Thermodiffusion in liquid Al-In

Abstract

Thermodiffusion (or Soret effect) describes the formation of a concentration gradient induced by a temperature gradient. In liquid metals, this cross-coupling effect of heat and mass transfer influences e.g. the growth of crystals, the behavior of solders and has recently been shown to be of use in creating metallic nanowires. The coefficient describing the concentration separation, the Soret coefficient, is defined as the ratio of thermodiffusion coefficient and interdiffusion coefficient. Close to a miscibility gap, a diverging behavior is expected as the interdiffusion coefficient approaches zero. To measure thermodiffusion in liquid metals, we combine x-ray radiography with a high temperature furnace. This technique enables in-situ observation excluding disturbances by solidification and revealing possible error sources as e.g. free surfaces. The time- and space-resolved information allows to simultaneously determine the interdiffusion coefficient and the thermodiffusion coefficient. Experiments in the demixing Al-In alloy for initial concentrations ranging from 3 to 30 at% In were performed at different temperatures in the liquid close to the miscibility gap. Al-In is especially well suited for these measurements as it shows a distinct contrast in x-ray absorption. A strong decrease in the interdiffusion coefficient is observed close to the miscibility gap. Soret coefficients of up to 8×10-3 K-1 were obtained, with indium migrating towards the cold side, which is among the highest Soret coefficients ever measured in a liquid alloy.