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Interplay between the magnetic and magneto-transport properties of 3D interconnected nanowire networks
Get PDF""We have explored the interplay between the magnetic and magneto-transport properties of 3D interconnected nanowire networks made of various magnetic metals by electrodeposition into nanoporous membranes with crossed channels and controlled topology. The close relationship between their magnetic and structural properties has a direct impact on their magneto-transport behavior. In order to accurately and reliably describe the effective magnetic anisotropy and anisotropic magnetoresistance, an analytical model inherent to the topology of 3D nanowire networks is proposed and validated. The feasibility to obtain magneto-transport responses in nanowire network films based on interconnected nanowires makes them very attractive for the development of mechanically stable superstructures that are suitable for potential technological applications.
Complete stability region of PD controllers for TCP/AQM networks
Get PDF"This paper addresses the stabilization problem of delay models of Transmission Control Protocol/Active Queue Management (TCP/AQM) by using a Proportional- Derivative (PD) controller as AQM strategy. The complete set of PD controllers that exponentially stabilizes the linearization is determined in counterpart with the existing works in the literature which only give an estimate of it. Additionally, a simple procedure for determining a non-fragile PD controller that admits controller coefficient perturbations is provided.
Strange attractors generated by a fractional order switching system and its topological horseshoe
Get PDF"Chaos generation in a new fractional order unstable dissipative system with only two equilibrium points is reported. Based on the integer version of an unstable dissipative system (UDS) and using the same systems parameters, chaos behavior is observed with an order less than three, i.e., 2.85. The fractional order can be decreased as low as 2.4 varying the eigenvalues of the fractional UDS in accordance with a switching law that fulfills the asymptotic stability theorem for fractional systems. The largest Lyapunov exponent is computed from the numerical time series in order to prove the chaotic regime. Besides, the presence of chaos is also verified obtaining the topological horseshoe. That topological proof guarantees the chaos generation in the proposed fractional order switching system avoiding the possible numerical bias of Lyapunov exponents. Finally, an electronic circuit is designed to synthesize this fractional order chaotic system.
Internal architecture of the Tuxtla volcanic field, Veracruz, Mexico, inferred from gravity and magnetic data
Get PDF"The Tuxtla Volcanic Field (TVF) is a basaltic volcanic field emerging from the plains of the western margin of the Gulf of Mexico in the Mexican State of Veracruz. Separated by hundreds of kilometers from the Trans-Mexican Volcanic Belt to the NW and the Chiapanecan Volcanic Arc to the SE, it stands detached not only in location but also in the composition of its rocks, which are predominantly alkaline. These characteristics make its origin somewhat puzzling. Furthermore, one of the large volcanoes of the field, San Martin Tuxtla, underwent an eruptive period in historical times (CE 1793). Such volcanic activity conveys particular importance to the study of the TVF from the perspective of volcanology and hazard assessment. Despite the above circumstances, few investigations about its internal structure have been reported. In this work, we present analyses of gravity and aeromagnetic data obtained from different sources. We present the complete Bouguer anomaly of the area and its separation into regional and residual components. The aeromagnetic data were processed to yield the reduction to the pole, the analytic signal, and the upward continuation to complete the interpretation of the gravity analyses. Three-dimensional density models of the regional and residual anomalies were obtained by inversion of the gravity signal adding the response of rectangular prisms at the nodes of a regular grid. We obtained a body with a somewhat flattened top at 16 km below sea level from the inversion of the regional. Three separate slender bodies with tops 6 km deep were obtained from the inversion of the residual. The gravity and magnetic anomalies, as well as the inferred source bodies that produce those geophysical anomalies, lie between the Sontecomapan and Catemaco faults, which are proposed as flower structures associated with an inferred deep-seated fault termed the Veracruz Fault. These fault systems along with magma intrusion at the lower crust are necessary features to understand the origin and structure of the TVF.
Temperature dependence of sensors based on silver-decorated nitrogen-doped multiwalled carbon nanotubes
Get PDF"Vapor sensors are easily fabricated onto alumina substrates using foils of silver-decorated nitrogen-doped multiwalled carbon nanotubes (CNX-MWNTs-Ag) as active sensing material. The vapor sensors are tested using carbon disulfide, acetone, ethanol, and chloroform vapors. The CNX-MWNTs are produced by chemical vapor deposition process and then decorated with 14 nm Ag nanoparticles (Ag-NPs). The samples are characterized using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Our results demonstrate that Ag-decorated CNX-MWNTs exhibit a better response and sensitivity when compared with pristine CNX-MWNTs based sensors, making them promising candidates for air-pollutants environmental monitoring. The temperature effect on the sensor performance is also studied; we found that the detection mechanism could be tuned from physisorption, at room temperature, to chemisorption at higher working temperature. Finally, first-principles density functional calculations are carried out to understand the interactions between the systems involved in the sensors, finding good agreement between experimental results and the theoretical approach.
