Interdiffusion of Bi in Liquid Sn

The diffusion coefficients for the interdiffusion of Bi in liquid Sn were determined using the thin layer, long capillary technique. The effect of 0.5, 0.8, and 1.6 mm diameter capillaries on the apparent diffusion coefficient was investigated. For any temperature up to 600 degrees C all three diameter capillaries yielded similar interdiffusion coefficients. At 700 degrees C, the 1.6 mm diameter capillaries yielded concentration-penetration profiles that exhibited considerable scatter and abnormally high diffusion coefficients, indicative of the presence of convective mixing. At 800 degrees C, both the 0.8 and 0.5 mm capillaries yielded considerable scatter in the concentration-penetration profiles and abnormally high diffusion coefficients. Because of agreement of the results for both the 0.5 and 0.8 mm capillaries up to and including 700 degrees C (and the 1.6 mm capillaries up to 600 degrees C), it was concluded that the interdiffusion coefficients obtained are accurate for the temperature range 300 to 700 degrees C with no significant contribution from buoyancy driven convection, capillary wall effects and/or Marangoni convection. The interdiffusion coefficient for the diffusion of Bi in liquid Sn can be represented by: D-Bi(Sn) = {3.4 +/- 0.6 x 10(-8)} {exp [-(13, 600 +/- 1200/RT)]} m(2)/s: The results show that Bi diffuses in Sn at a slower rate than does Sn itself. In addition, it is possible that the interdiffusion coefficient may not be predicated on simple temperature dependence in at least some binary liquid metal systems, and that the diffusion coefficient may be affected by liquid/liquid phase transformations occurring at some temperature within the liquid.