Interfacial Coordination Manifolds for High‐Temperature Electromagnetic Loss in SiC/SiOC Polymer‐Derived Ceramics

ABSTRACT The interaction between electromagnetic waves and condensed matter depends on polarization and magnetization processes that are sensitive to local electronic structure, whereby interfaces contribute strongly through symmetry breaking and charge redistribution. Consequently, tailoring interfacial states has become a widely explored strategy for controlling electromagnetic dissipation. However, interfaces are inherently chemically and structurally nonuniform, rendering conventional interfacial modifications unstable at high temperature. Here, a single oxidative etching‐reconstruction step is developed to generate Fe/FeO x– C nanophases and graded junctions within thermally robust SiC/SiOC polymer‐derived ceramics. Local oxygen coordination, valence, and interfacial‐gradient thickness are treated as descriptors of an interfacial coordination manifolds that govern loss. These stabilized ceramic interfaces regulate electromagnetic loss through charge redistribution, interfacial dipoles, and local magnetic moments. We further condense the influence of processing and interfacial structure into a dimensionless balance indicator, Δv, and organize interfacial configurations into distinct regimes. Optimized regimes deliver reflection loss minima approaching −70 dB with multi‐gigahertz bandwidth at ∼1.6 mm, maintain wide absorption bands up to 400°C, and retain strong attenuation following prolonged thermal exposure. Together, these findings demonstrate that SiC/SiOC polymer‐derived ceramics support robust interfacial coordination and that parameterizing these interfaces with a reduced descriptor enables rational high‑temperature electromagnetic materials design.