1,721,247 research outputs found
Influence of metal content on size, dispersion, and magnetic properties of iron-cobalt alloy nanoparticles embedded in silica matrix
No full text(FexCo1-x)(y)(SiO2))(1-y) nanocomposites were prepared by a sol-gel method, using tetraethoxysilane and metal acetates as precursors. Two sets of xerogel samples were prepared: in the first set x was kept equal to 0.5 and four y values were selected in the range 0.02-0.2; in the second set y was kept equal to 0.1 and three samples with x equal to 0.4.5. 0.50. and 0.70 were prepared. The samples were characterized by X-ray diffraction, TEM, and HREM to verify the formation of the FeCo alloy nanocrystals and to investigate their Structure, morphology, and composition. All samples contain spherical FeCo alloy nanoparticles uniformly dispersed in the silica matrix with average size increasing with metal content. Magnetic measurements carried out on selected samples show a superparamangetic behavior. with blocking temperature which depends on metal load, as a con-sequence of particle distance variation
Iron-cobalt alloy nanoparticles embedded in an alumina xerogel matrix
No full textNanocrystalline gamma-Al2O3 and FeCo-Al2O3 nanocomposite xerogels with high surface areas and pore volumes were prepared from alcogels obtained by a fast sol-gel procedure. The formation of gamma-Al2O3 occurs via a sequence of stages starting from a disordered pseudo-bohemite phase which around 700 degrees C gives rise to amorphous allumina; this progressively cristallizes as gamma-Al2O3, which is stable up to 1100 degrees C, when microcrystalline alpha-Al2O3 becomes the dominant phase; in the range 1000-1200 degrees C minor traces of delta-Al2O3 and theta-Al2O3 are present. Xerogels containing iron and cobalt are amorphous up to 700 degrees C; calcination at 800 degrees C gives rise to a spinel phase similar to gamma-Al2O3 where metal ions partially fill the vacancies; at 1000 degrees C gamma-Al2O3 progressively disappears to form alpha-Al2O3 and Co(Fe)Al2O4, which are the only phases present at 1200 degrees C. Reduction in hydrogen flow of the xerogel, previously calcined at 450 degrees C, leads to a nanocomposite constituted of FeCo alloy nanoparticles around 10 nm dispersed into alpha-Al2O3 nanocrystalline matrix. Zero-field-cooled (ZFC) and FC magnetic curves are typical of superparamagnetic materials and indicate the occurrence of high-strength particle interactions
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
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
A transmission electron microscopy study of CoFe2O4 ferrite nanoparticles in silica aerogel matrix using HREM and STEM imaging and EDX spectroscopy and EELS
No full textMagnetic nanocomposite materials consisting of 5 and 10 wt% CoFe 2O4 nanoparticles in a silica aerogel matrix have been synthesized by the sol-gel method. For the CoFe2O4-10wt% sample, bright-field scanning transmission electron microscopy (BF STEM) and high-resolution transmission electron microscopy (HREM) images showed distinct, rounded CoFe2O4 nanoparticles, with typical diameters of roughly 8 nm. For the CoFe2O4-5wt% sample, BF STEM images and energy dispersive X-ray (EDX) measurements showed CoFe2O4 nanoparticles with diameters of roughly 3 ± 1 nm. EDX measurements indicate that all nanoparticles consist of stoichiometric CoFe2O4, and electron energy-loss spectroscopy measurements from lines crossing nanoparticles in the CoFe2O4-10wt% sample show a uniform composition within nanoparticles, with a precision of at best than ± 0.5 nm in analysis position. BF STEM images obtained for the CoFe2O 4-10wt% sample showed many "needle-like" nanostructures that typically have a length of ? 10 nm and a width of ? 1 nm, and frequently appear to be attached to nanoparticles. These needle-like nanostructures are observed to contain layers with interlayer spacing 0.33 ± 0.1 nm, which could be consistent with Co silicate hydroxide, a known precursor phase in these nanocomposite materials
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Supported ionic liquid phase catalysis on functionalized carbon nanotubes
No full textHighly active rhodium catalysts have been prepared by immobilization of an ionic liquid film on carbon nanotubes functionalized with imidazolium-based ionic moieties
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