Papers by Author: Jana Grosse-Brauckmann

Abstract: . In the present work we used the sol-gel process to prepare Y2SiO5 precursor sols suitable for electrophoretic deposition (EPD). The sol synthesis was performed through the controlled hydrolysis of alkoxide solutions of tetraethoxysilane and yttriumoxoisopropoxide. During sol development emphasis was put on characterization of particles size and zeta potential of the formed aggregates. We succeeded in synthesizing a clear sol containing polymeric aggregates with acceptable particle charge. The electrophoretic deposition on glassy carbon or C/C-SiC slabs led to homogenous layers. At low sol concentrations micro cracks in the deposited layers were observed whereas higher concentrations led to thin and dense layers. During constant current EPD a constant voltage was recorded indicating that the deposited layer does not lead to an increase in resistivity in this kind of EPD system.

Abstract: The use of carbon fibre reinforced carbon composites in oxidizing atmospheres is limited to temperatures below 400 °C. To benefit from their excellent mechanical strength that is still preserved at high temperatures, suitable oxidation protection coating systems have to be developed. Composites which are capillary infiltrated with Si and coated with SiC via chemical vapour deposition show significantly enhanced oxidation resistance. For the increase of service temperature above 1300 °C, high temperature stable materials with low oxygen diffusivities such as yttrium silicates have to complement the SiC coating. The electrophoretic deposition performed under constant current conditions leads to relatively high green densities and therefore good sinterability of the applied coatings. In this work we present the preparation of suspensions, their characterization regarding particle size and electrophoretic mobility for yttrium silicate powder prepared by the solid state method. Depending on particle charge and conductivity of the investigated suspensions iodine is employed to increase particle charge. The use of current densities between 0.5-5mA/cm² leads to smooth and homogeneous layers. Layers sintered as low as 1400 °C for 2h already show promising protection of the C/C-Si-SiC substrate during thermogravimetric analysis.

Abstract: In this study we present the preparation of SOFC half cells by electrophoretic deposition. The cathode is deposited from a methyl ethyl ketone (MEK) suspension on dense apatite type lanthanum silicate (ATLS) pellets using a constant voltage. ATLS are a new class of electrolytes, which provides good oxygen permeability even at temperatures below 800°C. As cathode materials we used La2Ni0.8Cu0.2O4+δ (LNC) and La0.8Sr0.2Ni0.6Fe0.4O3 (LSNF). A conductive layer on the ATLS material was used to provide the necessary conductivity for electrophoretic deposition (EPD) process. For the preparation of suitable suspensions the charging behaviour and particle size of the different powders were investigated by means of zeta-potential and laser scattering measurements. When the electrophoretic mobility of the suspended particles was not sufficient iodine was added. Deposition kinetics were determined prior to preparation of the half cells. EPD parameters as well as sintering conditions were optimized in order to gain well adhering porous cathode layers.

Abstract: The present study uses electrophoretic deposition of yttrium doped barium cerate to prepare SOFC half cells on nickel/barium cerate (Ni/BaCe) and nickel / yttria stabilized zirconia (Ni/YSZ) substrates. Therefore we investigated charging and agglomeration behaviour of nanosized barium cerate powder in different solvents by means of a micro electrophoresis experiment and dynamic light scattering. Stable suspensions of barium cerate were prepared and deposition kinetics was examined prior to the preparation of thin membranes on the two cermets. Tests with different solvents and additive combinations showed that methyl-ethyl-keton (MEK) with addition of polyacrylic acid (PAA) is the best combination for the use of the suspension for electrophoretic deposition. Electrophoretic deposition in this case is a simple and low cost method to produce homogenous and dense layers in the scale of microns.

Abstract: In this work we investigated the possibility to use electrophoretic deposition from organic solvents in order to deposit catalytically active surface-stabilized nanoparticles on a substrate. This permits to functionalize a surface by depositing a catalytic layer or simply offers a way to collect the synthesized nanoparticles e.g. for subsequent characterization. As an example, Co nanoparticles were prepared using a sonochemical method, and subsequently deposited on a substrate by electrophoretic deposition. We used ionic liquids (IL) in order to increase the ζ-potential of the nano particles.

Abstract: Ionic Liquids (IL) are novel materials with a wide range of applications. The variable combination of organic cation and inorganic anion offers the possibility of tuning their properties. In the present investigation we test the application of different ionic liquids as charging agents for the preparation of stable oxide suspensions for direct use in electrophoretic deposition to produce oxide layers on different substrates. Main objective of these investigations was to prove that this substance class constitutes suitable additives for successful electrophoretic deposition (EPD). As one key parameter for the EPD process is the particle charge, a big part of our investigations involved the measurement of the zeta-potential of in alcoholic suspension under the influence of phosphonium and imidazolium based ionic liquids using a micro-electrophoresis measurement