Conductive network formation in the melt of carbon nanotube/thermoplastic polyurethane composite

This research concerns the effect of conductive network formation in a polymer melt on the conductivity of multi-walled carbon nanotube/thermoplastic polyurethane composite systems. An extremely low percolation threshold of 0.13 wt.% was achieved in hot-pressed composite film samples, whereas a much higher CNT concentration (3–4 wt.%) is needed to form a conductive network in extruded composite strands. This is explained in terms of the dynamic percolation behaviour of the CNT network in the polymer melt. The temperature and CNT concentration needed for dynamic percolation to take effect were studied by the conductivity versus temperature behaviour of extruded strands, in an attempt to optimise the processing conditions.     

Simultaneous global and local strain sensing in SWCNT-epoxy composites by Raman and impedance spectroscopy

Polymer composites can be benefited in many ways through the addition of carbon nanotubes (CNT). For instance, CNT can build up a percolated network within the polymer matrix, which results in a composite material with electrical conductivity and piezoresistive characteristics. This has very important implications for the realization of self-stress sensing structural composites. Moreover, the remarkable optical and transport properties of CNT permit to obtain information about the stress state of the composite at different scales. In the present work, the local and global stress response of SWCNT-epoxy composites is characterised by simultaneous Raman spectroscopic and electrical measurements on nanocomposite specimens submitted to different levels of surface strain. Both the Raman G′-band resonance frequency and the electrical resistance of the composite are found to change monotonically with strain until an inflection point is reached at ∼1.5% strain. Increased sensitivity of the piezoresistive network and lower load transfer efficiency occur beyond this strain level, and are considered to be the result of CNT slippage from the polymer. The reversibility of the stress sensitivity of the composites is verified by performing cyclic loading tests. Hysteresis loop are found to develop earlier on the Raman curves as in the resistance curves, which indicates that even at low strain levels, permanent damage is induced in the vicinity of carbon nanotubes. The use of Raman spectroscopy in combination with electrical methods provides a further insight on the stress sensing capabilities of CNT and the factors which affect the sensitivity and reproducibility of this behaviour.     

Preparation of continuous carbon nanotube networks in carbon fiber/epoxy composite

In order to optimize carbon nanotube (CNT) dispersion state in fiber/epoxy composite, a novel kind of CNT organization form of continuous networks was designed. The present work mainly discussed the feasibility of preparing continuous CNT networks in composite: Fiber fabric was immersed into CNT aqueous solution (containing dispersant) followed by freeze drying and pyrolysis process, prior to epoxy infusion. The morphologies of fabric with CNTs were observed by Scanning Electron Microscope. The relationship between CNT networks and flowing epoxy resin was studied. Properties of composite, including out-of-plane electrical conductivity and interlaminar shear strength (ILSS), were measured. The results demonstrated that continuous and porous CNT networks formed by entangled CNTs could be assembled in fiber fabric. Most part of them were preserved in composite due to the robustness of network structures. The preserved CNT networks significantly improved out-of-plane electrical conductivity, and also have an effect on ILSS value.     

Damage detection of glass fiber reinforced composites using embedded PVA–carbon nanotube (CNT) fibers

Polyvinyl alcohol–carbon nanotube (PVA–CNT) fibers were embedded in glass fiber reinforced plastic composites and used as strain sensors for damage monitoring of the composite. Sensing of the structural integrity of the composite was made by the in situ measurement of the electrical resistance of the embedded PVA–CNT fiber during the mechanical tests. The multi-functional materials were tested in tensile progressive damage accumulation (PDA) tests. These tests aimed to seek the electrical response of untreated and pre-stretched PVA–CNT fibers with known level of progressively induced damage to the composite. The advantages and disadvantages of each PVA–CNT fiber used as a sensor are analyzed; the electrical resistance readings of the PVA–CNT fibers were correlated with known parameters that express the induced damage of the composite.     

Electrostatic self-assembled carbon nanotube/nano carbon black composite fillers reinforced cement-based materials with multifunctionality

Electrostatic self-assembled carbon nanotube (CNT)/nano carbon black (NCB) composite fillers are added into cement mortar to fabricate smart cement-based materials. The grape bunch structure of CNT/NCB composite fillers is beneficial for dispersing CNT/NCB in cement mortar matrix and achieving cooperative improvement effect. The mechanical, electrically conductive, and piezoresistive behaviors of the cement mortar are investigated. The CNT/NCB composite fillers can effectively enhance the flexural strength and electrical conductivity of cement mortars, and endow stable and sensitive piezoresistivity to cement mortar at a low filler content. However, they weaken the compressive strength of cement mortar to some extent. The percolation threshold zone of cement mortar with CNT/NCB composite fillers ranges in the amount of 0.39–1.52 vol.%. The optimal content of CNT/NCB composite fillers is 2.40 vol.% for piezoresistivity and the stress and strain sensitivities can reach 2.69% MPa⁻¹ and 704, respectively.     

