Fig. 2

Charging of paraffin composites filled with Fe₃O₄@graphene nanoparticles. a Transmission electron microscopy (TEM) image of hybrid Fe₃O₄@graphene optical absorbers. The scale bar is 200 nm. The inset TEM image at a higher magnification shows the dense decoration of Fe₃O₄ nanoparticles on the surface of graphene sheets. The scale bar of the inset TEM image is 50 nm. b Schematic of experimental setup for charging of paraffin composites. A green laser (532 nm) with a power density of 4 W cm⁻² is used to illuminate a black aluminum foil in thermal charging (TC) mode or directly illuminate the surface of composites of paraffin-Fe₃O₄@graphene (0.02 wt%) in optical charging (OC) mode. c, d Infrared (IR) image of a thermally and optically charged sample after illumination for 100 s. The dashed line marks the boundary of the sample within the cuvette. e, f Time-sequential temperature distribution profiles of thermally charged sample and optically charged sample. The temperature profiles were extracted from the central line of the IR images. g Differential scanning calorimetry (DSC) curves of paraffin and paraffin composites with different loading of Fe₃O₄@graphene nanoparticles (composite 1: 0.02 wt%, composite 2: 0.04 wt%, composite 3: 0.1 wt%). h Comparison of fusion phase-change enthalpy, melting, and solidification temperatures of neat paraffin and paraffin-Fe₃O₄@graphene composites