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Astronomical silicate nanoparticle analogues produced by pulsed laser ablation on olivine single crystals
Full text linkSilicate nanoparticles, otherwise referred to as very small grains (VSGs) [1], occur in various
astrophysical environments. These grains experience substantial processing (e.g., amorphization)
during their lifetime in the diffuse interstellar medium due to events such as grain-grain collisions
and irradiation [2]. Moreover, several studies have pointed out that the main building blocks of
these silicates are O, Si, Fe, Mg, Al and Ca, all elements that are among the principal constituents of
the Earth’s surface [3], thus leading to the name “astronomical silicates”. However, the structure
and chemical evolution together with the origin of these grains are still poorly understood and
intensively debated [4,5].
The aim of this study is the simulation of space weathering processes on olivine single
crystals by liquid phase pulsed laser ablation (LP-PLA). The study of the resulting structure of both
the target and the ablated material together with their chemical evolution has been carried out by a
multiple technique characterization. In particular, spectroscopy and dynamic light scattering
measurements, analyses of the electrostatic properties and reactivity to acids and bases on the
obtained colloidal solutions of the ablated nanoproducts have been performed and coupled with highresolution transmission electron microscopy (HR-TEM).
Selected olivine target crystals (Fo87) from the São Miguel island (Azores) were analyzed
by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX). LP-PLA
experiments were performed with a Nd:YAG laser focused via a singlet lens onto the surface of the
target, which was fixed at the bottom of a polystyrene box filled with 4 ml of deionized water (type
1) to immerge it completely. Laser pulses of 5 ns and 100 mJ simulate the timeframe and energy
exchange occurring during grain-grain interstellar collisions [6] and they generate a plasma plume
at the crystal/liquid interface. The rapid cooling induced by the confining liquid layer brings about
the condensation of the chemical vapor it contains with production of a colloidal solution of
nanoparticles. These solutions were analyzed by dynamic light scattering techniques and optical
absorption spectroscopy in the range from 200 nm to 1100 nm (6.20 eV - 1.13 eV). Absorption
measurements on the colloidal solutions have been compared against reference colloidal solutions
dispersed in deionized water (i.e. mesoporous silica [SiO2] nanoparticles, brucite [Mg(OH)2]
nanoparticles, aluminum hydroxide [Al(OH)3] nanoparticles, chrysotile [Mg3Si2O5(OH)4] nanotubes,
and synthetic forsterite [Mg2SiO4] nanoparticles). Moreover, additional absorption analyses have
been carried out as a function of the addition of known aliquots of sulfuric acid and sodium
hydroxide solutions. TEM/EDS analyses were then performed on the ablated nanoparticles deposited
via electrophoresis on C-coated Cu grids and compositional variations of the ablated target were
determined by X-ray photo-emission spectroscopy analyses.
The size distribution of LP-PLA synthesized nanoparticles is typically multimodal due to
aggregation phenomena. Aggregation is consistent with the measured ζ-potential, which is negative
with a relatively low absolute value, within the range 30-50 mV. Nonetheless, a recurrent mode is
centered at about 2 nm (hydrodynamic diameter) and it is consistent with the measured size
distribution obtained by transmission electron microscopy analysis (average nanoparticles diameter
around 3-5 nm). Optical absorption measurements on the ejected material show a main band
around 215 nm. This feature is very similar to the “B2 band” reported in several studies on silica
glass [7] and ascribed to oxygen vacancies, but its nature is still far to be fully understood. We also
found that this feature at 215 nm is very common among both Si and Mg compounds (e.g., Sioxide, Mg-hydroxide, chrysotile). Moreover, additional absorption bands in the range 240-350nm are
observed suggesting the formation of new space weathering products as result of the ablation
process.
Therefore, these results suggest that substantial chemical processing might be expected
during space weathering of “typical” interstellar grains into VSGs. Moreover, coupling these
experimental results with remote sensing datasets will provide fundamental information about the
origin and evolution of these silicate grains
Laboratory Simulation of Space Weathering on Silicate Surfaces in the Water Environment
Full text linkSilicate nanoparticles occur in various astrophysical environments where they experience
substantial processing due to events such as grain-grain collisions and irradiation. However, the
structure and chemical evolution together with the origin of these grains are still poorly understood
and intensively debated. For this purpose, we performed liquid-phase nanosecond pulsed laser ablation on olivine single crystals to (i) simulate space weathering in a water environment (e.g.,
hydrous or volatile-rich bodies) and (ii) study the chemical and structural evolution of both the
target surface and the ablated material. In particular, optical spectroscopy analyses have been
performed on the ablated material and correlated with high-resolution transmission electron
microscopy and diffraction; whereas, compositional variations of the ablated target surface were
determined by X-ray photoelectron spectroscopy. Our results show that the target material is
enriched in Fe and depleted in Mg after the ablation process, with the water environment triggering
the oxidation of Fe2+ into Fe3+ in a region confined at the solid-liquid interface and thus promoting
the formation of magnetite on the sample surface. On the other hand, in the ablated material we
find olivine crystalline fragments with shock features together with Mg-rich crystalline
nanoparticles. Notably, no metallic iron nanoparticles have been detected in the ablated material.
Our simulation of space weathering in water environment revealed structural and chemical
changes which are expected to give rise to distinctive features in the reflectance spectra when
compared to those from airless bodies of the inner Solar System
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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