Characterizing the Conductivity and Enhancing the Piezoresistivity of Carbon Nanotube-Polymeric Thin Films.

The concept of lightweight design is widely employed for designing and constructing aerospace structures that can sustain extreme loads while also being fuel-efficient. Popular lightweight materials such as aluminum alloy and fiber-reinforced polymers (FRPs) possess outstanding mechanical properties, but their structural integrity requires constant assessment to ensure structural safety. Next-generation structural health monitoring systems for aerospace structures should be lightweight and integrated with the structure itself. In this study, a multi-walled carbon nanotube (MWCNT)-based polymer paint was developed to detect distributed damage in lightweight structures. The thin film's electromechanical properties were characterized via cyclic loading tests. Moreover, the thin film's bulk conductivity was characterized by finite element modeling

Strain Sensors, Methods of Making Same, and Applications of Same

In one aspect, the present invention relates to a layered structure usable in a strain sensor. In one embodiment, the layered structure has a substrate with a first surface and an opposite, second surface defining a body portion therebetween; and a film of carbon nanotubes deposited on the first surface of the substrate, wherein the film of carbon nanotubes is conductive and characterized with an electrical resistance. In one embodiment, the carbon nanotubes are aligned in a preferential direction. In one embodiment, the carbon nanotubes are formed in a yarn such that any mechanical stress increases their electrical response. In one embodiment, the carbon nanotubes are incorporated into a polymeric scaffold that is attached to the surface of the substrate. In one embodiment, the surfaces of the carbon nanotubes are functionalized such that its electrical conductivity is increased

Strain and Temperature Sensing Properties of Multiwalled Carbon Nanotube Yarn Composites

Strain and temperature response of Multiwalled Carbon Nanotube (MWCNT/CNT) yarns on a stainless steel test beam has been studied. The carbon nanotube yarns are spun from a multiwalled carbon nanotube forest grown on a silicon substrate to a 4-ply yarn with a diameter of about 15-20 microns. Four of the 4-ply CNT yarns are arranged in a Wheatstone bridge configuration on the stainless steel test beam using a thin layer of polyurethane resin that insulates and protects the yarns from the test beam. Strain sensitivities of the CNT yarn sensors range from 1.39 to 1.75 mV/V/1000 microstrain at room temperature, and temperature sensitivity of the CNT yarn bridge is 91 microA/degC. Resistance of the yarns range from 215 to 270 ohms for CNT yarn length of approximately 5 mm. Processes used in attaching the CNT yarns on the test beam and experimental procedures used for the measurements are described. Conventional metallic foil strain gages are attached to the test beam to compare with the CNT sensors. The study demonstrates multifunctional capability of the sensor for strain and temperature measurements and shows its applicability where engineering strain is less than 3%

Carbon nanotubes based multi-directional strain sensor

In this work a new carbon nanotubes (CNT) based multi-directional strain sensor capable of quantifying and indicate strain direction is foreseen. This work investigates the electromechanical behavior of an aligned CNT sensing patch strained at 45◦ in order to validate its multi-directional sensing capability. Vertically aligned CNT forests are produced by chemical vapor deposition (CVD) and then mechanically knocked down onto polyimide (PI) films. Two configurations, diamond (D sample) and square (Sq sample), are considered. The relative electrical resistance (ΔR/R0) and the electrical anisotropy (RB/RA) upon strain increments are analyzed and compared to previous work results (0◦ and 90◦ strain direction). Both 45◦ samples, D and Sq, are sensitive to strain. A correlation between electrical anisotropy behavior and strain direction (0◦, 45◦ and 90◦) is established. The results show that with only an aligned CNT small patch it is possible to quantify and indicate strain in three directions.This work was partially funded under the project “IAMAT – Introduction of advanced materials technologies into new product development for the mobility industries”, with reference MITP-TB/PFM/0005/2013, under the MIT-Portugal program exclusively financed by FCT – Fundação para a Ciência e Tecnologia. This work was also co-financed by national funds through FCT – Fundação para a Ciência e Tecnologia, with the scope of projects with references UIDB/05256/2020 and UIDP/05256/2020”

Multifunctional layer-by-layer carbon nanotube–polyelectrolyte thin films for strain and corrosion sensing

Since the discovery of carbon nanotubes, researchers have been fascinated by their mechanical and electrical properties, as well as their versatility for a wide array of applications. In this study, a carbon nanotube–polyelectrolyte composite multilayer thin film fabricated by a layer-by-layer (LbL) method is proposed to develop a multifunctional material for measuring strain and corrosion processes. LbL fabrication of carbon nanotube composites yields mechanically strong thin films in which multiple sensing transduction mechanisms can be encoded. For example, judicious selection of carbon nanotube concentrations and polyelectrolyte matrices can yield thin films that exhibit changes in their electrical properties to strain and pH. In this study, experimental results suggest a consistent trend between carbon nanotube concentrations and strain sensor sensitivity. Furthermore, by simply altering the type of polyelectrolyte used, pH sensors of high sensitivity can be developed to potentially monitor environmental factors suggesting corrosion of metallic structural materials (e.g. steel, aluminum).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/58148/2/sms7_2_022.pd