Structural health monitoring of glass fiber reinforced composites using embedded carbon nanotube (CNT) fibers

In the present work, carbon nanotube (CNT) fibers had been embedded to glass fiber reinforced polymers (GFRP) for the structural health monitoring of the composite material. The addition of the conductive CNT fiber to the non-conductive GFRP material aims to enhance its multi-function ability; the test specimen’s response to mechanical load and the insitu CNT fiber’s electrical resistance measurements were correlated for sensing and damage monitoring purposes. It is the first time this fiber is used in composite materials for sensing purposes; CNT fiber is easy to be embedded and does not downgrade the material’s mechanical properties. Various incremental loading–unloading steps had been applied to the manufactured specimens in tension as well as in three-point bending tests. The CNT fiber worked as a sensor in both, tensile and compression loadings. A direct correlation between the mechanical loading and the electrical resistance change had been established for the investigated specimens. For high stress (or strain) level loadings, residual resistance measurements of the CNT fiber were observed after unloading. Accumulating damage to the composite material had been calculated and was correlated to the electrical resistance readings. The established correlation between these parameters changed according to the material’s loading history.     

Evaluation and modelling of electrically conductive polymer nanocomposites with carbon nanotube networks

Superior electrical, thermal, and mechanical properties of carbon nanotubes (CNTs) have made them effective filler for multifunctional polymer nanocomposites (PNCs). In particular, electrically conductive PNCs filled with CNTs have been researched extensively. These studies aimed to increase the PNCs' electrical conductivity (σ) and to minimize the percolation thresholds (ϕ_c). In this work, we have developed an improved model to describe the CNT networks and thereby evaluate the PNCs' ϕ_c and σ. The new model accounts for the electrical conductance contributed by the continued CNT network across the boundary of adjacent representative volume elements. It more realistically represents the interconnectivity among CNTs and enhances the evaluation of the structure-to-property relationship of PNCs' σ.     

Study on the semi-empirical model for the complex permittivity of carbon nanocomposite laminates in microwave frequency band

A semi-empirical model is proposed for the complex permittivity of composites containing electrical conductive carbon nanomaterials such as carbon black (CB), carbon nanofiber (CNF) and multi-walled carbon nanotube (CNT). The composites were fabricated with E-glass fabric/epoxy prepregs. The model is based on the percolation theory. The model is available for the composite of filler content over the percolation threshold and applicable within the high frequency band in which AC electrical conductivity of the composite is continuously proportional to the frequency. The proposed model is composed of the numerical equations of the scaling law in percolation theory and constants obtained from experiments to quantify the model. The model describes the complex permittivity as a function of frequency and filler content. The model was verified when compared with the measurements. The measurements for the complex permittivities of the composites were performed at the frequency band between 0.5 and 18.0 GHz using a vector network analyzer with a 7 mm coaxial air line.     

Numerical investigation of mechanisms affecting the piezoresistive properties of CNT-doped polymers using multi-scale models

Carbon nanotube polymer nanocomposites exhibit conductive behavior due to the formation of conductive nanotube networks inside the polymer. Their electrical resistance is known to vary with strain. Two mechanisms that affect the conductivity and piezoresistive response of CNT nanocomposites are investigated using models at two discrete material scales: (a) nanoscale models to analyze the electromechanical response of carbon nanotubes and (b) nanotube percolation models to investigate the composites electrical resistance at microscale. Numerical studies determine the impact of each mechanism on the macroscopic response of the nanocomposite. Results suggest that the variation of nanotube resistance with strain is the dominant mechanism.     

A novel carbon-based nanocomposite plate as a counter electrode for dye-sensitized solar cells

This study develops cost effective and high performance composite conductive plates for use in dye-sensitized solar cells (DSSCs). Composite plates with various graphite contents at a constant carbon nanotube (CNT) loading were prepared by bulk molding compound (BMC) process. Results show that the bulk electrical resistance of the composite plate gradually decreases from 6.7 mΩ cm to 1.7 mΩ cm as the graphite content increases, which is due to the formation of efficient electronic conducting networks. For DSSCs, the composite plates may be suitable substitutes for the conductive glass plates in the counter electrodes substrates of DSSCs. Results reveal that composite plates at the optimum level (80 wt.% graphite loading) provide lower cell resistance, lower preparation cost and higher cell performance than common conductive glass plates. Therefore, in order to decrease the cost of the cells and to maintain good cell performance, this graphite-like composite plate prepared by the BMC process is a promising substitute component for DSSCs.     

