Experimental Research On The Microwave Property Of Novel Carbon Nano-materials And Its Applications

The applications of a material depend on its physical properties. In this dissertation, to find potential applications of carbon nanotube and graphene in microwave and millimeter-wave techniques, the anisotropic conductivity dispersion of horizontally aligned carbon nanotubes (HA-MWCNTs) thin film and the conduction of graphene film in microwave range are measured and characterized. In addition, some applications of graphene in X-band are discussed.Firstly, the anisotropic conductivity dispersion of HA-MWCNTS thin film is experimentally characterized at Ku-, Ka- and Q-band. The samples are obtained by mechanical rolling the vertically aligned multi-walled carbon nanotubes grown by CVD. The inspection of Raman spectrums of the sample indicates no destruction of carbon nanotube structure after the rolling procedure, while the SEM images show the axial directions of the HA-MWCNTs mainly orient in one direction. By comparing the sandwiched and sealed waveguide structures, the sealed waveguide is chose to characterize the samples for no energy leakage. Since our sample is composed of carbon nanotube thin film, kapton film and supporting foam, traditional Nicolson-Ross-Weir (NRW) method is not applicable for extracting the conductivity of our sample. Here, we developed an extraction algorithm based the signal flow chart to extract the conductivity of our sample. The results reveal the anisotropic conduction of the HA-MWCNTs thin film. Meanwhile, the real part of the extracted conductivity remains constant over the frequency band while the imaginary part increases almost linearly with the frequency. Therefore, the HA-MWCNTs thin film can be physically equivalent to a RC parallel circuit. Uncertainties of the measured S-parameters and physical parameters of the kapton film and supportive foam (thickness and relative permittivity) will lead to errors in the extracted surface conductivity. To estimate these errors, an error analysis is performed based on the uncertainties of parameters. The results indicates that the axial surface conductivity is more accurate than the radial conductivity.Secondly, the conduction of single layer graphene between 1GHz and 10GHz is experimentally characterized. The sample is grown by CVD method. The contact method is used to measure the S-parameter of graphene sample based on some analysis. After transferred to a coplanar waveguide (CPW) with center line slotted, the S-parameters of sample are measured. The impedance of sample are obtained via a de- embedding procedure erasing the parasitic effect of the CPW. An equivalent circuit model consisting of a RL series circuit and two RC parallel circuits is proposed to fit the impedance of sample and the surface impedance of graphene is extracted, which reveals that the inductance of graphene is negligibly small compared with its resistance in the measured band, as predicted by Kubo formula. In addition, the contact resistance between the graphene and the Au electrode is relatively small compared with the resistance of graphene. This indicates the contact method is effective to characterize the graphene sample.Finally, graphene based microwave absorber and microstrip antenna working in X-band are studied. Employing graphene to construct Salisbury screen, the new structure can not only absorb the electromagnetic energy efficiently but also its absorption can be controlled by tuning the surface impedance of graphene. By replacing the patch in microstrip antenna with graphene, the return losses and radiation patterns of the new antenna can be largely affected by tuning the surface impedance of graphene.