The aim of the present work is the preparation of thin (<20 m) zirconia layers on porous substrates with the electrophoretic deposition process. The preparation was completed with a cosintering step of substrate and layer. Through adjustment of shrinkage and the shrinkage rate of the deposited zirconia layer on the presintered porous substrate, thin, dense layers without cracks were prepared. A method for direct control of the layer thickness during the electrophoretic deposition process was developed. The solid oxide fuel cell application with porous anode substrates and thin zirconia electrolytes was chosen to demonstrate the potential of the electrophoretic deposition process.
Electrostatically stabilized alumina suspensions can be destabilized by the enzyme-catalyzed decomposition of urea (direct coagulation casting). Depending on the conditions, this reaction can shift the pH of a suspension to the buffer pH of the reaction products or increase the ionic strength at the buffer pH. The coagulation for both mechanisms was investigated using in situ rheological measurements. Using a vane tool in oscillation mode, the measuring conditions were optimized to find a reasonable method for time-dependent measurements. Constant parameters (stress or strain) proved to be unsuitable, because the linear viscoelastic region shifted considerably during the coagulation. Furthermore, the gel structure produced on coagulation via increase of ionic strength (⌬I) was very sensitive to the oscillation. Therefore, for long-time experiments, a short continuous measurement with a low strain was followed by amplitude sweeps with increased intervals to determine the linear values of G and G. In this way, the increase of the moduli G and G could be followed for longer times, and it was possible to demonstrate two results. First, the final G of the network was about 10 times higher for ⌬I-coagulated material than for suspensions coagulated via pH shift (⌬pH). Second, particle rearrangement processes took place in ⌬I-coagulated networks even after the chemical changes were finished, whereas ⌬pH-coagulated samples were "frozen-in" when approaching the isoelectric point and showed no further physical changes afterward.
powder mixtures was established. The surface charges and isoelectric points of the three different powders were investigated within the pH range from pH 3 to pH 12. Citric acid diammonium salt was found to be an effective deflocculant for shifting the isoelectric points to pH 3.5 for Al 2 O 3 and to pH 6 for Y 2 O 3 . Aluminum hydroxide (Al(OH) 3 ) showed strong interaction with the Si 3 N 4 powder, shifting the isoelectric point from pH 7 to pH 5.5. Low-viscosity, high-solids-loading suspensions (60-63 vol%) thus were possible at pH 9.7. The preparation of homogeneous and stable suspensions with a solids content of ≤61 vol% and a viscosity <1 Paؒs was limited to a pH regime between pH 9 and pH 10.5 because of the high solubility of yttria. The homogeneous suspensions were easily solidified by direct coagulation casting (DCC), using the urease-catalyzed decomposition of urea at pH 9 to pH 10, by forming salt. No shrinkage cracking, sedimentation, or phase separation was observed during coagulation or drying. The green-density distribution was homogeneous throughout all bodies, even for complex geometries.
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