Robocasting of alumina hollow filament lattice structures

Robotic controlled deposition (robocasting) of an aqueous colloidal α-Al2O3 gel for manufacturing of cellular ceramics with periodical lattice structure was investigated. The colloidal gel was loaded with 50 vol% α-Al2O3 and exhibits shear-thinning behavior, a shear modulus of 288 kPa and a yield-stress of ∼700 Pa. Tubular filaments of circular and rectangular cross section having an outer diameter of 1.5 mm and a capillary diameter of 0.75 mm were deposited in an oil bath to fabricate lattice truss structures with free spanning filaments. After freeze drying the robocast grids were sintered in air at 1550 °C. X-ray μ-CT revealed continuity of the tubular filaments for long distances (∼650 mm). Critical conditions to avoid capillary collapse were discussed by considering bulging stress and pressure distribution within the hollow filament. At short filament length oil infiltration into the capillary driven by capillary suction supports the tubular filament whereas oil flow driven by movement of the tool nozzle causes pressure difference to increase linearly with increasing filament length.