1,721,054 research outputs found
Les régions problèmes de l'Europe, Alan B. Mountjoy, G. R. P. Lawrence, Hugh Clout, Ian B. Thompson, Kenneth Warren
No full textBeaujeu-Garnier Jacqueline. Les régions problèmes de l'Europe, Alan B. Mountjoy, G. R. P. Lawrence, Hugh Clout, Ian B. Thompson, Kenneth Warren. In: Annales de Géographie, t. 83, n°458, 1974. pp. 465-466
A transmission electron microscopy study of fe-Co alloy nanoparticles in silica aerogel matrix using HREM, EDX, and EELS
No full textMagnetic nanocomposite materials consisting of 5.5 wt% Fe-Co alloy nanoparticles in a silica aerogel matrix, with compositions FexCo1x of x = 0.50 and 0.67, have been synthesized by the sol-gel method. The high-resolution transmission electron microscopy images show nanoparticles consisting of single crystal grains of body-centered cubic Fe-Co alloy, with typical crystal grain diameters of approximately 4 and 7 nm for Fe0.5Co0.5 and Fe0.67Co0.33 samples, respectively. The energy dispersive X-ray (EDX) spectra summed over areas of the samples gave compositions FexCo1x with x = 0.48 0.06 and 0.68 0.05. The EDX spectra obtained with the 1.5 nm probe positioned at the centers of 20 nanoparticles gave slightly lower concentrations of Fe, with means of x = 0.43 0.01 and x = 0.64 0.02, respectively. The Fe0.5Co0.5 sample was studied using electron energy loss spectroscopy (EELS), and EELS spectra summed over whole nanoparticles gave x = 0.47 0.06. The EELS spectra from analysis profiles of nanoparticles show a distribution of Fe and Co that is homogeneous, i.e., x = 0.5, within a precision of at best 0.05 in x and 0.4 nm in position. The present microscopy results have not shown the presence of a thin layer of iron oxide, but this might be at the limit of detectability of the methods
AN ELECTRON-ENERGY-LOSS SPECTROSCOPY STUDY OF NI60B40 ALLOYS PREPARED BY CHEMICAL-REDUCTION AND MELT-SPINNING
No full textEffect of the Support on the Formation of FeCo Alloy Nanoparticles in an SBA-16 Mesoporous Silica Matrix: An X-ray Absorption Spectroscopy Study
No full textA series of nanocomposites consisting of FeCo alloy nanoparticles supported on a three-dimensional cubic mesoporous silica matrix (SBA-16) were prepared by wet impregnation of the matrix with a solution of Fe and Co nitrates. FeCo alloy nanoparticles were obtained by heat treatment at 800 °C in reducing atmosphere of the impregnated SBA-16 previously calcined at 500 °C. Three different SBA-16 types were used as a support of the nanophase. The influence of the matrix on the absorption of Fe and Co ions was investigated using X-ray diffraction and X-ray absorption spectroscopy. In particular, extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques at the Fe and Co K-edges were used in order to identify the intermediate products before reduction and to study the formation of the bcc FeCo alloy, which cannot be assessed unambiguously using X-ray diffraction. An important influence of the matrix has been observed in the phases formed before reduction, in the size of nanoparticles, and in the oxidation of the FeCo alloy nanoparticles
Insights into the Structure of Dot@Rod and Dot@Octapod CdSe@CdS Heterostructures
Full text linkCdSe@CdS dot@rods with diameter around 6 nm and length of either
20, 27, or 30 nm and dot@octapods with pod diameters of ?15 nm and lengths of ?50
nm were investigated by X-ray absorption spectroscopy. These heterostructures are
prepared by seed-mediated routes, where the structure, composition, and morphology of
the CdSe nanocrystals used as a seed play key roles in directing the growth of the second
semiconducting domain. The local structural environment of all the elements in the
CdSe@CdS heterostructures was investigated at the Cd, S, and Se K-edges by taking
advantage of the selectivity of X-ray absorption spectroscopy, and was compared to pure
reference compounds. We found that the structural features of dot@rods are
independent of the size of the rods. These structures can be described as made of a
CdSe dot and a CdS rod, both in the wurtzite phase with a high crystallinity of both the
core and the rod. This result supports the effectiveness of high temperature colloidal
synthesis in promoting the formation of core@shell nanocrystals with very low
defectivity. On the other hand, data on the CdSe@CdS with octapod morphology suggest the occurrence of a core composed of
a CdSe cubic sphalerite phase with eight pods made of CdS wurtzite phase. Our findings are compared to current models
proposed for the design of functional heterostructures with controlled nanoarchitecture
Formation and cation distribution in supported manganese ferrite nanoparticles: An X-ray absorption study
No full textExtended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques at both Fe and Mn K-edges were used to investigate the formation of MnFe2O4 nanoparticles embedded in a silica aerogel matrix as a function of calcination temperature (at 450, 750 and 900°C). Up to 450°C, two separated highly-disordered phases of iron and manganese are present. With increasing the temperature (to 750 and 900°C), the structure of aerogel nanoparticles becomes progressively similar to that of the spinel structure MnFe2O4 (jacobsite). Quantitative determination of cations distribution in the spinel structure shows that aerogels calcined at 750 and 900°C have a degree of inversion i = 0.20. A pure jacobsite sample synthesised by co-precipitation and used as a reference compound shows a much higher degree of inversion (i = 0.70). The different distribution of iron and manganese cations in the octahedral and tetrahedral sites in pure jacobsite and in the aerogels can be ascribed to partial oxidation of Mn2+ to Mn3+ in pure jacobsite, confirmed by XANES analysis, probably due to the synthesis conditions
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
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 the NiK edge in NiO-SiO2 nanocomposite materials prepared by the sol-gel method
No full textEXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) techniques have been used to study the structural evolution of nanocomposites constituted of nickel oxide nanoparticles embedded into an amorphous silica matrix during their sol-gel preparation. EXAFS data show that no interaction between the metal oxide nanoparticles and the silica network develops since the spectra of all the samples have a structure very similar to that of crystalline NiO. Quantitative information obtained from the fitting of the data using a full multiple scattering calculation shows that the reduction of the EXAFS oscillations in the nanocomposite samples is mainly related to an increase of disorder of the superficial sites while there was no evidence of a significant reduction of coordination numbers due to the size of the crystallites. XANES results support the conclusion that the structure of the nanocomposites is very similar to that of crystalline NiO
- …
