Departamento de Ciencias Naturales

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https://hdl.handle.net/20.500.12590/17953

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Browsing Departamento de Ciencias Naturales by Description "Trabajo de investigación"

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    Compressive creep behavior of zirconia/nickel oxide composites
    (International Committee on Composite Materials, 2017) Huamán Mamani, Fredy; Jiménez Melendo, Manuel
    Nickel oxide/zirconia composites with different NiO contensts have been fabricated by mechanical mixing of nickel oxide and zirconia powders and sintering at 1500 ºC for 10 h in air. The resulting microstructures have been characterized by scanning and transmission electron microscopy. The mechanical behaviour of the composites has been characterized by compressive tests at high temperatures between 1100 and 1350 ºC. For NiO contents above the percolation limit, the softer phase was found to be rate-controlling in oxidizing atmospheres, achieving extended steady states of deformation without macroscopic failure. By contrast, the overall creep strength of the composites decreased severely in reducing conditions. © 2017 International Committee on Composite Materials. All rights reserved.
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    Creep properties of polycrystalline Al2O3 – Based ceramic composites
    (International Committee on Composite Materials, 2017) Huamán Mamani, Fredy; Jiménez Melendo, Manuel
    Three-phase Al2O3 - ZrO2 - Er2O3 composites with the ternary eutectic composition were fabricated by a conventional solid-state reaction route starting from commercial powders of alumina, erbia and monoclinic zirconia. Bulk composites with relative densities higher than 98% were obtained after calcining at 1400 ºC for 10 h in air and sintering at 1500 °C in air for 10 h. X-ray diffraction and energy-dispersive X-ray spectroscopy showed the presence of the three expected phases: alumina, EAG and zirconia. The composites exhibit a homogeneous microstructure with equiaxed grains of about 1 µm in size. Mechanical tests were carried out in compression at temperatures between 1300 – 1450 °C in air. Extended steady states of deformation were attained without macroscopic damage. The stress exponent in such experimental conditions was close to 2, suggesting that grain boundary sliding is the main deformation mechanism, as found in other fine-grained ceramics. © 2017 International Committee on Composite Materials. All rights reserved.
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    Study of the Equilibrium, Kinetics, and Thermodynamics of Boron Removal from Waters with Commercial Magnesium Oxide
    (Hindawi Limited, 2018) Montalvo Andia, Javier; Yokoyama, Lidia; Cesar Teixeira, Luiz Alberto
    In the present work, the equilibrium, thermodynamics, and kinetics of boron removal from aqueous solutions by the adsorption on commercial magnesium oxide powder were studied in a batch reactor. The adsorption efficiency of boron removal increases with temperature from 25°C to 50°C. The experimental results were fitted to the Langmuir, Freundlich, and Dubinin-Radushkevich (DR) adsorption isotherm models. The Freundlich model provided the best fitting, and the maximum monolayer adsorption capacity of MgO was 36.11 mg·g-1. In addition, experimental kinetic data interpretations were attempted for the pseudo-first-order kinetic model and pseudo-second-order kinetic model. The results show that the pseudo-second-order kinetic model provides the best fit. Such result suggests that the adsorption process seems to occur in two stages due to the two straight slopes obtained through the application of the pseudo-first-order kinetic model, which is confirmed by the adjustment of the results to the pseudo-second-order model. The calculated activation energy (Ea) was 45.5 kJ·mol-1, and the values calculated for ?G°, ?H°, and ?S° were -4.16 kJ·mol-1, 21.7 kJ·mol-1, and 87.3 kJ·mol-1, respectively. These values confirm the spontaneous and endothermic nature of the adsorption process and indicated that the disorder increased at the solid-liquid interface. The results indicate that the controlling step of boron adsorption process on MgO is of a physical nature. © 2018 Javier Paul Montalvo Andia et al.