Publikationen

In order to shed more light on the role of magnetic gradient forces and Lorentz forces on the deposition pattern found recently at copper electrodes, experiments and numerical simulations have been performed in a simple geometry that consists of a single small cylindrical permanent magnet which is placed behind the cathode. The cylinder axis coincides with the magnetization direction and points normal to the electrode surface. The electrode is oriented vertically which allows a separate discussion of the influence of both forces. - Experiments and numerical simulations are found to give very good qualitative agreement with respect to the deposition pattern. Our analysis clearly shows that the major influence is due to the action of the magnetic gradient force. Numerical simulations prove that the separate action of the Lorentz force does not reproduce the deposition structure. A detailed analytical discussion of the motion forced by the different magnetic forces in superposition with natural convection is given.

The effects induced by a magnetic field on the oxide film growth on aluminum in sulfuric, oxalic, phosphoric and sulfamic acid, and on current transients during re-anodizing of porous alumina films in the barrier-type electrolyte, were studied. Aluminum films of 100 nm thickness were prepared by thermal evaporation on Si wafer substrates. We could show that the duration of the anodizing process increased by 33% during anodizing in sulfuric acid when a magnetic field was applied (0.7 T), compared to the process without a magnetic field. Interestingly, such a magnetic field effect was not found during anodizing in oxalic and sulfamic acid. The pore intervals were decreased by ca. 17% in oxalic acid. These findings were attributed to variations in electronic properties of the anodic oxide films formed in various electrolytes and interpreted on the basis of the influence of trapped electrons on the mobility of ions migrating during the film growth. The spin dependent tunneling of electrons into the surface layer of the oxide under the magnetic field could be responsible for the shifts of the current transients to lower potentials during re-anodizing of heat-treated oxalic and phosphoric acid alumina films.

Current transients and mass variations in asprepared and heattreated anodic alumina films were measured during reanodizing by means of voltammetry and electrochemical quartz crystal microbalance (EQCM), respectively. Aluminum electrodes (100 nm) on quartz crystals were prepared by thermal evaporation. Anodic alumina films were formed on the surface of Al electrodes in aqueous solutions of oxalic (0.3 M) and phosphoric (0.6 M) acid in the potentiostatic regime. The EQCM experiments did not detect an overshoot in the mass variation of the Al electrode during reanodizing of heattreated anodic alumina films. The observed current overshoot in transients proved the presence of electrons and electron holes injected from the contacts in the bulk of the oxide. This can be explained by the emergence of excess electrons in the barrier layer of the alumina films due to a change in the mobility of the electrons.