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abstracts
During the last decades an increasing demand for the development of new methods of preparation of nano-composite materials suited for technological applications has been observed [1]. The interest on these nanomaterials is related to the fact that many of their properties can be varied in a continuous way by changing size, shape and/or spatial ordering. Silicides of transition metals such as CoSi2 are of great interest because of the possibilities that they open to applications as a contact material for microelectronic devices [2]. A simple method for obtaining buried arrays of CoSi2 plates coherently embedded in a Si host lattice was described in a previous study [3]. Grazing-incidence small-angle X-ray "GISAXS" and transmission electron microscopy "TEM" results indicated that the nanoplates exhibit a hexagonal lateral shape and a remarkable uniform thickness. The lattice of each CoSi2 nanoplates was shown to be coherent with the host Si lattice, and parallel to one of the four planes belonging to the Si(111) crystallographic planes family. In this work we present a diffusion dynamics study of Co into Si single crystal lattice using in situ GISAXS setup designed ad-hoc. The Co atoms were initially embedded in a SiO2 thin film deposited on Si(001) substrates subjected to a isothermal treatment during 1 hour at several temperatures between 650-850 oC, The diffused Co react with Si atoms and form CoSi2 nanoplates. GISAXS intensity was modeled assuming that the total scattering intensity is the sum of the intensity produced by a set of spatially uncorrelated and spherical Co nanoparticles - embedded in the SiO2 layer - plus the intensity coming from the hexagonal nanoplates parallel to the different planes of the Si(111) family.
abstracts
SAXS technique alone and combined with XRD can be applied to determine the size dependences of crystal-to-liquid and liquid-to crystal transition temperatures of nanoparticles in dilute state. Since the volume of nanoparticles at the transition temperatures are expected to exhibit (weak) discontinuities, SAXS can be applied to determine the melting and freezing temperatures of nanoparticles as functions of their size. For this purpose, a set of samples, each of them with different nanoparticle size, is studied by in situ SAXS at varying temperatures. The variations in nanoparticle volume at the transitions lead to discontinuities in the 3D integral of SAXS intensity, ΔQ, in the slope of Guinier plots, ΔS, and in SAXS intensity at any q except at q=0, ΔI(q>0). Thus, the transition (melting and freezing) temperatures of a nearly monodisperse set of nanoparticles, can be derived from the discontinuities observed in the temperature dependence of V, S or I(q>0). Since the transition temperatures are strongly dependent on the size of the nanoparticles, for samples containing nanoparticles with a wide size distribution the size dependences of the melting and freezing temperatures cannot be determined by applying the method outlined above. However, in the particular case of systems consisting of a dilute set of spherical nanoparticles with a broad radius distribution, N(R), the combined use of SAXS and XRD makes it possible to determine the radius dependences of the melting and freezing temperatures, MT(R) and FT(R), respectively. This is achieved by studying a single sample in situ, along a heating/cooling cycle, and simultaneously determining the temperature dependences of the SAXS intensity pattern and the area of XRD Bragg peaks. A few applications of these procedures will be described, namely the determination of the radius dependences MT(R) and FT(R) of (i) a nearly monodisperse set of Pb nanoparticles embedded in a lead-borate glass (Gorgeski et al, 2014) and (ii) a polydisperse set of Bi nanoparticles embedded in a sodium-borate glass [Kellermann and Craievich, 2002). Other relevant structural features of Bi nanoparticles embedded in borate glass could also be derived from the analysis of the same experimental results [Kellermann and Craievich, 2008].
abstracts
In the last years, extensive research has been devoted to develop novel materials and structures with high electrochemical performance for intermediate-temperatures solid-oxide fuel cells (IT-SOFCs) electrodes. In recent works, we have investigated the structural and electrochemical properties of nanostructured La0.6Sr0.4CoO3 (LSCO) and La0.6Sr0.4(Co;Fe)O3 (LSCFO) cathodes, finding that they exhibit excellent electrocatalytic properties for the oxygen reduction reaction [1,2]. These materials were prepared by a pore-wetting technique using polycarbonate porous membranes as templates. Two average pore sizes were used: 200 nm and 800 nm. Our scanning electronic microscopy (SEM) study showed that the lower pore size yielded nanorods, while nanotubes were obtained with the bigger pore size. All the samples were calcined at 10000C in order to produce materials with the desired perovskite-type crystal structure. In this work, we analyze the oxidation states of Co and Fe and the local atomic order of LSCO and LSCFO nanotubes and nanowires for various compositions by X-ray absorption spectroscopies. For this purpose we performed XANES and EXAFS studies on both Co and Fe K edges. These measurements were carried out at the D08B-XAFS2 beamline of the Brazilian Synchrotron Light Laboratory (LNLS). XANES spectroscopy showed that Co and Fe only change slightly their oxidation state upon Fe addition. Surprisingly, XANES results indicated that the content of oxygen vacancies is low, even though it is well-known that these materials are mixed ionic-electronic conductors. EXAFS results were consistent with those expected according to the rhombohedral crystal structure determined in previous X-ray powder diffraction investigations.
