High-entropy ceramics (HECs) have attracted considerable attention for their potential in electromagnetic wave absorption because of their tunable composition and complex microstructures. However, the current challenges in this field include a limited understanding of the relationships among the composition, microstructure, and electromagnetic properties, as well as the difficulty in achieving a good balance between strong absorption intensity and broad bandwidth. To address these issues, (Hf_(1−X)/4Zr_(1−X)/4Nb_(1−X)/4Ta_(1−X)/4Co_X)C (X = 0.14, 0.18, and 0.20) high-entropy ceramic powders were successfully synthesized via a polymer-derived ceramic (PDC) method at 1700–1900 °C. Structural analysis confirmed the formation of single-phase rock‒salt structures with homogeneous elemental distributions and significant lattice distortion. The (Hf_0.215Zr_0.215Nb_0.215Ta_0.215Co_0.140)C ceramic prepared at 1700 °C exhibited excellent reflection loss (RL) of −37.95 dB at 14.01 GHz with a thickness of 3.10 mm. The introduction of the magnetic element cobalt optimized the permeability and dielectric constant of the sample, significantly enhancing the dielectric–magnetic loss synergy. This work bridges the gap in systematic research on incorporating Co into high-entropy carbide ceramics and provides new insights for designing high-performance electromagnetic wave absorbing materials.

The development of advanced and efficient microwave-absorbing materials through the precise regulation of dielectric loss and impedance matching remains a significant challenge. In this study, (Hf_0.25Zr_0.25Ta_0.25Nb_0.25)C–SiC (HEC–SiC) biphasic ceramic powders were synthesized via a single-source-precursor route. The SiC content was systematically controlled by adjusting the amount of methyltrimethoxysilane. The resulting polymer-derived HEC–SiC composite exhibited a unique microstructure, with nanosized SiC particles uniformly distributed throughout the HEC matrix. As a result, the HEC–SiC-2 composite, containing approximately 21.21 wt% SiC, achieved a minimum reflection loss value (RL_min) of −54.28 dB at 12.39 GHz with a thickness of 3.14 mm. The superior microwave attenuation capability is attributed to optimized impedance matching, enhanced interfacial polarization between the HEC matrix and nanosized SiC, and dipole polarization induced by defects within HEC. This study offers a novel strategy for the fabrication of high-entropy ceramic–SiC biphasic composites with excellent microwave absorbing properties, paving the way for their application in electromagnetic interference shielding and stealth technologies.