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Nanoparticles of nickel-doped iron chromite (Fe1-xNixCr2O4) with composition x = 0.2-0.8 have been prepared by the sol–gel method using citric acid as a complexing agent. X-ray diffraction (XRD) re...

International audience; Hydrophobic elastomers are capable of absorbing a small amount of water that forms droplets around hydrophilic sites. These systems allow the study of confinement effects by a hydrophobic environment on the dynamics and thermodynamic behaviour of water molecules. The freezing–melting properties and the dynamics of water inside nano-droplets in butyl rubber are affected, as revealed by differential scanning calorimetry (DSC) and deuterium nuclear magnetic resonance (2H-NMR). Upon cooling down, all water crystalizes with a bimodal droplet population (da = 3.4 nm and db = 4.4 nm) in a temperature range associated with the droplet size distribution. However, the melting temperature is not shifted according to the Gibbs–Thomson equation. The relative decrease of the 2H-NMR longitudinal magnetization is not a single exponential and, by inverse Laplace transformation, it was deduced to be bimodal in agreement with the DSC measurements (T1,a ∼ 10 ms and T1,b ∼ 200 ms). The deduced correlation time of molecular reorientation is longer than that of bulk water and the behaviour with temperature follows the Vogel-Fulcher-Tammann (VFT) equations with a changing fragility as the droplet size is reduced when reducing hydration.

The electric conductivity and thermoelectric power for different compositions of the chalcogenide glasses (SbSn)xSe100−x (x = 4, 8, 12, 16 and 20 at.%) were measured experimentally. The dc conductivity is found to have an Arrhenius behavior in the temperature range (300–450) K with typical activation energies decreasing from 0.87 to 0.74 eV with the SbSn content. The concentrations of charge carriers, as well as their mobility and characteristic relaxation time were deduced in the same temperature range and compositions. The variations of activation energy calculated from measured electric conductivity versus SbSn content are compared to the behavior of calculated half band gap. From the measured Seebeck coefficient, we deduced its specific activation energy and found that it is lower than that related to electric conductivity. This difference is assumed to be the activation energy needed for polaron hopping. The increase in cross-linking with SbSn content is proposed as an explanation for the increase in polaron-hopping activation energy.

The structure and porosity of carbon black (CB) grades determine the ultimate applications of these most industrially used members of carbon materials. Hence, one uses complimentary methods to investigate these features for two conductive CB grades in order to understand their effects on the electric properties of polymeric nanocomposites. The mean particle size, the aggregate structure and voids morphology are studied by scanning electron microscopy (SEM) whereas high resolution transmission electron microscopy (HRTEM) reveals the ordering of the graphitic layers inside a CB particle as well as particles interconnections. The specific surface, voids size distribution and specific voids volume are studied using nitrogen adsorption, differential scanning calorimetry (DSC) thermoporosimety and nuclear magnetic resonance (NMR) relaxometry. This comparative study revealed a relatively smaller particle size, higher specific surface and specific volume with more disorder graphitic layers in Ketjenblack EC600JD than in Printex XE2. Finally, one presents the effect of such structural and porosity differences on the electric conductivity of nanocomposites made of these two grades.

The effects of the Aluminium concentration on the emission of Al 0.45 In 0.55 As/Al y Ga 1− y As quantum dots (QDs) are investigated by photoluminescence (PL), with the excitation power density as a variable parameter. The influence of a varying barrier composition on the QD emission is investigated theoretically and discussed with respect to PL measurements. For the highest barrier composition value ( y = 0.77), we interpret the QD emission as originating from indirect type-II transitions involving electrons in the barrier X valley and heavy holes (HH), with S and P symmetry, in Al 0.45 In 0.55 As QDs. The PL spectra of the y = 0.38 sample exhibits three lines: two of them are related to indirect type-II transitions, in which the electron ground state belongs to the indirect gap (L and X) minima in the barrier conduction band, whereas the third transition is attributed to a direct type-I transitions.

