Robert Deltour

Electrical and dielectric properties of carbon black filled co-continuous two-phase polymer blends

The electrical and dielectric properties of co-continuous polystyrene (PS)/poly(methylmethacrylate)(PMMA) blends loaded with carbon black (CB) of a special grade (BP 1000) have been studied. They strongly depend on the selective localization of the CB particles at the blend interface quite consistently with the double percolation concept. The interfacial localization of CB contributes to the stabilization of the phase morphology against thermal post treatment. Nevertheless, the sample annealing has a very favourable effect on the percolation threshold that decreases. The composition range in which phases are co-continuous is also increased by the addition of the filler. Dispersion of the conductance and the dielectric constant has been measured in the wide frequency range 10-4-108 Hz for blends containing 0-5 wt% CB. On the basis of these results, the mechanism of electrical transport (trapping, hopping, tunnelling, percolation) has been discussed in relation to the CB loading and the optimum loading has been identified for the electrical and dielectric properties to be stable and reproducible.

“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.

Deviations from plastic barriers in Bi2Sr2CaCu2O8+delta thin films

The crystal and the magnetic structures as well as magnetostriction of the Laves-phase compound NdCo2 are investigated by means of temperature-dependent high-resolution neutron-powder diffraction. The compound crystallizes in the cubic Laves phase C15 structure at high temperature, undergoes a tetragonal distortion (space group I4l/amd) around T(C)approximate to 100 K and an orthorhombic distortion (space group Fddd) at T 42 K. The temperature dependence of lattice constants, magnetostriction constant, and magnetic moment indicate that the magnetic and structural transitions are second order in the vicinity of T-C and are first-order around 42 K. Refinements of magnetic structure reveal that the Nd moment distinctly exhibits an abrupt increase at the first-order transition and the easy magnetization direction of the compound changes from [001] in the tetragonal lattice to [011] in the orthorhombic lattice, indicating a strong coupling between crystal structure and magnetic properties at zero field. Analysis of the temperature dependence of bondlength suggests a strong magnetic exchange striction in the tetragonal structure and that the abrupt increase of the Nd moment is attributed essentially to a change in crystal electric field. Field-dependent neutron diffraction reveals a decoupling of the magnetic and structural transitions under relatively modest magnetic fields.

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.

Evidence for a sharp metal-semiconductor phase transition in K2Pt(CN)4Br0.33H2O under pressure☆

Abstract A well defined metal-semiconductor has been observed at high pressure in K2Pt(CN)4Br0.33H2O by resistivity measurements up to 32 kbar. The transition temperature corresponds to a narrow peak in the logarithmic derivative ∂ 1n ϱ/∂ ( 1 T ) and its width is connected with the pressure enhanced 3-d coupling. The 32 kbar data are discussed in terms of a mean-field treatment of the Peierls transition (Horovitz, Gutfreund, Weger). This high pressure investigation suggest a possible suppression of the Peierls transition in KCP, above 70 kbar. Under the same conditions a superconductor, Tc ≅ 6Kcould possible exist.

Electrical properties of poly(2-vinylpyridine)-iodine films

Abstract The d.c. electrical conductivity of thick films of poly(2-vinylpyridine)-iodine is measured as a function of iodine concentration and temperature. The results are interpreted in terms of electronic hopping between ionic centers dispersed at random in the polymer.

Effect of Fe substitution for Cu in the mixed state of (Bi, Pb):2223 superconductor

The effect of the partial substitution of x = 0.02 Fe for Cu in the stabilized (Bi, Pb):2223 phase has been investigated by magnetoresistivity measurements up to 0.7 T. The intra- and intergranular transitions as a function of magnetic field were analysed using versus T curves. The Arrhenius plots are represented by a family of straight lines. The activation energy is proportional to . The main dissipation process is related to the vortex cutting mechanism.

The incommensurate structure of (Sr, Ca)14Cu24O41: a study by electron diffraction and high-resolution microscopy

The modulated structure in (Sr, Ca)14Cu24O41 has been studied using electron diffraction and high-resolution microscopy. The structure can be considered as consisting of two interpenetrating substructures. The first sheet-like substructure is shown to be hardly modulated while the second substructure, consisting of c-oriented chains, contains most of the modulation. High-resolution electron microscopy allows either separate imaging of the two substructures or identification of the misfit between them.

Influences of thermal annealing and humidity exposure on surface structure of (100) single-crystal MgO substrate

We studied the influence of thermal annealing on the surface structure of (100) singlecrystal MgO substrates by atomic force microscopy (AFM). By annealing MgO substrates at various temperatures for 4 h in flowing oxygen, we showed that the surface reconstruction could be explained by considering surface diffusion, surface evaporation, and condensation. At an annealing temperature of 1473 K, a stepped structure was formed with screw dislocations acting as step sources. The influence of humidity on the surface morphology of MgO substrates was also studied by exposing them to a constant humidity of 40 and 80% for different times. After an exposure time of 1.5 h in 80% humidity, the substrate surface was already covered by reaction products. For the 40% humidity, the corresponding time is 10 h. The major reaction product was identified as Mg(OH) 2 by x-ray diffraction.