Thermoelastic properties of highly porous mullite ceramics

SCHREUER, J.; HÖRSTER, J-F.; TALOU, M. H.; CAMERUCCI, M. A.; SCHNEIDER, H.

Colonia

Conferencia; Annual Conference of the Deutsche Mineralogische Gesellschaft (DMG) and the Deutsche Geologische Gesellschaft - Geologische Vereinigung (DGGV): GeoMinKöln 2022; 2022

Mullite is one of the most important constituents of oxide ceramics because of its outstanding thermo-mechanical properties in oxidizing environments at high temperatures. In order to study the influence of porosity on elastic stability, mullite ceramics with porosity up to about 57% were prepared with a novel technique based on starch consolidation casting and sintering of commercial mullite powders with approximate 3/2-composition at temperatures up to 1923 K. Their coefficients of thermal expansion and elastic stiffnesses were determined between 100 K and 1600 K employing dilatometry and resonance ultrasound spectroscopy, respectively. Additionally, the microstructures were investigated employing scanning electron microscopy and X-ray tomography. Although size (ca. 5 – 20 µm), shape and distribution of pores appear chaotic in high magnification scanning electron microscopic images, their distribution is quite homogeneous at a larger scale. The thermal expansion of the porous ceramics is very similar to the ones of dense mullite ceramics. However, the bulk and Young’s moduli decrease strongly with the porosity as expected for materials having spherical pores. A critical porosity of about 71% is derived where the mullite ceramics probably lose integrity.  The deviations from Cauchy-relations of the porous mullite ceramics are significantly smaller compared to the ones of dense mullite materials, reflecting the limitation of elastic interactions to the thin pore walls of the porous ceramics.