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Desde finales del siglo XX, la crisis ambiental ha tomado relevancia tanto en espacios académicos como en la sociedad. La utilización de combustibles fósiles y su consecuente emisión de gases de efecto invernadero, provocan una creciente inquietud a nivel mundial. Una solución a este problema, es la utilización de hidrógeno como vector energético, debido a su notable rendimiento. Sin embargo, el principal desafío radica en el almacenamiento y liberación eficiente para su posterior utilización. Con el objetivo de abordar esta problemática, la presente tesis doctoral se enfoca en el estudio de dos sistemas prometedores vinculados a esta cuestión. El primer sistema estudiado es hidruro de magnesio dopado con niobio (MgH2+Nb) con vacancias cargadas. Se analizaron los cambios estructurales, magnéticos y energéticos. En una segunda parte, se analizó el efecto de la adsorción de distintos metales alcalinos sobre nanotubos de carbono de pared simple (8,0) prístinos y con una vacancia de carbono. Los metales alcalinos aumentan la cantidad de sitios activos para la adsorción de hidrógeno. Un estudio detallado de la adsorción de hidrógeno en nanotubos de carbono, requiere un entendimiento completo del sustrato para lograr mejoras significativas en las propiedades de adsorción y así maximizar el potencial de almacenamiento. En este sistema se estudiaron las geometrías de adsorción, la naturaleza de los enlaces y la estructura electrónica. Todos los cálculos fueron realizados utilizando la teoría del funcional de la densidad (DFT) con spín polarizado implementados por el código VASP. / Since the end of the 20th century, the environmental crisis has taken on relevance both in academic spaces and in society. The use of fossil fuels and their consequent emission of greenhouse gases, cause a growing concern worldwide. A solution to this problem is the use of hydrogen as an energy vector, due to its remarkable performance. However, the main challenge lies in efficient storage and release for later use. In order to address this problem, this doctoral thesis focuses on the study of two promising systems related to this issue. The first system studied is magnesium hydride doped with niobium (MgH2+Nb) with charged vacancies. Structural, magnetic and energetic changes were analyzed. In a second part, the effect of the adsorption of different alkali metals on pristine single-walled carbon nanotubes (8,0) and with a carbon vacancy, was analyzed. Alkali metals increase the number of active sites for hydrogen adsorption. A detailed study of hydrogen adsorption on carbon nanotubes requires a thorough understanding of the substrate in order to achieve significant improvements in adsorption properties and thus maximize storage potential. In this system, the adsorption geometries, the nature of the bonds and the electronic structure were studied. All calculations were performed using spin polarized density functional theory (DFT) implemented by the VASP code.

The use of carbon nanotubes (CNTs) as a material for construction of sensors is a promising effort. This is due to some unique characteristics of CNTs. In recent years, strain sensors built from CNT composite films have been developed. This thesis study first proposed that the piezoresistive sensitivity of CNT composite films can be limited due to the presence of one of the constituent elements in the CNT composite films, that is, surfactant. CNT films free of surfactants were thus hypothesized to have a great promise to improve piezoresistive sensitivity. The motivation of this thesis study was to explore this promise.<p> This thesis presents an experimental study on Single-Wall CNT (SWNT) films free of surfactants. Such SWNT films are called pure SWNT films. The study has concluded: (1) the gauge factor of one layer SWNT film is much higher than that of CNT composite film; (2) the fabrication of multilayered pure CNT films is highly possible; (3) the gauge factor of multilayered pure SWNT films (10 layers and 0.8mg/ml concentration) can reach as high as 2.59 with non-linearity of 0.89% and repeatability of 0.1%, which outperforms the strain sensor built from CNT composite films; (4) the role of surfactants is indeed restrictive to piezoresistive response, and (5) the junction theory is likely applicable to pure SWNT film sensors.<p> The main contributions of this thesis study are: (1) the finding of a new type of strain sensors built from pure CNT films and (2) the development of a fabrication process for multilayered pure SWNT films.

The use of carbon nanotubes (CNTs) as a material for construction of sensors is a promising effort. This is due to some unique characteristics of CNTs. In recent years, strain sensors built from CNT composite films have been developed. This thesis study first proposed that the piezoresistive sensitivity of CNT composite films can be limited due to the presence of one of the constituent elements in the CNT composite films, that is, surfactant. CNT films free of surfactants were thus hypothesized to have a great promise to improve piezoresistive sensitivity. The motivation of this thesis study was to explore this promise.<p> This thesis presents an experimental study on Single-Wall CNT (SWNT) films free of surfactants. Such SWNT films are called pure SWNT films. The study has concluded: (1) the gauge factor of one layer SWNT film is much higher than that of CNT composite film; (2) the fabrication of multilayered pure CNT films is highly possible; (3) the gauge factor of multilayered pure SWNT films (10 layers and 0.8mg/ml concentration) can reach as high as 2.59 with non-linearity of 0.89% and repeatability of 0.1%, which outperforms the strain sensor built from CNT composite films; (4) the role of surfactants is indeed restrictive to piezoresistive response, and (5) the junction theory is likely applicable to pure SWNT film sensors.<p> The main contributions of this thesis study are: (1) the finding of a new type of strain sensors built from pure CNT films and (2) the development of a fabrication process for multilayered pure SWNT films.