Fotoánodos modificados con óxido de grafeno reducido para mejorar el rendimiento fotoelectrocatalítico de B-TiO2 bajo luz visible
Get PDF"The effect of reduced graphene oxide (rGO) content in boron-modified TiO2 nanocrystalline films on their photocatalytic activity in phenol oxidation is investigated. Visible-light-active TiO2 modified photoanodes were prepared by incorporating graphene sheets into the sol-gel reaction of B-TiO2, followed by depositing the reaction products on 304 stainless steel plates by dip-coating technique. Thin films obtained by in situ sol-gel synthesis were characterized by FESEM, GIXRD and UV–vis diffuse reflectance spectroscopy. FESEM examination showed cracked films due to the tensile stress generated by solvent evaporation. GIXRD results showed that boron in the films inhibits the growth of crystallites. Comparing to unmodified TiO2, B-TiO2/rGO showed a red shift in the band gap. The potentiodynamic anodic polarization measurements showed that graphene incorporation improved the photogenerated electron transport within the film, hence increasing the photocurrent. These enhancements are explained on the basis of the ability of graphene in promoting the charge carrier separation by transferring the photogenerated electrons from the illuminated photoanode to the substrate. The film B-TiO2/rGO obtained from the sol solution containing 0.03 wt/v% boron and 3 wt/v% graphene exhibited the highest photocurrent, which was 30 times larger compared with the photocurrent of TiO2 film.""Se investiga el efecto del contenido de óxido de grafeno reducido (rGO) en películas de TiO2 modificadas con boro sobre su actividad fotocatalítica en la oxidación de fenol. Fotoánodos modificados de TiO2 activos a la luz visible fueron preparados incorporando hojas de grafeno en la reacción sol-gel de B-TiO2, seguido por el depósito de los productos de la reacción sobre láminas de acero inoxidable 304 por la técnica dip-coating. Las películas delgadas obtenidas por síntesis sol-gel in-situ fueron caracterizadas por FESEM, GIXRD y espectroscopia de reflectancia difusa UV-vis. La observación por FESEM mostró películas agrietadas debido al estrés mecánico generado por la evaporación del solvente. Los resultados de GIXRD mostraron que el boro en las películas inhibe el tamaño de los cristalitos. Comparando con el TiO2, el dióxido de titanio modificado presentó un desplazamiento de la banda de energía prohibida hacia el rojo. Las mediciones de polarización anódica potenciodinámica mostraron que la incorporación de grafeno mejora el transporte de electrones fotogenerados dentro de las películas compuestas incrementando así la fotocorriente. Estas mejoras se explican en base a la habilidad del grafeno para facilitar la separación de portadores de carga, transfiriendo los electrones fotogenerados desde la película iluminada de B-TiO2 hasta el sustrato. La película compuesta B-TiO2/rGO obtenida a partir de la solución con 0.03 % p/v de boro y 3 % p/v de grafeno presentó la fotocorriente más alta, la cual fue 30 veces mayor comparada con la fotocorriente de la película de TiO2.
