Polarized infrared reflectivity study of an oriented ceramic of Bi 2 Sr 2 Ca 2 Cu 3 O 10 + δ (Bi-2223)

The reflectivity spectra of an oriented ceramic of Bi-2223 has been investigated by polarized infrared reflectivity spectroscopy in the energy range 0.005-2.2 eV. It is shown that the data for the polarization parallel to the c axis cannot be fitted with a one-component Drude or extended-Drude model. The conductivity spectrum is then obtained from the best fit of a “double-damping Drude” model to reflectivity spectra, itself derived from the factorized form of the dielectric function, and by a Kramers-Kronig inversion as well. The data and their analysis give a new insight of the 2D character of the system. Received 26 April 2001 and Received in final form 28 August 2001     

Reaction de metathese des olefines induite photochimiquement en presence d'un complexe de metal de transition. IV.- etude cinetique de l'interconversion catalysee cis-trans du butene-2.

The cis-trans interconversion of olefins in the system W(CO)₆ + CCl₄ + 2-butene is studied, both with initial irradiation of a solution of W(CO)₆ in CCl₄ (photoinduction), and with continuous irradiation of the system, for cis- and trans-butene concentrations between 0.09 and 6.0 M. Analysis of the results of the photoinduction experiments (rate of conversion and kinetic law as a function of the initial concentration of the olefin) allowed us to propose a simple kinetic scheme comprising a cis-trans interconversion of 2-butene and olefin-catalyzed destruction of the catalytic entity (k₂ = (0.62 ± 0.06)x10^?4 M s^?1). In the continuous irradiation experiments the final distribution of the olefin was independent of the initial butene concentration (cis-2-butene/trans-2-butene 3.0) and the reaction kinetics are of first-order (k_obs = (3±1) x10^?4 s^?1. Comparison of the two experiments suggests continuous photochemical regeneration of the catalytically active entity. The first-order reaction kinetics are in agreement with a carbene-metal carbonyl structure of the intermediate     

Dilatation thermique anormale et caractere en couches des metavanadates MV2O6 (M = Ca,Cd, Zn,Mg, Pb)

The variations of cell parameters and thermal expansion tensors of metavanadates MV₂O₆ have been measured in the range 77–295 K. The thermal expansion anisotropy is characteristic of layer structures especially for brannerite-type structures (M = Cd, Zn, Mg); this anisotropy is explained by the presence of [VO₅] polyhedra, such as occur in the V₂O₅ layer structure. For CaV₂O₆ the variation of thermal expansion as a function of temperature is abnormal: peaks, typical of a diffuse transition, are observed at 260 K for α₁(T) and α₃(T) curves. The temperature anomaly is reduced when cadmium is substituted for calcium.     

Preparation,analyses et etudes thermochimiques du tetrafluorure de tellure

Tellurium Tétrafluoride has been prepared and various analysis (chemical, thermal, Xray) have been performed. Heat of fusion (δH_fus = 3,02 Kcal.mol^-1), and calorific capacity of solid TeF₄ from 298 to 402°K (Cp_s = 30 cal.K^-1. mol^-1) and of liquide TeF₄ at 423°K (Cp₁ = 31, 1 cal.K.^-1.mol^-1) have been determined Enthalpic values are given.     

Shape-dependent electrocatalysis: methanol and formic acid electrooxidation on preferentially oriented Pt nanoparticles

Reactivity towards methanol and formic acid electrooxidation on Pt nanoparticles with well characterised surfaces were studied and compared with the behaviour of single crystal electrodes with basal orientations. Polyoriented and preferential (100), (111) and (100)-(111) Pt nanoparticles were synthesised, cleaned preserving its surface structure, characterised and employed to evaluate the influence of the surface structure/shape of the Pt nanoparticles on these two relevant electrochemical reactions. The results pointed out that, in agreement with fundamental studies with Pt single crystal electrodes, the surface structure of the electrodes plays an important role on the reactivity of both oxidation processes, and thus the electrocatalytic properties strongly depend on the surface structure/shape of the nanoparticles, in particular on the presence of sites with (111) symmetry. These findings open the possibility of designing new and better electrocatalytic materials using decorated shape-controlled Pt nanoparticles as previously described with Pt single crystal electrodes.     

