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Multifilament continuous hollow carbon fiber tows with a honeycomb cross-section have been produced using a gel-spun bicomponent islands-in-a-sea precursor with polyacrylonitrile (PAN) as the sea component and polymethylmethacrylate (PMMA) as the sacrificial island component. The density of the hollow carbon fibers is 1.15 g/cm3, more than 30% lower compared to commercial carbon fibers. The tensile strength of these fibers is in the range of 2.3–3.0 GPa and tensile modulus between 202 and 234 GPa. The tensile strength is over 80% higher than the 1.6 GPa strength value reported earlier for hollow carbon fibers. The specific tensile strength is 30% higher than T300 carbon fibers. The specific tensile modulus is 60% higher than T300 carbon fibers and 20% higher than aerospace grade IM7 carbon fibers. The manufacturing was successfully scaled up from a single filament to 740 filament tow without affecting the carbon fiber structural parameters. The tensile strength is limited by the size of the largest defects present, estimated to be in the range of 40–65 nm for the hollow carbon fibers.

The growing commercial availability of carbon nanotube (CNT) macro-assemblages such as sheet and yarn is making their use in structural composite components increasingly feasible. However, the mechanical properties of these materials continue to trail those of state-of-the-art carbon fiber composites due to relatively weak inter-tube load transfer. Forming covalent links between adjacent CNTs promises to mitigate this problem, but it has proven difficult in practice to introduce them chemically within densified and aligned CNT materials due to their low permeability. To avoid this limitation, this work explores the combination of pulsed electrical current, temperature, and pressure to introduce inter-CNT bonds. Reactive molecular dynamics simulations identify the most probable locations, configurations, and conditions for inter-nanotube bonds to form. This process is shown to introduce covalent linkages within the CNT material that manifest as improved macroscale mechanical properties. The magnitude of this effect increases with increasing levels of pre-alignment of the CNT material, promising a new synthesis pathway to ultralight structural materials with specific strengths and stiffnesses exceeding 1 and 100 GPa cm3 g−1, respectively, comparable to current carbon fiber reinforced polymer composites.

Carbon nanotubes (CNTs) were mixed with silver particles and nanowires in an effort to produce lightweight conductors of Ag/CNT composites. Silver nanostructures composites with CNTs were achieved through sonication, filtration and infusion processes. Uniform dispersion of silver nanostructures in the composite samples was achieved using different amounts of silver (Ag), and then the samples were characterized under scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). Both electrical and thermal conductivities were improved by increasing the amount of silver particles and sizes. The samples with silver nanowires (AgNWs) with high aspect ratios had a lower percolation threshold and achieved higher electrical and in-plane thermal conductivity of 1.3 × 104 S/cm and 126 W/mK at 9 vol % of Ag content, respectively, at room temperature. Temperature dependence of resistivity of the resultant CNT/Ag composites can be described using a heterogeneous model with metallic and non-metallic terms. The CNT/AgNW composites showed the dominant metallic transport mechanism down to 17 K. Also, a high in-plane thermal conductivity of the AgNW samples demonstrated good thermal dissipation. The resultant Ag/CNT composites conductors demonstrated low density and high transport properties for potential engineering applications.

Carbon nanotube/carbon (CNT/C) composites show potential for lightweight structural materials and non-metal electrical conductors for aerospace, military, and other industries where the combination of lightweight, high strength and excellent conductivity are required. Most research attempts have been reported to fabricate CNT/C composite focusing on the high CNT alignment and dense carbon matrix. However, simultaneous improvement of strength and electrical conductivity in materials still presents a great challenge. In this study, pyrolyzed polydopamine (py-PDA) with selected surface treatments is introduced as an interface enhancer between CNTs and the carbon matrix. Due to the presence of py-PDA, the effective physical interlocking and conductive pathways are rebuilt at the interface area between CNTs and carbon matrix, resulting in better load transfer and electron transport. The CNT/py-PDA/C composite fibers demonstrated remarkable improvements in electrical conductivity (2.1 × 103 S cm−1 or 228 S m2 kg−1) and tensile strength (up to 727 MPa or 790 MPa/(g·cm−3)), which should prove to be vastly advantageous as compared to the previously reported CNT/C composites. The outstanding thermal stability of fully carbonized materials is also an attractive feature. Coupled with scalable manufacturing methods, these integrated characteristics of CNT/py-PDA/C composite fiber can potentially have broad applications for lightweight structural materials and non-metal conductors.

