Differential shrinkage induced formation of yolk-shell carbon microspheres toward enhanced microwave absorption

Rational design of the microstructure paves new ways for microwave absorbing materials because it can create more facilities for the attenuation of incident electromagnetic waves. In this study, a simple method is proposed to prepare yolk-shell carbon microspheres through differential shrinkage in the internal cores and external shells of polypyrrole microspheres with the assistance of outermost SiO2 coating. This method simplifies the preparation procedures and avoids strictly controlled conditions. The electromagnetic parameters, such as relative complex permittivity and permeability, of the as-prepared yolk-shell carbon microspheres, are investigated in the frequency range of 2.0–18.0 GHz. Compared with solid carbon microspheres, yolk-shell carbon microspheres exhibit significantly enhanced microwave absorption properties in terms of both the reflection loss intensity and absorption bandwidth. The minimum reflection loss value can reach up to −27.5 dB at 8.32 GHz with an absorber thickness of 2.96 mm. The absorption bandwidth over −10.0 dB is in the range of 11.3–16.2 GHz at the typical thickness of 2.0 mm. The enhanced microwave absorption properties may be attributed to the good attenuation ability and well matched characteristic impedance. This work not only provides a promising candidate for microwave absorption, but also provides an attractive strategy to prepare various yolk-shell composites.