Zirconia microrods and sodium titanate nanostructures have been synthesized hydrothermally. The process is performed in an autoclave at very high pH using NaOH as a mineralizer. The temperature of the treatments spanned from 130 oC to 200... more
Zirconia microrods and sodium titanate nanostructures have been synthesized hydrothermally. The process is performed in an autoclave at very high pH using NaOH as a mineralizer. The temperature of the treatments spanned from 130 oC to 200 oC and their duration from 18 h to 1 week. The use of titania as base material leads to the formation of sodium titanate nanowires. The use of zirconium monoxide as base material lead to ZrO2 bara, 200 nm in diameter, with no sodium incorporation in the structure. Titanium dioxide-based nanostructures have been produced under hydrothermal techniques by several authors, many of them based on Kasuga`s method [1]. It consists in placing fine TiO2 particles (rutile or anatase) in an aqueous Na(OH) solution inside a sealed reactor, at temperatures between 100 and 160 oC. Depending on the subsequent treatment (annealing, washing with HCl) several compounds are produced, mainly sodium and hydrogen titanates. There are fewer reports on the hydrothermal pre...
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The H2 uptake performance at room temperature of porous tungsten oxide nanomaterials with and without a catalytic Pd coating was studied by the quartz crystal microbalance technique. Tungsten oxide composed mainly of WO3 was synthesized... more
The H2 uptake performance at room temperature of porous tungsten oxide nanomaterials with and without a catalytic Pd coating was studied by the quartz crystal microbalance technique. Tungsten oxide composed mainly of WO3 was synthesized by inert gas condensation method using He. The samples consisted of semi-amorphous nanomaterials of low crystallinity and high porosity, as revealed by SAED, TEM, XRD, Raman spectroscopy and N2 adsorption–desorption isotherms. The H2 uptake capacity of the porous oxide was studied under increasing H2 exposure pressures (1000–7000[Formula: see text]Pa), with and without Pd coating. After reaching a maximum value of 1.2 H2[Formula: see text]wt.%, at 1160[Formula: see text]Pa, the H2 uptake capacity of the Pd-coated oxide consistently decreased. Successive hydrogenation cycles were carried out on the Pd-coated oxide at 3300 Pa and 6000 Pa to evaluate the H2 uptake performance of the sample under this H2 loading and unloading process. It was found that t...
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The oxidation of 99 wt.-% purity (low purity – main impurities were Fe and Si) and 99.999 wt.-% purity (high purity) Al foil was studied using a thermo-gravimetric method. The Al oxide was characterized with several techniques such as... more
The oxidation of 99 wt.-% purity (low purity – main impurities were Fe and Si) and 99.999 wt.-% purity (high purity) Al foil was studied using a thermo-gravimetric method. The Al oxide was characterized with several techniques such as scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS). The Al foils were oxidized in a 50 % O2 – 50 % Ar mixture between 773 and 843 K. Microscopic examination (SEM) of the oxide revealed that “pitting” occurred on the low purity Al. High Si and Fe concentration were found on the surface or near-surface of the oxide. The presence of Fe, as precipitates, on the surface of the oxide accounts for the pitting of the oxide. XPS inspection revealed the formation of γ-Al2O3 with the presence of some Al hydroxide.
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The use of solid lubricants such as graphene, graphene oxide, and other nanoparticles have gained notable attention in the tribological community to reduce friction and wear thus aiming at improved energy efficiency and sustainability.... more
The use of solid lubricants such as graphene, graphene oxide, and other nanoparticles have gained notable attention in the tribological community to reduce friction and wear thus aiming at improved energy efficiency and sustainability. Tribological experiments unify rather extreme conditions such as high contact pressures, small contact areas, relative sliding motion, and rapid heating. This combination leads to mechanically- and/or thermally induced chemical, structural and microstructural modifications of the lubricating nanoparticles during rubbing thus altering their material’s properties. Due to the high sensitivity, we propose nanocalorimetry as the method of choice to shed more light on the thermally-induced processes and changes. As a model material for solid lubricants, we explore the transitions of graphene oxide under heating with 1000 °C/s up to 600 °C using quasi-adiabatic nanocalorimetry. We identify a strong exothermic runaway reaction at 317 °C. This runaway is prece...
