Microinfiltration of Mg(NO3)2·6H2O into g-C3N4 and macroencapsulation with commercial sealants: A two-step method to enhance the thermal stability of inorganic composite phase change materials

Mg(NO3)2·6H2O is a promising thermal energy storage material owing to its suitable melting point and high latent heat; however, it suffers from poor thermal stability owing to dehydration. This paper presents a two-step encapsulation method to prepare a shape-stabilized Mg(NO3)2·6H2O composite phase change material (CPCM). First, Mg(NO3)2·6H2O is infiltrated into a novel porous matrix – the graphitic carbon nitride (g-C3N4). The g-C3N4 provides a microhousing for Mg(NO3)2·6H2O to prevent liquid from leaking out during the solid–liquid phase change. The g-C3N4 also significantly reduces the sub-cooling degree of Mg(NO3)2·6H2O from 29.2 °C to 1.9 °C. The Mg(NO3)2·6H2O/g-C3N4 composite with 80 wt% Mg(NO3)2·6H2O has a phase change temperature of 87.0 °C and a specific phase change enthalpy of 112.30 kJ kg−1. Second, the Mg(NO3)2·6H2O/g-C3N4 composite is shaped into a cylinder and then macroencapsulated with commercial adhesive sealants (an epoxy resin structural adhesive and silicon sealant). The sealants provide a shell for the hydrated salt to prevent dehydration. After 100 thermal cycles, the composite phase change material only lost 0.84% and 6.25% in weight with coatings of epoxy resin and silicon sealant, respectively, which are much lower than the 22.92% for the uncoated composite phase change material. The specific phase change enthalpy of the composite phase change material barely changed after 100 cycles; however, the loss for the uncoated composite phase change material reached 22.47%. The joint effect of the g-C3N4 matrix and the sealants improve the thermal stability and reliability of the Mg(NO3)2·6H2O. This sequential multi-scale encapsulation method is promising for solving the typical problems of hydrated salts.