Freezing kinetics of vacuum-induced surface directional freezing in a glass vial

•A mechanism for vacuum-induced surface directional freezing of ceramic dispersions is proposed.•The Stefan equation is used to describe freezing rate.•The temperature of the cold source is predicted to be −14 °C declining to −37 °C.•After surface freezing it is sublimation that acts as a cold source for directional freezing.

Directional freezing is used to prepare aligned porous materials made from ceramic dispersions. The alternate procedure of vacuum-induced surface-freezing uses a reduction in pressure to achieve freezing rather than the use of a cryogen-driven cold-finger. In this work the rate of directional freezing that occurs during vacuum-induced surface freezing of alumina dispersions in glass vials was measured using a video camera technique. The rate of advancement of the freezing front is broadly similar to that seen with conventional freeze casting: initially 900 µm/s caused by spontaneous freezing of a surface layer at 250 mTorr, followed by steady state at approximately 12-15 µm/s during directional freezing. The steady state phase could be described by Stefan's equation where the ice layer thickness grows with the square root of time. The temperature differences across the sample necessary to give the measured rates of advancement of the freezing front were calculated to be 18.8 °C, 13.9 °C and 9.1 °C for volume fractions of dispersed phase of 0.032, 0.064 and 0.128, respectively. The temperature of the slurry below the freezing front declined from 0 °C at the start of directional freezing to around −23 °C at its conclusion. Stefan's equation predicts that the temperature of the cold source, i.e. a sublimation front, is −14 °C declining to −37 °C, which are plausible values. The mechanism of vacuum-induced surface directional freezing is suggested to be undercooling of the slurry surface to below −10 °C at reduced pressure by evaporative water loss. After rapid formation of a continuous, frozen surface-layer, sublimation starts and the enthalpy of sublimation assumes the role of a cold source for directional freezing. A sublimation front moves down through the frozen region behind the freezing front and has a rapidly declining temperature down to below −35 °C. This is the cold source for directional freezing.