Photonic devices based on preferential etching

We introduce a design concept of optical waveguides characterized by a practical and reproducible process based on preferential etching of crystalline silicon substrates. Low-loss waveguides, spot-size converters, and power dividers have been obtained with polymers. We have also aligned liquid crystals in the waveguides and demonstrated guided propagation. Therefore this technology is a suitable platform for soft-matter photonics and heterogeneous integration.     

Aluminum buffer rods for ultrasonic monitoring at elevated temperatures

Aluminum (Al) buffer rods, having circular, square, and rectangular cross-section shapes with and without cladding, were experimentally investigated to achieve high performance in ultrasound for industrial process monitoring. Focus was devoted to the signal-to-noise ratio (SNR) aspect in pulse-echo mode because high SNR can make many practical ultrasonic monitoring applications feasible. If the size of the rod cross section can be properly selected, spurious echoes induced in the rod having finite dimensions--due to mode conversion, wave reverberation, and diffraction--do not overlap the desired echoes during pulse-echo measurements. Thus, a sufficient SNR may be obtained using nonclad rods for certain applications, which is of low cost. Clad buffer rods, having the above three shapes and consisting of an Al core and a stainless steel cladding, also were manufactured through thermal spray method. These clad Al rods achieved a SNR of more than 40 dB and had better SNR than clad steel rods. It was shown that the rod with a larger cross-section area exhibited a wider ultrasonic beam. The Al rods are handy in real-time, nonintrusive and nondestructive ultrasonic monitoring.     

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.