Síntesis de materiales a base de renio y sus propiedades catalíticas en la producción de hidrógeno a partir de biomasa
Get PDF"En este trabajo experimental se sintetizo la fase de carburo de renio (ReC640) y diboruro de renio (ReB640) por molienda mecánica a partir de mezclas de los elementos químicos constitutivos. La fase de Re2C fue obtenida a 640 minutos de molienda de una mezcla molar 2:1 Re:C en condiciones de baja temperatura y presión atmosférica utilizando medio de molienda y tazón de carburo de tungsteno. Ésta es la primera vez que se obtiene una fase de carburo de renio en condiciones de baja temperatura y presión atmosférica. También se sintetizo micropartículas octaédricas de diboruro de renio a baja temperatura y presión atmosférica utilizando medio de molienda y tazón de carburo de tungsteno durante 640 min de tiempo de molienda de una mezcla de molar 2:1 B:Re. 1160 minutos menos que lo reportado por otros autores. Los materiales fueron probados en la producción catalítica de hidrógeno a partir de la descomposición térmica de endocarpo de coco a 500 ºC. El catalizador ReB2 registro una máxima producción de hidrógeno de 0.72 mgH para una masa de 100 mg, mientras que el material Re2C registro su máxima producción en relación con la masa de catalizador de 1.91 mgH para 50 mg, que es 5.3 veces más que la mayor producida con el catalizador ReB2 y 5.02 veces más que la producción de hidrógeno a partir de endocarpo de coco solamente por el efecto de la temperatura (500 ºC, 0.38 mgH). El Re2C es un catalizador para su uso en la producción de hidrógeno a partir de biomasa. Los materiales se caracterizaron por difracción de rayos-X (XRD), microscopia electrónica de barrido (SEM), espectroscopia de dispersión de energía (EDS), análisis termogravimétrico (TGA), espectroscopia de infrarrojo (FTIR) y propiedades superficiales (BET).""In this experimental work are synthesized the phases of rhenium carbide (ReC640) and rhenium diboride (ReB640) by mechanical milling from the constituent elements. Re2C was obtained at 640 minutes of grinding from a molar mixture Re:C = 2:1, under low temperature and atmospheric pressure using grinding media and tungsten carbide bowl. This is the first time a rhenium carbide phase is obtained at low temperature and at atmospheric pressure. The ReB2 phase is synthesized octahedral shaped microparticles at low temperature and atmospheric pressure during 640 min milling time of a molar mixture B:Re = 2:1 using grinding media and tungsten carbide bowl. The reaction time used here is 1160 minutes less than that reported by other authors. The materials were tested in catalytic reaction of hydrogen production from decomposition of coconut endocarp at 500 °C. 100 mg of catalyst ReB2 achieved 0.72mgH hydrogen production, however, 50 mg of catalyst Re2C achieved 1.91mgH. Hydrogen production by Re2C is 5.3 times that the hydrogen obtained by ReB2 and 5.02 times that the hydrogen production from coconut endocarp without catalyst at 500 °C (0.38 mgH). The Re2C is an excellent catalyst for use in the production of hydrogen from biomass. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), Fourier transformed of infrared radiation (FTIR) and surface properties (BET).
Nuclear localization of the dehydrin OpsDHN1 is determined by histidine-rich motif
Get PDF"The cactus OpsDHN1 dehydrin belongs to a large family of disordered and highly hydrophilic proteins known as Late Embryogenesis Abundant (LEA) proteins, which accumulate during the late stages of embryogenesis and in response to abiotic stresses. Herein, we present the in vivo OpsDHN1 subcellular localization by N-terminal GFP translational fusion; our results revealed a cytoplasmic and nuclear localization of the GFP::OpsDHN1 protein in Nicotiana benthamiana epidermal cells. In addition, dimer assembly of OpsDHN1 in planta using a Bimolecular Fluorescence Complementation (BiFC) approach was demonstrated. In order to understand the in vivo role of the histidine-rich motif, the OpsDHN1 - Delta His version was produced and assayed for its subcellular localization and dimer capability by GFP fusion and BiFC assays, respectively. We found that deletion of the OpsDHN1 histidine-rich motif restricted its localization to cytoplasm, but did not affect dimer formation. In addition, the deletion of the S-segment in the OpsDHN1 protein affected its nuclear localization. Our data suggest that the deletion of histidine-rich motif and S-segment show similar effects, preventing OpsDHN1 from getting into the nucleus. Based on these results, the histidine-rich motif is proposed as a targeting element for OpsDHN1 nuclear localization.
Estructura-evolución de un modelo de sistema dinámico de terremotos: análisis de correlaciones ocultas de largo alcance
Get PDF"Este trabajo se apoya en el análisis-comparación de las series de tiempo generadas por un modelo dinámico de terremotos el cual se presenta como un sistema no lineal de tercer orden que acopla leyes de fricción y heterogeneidades del medio. El análisis-comparación de la estructura del modelo se realiza con tres métodos de análisis de series de tiempo que buscan correlaciones ocultas de largo alcance y además proporcionan información de la estructura-evolución del sistema dinámico. Las series de tiempo analizadas son las generadas por el modelo de Fricción Dieterich-Ruina-Stribeck donde ejemplica los desplazamientos relativos entre los bloques tectónicos en contacto. El primer método de análisis es la transformada ondoleta (TO) que da información de la distribución de la energía en los diferentes niveles ondoleta, el segundo método es el análisis de fuctuaciones sin tendencia (DFA por sus siglas en ingles) que da información de la estructura-comportamiento de la serie de tiempo y por último el exponente de Hurst que se utiliza para determinar la persistencia de señales además investiga el comportamiento caótico o auto-similar de series de tiempo.
