Role of Sodium Acetate Anhydrate in the Nucleation of the Sodium Acetate Trihydrate Phase-Change Material

High Resolution Image Download MS PowerPoint Slide Sodium acetate trihydrate (SAT) is a widely used phase-change material for thermal energy storage, but its tendency to supercool requires the use of nucleating agents to ensure reliable crystallization during operation. Upon heating, SAT undergoes peritectic decomposition to form a liquid phase and sodium acetate anhydrate (SAA). In this study, the role of SAA in the nucleation of SAT was investigated in detail. Two forms of SAA were compared: SAA-1, obtained by peritectic decomposition of SAT, and SAA-2, produced by thermal dehydration of SAT. Consistent with previous findings, SAA-1 was found to be inactive as a nucleating agent. Surprisingly, SAA-2 promoted reproducible crystallization of SAT over multiple heating–cooling cycles. Yet, the nucleation activity of SAA-2 gradually decreased during thermal cycling and was lost rapidly at elevated temperatures. Structural characterization with X-ray diffraction revealed that both SAA materials consist predominantly of the same polymorphs and they also displayed very similar Raman spectra. In contrast, pronounced differences in crystal morphology were observed in optical microscopy. SAA-1 forms large, well-faceted lath-shaped crystals, whereas SAA-2 consists of smaller, irregular crystallites with a high density of surface defects. These results indicate that SAT nucleation is controlled by the crystal morphology and surface structure of SAA, and that dissolution–recrystallization processes gradually convert the active SAA-2 into SAA-1 during thermal cycling, thereby eliminating its ability to nucleate SAT. More generally, the findings demonstrate that the performance of nucleating agents can be governed by the evolution and “healing” of their surface structures during operation, an aspect that should be considered in the design of durable phase-change materials for thermal energy storage.