Carbon nanotubes (CNTs) possess exceptional mechanical, thermal and electrical properties along their main axis, superior to those of most materials. These can be exploited on a macroscopic scale by assembling the CNTs into a fibre with the nanotubes preferentially oriented parallel to each other and to the fibre axis. CNT fibres can be produced continuously, directly from the gas phase during CNT synthesis by chemical vapour deposition (CVD), and spun at rates of up to 70 m/min. Their combination of outstanding mechanical, electrical and thermal properties and low density (1 g/cm3) makes CNT fibres a potential candidate for high-performance applications. The fibre specific strength and stiffness are typically 1 GPa/SG and 50 GPa/SG, respectively; however, at small gauge lengths (> 2mm) they also show values of 6-9 GPa/SG strength and 180-390 GPa/SG stiffness. The electrical conductivity of the CNT fibres is approximately 8 x 10 5 S/m and their thermal conductivity of the order of 50 W/mK. These properties derive from the long length, high alignment and efficient packing of the nanotubes in the fibre. Further improvements to the fibre structure and properties at long gauge lengths are possible through removal of impurities from the fibre by annealing.

No description available.

Carbon nanotube templated microfabrication (CNT-M) is a term describing a grouping of processes where carbon nanotubes (CNTs) serve a structural role in the fabrication of a material or device. In its basic form, CNT-M is comprised of two steps: produce a template made from carbon nanotubes and infiltrate the porous template with an additional material. Vertically aligned carbon nanotube (VACNT) templates can be grown to heights ranging from microns to millimeters and lithographically patterned to a desired form. Deposition of an existing thin film material onto a CNT template will coat all template surfaces and can produce a near solid material with dimensions on the millimeter scale with resulting material properties coming primarily from the thin film. Progress within CNT-M falls broadly within one of two categories: control of the CNT template's properties and form, or control of infiltration and new materials.Three-dimensional CNT templates were developed to allow patterned multilayer VACNT structures. In one embodiment, VACNTs were grown below an existing, patterned and capillary-formed VACNT structure by reusing the original catalyst in combination with newly deposited catalyst to create a CNT-based microneedle array on a VACNT support. In another embodiment, VACNTs were mechanically coupled from the initial stages of growth to create a smooth, low porosity surface on which a secondary, patterned CNT forest was grown using standard film deposition and lithographic techniques.A microfabrication compatible thermal barrier was produced using CNTs as a sacrificial template for silicon oxide. The resulting thermal barrier exhibited a thermal conductivity that could be tuned across 2 orders of magnitude based on the degree to which the sacrificial template was removed. Carbon infiltrated carbon nanotubes (CI-CNTs) were produced that exhibited a Young's modulus ranging from 5GPa to 26GPa based on controlled process parameters. Porosity, centroid position, and the second moment of area was calculated from SEM images of CI-CNT structures using an automatic pore identification technique. The porosity results suprisingly show little to no porosity gradient across the width of the structure and a nearly linear increase in porosity from the top to bottom. This work advances the understanding of existing CNT-M processes and demonstrates novel techniques for producing future CNT templates.

No description available.

Thermal transport behavior of austenite stainless steel stripe (304) and the carbon nano-tubes (CNTs) films of varying thickness are studied using a micropipette thermal sensor. Micropipette sensors of various tip sizes were fabricated and tested for the sensitivity and reliability. The sensitivity deviated by 0.11 for a batch of pipette coated under same physical vapor deposition (PVD) setting without being affected by a tip size. Annealing, rubber coating and the vertical landing test of the pipette sensor proved to be promising in increasing the reliability and durability of the pipette sensors. A micro stripe (80µm × 6µm × 0.6µm) of stainless steel, fabricated using focused ion beam (FIB) machining, was characterized whose thermal conductivity was determined to be 14.9 W/m-K at room temperature. Similarly, the thermal characterization of CNT films showed the decreasing tendency in the thermal transport behavior with the increase in the film thickness.

The study presented in this thesis was aimed to gain the fundamental knowledge regarding the mechanism of H2S measurement in sera by using carbon nanotubes (CNT) and fluorescence response. Characterization techniques such as Raman spectroscopy, X-ray absorption spectroscopy (XAS) and confocal laser scanning microscopy (CLSM) were employed to achieve this goal. The model system used for this study was composed of H2S, distilled water, two major serum proteins (albumin and globulin), serum, hemoglobin, and CNT. The results of this study showed that: (1) Two major serum proteins (albumin and globulin) are physically adsorbed on the sidewall of the CNTs; while H2S is adsorbed on the defect site of the CNTs. (2) Presence of the proteins on the CNTs did not affect the CNTs adsorption of H2S. (3) Using CLSM with the incident wavelength of 514 nm and the emission wavelength of 530 to 580 nm to acquire the fluorescence response of the H2S adsorbed on the CNTs is a reliable approach to measure H2S in sera. (4) Single-wall carbon nanotubes (SWNTs) outperform multi-wall carbon nanotubes (MWNTs) in measurement sensitivity. (5) Presence of hemoglobin in a H2S solution did not affect the measurement of H2S with CNTs and CLMS. The study described in this thesis has provided new knowledge of the interaction behaviors of CNTs with H2S and major proteins in sera along with the mechanism which governs these behaviors. Such knowledge is very useful to further advance the CNT approach to sensing H2S in sera and water solution and to further extend the approach to sensing H2S in other mammalian tissues such as blood.