Nanocomposite Strain Sensors Research Articles

Carbon nanotube thin film strain sensors: comparison between experimental tests and numerical simulations

Carbon nanotubes can be randomly deposited in polymer thin film matrices to form nanocomposite strain sensors. However, a computational framework that enables the direct design of these nanocomposite thin films is still lacking. The objective of this study is to derive an experimentally validated and two-dimensional numerical model of carbon nanotube-based thin film strain sensors. This study consisted of two parts. First, multi-walled carbon nanotube (MWCNT)-Pluronic strain sensors were fabricated using vacuum filtration, and their physical, electrical, and electromechanical properties were evaluated. Second, scanning electron microscope images of the films were used for identifying topological features of the percolated MWCNT network, where the information obtained was then utilized for developing the numerical model. Validation of the numerical model was achieved by ensuring that the area ratios (of MWCNTs relative to the polymer matrix) were equivalent for both the experimental and modeled cases. Strain sensing behavior of the percolation-based model was simulated and then compared to experimental test results.

Strain Sensors with Adjustable Sensitivity by Tailoring the Microstructure of Graphene Aerogel/PDMS Nanocomposites.

Strain sensors with high elastic limit and high sensitivity are required to meet the rising demand for wearable electronics. Here, we present the fabrication of highly sensitive strain sensors based on nanocomposites consisting of graphene aerogel (GA) and polydimethylsiloxane (PDMS), with the primary focus being to tune the sensitivity of the sensors by tailoring the cellular microstructure through controlling the manufacturing processes. The resultant nanocomposite sensors exhibit a high sensitivity with a gauge factor of up to approximately 61.3. Of significant importance is that the sensitivity of the strain sensors can be readily altered by changing the concentration of the precursor (i.e., an aqueous dispersion of graphene oxide) and the freezing temperature used to process the GA. The results reveal that these two parameters control the cell size and cell-wall thickness of the resultant GA, which may be correlated to the observed variations in the sensitivities of the strain sensors. The higher is the concentration of graphene oxide, then the lower is the sensitivity of the resultant nanocomposite strain sensor. Upon increasing the freezing temperature from -196 to -20 °C, the sensitivity increases and reaches a maximum value of 61.3 at -50 °C and then decreases with a further increase in freezing temperature to -20 °C. Furthermore, the strain sensors offer excellent durability and stability, with their piezoresistivities remaining virtually unchanged even after 10 000 cycles of high-strain loading-unloading. These novel findings pave the way to custom design strain sensors with a desirable piezoresistive behavior.

A Spray-On Carbon Nanotube Artificial Neuron Strain Sensor for Composite Structural Health Monitoring

We present a nanocomposite strain sensor (NCSS) to develop a novel structural health monitoring (SHM) sensor that can be easily installed in a composite structure. An NCSS made of a multi-walled carbon nanotubes (MWCNT)/epoxy composite was installed on a target structure with facile processing. We attempted to evaluate the NCSS sensing characteristics and benchmark compared to those of a conventional foil strain gauge. The response of the NCSS was fairly good and the result was nearly identical to the strain gauge. A neuron, which is a biomimetic long continuous NCSS, was also developed, and its vibration response was investigated for structural damage detection of a composite cantilever. The vibration response for damage detection was measured by tracking the first natural frequency, which demonstrated good result that matched the finite element (FE) analysis.

Processing and Characterization of a Novel Distributed Strain Sensor Using Carbon Nanotube-Based Nonwoven Composites.

This paper describes the development of an innovative carbon nanotube-based non-woven composite sensor that can be tailored for strain sensing properties and potentially offers a reliable and cost-effective sensing option for structural health monitoring (SHM). This novel strain sensor is fabricated using a readily scalable process of coating Carbon nanotubes (CNT) onto a nonwoven carrier fabric to form an electrically-isotropic conductive network. Epoxy is then infused into the CNT-modified fabric to form a free-standing nanocomposite strain sensor. By measuring the changes in the electrical properties of the sensing composite the deformation can be measured in real-time. The sensors are repeatable and linear up to 0.4% strain. Highest elastic strain gage factors of 1.9 and 4.0 have been achieved in the longitudinal and transverse direction, respectively. Although the longitudinal gage factor of the newly formed nanocomposite sensor is close to some metallic foil strain gages, the proposed sensing methodology offers spatial coverage, manufacturing customizability, distributed sensing capability as well as transverse sensitivity.

