The relevance of microgravity to the interaction of a solidifying planar interface with an insoluble particle

Abstract

Liquid convection ahead of the solidifying interface alters particle behavior in the vicinity of the interface. Relevant directional solidification experiments were conducted using a mixture of transparent biphenyl matrix and spherical glass particles, as well as one of succinonitrile matrix with polystyrene particles. The convection level during solidification was varied by either changing the distance between the glass slides containing the composite sample or by alternating low-gravity and high-gravity solidified regions during parabolic flight. It was found that at higher natural convection during solidification the critical velocity increases by up to 40 percent. At very high convection levels engulfment may become impossible because particles fail to interact with the interface. Localized deviations from interface planarity in the proximity of an insoluble particle also affects the nature of interaction between a solid/liquid interface and a particle. Such deviations are caused by the thermal conductivity mismatch between the particle and the liquid melt. This phenomenon was investigated for a metallic system (Al with ZrO2 particles) in real-time using a state-of-the-art X-ray transmission microscope (XTM). The nature and extent of the deviations were quantified as a function of thermal conductivity ratio between the particle and the liquid, distance of interface from particle, radius of particle, and interface velocity. Based on these results, a hypothesis for particle engulfment or pushing is presented. (Author)