Interfacial effects in electrochemical cells for oxygen ionic conduction measurements I. The e.m.f. method

Significant electrode polarization leads to the underestimation of apparent oxygen ion transference numbers of mixed ionic–electronic conductors, determined by measurement of either e.m.f. of oxygen concentration cells or Faradaic efficiency. In addition to e.m.f. data under open-circuit conditions, the exact measurement of the transference numbers requires no less than two parameters describing the system such as oxygen permeation flux, bulk resistance, voltage or current under closed-circuit conditions. Selected examples of experimental routes, aimed at taking electrode polarization into account, are considered. An appropriate choice of experimental procedures used in e.m.f. or Faradaic efficiency measurements, in combination with the use of electrodes with high polarization resistance, enables significant accuracy improvement in the determination of very small electronic contributions to the total conductivity. For potentiometric oxygen sensors, the presence of minor electronic conduction was shown to result in great experimental errors in the oxygen partial pressure measurement if the polarization resistance of sensor electrodes is high. Analogously, minor electronic conductivity in solid electrolytes may lead to a large error of the polarization resistance determination by impedance spectroscopy in conditions where the polarization resistance significantly exceeds the bulk resistance of the electrolyte.     

Data on light-fragment correlations in 40 Ar + 58 Ni at 77 MeV/nucleon

Results of the experimental study of p ⁴ He, d ³ He, dT, pT, p ³ He and, for the first time, nT, n ³ He, n ⁴ He correlations in the region of small relative momenta are presented. The data analysis provides an estimate of the effective source size or emission time.Received: 26 May 2003, Published online: 2 December 2003PACS: 25.70.Pq Multifragment emission and correlations     

Application of the monophase Zr-Mg-Y oxide system as catalyst for gas-phase phenol methylation

The investigation presented attempts to apply the Zr-Mg-Y oxide system as a catalyst for phenol alkylation with methanol by a gas-phase reaction. The studies were carried out in a continuous process performed at atmospheric pressure. It was indicated that the catalyst obtained is active and ortho-selective in the methylation of phenol. At 370°C, in the presence of Zr-Mg-Y oxide catalyst under the experimental conditions studied, an interesting yield of ortho-cresol (over 46% with 66.5% selectivity) was attained. Zr-Mg-Y-O catalyst was prepared by low temperature method and it was characterised by X-ray powder diffraction, scanning microscopy and nitrogen physisorption. In addition, in this reaction the catalytic specificity of Zr-Mg-O, ZrO₂, MgO and Y₂O₃ was examined.     

The 181Hf/181Ta Probe in the Li and Nb Sites of Congruent Linbo3 Co-doped with Mg and Cr Ions Studied by γ–γ PAC

The results reported in this work show the relevance of the lithium deficiency for the Li lattice site occupation of dopants in congruent lithium niobate crystals co-doped with 0.1 mol% Cr₂O₃ and different concentrations of HfO₂, up to 3 mol%. The same behavior is observed in a crystal doped with 1 mol% of HfO₂ and co-doped with 4 mol% MgO where after annealing most of the Hf/Ta probes occupy the Nb lattice site.     

The influence of catalytic activity on the phase transition governed binary switch point of MISiC-FET lambda sensors

A metal insulator silicon carbide field effect transistor (MISiC-FET) sensor with a catalytic metal gate is currently under development for detecting the lambda value, or air-to-fuel ratio, of gasoline exhausts. It has been noticed that a change from a low to a high signal level of the sensor occurs at a lambda value above 1.00, which is an oxidizing atmosphere. The exact location of the switch point depends both on the kind of gas and gas concentrations chosen to obtain a specific lambda value. The switch point would rather have been expected at 1.00, which is at stoichiometry, irrespective of the composition of the gas mixture. The origin of this phenomenon is studied here by exposing the sensor to lambda stairs while changing different operating parameters. An increase in catalytic activity has been observed to move the switch point of the device towards a lambda value of 1.00. A similar effect is achieved when decreasing the flow or increasing the temperature of operation of the device. The behavior is explained through the introduction of mass transport limitations in the measurement cell, and the difference in diffusion constants and sticking coefficients among the gases when reaction limitation prevails.     

Semi-Constant-Time HMSQC (SCT-HMSQC-HA) for the Measurement of JHH Couplings in N-Labeled Proteins

A simple method for accurately measuring ³J_H^NH^α coupling constants in ¹⁵N-labeled proteins is described. This semi-constant-time HMSQC-HA experiment combines the rapidity and convenience of the recently introduced CT-HMQC-HA scheme (Postingl and Otting, J. Biomol. NMR 12, 319–324 (1998)) with the high resolution and robustness of the HSQC experiment. The proposed method is demonstrated for the 76-residue human ubiquitin and Saccharopolyspora erythraea calerythrin (176 residues). Our results imply that the SCT-HMSQC-HA experiment is suitable also for proteins with less favorable NMR properties due to its good resolution and sensitivity.     

Interwire element of an impurity spectral function in coupled asymmetric quantum wires

We calculate the interwire element of the impurity spectral function in coupled double quantum wires at finite temperatures. Simple anisotropy in the direction perpendicular to the wires gives rise to this element, which is responsible for tunneling-induced effects. We find interwire particle–hole (single-particle) excitations contributing to the total impurity spectral function for temperatures up to 20 K.