Speciation of trace metals in natural waters: the influence of an adsorbed layer of natural organic matter (NOM) on voltammetric behaviour of copper

The influence of an adsorbed layer of the natural organic matter (NOM) on voltammetric behaviour of copper on a mercury drop electrode in natural water samples was studied. The adsorption of NOM strongly affects the differential pulse anodic stripping voltammogram (DPASV) of copper, leading to its distortion. Phase sensitive ac voltammetry confirmed that desorption of adsorbed NOM occurs in general at accumulation potentials more negative than −1.4 V. Accordingly, an application of negative potential (−1.6 V) for a very short time at the end of the accumulation time (1% of total accumulation time) to remove the adsorbed NOM was introduced in the measuring procedure. Using this protocol, a well-resolved peak without interferences was obtained. It was shown that stripping chronopotentiogram of copper (SCP) in the depletive mode is influenced by the adsorbed layer in the same manner as DPASV. The influence of the adsorbed NOM on pseudopolarographic measurements of copper and on determination of copper complexing capacity (CuCC) was demonstrated. A shift of the peak potential and the change of the half-peak width on the accumulation potential (for pseudopolarography) and on copper concentration in solution (for CuCC) were observed. By applying a desorption step these effects vanished, yielding different final results.     

Entropy production and time asymmetry in nonequilibrium fluctuations

The time-reversal symmetry of nonequilibrium fluctuations is experimentally investigated in two out-of-equilibrium systems: namely, a Brownian particle in a trap moving at constant speed and an electric circuit with an imposed mean current. The dynamical randomness of their nonequilibrium fluctuations is characterized in terms of the standard and time-reversed entropies per unit time of dynamical systems theory. We present experimental results showing that their difference equals the thermodynamic entropy production in units of Boltzmann's constant.     

Applying nonlinear resonant ultrasound spectroscopy to improving thermal damage assessment in concrete

Nonlinear resonant ultrasound spectroscopy (NRUS) consists of evaluating one or more resonant frequency peak shifts while increasing excitation amplitude. NRUS exhibits high sensitivity to global damage in a large group of materials. Most studies conducted to date are aimed at interrogating the mechanical damage influence on the nonlinear response, applying bending, or longitudinal modes. The sensitivity of NRUS using longitudinal modes and the comparison of the results with a classical linear method to monitor progressive thermal damage (isotropic) of concrete are studied in this paper. In addition, feasibility and sensitivity of applying shear modes for the NRUS method are explored.     

Engineering paper as a substrate for blood typing bio-diagnostics

The effect of paper structure on blood typing visualization was quantified and analyzed to engineer low-cost diagnostics. Commercial and experimental papers varying in fibre composition, basis weight, density and porosity were investigated for their ability to separate agglutinated (blood interacted with specific antibodies) from non-agglutinated (blood interacted with non-specific antibodies) red blood cells (RBCs). Antibodies solutions and blood samples were sequentially absorbed on paper, allowed to interact, eluded with a saline solution, and the intensity of the remaining blood spot was quantified by image analysis. The efficiency and clarity of RBC separation was quantified with the relative intensity (R.I.) index defined as the intensity ratio of the non-specific test over the specific system; the lower the R.I., the better is the separation between a positive from a negative test. Thick and dense papers are improper for blood typing as they retain indiscriminately both agglutinated and non-agglutinated RBCs. Thin and porous papers provided the best performance. The R.I. index (the lower the better) increased fairly linearly with paper density and thickness but inversely proportionally with paper pore size. The type of fibres played a minor role. The paper structure is critical in the design of blood typing assay. However, it is only one element of the diagnostic system to engineer with the interactions RBC-antibody-paper.