Freeze-casting of highly porous cellulose-nanofiber-reinforced γ-Al2O3 monoliths

Freeze-casting is a powerful consolidation technique for the fabrication of highly porous and layered-hybrid materials, including ceramic-metal composites, and porous scaffolds for catalysis, bone substitutes and high-performance membranes. The aqueous suspensions to be freeze-casted usually contain dense particles facilitating macroporous, layered ceramics with dense (nonporous) struts. In the present study, hierarchical macro-mesoporous alumina (HMMA) monoliths were successfully prepared by freeze-casting of aqueous suspensions containing hierarchically-assembled, mesoporous γ‒Al2O3 (MA) powder and cellulose nanofibers (CNF). As-prepared monoliths were ultra-porous (93.1–99.2%), had low densities (0.01–0.25 ​g/cm3), and displayed relatively high surface areas (91–134 ​m2/g), but were still remarkably rigid with high compressive strengths (up to 52 ​kPa). Owing to the columnar porosity and mesoporous nature of the struts the freeze-casted HMMA monoliths exhibited high permeability and high thermal insulation, the latter ranging from 0.039 ​W/m∙K to 0.071 ​W/m∙K, depending on pore orientation.