Papers by Author: Alexander Nold

Abstract: The processing of ceramic thick and thin films, nano- and micro-scaled ceramic structures as well as bulk ceramics of high quality and precise dimensions under electrophoretic boundary conditions requires a full understanding of the dynamics of relevant interfacial mechanisms and interactions of colloidal phases at the nano- and micro-scale. Recent findings and latest insights on the importance of electrokinetic and electrohydrodynamic interfacial processes for membrane electrophoretic depositon in aqueous media are summarised. In this context, the paper addresses the fundamental importance of surficial charge heterogeneities, electric double layer instabilities, electrokinetically induced micro-vortex dynamics, as well as lateral and medial effective electrical field gradients. These phenomena are evaluated in terms of reasonable correlations and mechanistic coincidences of general EPD deposition principles. The experimental results are based on potentiometry, in-situ videomicroscopy, high-resolution as well as secondary electron microscopy. A numerical method for the simulation of the electrophoretic deposition process is suggested based on a multiphysical Finite Element approach given by Nernst-Planck, Poisson- and Navier-Stokes equations. The results of the simulations provide adequate agreement with experimental findings.

Abstract: Shaping of structures is of growing importance so that innovative processes of fabrication are needed. In this sense, electrophoretic deposition (EPD) of local structures from aqueous suspensions on a membrane has proved to be a suitable process. In this work, the electrical field between two point electrodes was simulated employing the finite element method. Different electrode configurations were studied, seeking to attain a strong focusing of the electric field in order to achieve a local deposition. Aqueous suspensions are preferred for industrial application due to the high polarity of water enabling high solid loadings and also for environmental reasons. Nevertheless, the bubbles generating at the point electrodes surface (due to water electrolysis) disturb the electric field. It is therefore very important to minimize the bubbles size. In this respect, we applied a measuring technique to detect and monitor the bubbles formation and first results are presented.

Abstract: In this paper, the feasibility of obtaining well-defined compact EPD deposits in a short time was studied. EPD from organic and aqueous suspensions were tested. The formation of bubbles coming from solvent electrolysis damages the deposits. 4 different EPD geometric configurations were developed and tested with different experimental conditions. One of them, consisting of both electrodes tilted 30° helped to avoid bubbles getting trapped in the deposit. Our preliminary results showed that there is a very strong influence of the voltage applied, and the separation distance between the electrodes, over the width and height of the structures deposited. Working with aqueous suspensions is more suitable than with organic solvents from the environmental point of view. More work needs to be done in this direction to improve the quality of the deposits and to fine-tune the experimental conditions.

Abstract: Electrophoretic deposition (EPD) is a potential-enhanced technique for fabricating near-netshaped geometries. Its advantage is the independence of particle velocity from particle size so that optimum package densities are achieved by using powder mixtures. If the deposit is formed on a membrane that is located between the two electrodes, aqueous suspensions can be used because the formation of bubbles caused by the electrolytic decomposition of water and the deposition are separated in space. Combining EPD with a CAM system, any structure can be individually near-netshaped. In the present study, the geometry for an electrode for the use with a CAM system is theoretically developed by simulating the distribution of the electric field in EPD and finally fabricated in order to investigate its deposition properties. First of all, spot-wise deposits are fabricated on a membrane so that theoretical predictions and experimental results can be compared. Secondly, a translative motion of the electrode along the membrane is controlled by a CAM system. The so fabricated two-dimensional structures are rectangles and circles whereas the fineness as well as the structural integrity are investigated. This approach will be further developed in future to enable the fast fabrication of individual geometries with excellent green body properties.