Analysis and optimisation of the HIPERLAN Channel Access Contention Scheme

This paper analyses the HIPERLAN channel access mechanism. Both a purely mathematical and a simulation model are built in order to evaluate the two successive phases of the contention scheme. Several optimisations, based on the choice of parametric values, are proposed, evaluated and compared.     

Sur de nouvelles phases séléniées ternaires du molybdène

The synthesis and the crystal properties of new selenides of formula M_xMo₃Se₄ are described. If M = Zn, Ag, Cd, Sn and Pb, they are stoichiometric with x = 0.6; if M = Fe, Mn, Cr, V, Ti, triclinic solid solutions are observed with 0.5 < x < 0.7; if M = Cu, Co, Ni, rhombohedral solid solutions are obtained with 0 < x < 1.4 for M = Cu, 0 < x < 0.7 for M = Co and 0 < x < 0.8 for M = Ni All these phases can be deduced from the Mo₃Se₄ structure by introducing metal atoms into the tunnels between the “Mo₆Se₈” metal atom cluster configuration.     

Plasma-enhanced atomic layer deposition of Co using Co(MeCp)2 precursor

Cobalt (Co) thermal or plasma enhanced atomic layer deposition (PE-ALD) was investigated using a novel metal organic precursor, Co(MeCp)₂, and NH₃ or H₂ or their plasma as a reactant. The growth characteristics, electrical and microstructural properties were investigated. Especially, PE-ALD produced Co thin films at low growth temperature down to 100 °C. Interestingly, the low temperature growth of Co films showed the formation of columnar structure at substrate temperature below 300 °C. The growth characteristics and films properties of PE-ALD Co using bis(η-methylcyclopentadienyl) Co(II) (Co(MeCp)₂) was compared with those of PE-ALD Co using other Cp based metal organic precursors, bis-cyclopentadienyl cobalt (II) (CoCp₂) and cyclopentadienyl isopropyl acetamidinato-cobalt (Co(CpAMD)).     

A new increasing delocalization of M=3d-elements (Ti, Fe, Co) in the channels network of the ternary MyMo6Se8 Chevrel phases

A structural investigation on single crystals of M_yMo₆Se₈ Chevrel phases (M=3d: Ti, Fe, and Co) has been carried out. These latter compounds as well as others with Cr, Mn, and Ni atoms make part of a new original family of Chevrel phases. The M ions occupy new interstices in the tridimensional channels network never observed in the other classical Chevrel phases as Li_y, Cu_y Mo₆X₈. They occupy only cavity 2 centered on and with a progressive delocalization of the M cation versus the nature of the cation from the center of the cavity to the outside . The position in cavity 2 is different from the position of the Cu (2) atoms, the second position of the copper atoms in the channels of the Cu_yMo₆X₈ compounds. This new position reinforces the interaction with the Mo₆ cluster, and may be able to involve new remarkable physical properties as thermoelectric properties.     

In situ monitoring of styrene polymerization using Raman spectroscopy. Multi‐scale approach of homogeneous and heterogeneous polymerization processes

Free radical polymerization of styrene was monitored in situ by combining Raman spectroscopy to other experimental techniques (gravimetry and rheology). Three different processes were investigated: bulk, emulsion and miniemulsion polymerization. A complete analysis of the evolution of Raman spectrum during the course of reaction showed that a lot of information about molecular dynamics could be extracted and related to chemical phenomena. In addition, we report for the first time the coupling of Raman spectroscopy to a rheometer in order to monitor styrene bulk polymerization both at the scale of chemical bonds and at the scale of macroscopic phenomena (viscosity variation). Copyright © 2013 John Wiley & Sons, Ltd.     

New pseudo-one-dimensional metals: M2Mo6Se6 (M = Na,In, K,TI), M2Mo6S6 (M = K,Rb, Cs), M2Mo6Te6 (M = In,TI)

We present a new series of ternary chalcogenides, derived from divalent molybdenum: M₂Mo₆X₆. These compounds crystallize in a hexagonal lattice with a ≈ 9 Å, c ≈ 4.5 Å, and space group $\text{P6}{}_3\text{m}$. The compounds are characterized by clusters (Mo₃)¹_∞ in the form of linear chains, resulting from a linear condensation of Mo₆ octahedral clusters. The (Mo₃)¹_∞ clusters are well separated from each other, with the shortest MoMo intercluster distance larger than 6 Å. The resulting pseudo-one-dimensional structures show remarkable anisotropy of physical properties.     

Hot pressing sintered CuxMo6S8 targets for laser ablation thin films deposition

Sintering conditions of Cu₄Mo₆S₈ pellets, 20 mm in diameter and about 4 mm thick have been determined. The expulsion of copper out of the pellet for an annealing temperature of 1100 ° C has been observed, in agreement with the evolution of the phase diagram when the temperature increases. The appearance of Cu₂S cristallites on the surface of the films deposited from highly densified targets associated with a high excess of copper and sulfur can be correlated with the behavior observed on the pellets. In fact the increase of the temperature of the target under the laser beam impact is likely responsible for a copper migration on its surface, which leads to the observed composition of the film.     

A new pseudo-one-dimensional superconductor: Tℓ2Mo6Se6

The properties of a new superconducting pseudo-one-dimensional compound, T?₂Mo₆Se₆ are presented. The crystal structure of this material contains linear chains in the form of infinite stacks of Mo₆ clusters. The anisotropy in the normal state resistivity is found to be of the order of 10³ and the parallel upper critical field is 26 times higher than the perpendicular one.     

Neutron diffraction study of the magnetic order in the ternary superconductor GdMo6S8

The coexistence of antiferromagnetism and superconductivity has been demonstrated in GdMo₆S₈ by neutron diffraction experiments. Although the magnetic unit cell (2a,a,a) is determined by the RKKY interactions, the direction of magnetic moments is determined by the anisotropic dipolar interactions i.e. in the (100) cubic plane.