Microwave characterization and shielding properties of biochar based polymers and cements

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Graphene tunability at microwave frequency

Graphene is the most notable among carbon-based materials and it has been greatly studied. One of the most important properties of graphene is its tunable conductivity. The electron mobility of graphene is varied by a DC voltage causing a change in its electrical conductivity. The variation of the electrical conductivity is valid over a large frequency band from DC up to millimeter waves. It makes graphene suitable for a wide range of different applications. The electrical conductivity of multilayered graphene just like monolayer graphene can be tuned by the help of a DC bias voltage. Several tunable passive microwave components were designed based on lab grown multilayered graphene. Recently commercial graphene was used to design microwave tunable attenuators, phase shifters and reconfigurable antennas

The Role of Probe Attenuation in the Time-Domain Reflectometry Characterization of Dielectrics

The influence of the measurement setup on the estimation of dielectric permittivity spectra from time-domain reflectometry (TDR) responses is investigated. The analysis is based on a simplified model of the TDR measurement setup, where an ideal voltage step is applied to an ideal transmission line that models the probe. The main result of this analysis is that the propagation in the probe has an inherent band limiting effect, and the estimation of the high-frequency permittivity parameters is well conditioned only if the wave attenuation for a round trip propagation in the dielectric sample is small. This is a general result, holding for most permittivity model and estimation scheme. It has been verified on real estimation problems by estimating the permittivity of liquid dielectrics and soil samples via an high-order model of the TDR setup and a parametric inversion approac

Permittivity measurement of sand and clay soil with a capacitive sensor

The determination of permittivity of sand and clay soil over a wide frequency band can be useful in several applications. Fringing Capacitive sensors can be used to measure the real and imaginary part of the permittivity of materials in the RF and microwave frequency bands. In this paper the use of a commercial capacitive sensor has been exploited in order to characterize sand and clay soils with different water content. Liquid and granular materials are particular suited for this kind of sensor because the sensor can be dipped into the sample thus avoiding contact problems between the surface of the sensor and the material as for solid one. The measurement setup is composed by an Agilent Coaxial Probe kit 85070D, an HP 8720B Network analyzer and PC for data acquisition. This sensor works in the frequency range 200MHz-20GHz. The calibration procedure is based on three reference measurements (air, short circuit and deionized water). The setup and the calibration procedure has been tested by measuring the permittivity of several reference liquids (methanol, ethanol, acetone, water with different salt concentration). The comparison with the Cole-Cole model was also performed. Then, several samples of sand and clay soil with different water content have been considered. This measurement technique has also been compared with a frequency domain approach for the permittivity determination based on the double-delay method

Enhancing the phase shift of tunable phase shifters based on graphene nanoplatelets

Graphene exhibits tunable conductivity in a wide frequency band ranging from DC to microwaves [1]. The tunable conductivity of graphene opens a new paradigm of innovative microwave components that can be dynamically tuned by a DC voltage. The tunable conductive behavior of graphene exists not only in monolayer graphene but also in few layer graphene nanoplatelets [2]. Few layer graphene nanoplatelets are easier to fabricate and deposit as compared to monolayer graphene. The availability of commercial graphene nanoplatelets paves the way for mass scale usage. Graphene nanoplatelets were used in designing tunable attenuators [3] and phase shifters. By applying a DC bias voltage across graphene nanoplatelets, their sheet resistance drastically reduces. This variation of resistance can be converted to a variation of reactance by the help of stubs and tapered transmission line sections. The introduction of a variable reactance in the middle of a two-port transmission line causes a shift in the phase of the signal passing through the line. The lengths of the tapered lines are optimized to maximize the phase shift variation and minimize the additional insertion loss caused by the variation of the resistance of graphene. By adopting such structure, a phase shift of 40 degrees was demonstrated with an additional insertion loss of 3 dB [4]

Graphene-based Radiofrequency resonator for non-invasive glucose detection

This paper presents a radio-frequency (RF) sensor to detect glucose oxidase. At the core of the proposed approach is a graphene film deposited on a gap connected to a split ring resonator. The graphene film is doctor bladed on the gap. The film is then properly chemically functionalized in order to detect the presence of glucose. In this paper, we validate the proof-of-concept operation of glucose concentration detection by measuring the frequency shift of the transmission coefficient of the sensor

Dynamically tunable phase shifter with commercial graphene nanoplatelets

In the microwave frequency band the conductivity of graphene can be varied to design a number of tunable components. A tunable phase shifter based on commercial graphene nanoplatelets is introduced. The proposed configuration consists of a microstrip line with two stubs connected with a taper. On each side of the stubs there is a gap, short circuited through a via, where the commercial graphene nanoplatelets are drop casted. By applying a DC bias voltage that alters the graphene resistance the phase of the transmitted signal through the microstrip line can be varied. In order to maximize the phase shift of the transmitted signal and minimize the insertion loss, the length of the taper and the stubs are optimized by the help of circuit model and full-wave simulations. A prototype working at 4GHz is fabricated and measured. A phase variation of 33 degrees is acquired with an amplitude variation of less than 0.4dB

Analysis of Transmission Properties of Sludge Biochar Composites in the C-Band

In order to pave the way for the widespread use of eco-friendly materials for novel applications, there is a growing need for morphological, mechanical and electrical, and microwave characterization techniques. The measurement of scattering parameters is an accurate technique for the microwave characterization of novel materials, but it requires a number of components, including waveguides, adapters, and so on. The band of measurements is also limited to the working band of the waveguide used. A method of retrieving waveguide scattering parameters from permittivity values is devised. The method is validated with measurements of the scattering parameters in a waveguide. The method is tested with composites based on biochar derived from sewage sludge and a standard epoxy sample. The addition of biochar considerably reduces transmission scattering and is found to be a suitable candidate for filling composite materials

Single and Multi-Piece Behavioral Models of IC Output Buffers

This paper discusses the switching properties of the behavioral models of integrated circuit output buffers. Present behavioral models are based on a two-piece structure defined by a linear combination of two submodels for the two logic states. These models are very accurate in the two logic states, but their state switching is sensitive to the driven loads. The analysis carried out in this paper shows that this load sensitivity stems from the linear combination defining the two-piece model. A new single-piece behavioral model is proposed, that solves this problem and has efficiency and complexity levels comparable to those of two-piece model