Characterization of Intermediate Phases formed between Solid Nickel and Liquid Zinc during Cycling for a Solar Thermal Energy Storage System

As fossil fuels and traditional energy sources become depleted, the need for renewable alternatives increases. Solar energy is one of the most popular amongst the renewable energy sources considered as alternative energy sources due to its seemingly endless supply. Recently, the U.S. Department of Energy decided to further explore how to increase the storage capacity of solar thermal energy storage systems by using higher temperature storage media. In an effort to develop a thermal energy storage system capable of operating at temperatures higher than current thermal energy storage systems, a team at Lehigh University has decided to use Zinc as a phase change storage medium encapsulated in Nickel at an operating temperature of 450°C. Preliminary results obtained from drop calorimetry tests performed on lab-scale specimen showed that the efficiency and amount of pure Zinc available to store energy decreased with increasing numbers of melting/solidification cycles. It was found that interaction at the interface between molten Zinc and solid Nickel produced intermediate phases, which consumed the Zinc and Nickel.;In order to obtain conclusive evidence of the formation of the intermediate phases between Nickel and Zinc, this study serves to identify the intermediate phases and determine a relationship between their thicknesses and exposure durations. Tests were conducted, in which Nickel rods were dipped into molten Zinc at 450°C then removed and quenched in water after exposure times of 1 hour, 2 hours, 4 hours, 8 hours, and 16 hours. The specimens were then metallographically prepared and analyzed using light optical microscopy, electron probe microanalysis (EPMA), and electron backscattered diffraction (EBSD).;It was found that two intermediate phases formed as sequential layers between the Nickel and Zinc. These layers were identified by a combination of electron probe microanalysis (EPMA) and electron backscattered diffraction (EBSD) as the gamma and delta phase homogeneity regions according to the binary Ni-Zn equilibrium phase diagram. The average composition ranges associated the gamma and delta phases, as determined by EPMA, were 78.54 wt.% Zn to 85.46 wt.% Zn and 87.59 wt.% Zn to 90.12 wt.% Zn, respectively. EBSD confirmed the presence of the gamma and delta phases regions and the two phases were indexed as NiZn3 and Ni3Zn22, respectively. The thicknesses of the gamma and delta phases were found to be proportional to the exposure time; however the growth rate of each phase was found to decrease with increasing exposure time. This can be attributed to further growth becoming dependent on diffusion of Nickel and Zinc atoms through the thickness of each layer. The total combined thickness ranged from 25 microm in the specimen exposed to molten Zinc for 1 hour to 335 microm in the specimen exposed for 16 hours.;This work shows that the formation of the two intermediate phases, gamma and delta, consumes Zinc and Nickel in the process and that the total layer thickness increase as the exposure time increases. Combined with previous results from drop calorimetry, these results suggest that Zinc-based thermal energy storage systems are not feasible. The capacity of the Zinc to store energy upon successive phase changes decreases due to the formation and growth of the gamma and delta phases, which limit the amount of Zinc available for energy storage.