ABSTRACT
The fundamentals, performance, and applications of piezoresistive strain sensors based on polymer nanocomposites are summarized herein. The addition of conductive nanoparticles to a flexible polymer matrix has emerged as a possible alternative to conventional strain gauges, which have limitations in detecting small strain levels and adapting to different surfaces. The evaluation of the properties or performance parameters of strain sensors such as the elongation at break, sensitivity, linearity, hysteresis, transient response, stability, and durability are explained in this review. Moreover, these nanocomposites can be exposed to different environmental conditions throughout their lifetime, including different temperature, humidity or acidity/alkalinity levels, that can affect performance parameters. The development of flexible piezoresistive sensors based on nanocomposites has emerged in recent years for applications related to the biomedical field, smart robotics, and structural health monitoring. However, there are still challenges to overcome in designing high-performance flexible sensors for practical implementation. Overall, this paper provides a comprehensive overview of the current state of research on flexible piezoresistive strain sensors based on polymer nanocomposites, which can be a viable option to address some of the major technological challenges that the future holds. .
ABSTRACT
The temperature effect on electronic transport mechanisms in graphene nanoplatelet (GNP) doped polydimethylsiloxane (PDMS) for temperature sensing applications has been investigated under electrical impedance spectroscopy (EIS) analysis. AC measurements showed a very prevalent frequency-dependent behavior in low filled nanocomposites due to the lower charge density. In fact, 4 wt % GNP samples showed a nonideal capacitive behavior due to scattering effects. Therefore, the standard RC-LRC circuit varies with the substitution of capacitive elements by CPEs, where a CPE is a constant phase element which denotes energy dissipation. In this regard, the temperature promotes a prevalence of scattering effects, with an increase of resistance and inductance and a decrease of capacitance values in both RC (intrinsic and contact mechanisms) and LRC (tunneling mechanisms) elements and, even, a change from ideal to nonideal capacitive behavior as in the case of 6 wt % GNP samples. In this way, a deeper understanding of electronic mechanisms depending on GNP content and temperature is achieved in a very intuitive way. Finally, a proof-of-concept carried out as temperature sensors showed a huge sensitivity (from 0.05 to 11.7 °C-1) in comparison to most of the consulted studies (below 0.01 °C-1), proving, thus, excellent capabilities never seen before for this type of application.
ABSTRACT
A novel nanoreinforced adhesive film has been developed to detect adhesive deformation and crack propagation along the bonding line by means of the electrical response of the material. Adhesive films were doped by spraying an aqueous dispersion of carbon nanotubes (CNTs) over the surface. To determine the sensitivity of bonded joints, single lap shear (SLS) and mode-I fracture energy tests have been carried out while their electrical response has been measured. It has been found that CNT-doped adhesive films are able to detect adhesive deformation and final failure for SLS specimens and crack initiation and propagation along the bonding line for mode-I specimens with a high sensitivity. Sudden increases on electrical resistance are correlated to a rapid growing of the crack length due to instability on crack propagation in a tick-slip case, whereas specimens with a more uniform crack propagation are linked to a steadier increase on electrical resistance, and both of them are properly correlated to the mechanical response. By analyzing more in detail the electrical response and comparing with theoretical approaches, the stick-slip behavior is associated with the presence of porosity and lack of adhesives because of possible manufacturing issues such as adhesive overflowing. These statements are also validated by microstructural analysis. Therefore, the potential and applicability of the proposed adhesive films for evaluating the structural integrity has been demonstrated.
