Investigation of the anisotropic thermal properties of the cuboid-like Ca(NO3)2-NaNO3/EG composite

Abstract

Molten salts/expanded graphite (EG) composite phase change materials (PCMs) could have an anisotropic structure due to the intercalation structure of EG, which may lead to anisotropic thermal properties of the composite. In this study, the anisotropic thermal properties of the cuboid-like Ca(NO3)2-NaNO3/EG composite were investigated. The effects of bulk density and temperature on the anisotropic thermal conductivities were studied, and suitable models for prediction of the anisotropic thermal conductivities were analyzed and developed. The results showed that a layer-by-layer structure assembled by EG microsheets with the deposition of salt particles was formed. When the compressing pressure was 4 MPa, the composite showed an isotropic thermal conductivity, while the composites became more and more anisotropic with the increase of compressing pressure. At the room temperature, the axial thermal conductivity increased from 4.016 to 7.694 W/(m K), and the radial thermal conductivity increased from 4.040 to 5.645 W/(m K), when the compressing pressure was increased from 4 to 20 MPa. Contrarily, the working temperature showed little effect on the thermal conductivities. Finally, it was shown that the Maxwell-Eucken model and the modified Parallel model proposed in this work could be used to predict the axial and radial thermal conductivities of the Ca(NO3)2-NaNO3/EG composite, respectively.