1,720,957 research outputs found
Raman and XANES spectroscopy of permanently densified vitreous silica
No full textVitreous SiO2 was permanently densified at pressures ranging up to 8 GPa at 700 °C, where its density reached a value of 2.58 g/cm3. The series of glasses formed were analyzed by Raman spectroscopy and X-ray absorption near edge structure (XANES)
spectroscopy at both the oxygen and silicon K-edges. Changes in the Raman spectra are most evident in the Si–O–Si bending and
rocking region near 500 cm-1, as the broad band becomes narrower and shifts to higher frequency, indicating a narrower distribution
of Si–O–Si bond angles in the compacted glasses. A pronounced shift to lower frequency of the weak high-frequency bands
associated with Si–O stretching motions is also observed, indicating a lengthening of Si–O bonds with increasing densification. More
subtle changes are observed in both the Si and O K-edge spectra, with the O K-edge appearing more sensitive to short-range structural modifications such as narrowing of the intertetrahedral angle. In contrast to the structure of permanently densified v-SiO2
formed at room temperature, in which medium-range order is mostly affected, densification at higher temperatures is accomplished
at shorter range, although at the pressures of this study, the glass remains principally a fully polymerized tetrahedral framework
glass
Electrical conductivity of a phonotephrite from Mt Vesuvius: The importance of chemical composition on the electrical conductivity of silicate melts
No full textThe bulk electrical conductivity of the phonotephritic lava from the 1944 eruption of Mt Vesuvius was measured using complex impedance spectroscopy in a multianvil apparatus at 1 GPa and temperatures up to
700 °C. Melting experiments prior to the electrical measurements were also performed on this sample in a
piston cylinder apparatus in order to gauge how bulk conductivity varies as a function of its melt fraction.
Unlike the behaviour found in basaltic rocks in which conductivity increases with increasing melt fraction,
we observe a conductivity decrease of the order of a factor of ten for samples at 700 °C ranging in melt
fraction from 32 vol.% to completely molten.We attribute this anomalous behaviour to the progressive loss of highly conductive leucite upon melting. The addition of potassium to the melt phase, however, does not
result in an increase of the total alkali concentration due to the melting of other mineral components. We
also present an empirical model to predict the electrical conductivity of fully molten silicate liquids as a function of temperature and chemical composition, based on conductivity data for natural silicate liquids
found in the literature. The inclusion of compositional terms reduces the error by more than a factor of four
with respect to a composition independent, temperature-only parameterization
The effect of Fe-Mg substitution on the electrical conductivity of pyrope (Mg3Al2Si3O8)-almandine(Fe3Al2Si3O8) garnets
No full textThe combined effects of water and fluorine on the viscosity of silicic magmas
No full textThe Newtonian viscosity of water-plus-fluorine-bearing silicate melt of haplogranitic composition
(HPG8) has been determined. Viscosities of HPG8 melt with addition of 3.11 and 4.25 wt.% of F and up to
3 wt.% H2O have been obtained using a micropenetration technique in the interval 109.74 to 1011.84 Pa s and
temperatures varying from 370 to 700°C, at ambient pressure. Determination of the temperature dependence
of viscosity from this and previous studies permits the parameterization of the viscosity of melts containing
water and fluorine, having similar composition, within a 0.3 log units standard error. The viscosity of water-bearing, F-rich haplogranitic samples is represented by a modified Vogel-Fulcher-Tammann (VFT)
equation which provides a non-Arrhenian description of the temperature dependence of the viscosity. The results of this study indicate that, taken individually or together, both H2O and F- have a strong and similar
effect on the viscosity of SiO2-rich compositions. This similarity between F2O-1 and H2O greatly simplifies
the task of predicting viscosity for volatile-rich, highly silicic magmas. The low viscosities of hydrous
fluorine-bearing granitic melts favour efficient crystallization-fractionaction paths for these liquids, controlling
degassing paths and consequently the eruptive behaviour. Numerical simulations of eruptive events normally do not take into account the contribution of fluorine; this may introduce a significant error in the description of the fluid-dynamic properties of magma and, therefore, in the accurate prediction of eruptive scenarios, as
well as in hazard assessment studies. Fluorine, unlike water, remains dissolved in the melt at high concentrations
and low confining pressures. The incorporation of fluorine data and the modelling of fluorine-bearing
viscosity data are therefore of fundamental importance for simulations of magma dynamics and prediction of eruptive scenarios
