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High-porosity dendritic porous alumina was fabricated by using tertiary butanol (TBA) hydrate crystals combined with directional freeze casting. The porosity of this porous alumina approximated 80 %, and its high porosity resulted in high water flux. Dendritic pores improved the physical interception capability of porous ceramics due to the intrinsic moving paths and intercepts from the pore structure. Changes in the TBA content (from 70 vol.% to 85 vol.%) caused a change in pore size from 36.58 μm to 11.54 μm and pore structure (change order: snowflake, dendritic, rod-like, and needle-like), which are important factors affecting water flux and interception capability. The interception and removal of Escherichia coli by 7 mm-height porous ceramics with dendritic structure and an average pore size of 27.90 μm reached 100 % at pH 7.2. This study provides a simple and low-cost method for the effective removal of bacteria.

The effects of the annealing methods and annealing temperatures on the pore structures and mechanical properties of porous alumina were investigated. The amorphisation behavior and solidification behavior of the sucrose solutions during annealing were discussed. The pore morphology of porous alumina changed noticeably after uniform annealing. As annealing temperature increased from −25 ℃ to −5 ℃, the pore morphology of porous alumina changed gradually from irregular lamellar channels to circular channels. After directional annealing, the pore morphology of porous alumina was similar to that after uniform annealing; however, the uniformity of pore channels and the density of pore walls were increased. During directional annealing at −15℃, the compressive strength of porous alumina reached 58.8 MPa, which was 35% higher than that of unannealed porous alumina.

Tert -butyl alcohol (TBA)–water (H 2 O) crystals are used as template to fabricate aligned porous alumina with special pore structures. Alumina slurries were prepared using TBA–H 2 O mixtures, which could undergo continuous crystallization by unidirectional freezing. The pore shape of porous alumina constantly changed from lamellar to snowflake, dendritic, needle-like, and hexagonal with the increase in the TBA content in alumina slurries. The hexagonal porous structure obtained by pure TBA slurry exhibited the best mechanical properties, whereas the mixture structure with irregular pore walls showed the worst, exhibiting inferior properties to lamellar pore structures.

A novel directional solidification method was used to fabricate lamellar porous alumina with an axisymmetrical structure by combining multiple cold sources, electrostatic field, and magnetic field during the freezing of aqueous alumina slurries. Multiple cold sources with mutually perpendicular directions formed by the bottom and parallel sides of rectangular copper molds induced a specific growth direction for ice crystals. The pore channels of porous ceramic are arranged parallel to the direction of electric field lines, and the axisymmetrical structure was obtained after applying the electrostatic field. The applied magnetic field can reduce pore channel size; however, the field has no effect on pore channel direction and distribution, and leads to cracking in the central region of porous ceramics. The compressive strength along the electrostatic field direction is higher than that achieved by conventional freeze casting.

A novel freeze-casting method is used to fabricate aligned lamellar porous alumina with a centrosymmetric structure from aqueous alumina slurries. Two cold fronts oriented perpendicularly to each other, originating from the bottom and side of the cylindrical copper mold, induce the growth of ice crystals in specific directions along the radius of the cylindrical mold. Lamellar channels of porous alumina are arranged centrosymmetrically along the radial axis. The pore distribution of the currently prepared porous ceramics is more regular when compared with that of porous ceramics prepared by conventional freeze casting. This affords porous ceramics with improved mechanical properties and stability. The current method addresses the issue of partial failure as induced by the randomly distributed channels in lamellar porous ceramics.