Strain sensing in composites using aligned carbon nanotube sheets embedded in the interlaminar region

This paper introduces a novel technique for embedding aligned sheets of two millimeters long, interconnected CNTs into the interlaminar region of composite structures. The potential of these embedded CNT sheets to function as damage detecting and strain sensing elements was demonstrated via various mechanical tests that were accompanied by real time electrical resistance change data acquisition. The experimental results suggested that the CNT sheet sensitivity could be further enhanced by an oxygen plasma treatment and also by pre-straining the CNT sheets before embedding them. The samples containing two CNT sheets layers exhibited long term stability, sensitivity and repeatability which are vital features for health monitoring.     

Mechanical and electrical response of carbon nanotube-based fabric composites to Hopkinson bar loading

The numerous structural applications of composites, coupled with their complex, rate-dependent mechanical behavior necessitate research into their mechanical response under dynamic loading scenarios. While the damage mechanisms of composites under dynamic compression loading are well-understood, measuring the occurrence of damage in a non-invasive manner is challenging. Toward this end, we investigate the electrical response of an embedded percolating carbon nanotube network in woven fabric/epoxy composites to dynamic compression loading. The percolating network is established through the use of a non-uniform dispersion of carbon nanotubes, achieved using a fiber sizing agent. The resulting conductive network is sensitive to delamination and damage occurring near the fiber surfaces. The dynamic mechanical response of the composite specimens is explored using Hopkinson bar methodology. Definite increases in baseline resistance of the conductive composite specimens are seen after repeated impacts demonstrating the ability of the carbon nanotube network of these conductively modified composites to respond electrically to damage induced during dynamic loading.     

Formicary-like carbon nanotube/copper hybrid nanostructures for carbon fiber-reinforced composites by electrophoretic deposition

An electrophoretic deposition process has been applied to produce unique carbon nanotube (CNT)/copper nanostructures on the carbon fiber surfaces. During the deposition process, ionized copper and positively charged CNTs are accelerated towards the carbon fiber under applied electric fields. An interconnected formicary-like network of nanotubes and nanoparticles is formed where copper nucleation and growth occurs predominantly at nanotube crossing and edge-contact locations. When embedded in a structural composite the CNT/copper structures create a highly conductive and strongly bonded network shown by significant enhancements in both electrical conductivity and interlaminar shear strength as compared to composites without the CNT/copper nanostructures.     

Highly strong and conductive carbon nanotube/cellulose composite paper

Carbon nanotube (CNT)/cellulose composite materials were fabricated in a paper making process optimized for a CNT network to form on the cellulose fibers. The measured electric conductivity was from 0.05 to 671 S/m for 0.5–16.7 wt.% CNT content, higher than that for other polymer composites. The real permittivities were the highest in the microwave region. The unique CNT network structure is thought to be the reason for these high conductivity and permittivity values. Compared to other carbon materials, our carbon CNT/cellulose composite material had improved parameters without decreased mechanical strength. The near-field electromagnetic shielding effectiveness (EMI SE) measured by a microstrip line method depended on the sheet conductivity and qualitatively matched the results of electromagnetic field simulations using a finite-difference time-domain simulator. A high near-field EMI SE of 50-dB was achieved in the 5–10 GHz frequency region with 4.8 wt.% composite paper. The far-field EMI SE was measured by a free space method. Fairly good agreement was obtained between the measured and calculated results. Approximately 10 wt.% CNT is required to achieve composite paper with 20-dB far-field EMI SE.     

Poly(vinyl alcohol) reinforced with large-diameter carbon nanotubes via spray winding

For practical application of carbon nanotube (CNT)/polymer composites, it is critical to produce the composites at high speed and large scale. In this study, multi-walled carbon nanotubes (MWNTs) with large diameter (∼45 nm) and polyvinyl alcohol (PVA) were used to increase the processing speed of a recently developed spraying winding technique. The effect of the different winding speed and sprayed solution concentration to the performance of the composite films were investigated. The CNT/PVA composites exhibit tensile strength of up to 1 GPa, and modulus of up to 70 GPa, with a CNT weight fraction of 53%. In addition, an electrical conductivity of 747 S/cm was obtained for the CNT/PVA composites. The good mechanical and electrical properties are attributed to the uniform CNTs and PVA matrix integration and the high degree of tube alignment.     

Morphology and electrical properties of polyamide 6/polypropylene/multi-walled carbon nanotubes composites

Morphology, electrical properties and conductive mechanisms of polyamide 6/polypropylene/muti-walled carbon nanotubes (PA6/PP/MWNTs) composites with varied compositions and different blending sequences were investigated. The MWNTs were found to be located preferentially in the PA6 phase in the composites, whatever the PA6 was continuous or dispersed phase. While the incorporation of MWNTs changed the dispersed PA6 phase from spherical to elongated or irregular shape. The PA6/PP/MWNTs (20/80/4) composite with a dispersed PA6 phase exhibited a higher electrical conductivity in comparison with the PA6/PP/MWNTs (50/50/4) composite which has a co-continuous phase and exhibits double percolation. This was due to the formation of a conductive MWNTs networks in the PA6/PP/MWNTs (20/80/4) composite as proved by means of field emission scanning electron microscopy and rheological measurements. The morphology and electrical properties of the PA6/PP/MWNTs (20/80/4) composites were significantly influenced by blending sequences. When blending 3.9 phr MWNTs with a pre-mixed PA6/PP/MWNTs (20/80/0.1) composite, the dispersed PA6 phase formed an elongated structure, which was beneficial to the electrical properties.     

