Sur un Théorème de Steiner

Sans résumé     

DX centers in selectively Si-doped GaAs---AlAs superlattices

DX center has been characterized in four GaAs---AlAs superlattices grown by MBE at 580°C. The structures are uniformly Si-doped or selectively Si-doped in the AlAs layers or in the middle of the GaAs layers or on both sides of the interfaces. Deep level transient spectroscopy measurements (DLTS) put in evidence one dominant electron trap, with an activation energy for thermal emission of about 0.42eV for all the superlattices. This defect shows a thermally activated capture cross section and a large concentration except for the case where the only GaAs layers are doped. For the first time, a study of the capture reveals a capture activation energy of 0.36-0.37 eV, which allows us to locate the DX level nearly 60 meV below the conduction miniband. From these results, we show that the observed DX center is related to silicon in the AlAs layers. For the case when the AlAs barriers are not doped, the DX level is due to the Si diffusion from the middle of the wells towards the barriers, the Si atoms having diffused during the growth.     

Structure of the product formed via Cu(I)-promoted oxidative dimerization ofexo-8-ethynylpentacyclo[5.4.0.02,6.03,10.05.9]undecan-endo-8-ol

The structure consists of two acetyl-substituted PCU cages linked by a diyne fragment. The conformation about the linker is midway between staggered and eclipsed, and the acetyl groups are somewhat distorted due to the proximity to the bulky cage units.     

Generation of Silicone Poly‐HIPEs with Controlled Pore Sizes via Reactive Emulsion Stabilization

Macrocellular silicone polymers are obtained after solidification of the continuous phase of a poly(dimethylsiloxane) emulsion, which contains poly(ethylene glycol) drops of sub‐millimetric dimensions. Coalescence of the liquid template emulsion is prohibited by a reactive blending approach. The relationship is investigated in detail between the interfacial properties and the emulsion stability, and micro‐ and millifluidic techniques are used to generate macrocellular polymers with controlled structural properties over a wider range of cell sizes (0.2–2 mm) and volume fractions of the continuous phase (0.1%–40%). This approach could easily be transferred to a wide range of polymeric systems.

     

Design trade-offs in optoelectronic parallel processing systems using smart-SLMs

Optoelectronic devices with free-space optical interconnections offer new possibilities in massively parallel processing. The trade-offs involved in system design and device selection for optoelectronic implementations are examined. System design trade-offs are approached from algorithmic and technological standpoints. From the algorithmic standpoint, new architectures based on expander graphs, that have been shown to provide low-contention fault-tolerant communication, are discussed. Optoelectronic systems which implement such random graphs can be folded to reduce the hardware cost or unfolded to increase bandwidth. They can also be partially folded by increasing the grain size or by reducing the randomness of the graph topology to reduce the complexity of the interconnection holograms. An optoelectronic and a VLSI implementation of a multistage interconnection network are compared from a technological standpoint. Physical design parameters, such as the chip size or the number of phase levels of the interconnection holograms, are related to the system design metrics such as bandwidth, volume, area and power. It is shown that the optoelectronic implementations have higher performance and are more cost-effective than VLSI implementations. These results are also used to provide general guidelines for device selection in the design of smart pixels/smart spatial light modulators based optoelectronic systems.     

Preliminary study of some “pyro”-silico-nitrides

Two oxynitrides of the LnSiDN system were studied by IR spectroscopy : Nd₂Si₃O₃N₄, with a melilite-like structure and Nd₄Si₂O₇N₂ with a cuspidine-like structure. The first compound belongs to the space group D³_2d(P4̄2₁m) with two formula units per unit cell. There are 10 vibrations of B₂ species and 18 of E species active in the IR. A preliminary study by neutron diffraction displayed that the oxygen and nitrogen atoms are perfectly ordered, and that two silicon atoms are bridged by an oxygen. In the present work, we compare the (SiON₂)₂O IR vibrations with those of silicon oxynitride previously studied and |Si₂O₇|^6? ion in akermanite. The space group of cuspidine is C⁵_2h (P2₁/c with 4 formula units per unit cell. So, 21 vibrations of A_u species and 21 of B_u species are active in IR absorption for Nd₄Si₂O₇N₂. The absorption bands are not as sharp as the bands in Nd₂Si₃O₃N₄. This fact might be explained by a disordered structure as the nitrogen position has not been exactly determined.     

Kinetics of the autocatalytic oxidation of N-amino-3-azabicyclo[3,3,0]octane by chloramine in aqueous medium

The kinetics of the degradation of N-amino-3-azabicyclo[3,3,0]octane by chloramine has been studied by GC and HPLC in stoichiometric conditions in a solution buffered with NaOH/KH₂PO₄ and Na₂B₄O₇.10 H₂O between pH = 10.5 and 13.5. The second-order reaction exhibits specific acid catalysis which indicates competitive oxidation between the haloamine and the neutral and ionic forms of the bicyclic hydrazine. The enthalpy and entropy of activation were determined at pH = 12.89. In a nonbuffered solution, the interaction is autocatalyzed due to acidification of the mixture by the ammonium ions. In basic medium, the reaction forms an endocyclic hydrazone. A mathematical treatment based on an implicit equation allows a quantitative interpretation of all the phenomena observed over the above pH interval. This takes both the acid/base dissociation equilibria and the alkaline hydrolysis of the chloro-derivative into account. © 1995 John Wiley & Sons, Inc.