[Introduction To Partial Differential Equations By Sankara Rao 40

[Christel Malden]

Storage of hydrogen in metal hydride alloys is a promising technology which offers high potential storage density in a safe solid state. A major factor to be considered for the use of these alloys in storage applications is the significant amount of heat which is released during the exothermic hydrogen absorption process, as well as the consequent need for effective cooling methods. Various difficulties are involved in large-scale experimentation of these phenomena in storage vessels. Therefore numerical analysis of simulation models is a more practical and effective approach. The thermal conductivity of a metal hydride bed has a major impact on its thermal profile during the hydrogen absorption process. Because of the complex nature of the metal hydride powders, an accurate simulation model requires the use of an effective thermal conductivity value (Kef) which reflects the conductivity of the bed as a whole. This value takes into account both the metal hydride material itself and the gas occupying the spaces between the powder particles. In this study, we incorporate experimentally obtained Kef values for Ti-Cr-V and Ti-Cr-V-Fe solid solutions into numerical model simulations to predict the heating mechanisms and temperature profiles for various bed formations of these alloys. The results are then used as a reference to define appropriate parameters for simulation models which take the hydride powder and gas elements into account separately, in order to treat the metal hydride beds more accurately as porous media. Based on the resulting thermal profiles, a number of possible cooling facilitation methods and their effects on the alloy bed are investigated through additional numerical simulations.We were able to conclude through these simulations that higher effective conductivities result in significantly enhanced cooling behavior for most configurations of Ti-Cr-V and Ti-Cr-V-Fe beds. Consequently, a number of methods are proposed to increase the effective thermal conductivity Kef, and thereby facilitate the cooling of the beds.

Yun Liu 1 2 , Houria Kabbour 3 , Craig Brown 1 4 , Dan Neumann 1 , Channing Ahn 3
1 Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Department of Materials Science and Engineering, University of Maryland, College Park, Maryland, United States, 3 Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, United States, 4 Indiana University Cyclotron Facility, Indiana University, Bloomington, Indiana, United States

introduction to partial differential equations by sankara rao 40

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Metal-organic frameworks (MOFs) are promising hydrogen storage candidates due to their high surface areas. To achieve technologically relevant levels, the hydrogen packing density as well as the total available surface area are critical metrics. MOF-74 is used as a model material showing significant packing density. The hydrogen adsorption sites in MOF-74 are identified for the first time using neutron powder diffraction and a shorter H2-H2 interaction distance is observed than is expected given the nearest neighbor distance of solid H2. Of significance is the presence of coordinatively unsaturated metal centers. Inelastic neutron scattering spectra measured at different temperatures reveal large binding energy differences between some adsorption sites. Moreover, the adsorbed hydrogen molecules form a one-dimensional nanotube-like structure. These results extend our insights into physisorbed hydrogen molecules in micro-porous media including MOFs, carbon nanotubes/nanohorns and amorphous carbons.

Kouji Sakaki 1 , Kenji Iwase 1 , Yumiko Nakamura 1 , Yasuharu Shirai 2 , Etsuo Akiba 1
1 Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Japan, 2 Department of Materials Science and Engineering, Osaka University, Suita Japan

Ti-based BCC alloys developed by Iba and Akiba in 1995 [1] have twice larger gravimetric hydrogen capacity than that in conventional hydrogen storage alloys such as AB5 and AB2 type alloys. Ti-based BCC alloys are one of the candidates for hydrogen storage media in fuel cell vehicles. Although these have higher gravimetric density of hydrogen, the decrease of capacity with cycles should be improved for the practical use. Our previous results for AB5 alloys suggested that the concentration of the lattice defects introduced by hydrogenation is close related with the cyclability [2]. In this study, we investigated the introduction of the lattice defects in Ti45Cr55-xMox (x=15, 30) alloys and their thermal stability by the positron lifetime measurement.In-situ positron lifetime measurement showed that lattice defects started to be introduced even in the hydrogen solid solution region and they remained after dehydrogenation. Further hydrogenation to hydride-phase region increased the concentration of lattice defect. The annealing experiments showed that introduced lattice defects were vacancy and dislocation. The onset temperatures of their migration were 573 and 1173 K, respectively. They were completely recovered around 773 and 1573K, respectively.The release behavior of the residual hydrogen that is settled down in the activated sample even after evacuation was investigated by TG-DTA measurement in Ti45Cr25Mo30. The release of the residual hydrogen started around 550 K and then was completed below 773 K. This temperature region well agrees with the vacancy migration temperature. It suggests that the vacancy introduced by hydrogenation is coupled with the residual hydrogen. Similar result has been reported in LaNi5 alloy [3].We found the reversible phenomenon of vacancy introduction and recovery during hydrogenation and dehydrogenation in LaNi4.93Sn0.27 [2]. If we appended this reversible property into Ti-based BCC alloys, we could dramatically improve the hydrogen storage capacity and degradation behaviors of Ti-based BCC alloys.[1] H. Iba and E. Akiba: J. Alloys Compd. 231 (1995) 508.[2] Kouji Sakaki, Ryosuke Date, Masataka Mizuno, Hideki Araki, Yumiko Nakamura, Yasuharu Shirai, Robert C. Bowman, Jr. and Etsuo Akiba: to be published.[3] K. Sakaki, H. Araki and Y. Shirai: Mater. Trans. 43 (2002) 1494.

