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][NTf2] 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.