Effect of particle size on dielectric and microwave absorption properties of starch-derived micron-carbon spheres

The particle size of absorbents is critical to the electromagnetic parameters and microwave absorption. A tunable particle size and related properties are still a challenge for the study of microwave absorption materials. Here, micron-carbon spheres (MCs) with tunable particle size were designed and prepared to study the size effects. The ε′ decreased from 17.88 to 12.21 at 8.2 GHz as the mean size increases from 1.29 to 1.75 µm, and the ε″ was positively related to D^{− 1} (diameter) in accordance with the electron mean free path theory. An increasing percolation threshold with increasing particle size was observed from direct current (DC) conductivity analysis. An excellent microwave absorption was obtained from the synergistic effect of polarization loss and conductive loss corresponding to DC conductivity of around 0.02 S/m. The strongest microwave absorption with the reflection loss (RL) of − 71.9 dB at 9.25 GHz was achieved with a thickness as a thin as 1.97 mm (filled with 35 wt% MCs). The RL_min of paraffin-based composites loaded with 25 wt% MCs with the mean size of 1.29 and 1.48 μm reaches − 50.5 and − 54.0 dB, at 9.44 and 10.64 GHz with a thickness of 2.14 and 1.9 mm, respectively.