Hiroyuki Oguchi 1 , Ichiro Takeuchi 1 , Daniel Josell 2 , Edwin Heilweil 2 , Leonid Bendersky 2
1 Department of Materials Science and Engineering, University of Maryland, College Park, Maryland, United States, 2 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States

Hydrogen storage is one of the stumbling blocks for the practical use of hydrogen as a new source of energy for transportation. Absorption and desorption of hydrogen by a storage material depends on the materials precise composition and microstructural state. Combinatorial thin films with continuously changing composition provide an opportunity for studying a wide range of compositions and microstructures (amorphous, nanocrystalline, single crystal, multiphases) on a single substrate. Here we report preparation, characterization and hydrogenation of MgxNi1-x thin films with a Pd overlayer. Mg-rich MgxNi1-x, has good gravimetric density of hydrogen. Capping the film with a Pd layer is known to both prevent oxidization and catalyze hydrogenation reaction. These 200 nm-thick, square-shape films have compositional variation in one in-plane direction and variation in Pd thickness (from 0 to 20 nm) in the other direction. Therefore rapid study of hydrogen absorption/desorption properties of various compositions with various thicknesses of Pd catalytic layer is possible in just one experiment. Structures of the films were characterized by scanning x-ray and cross-sectional TEM. Hydrogen absorption/desorption of the films was monitored with an infrared (IR) CCD camera that could image the full area of the substrate. The observed changes in infrared intensity were attributed to changing electronic states of the MgxNi1-x thin films upon accommodation of hydrogen. In the study reaction temperature and kinetics dependence on the alloy composition and Pd layer thickness were obtained.

A Mg-based composite material containing 2 wt.% of multiwall carbon nanotubes (MWCNT) was synthesized by hot isostatic pressing of a mixture of Mg powder with MWCNT. In addition, the as-sintered composite was processed by one or two passes of equal channel angular pressing (ECAP). The hydrogen storage properties of the prepared materials were determined by volumetric method and compared with those of pure Mg. It was found that addition of MWCNT to Mg eliminates the pressure hysteresis and increases the slope of the pressure plateau in the "pressure-composition" isotherms measured at 573 K. The kinetics of hydrogen desorption is also significantly enhanced. The hydrogen desorption pressure in the middle of plateau region for the as-sintered composite is by about 50% higher than that for pure Mg. Surprisingly, ECAP processing of the as-sintered composite lowers hydrogen desorption pressure, although it still remains higher than for pure Mg. This is in contrast with the results of previous studies in which it was shown that ECAP processing of Mg-based alloys increases hydrogen desorption pressure. Transmission electron microscopy observations of the ECAP processed composite indicate that the changes in morphology of Mg-MWCNT interface upon ECAP may be responsible for the deterioration of hydrogenation properties.

Magnesium-transition metal alloys are interested because of their ability of storing and releasing hydrogen. The reactions involve reversible transitions between a metallic state and a hydride state, which are accompanied by drastic changes in electrical conductivity and optical transmission. Thin films of these materials are further investigated because of the potential of being used in switchable mirrors, hydrogen sensors, and many other conceived applications. In this study, we focused on the Mg-cobalt (Co) thin film system. Film samples were prepared by co-sputtering a Mg target and a Co target. The ratio of the sputtering powers was set such that films of various compositions covering a broad range were fabricated. Each film was covered with a palladium (Pd) overcoat with a thickness less than 10 nm to protect the film from oxidation. The film structure was investigated by using X-ray diffraction (XRD). Except the films containing predominantly one metal, the diffraction patterns of Mg-Co films with other compositions in between only showed broad halos. This suggests that the structure of Mg-Co films is highly disordered and not isostructural to any known stochiometric Mg-Co compounds. X-ray photoelectron spectroscopy (XPS) analysis provided information on the depth profiles of elemental composition. Pd was only detected at shallow depths. Conspicuous amount of oxygen was found in the Pd layer, and started to drop from the Pd/Mg-Co interface with increasing depth. Mg showed a peak value near the Pd/Mg-Co interface and a coherent drop with oxygen content along depth. This leads one to suggest that oxygen atoms can penetrate through the Pd layer and react with Mg, inducing more Mg atoms to diffuse towards the top surface. At deeper regions, the Mg:Co ratio approached some stable value in accordance with the power ratio used to prepare the sample. Prolonged exposure to air resulted in significant oxidation, as reflected by XPS data and the appearance of pits on the film surface. The change of electrical conductivity (σ) of a sample in a small chamber was continuously monitored during hydridation and dehydridation at room temperature. Each cycle consisted of exposure to 15% hydrogen in argon at 105 Pa (10 min), followed by evacuation to rough vacuum and then exposure to air (10 min). In general, the value of σ of a film with a higher Mg content showed a stronger drop in the hydridation process, but the response times for both hydridation and dehydridation processes were longer, such that the range of the change in σ in the subsequent cycles appeared to be smaller.

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