Synthesis and characterisation of highly dispersed Ni/SiO2 catalysts prepared by gas-phase impregnation/decomposition (GPI/D) of a Ni(II) β-diketonate precursor complex

Silica-supported nickel catalysts have been prepared by a solventless technique involving the deposition of the [Ni(tmen)(acac)₂] precursor from the gas phase. According to UV–vis and IR spectroscopy and to elemental analysis, the initial deposition step results in the grafting of Ni(II) complexes on silanol groups to give predominantly [Ni(acac)₂ (Si-OH)₂]⁰ monomeric inner-sphere complexes, with elimination of the neutral (tmen) ligands into the gas phase. Subsequent thermal treatments in oxidising atmospheres causes an oxidative decomposition of the (acac) ligands without nickel desorption or coalescence into oxide crystallites. The ensuing coordinatively unsaturated Ni(II) centres are reduced to Ni(0) by flowing hydrogen at low temperatures (300 °C), yielding nanosized Ni particles as evidenced by TEM and O₂ titration. Thus, the gas-phase deposition technique represents an interesting alternative to conventional wet deposition procedures since it allows one to obtain well-dispersed supported nickel catalysts by reduction at low-temperatures.     

Conductivite ionique des oxydes Li8MO6 (M = Ce,Hf)

The electrical conductivity of the oxides Li₈CeO₆ and Li₈HfO₆, which are thermodynamically stable against Li (1), has been studied under oxygen atmosphere in the temperature range between room temperature and 200°C. As for Li₈ZrO₆ and Li₈SnO₆ (2), the materials can be considered as solid electrolytes with medium conductivity. The interpretation of the results, considering the Ca²⁺ ions as impurities, are in agreement with those based on the diffusion mechanisms of the Li⁺ ions recently established by ⁷Li NMR in isostructural oxides by Simpson et al. (3).     

Correlation between ionic conductivity and7Li-NMR of polyether-polyurethane networks containing lithium perchlorate

Summary The effect of temperature on the ionic conductivity and the⁷Li NME line width of a polyether-polyurethane network containing lithium perchlorate was studied. A treatment of the results based on free-volume principles yielded a linear correlation between the logarithm of the reduced conductivity, _T /_{T 0}, and the logarithm of the reduced⁷Li NMR line width, _T/_{T 0}, for a given reference temperature, T_o=323 K.     

Étude des séquences sphéroïdales discrètes dans le cas d’un échantillonnage à la fréquence de Shannon

Representation of band-limited, time sampled signals, on a finite-time support, leads to privilege sets, of finite dimension, called discrete spheroidal sequences. The obtention of such sets is, on numerical point of vue, generally quite fine. Here the authors show that sampling at Shannon rate can supply a very simple algorithm for calculation of these spheroidal sequences in the discrete case.     

On an intermediate electronic configuration of cobalt (+III)

In a D_4h site with a strong elongation the ³A_2g term of cobalt (+III) can be stabilized with an electronic configuration intermediate between low spin (¹A_1g) and high spin (⁵E_g): d²_xzd²_yzd¹_xyd¹_z2d⁰_x2_?y2. The La₂Li_0.5Co_0.5O₄ phase, whose layer structure favours strongly such a distortion of the CoO₆ octahedra, illustrates very well this possibility for cobalt (+III). ¹A_1g → ³A_2g and ¹A_1g → ⁵E_g transitions appear as the temperature increases.     

Optimalisation des investissements dans les réseaux téléphoniques locaux

Local telephone networks are composed of cables and equipments used between the subscriber’s premises and his local exchange. This paper describes a method to determine an optimal extension policy for local networks, which minimizes the present worth of annual costs over a given study period, of investments of cables, supporting structures (poles, trenches, conduits...) and subscriber pair gain systems. Using graph and network flow theory, the problem is represented as a non linear integer mathematical program. This problem is solved using a depth firsttree search method. Results obtained on a few typical examples are presented.     

Initial mechanisms involved in substituted magnetite oxidation

The oxidation kinetics of slightly-substituted and pure magnetites, at low conversion ratio is controlled, both by diffusion and surface reaction. However, for highly-subtituted magnetites, diffusion is the sole regulating process. The results are related to the semiconducting properties of the compounds.     

Point target detection and subpixel position estimation in optical imagery

We address the issue of distinguishing point objects from a cluttered background and estimating their position by image processing. We are interested in the specific context in which the object's signature varies significantly relative to its random subpixel location because of aliasing. The conventional matched filter neglects this phenomenon and causes a consistent degradation of detection performance. Thus alternative detectors are proposed, and numerical results show the improvement brought by approximate and generalized likelihood-ratio tests compared with pixel-matched filtering. We also study the performance of two types of subpixel position estimator. Finally, we put forward the major influence of sensor design on both estimation and point object detection.