Exploring additives for improving the reliability of zinc nitrate hexahydrate as a phase change material (PCM)

Phase change materials (PCM) store large quantities of energy in small volumes due to the high values of latent heat involved during phase change (e.g., from solid to liquid). However, they often require low temperatures (i.e., supercooling below the melting point) to initiate solidification. Heterogeneous nucleation enables reduction in the values of sub-cooling, (ΔT)sub, during the solidification of PCMs. Identification of material-specific nucleation promoters (or “nucleators”) remains non-trivial. In this paper, we investigate heterogeneous nucleation in a chosen thermal energy storage material, Zinc nitrate hexahydrate, Zn(NO3)2.6H2O. Previously recommended heterogeneous nucleators were zinc oxide (ZnO) and impurities without any specific causality being identified that justifies the specific selection of this nucleator. In last 30 years, no systematic study was conducted to study the effects of different heterogenous nucleators on zinc nitrate hexahydrate. In one study, the heterogenous nucleators were selected based on the relation between supercooling and lattice mismatch between the planes of closely packed polyhedral sites of PCMs. In this study, the operational performance of zinc nitrate hexahydrate as PCM candidate was improved through heterogenous nucleation (i.e. reduction in supercooling) and the stability was studied to ensure that the PCM survived over 1000 thermal cycles (i.e. repeated melt-freeze cycles). The heterogeneous nucleators considered in this study were zinc oxide [ZnO] and zinc hydroxyl nitrate [Zn3(OH)4(NO3)2]. The results have shown that the heterogenous nucleators reduce to 3.0 °C supercooling, increased the energy storage (i.e., recovery during solidification), and survived up to 750 cycles of repeated melting and solidification.