Here, the microwave-absorbing properties of 1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium ([E and BMIm] ⁺ -based) room-temperature ionic liquids (ILs) with different anions were systematically investigated by measuring dielectric properties in the 1–14 GHz microwave-frequency range. First, the dielectric properties of the [E and BMIm] ⁺ -based ILs were studied with a typical open-ended coaxial probe system. The results showed that the real part of permittivity values decreased from 13 to 4 and the values of loss tangent decreased from 5.91 to 0.24, meaning that ILs displayed high dielectric-loss characteristics at microwave-frequency ranges. Furthermore, it was found that the conductivity loss was dominant for dielectric loss in the low-frequency band, and the polarization loss played a major role in high-frequency bands. For the same anion ILs with different cations, the permittivity became higher as the length of the alkyl chain decreased. The absorption properties of [E and BMIm] ⁺ -based ILs could be obtained based on their dielectric properties, and found the ILs absorption bands were mainly concentrated in the C band and X band. It was noted that the maximum reflection loss (RL) of [BMIm][NTf ₂ ] with a thickness of 4 mm reached −27.6 dB at 8.73 GHz, and the bandwidth of an RL less than −10 dB could be up to 4.3 GHz. The absorption peak frequency of the ILs gradually shifted to lower frequencies with increasing temperature and coating thickness, which might be caused by the change in impedance matching.

We propose an all-dielectric hollow annular groove ultra-broadband metamaterial absorber (A-D ILMMA) based on imidazole ionic liquids, which is freed of metallic substrate and can offer an extremely wide bandwidth of electromagnetic absorption and prominent wide polarization angle range. When the ionic liquid 1-ethyl-3-methylimidazolium Tetrafluoroborate ([EMIM][BF ₄ ]) with a high conductivity and dielectric loss factor is arranged into the hollow annular groove unit cells with a 6 mm total thickness, nearly perfect impedance matching can be achieved in a wide frequency range. Research results show that the simulated absorptivity is over 90% in 13.9–50 GHz with the relative bandwidth of 113%, and the corresponding microwave experiment is performed to support the results of the simulation. Moreover, the A-D ILMMA can work efficiently at arbitrary polarization angles from 0° to 90° and wide incident angles from 0° to 60°.

In this paper, a temperature tunable ultra-wideband absorber based on ionic liquid is proposed for the microwave frequency band. The absorber consists of a band-resistive frequency selective surface, a 3D resin cavity vessel, and an ionic liquid ([EMIm][N(CN)₂]) layer. Numerical simulation analysis shows that the absorptivity is more than 90% and a relative bandwidth is 113.04% in the range of 7.5–27 GHz. Meanwhile, the absorber absorptivity has different tuning effects in different frequency bands with the change of temperature. Along with the temperature going up, the absorptivity decreases in the low-frequency band of 6.5–14 GHz, the absorptivity increases in the high-frequency band of 28 GHz–40 GHz. It is worth mentioning that the proposed ionic liquid-based absorber has the characteristics of wide incidence angle and polarization insensitivity. Finally, the temperature tunable absorber model based on ionic liquid is fabricated by 3D printing technology. The experimental results are consistent with the simulation results, demonstrating that the absorber is practically feasible. In summary, the absorber achieves a wide frequency tuning range, which gives it great potential application prospects in fields such as frequency-selective thermal radiators.