Search results for "defect"

A computational study of point defects and diffusion in enstatite

2009

In order to contribute to the understanding of diffusion processes in enstatite (Mg2Si2O6), we have used atomistic simulation techniques to study point defects in this mineral. We present results for a variety of Scottky and Frenkel defects of all atomic species present in it. We have furthermore calculated the activation energy for magnesium diffusion. We break diffusion down into a succession of hops between neighboring sites. Each hop is associated with a migration energy barrier. By making a series of hops, the diffusing ion may cross the unit cell. The maximum migration energy necessary to attain movement in a direction is the activation energy for diffusion in that direction. The defe…

Computational study of defects in ideal phengite

2009

Phengite is the name of a series designating the “potassic dioctahedral micas between, or close to, the joins muscovite-aluminoceladonite and muscovite-celadonite” (Rieder et al. 1998). These micas play a important role in most petrogenetic reactions occurring in high-pressure (HP) metamorphic environments; they are useful geothermobarometers and participate in reactions as H2O carriers in the subduction zone. In this work we have employed atomistic simulations techniques to model defects, evaluate the most stable defect species and determine the most likely diffusion mechanism in crystals. We have calculated the defect formation energies for vacancies, impurities and interstitials, Frenkel…

Visual field defects,type A personality,anxiety and coping styles in patients with primary open angle Glaucoma

2012

Peritonite meconiale in una rara associazione di atresia ileale apple-peel con difetto di parete addominale

2014

L’atresia intestinale tipo III B (apple peel) e la gastroschisi sono entrambe malformazioni congenite che necessitano di correzione chirurgica precoce in epoca neonatale. La loro associazione è un evento molto raro. Presentiamo il caso di un neonato a termine con l’associazione di atresia ileale apple-peel parziale e piccolo difetto della parete addominale anteriore, complicate con perforazione intestinale in utero e conseguente peritonite meconiale. Abbiamo riscontrato un’atresia parziale delle anse intestinali, coninteressamento dell’ileo terminale e risparmio del digiuno e di buona parte dell’ileo prossimale, difetto della parete addominale di piccole dimensioni e con poche anse erniate,…

Diffusion of point defects in shocked molecular crystals

2005

The dynamic response of a molecular crystal containing defects to shock wave loading is modeled and the resulting diffusion of point defects is simulated. It is shown that diffusion proceeds not only via the stress assisted diffusion, which is point defect diffusion in a stress field, also known as the Gorsky effect, but also through defect diffusion in the field of inertial forces. The method for modeling diffusion of point defects in shocked solids is developed. It is shown that diffusion in the inertial field significantly exceeds the stress diffusion in organic molecular crystals. Interplay between stress-assisted and inertial diffusion leads to the separation of light particles from he…

Improving the understanding of the melting behaviour of Mo, Ta, and W at extreme pressures

2004

We discus existent conflicts between experimentally measured and theoretically calculated melting curves of Mo, Ta, and W. By assuming that vacancy formation plays a fundamental role in the melting process, an explanation for the measured melting curves is provided. Furthermore, we show that the Lindemann law fits well all the measured melting curves of bcc transition metals if the Grueneisen parameter is written as a power series of the interatomic distance. For completeness, we examine possible reasons for current disagreements between shock-wave and DAC experiments. To solve them, we propose the existence of an extra high P-T phase for Mo, Ta, and W

Defect-related visible luminescence of silica nanoparticles

2013

The high photon emissivity in the visible spectral range is one of the most relevant phenomena emerging from the reduction of silica down to nanoscale; hence it is promising for the development of optical nanotechnologies (down converter, probes, display). It is well accepted that the origin of this luminescence is related to the high specific surface (~100 m2/g) that favors the formation of optically active defects at the nanosilica surface. With the aim to clarify the role of specific luminescent defects, here we report a detailed study of spectral and decay features by time-resolved photoluminescence spectra under a visible-UV tunable laser excitation. Our study is carried out on differe…

Vibrational properties of the surface-nonbridging oxygen in silica nanoparticles

2008

By studying the site-selective luminescence spectra of oxidized silica nanoparticles we identify the electronic and the vibrational lines associated with the surface nonbridging oxygen, $\ensuremath{\equiv}{\text{Si-O}}^{\ifmmode\bullet\else\textbullet\fi{}}$. This defect emits a zero-phonon line inhomogeneously distributed around 2.0 eV with full width at half maximum of 0.04 eV, weakly coupled with the local ${\text{Si-O}}^{\ifmmode\bullet\else\textbullet\fi{}}$ stretching mode whose frequency is measured to be $920\text{ }{\text{cm}}^{\ensuremath{-}1}$. These findings are different from those of the well-characterized defect in the bulk silica thus evidencing structural peculiarities of …

β-ray irradiation effects on silica nanoparticles

2015

By electron paramagnetic resonance (EPR) measurements, we examine the amplitude of the signal typically due to a combination of NBOHC (Non Bridging Hole Center) and POR (Peroxy Radical) defects induced by β-ray irradiation (from 1.2 to 1200 MGy) in silica nanoparticles with diameter ranging from 7 to 20 nm. Our data indicate that the signal line-shapes recorded at different doses is quite independent from the particles sizes and from the dose. Furthermore, for each considered nanoparticles size, the concentration of defects is also almost constant with respect to dose, and it does not change significantly if measured after 2 or 9 months from the irradiation. By contrast, we observed that th…

Self-limiting and complete oxidation of silicon nanostructures produced by laser ablation in water

2016

Oxidized Silicon nanomaterials produced by 1064 nm pulsed laser ablation in deionized water are investigated. High-resolution transmission electron microscopy coupled with energy dispersive X-ray spectroscopy allows to characterize the structural and chemical properties at a sub-nanometric scale. This analysis clarifies that laser ablation induces both self-limiting and complete oxidation processes which produce polycrystalline Si surrounded by a layer of SiO2 and amorphous fully oxidized SiO2, respectively. These nanostructures exhibit a composite luminescence spectrum which is investigated by time-resolved spectroscopy with a tunable laser excitation. The origin of the observed luminescen…