  A new method to characterize microwave electromagnetic absorption of a bulk
carbon nanotube material is proposed and experimentally evaluated in this
paper. The method is based on the measurement of microwave transmission through
a capacitive-resonator aperture in a conductive screen loaded with a CNT sample
under test. This method allows to measure microwave permittivity and absorption
of thin samples, several atomic layers to few micrometers thick, with linear
dimensions much smaller than the wavelength of radiation in free space. This
minimal sample requirement restricts the application of conventional microwave
characterization methods such as free-space or waveguide permittivity
characterization. It is demonstrated that the resonance E-field enhancement
inside the CRA leads to strong EM interaction of the microwave E-field with the
CNT sample under test thus enabling high sensitivity and dynamic range of the
measurement procedure. Another advantage of the proposed technique over
conventional non-resonance characterization methods is that in the resonance
transmission band, the CRA operation is reflection-less which leads to a
relatively simple qualitative algebraic de-embedding procedure of the material
parameters based on the principle of energy conservation. The experimental
microwave absorption data of the multiwall CNT samples are presented in the S
frequency band (2-4GHz), demonstrating microwave absorption properties of the
multiwall CNT ribbons.