Polyacrylonitrile (PAN) fibers containing up to 40 wt% cellulose nanocrystals (CNC) have been successfully stabilized and carbonized to make carbon fibers. Under the batch process, PAN/CNC based carbon fibers exhibit a tensile strength in the range of 1.8–2.3 GPa, and tensile modulus in the range of 220–265 GPa, which are comparable to the control PAN-based carbon fibers. Based on the transmission electron microscopy study, individually distributed CNC regions surrounded by PAN matrix were observed in the stabilized and carbonized fibers. Carbonized fibers show two distinct stabilized and carbonized regions. This study shows that biphasic carbon fibers can be made from the PAN/CNC system.

Microstructures of typical carbon fibers (CFs) from polyacrylonitrile (PAN) and pitch-based precursors were studied using a novel digital twin approach with individual carbon fibers for a local crystal scale model. The transmission electron microscopy (TEM) samples were prepared using a focused-ion beam (FIB) for both longitudinal and transverse directions of carbon fibers. Measurements of the crystal size and orientation were estimated from X-ray scattering. TEM imaging of graphitic packing facilitated further comprehension of associations between processing and final material properties, which could enable customization of microstructures for property targets. Then the detailed microstructural information and their X-ray scattering properties were incorporated into the simulation model of an individual carbon fiber. Assuming that graphene properties are the same among different forms of carbon fiber, a reasonable physics-based explanation for such a drastic decrease in strength is the dislocations between the graphitic units. The model reveals critical defects and uncertainty of carbon fiber microstructures, including skin/core alignment differences and propagating fracture before ultimate failure. The models are the first to quantify microstructures at the crystal scale with micromechanics and to estimate tensile and compressive mechanical properties of carbon fiber materials, as well as potentially develop new fundamental understandings for tailoring carbon fiber and composites properties.

Using a floating catalyst synthesis process, carbon nanotubes (CNTs) can be produced to form randomly oriented networks. However, to realize their potential high structural performance, the nanotubes must be aligned and closely packed to eliminate molecular and microscale defects, which would be similar to carbon fiber microstructures. This paper describes a mechanical stretching technique using bismaleimide (BMI) resin to transform the randomly oriented networks into aligned networks. The BMI resin acts as a lubricant to decrease the friction between the nanotube bundles within the network during the stretching process. The unique flattening and self-assembling behaviors and the resultant graphitic crystal packing of CNTs were observed. The nanotubes' degree of alignment, measured by Raman and X-ray scattering drastically increased at approximately 40% stretch strain, plateaued at a 60% stretch strain, and achieved a maximum of 0.92 degree of alignment with noticeable graphitic crystal packing at 80% stretch strain. Both TEM and SEM observations indicate that as the stretch strain increased, the CNTs started to align along the stretched direction and self-assembled into large bundles. Additionally, high-resolution TEM analysis indicated that the CNTs exhibited flattening and polygonization self-assembling to form graphitic crystal packing. Tensile testing on the stretched CNT/BMI composite samples revealed an increase in Young's modulus, with a maximum of 252 GPa at 80% stretch strain, while an ultimate tensile strength of 1.58 GPa was reached at 70% stretch strain. The high degree of alignment and polygonization packing resulted in a better load transfer among CNTs, and thus a higher mechanical performance in the resultant CNT composites. Furthermore, this stretching process is scalable and has the potential to realize greater performance for applications using CNTs.

