Thermal transport and crystallisation in octadecane composites with porous framework coatings and graphene

Low thermal conductivity and leakage limit the use of organic phase change materials for latent heat storage. Confinement can also delay crystallisation. Polyurea capsules containing n-octadecane were coated with either zeolitic imidazolate framework-8 (ZIF-8) or Universitetet i Oslo-66 (UiO-66) and embedded in a polyurethane matrix containing graphene nanoplatelets. Modulated differential scanning calorimetry characterised heat-flow components, while fast scanning calorimetry examined cooling-rate dependence. At a graphene loading of 5 wt%, the ZIF-8-coated composite had an effective thermal conductivity of 0.78 ± 0.04 W m −1 K −1 at 25 °C and an onset-based supercooling of 1.2 ± 0.2 °C. Corresponding values for the UiO-66-coated composite were 0.72 ± 0.03 W m −1 K −1 and 2.4 ± 0.3 °C. Control composites containing only the coating or only the graphene reached 0.31 ± 0.02 and 0.44 ± 0.03 W m −1 K −1 , so neither component accounts for the transport of the complete formulation. The ZIF-8-coated composite retained 98.2 ± 1.1% of its initial melting enthalpy after 1000 cycles between 10 and 60 °C, losing 0.34 ± 0.08% of its mass and showing unchanged infrared and diffraction signatures. Values are means and standard deviations across three independently prepared batches. The two coatings differ in thickness, crystallite size, surface area and growth conditions, so the measurements compare complete formulations rather than isolating a topology-dependent nucleation mechanism. Discharge power at device scale remains untested.

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