Entropy Modulation Boosted Ultra-Broadband Electromagnetic Wave Absorption of Si(Zr,Hf,Ta,Ti,Nb)BCN Ceramics

SiMBCN ceramics containing one or multiple metallic elements are synthesized via chemical modification of polyborosilazane using one to five metal alkylamino compounds with an equimolar ratio (i.e., M = Zr, Hf, Ta, Ti, and/or Nb). The effects of the number and types of metallic elements on the local lattice distortion, microstructure, dielectric properties, and electromagnetic wave absorption (EMA) performance are systematically investigated via experiments and ab initio calculations. These results show that increasing the number of metallic elements remarkably increases the local lattice distortion and suppresses the crystallization of the ceramics, which strongly affects the dielectric properties, showing a significant entropy effect. Particularly, the SiZrHfTaTiNbBCN ceramics remain in nearly amorphous state even after annealing at 1600°C for 5 h. Under the synergistic effect of annealing temperature and entropy engineering, the SiZrHfNbBCN ceramic develops a unique microstructure consisting of a ZrHfNbCN/Si₃N₄/C_free core with abundant interfaces and a low-dielectric β-Si₃N₄-based shell. This microstructure balances the impedance matching and dielectric loss, resulting in an ultra-broad effective absorption bandwidth at 1.90 mm (7.65 GHz), which is superior to all previously reported polymer-derived ceramics (PDCs) at thickness < 2 mm and therefore presents a significant advance in ceramic-based EMA materials for high-temperature applications.