Ultrasensitive strain sensors of multiwalled carbon nanotube/epoxy nanocomposite using dielectric loss tangent

In this work, the dielectric loss tangent (tan δ) of a series of strain sensors, fabricated from an epoxy nanocomposite with multi-wall carbon nanotube (MWCNT) content varying at 1 wt. % – 5 wt. %, was characterized experimentally. The effects of four parameters including frequency, strain of nanocomposite, MWCNT content, and loading voltage were investigated extensively. Moreover, an alternative current gauge factor KAC was developed. The largest value of KAC was found to be 256 for the nanocomposite strain sensor with 1 wt. % MWCNT content at 0.6% tensile strain, which indicates the ultra-sensitivity of the present strain sensor.

Deformable strain sensors based on patterned MWCNTs/polydimethylsiloxane composites

ABSTRACTPatterned MWCNT/polydimethylsiloxane (PDMS) nanocomposite strain sensors were achieved by a microelectromechanical system assisted electrophoretic deposition (EPD) technique. With the combined effect of superior intrinsic piezoresistivity of the individual MWCNT and the tunneling effect of the MWCNT network, the stretchable composite demonstrates high sensitivity to the tensile strain. The gauge factor shows a strong dependence on both the initial resistance of the CNT/PDMS composite and the applied strain level. The mechanism is elucidated by analyzing the structure‐property‐function of patterned CNT networks. When the entanglement of a MWCNT network allows effective load transfer, the sensitivity is primarily dominated by the intrinsic piezoresistivity of individual MWCNTs. Conversely, when the MWCNTs interpenetrate loosely, the tunneling effect prevails. The sensitivity of the device can be tailored by the proposed technique since MWCNT film thickness/density can be readily controlled by means of the patterning parameters of the EPD process. The work provides useful guidance for design and development of strain/stress sensors with targeted sensitivity for flexible electronics applications. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1505–1512

High strain biocompatible polydimethylsiloxane-based conductive graphene and multiwalled carbon nanotube nanocomposite strain sensors

High performance strain sensors were achieved featuring simple, low-cost construction involving the screen printing of combinations of multi-walled carbon nanotube and graphene nano-platelet nanocomposites on biocompatible and flexible polymer substrates. Conductivity and thermal coefficients of resistance of different conductive nanocomposite sensor materials were measured. The zero current resistance and gauge factor of printed sensors was characterized. The combination of high strain operation (up to 40%), high gauge factor (GF > 100), and biocompatible construction pave the way for applications such as minimally invasive in vivo strain measurements.

Performance characterization of VGCF/epoxy nanocomposite sensors under static load cycles and in static structural health monitoring

Compared to conventional metal-foil strain gauges, nanocomposite piezoresistive strain sensors have demonstrated high strain sensitivity and have been attracting increasing attention in recent years. To fulfil their ultimate success, the performance of vapor growth carbon fiber (VGCF)/epoxy nanocomposite strain sensors subjected to static cyclic loads was evaluated in this work. A strain-equivalent quantity (resistance change ratio) in cantilever beams with intentionally induced notches in bending was evaluated using the conventional metal-foil strain gauges and the VGCF/epoxy nanocomposite sensors. Compared to the metal-foil strain gauges, the nanocomposite sensors are much more sensitive to even slight structural damage. Therefore, it was confirmed that the signal stability, reproducibility, and durability of these nanocomposite sensors are very promising, leading to the present endeavor to apply them for static structural health monitoring.

