Microwave-absorbing properties of room-temperature ionic liquids

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.