Effects of heat transfer in a growing particle layer on microstructural evolution during solidification of colloidal suspensions*

Abstract

Heat transfer is the foundation of freezing colloidal suspensions and a key factor for the interface movement. However, how the thermal conductivity of particles affects freezing microstructural evolution remains unknown. Here in this work, a mathematical model is built up to investigate thermal interactions among a growing particle layer, pulling speeds, and the freezing interface under a thermal gradient. Experiments are conducted to confirm the tendency predictions of the model. With the increase of pulling speeds, the drifting distance of the freezing interface increases and the time to finish drifting decreases. When the thermal conductivity of particles (k p) is smaller than that of the surrounding (k w), the freezing interface tends to go forward to the warm side. Contrarily, the freezing interface tends to go back to the cold side when the thermal conductivity of particles is larger than that of the surrounding (α = k p/k w > 1). It originates from the shape of the local freezing interface: convex (α < 1) or concave (α > 1). These morphological changes in the local interface modify the premelting drag force F f. When α < 1, F f decreases and the freezing morphology tends to be the frozen fringe. When α > 1, F f increases and the freezing morphologies tend to be ice spears. These understandings of how the thermal conductivity of particles affect microstructural evolution may optimize the production of freeze-casting materials and their structural-functional properties.