J. F. Pelletier

Investigation of dehydrogenation mechanism of MgH2–Nb nanocomposites

Nanocomposites of composition MgH2–V and MgH2–Nb have been shown to have very fast hydrogen sorption kinetics. This could be explained by the presence of vanadium, which eases hydrogen penetration into the material, and by the particular microstructure of this nanocomposite. In this paper, we report a systematic structural study of the nanocomposite MgH2–Nb5at.%. To see the effect of hydrogenation/dehydrogenation process on the crystal structure, X-ray diffraction under hydrogen pressure was carried out at different temperatures. Crystallite size was evaluated by X-ray powder diffraction peak broadening. In order to see the evolution of crystal phases during the dehydrogenation process, real time X-ray investigation of dehydrogenation in MgH2–Nb nanocomposite was carried out using synchrotron radiation. It was found that a niobium hydride metastable phase, closely related to the low temperature e-NbH phase, acts as a gateway for hydrogen. Activation energies of the hydrogenation and dehydrogenation processes are also estimated.

Structure of nanocomposite metal hydrides

Abstract It has recently been shown that MgH2–(V, Nb) nanocomposite has very fast hydrogen sorption kinetics. This could be explained by the presence of vanadium which eases hydrogen penetration into the material and by the particular microstructure of this nanocomposite. In order to have a better understanding of the nature and hydrogen desorption mechanism, a systematic structural study has been undertaken. The powder morphology and chemical phase distribution were observed by SEM. X-ray diffraction under hydrogen pressure was performed at different temperatures in order to see the effect of absorption/desorption process on the crystal structure. Crystallite size was evaluated by X-ray powder diffraction peak broadening. Real time X-ray investigations of hydrogen desorption in MgH2–Nb nanocomposite were performed using synchrotron radiation. For the first time, we were able to get direct evidence of the dehydrogenation mechanism. A metastable new niobium hydride phase associated with the desorption process was detected. This metastable hydride is probably the result of long-range ordering of hydrogen in the niobium hydride lattice in order to facilitate the hydrogen flow.

Design and characterization of an undulator beamline optimized for small-angle coherent X-ray scattering at the Advanced Photon Source

An undulator beamline and small-angle-scattering spectrometer have been implemented at the Advanced Photon Source. The beamline is optimized for performing small-angle wide-bandpass coherent X-ray scattering measurements, and has been characterized by measuring static X-ray speckle patterns from isotropically disordered samples. Statistical analyses of the speckle patterns have been performed from which the speckle widths and contrast are extracted versus wavevector transfer and sample thickness. The measured speckle widths and contrast are compared with an approximation to the intensity correlation function and found to be in good agreement with its predictions.

Hydrogen absorption in thermally prepared RuO2 electrode

Electrochemical in situ X-ray diffraction measurements were performed in 1 N H 2 SO 4 electrolyte on a thermally prepared RuO 2 thin layer deposited on a Ti substrate. As evidenced by a shift of the characteristic diffraction peaks of RuO 2 , a volume increase of the tetragonal unit cell occurs when the electrode is polarized in the potential region where hydrogen evolution occurs. These changes in the lattice parameter are thought to occur as a result of hydrogen absorption in the oxide layer. These modifications are not totally reversible, as the positions of the diffraction peaks of the used electrode do not coincide with that of pristine material.

Subpicosecond time-resolved x-ray spectroscopy of plasmas produced by high-intensity ultrashort laser pulses

We present and discuss time resolved spectra (KeV range) of solid density plasmas produced by the interaction of clean high intensity laser pulses (400 fs, 0.53 micrometers light) with solid and foil targets at intensities between 1018 W/cm2 and 1020 W/cm2. Results have been obtained with the new PX1 x-ray stream camera which has a temporal resolution of 750 fs in the KeV x-ray range.

Wide-angle monochromatic x-ray beam shutter: a design study

A novel design of a wide-angle monochromatic x-ray beam shutter is discussed. The shutter is designed as a compact unit capable of providing users with the means of shutting off the beam in secondary beamlines that are at an angle to the primary beamline and to each other. The single-unit design used the fact that all the secondary beamlines will be closed at the same time. The main challenge was to fit the shutter in the limited space of the existing Advanced Photon Source IMMW-CAT hutch. Space limitations led to the change in position of the actuator subassembly as compared to the standard shutter design. Although the actuator subassembly is placed underneath the shutter, fail-safe shutting is achieved by placing tungsten blocks above the beam while the shutter is open and using gravity to close the shutter in case of pneumatic failure. Redundancy required by safety concerns was achieved by duplicating the tungsten block/actuator subunits. Tungsten blocks of uneven length were used to counteract the increase in the center-to-center distance among secondary beamlines due to their angular offset. A special support table was designed to facilitate assembly and adjustability of the shutter position in the available space. To provide a radiation-tight hutch, a non-standard guillotine system was designed. In this paper, the design, specifications and optical ray tracing of the shutter assembly are presented.

A time-resolved X-ray scattering experiment for the study of phase transitions and crystallization processes in metallic alloys

An experimental setup to perform high-resolution time-resolved X-ray scattering has been commissioned on the side station of beamline 8-ID at the Advanced Photon Source. A Peltier-cooled diode detector array covering an angle range of 20 degrees is mounted on a 4-circle goniometer and is used to temporally resolve X-ray scattering patterns with a resolution up to 10 ms. Metallic ribbon samples can be quickly heated and cooled from temperatures up to 500 °C inside a furnace with controllable atmosphere and equipped with a beryllium window. A description of the setup is presented along with actual results showing time-resolved phase transitions and crystallization processes in AlYNi metallic alloys. These results demonstrate the power of this technique to investigate complex crystallization processes as well as the versatility of this time-resolved X-ray scattering spectrometer.