Freeze‐cast porous alumina tubes for membrane applications: Role of niobium pentoxide in microstructure and gas transport properties

This study investigates the porosity, microstructure, mechanical strength, and gas permeation behavior of high‐purity α‐Al 2 O 3 tubes doped with 0.25‒5.0 wt.% of monoclinic or amorphous Nb 2 O 5 , fabricated via freeze‐casting and pressureless sintering. X‐ray diffraction revealed the formation of the AlNbO 4 secondary phase in samples containing 5 wt.% Nb 2 O 5 . Nb 2 O 5 addition enhanced sintering and increased flexural strength, with maximum values of 58.1 MPa for 3 wt.% amorphous Nb 2 O 5 and 38.4 MPa for 5 wt.% monoclinic Nb 2 O 5 . Open porosity ranged from 74% (undoped) to 23% (5 wt.% amorphous Nb 2 O 5 ), and average pore diameter varied between 1.1 and 2.8 µm. The presence of a transient liquid phase, observed from 1400°C, contributed to particle rearrangement, enhanced diffusion, and microstructural refinement. Grain growth reached up to 7.8 µm in alumina samples doped with 5 wt.% monoclinic Nb 2 O 5 . However, micro‐CT analysis revealed the presence of internal cracks in samples containing 5 wt.% amorphous Nb 2 O 5 , attributed to phase transformation and internal stress development during sintering. Gas permeation tests demonstrated high nitrogen and carbon dioxide permeance, with values on the order of 10 ‒5 mol m ‒2 s ‒1 Pa ‒1 . These findings confirm that Nb 2 O 5 ‐doped freeze‐cast alumina tubes are suitable candidates for porous ceramic supports in gas and liquid separation membranes.