A. Rinaldi

A flexible and highly sensitive pressure sensor based on a PDMS foam coated with graphene nanoplatelets

The demand for high performance multifunctional wearable devices is more and more pushing towards the development of novel low-cost, soft and flexible sensors with high sensitivity. In the present work, we describe the fabrication process and the properties of new polydimethylsiloxane (PDMS) foams loaded with multilayer graphene nanoplatelets (MLGs) for application as high sensitive piezoresistive pressure sensors. The effective DC conductivity of the produced foams is measured as a function of MLG loading. The piezoresistive response of the MLG-PDMS foam-based sensor at different strain rates is assessed through quasi-static pressure tests. The results of the experimental investigations demonstrated that sensor loaded with 0.96 wt.% of MLGs is characterized by a highly repeatable pressure-dependent conductance after a few stabilization cycles and it is suitable for detecting compressive stresses as low as 10 kPa, with a sensitivity of 0.23 kPa−1, corresponding to an applied pressure of 70 kPa. Moreover, it is estimated that the sensor is able to detect pressure variations of ~1 Pa. Therefore, the new graphene-PDMS composite foam is a lightweight cost-effective material, suitable for sensing applications in the subtle or low and medium pressure ranges.

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.

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.

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.

Wearable graphene-based sensor array for finger tracking

In this work, we have developed novel flexible strain sensors based on an highly interconnected three-dimensional Polyurethane (PU) foam coated with a Polyvinyl alcohol (PVA)-multilayer graphene nanoplatelets (MLGs) paint. The electromechanical response of the sensors embedded in a biocompatible elastomer is firstly investigated through quasi-static tensile tests. The measured resistance variation under an applied stress confirmed the piezoresistive behavior of the fabricated sensors, showing a gauge factor of 8 at strain of 20%. Finally, we also demonstrated the feasibility to use an array of three sensors to monitor the flexion/ extension of finger joints.

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.

RF shielding performance of thin flexible graphene nanoplatelets-based papers

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.