NaNO3-KNO3/EG/Al2O3 shape-stable phase change materials for thermal energy storage over a wide temperature range: Sintering temperature study

Abstract

The coefficient of thermal expansion of inorganic salts increases with increasing temperature. The sintering temperature may affect the properties of the shape-stable phase change materials (SSPCMs). We study the SSPCMs with NaNO3-KNO3 as the phase change material and Al2O3 and expanded graphite (EG) as the skeletal support materials (SSMs) to achieve thermal energy storage over a wide temperature range. The effects of different sintering temperatures on the SSMs content, thermal conductivity and compressive strength of SSPCMs were studied. The results showed that as the sintering temperature increased, more SSMs were required to support the SSPCMs. The maximum NaNO3-KNO3 mass percentages of the SSPCMs sintered at 300 °C and 450 °C were 82.8 wt% and 66.4 wt%, respectively. Samples with application temperatures higher than the sintering temperature may have problems stabilizing the shape. Sintering temperature had little influence on the thermal conductivity of the same SSPCMs. However, the compressive strength of the SSPCMs decreased with increasing sintering temperature. The addition of EG decreases the decomposition temperature of the composite and limits its application temperature range. The SSPCM-10/30 sintered at 450 ℃ showed good chemical stability performance, and the latent heat decreased from 56.01 J/g to 53.44 J/g. The compressive strength dropped from 30.7 MPa to 25.17 MPa before and after 20 cycles due to larger internal gaps and did not change much after 20 cycles.