DOI: 10.1002/adem.201000114 Processing of Ceramic Foams with Hierarchical Cell Structure** By Bruno Ceron-Nicolat, Tobias Fey * and Peter Greil Lightweight cellular ceramics with fractional densities lower than 0.3 exhibit a high innovation potential for a variety of emerging fields of applications including mobility, energy, environment, defense, and medicine. [1] Cellular ceramics are characterized by a porous interconnected network of solid material forming edges and faces of cells, such as an open-cell or reticulated foam. [2] Open cellular ceramics with inter- connected porosity offering a high permeability, high surface area, and high mechanical stability at elevated temperatures are increasingly used as filters for molten metal casting and for particle loaded exhaust gases such as in diesel particle filtration systems. [3,4] It has been suggested that reticulated ceramic foams could also be used as flame diffusers [5] and, they have attracted considerable attention as potential substrates for heterogeneous catalysts due to enhanced mass and heat transfer triggered by high convection in the tortuous pore structure. [6] Tailoring the properties of cellular ceramics may be achieved either by controlling the amount and connectivity of porosity (open or closed) or by impregnation the accessible porosity with an interpenetrating phase (interpenetrating phase composites, [7] ). For example, reinforcement of light metals such as Mg, Al by forming interpenetrating phase composite with a open cellular Si–O–C ceramic foam resulted in significant improvement of light metal mechanical proper- ties particularly at higher application temperatures. [8] Inter- penetrating phase composites of a cellular piezoelectric PZT ceramic skeleton impregnated with a passive polymer phase have shown high hydrophone figures of merit (HFOM) compared with dense PZT with optimum HFOM values at 80% porosity to be used as transducers. [9] The high porosity of light weight cellular ceramics offers the possibility of matching the physical properties of the matrix material to specific loading conditions. For example, lightweight and heat resistant SiC foams containing closed porosity exhibit good impedance match with the free space, and therefore, they are a good candidate as a wide-range frequency absorbent medium (radar absorbent materials). [10] Porous COMMUNICATION [*] Dr. T. Fey, B. Ceron-Nicolat, Prof. P. Greil Department of Materials Science (Glass and Ceramics), University of Erlangen-Nuermberg Martensstr. 5, D-91058 Erlangen, Germany E-mail: tobias.fey@ww.uni-erlangen.de [**] Acknowledgements, The Cluster of Excellence ‘‘Engineering of Advanced Materials’’ funded by DFG is gratefully acknowl- edged for financial support. Silicon carbide-based cellular ceramics characterized by a hierarchical pore structure were processed. An open-cellular PU foam template with a mean cell density of 10 pores per inch (ppi), equivalent to a cell diameter of 4 mm and a strut thickness of 250 mm, was coated with a primary SiC slurry. Cross-linking of a polysiloxane binder at 190 h resulted in a SiC filled reticulated thermoset foam (first generation). Subsequently, this matrix foam was infiltrated with a second slurry of slightly different polysiloxane composition which upon heating to 290 8C caused bubble nucleation and formation of a second generation foam filling the cell space in the matrix foam skeleton. After pyrolysis at 1000 8Ca SiC/SiOC reaction bonded composite foam with a hierarchical cell structure and a fractional density of 0.31 was obtained. SEM and X-ray m-CT analyses confirm both generations of matrix as well as infiltrated foam to be open cellular but with a pronounced difference in cell size (matrix foam cell size 2.5 mm versus infiltrated foam cell size 0.29 mm). While the matrix foam skeleton provides control of macroscopic shape as well as density distribution in the component, single- or multistep foam infiltration may offer a high potential for improving the mechanical properties of hierarchical cellular materials. Thus, hierarchical structure formation offers a high potential to fabricate low density cellular ceramics which might be of interest for lightweight design of novel engineering materials. 884 wileyonlinelibrary.com ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ADVANCED ENGINEERING MATERIALS 2010, 12, No. 9