Volume 217, Issue 4

Application of ultrashort voltage pulses to a tiny tool electrode under suitable electrochemical conditions enables precise three-dimensional machining of stainless steel. In order to reach submicrometer precision and high processing speed, the formation of a passive layer on the workpiece surface during the machining process has to be prevented by proper choice of the electrolyte. Mixtures of concentrated hydrofluoric and hydrochloric acid are well suited in this respect and allow the automated machining of complicated three-dimensional microelements. The dependence of the machining precision on pulse duration and pulse amplitude was investigated in detail.

Adsorption behaviours of some structurally related aromatic heterocyclic compounds at an high-area carbon-cloth material have been investigated in relation to waste-water purification. Adsorption processes were followed by monitoring the concentration of adsorbates in a specially designed cell using an in situ UV spectrophotometric method. The rates of adsorption were compared by treating the data according to first-order kinetics. Among the seven heterocyclic compounds studied, thiophene was found to exhibit the highest adsorption rate. This was attributed to the presence of the electron-donative S heteroatom in the structure of this compound. The influences of the dipole moment, the orientation at the C-cloth surface and the size of the heterocyclic adsorbate compound, as well as the type of heteroatom in the ring and the adsorbates’ hydration parameters, on the extent of adsorption of these compounds at the C-cloth were investigated.

Interpretation of the impedance of the silicon electrolyte interface is particularly complicated under conditions in which anodic silicon dissolution occurs. In the present work the frequency-resolved potential modulated microwave reflectivity (PMMR) technique has been combined with electrochemical impedance spectroscopy (EIS) to study the behaviour of low-doped p -type silicon in buffered ammonium fluoride. The EIS response is determined by the total impedance of the system, whereas the PMMR response originates from modulation of the hole density in the silicon space charge region. This difference has led to the development of a novel approach to deconvolution of the interfacial impedance. It involves multiplication of the PMMR and EIS responses to give the space charge impedance. Particular attention is focussed on accumulation conditions in the electropolishing region, where the surface of the silicon is covered by a duplex oxide film. The impedance of the accumulation layer is obtained using the new approach, and the oxide impedance is then derived. Analysis of the PMMR results provides convincing evidence that the hole mobility in the accumulation layer is more than an order of magnitude smaller than in the bulk.

The voltammetric behaviour of Pt(111) electrodes in neutral and near-neutral aqueous KClO 4 solutions (4 < pH < 9.7) is discussed. In this system, within the ideally polarizable potential region, the only processes affecting voltammetry and impedance are the adsorption and desorption of the H + and OH − ions. Due to the low bulk concentrations of these ions, the desad processes are diffusion controlled. This situation is analysed using cyclic voltammetry, transient and impedance measurements.

A quantitative model of oscillations observed during hydrogen oxidation on platinum in the presence of electrosorbing metal ions and specifically adsorbing anions is presented and the model predictions are compared with experiments. Mass and charge balances of all reactants lead in a first step to a seven variable model which is governed by reaction steps that have been widely studied. We demonstrate that attractive interactions between metal and halide ions on the electrode surface, which were recently reported, are crucial for the observed dynamics. The model parameters were almost exclusively taken out of the literature. The model is then reduced to its minimal form without losing dynamic features arriving at a four variable system. Experimental time series of three of the four variables of the model and measured bifurcation diagrams are presented. It is shown that the integrated time evolution and the calculated bifurcation diagrams of the model agree almost quantitatively with the experiment.

A model of alloy deposition was developed based on a concept of dwell times of atoms in kink site positions . It was shown that the dwell times, together with the deposition rates of the alloy components are the steering parameters deciding, which atom will be deposited in the different kink sites of the growing alloy. The model provides a basis for quantitative modelling of alloy composition and alloy structure.

A kinetic model of the processes during hydrogen evolution at a single Pd nanoparticle supported on an Au substrate is studied. It builds the theoretical framework for results of currently performed STM measurements on this system. The objective of the theory is to establish relations between measured transients and underlying processes. It, thereby, helps to implement the routines for the determination of kinetic parameters from transient currents and supplements the characterization of the Pd/Au system. The balance of hydrogen involves interfacial processes on the Pd/Au substrate surface, hydrogen diffusion in the bulk electrolyte and hydrogen oxidation at the tip. The key parameter of the model is the rate of hydrogen desorption from the Pd surface. If this rate is small hydrogen will spillover from the Pd particle and diffuse on the Au surface from where it will be subsequently released. Thereby, large amounts of hydrogen can be stored on the surface. It is demonstrated, that this mechanism complies with the high turnover rates at the smallest catalyst particles and characteristic time scales observed in experiment.

The significance of nano-size effects for ion transport in solids is highlighted both experimentally – by presenting various results of recent investigations – and theoretically – by considering expected size effects using a top-down approach. It is helpful to distinguish between trivial size effects (effects that equally occur at single interfaces but are augmented by the high interfacial density) and true size effects (involving local modifications due to the finite boundary conditions). The latter ones essentially include space charge overlap and structural interference leading to two characteristic sizes with respect to defect chemistry. The contribution also briefly touches upon effects of dimensionality and shape as well as of discreteness of carrier distribution.