Melting Temperature Depression and Phase Transitions of Nitrate-Based Molten Salts in Nanoconfinement.

Hybrids of nitrate-based molten salts (KNO 3 , NaNO 3 , and Solar Salt) and anodic aluminum oxide (AAO) with various pore sizes (between 25 and 380 nm) were designed for concentrated solar power (CSP) plants to achieve low melting point (<200 °C) and high thermal conductivity (>1 W m -1 K -1 ). AAO pore surfaces were passivated with octadecyl phosphonic acid (ODPA), and the results were compared with as-anodized AAO. The change in phase transition temperatures and melting temperatures of salts was investigated as a function of pore diameter. Melting temperatures decreased for all salts inside AAO with different pore sizes while the highest melting temperature decrease (Δ T = 173 ± 2 °C) was observed for KNO 3 filled in AAO with a pore diameter of 380 nm. Another nanoconfinement effect was observed in the crystal phases of the salts. The ferroelectric phase of KNO 3 (γ-phase) formed at room temperature for KNO 3 /AAO hybrids with pore size larger than 35 nm. Thermal conductivity values of molten salt (MS)/AAO hybrids were obtained by thermal property analysis (TPS) at room temperature and above melting temperatures of the salts. The highest increase in thermal conductivity was observed as 73% for KNO 3 /AAO-35 nm. For NaNO 3 /AAO-380 nm hybrids, the thermal conductivity coefficient was 1.224 ± 0.019 at room temperature. To determine the capacity and efficiency of MS/AAO hybrids during the heat transfer process, the energy storage density per unit volume (J m -3 ) was calculated. The highest energy storage capacity was calculated as 2390 MJ m -3 for KNO 3 /AAO with a pore diameter of 400 nm. This value is approximately five times higher than that of bulk salt.