Moro, María Belén (2017) Fases metaestables con estructura BCC en el sistema Mg-Nb-H. / Metaestable phases with BCC structure in the Mg-Nb-H. Maestría en Ingeniería, Universidad Nacional de Cuyo, Instituto Balseiro.
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Resumen en español
En esta tesis se estudian fases metaestables del sistema Mg-Nb con estructura cristalina bcc, sintetizadas por medio de molienda mecánica. La motivación del trabajo son las interesantes propiedades para almacenamiento de hidrógeno reportadas para el compuesto metaestable Mg_3Nb, obtenido en forma de película delgada. Los materiales se prepararon moliendo en atmósfera controlada mezclas Mg-Nb con distintas relaciones atómicas en el rango 0,5:1-3:1. Durante la síntesis se utilizaron estrategias combinadas de hidruración, deshidruración, y molienda para lograr una mezcla eficiente de los metales. Los materiales se caracterizaron por XRD, DSC, TG, TEM y SEM. De manera complementaria, se realizó el refinamiento de los difractogramas mediante el método de Rietveld. Asimismo, se estudió la interacción de las muestras con H2 utilizando técnicas volumétricas. El resultado principal del trabajo es la obtención de una solución sólida sustitucional Mg-Nb en el caso de la mezcla equimolar, que mantiene la estructura bcc del Nb. Esta fase tiene un parámetro de red (3,341 Å) mayor al del Nb metálico (3,302 Å), en acuerdo con el mayor radio atómico del Mg respecto al del Nb. La composición de esta solución se ha estimado en Mg_0,46Nb_0,54. Los materiales preparados con un mayor contenido de Mg muestran una fase bcc muy similar a la obtenida en el caso de la mezcla equimolar y un exceso de Mg, lo cual sugiere que no es posible incorporar más Mg en la estructura del Nb. Los materiales con defecto de magnesio presentan dos fases con estructura bcc y distinta proporción Mg:Nb, probablemente debidas a inhomogeneidad en la muestra. Las soluciones incorporan hidrógeno si se las mantiene a temperaturas en el rango 200-300 ºC y a presiones de H_2 de 6 MPa durante 3 días, dependiendo de la composición de la mezcla. Durante este proceso la estructura bcc se mantiene, pero aumenta el tamaño de celda. El contenido de hidrógeno absorbido por las soluciones es bajo, del orden del 0,3 % en peso en el caso de la mezcla equimolar hidrurada a 275 ºC. Los materiales con Mg en exceso no absorben hidrógeno, aún a temperaturas de 400 ºC; mientras que las mezclas con menor contenido de Mg son más reactivas, pudiendo hidrurarse la solución a 200 ºC. Tanto la solución sólida como su hidruro son metaestables, pero requieren temperaturas cercanas a 300 ºC para descomponerse.
Resumen en inglés
n this thesis, metastable phases of the Mg-Nb system with bcc crystalline structure synthesized by mechanical milling are studied. The motivation of the work are the interesting properties of hydrogen storage reported for the metastable compound Mg_3Nb, obtained as a thin film. The materials were prepared by milling Mg-Nb mixtures with atomic ratios in the 0.5:1-3:1 range in a controlled atmosphere. During the synthesis, combined strategies of hydriding, dehydriding and milling were used to achieve an efficient mixture of metals. The materials were characterized by XRD, DSC, TG, TEM, and SEM. In addition, the diffractograms were refined by the Rietveld method. The interaction of said samples with H2 was also studied, using volumetric techniques. The main result of the work is the synthesis of a solid substitutional solution Mg-Nb in the case of the equimolar mixture, which maintains the bcc structure of Nb. This phase has a lattice parameter (3.341 Å) larger than that of the metallic Nb (3.302 Å), in agreement with the larger atomic radius of Mg compared to Nb. The composition of this solution has been estimated to be Mg_0.46Nb_0.54. The materials prepared with a higher Mg content show a very similar bcc phase to the one obtained in the case of the equimolar mixture, and also an excess of Mg, suggesting that it is not possible to incorporate more Mg in the structure of Nb. Materials with lower magnesium content show two phases with bcc structure and different Mg:Nb ratio, probably due to inhomogeneity in the sample. The solid solutions incorporate hydrogen if maintained at temperatures in the range from 200 to 300 °C and at H_2 pressures of 6 MPa for 3 days, depending on the composition of the mixture. During this process the bcc structure is maintained, but the cell size increases. The hydrogen content absorbed by the solutions is low, of the order of 0.3 wt. % in the case of the equimolar mixture hydrided at 275 °C. Materials with excess of Mg do not absorb hydrogen, even at temperatures of 400 °C; wheareas the mixtures with lower Mg content are more reactive, and the solution can be hydrided at 200 °C. Both the solid solution and its hydride are metastable, but require temperatures close to 300 °C to decompose.
| Tipo de objeto: | Tesis (Maestría en Ingeniería) |
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| Palabras Clave: | Magnesium; Magnecso; Hydrogen; Hidrógeno; [Metastability; Metaestabilidad; Mechanical milling; Molienda mecánica; Hydrogen storage; Almacenamiento de hidrógeno; BCC structure] |
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| Materias: | Ingeniería > Ciencia de los materiales |
| Divisiones: | Aplicaciones de la energía nuclear > Tecnología de materiales y dispositivos > Fisicoquímica de materiales |
| Código ID: | 636 |
| Depositado Por: | Tamara Cárcamo |
| Depositado En: | 27 Oct 2017 15:14 |
| Última Modificación: | 30 Oct 2017 09:29 |
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