369 research outputs found
High-pressure study of X-ray diffuse scattering in ferroelectric perovskites
We present a high-pressure x-ray diffuse scattering study of the ABO
ferroelectric perovskites BaTiO_3 and KNbO_3. The well-known diffuse lines are
observed in all the phases studied. In KNbO_3, we show that the lines are
present up to 21.8 GPa, with constant width and a slightly decreasing
intensity. At variance, the intensity of the diffuse lines observed in the
cubic phase of BaTiO_3 linearly decreases to zero at GPa. These
results are discussed with respect to x-ray absorption measurements, which
leads to the conclusion that the diffuse lines are only observed when the B
atom is off the center of the oxygen tetrahedron. The role of such disorder on
the ferroelectric instability of perovskites is discussed.Comment: 4 pages, Accepted in PR
Hydrogen Energy Storage: New Techno-Economic Emergence Solution Analysis
International audienceThe integration of various renewable energy sources as well as the liberalization of electricity markets are established facts in modern electrical power systems. The increased share of renewable sources within power systems intensifies the supply variability and intermittency. Therefore, energy storage is deemed as one of the solutions for stabilizing the supply of electricity to maintain generation-demand balance and to guarantee uninterrupted supply of energy to users. In the context of sustainable development and energy resources depletion, the question of the growth of renewable energy electricity production is highly linked to the ability to propose new and adapted energy storage solutions. The purpose of this multidisciplinary paper is to highlight the new hydrogen production and storage technology, its efficiency and the impact of the policy context on its development. A comprehensive techno/socio/economic study of long term hydrogen based storage systems in electrical networks is addressed. The European policy concerning the different energy storage systems and hydrogen production is explicitly discussed. The state of the art of the techno-economic features of the hydrogen production and storage is introduced. Using Matlab-Simulink for a power system of rated 70 kW generator, the excess produced hydrogen during high generation periods or low demand can be sold either directly to the grid owners or as filled hydrogen bottles. The affordable use of Hydrogen-based technologies for long term electricity storage is verified
Inventaire des mollusques d'eau douce en Ituri (Haut-Zaïre) : conséquences sanitaires pour l'homme et le bétail
Cent soixante-dix points d'abreuvement pour le bétail ont été prospectés de juin 1986 à août 1988 en Ituri (nord-est du Zaïre) afin de déterminer les principaux mollusques d'eau douce pouvant intervenir dans la transmission des trématodoses animales et humaines. Dix-neuf espèces de gastéropodes ont été rencensées. Quatre gîtes sur dix n'hébergent pas ou très peu de mollusques. Les trois espèces les plus fréquemment rencontrées sont #Lymnaea natalensis, #Biomphalaria pfeifferi (15,3 %). La distribution des deux premières espèces semble très homogène au niveau régional et ne permet pas l'établissement d'une relation avec le risque parasitaire (#Fasciola gigantica et #Schistosoma mansoni). Inversement, la répartition de #Bulinus africanus (et secondairement celle de #B. forskalii) correspond globalement à la distribution de #Schistosoma bovis chez les bovins. L'association de #Lymnaea natalensis et de #Bulinus africanus au sein des gîtes rend compte de l'association des infestations à #Fasciola gigantica et #Schistosoma bovis$ chez les bovins. (Résumé d'auteur
Study of the Galactic Interstellar Medium from High Resolution X-Ray Spectroscopy: X-Ray Absorption Fine Structure and Abundances of O, Mg, Si, S, and Fe
We study the composition of the Galactic interstellar medium (ISM) toward the
Galactic center region (5 < |l| < 20 degree) by utilizing X-ray absorption
features of three bright low-mass X-ray binaries (LMXBs), GX 13+1, GX 5-1, and
GX 340+0, observed with the Chandra HETGS. We detect X-ray absorption fine
structure (XAFS) of the Si K-edge, characterized by a narrow and a broad
absorption feature at 1846 and ~1865 eV, respectively. Comparison with ground
experimental data indicates that most of the ISM Si exists in the form of
silicates, although a composition of "pure" forsterite is ruled out. The XAFS
spectra of the sulfur K-edge indicate that a significant fraction of S exists
in the gas phase. From each source, we derive the column densities of Mg, S,
Si, and Fe from the K-edge depth and that of O (or H) from the absorption of
the continuum. The elemental abundance ratios are found to be consistent
between the three targets: the mean values of O/Si, Mg/Si, S/Si, and Fe/Si are
determined to be 0.55+-0.17, 1.14+-0.13, 1.03+-0.12, and 0.97+-0.31 solar,
respectively (90% error in the mean value). We discuss the origins of the
overabundances of the heavy metals relative to O in the Galactic ISM by
comparison with the abundance pattern of the intracluster medium in clusters of
galaxies. Assuming that most of the Mg and Si atoms are depleted into silicates
of either the proxine or olivine family, we estimate that the number ratio of
Mg to Fe in olivine is >~1.2 and that 17%-43% of the total O atoms in the ISM
must be contained in silicate grains.Comment: 31 pages, 15 figures, accepted for publication in ApJ, vol. 620,
2005. Proof corrections are reflected (column densities of O and H were
overestimated in the previous version.). Figures 1(a)-(c) are revise
Influence of the 6^1S_0-6^3P_1 Resonance on Continuous Lyman-alpha Generation in Mercury
Continuous coherent radiation in the vacuum-ultraviolet at 122 nm
(Lyman-alpha) can be generated using sum-frequency mixing of three fundamental
laser beams in mercury vapour. One of the fundamental beams is at 254 nm
wavelength, which is close to the 6^1S_0-6^3P_1 resonance in mercury.
