1,720,976 research outputs found
A reverse Monte Carlo study of a zinc-europium phosphate glass
No full textA combination of seven independent experimental functions obtained by X-ray diffraction and EXAFS from synchrotron radiation and neutron diffraction techniques were used as references in a reverse Monte Carlo simulation of a glassy sample of 0.1 Eu(PO3)(3)-0.9 Zn(PO3)(2) composition. The final model shows that the phosphate chains essentially retain the structure they have in pure zinc metaphosphate, while zinc and europium ions are set among the chains, so enveloped that oxygens from the PO4 groups create coordination cages. The zinc ions maintain the same environment they have in pure zinc metaphosphate. The europium sites are more complex, made up by a nearest neighbour shell of non-bridging oxygens, to which also a minor number of bridging oxygens contribute at longer distances
Non-destructive in-depth composition profile of oxy-hydroxide nanolayers on iron surfaces from ARXPS measurements
No full textIn this work the maximum entropy method (MEM) is applied, for the first time, to angle-resolved X-ray photoelectron spectroscopy (ARXPS) data from oxy-hydroxide films on iron surfaces. This nondestructively derives information on the in-depth distribution of the composition and chemical state. An MEM algorithm was created and first tested on the simulated data. The reconstructed composition depth profiles agreed very well with the theoretical ones up to 5% Gaussian noise added to the data. The same algorithm was then applied to ARXPS data from iron samples to investigate the in-depth variations in the composition and chemical state of the nanosized oxy-hydroxide film naturally grown on the iron surface. The resulting surface film presents a complex multilayer structure with concentration gradients. The effect of air exposure on the structure was also investigated. Copyright (C) 2006 John Wiley & Sons, Ltd
Nondestructive Surface Depth Profiles from Angle-Resolved X-ray Photoelectron Spectroscopy Data Using the Maximum Entropy Method. I. A New Protocol
No full textThe knowledge of the depth concentration profile of thin-layered Surfaces a few nanometers thick is very important for research and applications in microelectronics, corrosion, wear, and tribology. In-depth profiling methods reported in the literature are either destructive (ion sputtering), based on severe approximations (concentration gradients are not taken into account, and electron inelastic mean free paths (IMFPs) are calculated for electrons traveling throughout pure elemental materials) or limited to relatively simple profiles (less than three components and constant IMFPs). A reconstructed depth profile should be consistent with the angle-resolved X-ray photoelectron spectroscopy (ARXPS) data acquired, but transformation of XPS signal intensities vs emission angle into chemical species concentrations vs depth is an ill-posed mathematical problem which requires inversion of a Laplace transform. The main goal of this work was thus to develop a new, iterative protocol based on the maximum entropy method (MEM) that allows obtaining in-depth concentration profiles of layered surfaces from nondestructive ARXPS measurements. Numerical experiments were performed on a large series of computer generated, ideal, and error-containing ARXPS data from model depthprofiles with lip to four layers and up to eight components. The new algorithm enabled LIS to reconstruct these depth profiles with a maximum uncertainty of +/-20% for layer thickness and of +/-30% for composition of the individual layers. Moreover, the new protocol involves an iterative procedure for calculating the IMFP values of the different components, taking into account the actual depth concentration profile of the sample surface under investigation. The new protocol proved to be more powerful than any of the existing algorithms since it has been successfully applied for reconstructing depth profiles with up to eight components
The structure of a zinc metaphosphate glass. A reverse Monte Carlo study
No full textA combination of six sets of independent experimental data has been used in a reverse Monte Carlo procedure to obtain information about the structure of a zinc metaphosphate glass. The model structural functions have been fitted simultaneously against structural functions obtained from neutron diffraction data, from four X-ray diffraction experiments carried out at different energies, and from an extended X-ray absorption ne structure (EXAFS) spectroscopy data set. The final model structure is consistent with a description of the vitreous structure made of long phosphate chains, with the zinc ions interposed in between
