Vibrational coherence of self-organized silver nanocrystals in f.c.c. supra-crystals

Fabrication of devices from inorganic nanocrystals normally requires that they are self-organized into ordered structures. It has now been demonstrated that nanocrystals are able to self-organize in a 'supra'-crystal with a face-centred cubic (f.c.c.) structure. The physical properties of nanocrystals self-organized into compact arrays are quite different from those of both isolated nanocrystals and the bulk phase. The collective optical and magnetic properties of these nanocrystal assemblies are governed mainly by dipolar interactions. Here, we show that nanocrystals vibrate coherently when they are self-organized in f.c.c. supra-crystals. Hence, a phase relation exists between the vibrations of all of the nanocrystals in a supra-crystal. This vibrational coherence can be observed by a substantial change of the quadrupolar low-frequency Raman scattering peak. Although a change in electronic transport properties has previously been observed on self-organization of silver nanocrystals, vibrational coherence represents the first intrinsic property of f.c.c. supra-crystals.     

Kovacic's algorithm and its application to some families of special functions

We apply the Kovacic algorithm to some families of special functions, mainly the hypergeometric one and that of Heun, in order to discuss the existence of closed-form solutions. We begin by giving a slightly modified version of the Kovacic algorithm and a sketch proof.     

Analysis of the historical collapse of an abandoned underground chalk mine in 1961 in Clamart (Paris,France)

In chalk mines, the method of room and pillar mining has been widely used in France. Yet, many large collapses have occurred during or after their exploitation, with the collapse of Clamart (south of Paris, France) being well known as one of the most catastrophic. Today, 50 years after the collapse, the main causes of the event are still not well understood. This paper presents a back-analysis of the case study, using both an empirical approach and a numerical approach. Based on a set of hypotheses, an empirical approach and 3D numerical modeling have indicated a plausible scenario that may explain the collapse: The large collapse may have been caused by a lack of bearing capacity of the pillars and the sudden rupture of a hard limestone bed on which the load would have accumulated until overload occurred. The bed weakness between the two mine levels may also have had an influence on the mine’s general lack of stability. Moreover, under the given hypothesis, the study shows a very low influence of the water table due to flooding of the mine.

     

Hydration heat kinetics of concrete with silica fume

The objective of this study was to evaluate the influence of silica fume on the hydration heat of concrete. Portland cement was replaced by silica fume in amounts from 10 % to 30 % by mass in concrete with w/(c+sf) ratios varying between 0.25 and 0.45. A superplasticizer was used to maintain a fluid consistency. The heat of hydration was monitored continuously by a semi-adiabatic calorimetric method for 10 days at 20 °C. The calorimetric study indicated that the hydration was modified by the presence of silica fume. In the early stages, the silica fume showed a high activity and accelerated the hydration rate as compared to that of the reference concrete. The fine silica fume particled provided nucleation sites for hydrates growth. Then the pozzolanic activity took over and increased both strength and the hydration heat. A substitution of Portland cement by 10% with silica fume produced greater strength and cumulative heat of hydration as compared to that of the reference concrete.     

Ultra-Low Noise Charge Sensitive Preamplifier for Scintillation Detection with Avalanche Photodiodes in PET Applications

The need for compact, fast, low-noise front-end electronics in high resolution positron emission tomography (PET) has prompted this effort to design a preamplifier suitable for avalanche photodiode-based scintillation detectors. Due to the small signals from the detectors (< .03 pC/MeV), a preamplifier with ultra-low noise performance in the 5 to 20 MHz range is essential to achieve the timing resolution required by the PET application. Out of many available technologies, a new third generation MOSFET wqas selected and implemented as input transistor in an original charge sensitive (CSP) design. Performance among the best reported to date are obtained. The new design was implemented as a dual-channel preamplifier in high density hybrid (thick film) technology.     

Lamb shift measurements in high Z hydrogenic ions

This paper presents a possible technique for the measurement of the ground state Lamb shift in hydrogenic krypton, Kr³⁵⁺, via a comparison of the Lyman α and Balmer β wavelengths. Measurements of the yields of the Lyman α transitions in Kr³⁵⁺ using krypton beams at 18.6, and $\text{34 MeV}\text{A}$ are described.     

Decomposition du methane sur le carbone aux tres hautes temperatures et sous basses pressions

The decomposition of methane on pyrocarbon as deposited in the course of the reaction has been investigated at high temperatures (1000–2000°C) and low pressures ( 10⁻² Torr). Experiments have been carried out with electrically heated filaments made of amorphous, pyrolytic or graphitic carbon. Rates of decomposition are calculated from rates of molecular hydrogen formation. Results are very different from those reported previously about the decompositions of ethylene, acetylene, propadiene and benzene on the same substrates.

