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The fabrication of a sandwich-like composite that consists of reduced graphene oxide (RGO) and Si 3 N 4 ceramic (RGO/Si 3 N 4 ) 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/Si 3 N 4 exhibits an unprecedented high polarization relaxation loss (PRL), which accounts for 32% of the whole dielectric loss. The outstanding PRL endows the RGO/Si 3 N 4 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/Si 3 N 4 composites can cover the whole Xband (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 conductive loss was explicitly investigated. The outstanding absorption performance toward EM waves indicated that the resultant porous RGO/Si 3 N 4 composite can be a promising candidate for the applications under elevated temperature.

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