The effect of iodine content in Ix (As20Se80)100-x where x=0, 4, 8, 12, and 16 atomic % was studied both theoretically and experimentally. The calculated mean coordination number, cohesion energy, mean overall bonding energy, and band gap decrease with iodine content. This is correlated to the decrease of crosslinking by progressive incorporation of iodine. The impact of increasing iodine content on optical properties was studied through the analysis of a single reflection spectrum using Minkov's method. The experimentally evaluated band gap decreases from 1.7 eV to 1.41 eV in good agreement with the different estimations based on Manca's relation. The Urbach energy, related to the structural disorder, increases with the incorporation of iodine. From the measured refraction index, one deduces that the single oscillator energy decreases with iodine content while its dispersion energy increases. The nonlinear optical properties increase substantially with the incorporation of the more polarizable iodine atom.

We report photoluminescence (PL) characterization and model simulation of AlInAs/AlGaAs type-II quantum dots (QDs). A thorough and precise determination of the band parameters for QD and matrix materials is given, focusing on the effects of alloy composition and strain state on the electronic properties. Origins of experimentally observed PL emission peaks are identified through a comparison with the band lineup theoretically determined in this work. We interpret the QD emission as originating from indirect type-II transitions involving electrons in the barrier X valley and heavy holes with S and P symmetry.

A systematic investigation into the surface properties of siloxane rubber/carbon black (CB) nanocomposites has been performed, using an automated scanning probe microscope. In this way the influence of CB concentration and curing rate of the siloxane rubber matrix on roughness and conductivity of the composites was studied. Decreasing the curing rate while keeping the CB concentration resulted in a decrease in both roughness and surface conductivity, which can be explained by an additional siloxane-rubber layer formed during curing.

Mobility and orientation of amorphous chains in cold drawn films of isotactic polypropylene (PP) are studied by 2 H-NMR as a function of the annealing temperature. It is shown that the deformation is not affine. The relaxation of amorphous chains in materials annealed with free and clamped ends appears respectively at T *≈ T g +50°C and at T α c ≈80°C. Above these transition temperatures, corresponding to the onset of the chain mobility in the amorphous and crystalline states, the orientation of amorphous chains decreases linearly with the temperature. These reports show that the 2 H-NMR technique provides important information on the plastic deformation of semi-crystalline polymers.

It is shown that for porous systems filled with a solvent, if the temperature domains of crystallization and melting of the solvent are well separated, DSC technique, combined with suitably chosen thermal cycles, provides crystallization and melting curves which are independent of a) the mass of the material, b) the thermal contact between DSC pan and material and c) the thermal conductivity of the material. This method called DSC fractionation is applied to butyl rubber containing small water nodules. Thermoporosimetry is then applied to get the size distribution of mesoscopic solvent droplets.

The thermal transitions observed in hydrogels based on interpenetrating networks of poly(ethyl acrylate) and poly(hydroxyethyl acrylate) are explained in a simple thermodynamic framework based on the phase diagram that avoids making use of the hypothesis of different “kinds” of water present in the hydrogel. The features that can be explained on its basis include the melting temperature depression of water, the occurrence of crystallization on heating (even when it was absent upon cooling), and the existence of an amount of noncrystallizable water. Three water concentration regimes can be predicted to exist, which give rise to qualitatively different kinds of thermograms.

We analyze the electronic energy band structure of strained and unstrained III-V semiconductors alloys within full-zone k.p approach in order to reach a realistic and minimal k.p model, parametrized to provide an accurate description of both valence and conduction bands. We show that a 40-band k.p model is fairly sufficient to reproduce accurately the overall band structure, obviating the use of any fictitious s* orbital. As an application, the 40-level k.p model is used to describe the band offsets as well as the band parameters in the strained AlxIn1−xAs/AlyGa1−yAs system.