The Impurity Effect in the Space Shuttle Dendritic Growth Experiments with Succinonitrile

The high purity succinonitrile (SCN) used in the recent space shuttle dendritic growth experiments of Glicksman and co-workers was reported to be 99.999% pure. This material is treated as a very dilute alloy with a composition Co = 0.001%. The dendrite tip temperature (Tt) is determined from both heat flow and interfacial equilibrium considerations. Only the latter is affected by the presence of impurities. For the experimentally measured values of the tip growth rate (R) and tip radius (rt), the values of Tt estimated on the basis of the Ivantsov heat flow solution are in excess of the equilibrium melting point of succinonitrile. A consideration of the impurity effect suggests that the microgravity experiments correspond to what is considered to be the rapid solidification regime. The chemical Peclet number, p = Rrt/2DL, where DL is the diffusivity of the impurity, is greater than one-half (p > 0.50) in a large number of experiments and greater than one (p > 1) in some experiments.