We study the thermal properties and internal microstructures of n-hexadecane alkane containing nanoinclusions of copper nanowire, multi walled carbon nanotube, and graphene nanoplatelets of different volume fractions. Just below the freezing point, a large thermal contrast is observed in all the three systems. The thermal conductivity decreases with temperature below the freezing temperature and stabilizes at ∼10 °C below the freezing point. More than 100% of thermal conductivity enhancement is observed with 0.01 wt. % of nanofillers during the liquid to solid phase change. It is speculated that the reduction in the interfacial thermal resistance and the internal stress generated during the first order phase transition, due to the presence of nanoinclusions at grain boundaries of alkane crystals, led to the observed increase in the thermal conductivity. We found that an optimal nanoparticle loading with the space filling agglomerates in a phase change alkane can provide an extremely large thermal conductivity. Though the thermal conductivity enhancement at higher particle loading was independent of the bulk thermal conductivity of dispersed nanomaterials, an anomalously large thermal contrast is observed at a very low concentration in copper nanowire suspension. These results provide new approaches to achieve large thermal storage in organic phase change materials.
We report extremely large tunable thermal conductivity (k) in alkanes using inverse micellar templating and nanofillers. The thermal properties of n-hexadecane containing inverse micelles of different volume fractions (ϕ) have been studied during freezing and melting. The k enhancement between the solid and liquid phase in the presence of oleic acid, dioctyl sodium sulfosuccinate, and sorbitan oleate inverse micelles (size ∼1.5−6 nm) are found to be 185, 119, and 111%, respectively. Unlike the conventional nanofluids, the k enhancement in micellar templated alkanes is perfectly reversible under repeated thermal cycling owing to the monodispersity and nonaggregating nature of micelles. Our results suggest that during the first-order phase transition, the inverse micelles with highly packed linear chain surfactant are pushed to the intercrystal boundaries of alkanes, thereby reducing the interfacial thermal resistance. The k contrasts in surface modified graphite nanofibers and multiwalled carbon nanotube in n-hexadecane at 15°C for a ϕ ∼ 0.0039 are found to be 161 and 157%, respectively. The surface modified nanofillers dispersed in alkanes showed a higher thermal contrast compared to bare ones, owing to their uniform dispersibility in intercrystal regions. Our findings of the large thermal contrast using inexpensive surfactant micelles in alkane should have interesting applications in heat management.
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