Processing and electrical properties of carbon nanotube/vinyl ester nanocomposites

Vinyl ester resins are often utilized in advanced naval composite structures due to the relatively low viscosity of the resin and the capability to cure at ambient temperatures. These qualities facilitate the production of large naval composite structures using resin infusion techniques. Vinyl ester monomer was synthesized from the epoxy resin to overcome processing challenges associated with volatility of the styrene monomer in vinyl ester resin. In this research we have investigated the use of a calendering approach for dispersion of multi-walled carbon nanotubes in vinyl ester monomer, and the subsequent processing of nanotube/vinyl ester composites. The high aspect ratios of the carbon nanotubes were preserved during processing and enabled the formation of a conductive percolating network at low nanotube concentrations. An electrical percolation threshold below 0.1 wt.% carbon nanotubes in vinyl ester was observed. Formation of percolating carbon nanotube networks at low concentration holds promise for the utilization of carbon nanotubes as in situ sensors for detecting deformation and damage in advanced naval composites.     

Micromechanics modeling of the electrical conductivity of carbon nanotube (CNT)–polymer nanocomposites

A mixed micromechanics model was developed to predict the overall electrical conductivity of carbon nanotube (CNT)–polymer nanocomposites. Two electrical conductivity mechanisms, electron hopping and conductive networks, were incorporated into the model by introducing an interphase layer and considering the effective aspect ratio of CNTs. It was found that the modeling results agree well with the experimental data for both single-wall carbon nanotube and multi-wall carbon nanotube based nanocomposites. Simulation results suggest that both electron hopping and conductive networks contribute to the electrical conductivity of the nanocomposites, while conductive networks become dominant as CNT volume fraction increases. It was also indicated that the sizes of CNTs have significant effects on the percolation threshold and the overall electrical conductivity of the nanocomposites. This developed model is expected to provide a more accurate prediction on the electrical conductivity of CNT–polymer nanocomposites and useful guidelines for the design and optimization of conductive polymer nanocomposites.     

In-situ damage sensing of woven composites using carbon nanotube conductive networks

We report an in situ analysis of the microstructure of woven composites using carbon nanotube (CNT)-based conductive networks. Two types of specimens with stacking sequences of (0/90)_s (on-axis) and (22/85/−85/−22) (off-axis) were manufactured using ultra-high-molecular-weight polyethylene fibers and a CNT-dispersed epoxy matrix via vacuum-assisted resin transfer molding. The changes in the electrical resistance of the woven composites in response to uniaxial loading corresponded to the changes in the gradient of the stress–strain curves, which is indicative of the initiation and accumulation of microscopic cracking and delamination. The electrical resistance of the woven composites increased due to both elongation and microscopic damage; interestingly, however, it decreased beyond a certain strain level. In situ X-ray computed tomography and biaxial loading tests reveal that this transition is due to yarn compaction and Poisson’s contraction, which are manifest in textile composites.     

Electrical and thermal property enhancement of fiber-reinforced polymer laminate composites through controlled implementation of multi-walled carbon nanotubes

Aligned carbon nanotubes (CNTs) are implemented into alumina-fiber reinforced laminates, and enhanced mass-specific thermal and electrical conductivities are observed. Electrical conductivity enhancement is useful for electrostatic discharge and sensing applications, and is used here for both electromagnetic interference (EMI) shielding and deicing. CNTs were grown directly on individual fibers in woven cloth plies, and maintained their alignment during the polymer (epoxy) infiltration used to create laminates. Using multiple complementary methods, non-isotropic electrical and thermal conductivities of these hybrid composites were thoroughly characterized as a function of CNT volume/mass fraction. DC and AC electrical conductivity measurements demonstrate high electrical conductivity of >100 S/m (at 3% volume fraction, ∼1.5% weight fraction, of CNTs) that can be used for multifunctional applications such as de-icing and electromagnetic shielding. The thermal conductivity enhancement (∼1 W/m K) suggests that carbon-fiber based laminates can significantly benefit from aligned CNTs. Application of such new nano-engineered, multi-scale, multi-functional CNT composites can be extended to system health monitoring with electrical or thermal resistance change induced by damage, fire-resistant structures among other multifunctional attributes.