Synergistic pre-carbonization and flash Joule heating of cassava stalks for ultra-broadband microwave absorption

The widespread deployment of 5G communication systems and electronic devices has led to severe electromagnetic radiation pollution, creating an urgent demand for high-performance, green, and low-cost microwave absorbing materials. Biomass-derived carbon materials show great promise in this field, but conventional carbonization processes suffer from high energy consumption and poor microstructure controllability, resulting in an imbalance between dielectric loss and impedance matching. Herein, we report a synergistic pre-carbonization and flash Joule heating (FJH) strategy to fabricate high-performance microwave absorbing carbon materials from agricultural waste cassava stalks. The pre-carbonization process effectively removes volatile components and constructs stable carbon precursors, avoiding catastrophic structural collapse during subsequent ultrafast FJH treatment. By precisely tuning the pre-carbonization temperature (320–800 °C) and FJH temperature (1000–2000 °C), we achieve optimal balance among defect density, graphitization degree, and hierarchical porous structure. The optimal CS400–2000 sample exhibits exceptional microwave absorption performance with a minimum reflection loss of −56.2 dB at 7.5 GHz and an ultra-broad effective absorption bandwidth of 8.7 GHz at a thickness of 5 mm. Mechanistic investigations reveal that the outstanding performance originates from the synergistic effect of favorable impedance matching and multiple dissipation mechanisms including dipole polarization, interfacial polarization, conductive loss, and multiple reflections. This work provides an efficient and scalable approach for the fabrication of biomass-derived carbon materials with tunable microstructures and excellent microwave absorption properties.