1,720,996 research outputs found
Hydrogen trapping by defects in semiconductors studied by anelastic spectroscopy
No full textWe review some results obtained by anelastic spectroscopy on H-related defects in III-V semiconductors. Anelastic measurements on InP lead to the formulation of a model explaining the conversion to the semi-insulating (SI) state. Moreover, in GaAs:Zn an extraordinarily fast relaxation rate has been measured and a possible explanation has been suggested. This results are reviewed and discussed in the light of new experiments on InP:Zn, whose spectrum shows a relaxation process similar to the one in GaAs:Zn. (C) 2003 Elsevier B.V. All rights reserved
Binary mixtures of choline acetate and Tetrabutylammonium acetate with natural organic acids by vibrational spectroscopy and molecular dynamics simulations
No full textWe present a study of several mixtures obtained by the mixing of two organic acetate-based salts (choline acetate, ChAc, or tetrabutylammonium acetate, TBAAc) with three different natural organic acids (ascorbic acid, AA, citric acid, CA, and maleic acid, MA). The structures of the starting materials and of the mixtures were characterized by infrared spectroscopy (FT-IR) and classic molecular dynamics simulations (MD). The thermal behavior was characterized by differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA). The obtained mixtures, especially the ChAc-based ones, strongly tend to vitrify at low temperatures and are stable up to 100-150 °C. The FTIR measurements suggest the formation of a strong H-bond network: the coordination between acids and ChAc or TBAAc takes place by the donation of the H-bond by the acids to the oxygen of the acetate anion, which acts as an acceptor (HBA). The comparison with MD analysis indicates that acids predominantly exploit their more acidic hydrogens. In particular, we observe the progressive shift of νC═O and νOH when the ratios of acids increase. The structural differences between the two studied cations influence the spatial distribution of the components in the mixture bulk phases. In particular, the analysis of the theoretical structure function I(q) of the TBAAc-based systems shows the presence of important prepeaks at low q, a sign of the formation of apolar domain, due to the nanosegregation of the alkyl chains
Thermodynamic Analysis of the Hydrolysis of Borate-Based Lithium Salts by Density Functional Theory
Full text linkHere we discuss the thermodynamics of the hydrolysis of three borate-based lithium salts commonly used in aprotic electrolytes for
lithium-ion batteries: lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalate)borate (LiODBF) and lithium bis(oxalate)borate
(LiBOB). We performed density functional theory calculations at the ωB97M-V/6–31++G** level to compute the thermodynamic
stability of reagents, intermediate and products in solution phase. The variations of Gibbs free energy in heterogeneous reactions
were evaluated by coupling DFT values with thermochemical cycles. LiBF4 and LiODBF can be easily hydrolyzed by the direct
reaction with water at room temperature: the salts degradation is driven by the precipitation of LiF(s) and by the release of HF. On
the contrary, LiBOB is much more stable and only weakly reacts with water: it is therefore more resilient to hydrolyzation and therefore suitable for application in Li-ion battery electrolytes
A study of the conformers of the N,N-diethyl-N-methyl-N-propylammonium ion by means of infrared spectroscopy and DFT calculations
No full textThe infrared absorption spectrum of the N,N-diethyl-N-methyl-N-propylammonium (DMPA) bis
(fluorosulfonyl)imide (FSI) ionic liquid is measured as a function of temperature between 165 and
307 K. In the frequency range between 900 and 1070 cm1 only the cation gives rise to infrared bands. A
conformational analysis of DMPA is performed by means of DFT calculations at the B3LYP/6-31G** level.
The comparison of the experimental and calculated spectra provides evidence that the lowest energy
conformers coexist in the liquid. Experimentally, we
find that the energy difference between the most
stable rotamer and the next energy conformers is only 1.56 0.05 kJ/mol, in good agreement with the
calculated value. Finally, we show that in the solid state only the most stable isomer is retained
Intermolecular Interactions in Mixed Choline Acetate and Maleic Acid Systems
Full text linkMixed systems based on ionic liquids are promising innovative solvents due to their properties, which are strictly connected to the interactions that arise among the components. The present work investigates the intermolecular interactions of a mixed choline acetate and maleic acid system and their modifications with increasing acid content. MM/DFT calculations provided indications about the possible geometric configurations of the systems while Non-Covalent Interaction analysis was useful to describe and distinguish secondary interactions. The number of available configurations decreases at high acid concentration. Moreover, intramolecular hydrogen bonding was observed in all configurations except that in the most stable one of the lowest acid content mixture. Concomitantly, far-infrared spectroscopy was used to investigate intermolecular interactions and provided support to the computational results
Promising Isotope Effect in Pd77Ag23 for Hydrogen Separation
Full text linkPd–Ag alloys are largely used as hydrogen separation membranes and, as a consequence, the
Pd–Ag–H system has been intensively studied. On the contrary, fewer information is available for the
Pd–Ag–D system; thus, the aim of this work is to improve the knowledge of the isotope effect on the
commercial Pd77Ag23 alloy, especially for temperature above 200 C. In particular, deuteriumabsorption
measurements are carried out in the Pd77Ag23 alloy in the temperature range between 79 and 400 C
and in the pressure range between 102 and 16 bar. In this exploited pressure (p) and composition (c)
