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Highly porous, lightweight ceramics with high surface area are and high enough rigidity important in applications such as catalysis, thermal insulation and adsorption. There is a strong incentive to employ simple and green processes for the preparation of this family of ceramics. Unfortunately, conventional shaping techniques require sintering, which lowers porosity and surface area.

The goal of our research was to prepare light and porous γ-Al2O3 ceramic foams exhibiting a high surface area and permeability and a low thermal conductivity. To achieve that, micron-sized, hierarchically assembled, mesoporous γ-Al2O3 was synthesized, by exploiting the hydrolysis of AlN powder, and was redispersed in water, where we triggered highly stable aqueous interparticle association networks by using divalent cations and cellulose nanofibers.

Freeze casting of such aqueous stable slurries yielded foams with exceptionally high porosities (≈ 98-91%) and thermal conductivities as low as 0.03 W∙m-1∙K-1. Cellulose nanofibers helped to reinforce γ˗Al2O3 foams resulting in high strength (and even limited flexibility) rarely seen for as porous green bodies.

Presented research demonstrates the beneficial complementary contribution of cellulose nanofibers, as reinforcing agent, and freeze casting, as a very useful consolidation path, for shaping of rigid ceramic porous green foams.