Huntsman–Merrimack MIRALON® carbon nanotubes (CNTs) are a novel, highly entangled, commercially available, and scalable format of nanotubes. As-received and acid-treated CNTs were added to aerospace grade epoxy (CYCOM® 977-3), and the composites were characterized. The epoxy resin is expected to infiltrate the network of the CNTs and could improve mechanical properties. Epoxy composites were tested for flexural and viscoelastic properties and the as-received and acid treated CNTs were characterized using Field-Emission Scanning and Transmission Electron Microscopy, X-Ray Photoelectron Spectroscopy, and Thermogravimetric Analysis. Composites containing 0.4 wt% as-received CNTs showed an increase in flexural strength, from 136.9 MPa for neat epoxy to 147.5 MPa. In addition, the flexural modulus increased from 3.88 GPa for the neat epoxy to 4.24 GPa and 4.49 GPa for the 2.0 wt% and 3.0 wt% as-received CNT/epoxy composites, respectively. FE-SEM micrographs indicated good dispersion of the CNTs in the as-received CNT/epoxy composites and the 10 M nitric acid 6 h treatment at 120 °C CNT/epoxy composites. CNTs treated with 10 M nitric acid for 6 h at 120 °C added oxygen containing functional groups (C–O, C=O, and O=C–O) and removed iron catalyst present on the as-received CNTs, but the flexural properties were not improved compared to the as-received CNT/epoxy composites.

Carbon nanotube (CNT) electrically conductive cables, fibers, or tapes are undergoing extensive research. However, the research conducted on these materials with high conductivity has been primarily limited to small samples, such as fibers around 5–10 μm in diameter or sheets less than 1 μm thick. The need for scaling up these samples to manufacture CNT lightweight conducting materials is urgent, yet challenging. This paper reports on our study to produce large-scale continuous CNT sheets with ultra-high electrical conductivity. After mechanical stretching to achieve a high degree of CNT alignment, iodine doping and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) capping was performed, resulting in high electrical conductivity in the range of 10,000 S/cm (the highest value achieved as ∼13,000 S/cm). High CNT alignment and carrier concentration after iodine doping significantly improved the overall conductivity. The superior open air stability of samples, benefiting from the capping layer of PEDOT:PSS, was demonstrated by the very small changes of the conductivity during test period. Additionally, this doping process is scalable for producing continuous highly conductive and lightweight CNT sheets. Metallic conduction behaviors of the aligned and doped large size CNT sheets could provide lightweight conductor applications of CNT materials.

Conductive metals such as copper are heavily used as conductors or lightning protection materials in aircrafts and other industrial areas. But the high bulk density and corrosion problem make non-metal conductors with low density and corrosionresistance more promising such as carbon nanotubes (CNTs). However, the small sample size of previously reported highly conductive CNT fibers (1-2 μm in diameter) or films (less than 1 μm in thickness) limits the practical engineering applications. In this work, a macroscopic continuously stretched CNT tape was prepared with excellent CNT alignment, high conductivity, and large dimensional size (~ 9 mm in width and ~ 30 μm in thickness). Through combined post-treatments of mechanical stretching, iodine doping and polymer capping layer, the macroscopic CNT tape shows high conductivity in the range of 10,000 S/cm (the highest is ~ 13,000 S/cm) and this excellent performance is stable in open air without any sign of degradation. Various characterizations were conducted including SEM observation, Raman, TGA, PPMS, electrical, and mechanical measurements. Overall, the high conductivity was ascribed to better CNT alignment after stretching and high carrier density after iodine doping. More importantly, the substantial stability of electrical property was demonstrated, benefitting from the polymer capping layer. Additionally, this process is based on commercially available raw materials and scalable for industrial production. And the proposed method here can be readily transferred to our buckypaper (CNT sheets) with roll-to-roll production capability. Furthermore, these highly conductive CNT tape and sheet can be directly incorporated into fiber-reinforced composite to improve the conductivity which could serve multiple functionalities as structural, electromagnetic interference shielding and lightning protection materials in aircrafts. This work opens a new path to optimize the electrical performance of CNT assemblies with a wide range of engineering applications including conductive composites..

