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Polymer-derived SiBCN ceramics (PDCs-SiBCN) are promising ultrahigh-temperature ceramics owing to their excellent high-temperature oxidation resistance and electromagnetic wave (EMW)-absorbing capability. In this paper, the microstructure evolutions, the dielectric properties, and EMW absorption properties of Y2O3-doped SiBCN ceramics were investigated. The results reveal that Y2O3 acting as a catalyst promotes the formation of SiC, BN(C), and graphite crystalline phases in the SiBCN ceramics, and these crystalline phases are constructed as conduction phases and polarization phases to enhance the EMW-adsorbing properties. The minimum reflection loss (RLmin) reaches −42.22 dB at 15.28 GHz, and the effective absorption bandwidth is 4.72 GHz (13.28–18.00 GHz). In addition, there is only 0.56 wt % mass loss for the Y2O3-doped SiBCN ceramics when they are heated from ambient temperature to 1500 °C in air, indicating that the Y2O3-doped SiBCN ceramics obtain excellent oxidation resistance at high temperature. We believe that rare metal oxidation is beneficial for the growth of crystalline phases in the PDCs, resulting in high EMW-absorbing properties and oxidation resistance. Thus, the research extends a novel method and design strategy for microstructure regulation and property enhancement of PDCs.
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