Eric Vanlathem

“Tweed” structure of Fe-doped YBa2Cu3O7−δ

Abstract Fe-doped YBa 2 Cu 3 O 7−δ single crystals have been prepared by a flux method from an Y-Ba-Cu-O precursor with copper partially substituted by iron. Electron diffraction and microscopy were applied to study the domain structure of the material. Superposed on a broad twin band texture, as also observed in undoped material, all samples reveal a finer “tweed” structure of overlapping lenticular domains oriented along [120] and [1 1 0]. This suggests a two-step formation process of the domain structure. The displacement field, corresponding with the tweed structure, resembles that of microtwinning in undoped, quenched YBa 2 Cu7 3 O 7−δ samples. The randomly dispersed Fe-ions act as pinning centers or fragment the structure by the induction of twin interfaces. Monte Carlo simulations support this idea.

Electrical conductivity in carbon black-loaded polystyrene-polyisoprene blends. Selective localization of carbon black at the interface

SummaryThe electrical conductivity of carbon-black loaded polystyrene-polyisoprene blends has been studied. In this ternary system, the filler is at the interface of co-continuous polyblends as confirmed by the very low value of the filler percolation threshold (0.2 vol % for blends compression molded at 250°C) and by optical microscopy. As a result of the selective localization of carbon black at the interface, the percolation threshold is very sensitive to the compression molding temperature.

Electrical conductivity of polystyrene-rubber blends loaded with carbon black

Abstract Polystyrene/rubber blends have been loaded with carbon black (CB) and the filler localization in the two-phase polyblends has been studied in relation to the chemical structure of the rubber. The CB localization and the electrical conductivity are greatly influenced by the substitution of the rubber chains. In polystyrene/polybutadiene blends, the filler is localized within the polybutadiene phase. In contrast, in polystyrene/polyisoprene and polystyrene/ethylene—propylene rubber (EPM) blends, CB is mainly localized at the interface, so that the CB percolation threshold in cocontinuous two-phase polyblends is dramatically decreased.

Superconductivity and microstructure in Fe-doped Bi-Sr-Ca-Cu-O compounds

Abstract We have studied the influence of Fe impurities in the superconducting compound Bi-Sr-Ca-Cu-O prepared from two basic nominal compositions Bi 2 Sr 2 Ca(Cu 1− x Fe x ) 2 O 8+δ and Bi 4 Sr 3 Ca 3 (Cu 1− x Fe x ) 4 O 16+δ . The superconducting and secondary phases have been analyzed by SEM, EPMA, HREM and X-ray diffraction. The composition of the doped superconducting phase is best described by the formula: Bi 2+ y (Sr, Ca) 3 (Cu 1− x Fe x ) 2− y O 8+δ , where y increases as a function of x ( x Annealing of the samples in a poor oxygen atmosphere improves the T c values of the doped samples. The downward shift of T c for increasing Fe content follows a linear relationship with a slope of 5 K/(Fe/Fe+Cu) at.%. The importance of the Bi-O-Cu-O electron coupling in the superconductivity mechanism is discussed.

Neutron scattering study of the dynamics of iron spins in iron doped Bi2CaSr2Cu2O8 superconducting compounds

Abstract We have studied the dynamics of the iron spins in a 6.5% iron doped Bi 2 CaSr 2 Cu 2 O 8 superconducting compound with T c =46.5 K by time of flight (TOF) neutron scattering experiments. The quasielastic and elastic intensities and the width of the quasielastic signal strongly vary with temperature. At low temperature, a slowing down of the iron spin dynamics is observed, starting around 60 K, i.e. above T c . At higher temperature, an unusual temperature dependence of the quasielastic intensity is observed in the doped Bi compound as previously described in Fe doped YBa 2 Cu 3 O 7 .

Effects of substitution of copper by iron and cation distribution in the Bi-Ca-Sr-Cu-O superconducting compounds

Abstract We have investigated the effects of Fe on the structure, the phase composition and the superconducting properties of the Bi-Sr-Ca-Cu-0 compounds. The composition of the doped, low temperature (85K), superconducting phase is described by the formula: Bi 2 + y y(Sr,Ca) 3 (Cu 1 − x Fe x ) 2 − y O 8 + δ where y increases as a function of x, ( x For low iron concentration, X-ray and neutron diffraction show no important modification of the lattice primitive cell. However, X-ray line broadening is observed with increased iron content. The EDAX results confirm a progressive replacement of the copper atoms by the iron impurities. This phase is semiconducting with an activation energy of 50 meV.

Neutron scattering study of the dynamics of Fe spins in Bi2 Ca Sr2 (Cu1−xFex) 2 O8

Abstract A quasielastic neutron diffusion line is observed in a 6.5% Fe substituted Bi 2 Ca Sr 2 O 8 high Tc superconductor, whose intensity and linewidth are strongly temperature dependent, evidencing a slowing down of the spin dynamics at low temperature.