Effect of expanded graphite and carbon nanotubes on the thermal performance of stearic acid phase change materials

Carbon materials were added into stearic acid (SA) to improve the low thermal conductivity of SA, and their effect on thermal performance was investigated and compared. Stearic acid (SA)/expanded graphite (EG) and stearic acid (SA)/carbon nanotubes (CNTs) composite phase change materials (PCMs) were prepared via melt blending, vacuum adsorption and ultrasonication methods. Carbon additives were blended into SA with mass ratio from 1 to 9%. The leakage rate dropped dramatically with the increase in EG, and it was only physical adsorption in the preparation of both SA/EG and SA/CNTs composite PCMs. Thermophysical properties measured by TG-DSC showed that phase change temperatures of composite PCMs almost stayed the same and latent heat decreased slightly with increasing loadings of EG or CNTs. Thermal diffusivity and conductivity of pure SA were improved by adding EG and CNTs, while EG was far more effective than CNTs owing to the heat conduction network formed inside SA/EG composite PCMs, which was also the reason for the enhancement of thermal stability of SA/EG composite PCMs. Compared with pure SA, the thermal conductivity of composite PCMs was 6.2 times higher with 9 wt% EG and 1.4 times with 9 wt% CNTs, respectively. The heat storage and heat release efficiencies were also improved by carbon additives. This work demonstrates that EG is more potential to enhance the thermal performance of SA than CNTs, and SA/EG composite PCMs are quite promising for low-temperature solar energy storage.