Carbon nanotubes (CNTs) can be produced from floating catalyst synthesis to form random networks. However, the CNTs must be aligned and closely packed to eliminate molecular and microscale defects to reach levels of mechanical performance similar to carbon fiber systems. Previous research has shown that strain-induced alignment methods using bismaleimide (BMI) infiltration can effectively modify randomly oriented CNT networks. The BMI resin lubricates the network decreasing the friction between the CNT bundles and assists with load transfer between CNT bundles during the stretching process. This process yields unique CNT collapsing, self-assembling behaviors, and graphitic crystal packing at the nanoscale. In this study, the CNTs’ alignment degree was measured by Raman spectroscopy and X-ray scattering. Alignment degree analysis results showed a considerable increase at 40% stretch ratio with a plateau at 60% stretch ratio. An 80% stretch ratio achieved the highest alignment degree of 0.93 with noticeable graphitic crystal packing. Both scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) images of the CNT sheet surfaces and cross-sections show that as the stretch ratio increased, the CNTs started to align along the stretching axis and self-assembled into large bundles. Additionally, HRTEM analysis indicated that the CNTs exhibited collapsing and self-assemblage to form graphitic crystal packing. Tensile testing on the aligned CNT/BMI composite samples measured an increase in tensile properties. The ultimate tensile strength (UTS) and Young's modulus reached maximums of 1.58 GPa at 70% stretch ratio and 252 GPa at 80% stretch ratio, respectively. The high alignment degree and graphitic packing improved load transfer throughout the CNT network, and consequently, better mechanical performance in the CNT composites were achieved. Furthermore, this alignment process proved its scalability and potential to transcend into industrial-scale applications that utilize CNT material systems.

Carbon fiber is widely used reinforcement material for composite applications. Modulus and strength of carbon fiber can be tailored depending on the precursor materials and processing conditions. However, microstructure-property relationships and defects on the crystal scale induced by processing still need to be explored. We studied microstructures of two typical carbon fibers from polyacrylonitrile (PAN) and pitchbased precursor and compared with its X-ray scattering properties and preliminary simulation results including crystal size, orientation and spacing. For this purpose, transmission electron microscopy (TEM) sample was prepared using focused-ion beam (FIB) for both longitudinal and transverse directions of carbon fiber. Pitch-based carbon fiber shows large graphitic structures with triangular defects, meanwhile PAN-based carbon fiber shows smaller graphitic structures with random distribution. In the case of pitch-based carbon fiber, crystalline size ~40 nm can be observed from the low magnification TEM image. X-ray scattering pattern also exhibit the differences between two fibers. The wide-angle X-ray scattering (WAXS) pattern of each carbon fiber shows graphitic peak at 26 and that is sharper for the pitch-based carbon fiber for both radial (2) and azimuthal () direction. This corresponds to the large graphitic crystalline size and high alignment degree of fiber structures. Full-width half maximum (FWHM) of the azimuthal angle for pitch-based and PAN-based carbon fiber is 5.9 and 31.6, respectively. This confirms the high alignment degree of pitch-based carbon fiber ~ 0.99. Precise measurements of the crystal size, orientation and TEM imaging of graphitic packing allows for further comprehension of associations between processing and final material properties which will enable customization of microstructures for property targets.

Sensors placed along various stages of manufacturing processes can identify errors and ensure the quality of a product. Specially, sensors are becoming essential for more complicated composite fabrication due to coupling together heat transport, curing kinetics, resin rheology and flow as well as pressure effects. In this research, embedded composite sensor fabrication, maskless mesoscale materials deposition system has been used to print electrically conductive silver patterns on the carbon nanotube (CNT) sheets or buckypaper (BP) to measure its resistance change, and thus used as embedded sensors in composite laminates for both manufacturing process and structure health monitoring. Three different samples of nanotube sheets were used in this work and compared. The samples exhibited slightly different behaviors in terms of resistive response during the resin infusion and curing process with vacuum bagging, yet also demonstrated similar patterns, which indicated their sensing capabilities. Upon the resin flow, the sensor resistance dropped ∼20–30% from the initial value, then increased up to 120–145% compared to its minimum value at the final curing stage. After curing process, embedded sensors could monitor the impact or structural deformation of a composite part. Keywords: Carbon nanotube buckypaper, Printed sensors, Aerosol jet, Composite life cycle monitoring

Stretchable materials are indispensable for applications such as deformable devices, wearable electronics, and future robotics. However, designs for new elastomers with high stretchability have undergone only limited research. Here we have fabricated highly stretchable Ag+/polyacrylonitrile elastomer with nanoreservoirs of lubricant using cyano-silver complexes. The prepared products feature nanoconfinement structures of lubricant surrounded by polymer chains with coordination bond through chelates of cyano-silver, resulting in an enhanced stretchability of more than 600% from 2%. The elastomeric properties were investigated, and a mechanical response model was proposed, which explained the structural evolution including the polymer chain fluidity under external deformation. Also, the easy breakage and dynamic reformation of cyano-silver coordination complexes promises a strain recovery under various stretching conditions. This elastomer itself can directly work as sensors and open paths to alternative substrates for soft electronics development.