Experiments have been performed to investigate the effect of this one-photon
resonance on phasematching, absorption and the nonlinear yield. The efficiency
of continuous Lyman-alpha generation has been improved by a factor of 4.5.Comment: 8 pages, 7 figure
X-ray Absorption Near-Edge Structure calculations with pseudopotentials. Application to K-edge in diamond and alpha-quartz
We present a reciprocal-space pseudopotential scheme for calculating X-ray
absorption near-edge structure (XANES) spectra. The scheme incorporates a
recursive method to compute absorption cross section as a continued fraction.
The continued fraction formulation of absorption is advantageous in that it
permits the treatment of core-hole interaction through large supercells
(hundreds of atoms). The method is compared with recently developed
Bethe-Salpeter approach. The method is applied to the carbon K-edge in diamond
and to the silicon and oxygen K-edges in alpha-quartz for which polarized XANES
spectra were measured. Core-hole effects are investigated by varying the size
of the supercell, thus leading to information similar to that obtained from
cluster size analysis usually performed within multiple scattering
calculations.Comment: 11 pages, 4 figure
Deformation change in light iridium nuclei from laser spectroscopy
Laser spectroscopy measurements have been performed on neutron-deficient and stable Ir isotopes using the COMPLIS experimental setup installed at ISOLDE-CERN. The radioactive Ir atoms were obtained from successive decays of a mass-separated Hg beam deposited onto a carbon substrate after deceleration to 1kV and subsequently laser desorbed. A three-color, two-step resonant scheme was used to selectively ionize the desorbed Ir atoms. The hyperfine structure (HFS) and isotope shift (IS) of the first transition of the ionization path 5d^{7}6s ^{2}^{4}F_{9/2} \to 5d^{7}6s6p ^{6}F_{11/2} at 351.5nm were measured for Ir, and the stable Ir. The nuclear magnetic moments μI and the spectroscopic quadrupole moments Qs were obtained from the HFS spectra and the change of the mean square charge radii from the IS measurements. The sign of μI was experimentally determined for the first time for the masses 182≤A≤189 and the isomeric state . The spectroscopic quadrupole moments of Ir and Ir were measured also for the first time. A large mean square charge radius change between Ir and and between and was observed corresponding to a sudden increase in deformation: from β2 ≃ + 0.16 for the heavier group A = 193, 191, 189, 187 and 186m to β2 ≥ + 0.2 for the lighter group A = 186g, 185, 184, 183 and 182. These results were analyzed in the framework of a microscopic treatment of an axial rotor plus one or two quasiparticle(s). This sudden deformation change is associated with a change in the proton state that describes the odd-nuclei ground state or that participates in the coupling with the neutron in the odd-odd nuclei. This state is identified with the π3/2+[402] orbital for the heavier group and with the π1/2-[541] orbital stemming from the 1h _9/2 spherical subshell for the lighter group. That last state seems to affect strongly the observed values of the nuclear moments
Charge-radius change and nuclear moments in the heavy tin isotopes from laser spectroscopy: Charge radius of Sn
NESTER ACCLaser spectroscopy measurements have been carried out on the neutron-rich tin isotopes with the COMPLIS experimental setup. Using the optical transition, hyperfine spectra of Sn and were recorded for the first time. The nuclear moments and the mean square charge radius variation (\delta, the absolute charge radii of these isotopes were deduced in particular that of the doubly magic Sn nucleus. The comparison of the results with several mean-field-type calculations have shown that dynamical effects play an important role in the tin isotopes
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