The rheology of peralkaline rhyolites from Pantelleria Island
No full textThe viscosity of pantelleritic melts from the Khaggiar lava flow (5.5 ka — Pantelleria Island) was investigated as a function of temperature and water content. High (1673–1323 K) and low (973–613 K) temperature dry and hydrous liquid viscosities were determined by a combination of concentric cylinder (102.44 to 104.56 Pa s)
and micropenetration (108.67 to 1011.37) viscometric techniques. The effect of water, from 0.02 to 3.55 wt.%
H2O, was explored in the temperature range of 973–613 K. Liquid viscosities have been parameterized by
means of a modified Vogel–Fulcher–Tammann equation (VFT) which describes viscosities and derivative
properties (glass transition temperature Tg, fragility m) of silicic liquids as a function of T–X (H2O). The results yield the expected strong decrease of viscosity with temperature and water content. Fragility, as a mea-
sure of the deviation from Arrhenian behavior, increases with H2O content from m=23 to m=28. The peralkalinity (molar alkali excess over alumina), which characterizes the pantelleritic magmas, exerts a primary control over the rheological behavior of these melts. The excess of alkalies over alumina content is responsible for the peculiarly low viscosities of pantelleritic liquids compared to their metaluminous counterpart in the high temperature range, and leads to an even more dramatic decrease in the viscosities of these rhyolites in the low temperature range.
Comparison with available models shows substantial differences between the measured and calculated vis-
cosities. Based on our new viscosity results combined with existing literature data, we propose a new param-
eterization based on a modified Vogel–Fulcher–Tammann equation, accounting for the effect of water and
composition, which can be used to determine the viscosity of pantelleritic melts. The derived relationship reproduces the experimental data (58 in total) in the viscosity range from 102.44 to 1011.37 Pa s and in the temperature range from 613 to 1673 K within a root-mean-square-error (RMSE) of 0.12 log units. Application of this calculation model for high temperature low viscosity hydrous melt is limited by the lack of experimental
data and needs verification by additional measurements. Volcanological implications for welding and rheomorphic processes as well as conduit dynamic modeling are also discussed in light of the results presented in this workThe viscosity of pantelleritic melts from the Khaggiar lava flow (5.5 ka — Pantelleria Island) was investigated as a function of temperature and water content. High (1673–1323 K) and low (973–613 K) temperature dry and hydrous liquid viscosities were determined by a combination of concentric cylinder (102.44 to 104.56 Pa s) and micropenetration (108.67 to 1011.37) viscometric techniques. The effect of water, from 0.02 to 3.55 wt.% H2O, was explored in the temperature range of 973–613 K. Liquid viscosities have been parameterized by means of a modified Vogel–Fulcher–Tammann equation (VFT) which describes viscosities and derivative properties (glass transition temperature Tg, fragility m) of silicic liquids as a function of T–X (H2O). The results yield the expected strong decrease of viscosity with temperature and water content. Fragility, as a measure of the deviation from Arrhenian behavior, increases with H2O content from m = 23 to m = 28. The peralkalinity (molar alkali excess over alumina), which characterizes the pantelleritic magmas, exerts a primary control over the rheological behavior of these melts. The excess of alkalies over alumina content is responsible for the peculiarly low viscosities of pantelleritic liquids compared to their metaluminous counterpart in the high temperature range, and leads to an even more dramatic decrease in the viscosities of these rhyolites in the low temperature range.
Comparison with available models shows substantial differences between the measured and calculated viscosities. Based on our new viscosity results combined with existing literature data, we propose a new parameterization based on a modified Vogel–Fulcher–Tammann equation, accounting for the effect of water and composition, which can be used to determine the viscosity of pantelleritic melts. The derived relationship reproduces the experimental data (58 in total) in the viscosity range from 102.44 to 1011.37 Pa s and in the temperature range from 613 to 1673 K within a root-mean-square-error (RMSE) of 0.12 log units. Application of this calculation model for high temperature low viscosity hydrous melt is limited by the lack of experimental data and needs verification by additional measurements. Volcanological implications for welding and rheomorphic processes as well as conduit dynamic modeling are also discussed in light of the results presented in this work
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
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