The effect of Ni impurities on the nucleation rates of undercooled Sn droplets

Although high purity Sn can undercool more than forty percent of its melting temperature, small amount of Ni in Sn decrease undercooling to 10 K. We report on measurements of the nucleation rate of undercooled Sn droplets as a function of temperature and Ni content, within the context of classical nucleation theory. The undercooling of these Sn-Ni alloys was found to decrease linearly up to 0.5at% of Ni. Analysis of the dependence of nucleation rates of Sn-Ni alloy on temperature revealed that the barrier height to nucleation systematically decreased with Ni concentration, by more than three orders of magnitude upon the addition of Ni. Such a huge increase in nucleation rate is difficult to explain with a simple picture of heterogeneous nucleation of Sn on Ni₃Sn₄ which eventually forms during solidification process. Previous work has revealed that Ni₃Sn₄ is a poor inoculant for Sn nucleation. In contrast, the huge increase in nucleation rate observed in this work would require very low wetting angles to explain with such a heterogeneous nucleation picture. The increase in nucleation rate and hence the decrease in energy barrier can be explained by a decrease in surface energy between liquid and nucleating solid.