Ground state of the quasi-1D correlated electronic system BaVS3

In this paper we review the salient features of the different instabilities exhibited by the quasi-1D system BaVS₃ and show that there is a subtle interplay between the different phases stabilized. The analysis of the Peierls instability shows that the mobile dz² electrons are more localized than calculated because of their strong correlation with the localized e(t_2g) electrons. The complex AF magnetic structure of BaVS₃ incorporates the magnetization of the e(t_2g) electrons with the Peierls pairing of the dz² electrons into magnetic singlets. Finally, we propose that the zig-zag disorder remaining after an incomplete orthorhombic phase transition could change the sign of the magnetic coupling and thus help to stabilize the canted ferromagnetism observed in non stoichiometric BaVS_3−δ and Sr and Ba substituted compounds.     

Laser ultrasonics detection of an embedded crack in a composite spherical particle

Laser ultrasonics was applied to the manufacturing control of the integrity (no failure) of coated spherical particles designed for High Temperature Reactors (HTR). This control is of major importance, since the coating of the nuclear fuel kernel is designed to prevent from the diffusion of fission products outside the particle during reactor operation. The SiC layer composing the coating is particularly important, since this layer must be an impenetrable barrier for fission products. The integrity of the SiC shell (no crack within the shell) can be assessed by the ultrasonic vibration spectrum of the HTR particle, which is significantly changed, compared to the reference spectrum of a defect-free particle. Spheroidal vibration modes of defect-free dummy particles with a zirconium dioxide (ZrO₂) core were observed in the 2-5 MHz range. A theoretical analysis is presented to account for the observed vibration spectra of defect-free or cracked HTR particles.     

Some special aspects of the copolymer synthesis with Ziegler-Natta catalysts

An approach of the special problems observed in copolymerization with Ziegler-Natta heterogeneous catalysts is given on the basis of linear low-density polyethylene synthesis in gas phase reaction. The reaction is controlled by chemistry and not by diffusion. The ethylene-l-butene copolymerization reactivity computed from the kinetic study varies with the copolymer composition, and this is due to the fact that the comonomer acts as a ligand of the active centers. The reactivities computed from NMR analysis are also not really reliable as the copolymer is a mixture of copolymers with different compositions.