Well-established techniques for tailoring the microstructure and enhancing the properties of porous ceramics are freeze-casting and gelcasting, which offer a wide range of technological applications such as thermal insulation, filtration or biomedical implants.

These techniques were used to produce porous alumina samples with varying amount of macroporosity. Within the samples the amount of solid content, equivalent to the ceramic powder content was varied between 25 – 55 vol% and thus the porosity, compressive strength and thermal conductivity were investigated. In order to avoid the lamellar pore structure resulting from static freezing in the freeze-casting process, cooling down under continuous motion was also investigated. A porosity of 38% resulted in a compressive strength of 7 MPa for static freezing, whereas an increase to 54 MPa was observed for freezing under continuous motion. In addition, an increase of 1.45 W/mK was observed in the thermal conductivity of the porous alumina from 8.89 W/mK for static freezing to 10.34 W/mK using the continuous motion approach. Compared to gelcasting, which is also a proven method for producing ceramics with porosities between 40 and 95%, the properties of the pore network can differ in terms of size, morphology and connectivity and freeze-casting. The mechanical and thermal properties as well as the microstructure of porous samples produced by the different methods are analysed and compared.

Isabella Klösel (M.Sc.)