[Show abstract][Hide abstract] ABSTRACT: Innovative graphene-based sensors are produced through the spray deposition of multilayer graphene suspension in 1-propanol. They are cost effective and suitable for large-scale integration in aeronautical-grade composite structures. The piezoresistive characteristic of the new sensor has been assessed through static, quasi-static, and dynamic tests. The obtained results show that the new sensors are suitable for detecting strains in any directions due to the isotropic piezoresistive characteristic of the graphene-based material. Moreover, the potentiality of the proposed sensors for the in situ non-destructive investigations of carbon-fiber-reinforced composite (CFRC) is assessed through the response analysis of an eight-sensor array deposited over a CFRC plate, and excited by a mechanical vibration with a spectral content up to 100 kHz.
Full-text · Article · Dec 2015 · IEEE Sensors Journal
[Show abstract][Hide abstract] ABSTRACT: Papers presented at the 2nd Bilateral Indo-Italian Workshop NEEM 2015 Rome, Italy October 12-14, 2015
Organized with the support of the Directorate General for the Country Promotion Ministry of Foreign Affairs and
International Cooperation by Consorzio Hypatia Department of Science & Technology of India IMEM-CNR Institute for Plasma Research (Ghandinagar) Istituto Nazionale di Fisica Nucleare (INFN) – Laboratori Nazionali di Frascati Rome International Center for Materials Science Superstripes RICMASS Università Sapienza Università di Roma Accademia dei Lincei and Regione Lazio
[Show abstract][Hide abstract] ABSTRACT: Graphene films were produced by chemical vapor deposition (CVD) of pyridine on copper substrates. Pyridine-CVD is expected to lead to doped graphene by the insertion of nitrogen atoms in the growing sp2 carbon lattice, possibly improving the properties of graphene as a transparent conductive film. We here report on the influence that the CVD parameters (i.e., temperature and gas flow) have on the morphology, transmittance, and electrical conductivity of the graphene films grown with pyridine. A temperature range between 930 and 1070 °C was explored and the results were compared to those of pristine graphene grown by ethanol-CVD under the same process conditions. The films were characterized by atomic force microscopy, Raman and X-ray photoemission spectroscopy. The optical transmittance and electrical conductivity of the films were measured to evaluate their performance as transparent conductive electrodes. Graphene films grown by pyridine reached an electrical conductivity of 14.3 × 105 S/m. Such a high conductivity seems to be associated with the electronic doping induced by substitutional nitrogen atoms. In particular, at 930 °C the nitrogen/carbon ratio of pyridine-grown graphene reaches 3%, and its electrical conductivity is 40% higher than that of pristine graphene grown from ethanol-CVD.
Full-text · Article · Oct 2015 · Beilstein Journal of Nanotechnology
[Show abstract][Hide abstract] ABSTRACT: A graphene-based porous paper made of multilayer graphene (MLG) microsheets is developed for application as a flexible electrically conducting shielding material at radio frequency. The production process is based on the thermal expansion of a graphite intercalated compound, the successive liquid-phase exfoliation of the resulting expanded graphite in a proper solvent, and finally the vacuum filtration of the MLG-suspension using a nanoporous alumina membrane. Enhancement of the electrical conductivity and electromagnetic shielding properties of the MLG paper is achieved by gentle annealing at 250 °C overnight, and by mechanical compression at 5 MPa. The obtained results show that the developed MLG papers are characterized by an electrical conductivity up to 1443.2 S/cm, porosity around 43%, high flexibility, shielding effectiveness up to 55 dB at 18 GHz with a thickness of 18 μm. Numerical simulations are performed in order to understand the main factors contributing to the shielding performance of the new material.
[Show abstract][Hide abstract] ABSTRACT: In this work we analyze the feasibility of a broadband radar absorbing material made with an aeronautical grade honeycomb panel, coated with a thin conducting film of multilayer graphene nanoplatelets (MLGs). The film is deposited over the surface of a phenolic-aramid sheet and characterized in terms of sheet resistance and thickness. The morphology of the multilayer graphene flakes is analyzed by atomic force and scanning electron microscopies. The deposition parameters are optimized in order to control the sheet resistance of the conducting MLG film. The microwave broadband radar absorbing properties of a MLG-coated phenolic-aramid honeycomb panel are predicted through 3D electromagnetic simulations. It is shown that with a panel thickness of 10 mm and a sheet resistance of the MLG coating of 1238 Ω/□, the reflection coefficient RdB has a minimum at ~9 GHz. Moreover, it results that the absorbing panel is broadband, with RdB ≤ -10 dB in the range from 5.5 GHz to 27 GHz.
[Show abstract][Hide abstract] ABSTRACT: The issue concerning the measurement of the shielding effectiveness (SE) of planar materials over a wide frequency range is of crucial relevance in several electromagnetic compatibility applications. This paper describes three different coaxial specimen holders for the measurement of the SE of thin metallic films over a nonconducting substrate or sandwiched between two insulating layers from a few kHz up to 18 GHz. Besides the well-known ASTM D4935 flanged coaxial cell, two novel versions of coaxial fixtures with an interrupted and continuous inner conductor are presented and compared. Their limits of applicability, advantages, and drawbacks are discussed with respect to frequency, sample characteristics, and test procedure. The analysis is performed by the use of simple equivalent circuit models, experimentally validated measuring thin copper films of different thicknesses which are deposited on kapton substrates by magnetron sputtering. It is demonstrated that the use of the three methods, properly combined, provides reliable SE results in the overall considered frequency range. It is also shown that the measurement of conducting films between two dielectric layers is critical at frequencies lower than some tens of MHz.
