The demands on Hydrogen fuel based technologies is ever increasing for substitution or replacing fossil fuel due to superior energy sustainability, national security and reduced greenhouse gas emissions. Currently, the polymer based proton exchange membrane fuel cell (PEMFC), is strongly considered for on-board hydrogen storage vehicles due to low temperature operation, efficiency and low environmental impact. However, the realization of PEMFC vehicles must overcome the portable hydrogen storage barrier. DOE and FreedomCAR technical hydrogen storage targets for the case of solid state hydrides are: (1) volumetric hydrogen density > 0.045 kgH2/L, (2) gravimetric hydrogen density > 6.0 wt%, (3) operating temperature < 150 degrees C, (4) lifetimes of 1000 cycles, and (5) a fast rate of H2 absorption and desorption. To meet these targets, we have focused on lithium borohydride systems; an alkali metal complex hydride with a high theoretical hydrogen capacity of 18 wt.%. It has been shown by Vajo et al. that adding MgH2, improves the cycling capacity of LiBH4. The pressure-composition-isotherms of the destabilized LiBH4 + MgH2 system show an extended plateau pressure around 4-5 bars at 350 degrees C with a good cyclic stability. The mentioned destabilizing mechanism was successfully utilized to synthesize the complex hydride mixture LiBH4 + 1/2MgH2 + Xmol% ZnCl2 catalyst (X=2, 4, 6, 8 and 10) by ball milling process. The added ZnCl2 exhibited some mild catalytic activity which resulted in a decomposition temperature reduction to 270 degrees C. X-ray powder diffraction profiles exhibit LiCl peaks whose intensity increases proportionately with increasing ZnCl2 indicating an interaction between catalyst and hydride system, possibly affecting the total weight percent of desorbed hydrogen. Thermal gravimetric analysis profiles for MgH2 + 5mol% nanoNi and LiBH4 + ZnCl2 + 3mol% nanoNi indicate that small concentrations of nano-nickel acts as an effective catalyst that reduces the mixture desorption temperature to around 225 degrees C and 88 degrees C, respectively. Future work will be focused on thermodynamic equilibrium studies (PCT) on the destabilized complex hydrides.

The purpose of the work presented in this thesis was to explore and understand the mechanisms governing toughness, ductility and ballistic performance in a class of nanostructured carbide-free bainite-austenite steels, sometimes known as ‘superbainite’. The mechanical properties of these alloys have been extensively reported, but their interpretation is not clear. The thesis begins with an introduction to both the relevant nanostructures and some of the difficulties involved in explaining observed properties, alongside a summary of the role of mixed- microstructures in alloy development. An overview of the debate regarding the mechanism of bainite formation is pre- sented in Chapter 2, in the form of a literature survey encompassing the period of explicit recognition of the bainite microstructure. Of note is the role played by the displacive theory of formation in the development of the alloy structures investigated in this thesis. A characterisation of a commonly available bainitic alloy forms the basis for Chapter 4. Observations confirm the nanoscale nature of the structure, although additional phases are found to be present, namely: cementite and martensite. This is explained as resulting from relatively low alloying additions and chem- ical segregation effects, which are modelled using thermodynamic and kinetic approaches. Chapters 5 and 6 contain a comprehensive study of the response of this alloy to the stress concentration present at the notch root of a Charpy impact sample. The work provides evidence of notch root embrittlement due to stress-induced martensite transformation. Results from synchrotron and laboratory X-ray experiments in particular reveal that machining, as well as applied stress, can initiate the austenite-martensite transformation, and methods to mitigate this effect are suggested. An innovative approach is harnessed in Chapter 7, in order to identify exper- imentally the volume fraction at which three-dimensional connectivity (‘percolation’) of austenite is lost in a superbainitic steel. Hydrogen thermal desorption techniques are applied to this problem, inspired by the tendency of such alloys to undergo tensile failure with limited or zero necking. The striking result sheds light on the importance of austenite morphology in restricting the diffusion of hydrogen into a mixed structure. The final set of experimental work is directed towards understanding the damage mechanisms that occur during projectile penetration of a coarser bainitic armour- plate alloy. The formation of adiabatic shear bands is found to be a dominant factor governing the ballistic failure of the plate. The sheared material undergoes severe high-temperature deformation, but does not change phase upon cooling, leading to the proposal of certain methods that could be implemented to improve ballistic resistance of the steel. The totality of the research presented herein is summarised in Chapter 9, which draws attention to new areas of interest that have arisen from the current work, proposing several future directions of investigation. The broader issue of understanding, common to all studies performed thus far, is that of the causes, effects, and extent, of stress-induced transformation to martensite experienced by the retained austenite that is a key feature of superbainite and similar steels.

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Surface structure, chemical composition, bonding configuration, film polarity, and electronic properties of InN layers grown by high pressure chemical vapor deposition (HPCVD) have been investigated. Sputtering at an angle of 50-70 degrees followed by atomic hydrogen cleaning (AHC) was successful in removing the carbon contaminants. AHC is found to be the most effective cleaning process to remove oxygen contaminants from InN layers in an ultrahigh vacuum (UHV) system and produced a well ordered surface. Auger electron spectroscopy (AES) confirmed the cleanliness of the surface, and low energy electron diffraction (LEED) yielded a 1×1 hexagonal pattern demonstrating a well-ordered surface. High resolution electron energy loss spectra (HREELS) taken from the InN layers exhibited loss features at 550 cm-1, 870 cm-1 and 3260 cm-1 which were assigned to Fuchs-Kliewer phonon, N-H bending, and N-H stretching vibrations, respectively. Assignments were confirmed by observation of isotopic shifts following atomic deuterium dosing. No In-H species were observed indicating N-termination of the surface and N-polarity of the film. Broad conduction band plasmon excitations were observed centered at 3100 cm-1 to 4200 cm-1 in HREEL spectra acquired with 25 eV electrons, for a variety of samples grown with different conditions. Infrared reflectance data shows a consistent result with HREELS for the bulk plasma frequency. The plasmon excitations are shifted about 300 cm-1 higher in HREEL spectra acquired using 7 eV electrons due to the higher plasma frequency and carrier concentration at the surface than in the bulk, demonstrating a surface electron accumulation. Hydrogen completely desorbed from the InN surface upon annealing for 900 s at 425 ºC or upon annealing for 30 s at 500 ºC. Fitting the coverage versus temperature for anneals of either 30 or 900 s indicated that the desorption was best described by second order desorption kinetics with an activation energy and pre-exponential factor of 1.3±0.2 eV and 10-7.3±1.0 cm2/s, respectively. Vibrational spectra acquired from HREEL can be utilized to explain the surface composition, chemical bonding and surface termination, and film polarity of InN layers. The explanation of evidence of surface electron accumulation and extraction of hydrogen desorption kinetic parameters can be performed by utilizing HREEL spectra.

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科学研究費補助金 研究種目:基盤研究(A)(2) 課題番号:07558177 研究代表者:菅井 秀郎 研究期間:1995-1997年度