María J. Ariza

Electrodeposition of Cu on deeply reduced polypyrrole electrodes at very high cathodic potentials

Conducting polymers are considered to suffer metal–insulator transitions during reduction. After a deep electrochemical reduction, freestanding films of polypyrrole retain a conductivity as high as 0.05 S cm−1. Used as cathodes, they allow the flow of high cathodic currents from aqueous solutions, the hydrogen release being inhibited up to −3.0 V. Fast copper electrodeposition was obtained at −0.6, −1.5 and −2.1 V using freestanding films reduced for a long time. The electrochemically stimulated conformational relaxation (ESCR) model can explain the high conductivity of these “reduced” materials by the trapping of counterions when the structure shrinks and closes. Conductivity in the range of semiconductors and the inhibition of hydrogen release open new possibilities for the use of reduced conducting polymers as electronic conductors and cathodes for electrochemical studies in aqueous solutions.

Colloidal permeability of liquid membranes consisting of hard particles by nonequilibrium simulations

A novel particulate membrane, comprised of a confined fluid of colloidal hard spheres, is presented and studied by means of simulations. Using a fluid of smaller hard spheres as feed, the transport properties of the membrane are studied as a function of the volume fractions of both the feed solution and membrane and the size ratio between both types of particles. Our simulations show that the fluid in the membrane is compressed to the permeate side due to the pressure of the feed. This effect controls the permeability behavior of the membrane: impermeable when the feed pressure is too low, or when the pressure is high enough to induce crystallization of the membrane fluid. Thus, the permeability first increases and then decreases, upon increasing the feed concentration. Finally we focus in systems with high concentrations of the feed and membrane fluids, where completely impermeable membranes are obtained only when the feed spheres are big enough (sigma(f)>0.38sigma(m)).

Revisiting the electrochemical properties of conducting polymers

Free-standing polypyrrole (ppy) films 10-20mm thick were obtained by electropolymerization on metals and subsequent peeling off. The free-standing film can be reduced in aqueous solution up to -3.0 V without any presence of hydrogen release or polymer degradation, keeping a conductivity high enough to allow their use as electrode for elecromehanical reactions. Voltammetric experiments inside the polymer oxidation/reduction potential range show that the involved charge increases for decreasing sweep rates. A deep reduction of the film requires polarization times longer than 300 s at -0.6V or more cathodic potentials. According with the voltammetric (dynamic) reduction. Around -1.0V the polymeric structure is closed when still a 35 to 60% (depending on the scan rate) of the material remains oxidized. The reduction then goes on by slow migration of the counterions through the increasingly compacted polymetric entanglement by stimulating confrontational relaxation movements of the ppy chains. The cathodic maximum, appearing on the voltammograms between -.7 and -0.9V, is related to slow kinetic and structural processes since the film reduction is completely by long polarization time at -0.6V. Three potential windows are distinguished for these films in a aqueous solutions: from potentials a low as -3.0 to -0.6V the free-standing film is a compacted semiconducting electrode; from -0.6 to +0.5 V is a stationary oxidation/reduction region; at stationary higher potentials than +0.6V a degradation of the electromechanical activity occurs.

Study of cyclic thermal aging of tube type receivers as a function of the duration of the cycle

The tube type receivers are exposed to variable duration cyclic operating conditions, which can jeopardize its reliability, and make it hard to estimate its long term performance. The designers have to deal with this problem and estimate the receiver long term performance based on the poor available litterature and the data sheets of the material. In order to help the designer better estimate the performance of the receivers, in this paper the cyclic thermal aging is analyzed as a function of the cycle duration. For this purpose, coated and uncoated Inconel alloy 625 tubular samples, similar to those used in the commercial receivers, are cyclically aged with different thermal cycle duration. The aging of these samples has been analyzed by means of oxidation kinetics, microstructure examination and mechanical and optical properties. The effect of the thermal cycle duration is studied and discussed by comparison of the results.