Tektites and microtektites iron oxidation state and water content

Fe redox and water content of impact melt are important parameters as they can greatly affect melt density and viscosity which, in turn, are important parameters greatly affecting the melt evolution and fate. In this manuscript I briefly describe recent research on X-ray absorption spectroscopy (XAS) determination of Fe oxidation state and micro Fourier transform infrared spectroscopy (FTIR) determination of water content putting them in the context of previous research done with different techniques. In comparison with other techniques requiring large amount of samples (e.g. potassium dichromate titration, Mossbauer spectroscopy, Karl Fischer titration), XAFS and micro FTIR techniques allow to study both macroscopic samples of tektites as well as smaller microtektites giving the same error, making it possible to compare results between tektites and micro-tektites and, consequently, also to find differences between tektites and microtektites or between microtektites from different strewn fields. A brief introduction on impact cratering and melt formation during impact events is aimed to introduce the subject to non specialised readers

Economic Effects of Covid-19 and Non-Pharmaceutical Interventions: applying a SEIRD-RBC Model to Italy

We study the economic effects generated by the proliferation of the Covid-19 epidemic and the implementation of non-pharmaceutical interventions by developing a SEIRD-RBC model, where the outbreak and policy interventions shape the labor input dynamic. We microfoundan Epidemic-Macro model grounded on the RBC tradition, useful for epidemic and economic analysis at business cycle frequency, which is able to reproduce the highly debated health-output trade-off. Assuming a positive approach, we show the potential of our model by matching the epidemic and macroeconomic empirical evidence of the Italian case

Europium structural environment in a sodium disilicate glass by XAS

The structural environment of europium (Eu) in a synthetic silicate glass of Na2Si2O5 composition has been determined with the final purpose to carry out a detailed investigation of the role of europium in glasses by building a structural model as a function of glass composition. The sample analyzed in this study has been characterized through Eu L3-edge X-ray Absorption Spectroscopy (XAS), which allowed to obtain Eu oxidation state, coordination number and the bEu–ON bond distances in the glass. From EXAFS data analysis Eu results to be bonded to six oxygens (bEu–ON=2.26±0.01 Å) at the corners of an almost regular octahedron

Thermophysical parameters from laboratory measurements and tests in borehole heat exchangers

Besides the type of thermal regime, the performance of borehole heat exchangers relies on the overall thermal resistance of the borehole. This parameter strongly depends on the underground thermal conductivity, which accounts for most of the heat that can be extracted. The geometric configuration and the increase of thermal conductivity of the grout filling back the bore can yield a non-negligible enhancement in thermal performances. In this paper, we present a study on a pilot geothermal plant consisting of two borehole heat exchangers, 95 m deep and 9 m apart. Laboratory and in situ tests were carried out with the aim of investigating underground thermal properties, mechanisms of heat transfer and thermal characteristics of the filling grouts. Samples of grouting materials were analysed in the lab for assessing the thermal conductivity. An attempt to improve the thermal conductivity was made by doping grouts with alumina. Results showed that alumina large concentrations can increase the thermal conductivity by 25-30%

Exploring the Low Voltage Behavior of V2O5Aerogel as Intercalation Host for Sodium Ion Battery

Boosted by costs benefits, the development of room temperature Na-ion batteries is strongly desired for stationary applications. In this study we explore the possible use of V2O5 aerogel as anode material for sodium ion batteries. The aerogel is able to reversibly insert more than 3 Eq. of sodium in the voltage range 0.1 V–4 V vs. Na/Na+ demonstrating to possess additional capacity when cycled to lower voltage. The anode delivers about 200 mAh g−1 in the voltage range 0.01 V–1.5 V vs. Na/Na+. The preliminary characterization of a full Na-ion cell made coupling the V2O5 aerogel anode with carbon-coated Na3V2(PO4)3 cathode is also reported. The cell, showing an average voltage of 2.5 V, performed 200 cycles with good efficiency and a maximum specific capacity of 113 mAh per gram of anode material

