Observation of Phase Segregation Phenomenon of Latent Heat Storage Material Na₂HPO₄·12H₂O Melt and Supposition on Its Generation Mechanism

Disodium hydrogen phosphate dodecahydrate is as a latent heat storage material that is expected to be applied to indoor air conditioning and floor heating, especially medical applications. However, the heptahydrate, which has a different number of crystal water from the dodecahydrate, can exist stably at a temperature higher than the melting point of the dodecahydrate. Owing to the crystallization and the subsequent accumulation of the heptahydrate in the dodecahydrate melt, during a long-term thermal cycle of heat storage and heat release, the amount of latent heat of the dodecahydrate gradually decreases. In this study, in order to suppress the occurrence of such phase segregation, the melting and freezing behavior when applying a thermal cycle to the dodecahydrate was observed in detail. As a result, it was clarified that the occurrence of phase segregation not only reduces the latent heat amount of the dodecahydrate, but also lowers the nuclearization temperature thereof. The occurrence of phase segregation due to the crystallization of heptahydrate was thought to be due to the decrease in the diffusion rate during the dissolution process of the heptahydrate. It was suggested that, if the concentration polarization of the Na₂HPO₄ component forming in the melt was eliminated, for example, by installing a stirring mechanism in the heat storage device, the liquidus line of the heptahydrate would be substantially dissolved and the occurrence of phase segregation could be prevented.