Carbon nanotube/carbon (CNT/C) composites show potential for lightweight structural materials and non-metal electrical conductors for aerospace, military, and other industries where the combination of lightweight, high strength and excellent conductivity are required. Numerous research attempts have been reported to fabricate CNT/C composite focusing on high CNT alignment and dense carbon matrix. However, simultaneous improvements for mechanically strengthening and electrically improving properties of strength and conductivity in materials still presents a great challenge. In this study, pyrolyzed polydopamine (py-PDA) with selected surface treatments is introduced as an interface enhancer between CNTs and carbon matrix. Due to the presence of py-PDA, the effective physical interlocking and conductive pathways are rebuilt at the interface area between CNTs and carbon matrix, resulting in better load transfer and electron transport. The CNT/py-PDA/C composite fibers demonstrated remarkable improvements in electrical conductivity (2.1 × 103 S cm-1) and tensile strength (up to 727 MPa), which should prove to be vastly advantageous as compared to the previously reported CNT/C composites. The outstanding thermal stability of fully carbonized materials is also an attractive feature. Coupled with scalable manufacturing methods, these integrated characteristics of CNT/py-PDA/C composite fiber can potentially have broad applications for lightweight structural materials and non-metal conductors.

Aligned carbon nanotube (CNT) sheets reinforced with silicon carbonitride were prepared by the infiltration and pyrolysis of liquid polysilazane into mechanically stretched CNTs. The resultant nanocomposites contained a high volume fraction of CNTs (60 vol%), and due to alignment, reached an electrical conductivity of up to 2.2 × 105 S m− 1. The electrical conductivity was anisotropic based upon the CNT alignment and changed from 3.3 to 9.2 after the pyrolysis process. The high electrical conductivity of the nanocomposites contributes to their potential application in electromagnetic interference shielding.

We report the synthesis of flexible ceramic composites with a high tensile strength (536.33±7.23 MPa) using carbon nanotube sheet aligned by mechanically stretching process. The process is based on the infiltration and pyrolysis of liquid ceramic precursor into aligned carbon nanotube sheet. Mechanical properties and microstructure of the resultant composites are investigated. The resultant nanocomposites maintain well-aligned carbon nanotube morphology with high volume fraction (60%) and long pullout (15 µm), contributing to a high degree of load-transfer efficiency and toughening. Flexibility test reveals that such ceramic nanocomposites retain the original mechanical properties and microstructures after one thousand repetitions of 75% bending deformation, showing excellent compliance and durability. Applications requiring materials with high flexibility and mechanical properties can benefit from this research.

High-performance lightweight composites are sizably manufactured by impregnating continuous aligned carbon nanotube sheet (buckypapers) with self-reinforcing polyphenylene (Parmax) solution and followed by hot-press. The high processing pressure flattens nanotubes, which preferably π-stack with the aromatic rings of Parmax chain, and accordingly improve the load transfer. Both tensile strength and Young's modulus of the thermoplastic composites increase with the alignment degree of nanotubes, and can reach 950 MPa and 94 GPa, respectively, for the composite containing 50%-stretched buckypaper. The highly aligned nanotubes also boost phonon transfer (70 W mK−1) and the electric conductivity (425 S cm–1) of the composite along the alignment direction. These combined outstanding properties would enable the thermoplastic composites in wide applications as multifunctional material.

