Undercooling evolution of pure Sn droplets in various atmospheres based on fast scanning calorimetry

Undercooling of Sn droplets in different atmospheres was studied by fast scanning calorimetry (FSC) at cooling rate of 1,000 K/s. It is found that the undercooling decreased with increasing partial pressure of oxygen. Randomly distributed SnO₂ islands were observed to form on the droplet surface, which likely has promoted the heterogeneous surface nucleation. As the partial pressure of oxygen changes, the nucleation rate and growth of SnO₂ led to different oxide islands, which resulted in various potential catalytic sites for the nucleation of the molten Sn droplet. The results showed that the nucleation process of the Sn droplets was sensitive to the solidification environment, and therefore the atmosphere should be taken into account in the study of the nucleation behavior of the single Sn droplets.