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Abstract We have measured hydrogen absorption capacity for a series of Pd films grown by two deposition techniques: a) flat films made with e-beam evaporation of Pd and b) films made of assembled Pd clusters. The films range in thickness... more
Abstract We have measured hydrogen absorption capacity for a series of Pd films grown by two deposition techniques: a) flat films made with e-beam evaporation of Pd and b) films made of assembled Pd clusters. The films range in thickness from 9.5 to 45 nm. The technique implemented for measuring the amount of hydrogen absorbed by the films was a quartz crystal microbalance. The Pd films were grown on the quartz crystals and the change of mass due to the hydrogen absorption was determined from the shifting of the resonance frequency. Minute amounts of hydrogen absorbed of the order of nanograms per square centimeter can be detected by this technique. Plots of H/Pd atom ratio for each Pd film as a function of hydrogen pressure were obtained. The Pd e-beam grown films displayed a saturation H/Pd atomic ratio around 0.6 which is similar to bulk Pd. The Pd cluster film with a thickness of 14 nm displayed a remarkable absorption capacity with a ratio H Pd ≈ 0.84 .
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Porous MoO3 and MoO3−x were grown in vapor-phase under He and H2 at pressures between 100 and 1200Pa. MoO3 was vaporized from a tungsten boat heated between 650 and 1250°C. Growth of porous material was promoted at 100 and 200Pa of the... more
Porous MoO3 and MoO3−x were grown in vapor-phase under He and H2 at pressures between 100 and 1200Pa. MoO3 was vaporized from a tungsten boat heated between 650 and 1250°C. Growth of porous material was promoted at 100 and 200Pa of the carrier gas. At higher pressures and only in hydrogen under particular preparation conditions, samples rich in porous phase
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This work reports on the in situ-generation of twins due to the incidence of electron beams on copper nanoparticles when they are investigated under transmission electron microscopy (TEM).
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ABSTRACT Hydrogen absorption by palladium has been studied for decades due to the significant importance in a number of applications like production and storage of hydrogen and hydrogen sensors. Alloying Pd with just a 4% of Ru... more
ABSTRACT Hydrogen absorption by palladium has been studied for decades due to the significant importance in a number of applications like production and storage of hydrogen and hydrogen sensors. Alloying Pd with just a 4% of Ru drastically reduces the absorption properties of the Pd. The fcc crystal structure is preserved but the lattice constant is reduced slightly. In order to understand this phenomenon, we used three samples: a Pd foil, a Pd-Ru(4%) alloy foil, and a Pd foil with a Pd-Ru surface alloy. The surface alloy was made evaporating 8 nm of Ru using an e-beam evaporation technique on top of Pd, followed with a heating the sample up to 700 ^oC in a high vacuum system. We studied the changes in absorption properties of these samples using Thermal Program Desorption (TPD), resistance changes and grazing incidence X-ray Diffraction (GID).
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Research Interests: Organic Chemistry, Humans, Liver, Mice, Animals, and 7 moreMale, Naphthoquinones, Lung, Molecules, Diterpenes, Stomach, and Tabebuia
Research Interests: Organic Chemistry, Cell line, Humans, Mice, Animals, and 6 moreMale, Ethanol, Amino Acids, Molecules, Diterpenes, and Molecular Structure
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ABSTRACT The structural transformations induced by heating on MoO3 mesoporous samples grown in vapor-phase under helium and hydrogen were studied by in situ Raman spectroscopy. The samples were continuously irradiated by a He–Ne laser of... more
ABSTRACT The structural transformations induced by heating on MoO3 mesoporous samples grown in vapor-phase under helium and hydrogen were studied by in situ Raman spectroscopy. The samples were continuously irradiated by a He–Ne laser of 5.5 mW for 105 min and Raman spectra were dynamically acquired every 5 min, in order to evaluate the laser effects. The He-grown sample did not undergo structural transformations due to the laser irradiation while the H2-grown sample underwent changes after just 10 min of irradiation. On the other hand, each type of sample was heated in air from room temperature up to 450 °C using a heating rate of 5 °C min−1 and Raman spectra were recorded each 25 °C. The He-grown sample remained structural unchanged up to 250 °C revealing a high temperature state of its amorphous matrix whereas the H2-grown sample exhibited changes around 70 °C. These changes were attributed to the crystallization onset of its amorphous matrix to β-MoO3. However, this same transition was detected between 250 °C and 275 °C for the He-grown sample. A second transformation was detected in both samples in a temperature range more similar associated to the transition of the formed β-MoO3 phase to α-MoO3. After the heat treatment, the oxide of both samples was slightly reduced, as revealed by XPS analysis.The metastability at low temperatures of the amorphous phase of the H2-grown sample could be associated to release of hydrogen trapped inside the compound and to the break of weak bonds between Mo and OH groups. At high temperatures, above 225 °C, the rapid conversion to β-MoO3 of the amorphous matrix of the H2-grown sample occurred due to the dehydration of the sample.
