Search results for "Amorphous solid"
showing 10 items of 622 documents
Understanding the Acidic Properties of the Amorphous Hydroxylated Silica Surface
2019
Amorphous silica is an intrinsic challenging system to study. In the last decades, some particular chemical properties have been discovered and described, but their description and understanding at the molecular level are experimentally difficult. Therefore, theoretical quantum chemical methods and descriptors, combined with experimental input, are a very appropriate set up to tackle this topic. In this study, the acidity of silanol groups of amorphous silica in hydrated conditions is investigated. Special attention has been drawn to the chemical shift, but also Bader charges, and vibrational frequencies with their intensities. The known bimodal acidity behavior was recovered and rationaliz…
Structure development in poly(ethylene terephthalate) quenched from the melt at high cooling rates: X-ray scattering and microhardness study
2000
The structure and microhardness of poly(ethylene terephthalate) (PET) cooled from the melt, using a wide range of cooling rates, was studied. PET thin films rapidly cooled from the melt (cooling rates larger than 5°C/s) show a continuous variation of structure and properties depending on cooling rate. Results highlight differences in the micro-mechanical properties of the glass suggesting the occurrence of amorphous structures with different degrees of internal chain ordering. The comparative X-ray scattering study of two glassy PET samples (7500 and 17°C/s) reveals the occurrence of frozen-in electron density states giving rise to an excess of scattering for the amorphous sample solidified…
First-principles study of the amorphous In3SbTe2phase change compound
2013
Ab initio molecular dynamics simulations based on density functional theory were performed to generate amorphous models of the phase change compound In${}_{3}$SbTe${}_{2}$ by quenching from the melt. In-Sb and In-Te are the most abundant bonds with only a minor fraction of Sb-Te bonds. The bonding geometry in the amorphous phase is, however, strongly dependent on the density in the range 6.448--5.75 g/cm${}^{3}$ that we investigated. While at high density the bonding geometry of In atoms is mostly octahedral-like as in the cubic crystalline phase of the ternary compound In${}_{3}$SbTe${}_{2}$, at low density we observed a sizable fraction of tetrahedral-like geometries similar to those pres…
Heat shrinkable behavior, physico-mechanical and structure properties of electron beam cross-linked blends of high-density polyethylene with acryloni…
2016
Abstract In this study, heat-shrinkable composites of electron beam irradiated high-density polyethylene (HDPE) composites with acrylonitrile-butadiene rubber (NBR) were investigated. HDPE/NBR blends at a ratio of components 100/0, 90/10, 80/20, 50/50 and 20/80 wt% were prepared using a two-roll mill. The compression molded films were irradiated high-energy (5 MeV) accelerated electrons up to irradiation absorbed doses of 100–300 kGy. The effect of electron beam induced cross-linking was evaluated by the changes of mechanical properties, gel content and by the differences of thermal properties, detected by differential scanning calorimetry. The thermo-shrinkage forces were determined as the…
Chalcogen adsorption and surface magnetism
2000
Abstract Investigations concerning the electronic and magnetic properties of oxygen and sulfur adsorbed on magnetized surfaces were carried out by means of angle and spin resolving photoelectron spectroscopy. Iron(110), a polycrystalline iron alloy, and an amorphous metallic glass (Fe 79 B 16 Si 5 ) served as ferromagnetic substrates. Exchange splittings of the O 2p and S 3p derived levels could be detected, demonstrating a magnetic coupling between the chemisorbate and iron. This observation presents a prerequisite for an induced magnetic moment within the adsorbate overlayer. For sulfur an adsorbate-induced structure only in the minority spin channel near the Fermi level was observed whic…
Variability of the Si-O-Si angle in amorphous-SiO2 probed by electron paramagnetic resonance and Raman spectroscopy
2009
We report an experimental investigation by electron paramagnetic resonance (EPR) and Raman spectroscopy on a variety of amorphous silicon dioxide materials. Our study by EPR have permitted us to point out that the splitting of the primary hyperfine doublet of the Eγ′ center shows a relevant sample-to-sample variability, changing from ∼41.8 to ∼42.6 mT in the set of materials we considered. The parallel study by Raman spectroscopy has enabled us to state that this variability is attributable to the different Si-O-Si angle characterizing the matrices of the different materials. © 2009 Elsevier B.V. All rights reserved.
Structural properties of the range-II- and range-III order in amorphous-SiO2 probed by electron paramagnetic resonance and Raman spectroscopy
2010
In the present work we report an experimental investigation by electron paramagnetic resonance spectroscopy on the hyperfine structure of the E. point defect, probing the local arrangement of the network (range-II order), and by Raman spectroscopy on the D 1 and D 2 lines, probing mean features of the network (range-III order). Our studies, performed on a-SiO 2 samples thermally treated at 1000 °C in air for different time durations, show that changes of the hyperfine structure and of the D 1 and D 2 lines occur in a correlated way. These results give strong evidence that the range-II and range-III order properties are intimately related to each other and that these properties are determine…
Temperature dependence of the absorption properties of silanol groups in amorphous : Are silanol groups organized in clusters?
2011
Abstract We present a study on the vacuum-ultraviolet (VUV) and infrared (IR) absorption of silanol groups in amorphous silicon dioxide ( a - SiO 2 ) in the range of temperature from 4 to 300 K. The observed temperature induced modifications of IR and VUV absorption spectra are interpreted as due to a process of conversion of free into H-bonded silanol groups. The changes of the amplitude of the VUV absorption spectra are shown to be linearly correlated to the changes of the IR absorption of free Si–OH groups. This point together with the evidence that the shape of the Si–OH VUV absorption does not depend on temperature demonstrates that the different silanol group sub-species have differen…
Molecular dynamics studies of 3D quasicrystals
1993
Abstract The thermodynamic behaviour of monatomic and diatomic models of three-dimensional icosahedral quasicrystals has been studied in NVT and NPT ensembles using molecular dynamics simulations with atoms interacting via Lennard-Jones potentials. We also report on a microcanonical MD simulation of decagonal Al 65 Cu 20 Co 15 quasicrystals where an experimentally determined binary model has been used. To obtain stability in this latter case it is necessary to endow the atoms with effective pair potentials. No transitions to crystalline or amorphous phases were found for diatomic models. Monatomic models, however, display a certain stability only in the low-temperature region. In the range …
Short range ordering and phase separation in rapidly quenched Ni80 57Fe1P19 amorphous alloys solution treated at different temperatures
1990
Abstract Electrochemical measurements and Mossbauer spectroscopy were used to study a rapidly quenched Ni80 57Fe1P19 amorphous alloy solution treated between 920 and 1500 °C. Different short range orderings were shown in amorphous alloys solution treated at different temperatures. This finding can be associated with phase separation occurring in the liquid state. This phase separation can be inherited in slightly relaxed amorphous state.