta quality Gaussian-type basis set using the CRYSTAL09 computer program. The effect on the structural parameters due to the adoption of the present all-electron basis set for the Ca ion compared to the pseuodpotential adopted in previous work has also been discussed. Different orientations of the carbonate ion in the apatite unit cell have been considered. The B3LYP functional and Gaussian-type basis set with polarization have been adopted. The geometry of the model (lattice parameters and internal coordinates) has been fully optimized and resulted in very good agreement with XRD data reported in literature that suggest a "close" configuration (type A1) of the carbonate ion, i.e., with a C-O bond perpendicular to the c-axis of the apatite cell
The vibrational features of hydroxylapatite and type A carbonated apatite: A first principle contribution
Full text linkIn this work, the vibrational spectra of hexagonal hydroxylapatite OHAp (space group P63) and type A carbonated apatite [Ca10(PO4)6(CO3), space group P1] have been calculated with an ab initio approach by the density function method using the hybrid B3LYP functional and an all-electron polarized double-? quality Gaussian-type basis set using the CRYSTAL09 computer program. The effect on the vibrational properties due to improving the Ca pseudopotential, usually adopted in previous studies on hydroxylapatite, toward the present all-electron basis set has also been briefly addressed. The anharmonic correction for hydroxyl groups in OHAp has also been considered. The results of the modeling are in good agreement with the available FTIR and Raman data presented in the literature and can be useful to experimental researchers to assign unequivocally the bands in infrared and Raman spectra to specific fundamental vibrational modes
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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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