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ABSTRACT In this work we report studies of the photoluminescence emission in samples based on Si/SiOx films deposited by the Pulsed Electron Beam Ablation (PEBA) technique. The samples were prepared at room temperature using targets with... more
ABSTRACT In this work we report studies of the photoluminescence emission in samples based on Si/SiOx films deposited by the Pulsed Electron Beam Ablation (PEBA) technique. The samples were prepared at room temperature using targets with different Si/SiO2 concentrations. The samples were characterized using X-ray Absorption Edge Spectroscopy (XANES) at the Si―K edge, Raman spectroscopy, Photoluminescence (PL) and X-ray Photoelectron Spectroscopy (XPS). The concentration of a-Si and nc-Si in the film was dependent on the silicon concentration in the target. It was also observed that the PL is strongly dependent on the structural amorphous/crystalline arrangement.
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Molybdenum trioxide nanostructures were grown by direct evaporation of MoO3 from a tungsten boat resistively heated in the presence of hydrogen or helium as carrier gas at pressures from 100 to 600 Pa. Crystalline structures such as,... more
Molybdenum trioxide nanostructures were grown by direct evaporation of MoO3 from a tungsten boat resistively heated in the presence of hydrogen or helium as carrier gas at pressures from 100 to 600 Pa. Crystalline structures such as, nanoribbons, nanofibers, nanoneedles and nanoparticles were obtained at source temperatures below 900 degrees C. On the other hand, at source temperatures above 1000 degrees C, nanoporous structures were obtained. The latter were found more often when hydrogen was used as carrier gas.
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ABSTRACT The ferroelectric switching process in Pb(Nb,Zr,Ti)O3 thin films was studied by performing Ramanspectroscopy and x-ray photoelectron spectroscopy (XPS). Switching was achieved using a macroscopic polarization experiment above and... more
ABSTRACT The ferroelectric switching process in Pb(Nb,Zr,Ti)O3 thin films was studied by performing Ramanspectroscopy and x-ray photoelectron spectroscopy (XPS). Switching was achieved using a macroscopic polarization experiment above and below the Curie temperature. Two samples in opposite switching states were obtained and characterized in order to correlate both vibrational-bands distortions of the bulk and changes in the elemental states of the surface. We have assigned the symmetrical A1(2TO) (332 cm−1) and A1(3TO) (603 cm−1) vibrational modes to the ferroelectric phase. Their corresponding peaks-area showed symmetrical behavior when the sample was polarized in opposite directions, while the quantity of phonons associated to the ferroelectric phase was conserved. We found that binding energies in the XPS signals of the Ti 2p, Nb 3d, Zr 3d, and Pb 4f levels increased when comparing to the values found in a non-polarized sample. Moreover, a high population of oxygen vacancies diffused to the surface of the ferroelectric capacitor under the application of external electric fields. Our novel results show the correlation between vibrational and ferroelectric behaviors and highlight the possibility to perform in situ treatments to decrease thedegradation of current technological capacitors.
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ABSTRACT In this article, the evaporation of niobium, ruthenium, and titanium using a conventional mini electron-beam evaporator (Tectraé-flux) is described. These metals require high temperatures for evaporation due to their high melting... more
ABSTRACT In this article, the evaporation of niobium, ruthenium, and titanium using a conventional mini electron-beam evaporator (Tectraé-flux) is described. These metals require high temperatures for evaporation due to their high melting points, low vapor pressures, and large bond energies between atoms. Usually, a high-power electron-beam evaporator with a power exceeding 600 W is used in order to grow films of these metals. However, evaporation conditions for deposition using a mini electron-beam evaporator of low power (600 W at 2 kV) were obtained. Film thicknesses between 2 nm and 12 nm were obtained and the films were characterized with different analytical techniques. In the case of ruthenium, a comparison between the evaporation achieved when using a graphite crucible or a metal rod as a target is presented. The quality of the deposited films was ascertained by Auger electron spectroscopy. Niobium and titanium film's thickness and quality were determined by X-ray reflectivity and atomic force microscopy. Theoretical values of vapor pressure as a function of temperature were calculated for niobium, ruthenium, and titanium using the Clausius-Clapyeron equation to compare their evaporation behaviors.