No preview · Article · Dec 2014 · IEEE Transactions on Electromagnetic Compatibility
[Show abstract][Hide abstract] ABSTRACT: Flexible composite foils made up of multilayer graphene (MLG) and ZnO nanowires (ZnO-NW) are produced via vacuum filtration of acetone-based suspension. The sheet resistance and effective dc electrical conductivity of the foils, with increasing content of ZnO-NW over a fixed amount of MLG, is measured in order to investigate the effect of ZnO inclusion in MLG papers. An enhancement in the dc conductivity of the composite foils with respect to a plain MLG-foil is noticed only for very low amounts of ZnO loading. The peak conductivity of 16.8 kS/m, representing an increase of 31% with respect to the conductivity of a plain MLG-foil, is observed at the optimum concentration of 10%wt ZnO over the MLG content. This confirms the physical and electronic interaction at the interface between MLG and ZnO-NW. At higher concentration of ZnO-NW, a linear decay in the effective conductivity of the foils is observed.
[Show abstract][Hide abstract] ABSTRACT: The transfer of chemical vapor deposited graphene is a crucial process, which can affect the quality
of the transferred films and compromise their application in devices. Finding a robust and intrinsically clean material capable of easing the transfer of graphene without interfering with its properties remains a challenge.
We here propose the use of an organic compound, cyclododecane, as a transfer material. This material can be easily spin coated on graphene and assist the transfer, leaving no residues and requiring no further removal processes. The effectiveness of this transfer method for few-layer graphene on a large area was evaluated and confirmed by microscopy,
Raman spectroscopy, x-ray photoemission spectroscopy, and four-point probe measurements.
Schottky-barrier solar cells with few-layer graphene were fabricated on silicon wafers by using the
cyclododecane transfer method and outperformed reference cells made by standard methods.
[Show abstract][Hide abstract] ABSTRACT: The transfer of chemical vapor deposited graphene is a crucial process, which can affect the quality of the transferred films and compromise their application in devices. Finding a robust and intrinsically clean material capable of easing the transfer of graphene without interfering with its properties remains a challenge. We here propose the use of an organic compound, cyclododecane, as a transfer material. This material can be easily spin coated on graphene and assist the transfer, leaving no residues and requiring no further removal processes. The effectiveness of this transfer method for few-layer graphene on a large area was evaluated and confirmed by microscopy, Raman spectroscopy, x-ray photoemission spectroscopy, and four-point probe measurements. Schottky-barrier solar cells with few-layer graphene were fabricated on silicon wafers by using the cyclododecane transfer method and outperformed reference cells made by standard methods.
[Show abstract][Hide abstract] ABSTRACT: In this work we investigate the shielding properties of graphene-based films, produced by spray casting deposition, which is a cost-effective technique that can be directly scaled to industrial applications. The deposition process is optimized in order to produce uniform films made of well dispersed graphene nanoplatelets (GNP), having thicknesses in the range 3-8 microns. The sheet resistance and the shielding effectiveness up to 18 GHz of films produced using GNP-suspensions at different concentrations are measured and compared with the ones of carbon nanotube (CNT)-based films, produced in the same conditions. The obtained results demonstrate the superior shielding properties of graphene-based films with respect to CNT-ones, mainly due to the bi-dimensional shape of GNPs, which contributes to minimizing the contact resistances among adjacent nanostructures. It is also demonstrated that after a thermal annealing at 250°C, the final measured shielding effectiveness of the produced GNP-film triplicates in linear scale, reaching the value of 30.6 dB.
[Show abstract][Hide abstract] ABSTRACT: In this work we investigate the piezoresistive effect in multilayer graphene (MLG) based films produced by two different cost-effective techniques, spray coating and drop casting. Both techniques enable the direct deposition of the sensor over the structure to be monitored. The piezoresistive behavior of the MLG-based sensors has been investigated experimentally by measuring the variation of the electrical resistance during three point bending tests. The sensor response has been stabilized through an optimized mechanical treatment. The obtained results show that the produced sensors are characterized by a gauge factor in the range 20-50 at very small strains (i.e. below 0.2%).
[Show abstract][Hide abstract] ABSTRACT: Thin and flexible freestanding graphene nanoplatelets (GNPs)-based paper-like materials are fabricated and experimentally characterized to assess their employment as radiofrequency electromagnetic interference shields. Samples are obtained by vacuum filtration of ultrasonicated suspensions. Different suspensions are prepared by mixing either acetone/DMF or acetone/NMP and worm-like exfoliated graphite expanded at the temperature of 1150 °C or 1250 °C for 5s. We investigated the effect of thermal annealing and mechanical compression on the sheet resistance, thickness, dc electrical conductivity and electromagnetic shielding of the produced GNP papers. Their shielding effectiveness (SE) is measured in the frequency range 10 MHz-18 GHz and validated by simulations. Electrical conductivity of 144 kS/m and SE as high as 55 dB are reached for a few microns thick GNP paper subjected to a compression of 5 MPa.
