Formation of micro-plumes at a planar solid/liquid interface in a temperature gradient

Abstract

For directional solidification experiments with low withdrawal rates near and even below the constitutional undercooling limit of the corresponding alloy, a sample is needed which reveals a homogeneous concentration along the sample axis. As stirring in samples of a certain length is difficult, homogenization by rapid solidification is very common. Hereby, the liquid sample is rapidly moved from hot to cold and thus fine and long dendrites with segregated inter-dendritic liquid are formed. By subsequently melting and holding the sample in a resting position within a given temperature gradient, diffusion in the liquid and enhanced diffusion in the hot solid diminishes local concentration differences. In this paper, we report on homogenization and solidification experiences with transparent organic model compounds of TRIS-NPG. During one hour of homogenization we observed (i) formation of a coarse grain structure, (ii) liquid film and droplet migration by temperature gradient zone melting, and (iii) formation and eventually disappearance of liquid channels. During solidification experiments we occasionally found liquid channels, which revealed a connection to the solid/liquid interface; liquid that rose in the channel and thus formed micro-plumes at the planar solid/liquid interface. This upwards motion is explained by solutal buoyancy of NPG-rich liquid and a possible feeding through an interconnected network of channels.