Scanning electrochemical microscopy (SECM) studies of catalytic EC' processes: theory and experiment for feedback, generation/collection and imaging measurements

This paper describes the use of scanning electrochemical microscopy (SECM) in the tip generation/substrate collection (TG/SC), or feedback, mode and substrate generation/tip collection (SG/TC) mode to measure homogeneous kinetics in the catalytic EC' process. Theoretical analyses of both configurations have been developed numerically to allow the optimal conditions for sensitive kinetic measurements to be determined. This is shown to involve collection efficiency measurements as a function of tip-substrate electrode distance in the case of TG/SC measurements and tip (collector current) images in a plane normal to the substrate electrode for the SG/TC mode. An important consideration for the SECM configuration (particularly for TG/SC and feedback measurements) is that the electroinactive co-reactant may be depleted more significantly than with other electrode geometries, because of cycling of the redox couple in the tip/substrate electrode gap, while the co-reactant can only enter this gap by hindered diffusion. The approaches described are examined through studies of the oxidation of amidopyrine by electrogenerated Fe(CN) in 0.5 mol dm(-3) aqueous KOH solution. A second-order rate constant of 390 ± 80 dm(3) mol(-1) s(-1) is obtained from TG/SC measurements, consistent with SG/TC quantitative imaging measurements. The consistency of the kinetic measurements confirms the validity of the approaches described. The kinetic constant is lower than expected based on previous ultramicroelectrode (UME) studies, and this is attributed to the fact that background currents for the direct heterogeneous oxidation of amidopyrine are more significant with conventional UME measurements, which will tend to enhance the current measured and may therefore lead to an overestimation of kinetic constants. The TG/SC approach, on the other hand, provides a means of making dual-electrode collection efficiency measurements with diffusional feedback of the redox couple, leading to superior voltammetric responses and enabling more accurate kinetic determination.     

Study of structural and electronic transport properties of Ce-doped LaMnO3

The structural and electronic transport properties of La_1−x Ce _x MnO₃ (x=0.0–1.0) have been studied. All the samples exhibit orthorhombic crystal symmetry and the unit cell volume decreases with Ce doping. They also make a metal-insulator transition (MIT) and transition temperature increases with increase in Ce concentration up to 50% doping. The system La_0.5Ce_0.5MnO₃ also exhibits MIT instead of charge-ordered state as observed in the hole doped systems of the same composition.     

Fourier Transform Spectroscopy of the A'(1)Pi-X(1)Sigma(+) System of CaO

The A'(1)Pi-X(1)Sigma(+) near-infrared system of CaO was observed for the first time at high resolution using a Fourier transform spectrometer. The A'(1)Pi-X(1)Sigma(+) chemiluminescence was excited in a Ca + N(2)O flame produced in a Broida-type oven. More than 3000 rotational lines, classified into 19 bands involving the A'(1)Pi 0 /= 2) levels with the nearby b(3)Sigma(+) (v-2) levels has been detected. An extended set of A'(1)Pi (v = 0-3) data has been obtained which is suitable for use in a future multistate deperturbation analysis of the a(3)Pi approximately A'(1)Pi approximately b(3)Sigma(+) approximately A(1)Sigma(+) complex of excited states. The new near-infrared spectra of the A'(1)Pi-X(1)Sigma(+) transition of CaO also permits the first direct high-resolution linkage between the orange and green systems and the near-infrared bands. Copyright 2000 Academic Press.     

Internal kink instability during off-axis electron cyclotron current drive in the DIII-D tokamak

Experimental evidence is reported of an internal kink instability driven by a new mechanism: barely trapped suprathermal electrons produced by off-axis electron cyclotron heating on the DIII-D tokamak. It occurs in plasmas with an evolving safety factor profile q(r) when q(min) approaches 1. This instability is most active when ECCD is applied on the high field side of the flux surface. It has a bursting behavior with poloidal/toroidal mode number = m/n = 1/1. In positive magnetic shear plasmas, this mode becomes the fishbone instability. This observation can be qualitatively explained by the drift reversal of the barely trapped suprathermal electrons.     

Apparent spin polarization decay in Cu-dusted Co/Al2O3/Co tunnel junctions

Co/Al2O3/Co magnetic tunnel junctions with an interfacial Cu layer have been investigated with in situ growth characterization and ex situ magnetotransport measurements. Cu interlayers grown on Co give an approximately exponential decay of the tunneling magnetoresistance with xi approximately 0.26 nm while those grown on Al2O3 have a decay length of 0.70 nm. The difference in decay lengths can be explained by different growth morphologies, and in this way clarifies a present disagreement in the literature. For monolayer coverage of Cu, we show that the tunneling spin polarization is suppressed by at least a factor of 2 compared to Co and beyond approximately 5 ML it becomes vanishingly small.