Compósitos a base de grafeno oxidado: propiedades adsortivas y catalíticas
Get PDFTesis (Doctorado en Nanociencias y Materiales)"En el presente trabajo de investigación se sintetizaron compósitos a base de óxido de grafeno empleando procesos hidrotérmicos asistidos por microondas (HTAM). Inicialmente, se sintetizó óxido de grafeno reducido (RGO) a partir de la oxidación de grafito prístino por el método de Hummers Mejorado, teniendo la ventaja de disminuir la cantidad de ácidos utilizados en la oxidación. La expansión-exfoliación y reducción de las capas de óxido de grafito se realiza vía HTAM. La exfoliación resultante fue turbostática produciendo capas delgadas hasta 3 micras y corrugadas, debido a la presencia de dominios sp2-sp3 en las capas. La reducción modifica la química superficial de la capas, con grupos funcionales remanentes tales como hidroxilos, carbonilos y epóxidos. El RGO resultante presenta alta higroscopicidad, disminución en el número de capas, sin embargo, esto produce pliegues en los bordes de las capas, debido a su condición turbostática pero con mejor calidad que el RGO comercial. Para estabilizar las capas de RGO, se sintetizaron compósitos a base de este material, el primer compósito fue con nano-esferas de SiO2 (RGO-SNS). La técnica hidrotérmica utilizada favorece una alta dispersión de las nano-esferas de SiO2 sobre las capas de RGO. La condición estructural turbostática de RGO facilita una mejor dispersión y enlace de las nano-esferas sobre y entre las capas de RGO. El área superficial del compósito así como la presencia de los grupos funcionales confieren al material compósito, una alta capacidad de adsorción en la remoción de Rodamina B (RhB). Los experimentos de reversibilidad y reutilización del compósito muestran una fuerte interacción con el contaminante. Para evaluar sus propiedades reductivas se utilizó hidracina como fuente de electrones, alcanzando la reducción de RhB en 2 horas. Además, las propiedades adsortivas de RGO-SNS se evaluaron con azul de metileno (AzM) como contaminante modelo. Las isotermas resultantes se ajustan al modelo de Langmuir, es decir, la interacción es mediada por grupos funcionales en la superficie del compósito. Un segundo compósito fue obtenido mediante el crecimiento vía HTAM de nanoestructuras de CeO2 sobre las capas de RGO (RGO-CeO2). Los análisis de estructura y morfología muestran la formación de nano-cubos y nano-barras de CeO2, los cuales presentaron una estructura cúbica fcc tipo Fluorita.""In this doctoral thesis, the microwave-assisted hydrothermal processes (MAHP) to prepare some composites based on graphene oxide has been used. Initially, the reduced graphene oxide (RGO) was synthesized from the oxidation of pristine graphite through the Hummers Improved method. This has the advantage of decreasing the amount of acids oxidation, expansion-exfoliation, and chemical reduction of layers the graphite oxide sheets during the MAHP. We noticed that (i) exfoliation was turbostatic producing thin and corrugated layers until a length of 3 microns, because of the presence of sp2-sp3 domains in the layers, (ii) the reduction modifies the surface chemistry of the layers, leaving functional groups, such as hydroxyl, epoxides and carbonyls on the surface and at the edges of RGO sheets, (iii) the obtained RGO sheets have high hygroscopicity and a reduced number of layers. On the other hand, the reduction processes generates folds in the edges of the layers due to the turbostatic condition, but still the produced sheets are of higher quality than the commercial RGO sheets. In order to stabilize the RGO layers, a composite with SiO2 nanospheres has been prepared (RGO-SNS). The hydrothermal technique produces in this case a high dispersion of the SiO2 nanospheres on the RGO layers. Because the turbostatic arrangement facilitates better dispersion and bonding of SiO2 nanospheres on RGO surface, the SiO2 nanospheres are encountered at edges and between sheets. The high surface area of SiO2 and the functional groups of RGO make of this material a composite with high adsorption capacity, because it is able to remove 99% of Rhodamine B (RhB). Reversibility and recycle experiments prove a strong interaction with the contaminant. When hydrazine is used in our experiments as an electron donor, we have obtained the elimination of RhB 5 ppm in about 2 hours. We have also used two mathematical models to determine the adsorption properties of the RGO-SNS composite. Methylene blue (AzM) as contaminant model has been used. It has been found that the reaction could be adjusted to a Langmuir isotherm model, i.e., the interaction is mediated by the functional groups on the surface of RGO-SNS. A second composite has been obtained by growing via MAHP of CeO2 nanostructures over the RGO layers that we call RGO-CeO2. By structure and morphology analyses, we have determined the formation of nanocubes and nanorods of CeO2.