Thierry Pagnier

Effect of rapid quenching on electrical properties of lithium conductive glasses

Abstract A twin roller apparatus has been designed to be used in a controlled environment, so that even hydroscopic and oxidizable glasses may be prepared by rapid quenching. xLi2O(1−x)P2O5 and xLi2S(1−x)GeS2 glasses have been prepared and their electrical conductivity measured as a function of temperature. The electrical characteristics of rapidly quenched and conventional glasses are compared in order to study the influence of the cooling rate. The results are quite different for oxide and sulfide glasses. Rapid quenching does not much affect oxide glasses whereas for sulfide glasses important decreases in activation energies and pre-exponential factors are observed.

Reactivity of SnO2–CuO nanocrystalline materials with H2S: a coupled electrical and Raman spectroscopic study

Abstract Nanocrystalline CuO-doped SnO 2 powders produced by different ways have been subjected to H 2 S at 100°C. As for pure SnO 2 , the reversible formation of SnS x was deduced from Raman spectra and correlated with the changes in electrical resistance. When the initial resistance of the sample was high, adsorbed SO 4 was observed, and in this case only, the reversible formation of Cu 2 S. These various phenomena are discussed in the light of thermodynamic data and the influence of powder preparation is emphasized.

Oxygen reduction at La0.5Sr0.5MnO3 thin film/yttria-stabilized zirconia interface studied by impedance spectroscopy

La[sub 0.5]Sr[sub 0.5]MnO[sub 3] thin films were prepared by a pyrosol technique. Films were characterized by x-ray diffraction and scanning electron microscopy analysis. Their electrical resistance was measured, and they showed a semiconductive behavior. The [eta](i) curves were drawn at 800 and 900 C and impedance spectroscopy was performed during polarization. At low polarization, the oxygen reduction takes place at the triple contact points and is progressively delocalized over the whole surface of the electrode. At high polarizations, two components of the impedance have been identified. The first, at high frequency, is correlated with an atomic rearrangement of the electrode (either in the bulk or at the surface), which favors the electrode reaction. At low frequencies, the impedance has been attributed to a concentration impedance due to at least two adsorbed species.

Capping ligand effects on the amorphous-to-crystalline transition of CdSe nanoparticles.

Amorphous CdSe nanoparticles were prepared by a base-catalyzed room-temperature reaction between cadmium nitrate and selenourea, with dodecanethiol as a capping ligand. The nanoparticle size could be controlled from 1.9 to 3.6 nm by increasing the water concentration in the reaction. When the nanoparticles were heated in a pyridine suspension, excitonic peaks appeared in the initially featureless optical absorption spectra. By changing the suspension solvent and the capping ligand and its concentration, it was shown that the dynamic surface exchange between the ligand and pyridine controls the crystallization process. This phenomenon was interpreted as a surface rigidity effect imposed by the ligand, whose importance was separately evidenced on the dried nanoparticles by the evolution of X-ray diffraction patterns and Raman spectra. In particular, both techniques showed that a threshold temperature is needed before crystallization occurs, and such a threshold was related to ligand desorption. The surface effect was directly visualized by high-resolution transmission electron microscopy observations of the amorphous particles, where crystallization under the electron beam was observed to start by the formation of a crystalline nucleus in the nanoparticle interior and then to extend to the whole structure.

Electrical properties of La0.5Sr0.5MnO3 thin films

Abstract La 0.5 Sr 0.5 MnO 3 thin films were prepared by ultrasonic spraying and RF sputtering. The electrical conductivity and the activation energy depend on substrate composition. Electrical measurements performed under substrate bending were interpreted in terms of charge carrier blocking effects due to grain boundaries and cracks in the film.

Preparation of La1−xSrxMnO3 thin films by a pyrosol derived method

Abstract La 1−x Sr x MnO 3 films (0≤x≤1) have been synthetised by the Pyrosol method and characterized by powder X-ray diffraction and scanning electron microscopy. Precursors were aqueous solutions of La, Sr and Mn nitrates. Films composed of monosized oxide grains were obtained on polycrystalline alumina substrates. The films were very homogeneous and their compositions in good agreement with those of the starting solutions. X-ray investigations revealed that the films had the same crystallographic structure as bulk materials.

Temperature-dependent raman spectroscopy of lithium triflate-PEO complexes: phase equilibrium and component interactions.

Poly(ethylene oxide) and complexes of lithium trifluorosulfonate-poly(ethylene oxide) (LiTf-PEO) with 4 <or= O/Li <or= 18 have been studied by Raman spectroscopy from room temperature up to 160 degrees C. The Raman spectrum of the (PEO)(3)LiTf defined compound has been deduced from the experimental data. Subtraction of the Raman spectrum due to (PEO)(3)LiTf in each sample allowed us to determine for the first time the composition and the Li(+)-Tf(-) and Li(+)-PEO interactions in the part of the polymer not crystallized as (PEO)(3)LiTf. It is shown that the local interactions between cation and anion or between cation and PEO chain persist even in the melted state, up to near the liquidus temperature. In particular, the Li(+)-PEO interactions decrease significantly just below the liquidus temperature with a simultaneous strong increase in the ion pair concentration.

Phosphate-based glasses containing transition metal oxides: chemical intercalation of alkali metals and structural investigations by Mössbauer spectroscopy

Abstract Glasses of composition 0.6 MO 3 0.4 P 2 O 5 (M  Mo, W) and 0if0.4 Fe 2 O 3 0.6 P 2 O 5 were prepared. Chemical intercalation of Li and Na was carried out starting from NaI, LiBr and n -butyl lithium. The density of these glasses was measured prior to and after intercalation and the volume growth was found to be small (less than 5%) for an intercalation ratio of more than 50%. Mossbauer spectroscopy was used to determine the structure of the 0.4 Fe 2 O 5 0.6 P 2 O 5 glass and for the glasses obtained after intercalation. The results suggest that Fe(III) is present in a single, probably octahedral, environment.

Metal-oxide based nanocomposites as materials for gas sensors

Correlations between the composition, structure, and sensor properties of SnO2-MIIO (MIIO = Fe2O3, MoO3, V2O5) nanocomposites prepared by wet chemistry synthesis were elucidated. The elemental and phase compositions of the materials, distribution of components between the bulk and surface, particle size, and specific surface area were examined. Surface modification of semiconductor oxides allows controlling the type and density of surface acid centers and redox properties of materials. The result is an increase in the sensor selectivity.

An ab initio study of WO3 under pressure up to 30 GPa

High-pressure polymorphs of WO3 have been studied with a first-principles pseudopotential method. The medium-range (0.01–20 GPa) and high-range (20–30 GPa) polymorphs have been characterized and are compared with recent experimental results. The main new feature is the appearance of a sevenfold coordinated tungsten in the high-pressure polymorph. The subtle phase transitions that were induced from Raman spectra evolutions have not been confirmed. However, changes in the W–O distances and O–W–O and W–O–W angles may explain the changes in Raman spectra.