Effect of Applying a Carbon Coating on the Crystal Structure and De-/Lithiation Mechanism of Mn-Doped ZnO Lithium-Ion Anodes

The introduction of transition metal dopants such as Fe and Co in zinc oxide enables substantially enhanced reversible capacities and greater reversibility of the de-/lithiation reactions occurring. Herein, we report a comprehensive analysis of the electrochemical processes taking place in Mn-doped ZnO (Zn$_{0.9}$Mn$_{0.1}$O) and carbon-coated Zn$_{0.9}$Mn$_{0.1}$O upon de-/lithiation. The results shed light on the impact of the dopant chemistry and, especially, its coordination in the crystal structure. When manganese does not replace zinc in the wurtzite structure, only a moderate improvement in electrochemical performance is observed. However, when applying the carbonaceous coating, a partial reduction of manganese and its reallocation in the crystal structure occur, leading to a substantial improvement in the material\u27s specific capacity. These results provide important insights into the impact of the lattice position of transition metal dopants—a field that has received very little, essentially no attention, so far

Spherulites growth in trachytic melts: a textural quantitative study from synchrotron X-ray microtomography and SEM data

This study shows the first textural data on synthetic alkali-feldspar spherulites grown in trachytic melts during cooling and decompression experiments with water-saturated conditions. Previous textural studies have shown the shape evolution and the growth process of spherulites as a function of undercooling (deltaT) and water content, although just in basaltic and rhyolitic melts [1-3]. Spherulites are spherical clusters of polycrystalline aggregates that occur commonly in rhyolitic melts under highly non-equilibrium conditions [3-4]. Cooling and decompression experiments have been carried out on trachytic melts in order to investigate crys- tallization kinetics of alkali feldspars and the implications for magma dynamics during the ascent towards the surface. Experiments have been conducted using cold seal pressure vessel apparatus at pressure range of 30 - 200 MPa, temperature of 750 - 850 degrees C and time of 2 - 16 hours, thereby reproducing pre- and syn-eruptive conditions of the Campi Flegrei volcanoes. This study presents quantitative data on spherulite morphologies obtained both by scanning electron microscopy (SEM) and synchrotron X-ray microtomography. Size, aspect ratio, number and crystallographic misorientation of alkali feldspar crystals will be measured. Furthermore, experiments performed at different durations could allow us to follow the growth and the evolution of spherulites. The shape of spherulites changes as a function of delta T and experimental durations. Two kind of spherulites occured during experiments: open spherulites and close spherulites. The open spherulites are characterized by an structure with large (generally rectangular prismatic), widely spaced fibers with main axis converging towards a central nucleus, in agreement with previous observations [5-6]. Instead, the close spherulites consist of acicular and tiny fibers radially aggregated around a nucleus and single crystals are hardly distinguishable. First preliminary results show: a) spherulites grow between 70-200 MPa, thus the nucleation process was favored at higher water contents; b) open spherulites seem to be favored at low deltaT, whereas close spherulites were favored in experiments at higher delta T and long durations; c) estimated growth rates of spherulites were of 10-7 cm/s

Structural and Electrochemical Characterization of Zn1−x_{1-x}Fex_{x}O : Effect of Aliovalent Doping on the Li⁺ Storage Mechanism