Carbon nanotube (CNT) yarns have demonstrated promising mechanical properties and scalability for composite applications. In this study, a commercially available continuous CNT yarn and two modified versions at large gauge lengths (~110 mm) were tested to reveal engineering properties. The as-received yarns with a large cross-section demonstrated the highest specific tensile strength (0.911 ± 0.137 N/tex), while the highly densified yarns showed the lowest specific tensile strength (0.337 ± 0.217 N/tex). Results suggest that alignment governs the ultimate tensile strength, while defects dictate how yarns fail and the harnessed strength potential. Of the two failure modes detected, brittle edge failures experienced greater ultimate tensile strengths ranging from ~8 to 20% depending on the yarn type. The test results were fitted to 2-parameter Weibull distributions to further confirm the failure behavior. These results indicate valuable properties for industrial CNT yarn applications and provide a baseline for large gauge length CNT studies.

The Floating Catalyst Chemical Vapor Deposition (FCCVD) method has attracted extensive research interests from both academia and industry due to its high potential for carbon nanotube (CNT) mass production. As aerospace aims become increasingly demanding, there is an ever growing need for mass producing lightweight structural composites. By infiltrating bismaleimide (BMI) resin and curing under pressure, CNT/BMI nanocomposites with high CNT loading are exhibiting excellent mechanical properties. By further integrating CNT into carbon fiber (CF), the mechanical and electrical improvements in axial, transverse and through-thickness direction demonstrate potential for multifunctional applications. In this work, a FCCVD reactor designed and assembled at the High-Performance Materials Institute was used for CNT synthesis. The CNT aerogel formed was continuously spun onto a rotating mandrel while continuously spraying a BMI/acetone solution, forming an ultra-thin prepreg material ranging from 1-5 μm in thickness. The quality of the CNTs collected was characterized using Raman Spectroscopy, Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA), and Transmission Electron Microscopy (TEM). To evaluate the ultra-thin FCCVD-CNT prepregs, six layers were stacked in the same alignment direction, and cured using a vacuum-assisted bagging system and hot press. Electrical conductivity tests were conducted in both the aligned and the transverse directions. Furthermore, the ultra-thin CNT prepregs were integrated into the aerospace-grade unidirectional IM7/CYCOM® 5250-4 prepregs following an alternating order. The assembly was finally vacuum-bagged to expel air and cured with a hot press. The microstructure and interface of CF/CNT hybrid composites and CNT/BMI composites were observed under SEM. This paper reports on a fabrication process of CNT prepregs, which could be potentially used to scale-up the CNT/BMI composites and CF/CNT interply hybrid composites manufacturing.

Free-standing carbon nanotube films or buckypaper can provide a significant platform to develop practical applications of nanocarbon materials. For this research, buckypaper with high thermal conductivity (20 W/m K) and large surface area (350 m2/g) was mass produced in-house to investigate for use in lightweight thermal management devices. Floating catalyst chemical vapor deposition carbon nanotube sheets were also studied in this work. We introduced two manufacturing techniques to use the sheets for heat dissipation: (1) printing conductive composite ink on the sheets to make lightweight thermal devices, such as heat sinks and (2) assembling the sheets directly into 3D structures that were mounted on the back of heat-generating devices. These manufacturing techniques resulted in extremely lightweight, high-performance heat dissipation devices compared with other heat sink materials.

Large scale manufacturing of electrically conductive carbon nanotube (CNT) sheets with production capability, low cost, and long-term electrical performance stability is still a challenge. A new method to fabricate highly conductive continuous buckypaper (CBP) with roll-to-roll production capability and relatively low cost is reported. The electrical conductivity of CBP can be improved to 7.6 × 104 S m−1 by using an oxidant chemical (i.e. HNO3 and I2) doping method. To compensate for the conductivity degradation caused by the instability of the oxidant chemical doping, a polymer layer of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) was coated on the chemically doped CBP. The fabricated highly conductive CBP showed stable electrical performance in air for more than a month. This CBP material with high electrical conductivity, relatively low cost, and roll-to-roll manufacturing capability could enable a wide range of engineering applications including flexible conductors, electromagnetic interference (EMI) shielding materials, and electrodes in energy devices.

