Microstructure characterization of mullite foam by image analysis, mercury porosimetry and X-ray computed microtomography

Abstract

It is well known that the effective properties of porous ceramics and ceramic foams depend strongly on the microstructure. That means, the prediction of effective properties of real materials generally requires quantitative values of well-defined microstructural descriptors as input parameters. The present contribution shows that ceramic foams with hierarchical microstructure require a complex microstructural characterization procedure based on a combination of different complementary techniques. Taking a light-weight mullite foam – an important and versatile ceramic material (e.g. for furnace linings, kiln furniture and high-temperature filters) – as a paradigmatic example, a systematic methodology is proposed for extracting relevant microstructural information. The microstructure of mullite foam with hierarchical microstructure has been characterized by stereology-based image analysis, mercury porosimetry and computed microtomography (X-ray µCT). Three metric descriptors (porosity, surface density, mean curvature integral density) and a topological descriptor (Euler characteristic density) have been determined, the latter being 192.4 ± 17.7·10−9 µm−3. SEM micrographs taken at different magnification allow to distinguish the porosity attributable to pore cavities (62.3–63.0%) and matrix (47.8–49.5%). Mean chord lengths and Jeffries sizes of the pores are 159–164 µm and 164–180 µm, respectively, for the pore cavities and 4.6–5.1 µm and 4.9–5.1 µm, respectively, for the matrix pores. Different types of pore size distributions have been determined, and it is found that in this specific case the size distribution of pore cavity sizes obeys a Rayleigh distribution. Mercury porosimetry covers small and medium-sized pore throats (median 76 µm), whereas image analysis captures pore cavities (median 234–247 µm) and X-ray µCT combines information on both pore throats and cavities, resulting in an intermediate median of 206 µm. The procedure proposed is very general and systematic and can be applied to porous materials with a wide range of microstructures.