The vibrational properties of silica glass have been intensively studied experimentally and theoretically during the last four decades. However there are few theoretical studies of the evolution of the vibrational properties of more complex silicate glasses (binary, ternary etc...).We have used first principles simulations in order to investigate the properties of two silicate glasses: a sodium silicate of composition 4Na2O-6SiO2 (NS1.5) and a sodium borosilicate of composition 3Na2O-B2O3-6SiO2 (NBS). The studies were carried up using first principles molecular dynamics within the density functional theory framework as implemented in the VASP code. We have studied the structural and vibrational properties of these two glasses, and we have identified  the contributions of the various species to the vibrational density of states (VDOS) as well as to the IR spectra.In particular we will discuss how the presences of the sodium and boron atoms affect the relevant vibrational parameters: positions, shapes and intensities of the main peaks of the VDOS and IR spectra. We have found that 3- and 4-fold coordinated boron atoms give rise to distinguished spectral features. Moreover, the partial vibrational density of the 3-fold coordinated B atoms has been found to be a weighted sum of 2 specific contributions so-called 3-fold symmetric coordinated B atoms and asymmetric coordinated B atoms.

Controlled doping of active carbon materials (viz., graphenes, carbonnanotubes etc.) may lead to the enhancement of their desired properties. The leaststudied case of C/Be substitution offers an attractive possibility in this respect. Theinteractions of Be2 with Be or C atoms are dominated by the large repulsive Pauliexchange contributions, which in turn offsets the attractive interactions leading torelatively small binding energies. The Be2 dimer, e.g., after being doped inside a planarcarbon network, undergoes orbital adjustments due to charge transfer and unusualintermolecular interactions and is oriented perpendicular to the plane of the carbonnetwork with the Be−Be bond center located inside the plane. The present theoreticalinvestigation on the nature of bonding in C/Be2 exchange complexes, using state of theart quantum chemical techniques, reveals a sp2 carbon-like bonding scheme in Be2 arising due to the molecular hybridization of σand two π orbitals. The perturbations imposed by doped Be2 dimers exhibit a local character of the structural and electronicproperties of the complexes, and the separation by two carbon atoms between beryllium active centers is sufficient to considerthese centers as independent sites.

Binding energy between adsorbent and adsorbate strongly affects themechanism of adsorption. Porous systems are usually characterized by a distribution ofthis energy, which is not easy to determine experimentally. A coupled experimentalsimulationprocedure to estimate binding energy directly from experimentaladsorption isotherms is proposed. This new approach combines experimentalinformation (pore size distribution determined from nitrogen adsorption at 77 K)and numerical data (grand canonical Monte Carlo simulations of adsorption in pores)to explain an influence of binding energy on adsorption isotherms. The procedure hasbeen validated by analysis of hydrogen adsorption in a series of carbons activated withKOH:C ratio varying from 3 to 6. These carbons show high capacity of hydrogenstorage both at 80 and 303 K (115 gH2/kgC and 23 gH2/kgC at p = 100 bar,respectively, for carbon activated during 1 h at T = 790 C (T = 1361 K) with KOH:Cratio equal to 3, having the surface area above 2600 m2/g, 0.77 porosity, and largefraction (31%) of pores with average width below 1 nm). An additional energeticparameter has been introduced into the conventional fitting procedure to account for the distribution of adsorption energy inmeasured samples. The observed high consistency between experimental and simulated results validates/correlates thecharacterization procedures and proves the coherence and robustness of both the experimental results and the numericalsimulations.

This paper covers the optimization of methane volumetric storage capacity by controlling the sub-nanometre (<1 nm) and supra-nanometre (1–5 nm) pore volumes. Nanospace engineering of KOH activated carbon generates an ideal structure for methane storage in which gas molecules are adsorbed as a high-density fluid by strong van der Waals forces into pores that are a few molecules in diameter. High specific surface areas, porosities, sub-nanometre (<1 nm) and supra-nanometre (1–5 nm) pore volumes are quantitatively selected by controlling the degree of carbon consumption and metallic potassium intercalation into the carbon lattice during the activation process. The formation of tuneable sub-nanometre and supra-nanometre pores is validated by sub-critical nitrogen adsorption. Aberration-corrected scanning transmission electron microscopy data show the atomic structure of high-surface-area activated carbon (2600 m2/g). While high surface area and high porosity are optimal for gravimetric methane storage, the results indicate that an exclusive sub-nanometre region, a low porosity and an acceptable surface area (approximately 2000 m2/g) are ideal for methane volumetric storage, storing 120 g CH4/l (184 vol/vol) at 35 bar and room temperature (22 °C). High-pressure methane isotherms up to 150 bar at 30, −25 and −50 °C on optimal activated carbons are presented. Methane volumetric storage capacity at 35 bar reaches 176 g/l (269 vol/vol) and 202 g/l (309 vol/vol) at −25 and −50 °C, respectively.

The vibrational properties of silica glass have been intensively studied experimentally andtheoretically during the last four decades. However there are few theoretical studies of theevolution of the vibrational properties under pressure.We have calculated the parallel and perpendicular Raman spectra of the vitreous silica. The studyhas been carried out using first principles calculations within the density functional theoryframework. The calculated spectra at ambient pressure are in good agreement with theexperimental ones as well as with previous calculations reported in the literature. Themodifications of the Raman spectra under pressure also reproduce the experimental data. Apreliminary analysis of the resulting structural modifications and their correlations with the changesseen in the spectra will be reported.

Raman Investigation of Heavily Al Doped 4H-SiC Layers Grown by CVD