Browsing by Author "Roble, M."

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    Effect of O2 pressure on the hydrothermal growth of CuO hierarchical microstructures: characterization and hydrogen adsorption capacity
    (2017) Roble, M.; Diaz-Droguett, D.
    In the present study, the effect of injected O-2 pressure to a hydrothermal reactor on the growth, morphology, structure and hydrogen adsorption properties of CuO hierarchical microstructures was investigated. The synthesis consisted of a simple one-step hydrothermal reaction using copper chloride and aqueous ammonia. Hydrothermal reactions were carried at 200 degrees C at reaction times of 1 and 12 h injecting at the beginning of the synthesis O-2 pressures of 0, 30 or 60 bar. The samples were characterized by SEM and EDS for morphological and chemical information, XRD for determination of crystalline phases, lattice parameters and crystallinity grade using Rietveld refinement. Specific surface area was determined using BET method from nitrogen adsorption/desorption isotherm measurements. Hydrogen adsorption capacities at 20 degrees C were studied for key samples using the quartz crystal microbalance technique under gas exposure pressures between 3 and 100 Torr. It was found that the samples were of monoclinic CuO phase consisting mainly of sphere-like hierarchical microstructures of different average sizes assembled by sheets. The effect of the reactor pressure was mainly seen on average size of the CuO microstructures due to changes in the growth rate, specifically during the first stages of the hydrothermal reaction. Reactor pressure induced changes on the crystallinity of the samples were determined but no correlation was stated. Hydrogen adsorption capacities of studied samples measured at 100 Torr ranged from 0.27 to 0.6 wt%, indicating a good performance as compared with other reported values. Oxygen deficiency in CuO can influence on the hydrogen adsorption capacity.
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    Growth, characterization and thermo-mechanical analysis of Al/Al2O3 core/shell nanoparticles obtained under H2 atmosphere
    (2020) Diaz-Droguett, D. E.; Ramos-Moore, E.; Roble, M.; Muecklich, Frank
    In this work, growth, characterization and thermo-mechanical behavior of Al/Al2O3 core/shell nanoparticles (NP) is performed. The growth was carried out by gas condensation methods using H-2 as carrier gas at a pressure of 100 Pa and with a temperature of the evaporation source of 1315 degrees C. The prepared NP were characterized by Energy Dispersive X-ray Spectroscopy for chemical information, transmission electron microscopy for morphological study, and electron diffraction patterns for structural information. The prepared Al NP exhibit a thin Al2O3 passivating oxide shell due to the air exposure when removed from the preparation vacuum chamber. Afterwards, the growth of the Al2O3 oxide shell of the Al NP and the thermo-mechanical interaction between this growing oxide shell and its Al core was studied by performing in-situ thermal X-ray diffraction from RT up to 505 degrees C. Coefficients of thermal expansion of both Al core and Al2O3 oxide shell were obtained by means of X-ray strain analysis. It was found that the thickness of the Al2O3 shell increases with temperature and the thermal stress induced in the system increases linearly with temperature. Our results highlight that these NP overcome higher values of fracture toughness compared with Al2O3 NP used in micro-nano composites, thus improving their mechanical properties for nanofluid applications.
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    Structural, optoelectronic and photo-thermoelectric properties of crystalline alloy CuAlxFe1-xO2 delafossite oxide materials
    (2021) Wheatley, R. A.; Roble, M.; Gence, L.; Acuna, C.; Rojas-Aedo, R.; Hidalgo-Rojas, D.; Guzman-De La Cerda, D. E.; Vojkovic, S.; Seifert, B.; Wallentowitz, S.; Volkmann, U. G.; Diaz-Droguett, D. E.
    CuFeO2 and CuAlO2 are attractive candidate materials for solar energy harvesting applications such as photocatalysis and photovoltaics. This work describes the structural, optoelectronic, thermal and electric properties of alloyed CuAlxFe1-xO2 Delafossite material synthesized using solid-state sintering techniques. The alloyed samples of CuAlxFe1-xO2 Delafossite oxide consisted of substitution of Fe for Al ranging from x = 0.01 to x = 0.99. The inclusion of Al in low concentrations affects the crystallization rate during solid-state synthesis, dramatically changing the resultant sample morphologies. The addition of dilute amounts of Al also greatly improves the conductivity of the material to a maximum of 3.18 Scm(-1). The material absorption edge shows strong changes over the alloying range. The more highly conducting samples show a strong photoinduced thermoelectric response to Vis-NIR illumination. (C) 2020 Published by Elsevier B.V.
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    Study of thermally induced phase transformations and microstructural changes in porous MoO3 nanostructures for the design of tribological additives
    (2021) Diaz-Droguett, D. E.; Ramos-Moore, E.; Roble, M.; Muecklich, Frank
    The thermal behavior of MoO3-nanostructures is relevant for the design of tribological additives containing Mo-oxides. We report on thermal behavior of porous MoO3-nanostructures analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). The nanostructures were grown by gas condensation under H-2. Our results revealed 3 stages of phases behavior as a function of temperature. We found dehydration of MoO3-0,33 H2O after 180 degrees C followed by the formation of beta-MoO3 after 230 degrees C, then to alpha-MoO3 after 280 degrees C, and finally formation of suboxide gamma-Mo4O11 at similar to 330 degrees C. The formation of the latter is mainly due to the crystallization and reduction of the porous oxide matrix. Our results highlight that both thermal and chemical processes must be taken into account when designing tribological additives that contain MoO3. (C) 2021 Elsevier B.V. All rights reserved.