Characterization of Multifunctional MWCNT/PP Nanocomposite Films for Use in Structural Health Monitoring and Disassembly of Ultrasonically Welded Single Lap Joints

In this study, multi-walled carbon nanotubes (MWCNT) nanocomposites were characterized and used as energy directors (EDs) in ultrasonic welding (USW) to test their viability as embedded sensors for structural health monitoring (SHM) and as heating elements for disassembly of thermoplastic composite joints. Three MWCNT loadings were used in this study to manufacture MWCNT/polypropylene (PP) nanocomposites, 15, 20, and 25 wt% while only one MWCNT loading was used to manufacture MWCNT/Nylon-6 (PA-6) nanocomposites, 15 wt%. The masterbatch pellets were compression molded into 0.06, 0.25, and 0.50 mm thick films for characterization and welding. For the purposes of using these MWCNT nanocomposites as EDs in USW for SHM and disassembly, the parameters that were investigated were the electrical conductivity, resistive heating performance, melting temperature, specific heat, storage modulus, loss modulus, and gauge factor. Electrical characterization showed that the quenched 15 wt% MWCNT/PA-6 nanocomposites had the lowest but most consistent conductivity while the air cooled MWCNT/PP nanocomposites showed less ohmic behavior and increasing conductivity with increasing MWCNT loadings. Resistive heating characterization showed that more conductive films could reach higher temperatures under similar voltages. Results from differential scanning calorimeter (DSC) analysis showed that the melting temperature and specific heat of the as received and post processed nanocomposites did not significantly vary with MWCNT loadings. The storage and loss moduli both increased with higher MWCNT loadings, until the loading reached 25 wt%, whereupon both moduli behaved more similarly to that of the pure polymer. Electrodes were taped to the steel grips of the dynamic mechanical analyzer (DMA) to measure the gauge factor (GF) of each xii nanocomposite in tension. After using the PP nanocomposites as EDs in USW, their GF was measured as the single lap specimens were pulled apart via tension. The GFs of the nanocomposites in the weld interface were approximately two orders of magnitude lower than the films in tension, indicating that the nanocomposite films in their current form are not yet suitable as strain sensors. However, the resistance signal of the nanocomposite films in the weld interface could detect damage

Real-time strain monitoring and damage detection of composites in different directions of the applied load using a microscale flexible Nylon/Ag strain sensor

Composites are prone to failure during operating conditions and that is why vast research studies have been carried out to develop in situ sensors and monitoring systems to avoid their catastrophic failure and repairing cost. The aim of this research article was to develop a flexible strain sensor wire for real-time monitoring and damage detection in the composites when subjected to operational loads. This flexible strain sensor wire was developed by depositing conductive silver (Ag) nanoparticles on the surface of nylon (Ny) yarn by electroless plating process to achieve smallest uniform coating film without jeopardizing the integrity of each material. The sensitivity of this Nylon/Ag strain sensor wire was calculated experimentally, and gauge factor was found to be in the range of 21–25. Then, the Nylon/Ag strain sensor wire was inserted into each composite specimen at different positions intentionally during fabrication depending upon the type of damage to detect. The specimens were subjected to tensile loading at a strain rate of 2 mm/min. Overall mechanical response of composite specimens and electrical response signal of the Nylon/Ag strain sensor wire showed good reproducibility in results; however, the Nylon/Ag sensor showed a specific change in resistance in each direction because of the respective position. The strain sensor wire designed not only monitored the change in the mechanical behavior of the specimen during the elongation and detected the strain deformation but also identified the type of damage, whether it was compressive or tensile. This sensor wire showed good potential as a flexible reinforcement in composite materials for in situ structural health monitoring applications and detection of damage initiation before it can become fatal

Piezoresistive Hybrid Nanocomposites for Strain and Damage Sensing: Experimental and Numerical Analysis

Carbon nanomaterials such as carbon nanotubes (CNTs) and graphite nanoplatelets (GNPs) demonstrate remarkable electrical and mechanical properties, which suggest promising structural and functional applications as fillers for polymer nanocomposites. The piezoresistive behavior of these nanocomposites makes them ideal for sensing applications. Besides, hybrid nanocomposites with multiple fillers like carbon nanotubes (CNTs) and graphite nanoplatelets (GNPs) are known to exhibit improved electrical and mechanical performance when compared to mono-filler composites. To comprehensively understand the mechanisms of electrical percolation, conductivity, and piezoresistivity in hybrid nanocomposites, the author develops a two-dimensional (2D) and a three-dimensional (3D) computational Monte Carlo percolation network models for hybrid nanocomposites with CNT and GNP fillers. In the experimental studies correlated to the computational models, the author fabricates the hybrid nanocomposites made of both fillers using resin infiltration techniques and show an improvement of their electromechanical performance when compared to CNT nanocomposites. Due to the limitations of the resin infiltration techniques, the author develops an inkjet printing procedure with a new water-based CNT ink to fabricated printed nanocomposites on both polyimide film (Kapton) and paper with high device-todevice reproducibility. The ink formulation, as well as the substrate surface treatment, have been optimized to obtain conductive and piezoresistive devices. The author shows the effectiveness of the printed devices as strain sensors and impact damage sensors respectively under mechanical strains and hypervelocity impact damages. Devices printed with the minimum number of ink deposited layers lead to the best sensing performance