High-alumina fly ash-derived microwave absorbents with enhanced electromagnetic wave attenuation via alkaline...

To efficiently recycle high-alumina fly ash (HAFA), a solid waste from coal-fired power plants, and reduce the production costs of microwave absorbents, fly ash-based composites were synthesized using a simple alkaline activation and carbothermal reduction method. The results showed that the dielectric constant of the composites could be adjusted by varying the concentration of the alkaline solution, thereby improving their electromagnetic properties. The minimum reflection loss (RL_min) for FA50, FA100, FA150, and FA200 are -14.46 dB, -18.36 dB, -39.71 dB, and -54.91 dB at 7 mm, respectively. After activation, the composites were sintered at 1250 °C for 4 h, with S50 achieving an RL_min of -40.37 dB at 17.05 GHz and a maximum effective absorption bandwidth (EAB_max) of 3.86 GHz at 1.5 mm. Meanwhile, composite S200 exhibited improved performance, with RL_min values of -58.82 dB at 7.88 GHz and -53.47 dB at 9.11 GHz, along with EAB_max of 2.75 GHz at 3.0 mm and 3.81 GHz at 2.5 mm, achieving an absorption efficiency of approximately 99.9999%. The composites' strong wave attenuation results from excellent impedance matching and from various conduction and polarization losses (dipole and interfacial) within their components. Due to their simple preparation, outstanding electromagnetic wave absorption, and low cost, these composites are promising for high-performance electromagnetic wave absorption applications. This study also demonstrates the full recovery and utilization of HAFA.