One-step synthesis and electromagnetic absorption properties of high entropy rare earth hexaborides (HE REB₆) and high entropy rare earth hexaborides/borates (HE REB₆/HE REBO₃) composite powders

Considering the emergence of severe electromagnetic interference problems, it is vital to develop electromagnetic (EM) wave absorbing materials with high dielectric, magnetic loss and optimized impedance matching. However, realizing the synergistic dielectric and magnetic losses in a single phase material is still a challenge. Herein, high entropy (HE) rare earth hexaborides (REB₆) powders with coupling of dielectric and magnetic losses were designed and successfully synthesized through a facial one-step boron carbide reduction method, and the effects of high entropy borates intermedia phases on the EM wave absorption properties were investigated. Five HE REB₆ ceramics including (Ce_0.2Y_0.2Sm_0.2Er_0.2Yb_0.2)B₆, (Ce_0.2Eu_0.2Sm_0.2Er_0.2Yb_0.2)B₆, (Ce_0.2Y_0.2Eu_0.2Er_0.2Yb_0.2)B₆, (Ce_0.2Y_0.2Sm_0.2 Eu_0.2Yb_0.2)B₆, and (Nd_0.2Y_0.2Sm_0.2Eu_0.2 Yb_0.2)B₆ possess CsCl-type cubic crystal structure, and their theoretical densities range from 4.84 to 5.25 g/cm³. (Ce_0.2Y_0.2Sm_0.2Er_0.2 Yb_0.2)B₆ powders with the average particle size of 1.86 μm were found to possess the best EM wave absorption properties among these hexaborides. The RL_min value of (Ce_0.2Y_0.2Sm_0.2Er_0.2Yb_0.2)B₆ reaches -33.4 dB at 11.5 GHz at thickness of 2 mm; meanwhile, the optimized effective absorption bandwidth (E_AB) is 3.9 GHz from 13.6 to 17.5 GHz with a thickness of 1.5 mm. The introduction of HE REBO₃ (RE = Ce, Y, Sm, Eu, Er, Yb) as intermediate phase will give rise to the mismatching impedance, which will further lead to the reduction of reflection loss. Intriguingly, the HEREB₆/HEREBO₃ still possess wide effective absorption bandwidth of 4.1 GHz with the relative low thickness of 1.7 mm. Considering the better stability, low density, and good EM wave absorption properties, HE REB₆ ceramics are promising as a new type of EM wave absorbing materials.