Structures of Solid Salt Eutectics Using the Raman Microscope

摘要

The structures of salt eutectics have hitherto received little attention, though higher solid electrical conductivities and a melting enthalpy term have both suggested that a considerable area of contact between the constituent phases is present. Such high interphasial areas are commonly found with metallic eutectic systems. However Raman microscopy on six binary salt eutectics (mps. 142-368℃) has shown in eaeh case "conglomerate" structures (close packed roughly rounded zones of the limiting solid solutions, the size increasing with slower cooling) rather than lamellar structures. However the higher melting KCl/Na_2SO_4 eutectic (mp 520℃) does show a well developed lamellar structure. The structures have been confirmed by SEM. It is thought that the lower thermal conductivity of salts, as compared to metals, means that a higher thermal gradient is required (i.e. higher eutectic melting point when cooling to room temperature) to produce the linear crystallisation front required for lamellar growth.