Fabrication and Performance Evaluation of Piezoelectric Strain Sensors Made from PVDF/MWNT Nanocomposites

In this study, we fabricated piezoelectric strain sensors from multi-walled carbon nanotubes (MWNTs) and PVDF nanocomposites, i.e., PVDF/MWNT for measuring dynamic strains. The influence of MWNT loading on the sensor performance was evaluated by changing the MWNT loading as 0.0wt%, 0.05wt%, 0.2wt%, and 0.3wt%. To increase MWNT dispersion in PDVF matrix, a mixing process by using a planetary stirring machine and sonication processing by using an ultrasonic mixer were firstly employed together to produce nanocomposite films. Then, these films were stretched under uniaxial loading and poled under 60MV/m to fabricate the strain sensors. Moreover, crystallinity of the PVDF/MWNT nanocomposites was analyzed by using X-ray diffraction (XRD) analysis, and the fractured surfaces of pre-stretched PVDF/MWNT nanocomposites were observed by using a polarized optical microscope (POM). The piezoelectricity and signal tracking capability of the PVDF/MWNT nanocomposites sensors in vibration were investigated. From experimental results, the piezoelectricity, i.e., the sensor output voltage of the PVDF/MWNT nanocomposites reaches to a maximum peak value at 0.05wt% MWNT loading, and then decreases with further more addition of MWNTs. The result of XRD intensity was consistent with the piezoelectric sensor output results of PVDF/MWNT nanocomposites. From POM observations, compared to that of pure PVDF, the spherulite's size in the PVDF/MWNT nanocomposites becomes smaller and its number increases. Moreover, there should exist an optimum content of MWNTs, which can result in high piezoelectric properties of the nanocomposite strain sensors.

Flexible latex—polyaniline segregated network composite coating capable of measuring large strain on epoxy

A new conductive polymer nanocomposite (CPC) strain sensor, with a segregated network of polyaniline nanoparticles in a poly(vinyl acetate) latex matrix (PVAc–PANI), was created to improve upon existing systems. The strain sensing capabilities of these CPC transducers, attached to epoxy beams, were determined by subjecting them to loading/unloading cycles up to 1% strain (and straining them until the epoxy beam failed (around 5% strain)), in uniaxial tension. Microstructural images and visco-elastic properties show that these CPC are homogeneous and behave much like neat PVAc. PVAc with 4 wt.% PANI provided the best compromise among high sensitivity, small hysteresis and low noise. These environmentally friendly, flexible and low cost strain sensors are capable of monitoring strain above 5%, with a gauge factor between 6 and 8 (3 times those of classical metallic gauges). Moreover, this technology can be easily scaled up to monitor deformations of large composite structures, opening up many promising areas (e.g., damage detection for aircraft).

Multi-scale numerical simulations on piezoresistivity of CNT/polymer nanocomposites

In this work, we propose a comprehensive multi-scale three-dimensional (3D) resistor network numerical model to predict the piezoresistivity behavior of a nanocomposite material composed of an insulating polymer matrix and conductive carbon nanotubes (CNTs). This material is expected to be used as highly sensitive resistance-type strain sensors due to its high piezoresistivity defined as the resistance change ratio divided by the mechanical strain. In this multi-scale 3D numerical model, three main working mechanisms, which are well known to induce the piezoresistivity of strain sensors fabricated from nanocomposites, are for the first time considered systematically. They are (a) the change of the internal conductive network formed by the CNTs, (b) the tunneling effect among neighboring CNTs, and (c) the CNTs’ piezoresistivity. Comparisons between the present numerical results and our previous experimental ones were also performed to validate the present numerical model. The influence of the CNTs’ piezoresistivity on the total piezoresistivity of nanocomposite strain sensors is explored in detail and further compared with that of the other two mechanisms. It is found that the first two working mechanisms (i.e., the change of the internal conductive network and the tunneling effect) play a major role on the piezoresistivity of the nanocomposite strain sensors, whereas the contribution from the CNTs’ piezoresistivity is quite small. The present numerical results can provide valuable information for designing highly sensitive resistance-type strain sensors made from various nanocomposites composed of an insulating polymer matrix and conductive nanofillers.

