317 research outputs found
La relance du chĂȘne-liĂšge dans le Plan national de reboisement en AlgĂ©rie -
En AlgĂ©rie, la superficie forestiĂšre n'a cessĂ© de rĂ©gresser et, plus particuliĂšrement, durant les deux derniĂšres dĂ©cennies. Aux causes multiples dĂ©jĂ connues, se sont rajoutĂ©s le terrorisme et les problĂšmes que risquent de poser les changements climatiques dont les effets ne sont pas encore bien cernĂ©s. Devant cette situation, les pouvoirs publics ont lancĂ© un Plan national de reboisement (PNR), dont les rĂ©alisations Ă mi-parcours ne couvrent mĂȘme pas la moitiĂ© de ce qui devrait ĂȘtre rĂ©alisĂ©. Dans le mĂȘme temps, les incendies ont brĂ»lĂ© plus que ce qui a Ă©tĂ© plantĂ©. S'agissant du chĂȘne-liĂšge, les techniques en usage sont loin de contribuer Ă la reconstitution de la subĂ©raie. Sur la base des observations de terrain et l'analyse critique de ce qui s'est fait jusque-lĂ , nous suggĂ©rons une nouvelle approche devant mener Ă une amĂ©lioration des rĂ©sultat
Atomic motions in the -region of glass-forming polymers: Molecular versus Mode Coupling Theory approach
We present fully atomistic Molecular Dynamics simulation results on a
main-chain polymer, 1,4-Polybutadiene, in the merging region of the -
and -relaxations. A real space analysis reveals the occurrence of
localized motions (``-like'') in addition to the diffusive structural
relaxation. A molecular approach provides a direct connection between the local
conformational changes reflected in the atomic motions and the secondary
relaxations in this polymer. Such local processes occur just in the time window
where the -process of the Mode Coupling Theory is expected. We show that
the application of this theory is still possible, and yields an unusually large
value of the exponent parameter. This result might originate from the
competition between two mechanisms for dynamic arrest: intermolecular packing
and intramolecular barriers for local conformational changes
(``-like'').Comment: 10 pages, 6 figure
Real-time Spectroscopic Ellipsometry Study of Ultrathin Diffusion Barriers for Integrated Circuits
The objective of this work is to monitor the growth process and the thermal stability of ultrathin tantalum nitride barrier nanostructures against copper diffusion in integrated circuits using real-time spectroscopic ellipsometry (RTSE). Single layers of copper and bilayer films of copper and tantalum nitride were produced on Si(111) substrates using unbalanced magnetron sputtering. The RTSE data was simulated using the Bruggeman effective medium approximation and a combined Drude-Lorentz model to obtain information about the growth process, film architecture, interface quality, and the conduction electron transport properties for these structures. The results deduced from the RTSE were verified by characterizing the structural and the chemical properties of the fabricated films using x-ray diffraction, Auger electron spectroscopy, and Rutherford backscattering. The effectiveness of the tantalum nitride barrier to stop the diffusion of copper into silicon was evaluated, monitoring their optical properties when annealed at 720 degreesC. The dielectric function of the films changed from a metallic to an insulating character when the diffusion proceeded. Also, the RTSE provided valuable information about the microstructure and the kinetics of the phase transformations that occur during heat treatment. (C) 2004 American Institute of Physics
Nonlinear control of multicellular single stage grid connected photovoltaic systems with shunt active power filtering capability
Structural Relaxation and Mode Coupling in a Simple Liquid: Depolarized Light Scattering in Benzene
We have measured depolarized light scattering in liquid benzene over the
whole accessible temperature range and over four decades in frequency. Between
40 and 180 GHz we find a susceptibility peak due to structural relaxation. This
peak shows stretching and time-temperature scaling as known from
relaxation in glass-forming materials. A simple mode-coupling model provides
consistent fits of the entire data set. We conclude that structural relaxation
in simple liquids and relaxation in glass-forming materials are
physically the same. A deeper understanding of simple liquids is reached by
applying concepts that were originally developed in the context of
glass-transition research.Comment: submitted to New J. Phy
Real-time spectroscopic ellipsometry study of ultrathin diffusion barriers for integrated circuits
Bimodal Effect on Pancreatic ÎČ-Cells of Secretory Products From Normal or Insulin-Resistant Human Skeletal Muscle
OBJECTIVE: Type 2 diabetes is characterized by insulin resistance with a relative deficiency in insulin secretion. This study explored the potential communication between insulin-resistant human skeletal muscle and primary (human and rat) beta-cells. RESEARCH DESIGN AND METHODS: Human skeletal muscle cells were cultured for up to 24 h with tumor necrosis factor (TNF)-alpha to induce insulin resistance, and mRNA expression for cytokines was analyzed and compared with controls (without TNF-alpha). Conditioned media were collected and candidate cytokines were measured by antibody array. Human and rat primary beta-cells were used to explore the impact of exposure to conditioned media for 24 h on apoptosis, proliferation, short-term insulin secretion, and key signaling protein phosphorylation and expression. RESULTS: Human myotubes express and release a different panel of myokines depending on their insulin sensitivity, with each panel exerting differential effects on beta-cells. Conditioned medium from control myotubes increased proliferation and glucose-stimulated insulin secretion (GSIS) from primary beta-cells, whereas conditioned medium from TNF-alpha-treated insulin-resistant myotubes (TMs) exerted detrimental effects that were either independent (increased apoptosis and decreased proliferation) or dependent on the presence of TNF-alpha in TM (blunted GSIS). Knockdown of beta-cell mitogen-activated protein 4 kinase 4 prevented these effects. Glucagon-like peptide 1 protected beta-cells against decreased proliferation and apoptosis evoked by TMs, while interleukin-1 receptor antagonist only prevented the latter. CONCLUSIONS: Taken together, these data suggest a possible new route of communication between skeletal muscle and beta-cells that is modulated by insulin resistance and could contribute to normal beta-cell functional mass in healthy subjects, as well as the decrease seen in type 2 diabetes
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