A solid solution-based millimeter-wave absorber exhibiting highly efficient absorbing capability and ultrabroad bandwidth simultaneously via a multi-elemental co-doping strategy

The development of millimeter-wave absorbing materials is urgent due to the ever-severe electromagnetic (EM) pollution problems in the higher frequency range with the rapid advancement in 5G communication technologies. However, subject to impedance mismatch in the applied frequency window, it is still a challenge to realize a large reflection loss (RL) and broad absorption bandwidth simultaneously at a small thickness. Herein, we report a solid solution-based high-performance millimeter-wave absorbing material, BaZr0.2Ti0.2Ni0.2W0.2Fe11.2O19, via a selective multi-elemental co-doping strategy. To efficiently generate Fe2+ ions in barium ferrite, the selection of co-doping ions is based on the principle that their average valence state should be slightly higher than that of Fe3+ ions, and half of the ions should possess a larger ionic radius while the other half ions have a smaller radius than that of Fe3+ ions. As a result, the electrical conductivity increases significantly to be 124 times higher than that in single-ion doping. Meanwhile, the magnetic loss range is effectively broadened due to the formation of multiple natural resonances arising from multiple Lande factors adjusted by exchange coupling between Fe3+ and Fe2+ ions, together with the eddy current loss. Benefiting from these multiple EM-wave attenuation mechanisms, the absorber exhibits superior millimeter-wave absorption performance in the frequency range of 18-40 GHz under perfect impedance matching, featuring a RL value of -61.8 dB (>99.9999% EM waves effectively absorbed), an ultrabroad -20 dB bandwidth of 9.15 GHz at a small matching thickness of 0.97 mm, outperforming the state-of-the-art millimeter-wave absorbers.