Sandwiched carbon nanotube film as strain sensor

Two types of carbon nanotube nanocomposite strain sensors were prepared by mixing carbon nanotubes with epoxy (nanocomposite sensor) and sandwiching a carbon nanotube film between two epoxy layers (sandwich sensor). The conductivity, response and sensitivity to static and dynamic mechanical strains in these sensors were investigated. The nanocomposite sensor with 2–3 wt.% carbon nanotube demonstrated high sensitivity to mechanical strain and environmental temperature, with gauge factors of 5–8. On the other hand, a linear relationship between conductivity and dynamic mechanical strain was observed in the sandwich sensor. The sandwich sensor was also not sensitive to temperature although its strain sensitivity (gauge factor of about 3) was lower as compared with the nanocomposite sensor. Both sensors have excellent response to static and dynamic strains, thereby having great potential for strain sensing applications.

Direct-write fabrication of freestanding nanocomposite strain sensors

This paper deals with the design and microfabrication of two three-dimensional (3D) freestanding patterned strain sensors made of single-walled carbon nanotube (SWCNT) nanocomposites with the ultraviolet-assisted direct-write (UV-DW) technique. The first sensor consisted of three nanocomposite microfibers suspended between two rectangular epoxy pads. The flexibility of the UV-DW technique enables the sensor and its housing to be manufactured in one monolithic structure. The second sensor was composed of a nanocomposite network consisting of four parallel microsprings, which demonstrates the high capability of the technique when compared to conventional photolithographic technologies. The performances of the sensors were assessed under tension and compression, respectively. The sensors’ sensitivities were evaluated by correlating their measured resistivities to the applied displacements/strains. Electrical conductivity measurements revealed that the manufactured sensors are highly sensitive to small mechanical disturbances, especially for lower nanotube loadings when compared to traditional metallic or nanocomposite films. The present manufacturing method offers a new perspective for manufacturing highly sensitive 3D freestanding microstructured sensors.

J044115 ナノコンポジットによる圧電型ひずみセンサの創製と性能評価

In this study, we fabricated a piezoelectric strain sensor from multi-walled carbon nanotubes (MWNTs) and PVDF nanocomposites, i.e., PVDF/MWNT for measuring dynamic strains. The influence of MWNT loading on the sensor performance was evaluated by changing the MWNT loading as 0.0wt%, 0.05wt%, 0.2wt%, and 0.3wt%. To increase the MWNT dispersion in PDVF matrix, a mixing process by using a planetary stirring machine and sonication processing by using an ultrasonic mixer were firstly employed together to produce the nanocomposite films. Then, these films were stretched under uniaxial loading and poled under 60MV/m to fabricate the strain sensors. Moreover, crystallinity of the PVDF/MWNT nanocomposites was analyzed by using X-ray diffraction (XRD) analysis, and the fractured surfaces of the pre-stretching PVDF/MWNT nanocomposites were observed by using a polarized optical microscope (POM). In this research, the piezoelectricity and signal tracking capability for the PVDF/MWNT nanocomposites sensors in vibration were investigated. From the experimental results, the piezoelectricity, i.e., the sensor output voltage of the PVDF/MWNT nanocomposites reaches to a maximum peak value at 0.05wt% MWNT loading, and then decreases with further more addition of MWNTs. The result of XRD intensity was consistent with the result of the piezoelectricity of PVDF/MWNT nanocomposites. From POM observations, compared to that of pure PVDF, the spherulite size of the PVDF/MWNT nanocomposites becomes smaller. Moreover, there might be an optimum content of MWNTs in the nanocomposites, which can result in high piezoelectric properties of the nanocomposite strain sensors.

