Alessandro Proietti

Graphene-Based Strain Sensor Array on Carbon Fiber Composite Laminate

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.

Electrical and Electromechanical Properties of Stretchable Multilayer-Graphene/PDMS Composite Foils

Multilayer graphene (MLG)/polydimethylsiloxane (PDMS) composite foils are developed for possible application in wearable electronics as stretchable conductors or in strain sensing. The MLG/PDMS foils are prepared through infiltration of PDMS into porous freestanding MLG papers. The freestanding MLG papers are first produced through vacuum filtration of MLG-flake suspensions and are characterized by sheet resistance in the range from ~0.7 to ~1.1 Ω/□ and average thickness in the range from ~75 to ~110 μm. The electromechanical characteristic of the produced MLG/PDMS foils is assessed experimentally by measuring the dc electrical resistance of the produced specimen during tensile strength tests. It results that the breaking load of the new composite foils, occurring after an elongation of ~11 mm, which corresponds to a deformation of ~80%, is nearly doubled with respect to the one of neat PDMS. Moreover, for an elongation up to 2 mm the total dc resistance of the foil exhibits an increase lower than the 20% of its original value.

Multilayer Graphene-based films for strain sensing

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%).

Multilayer graphene-coated honeycomb as wideband radar absorbing material at radio-frequency

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.

Electrical, Mechanical and Electromechanical Properties of Graphene-Thermoset Polymer Composites Produced Using Acetone-DMF Solvents

Recently, graphene-polymer composites gained a central role in advanced stress and strain sensing. A fundamental step in the production of epoxy-composites filled with graphene nanoplatelets (GNPs) consists in the exfoliation and dispersion of expanded graphite in a proper solvent, in the mixing of the resulting GNP suspension with the polymer matrix, and in the final removal of the solvent from the composite before curing through evaporation. The effects of traces of residual solvent on polymer curing process are usually overlooked, even if it has been found that even a small amount of residual solvent can affect the mechanical properties of the final composite. In this paper, we show that residual traces of N,N′-Dimethylformamide (DMF) in vinylester epoxy composites can induce relevant variations of the electrical, mechanical and electromechanical properties of the cured GNP-composite. To this purpose, a complete analysis of the morphological and structural characteristics of the composite samples produced using different solvent mixtures (combining acetone and DMF) is performed. Moreover, electrical, mechanical and electromechanical properties of the produced composites are assessed. In particular, the effect on the piezoresistive response of the use of DMF in the solvent mixture is analyzed using an experimental strain dependent percolation law to fit the measured electromechanical data. It is shown that the composites realized using a higher amount of DMF are characterized by a higher electrical conductivity and by a strong reduction of Young’s Modulus.

Wideband radar absorbing panels with lossy multilayer graphene and carbon nanofiber-based coating

A novel lightweight wideband radar absorbing material (RAM) for radio-frequency is developed using a rohacell (RC) panel coated with a film of carbon nanostructures. The coating is produced through the dispersion of either commercial graphene nanoplatelets (GNPs) or carbon nanofibers (CNFs) in 1-propanol and the deposition of the so-obtained colloidal suspension onto the RC surface with a bristle brush. Finally, a polymeric film is deposited over the carbon-based lossy layer in order to protect the lossy coating from the external environment. The electromagnetic properties of the produced panels are investigated through reflection coefficient measurements in the X and Ku bands. It is demonstrated that the RAM with the GNP-based lossy layer has a -10 dB bandwidth of ~7 GHz, but the polymeric top layer degrades its performances, resulting in a minimum reflection coefficient of -9 dB. With the CNF-based lossy layer it is possible to fabricate a RAM with a -10 dB bandwidth of ~10 GHz. The application of the protective layer reduces the bandwidth to ~6 GHz.

Electromechanical characterization of flexible and highly conducting multilayer graphene/polydimethylsiloxane composite paper

Multilayer graphene (MLG)/polydimethylsiloxane (PDMS) composite has been prepared by infiltrating free-standing MLG paper, obtained through the vacuum filtration of MLG-suspension using a nanoporous filter, with PDMS prepolymer. Electrical properties of both free-standing MLG paper and MLG/PDMS composite paper were investigated by four-point probe measurements. The obtained results show that the annealed MLG paper is characterized by a sheet resistance of ~ 0,69 Ω/□ which does not increase significantly with polymer infiltration. Moreover the electromechanical behaviour of the composite paper has been investigated experimentally by measuring the DC electrical resistance of the produced specimen during a tensile strength test. It results that the breaking of composite paper occurs at ~ 80% strain, like for the neat polymer, but with a more than doubled applied load. Furthermore, it is noted that for strain smaller than ~ 10%, the electrical resistance of MLG/PDMS composite paper is nearly constant.