Papers by Author: Rolf Clasen

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: The electrophoretic deposition (EPD) is a very promising process for shaping compacts, especially for nanopowders. Up till now EPD is not used on a large-scale production of ceramic products. Additionally, the higher cost of nanopowders was also a barrier for the development of new products. As the deposition rate at EPD is independent of particle size, it is an ideal process for shaping compacts of powder mixtures with nanopowders. Silica and zirconia powders were used as model systems to demonstrate the advantages of EPD. Applications might be the preparation of high-purity silica glass for processing of semiconductors and zirconia ceramics for near net-shaped dental crowns. In both cases the optimum green density was achieved for approx. 10 % addition of nanopowders to a coarser matrix powder. The compacts were homogenous. Alternatively, compacts of different powders can be prepared in a two-step process. Examples are shown for the electrophoretic impregnation (EPI) and a reactive forming of nanoparticles inside the pore volume of a compact consisting of coarser particles.

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.

Abstract: Doped glasses are usually manufactured by melting of silica. As the melting point of silica is 2100 °C, most of the suitable dopants evaporate. Hence, alternative processes for the fabrication of glasses are needed. Sintering of nano-sized silica powders to full densety and transparently occurs at 1400 °C because of its large sintering activity. Green bodies can be doped by means of gas infiltration, by soaking of green bodies with salt solutions or by using mixtures of different powders. A further development of the electrophoretic deposition (EPD) is the reactive electrophoretic deposition (REPD), where soluble salts are added into a suspension. The dissolved ions are adsorbed on the surface of the particles and can be deposited with the particles resulting in a homogeneously doped green body. Shaping of a green body and doping can be achieved within only one process step via REPD. The feasibility of this process was shown first for a suspension of SiO2 containing different amounts of boric acid or/and cobalt chloride. First of all, the influence of different quantities of boron acid or cobalt chloride on suspension properties like viscosity, stability and zeta-potential was investigated. Secondly, green bodies were shaped by REPD and characterized regarding deposition rate, green density and amount of dopant. Finally, sintering was carried out at different temperatures. Thus, it was shown that green bodies doped with boric acid could be sintered to full density at lower temperatures compared to undoped ones. However, the sintering temperature depends on the amount of boric acid added into the suspension.

Abstract: Sub-micrometer scaled CGO (Ce0,9Gd0,1O1,95) powder is used in order to manufacture Ni/CGO functionally graded green bodies with high green densities. Thereby the near-shape manufacturing of Ni/CGO cermets by gel casting of CGO and Ni powder mixtures and electrophoretic impregnation of sub-micro scaled ceria doped particles into a Ni/CGO mesoporous matrix is described. Solids content of dispersed suspensions is varied as well as the mixing ratio of sub-micro sized and micro sized particles. The influence on green density and sintering behavior is investigated in order to achieve maximum CGO densities in combination with maximum ionic conductivity for potential use in SOFC. Shrinkage rates from sintering experiments with constant rates of heating (CRH) are analyzed to determine most suitable co-sintering conditions of Ni-Ni/CGO composite anode materials.

Abstract: Applying the Karlsruhe Microwave Plasma Process, alumina and zirconia particles with particle sizes in the range from 2 to 5 nm were synthesized. Additionally, the influence of small amounts of iron(III)-oxide, chromia, and magnesia on the crystallization and phase transitions in alumina powders was investigated. It is shown that these dopants may influence the crystallization behavior significantly. Especially, the addition of 1 mol% iron(III)-oxide reduced the temperature of the formation of a-alumina from 1200 °C to 300 °C.