Correlating Piezoelectric Polymer/Carbon Nanotubes Nanocomposite Strain Sensor with Reliability and Optimization Tools

Carbon nanotubes with polymers offers great advantages in improving material for both mechanical and electrical nanostructures. Design and fabrication have to consider that a local change in each compound accounts to the total change of physical properties in nanocomposite materials. This paper presents two parts of study. A model of strain nanosensor has been developed by using the polyvinylidne fluoride and trifluoroethylene P(VDF-TrFE) copolymer and carbon nanotubes in sandwich nanostructure [P(VDF-TrFE)/SWCNTs/ P(VDF-TrFE)] as a new application in nanotechnology domain. The experimental strain sensing was about 10-4. On the other hand, reliability-based optimization is assessed for an efficient tool to consider this nanosensors nanodevice. We put emphasis on the combination of physical modeling and reliability based design optimization of nanomaterials. After investigation, we could make suggestions such as how to improve the reliability of nanodevices and nanosystems, and how to reduce cost and economic rates.

Inductively coupled nanocomposite wireless strain and pH sensors

Recently, dense sensor instrumentation for structural health monitoring has motivated the need for novel passive wireless sensors that do not require a portable power source, such as batteries. Using a layer-by-layer self-assembly process, nano-structured multifunctional carbon nanotube-based thin film sensors of controlled morphology are fabricated. Through judicious selection of polyelectrolytic constituents, specific sensing transduction mechanisms can be encoded within these homogenous thin films. In this study, the thin films are specifically designed to change electrical properties to strain and pH stimulus. Validation of wireless communications is performed using traditional magnetic coil antennas of various turns for passive RFID (radio frequency identification) applications. Preliminary experimental results shown in this study have identified characteristic frequency and bandwidth changes in tandem with varying strain and pH, respectively. Finally, ongoing research is presented on the use of gold nanocolloids and carbon nanotubes during layer-by-layer assembly to fabricate highly conductive coil antennas for wireless communications.

Tunneling effect in a polymer/carbon nanotube nanocomposite strain sensor

A strain sensor has been fabricated from a polymer nanocomposite with multiwalled carbon nanotube (MWNT) fillers. The piezoresistivity of this nanocomposite strain sensor has been investigated based on an improved three-dimensional (3D) statistical resistor network model incorporating the tunneling effect between the neighboring carbon nanotubes (CNTs), and a fiber reorientation model. The numerical results agree very well with the experimental measurements. As compared with traditional strain gauges, much higher sensitivity can be obtained in the nanocomposite sensors when the volume fraction of CNT is close to the percolation threshold. For a small CNT volume fraction, weak nonlinear piezoresistivity is observed both experimentally and from numerical simulation. The tunneling effect is considered to be the principal mechanism of the sensor under small strains.

The Bulk Piezoresistive Characteristics of Carbon Nanotube Composites for Strain Sensing of Structures

The bulk piezoresistivity of carbon nanotube (CNT) in polymer matrix was discussed to develop a strain sensor for engineering applications. The polymer improves interfacial bonding between the nanotubes and the CNT composite and that enhances the strain transfer, repeatability, and linearity of the sensor. The largest contribution of piezoresistivity of the sensor may come from slippage of overlaying or bundled nanotubes in the matrix, from a macroscopic point of view. Nano interfaces of CNTs in a matrix polymer also contribute to the linear strain response compared to other micro size carbon filler. The strain sensor had a low bandwidth and adequate strain sensitivity. The nanocomposite strain sensor is particularly useful for detecting large strains which can monitor strain and stress on a structure with simple electric circuit for strain monitoring of structures.

The Bulk Piezoresistive Characteristics of Carbon Nanotube Composites for Strain Sensing of Structures

The bulk piezoresistivity of carbon nanotube (CNT) in polymer matrix was discussed to develop a strain sensor for engineering applications. The polymer improves interfacial bonding between the nanotubes and the CNT composite and that enhances the strain transfer, repeatability, and linearity of the sensor. The largest contribution of piezoresistivity of the sensor may come from slippage of overlaying or bundled nanotubes in the matrix, from a macroscopic point of view. Nano interfaces of CNTs in a matrix polymer also contribute to the linear strain response compared to other micro size carbon filler. The strain sensor had a low bandwidth and adequate strain sensitivity. The nanocomposite strain sensor is particularly useful for detecting large strains which can monitor strain and stress on a structure with simple electric circuit for strain monitoring of structures.