Papers by Author: Willi Pabst

Abstract: In nanocrystalline materials the grain boundaries must be considered as regions of finite thickness with properties different from the crystalline bulk material present in the crystallite cores. Thus, dense (i.e. pore-free) single-phase nanocrystalline materials can be considered as quasi-twophase systems whose effective properties can be calculated when quantitative thickness information is available and the property value of the grain boundary phase can be reliably estimated. Similarly, dense two-phase nanocomposites may be considered as quasi-three-phase systems and their effective properties can be predicted using an analogous phase mixture modeling approach. In this contribution this is done for the thermal conductivity of alumina-zirconia nanocomposites. A twostage homogenization procedure is applied, consisting of a first step in which the alumina-zirconia composite is treated as a symmetric-cell material, and a second step in which the highly disordered grain boundary phase is treated as a matrix-phase, coating the crystallite cores. The individual averaging steps are discussed with respect to the two- and three-point bounds, and the resulting grain size dependence is compared with that of pure alumina and zirconia, and literature data.

Abstract: This work deals with the preparation and characterization of macroporous alumina ceramics and permeable laminates with a stepwise (layerwise) porosity gradient in the range of approx. 20–50 %. Layered structures are made by sequential casting and draining of ceramic suspensions containing corn starch (median size approx. 14 micrometers), using both traditional slip casting (TSC) and starch consolidation casting (SCC). In both techniques starch acts as a poreformer, which is eliminated during firing. The influence of the alumina concentration and starch content in the suspension on the porosity, pore size and pore connectivity in the individual layers is studied. It is shown that differential shrinkage of the layers in the case of SCC, caused by the different starch content, may be avoided by controlling the alumina content. The distribution of pore throat diameters (cell window sizes) is determined by mercury porosimetry, whereas the distribution of pore cavity diameters (cell sizes) is measured by microscopic image analysis.