RF characterization and applications of carbon based composites

Graphene is a monolayer of carbon atoms with remarkable electronic and mechanical properties. The attractive electronic properties of thin and thick films made of carbon nanotubes (CNTs) and graphene are increasingly being exploited for environmental and biological sensors. In particular, their sensitivity, selectivity, fast response time, ability to operate at room temperature, and their passive nature (no power consumption) provide competitive advantages of CNTs in sensor applications. However, their design as RF wireless sensors requires the integration of an antenna with the sensor element. Moreover, while the plasmonic nature of graphene at terahertz frequency has been widely reported, investigations on the practical utility of graphene at the microwave frequencies used in wireless sensor nodes are sparse which is indicated in this thesis. First, an ink comprising graphene thick films of different concentrations (12.5%, 25% and 33% in weight) is prepared for deposition, by screen printing. Detailed investigation of the surface morphology of the films using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) reveals that the graphene films present a homogeneous dispersion of the filler with a comparatively lower surface roughness at higher concentrations, and negligible agglomerates. The films are then printed in between copper electrodes on FR-4 substrate, commonly used in RF circuits, and the measured scattering parameters analyzed. A measurement-based RF equivalent circuit model of the graphene film is developed using a microstrip transmission line with a gap loaded by the film. Second, investigation on various patch antennas with different substrates using Multi-Walled Carbon Nanotube (MWCNT) thin film deposition is addressed. Screen printing technique is used to insert a CNT film in a loading stub connected to the antenna patch. The variation of the CNT surface impedance modifies the resonant frequency from the reference value, as revealed by comparison of return loss measured with and without the CNT loading. This CNT stub loaded patch antenna can be used as a bio sensor. Third, a printed RF slot ring resonator is configured with a graphene thin-film for sensor application. The conductive losses in the graphene film are characterized by dielectric spectroscopy and considered in the design. The graphene sensing element comprising the slot ring can be integrated with control electronics as a passive wireless sensor node. The novelty of this research is that RF losses are minimized by capacitively loading the ring at selective locations along its periphery. Dielectric spectroscopy is used to study variation in surface impedance of the film for various graphene loadings, and RF simulations are corroborated with measurements on graphene loaded slot ring resonators used in ammonia gas sensor application. The measurement steps are taken into consideration. As mentioned, the ring resonant frequency shift in presence of the ammonia gas is the factor used to sense the gas. Fourth, a novel design of an aperture coupled antenna which is weakly coupled to an interdigitated capacitor (IDC) is presented that serves the dual purpose of antenna impedance matching and the sensing function, the latter enabled by a thick film of CNTs deposited on the IDC surface. Simulations using CNT films of varying conductivity (or surface impedance) reveal that a strong antenna resonance can be produced. Furthermore, a study of the patch antenna radiation pattern with and without the CNT film shows weak coupling between the film and the antenna (loss of 0.5 dB or less relative to patch alone). Thus, the sensor film and geometry can be independently optimized without affecting radiation pattern.