Abstract: In situ construction of multiphase dielectric units and regulation of interfacial polarization offer an effective strategy for simultaneously achieving strong absorption and broad bandwidth in lightweight carbon-based microwave absorbers. In this work, rod-like bismuth-based metal-organic frameworks (MOFs) were used as self-sacrificial precursors to fabricate Bi/Bi₂O₃@C composites through controlled carbonization and reduction, enabling the in situ integration of semimetallic Bi, semiconducting Bi₂O₃ and a carbon matrix. During carbonization, the organic ligands evolved into a continuous carbon phase, while the Bi-containing species underwent confined reduction and phase evolution to form Bi/Bi₂O₃ heterostructured nanounits embedded within the rod-like carbon framework, thereby generating multiscale heterointerfaces and abundant dielectric-loss sites. Varying the carbonization temperature simultaneously modulated the reduction degree of Bi₂O₃, the graphitic ordering of the carbon phase and the interfacial polarization response, thus balancing enhanced conductive loss against impedance mismatch. At a filler loading of 40 wt%, the sample carbonized at 850 ℃ exhibited a minimum reflection loss of -39.25 dB at a matching thickness of 4.50 mm, whereas the sample carbonized at 900 ℃ achieved an effective absorption bandwidth of 5.36 GHz at a reduced thickness of 1.72 mm, demonstrating a distinct thin-layer broadband absorption advantage. The enhanced microwave attenuation arose from the synergistic contributions of interfacial polarization at the Bi/C and Bi₂O₃/C heterointerfaces, dipolar polarization associated with defect-rich carbon, and conductive loss within the moderately developed carbon network. Radar cross-section simulations further demonstrated that the optimized Bi/Bi₂O₃@C coating effectively suppressed macroscopic electromagnetic scattering. This work provides a design strategy for engineering multiphase dielectric interfaces and electromagnetic parameters in Bi-MOF-derived microwave absorbers.

Key words: electromagnetic wave absorption, Bi-based metal-organic framework, carbon composite, heterointerface, dielectric loss