Metastability Limit of Hydrated Phases of Sodium Sulfate under Temperature Cycling for Heat Storage

Sodium sulfate is an inexpensive, earth-abundant salt with great potential for heat storage applications due to its high energy density. Phase transitions involving crystallization of sodium sulfate decahydrate (Na2SO4•10H2O, mirabilite) are accompanied by a large latent heat. Using laser-transmittance, confocal Raman microspectroscopy, and freezing experiments, we accurately define the dissolution and metastability lines of the heptahydrate (Na2SO4•7H2O) and decahydrate phases, and subsequently present the complete phase diagram of the binary system Na2SO4–water. The metastable zone width (MSZW) is found to be independent of concentration, in the range of ΔT ∼ 17 °C for the heptahydrate, and ΔT ∼ 40–46 °C for the decahydrate form. The latter phase can directly precipitate only from an undersaturated solution below 1.4 mol/kg at a temperature below 0 °C, as in this range of concentrations the metastability line of the heptahydrate phase passes behind that of mirabilite at lower temperatures. Its precipitation is however found not to be ice-assisted, as the decahydrate phase precipitates before ice following the experimentally defined MSZW. Lastly, at high salt concentrations (>1.4 mol/kg), we show that the addition of various types of impurities into salt solutions has no significant impact on the reduction of the activation energy barrier of mirabilite to bypass the formation of the heptahydrate with a lower energy density.