In order to further improve the energy and power density of state-of-the-art lithium-ion batteries (LIBs), new cell chemistries and, therefore, new active materials with alternative storage mechanisms are needed. Herein, we report on the structural and electrochemical characterization of Fe-doped ZnO samples with varying dopant concentrations, potentially serving as anode for LIBs (Rechargeable lithium-ion batteries). The wurtzite structure of the Zn1−xFexO samples (with x ranging from 0 to 0.12) has been refined via the Rietveld method. Cell parameters change only slightly with the Fe content, whereas the crystallinity is strongly affected, presumably due to the presence of defects induced by the Fe3+ substitution for Zn2+. XANES (X-ray absorption near edge structure) data recorded ex situ for Zn0.9Fe0.1O electrodes at different states of charge indicated that Fe, dominantly trivalent in the pristine anode, partially reduces to Fe2+ upon discharge. This finding was supported by a detailed galvanostatic and potentiodynamic investigation of Zn1−xFexO-based electrodes, confirming such an initial reduction of Fe3+ to Fe2+ at potentials higher than 1.2 V (vs. Li+/Li) upon the initial lithiation, i.e., discharge. Both structural and electrochemical data strongly suggest the presence of cationic vacancies at the tetrahedral sites, induced by the presence of Fe3+ (i.e., one cationic vacancy for every two Fe3+ present in the sample), allowing for the initial Li+ insertion into the ZnO lattice prior to the subsequent conversion and alloying reaction

Effect of alkalis on the Fe oxidation state and local environment in peralkaline rhyolitic glasses

International audienceIron oxidation state and coordination geometry have been determined by Fe K-edge X-ray absorption near edge spectroscopy (XANES) for three sets of silicate glasses of peralkaline rhyolitic composition with different peralkalinity values. These compositions were chosen to investigate the effect of alkali content (and oxygen fugacity) on the Fe oxidation state. The samples were produced by means of hydrothermal vessels at 800 °C with oxygen fugacity conditions ranging from NNO-1.61 to NNO+2.96 log units. Comparison of the pre-edge peak data with those of Fe model compounds of known oxidation state and coordination number allowed determination of the Fe oxidation state and coordination number in all glasses analyzed. Within each group of samples, Fe tends to oxidize with increasing oxygen fugacity as expected. However, alkali content is shown to have a strong effect on the Fe3+/(Fe3++Fe2+) ratio at constant oxygen fugacity: this ratio varies from 0.25 to 0.55 (±0.05) for the least peralkaline series, and from 0.45 to 0.80 (±0.05) for the most peralkaline series. Moreover, pre-edge peak data clearly indicate that Fe3+ is in fourfold coordination in the most peralkaline glasses. Extrapolation of pre-edge peak data suggests the presence of both fourfold and fivefold coordination for trivalent Fe in the other two series. Divalent Fe is suggested to be mainly in fivefold coordination in all the three glass series. The presence of minor amounts of sixfold- and fourfold-coordinated Fe cannot be ruled out by XANES data alone. XANES data suggest that the amount of alkalis also affects the Fe3+ coordination environment resulting in a decrease in the average coordination numbers. Extended X-ray absorption fine structure (EXAFS) data of the most oxidized and peralkaline sample indicate that Fe3+ is in tetrahedral coordination with = 1.85 Å (±0.02). This value compares well with literature data for [4]Fe3+ in crystalline phases (e.g., in tetra-ferriphlogopite or rodolicoite) or in silicate glasses (e.g., phonolite glasses) supporting the XANES-determined coordination number obtained for the most peralkaline glasses. Calculated NBO/T ratios decrease slightly with Fe oxidation because of the higher fraction of network forming Fe, thus increasing the polymerization of the tetrahedral network

Vanadium K-edge XANES in vanadium-bearing model compounds: a full multiple scattering study

A systematic study is presented on a set of vanadium-bearing model compounds, representative of the most common V coordination geometries and oxidation states, analysed by means of vanadium K-edge X-ray absorption near-edge spectroscopy calculations in the full multiple scattering (FMS) framework. Analysis and calibration of the free parameters of the theory under the muffin-tin approximation (muffin-tin overlap and interstitial potential) have been carried out by fitting the experimental spectra using the MXAN program. The analysis shows a correlation of the fit parameters with the V coordination geometry and oxidation state. By making use of this correlation it is possible to approach the study of unknown V-bearing compounds with useful preliminary information