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ABSTRACT The influence of barium and strontium starting reactants used in different mole ratios, BaCl2 and Ba(OH)2, SrCl2 and Sr(OH)2, on the chemical and structural properties of Ba1−xSrxTiO3 (x=0−1) (BST) nanoparticles prepared via... more
ABSTRACT The influence of barium and strontium starting reactants used in different mole ratios, BaCl2 and Ba(OH)2, SrCl2 and Sr(OH)2, on the chemical and structural properties of Ba1−xSrxTiO3 (x=0−1) (BST) nanoparticles prepared via sol-gel-hydrothermal synthesis in an oxygen atmosphere is discussed. The effect of the type of reactant on the relative amount of Sr incorporated in BST compound was also analysed. The synthesised BST nanoparticles showed differences in their structural and chemical characteristics, which were attributed to the presence of Cl− or OH− anions during the synthesis of the compound. The structure, morphology and oxidation state of the samples were studied by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy, respectively. In addition, theoretical calculations using cluster models were carried out to understand the possible phases formed of BST, the effect of the Sr incorporation and the possible presence of oxygen vacancies inside the BST structure.
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ABSTRACT The chemical and structural properties of Ba1−xSrxTiO3 (BST, x=0–1) nanoparticles synthesised via sol–gel–hydrothermal were analysed. Two types of salts of Ba (BaCl2 and Ba(OH)2) and Sr (SrCl2 and Sr(OH)2) as starting reactants... more
ABSTRACT The chemical and structural properties of Ba1−xSrxTiO3 (BST, x=0–1) nanoparticles synthesised via sol–gel–hydrothermal were analysed. Two types of salts of Ba (BaCl2 and Ba(OH)2) and Sr (SrCl2 and Sr(OH)2) as starting reactants were used to compare two synthesis methods. Chemical characterisation and oxidation states were obtained using X-ray photoelectron spectroscopy. Structural information was acquired by Raman spectroscopy, and calculations to obtain theoretical Raman spectra associated with the different formed phases of BST were performed for comparison. The results were consistent with the presence of oxygen vacancies in all of the compounds synthesised, but the use of hydroxide salts introduced a minor concentration of oxygen vacancies into the BST compounds. In addition, the presence of oxygen vacancies produced an increase of the intensity of first-order modes of vibration and a minor oxidation state of the Ti atom in the structure. Finally, the oxygen vacancies produced a distortion of the structure, inducing the existence of the non-perovskite phase.
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ABSTRACT Thermal programmed desorption (TPD) of hydrogen was performed in Pd and 5% Ru–Pd foils. The foils were inspected with Auger Electron Spectroscopy and Ar ion-sputter cleaned. Hydrogen dosage was 1000 L at 10− 3 Pa. Hydrogen... more
ABSTRACT Thermal programmed desorption (TPD) of hydrogen was performed in Pd and 5% Ru–Pd foils. The foils were inspected with Auger Electron Spectroscopy and Ar ion-sputter cleaned. Hydrogen dosage was 1000 L at 10− 3 Pa. Hydrogen evolving from Pd (TPD curves) was 12 times larger than for the 5% Ru–Pd foil. A 1.8 μm thick Ru–Pd surface alloy was grown on the surface of Pd and the TPD curves were very similar to those of the 5% Ru–Pd bulk alloy. The lattice parameter of the Pd fcc crystal structure decreased by 2.8%, when alloying with 5% Ru. The lattice parameter of the fcc cell of Pd increased by 2.3% when absorbing hydrogen. Hydrogen saturation amount at 4655 Pa was obtained using a quartz crystal microbalance in a 24 nm Pd film and in a 5% Ru–Pd film. A hydrogen–metal ratio (x = H/Pd) was calculated from these measurements yielding a ratio of xPd = 0.68 for Pd and xRuPd = 0.27 for ~ 2% Ru–Pd film.