Resistivity and Carrier Mobility of the SmBa2 Cu3 O6+x Superconductor with Different Oxygen Doping Levels

Abstract DC conductivity measurements between 15 and 300 K are reported for SmBa2 Cu3O6+x samples with different oxygen doping amounts (x) produced by annealing under appropriate high temperature and oxygen pressure conditions and quenching. Samples with x≥0.5 are superconductors: Tc ~60 K at x=0.7, Tc >80 K at ; x=0.9. The transition from superconduction to non-superconduction corresponds to the tetragonal to orthorhombic structural tran-sition and to the transition from semiconducting to metallic temperature dependence of the resistivity. Oxygen doping causes a sudden increase of hole mobility near x=0.5. Below this threshold, the be-havior of the carrier mobility is in agreement with an Anderson localization

Nanoscale effects on the ionic conductivity of highly doped bulk nanometric cerium oxide

Nanometric ceria powders doped with 30 mol % samaria are consolidated by a high-pressure spark plasma sintering (HP-SPS) method to form > 99 % dense samples with a crystallite size as small as 16.5 nm. A conductivity dependence on grain size was noted: when the grain size was less than 20 nm, only one semicircle in the AC impedance spectra was observed and was attributed to bulk conductivity. In contrast to previous observations on pure ceria, the disappearance of the grain-boundary blocking effect is not associated with mixed conductivity. With annealing and concomitant grain growth, the samples show the presence of a grain-boundary effect

Nanosized Sodium-Doped Lanthanum Manganites: Role of the Synthetic Route on their Physical Properties

In this paper we present the results of the synthesis and characterisation of nanocrystalline La1-xNaxMnO3+delta samples. Two synthetic routes were employed: polyacrylamide-based sol-gel and propellant synthesis. Pure, single phase materials were obtained with grain size around 35 nm for the sol-gel samples and around 55 nm for the propellant ones, which moreover present a more broaden grain size distribution. For both series a superparamagnetic behaviour was evidenced by means of magnetisation and EPR measurements with peculiar features ascribable to the different grain sizes and morphology. Preliminary magnetoresistivity measurements show enhanced low-field (< 1 T) magnetoresistance values which suggest an interesting applicative use of these manganites.Comment: 31 Pages 10 Figures to appear in Chem. Mate

Orientational Effects and Random Mixing in 1-Alkanol + Alkanone Mixtures

1-Alkanol + alkanone systems have been investigated through the data analysis of molar excess functions, enthalpies, isobaric heat capacities, volumes and entropies, and using the Flory model and the formalism of the concentrationconcentration structure factor (SCC(0)). The enthalpy of the hydroxyl-carbonyl interactions has been evaluated. These interactions are stronger in mixtures with shorter alcohols (methanol-1-butanol) and 2-propanone or 2-butanone. However, effects related to the self-association of alcohols and to solvation between unlike molecules are of minor importance when compared with those which arise from dipolar interactions. Physical interactions are more relevant in mixtures with longer 1-alkanols. The studied systems are characterized by large structural effects. The variation of the molar excess enthalpy with the alcohol size along systems with a given ketone or with the alkanone size in solutions with a given alcohol are discussed in terms of the different contributions to this excess function. Mixtures with methanol show rather large orientational effects. The random mixing hypothesis is attained to a large extent for mixtures with 1-alkanols ≠ methanol and 2-alkanones. Steric effects and cyclization lead to stronger orientational effects in mixtures with 3-pentanone, 4-heptanone, or cyclohexanone. The increase of temperature weakens orientational effects. Results from SCC(0) calculations show that homocoordination is predominant and support conclusions obtained from the Flory model.Ministerio de Ciencia e Innovación, under Project FIS2010-1695

THE RELAXED F CENTER

The relaxed state of the F center is described in its relations and differences from the unrelaxed state. The most important experimental data referring to the emission phenomena is briefly reviewed ; particular attention is devoted to the radiative lifetime of the relaxed excited state ; nevertheless, photoconductivity data, electric field and stress effects are critically examined. Most of the comments are given in the framework of the "large orbit model". Recent data on emission energy, lifetime and thermal ionization energy, of the relaxed excited F center in NaF, KF, RbF are shown and an interesting dependence of hυemiss and τR on ε∞ is discussed.L'état relâché du centre F est décrit, en rappelant sa relation et différence avec celui pas relâché. Les plus importants résultats expérimentaux qui se réfèrent aux phénomènes d'émission sont brièvement discutés et l'on donne une attention particulière au temps de vie moyenne radiative de l'état excité et relâché ; cependant on va examiner critiquement les résultats de photoconductivité, des effets du champ électrique et des efforts appliqués. La plupart des interprétations est donné dans le cadre du modèle à large orbite. On montrera des résultats nouveaux sur l'énergie d'émission, le temps de vie moyenne et l'énergie thermique d'ionization du centre F excité et relâché dans NaF, KF, RbF et l'on discutera enfin une intéressante relation de hυemiss et de τR avec ε∞

Erbium spectroscopy in photosensitive tin-doped silica glass

There is considerable interest in Er-doped silica-based materials with photosensitive properties for lasing and photorefractive applications. Tin doping has been recently found to induce strong and stable photosensitivity in silica glass. In this work, Er co-doped Sn-doped silica samples are obtained by sol-gel method by hydrolysis and condensation of TEOS and tin dibutyl diacetate, Er(NO3)3, or Er(CH3 COO)3 as dopant precursors. Samples with Er content ranging between 0.1 and 1 mol% are investigated. Sn doping (0.4 mol%) is employed to induce photorefractivity properties in the glassy host. Time resolved photoluminescence, optical absorption and refractive index measurements are carried out and analyzed as a function of the Er content. The specific effects of Er content and Sn doping on the rare earth emission efficiency and photosensitivity of the glass are discussed. In particular it is shown that the Er emission shows the typical features of Er doped sol gel silica and the strong photosensitivity induced by tin doping is not affected by rare earth ions

Surface characterization of Co3O4 electrodes prepared by the sol-gel method

Co3O4 layers on an Ni support were prepared by the sol-gel method. Their properties were characterized by TGA, XRD and SEM. For comparison, Co3O4 was also prepared by thermal decomposition of Co nitrate. The surface electrochemical response was studied in alkaline solution by cyclic voltammetry at several potential scan rates. The electrochemical analysis included the open circuit potential, the voltammetric peak potentials, and the voltammetric charge. Extrapolation of the charge to scan rates v = 0 and v = 1e allowed the 'electrochemical porosity' to be estimated. The results suggest that Co3O4 from the sol-gel method is less crystalline and more porous, although the highest difference in terms of surface charge with respect to the oxide from the nitrate is only a factor of 2. \ua9 Elsevier Science S.A