1. (1) Electron microscope observations never showed any evidence of pyrocarbon deposition (Fig. 4). However some indications do not exclude this possibility in the deep pores of the Acheson graphite.

2. (2) Electrical resistivity measurements confirm the smallness—or even the complete absence—of any pyrocarbon deposition (Fig. 5).

3. (3) Even after many series of ten hours reaction time, the kinetics of methane decomposition are still strongly dependent upon the type of sample. This fact supports also the conclusion that pyrocarbon deposition is absent (in contrast, the pyrocarbon deposited in similar experiments using unsaturated hydrocarbons makes all types of samples tend progressively to a standard chemical behavior).

4. (4) On smooth pyrocarbon surfaces, methane decomposition is hardly detectable; collision yields are of the order 10⁻⁷ (as compared to 10⁻⁴ for unsaturated hydrocarbons, Fig. 6). But with porous samples—especially Acheson graphite—appreciable decomposition occurs; the kinetics show then many similarities with those reported previously for unsaturated hydrocarbons decompositions: especially the reactivity of the carbon surface is temperature and pressure dependent (Figs. 1–3).

The peculiar behavior of methane is explained in terms of chemical reactivity: unlike unsaturated hydrocarbons, methane does not undergo weakly activated chemisorption on the active sites of the carbon surface, and decomposition may then only occur after thermal accommodation of the gas molecules.

Résumé

On étudie la décomposition du méthane sous basses pressions ( 10⁻² Torr) et à hautes températures (1000–2000°C) sur le carbone qui se forme durant la réaction. Les résultats obtenus sont très différents de ceux acquis antérieurement dans des expériences similaires menées avec l'éthylène, l'acétylène, l'aliène et le benzène.

1. (1) Les observations à l'oeil nu et au microscope électronique n'ont jamais permis de déceler un changement de la surface du filament de carbone de départ qui puisse être imputé à un dépôt de carbone pyrolytique. Cependant certains indices n'excluent pas la possibilité d'un tel dépôt dans les anfractuosités profondes des graphites Acheson.

2. (2) Les mesures de résistivité électrique de l'échantillon de carbone ont confirmé l'insignifiance—ou l'inexistence—d'un dépôt de carbone.

3. (3) Même après une dizaine d'heures de réaction la cinétique de la décomposition du méthane varie beaucoup selon le type de filament de départ, preuve supplémentaire que le dépôt de carbone pyrolytique est quasi inexistant (en opérant pareillement avec les hydrocarbures insaturés, le dépôt de carbone pyrolytique uniformise rapidement tous les échantillons et unifie leur comportement chimique).

4. (4) Avec les échantillons à texture fermée et à surface lisse une décomposition du méthane est à peine décelable (rendement de choc 10⁻⁷ contre 10⁻⁴ pour les hydrocarbures insaturés). Avec les échantillons poreux—notamment les graphites Acheson—une décomposition mesurable a lieu: la cinétique s'apparente alors à celles observées avec les hydrocarbures saturés sur des surfaces de carbone lisses, c'est-à-dire qu'elle est révélatrice d'une réactivité de la surface du carbone variable avec la température et la pression.

On interprète le comportement singulier du méthane en terme de réactivité chimique: le méthane, contrairement aux hydrocarbures insaturés ne donne pas lieu à une adsorption très peu activée à la surface du carbone. Sa chimisorption, et par conséquent sa décomposition nécessitent une accommo- dation thermique beaucoup plus complète que dans le cas des hydrocarbures insaturés.     

Highly efficient pilot biopiles for on-site fertilization treatment of diesel oil-contaminated sub-Antarctic soil

In order to determine the efficiency of on-site bioremediation of diesel contaminated subAntarctic soils, a pilot biopiles study was initiated in January 2006 with a selected soil of The Grande Terre (Kerguelen Archipelago, 69° 42′E–49° 19′S). Each 4 kg biopile received 180 g of diesel fuel and several of them were submitted to different treatments. All biopiles were sampled on a regular basis over a one-year period. A highly efficient biodegradation was observed in all piles. Analytical results show that less than 7% of total petroleum hydrocarbons (TPH) remained after one year in biopiles not treated with fertilizers. This proportion decreases to less than 0.2% in biopiles treated with fish compost. These biopiles revealed to be 100 times more efficient than previously reported in situ bioremediation study conducted under similar weather conditions with the same concentrations of similar fertilizers. After one year, a significantly larger amount of polycyclic aromatic hydrocarbons (PAH) was present in fertilized biopiles without a physical treatment compared to ventilated or agitated ones. Although biopiles were highly efficient to reduce TPH concentrations well below the target concentration of 100 ppm, a residual toxicity determined with Microtox assay was observed in a number of biopiles treated with fish compost.