Structural investigation of Fe2O3-SiO2 nanocomposites through radial distribution functions analysis
No full textTwo Fe2O3-SiO2 nanocomposites containing 50% by weight of iron oxide were prepared by a sol-gel method. By changing the surface to volume ratio of the gelling mass from 0.04 (sample A) to 0.3 cm(-1) (sample B), gelation times changed, resulting in nanoparticles of average size of about 10 and 4 nm, respectively. Accurate X-ray diffraction (XRD) data were collected and used to calculate total structure and correlation functions. The analysis of these functions unequivocally proved that in both samples maghemite is the iron oxide formed. The result demonstrated that decreasing the gelation time gives rise to the formation of smaller and smaller nanoparticles of the same phase. The possibility of interpreting the XRD pattern of sample B in terms of occurrence of 2-line ferrihydrite was discussed
Structural study of highly porous nanocomposite aerogels
No full textThe structural properties of CoFe2O4-SiO2 highly porous nanocomposite aerogels have been investigated by X-ray Absorption Spectroscopy and Transmission Electron Microscopy techniques. The aerogels are obtained by supercritical drying of composite gels obtained using a two step procedure where fast gelation is achieved using urea in the second step. The formation of CoFe2O4 nanocrystals in the silica matrix begins after calcination at 750 degrees C of the parent aerogel and is complete after calcination at 900 degrees C, while the high porosity of the sample is mostly retained
An EXAFS study on iron-cobalt-alumina nanocomposites prepared by the sol-gel method
No full textThe selectivity of the EXAFS (extended x-ray absorption fine structure) technique was used in order to gain insights on the structural evolution of the Fe and Co environment at different stages of the preparation of FeCo-Al 2O3 aerogels and xerogels. The gels, which were prepared using aluminium tri-sec-butoxide, iron nitrate and cobalt nitrates as precursors, were dried using different procedures in order to obtain xerogels and aerogels, respectively. After drying, the samples are treated either in air up to 900°C or in hydrogen up to 800°C
NiFe2O4 Nanoparticles Dispersed in an Aerogel Silica Matrix: An X-ray Absorption Study
No full textThe formation of NiFe2O4 nanoparticles dispersed in an aerogel silica matrix was investigated as a function of calcination temperature by X-ray absorption fine structure and X-ray absorption near edge structure at both the Fe and Ni K-edges. In particular, nanocomposite aerogels containing a relative NiFe2O4 amount of 10 wt % and calcined at 450, 750 (1 h and 20 h), and 900 °C were studied. A quantitative determination of the relative occupancy of iron and nickel cations in the octahedral and tetrahedral sites of the spinel structure was obtained. It has been found that nickel ferrite prepared by sol?gel has the classical inverted spinel structure found in bulk materials with nickel(II) cations fully occupying the octahedra sites and iron(III) equally distributed between octahedra and tetrahedra sites
An X-ray absorption spectroscopy study of FeCo alloy nanoparticles embedded in ordered cubic mesoporous silica (SBA-16)
No full textNanocomposites containing FeCo alloy nanoparticles dispersed in a highly ordered cubic mesoporous silica (SBA-16) matrix were prepared using two different synthetic methods, co-precipitation and impregnation. Extended X-ray Absorption Spectroscopy (EXAFS) technique at both Fe and Co K-edges was used to investigate the structure of FeCo nanoparticles and the presence of additional disordered oxide phases. EXAFS technique gives evidence of differences in the oxidation degree of the FeCo nanoparticles depending on the synthetic method used
An EXAFS study on Iron-Cobalt-Silica Nanocomposite Materials prepared by the sol-gel method.
No full textThe structural evolution of FeCo-SiO2 xerogel and aerogel nanocomposite samples during their sol-gel preparation have been studied by EXAFS (extended X-ray absorption fine structure) technique. Depending on the precursors of the alloy nanoparticles different intermediate are formed which have a strong influence on the formation of the FeCo alloy of the desired composition. The porous structure also plays an important role
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