Experiment of Solidification With Thermocapillary Convection in a Liquid Layer

To produce high-quality materials, a large number of studies have been carried out for the solidification and melting process of metals and semiconductors in both terrestrial and space environments. The previous studies focused mainly upon the solidification and melting processes themselves. In the real system, however, one cannot neglect effects of convection in molten materials, such as natural and thermocapillary convections, upon the resultant quality of the materials. This study aims to investigate experimentally the effect of the thermocapillary convection upon the directional solidification in a liquid layer with the free surface. Especially, the solidification morphology is emphasized. Three types of interface morphology, planar, cellular and dendrite, can be seen during the growth of solid-liquid interface (SLI). These three types emerge depending upon the increasing interface instability in the order of the planar to the dendrite. The interface instability is a function of the growth rate and the temperature gradient. In case of no convection, it is already known which morphology among the three types emerges according to the balance between the growth rate and temperature gradient. In this experiment, succinonitrile-acetone (SCN-ACE) system was used as test fluid for a model of an alloy. This is often used for solidification experiments, because its coagulation form is similar to real metal and its main physical properties and phase diagram are known in detail. Basically experimental set-up was a typical Bridgman-type furnace. The temperature gradient and the growth rate were able to be controlled independently. Thermocapillary convection was induced over the molten material surface, because a temperature gradient was imposed in a liquid layer with the free surface. The growth of SLI was observed with a microscope. All of experimental apparatus were placed into a glove box filled with Ar gas because SCN-ACE oxidized easily. In case of the experiment without the free surface, a single type of morphology emerged depending upon the solidification condition. On the other hand, in case of “with” the free surface, two types of morphology co-existed across the thickness in a fairly large range of experimental condition. Even if the growth rate of the SLI at the free and bottom surfaces is equal, a distinct difference is observed in the solidification morphology between the both surfaces. That is, either planar or cellular morphology appears in the free surface while the dendrite in the bottom surface (see Fig. A).