Use of nano-α-Al2O3 to improve binary eutectic hydrated salt as phase change material

- The new Na2SO4·10H2O-Na2HPO4·12H2O eutectic hydrated salt has no phase separation.
- Nano-α-Al2O3 can be used to inhibit supercooling and improve thermal conductivity.
- Multi-factors orthogonal test is proposed to evaluate the effect of nano-α-Al2O3.
- 4.5 wt% nano-α-Al2O3-modified eutectic hydrated salt displays the best properties.
- Crystallization nucleation mechanism is revealed via OM observation and SEM-EDS.

In this paper, a novel Na2SO4·10H2O-Na2HPO4·12H2O eutectic hydrated salt (EHS) with melting temperature of 31.2 ℃/no phase separation was firstly prepared for thermal energy storage applications at heating solar greenhouses and active solar heating systems in buildings. In order to inhibit supercooling and enhance thermal conductivity of EHS, nano-α-Al2O3 at different weight proportions was loaded into EHS. The multi-factors orthogonal test method has been employed to evaluate the nucleation effects of nano-α-Al2O3. Besides, various characterization techniques such as DSC, OM, SEM and FT-IR were adopted to investigate the thermal properties, microstructure and chemical stability. The results showed that nano-α-Al2O3 was a highly effective nucleating agent and excellent thermal conductive filler. The supercooling degree was reduced from 7.8 ℃ to 1.6 ℃ and the thermal conductivities were improved by up to 61.3% when adding 4.5 wt% nano-α-Al2O3 and 1.0 wt% borax. Moreover, the latent heat of 4.5 wt% nano-α-Al2O3 modified EHS decreased by only about 8% and its melting point maintained constant as that of pure EHS. In addition, the modified EHS had good thermal and chemical stability after 200 thermal cycles.