Hierarchical macroporous–mesoporous γ-alumina monolithic green bodies with high strength

Hierarchical macroporous–mesoporous alumina (HMMA) and hierarchical mesoporous alumina (HMA) monolithic green bodies were fabricated by cold isostatic pressing (CIP) of mesoporous alumina (MA) powder, which was prepared by a quick, facile and very pure synthesis route that involved exploitation of the naturally self-driven aluminum nitride powder hydrolysis. The hierarchically self-assembled, nanocrystalline, yet micron-sized MA powder exhibited a controlled porosity, a fine crystallite size, a relatively large surface area and large pore volumes. The textural characteristics of the MA powder were analyzed, while the effect of the consolidation pressure on the pore size evolution, mechanical and permeability properties of the consolidated HMMA and HMA monoliths were investigated. At the lowest CIP pressures, the HMMA monoliths possessed favorable permeability characteristics with Darcian permeability constants up to 2 × 10−15 m2, also exhibiting a low thermal conductivity (≥0.185 W/mK), sufficient flexural strength (≥6 MPa) with an accessible porosity of ≤65%, a pore volume of ≤0.69 cm3/g and a macropore opening diameter of ≤370 nm at a constant mesopore opening diameter of about 4.5 nm. Increasing the CIP pressure resulted in the shrinkage of the macropores and a consequent suppression of the permeability characteristics; however, the flexural strength of the HMA monolith CIPed at 800 MPa increased to 51 MPa, which is the highest reported strength for a dry-pressed, binderless ceramic green body with an accessible porosity of 42% and a theoretical density of 44.3%.