Technical Note Layer merging during solidification of supereutectic NH4Cl–H2O system

Abstract

In recent years, experimental and numerical treatments of solidification of a binary system have been stimulated by material processing problems such as semiconductor crystal growth and casting of metallic alloys [1–6]. Most of them focus on double-diffusive convection which is naturally present in the melt of a binary system. In particular, multiple double-diffusive layer formation has been well known in the horizontal directional solidification [3–6]. Nishimura et al. [6]studied experimentally the occurrence and development of multiple layer for a supereutectic NH 4Cl–H 2O system and found that the criteria for successive layer formation are determined by the buoyancy ratio and the Rayleigh number based on thermal buoyancy force in the diffusive interface separating two layers. However, depending on the thermal conditions imposed on the solidification process, flow patterns in the melt are quite different. For example, when solidification is induced at one of the vertical walls of the enclosure and the opposite wall is maintained at the initial (superheated) temperature of the solution, the break-up and merging processes of the layers occur. The layer merging has an impact on the solidification process because the concentration field in the melt suddenly changes [1, 2]. However, the initial temperature and concentration effects on the onset of layer merging have not been known and the mechanism has not been understood fully, which motivates the present investigation. In this communication, we examine the criteria for layer merging during solidification of a supereutectic NH 4Cl–H 2O system.