Characterization of Intermediate Phases Formed Between Solid Nickel and Liquid Zinc During Use as an Encapsulated Phase Change Material in Solar Thermal Energy Storage Systems

Abstract

Of the new material systems under investiga- tion for use in higher temperature phase change material- based solar thermal energy storage (TES) systems, Zn encapsulated in Ni is proposed to increase storage capacity and efficiency by allowing solar TES systems to operate at higher temperatures. However, it is possible that the for- mation of intermediate phases could impede such storage. This study serves to characterize the intermediate phases formed between Zn and Ni at approximately 450 C for various exposure times between 1 and 16 h. Out of four possible intermediate phases present in the Ni-Zn equi- librium phase diagram, only two were found to exist between Ni and Zn upon quenching under experimental conditions. The combined layer thicknesses for each specimen increased with increasing exposure time and ranged from 25 to 335 lm in the specimens exposed 1 and 16 h, respectively. Electron probe microanalysis showed that the compositions of each intermediate phase corre- sponded to the c and d phases of the Ni-Zn equilibrium phase diagram. Electron backscattered diffraction con- firmed the presence of the c and d phases and showed that their structures corresponded to the NiZn3 and Ni3Zn22 structures, respectively.