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The fabrication of a sandwich-like composite that consists of reduced graphene oxide (RGO) and Si₃N₄ ceramic (RGO/Si₃N₄) was achieved through the combination of modified freeze-drying approach and chemical vapor infiltration process. Due to a hierarchical structure and a high ratio of I_D/I_G (1.27), the RGO/Si₃N₄ exhibits an unprecedented high polarization relaxation loss (PRL), which accounts for 32% of the whole dielectric loss. The outstanding PRL endows the RGO/Si₃N₄ composites with unique temperature-independent dielectric properties and electromagnetic (EM) wave absorption performance. Even at a low absorbent content of only 0.16 wt %, the effective absorption bandwidth of RGO/Si₃N₄ composites can cover the whole X-band (8.2–12.4 GHz) at broad sample thicknesses ranging from 4.3 to 4.6 mm and temperatures ranging from 323 to 873 K. The mechanism for the enhancement of PRL and …