Towards understanding the microstructural and structural changes in natural hierarchical materials for energy recovery: In-operando multi-scale X-ray scattering characterization of Na- and Ca-montmorillonite on heating to 1150 °C.

Understanding the changes in the microstructures and structures of clays with varying intercalated metal ions at elevated temperatures is of importance for many applications ranging from the recovery of shale gas from unconventional formations to developing effective nuclear waste containment technologies, and engineering materials such as ceramics for fuel cell applications. In this study, synchrotron-based in-operando multi-scale X-ray scattering analyses are used to determine dynamic microstructural and crystal structural changes in Na- and Ca-montmorillonite on heating from 30 °C to 1150 °C. Larger cations such as Ca2+ confer more defined morphological regimes compared to Na+ ions in compacted clays, as evident from the ultra-small-angle X-ray scattering results. The hierarchical morphology of clays is characterized to distinguish between nano-scale interlayer swelling porosity, meso-scale porosity, and intergranular pore spaces between powdered clay grains. On heating from ambient temperature to 200 °C, the removal of interlayer water reduced the basal distances to 9.6 Å. On further heating to 800 °C, gradual dehydroxylation of the clay sheets is evident from the structural changes. The effects of sintering at temperatures greater than 800 °C are evident from significant reductions in the intrinsic porosities of the clay sheets, and the formation of newer phases such as mullite. By connecting the in-operando microstructural and structural changes across spatial scales ranging from micrometers to Angstroms, the possibility of engineering high temperature processes for achieving morphologies and chemical compositions of interest is presented.