range, above 300 C the pc isotherms display the typical shape of materials where only a solid solution
of deuterium is present while at lower temperatures these curves seem to be better described by the
coexistence of a solid solution and a deuteride in a large composition range. The obtained results are
compared and discussed with the ones previously measured with the lightest hydrogen isotope. Such a
comparison shows that the Pd77Ag23 alloy exhibits a clear inverse isotope effect, as the equilibrium
pressure of the Pd–Ag–D system is higher than in Pd–Ag–H by a factor of 2 and the solubility of
deuterium is about one half of that of hydrogen. In addition, the absorption measurements were used to
assess the deuteration enthalpy that below 300 °C is DHdeut = 31.9 +/- 0.3 kJ/mol, while for temperatures
higher than 300 °C, DHdeut increases to 43 +/- 1 kJ/mol. Additionally, in this case a comparison with
the lighter isotope is given and both deuteration enthalpy values result lower than those reported for
hydrogenation. The results described in this paper are of practical interest for applications operating
above 200 °C, such as membranes or packing column, in which Pd77Ag23 has to interact with a gas
stream containing both hydrogen isotopes
A Study of the Conformers of the (Nonafluorobutanesulfonyl)imide Ion by Means of Infrared Spectroscopy and Density Functional Theory (DFT) Calculations
Full text linkPyrrolidinium-based ionic liquids with anions of the per(fluoroalkylsulfonyl)imide family
are particularly interesting for their use as electrolytes in lithium batteries. These ions have several
geometric isomers and the presence of different ion conformers and their distribution affects the ILs
(Ionic liquids) physical and chemical properties. In the present work, we report the temperature
dependence of the infrared spectra of the N-butyl-N-methyl-pyrrolidinium(trifluoromethanesulfonyl)
(nonafluorobutanesulfonyl)imide (PYR14-IM14) ionic liquid; DFT (Density Functional Theory)
calculations performed with different models provides indications about the IM14 conformers and
their vibrational spectra. Moreover the temperature dependence of the intensity of the lines identified
as markers of different conformers provide indications about the conformers’ distribution and the
difference of their enthalpy in the liquid phase
Hydrogen and Deuterium Solubility, Diffusivity and Permeability from Sorption Measurements in the Ni33Ti39Nb28 Alloy
Full text linkThe hydrogen/deuterium sorption properties of Ni33Ti39Nb28 synthesized by the vacuum induction melting technique were measured between 400 and 495 °C for pressure lower than 3 bar. The Sieverts law is valid up to H(D)/M < 0.2 in its ideal form; the absolute values of the hydrogenation/deuteration enthalpy are ΔH(H2) = 85 ± 5 kJ/mol and ΔH(D2) = 84 ± 4 kJ/mol. From the kinetics of absorption, the diffusion coefficient was derived, and an Arrhenius dependence from the temperature was obtained, with Ea,d = 12 ± 1 kJ/mol for both hydrogen isotopes. The values of the alloy permeability, obtained by combining the solubility and the diffusion coefficient, were of the order of 10−9 mol m−1 s−1 Pa−0.5, a value which is one order of magnitude lower than that of Ni41Ti42Nb17, until now the best Ni-Ti-Nb alloy for hydrogen purification. In view of the simplicity of the technique here proposed to calculate the permeability, this method could be used for the preliminary screening of new alloys
Relaxation Dynamics and Phase Transitions in Ionic Liquids: Viscoelastic Properties from the Liquid to the Solid State
No full textIn the present work we performed low-frequency mechanical
spectroscopy experiments to measure the mechanical modulus of two ionic liquids
and its variation during the main phase transitions occurring by varying the
temperature, in the both liquid and the solid states. The liquids share the same
anion, the bis(trifluoromethanesulfonyl)imide, and present different cations, 1-
butyl-1-methylpyrrolidinium and 1-allyl-3-H-imidazolium. A thermally activated
relaxation process is found in the liquid phase and is analyzed in terms of a
modified Debye model. The obtained parameters provide indications about the
nature and the mechanism giving rise to the peak, which is attributed to the ions
motion by means of hopping processes. Moreover, density functional calculations
were performed, and the comparison with the analysis of the experimental data
suggests that the anion conformers are likely to be involved in the different
configurations among which the ions can rearrange
An eutectic mixture in the tetrabutylammonium bromide‐octanol system. Macroscopic and microscopic points of view
No full textAn eutectic mixture of tetrabutylammonium bromide and octanol in the molar ratio 1-10 exhibited a melting point of -17°C. This system was investigated by means of infrared spectroscopy, in the liquid and in the solid state. Classical molecular dynamics was performed to study the fine details of the hydrogen bond interactions established in the mixture. Both octanol and the mixtures displayed
an almost featureless far-infrared spectrum in the liquid state but it becomes highly structured in the solid phase. DFT calculations suggest that new vibrational modes appearing in the mixture at low
temperatures may be related to the population of the higher energy conformers of the alcohol. Mid-infrared spectroscopy measurements evidenced no shift of the CH stretching bands in the mixture compared to the starting materials, while the OH stretching are blue shifted by a few cm-1. Consistently, molecular dynamics provides a picture of the mixture in which part of the hydrogen bonds (HB) of pure octanol is replaced by weaker HB formed with the Br anion. Due to these interactions the ionic couple becomes more separated. In agreement with this model, the lengths of all HB are much larger than those observed in mixtures containing acids reported in previous studies
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