Abstract
The phase change enthalpy, thermal conductivity, thermal stability and thermal reliability of a novel reduced graphene oxide (r-GO) containing phase change material (PCM) r-GO/CaCl2·6H2O were investigated. The material was made by the aqueous dispersion of r-GO and calcium chloride dihydrate (CaCl2·2H2O) according to the mass ratio of CaCl2 and crystal water in CaCl2·6H2O. The thermal conductivity of the phase change material increased by ∼80% when using ∼0.018% (by weight) of r-GO with a ∼2.7% decrease of enthalpy (i.e., storage capacity), while using ∼0.018% of graphite led to an increase of thermal conductivity by ∼14% and a decrease of enthalpy by ∼5.6%. Additionally, the surface active agent for dispersing r-GO had the extra function of enhancing the system stability and reliability. The decomposing temperatures of r-GO/CaCl2·6H2O were higher than those of CaCl2·6H2O. After 100 cycles, the melting and crystallizing enthalpies of r-GO/CaCl2·6H2O decreased to 178.4 J g−1 and 150.7 J g−1 from 180.6 J g−1 and 153.7 J g−1, dropping by 1.2% and 2.0%, respectively, while for CaCl2·6H2O they decreased to 178.9 J g−1 and 147.8 J g−1 from 185.6 J g−1 and 161.8 J g−1, dropping by 3.7% and 8.7%, respectively. The thermal conductivity enhancement of CaCl2·6H2O with r-GO was markedly superior compared to that with graphite and other thermal conductive additives reported in previous literature, and the provided method (i.e., preparing aqueous dispersions of additives firstly and synthesizing hydrated salt PCMs with corresponding salts subsequently) was also applicable for other functional additives that cannot be directly dispersed well to modify the thermal properties of hydrated salt PCM systems.
Highlights
Thermal energy is one of the major forms of energy in the universe, and exists in sources such as solar radiation, natural geothermal energy and oceans in the natural environment
It is shown that preparing a hydrated salt phase change material (PCM) system by combining the reduced graphene oxide (r-Graphene oxide (GO)) aqueous dispersion and the salt is an efficient way to utilize the high thermal conductivity of graphene derivatives
The well-dispersed r-GO has a great effect in boosting the thermal conductivity without causing a large enthalpy reduction in the hydrated salts. $0.018% of r-GO increases the thermal conductivity by $80% and gives a $2.7% decrease in enthalpy, while the same amount of graphite incurs a $14% increase in thermal conductivity and a $5.6% decrease in enthalpy, and the performance of r-GO is superior to other thermal additives reported in previous literature
Summary
A honeycomb-structured sheet of carbon atoms, possesses excellent properties including high thermal. Dispersing graphene directly into hydrated salts is difficult due to its easy agglomeration caused by the Van Edward force between the carbon layers and hydrophobicity.[19] Graphene oxide (GO) including oxygencontaining functional groups on the surface has a good dispersibility in aqueous dispersions.[20] its thermal conductivity is reduced because of structural defects To overcome this drawback and keep good dispersivity simultaneously, the method is to reduce the aqueous dispersion of GO to the aqueous dispersion of r-GO which possesses good dispersivity and better thermal conductivity due to the reparation of structural defects.[21,22] The aqueous dispersion of r-GO with good dispersivity can be utilized directly to synthesize hydrated salts using corresponding salts according to the mass ratio of the salt and crystal water in hydrated salt PCMs. it is reported that r-GO can be used to improve thermal conductivity of materials such as polystyrene,[23] polyimide[24] and paper.[25] the aqueous dispersion of r-GO could be a promising candidate to enhance the thermal conductivity of hydrated salt PCMs. GO was made using a modi ed Hummers’ method and further reduced to make a stable r-GO aqueous dispersion using the reducing agent hydrazine hydrate and the surface active agent polyvinyl pyrrolidone (PVP). Other phase change behaviors such as the supercooling degree, phase change temperature, latent heat, thermal stability and reliability of the PCM in the presence of r-GO were studied
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