The microwave absorption performance of candle soots

While carbon-based materials have been widely studied for microwave absorption and candle soot full of carbon particles has displayed wonderful superhydrophobic characteristics, microwave absorption by candle soot has rarely been investigated yet. In this study, we have explored the microwave absorption of candle soot and candle soot heated at 200, 300, and 400 °C and tested any correlations between the microwave absorption properties and superhydrophobicity of candle soot. Candle soot heated below 400 °C displays superhydrophobic characteristics with a water contact angle larger than 150°, and candle soot heated above 400 °C shows superhydrophilic characteristics with a water contact angle less than 10°. Heating apparently changes the disordered/crystalline phase ratio in candle soot and the content of unpaired electrons, as indicated by Raman and ESR spectra, and changes its microwave permittivity/permeability and reflection loss values, even though it does not apparently change the UV–vis reflection properties. Candle soot also exhibits interesting microwave absorption properties. For example, candle soot heated at 300 °C presents a maximal reflection loss of −68.40 dB near 10.32 GHz with a 6.0 GHz bandwidth when the thickness is 2.722 mm. Some possible while weak correlations are observed between the microwave absorption and the imaginary part of the permittivity, the tangent of the permittivity or permeability, and the conductivity for candle soot and candle soot heated at 200 and 400 °C, however, such correlations are not straightforward for candle soot heated at 300 °C. Unpaired electrons seem to be related to such correlations, other than surface superhydrophobicity or superhydrophilicity, or the ratios of the disorder/crystalline phase in the candle soot. Intrinsic absorption along with interference-induced absorption are both observed based on the dependence of the peak position with the thickness of the absorber. The possible absorption mechanisms are also explored at the end based on dielectric and magnetic relaxations.