Freeze-Dried Carbon Nanotube Aerogels for High-Frequency Absorber Applications

A novel technique for millimeter wave absorber material embedded in a metal waveguide is proposed. The absorber material is a highly porous carbon nanotube (CNT) aerogel prepared by a freeze-drying technique. CNT aerogel structures are shown to be good absorbers with a low reflection coefficient, less than −12 dB at 95 GHz. The reflection coefficient of the novel absorber is 3–4 times lower than that of commercial absorbers with identical geometry. Samples prepared by freeze-drying at −25 °C demonstrate resonance behavior, while those prepared at liquid nitrogen temperature (−196 °C) exhibit a significant decrease in reflection coefficient, with no resonant behavior. CNT absorbers of identical volume based on wet-phase drying preparation show significantly worse performance than the CNT aerogel absorbers prepared by freeze-drying. Treatment of the freeze-dried CNT aerogel with <i>n</i>- and <i>p</i>-dopants (monoethanolamine and iodine vapors, respectively) shows remarkable improvement in the performance of the waveguide embedded absorbers, reducing the reflection coefficient by 2 dB across the band