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Open-porous alumina foams with additional strut porosity are fabricated by a two-step sponge-replication based manufacturing process. As the first step, open cellular ceramic foams are prepared following the Schwarzwalder sponge replication technique. Therefore, organic foam templates are coated with different aqueous alumina slurries with a solid load between 20 and 40 vol% In a second step, an additional porosity is generated inside the foam struts by freezing of the foams at temperatures between À196 and À20 C and subsequent sublimation drying. The hierarchical structure of both, cell pores and strut pores in the freeze-dried material remains intact after drying, template removal, and sintering. Size, connectivity, and morphology of the strut pores strongly depend on the solid load of the alumina slurry and on the freezing temperature. Cellular structures with a strut porosity between 50% and 60% and a total porosity exceeding 90% are prepared, which show a compressive strength in between 0.4 to 0.6 MPa. Due to the additional strut porosity, the specific surface area of freeze-dried replica foams increases from 70 cm 2 g À1 for conventionally prepared foams to 200 cm 2 g À1 for freeze-dried foams, respectively.

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Ceramic foams in the system Si-O-C, possessing different bulk densities and morphologies, were obtained from preceramic polymers using two different direct foaming approaches. The electric properties of the foams were varied by adding suitable fillers to the precursor mixtures in amounts up to 80 wt%. The electrical conductivity of the foams was varied by several orders of magnitude. The effects of the type of filler and preceramic polymer (methylsiloxane or methylphenylsiloxane resins), as well as the used filler precursor, on the properties of the ceramic foams were investigated.

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Fine-pored, 45 ppi (pores per linear inch) alumina foams are prepared from ceramic slurries with varying contents of additives (deflocculant, binder) and solid loading following a standardized procedure. Rheological key parameters (yield stress, high-shear viscosity) of the respective slurries are determined by approximation of the experimental flow curves with appropriate rheological models. The resulting ceramic foams are characterized by computed tomography (CT) followed by a morphometric analysis of the reconstruction volume data. The main scope of the work involves the development of a procedure to reliably define the binarization threshold during these morphometric calculations, which is based on the analysis of the differential course of the total porosity results from calculations performed at varying binarization threshold values ("differential thresholding"). A very good match of the CT porosity results with experimental data is achieved, despite the unfavorable CT voxel resolution to foam structure fineness relation. The CT evaluation results are finally correlated to the rheological properties of the respective slurries used in foam manufacturing. The dominant slurry composition parameters are the weight fraction of the ceramic powder and the binder concentration. Increasing binder and solid content result in an increased yield stress and viscosity of the respective dispersion and consequently in a decreased porosity and cell size of the finally manufactured cellular ceramic.