[Show abstract][Hide abstract] ABSTRACT: A graphene-based composite, consisting of a thermosetting polymeric matrix filled with multilayer graphene microsheets (MLGs), is developed for application in thin radar absorbing materials. An innovative simulation model is proposed for the calculation of the effective permittivity and electrical conductivity of the composite, and used for the electromagnetic design of thin radar absorbing screens. The model takes into account the effects of the MLG morphology and of the fabrication process on the effective electromagnetic properties of the composite. Experimental tests demonstrate the validity of the proposed approach and the accuracy of the developed simulation models, which allow to understand the interaction mechanism between the incident electromagnetic field radiation and the MLG-based composite. Two dielectric Salisbury screen prototypes with resonant frequency at 12 GHz or 12.5 GHz and total thickness of 1.8 mm and 1.7 mm, respectively, are fabricated and tested. The results and technique proposed represent a simple and effective approach to produce thin absorbing screens for application in stealth technology or electromagnetic interference suppression.
[Show abstract][Hide abstract] ABSTRACT: An effective-layer model is proposed to predict the EM shielding performances of metallic wire grids typically used as protective layers in carbon fiber-reinforced composite laminates for aeronautical applications. The model is valid over a wide frequency range and it takes into account the field penetration through the grating. The shielding performance of the wire grid is estimated by means of the average shielding effectiveness, which represents the response of the material to an incident plane wave having both transverse-magnetic and transverse-electric polarizations. The proposed model is validated by comparison with experimental data, which demonstrates its validity over a wide frequency range, up to 18 GHz.
No preview · Article · Jun 2014 · IEEE Transactions on Electromagnetic Compatibility
[Show abstract][Hide abstract] ABSTRACT: This paper presents a morphological and functional characterization of nanostructured thin films featuring high radio frequency shielding effectiveness and high optical transparency in the wavelength range 400–1500 nm. The film morphology is analyzed at the micro- and nanoscales by processing the images acquired by a scanning electron microscope. A software tool developed for this purpose analyzes the statistical distributions of the film surface grains. Fitting models and experimental evidences are presented in order to describe and predict the correlations between the film morphological and functional properties. The adopted approach and measurement methods are developed to model and optimize a particular transparent conducting oxide but can be easily extended to similar materials, deposition processes, and applications.
No preview · Article · Apr 2014 · IEEE Transactions on Electromagnetic Compatibility
[Show abstract][Hide abstract] ABSTRACT: The strain-dependent electrical resistance of polyvinyl ester-based composites filled with different weight fractions of graphene nanoplatelets (GNPs) has been experimentally investigated. The GNP synthesis and nanocomposite fabrication process have been optimized in order to obtain highly homogeneous filler dispersion and outstanding electrical properties. The produced nanocomposites showed a low percolation threshold of 0.226 wt% and electrical conductivity of nearly 10 S m(-1) at only 4 wt% of GNPs. The piezoresistive response of thin nanocomposite laminae has been assessed by measuring the variation of the electrical resistance as a function of the flexural strain in three-point bending tests under both quasi-static monotonic and dynamic cyclic loading conditions. The obtained results showed higher strain sensitivity than traditional metal foil strain gauges or recently investigated carbon-based nanocomposite films.
[Show abstract][Hide abstract] ABSTRACT: Graphene and its derivatives are nowadays gaining an ever increasing attention for diverse applications, such as supercapacitors, flexible screens and nanocomposites. One of the main challenges still limiting the wide spreading use of graphene-based nanomaterials in electrical and electromagnetic applications consists in the capability of controlling their morphological properties and electrical conductivity through the proper setting of the synthesis route and production process. In this paper graphene nanoplatelets (GNPs) are produced via thermochemical exfoliation of graphite intercalated compound (GIC). The crucial phase of the exfoliation process consists in the tip sonication of the solution containing thermally reduced GIC. The study presented here aims at evaluating the effect of several synthesis parameters, such as the sonication duty cycle and suspension temperature, on the DC conductivity of GNP thick films and on the morphological properties of GNPs.
[Show abstract][Hide abstract] ABSTRACT: Graphene films are grown by chemical vapour deposition on copper layer and then transferred onto a silicon substrate, coated with silicon dioxide. The topological characterization of the produced film is performed by atomic force microscopy, and the sheet resistance is measured by applying the four-probe test method. The equivalent single conductor model is then used in order to analyze the signal propagation along a nanointerconnect made with multilayer graphene over silicon dioxide, in a wide frequency range, up to 100 GHz. The comparison of the radio-frequency performances of the nanointerconnect, modeled by using either the measured value of effective resistivity or a theoretical estimation of the p.u.l. resistance, suggests that graphene films grown by chemical vapor deposition are more suitable for application as low frequency electrical interconnections in flexible electronics, than in high-speed integrated circuits.