While the mechanical properties of highly aligned carbon nanotube (CNT) thin films and their nanocomposites have been widely studied, the load transfer mechanisms and failure modes of aligned CNT composites have not been sufficiently explored and understood. In this research, super-aligned CNT thin films with a measured alignment fraction of up to 0.93 are fabricated by mechanical stretching. High concentration (50–60 wt% CNT) CNT reinforced bismaleimide (CNT/BMI) nanocomposites are fabricated from the aligned network to study mechanical properties and microstructures. Atomic resolution transmission electron microscopy (TEM) analysis reveal unusual CNT crystal packing and permit the observation of interesting structural features of the CNTs and their assemblages, including collapse, flattened packing, preferred stacking, folding and twisting phenomena, as well as CNT pullouts from bundles and the resin matrix. The large surface-to-surface contact areas between aligned and flattened nanotubes, driven by van der Waals interactions, give rise to a high density packing of the flattened CNTs in the nanocomposite, resembling a graphitic material. Molecular dynamics (MD) simulations are performed to model the packing structure and understand the dependence of density on the relative content of flattened nanotube and void space. The modeling results support the conclusions drawn from the experimental observations.

Due to the nanoscale size of carbon nanotubes (CNTs), observing the CNT/CNT and CNT/resin interactions in nanocomposites requires a high resolution transmission electron microscope (TEM) analysis. This research focuses on developing a protocol to provide an effective and reliable approach to obtain ultra-thin ( Two CNT nanocomposite samples made with different resins were studied for both microtome cutting and TEM analysis and it revealed the alignment direction of the nanotubes and numerous stacks of CNT bundles. In addition, there was visible flattening of CNT packing into dumbbell shapes similar to results obtain in CNT yarn materials. TEM for the 3-layer CNT BP/BMI nanocomposite revealed uniform cutting in the resin areas. However, when the diamond knife reached graphite crystalline regions, the nanotubes either became deformed into a cone-like structure, were cut thicker than the resin, or folded over onto itself. This is most likely due to the high mechanical properties of CNT in response to the stress of cutting.

Higher concentrations of long carbon nanotubes (CNTs) that are aligned and functionalized can improve the mechanical properties of CNT composites. For this purpose, pristine/random CNT sheets were mechanically stretched at different strains of 40 %, 60 % and up to 80 %. The higher degree of CNT alignment was confirmed by scanning electron microscope (SEM) images and the degree of alignment was quantified by X-ray scattering and polarized Raman spectroscopy. The highest degree of alignment of 0.93 was achieved by using this method. Aligned CNT sheets were functionalized and CNT/bismaleimide (BMI) composites were fabricated. The mechanical properties of the composites were improved with a higher degree of alignment achieving a modulus of 252 GPa and tensile strength of over 1.4 GPa for an 80% stretched CNT/BMI composite. To understand the internal CNT packing and the CNT/resin interface, thin composite cross sections that are perpendicular to the alignment direction were prepared using focused ion beam (FIB) with a lift-out process. Using a high-resolution transmission electron microscopy (HRTEM), relatively large diameter CNTs (> 5 nm) cross sections and double-walled CNT bundles were observed. An increased number of densely packed and collapsed CNT structures were also observed with higher stretch ratio, which confirmed the CNT alignment and dense packing along the stretch direction. The CNT/resin interface was also analyzed by electron energy loss spectroscopy (EELS). Carbon K-edge peak was used to identify the functionalization of CNT, and a ratio of 1s-* to 1s-* transition was mapped by a scanning TEM. A decreased */* transition ratio was observed at the edge of CNT packing and could be related to the reduced sp2 bonding after functionalization. This leads efficient load transfer between CNT and resin, thus higher mechanical properties.

This paper reports on highly sensitive aligned carbon nanotube network (CNTN)-based strain sensors produced by aerosol jet printed electrodes and polyimide substrates. Two types of sensing mechanisms were revealed. Type A sensors, whose electrodes were directly printed on the CNTNs and embedded in epoxy resin, take advantage of the intrinsic piezoresistive properties of highly oriented carbon nanotube bundles and exhibited large positive gauge factor. Type B sensors, whose electrodes were printed on the polyimide substrate, utilize the effects of applied strains on the contact resistances between layers of the CNTN structure and between CNTN and the electrodes, exhibited large negative gauge factors. Type A and B sensors achieved positive and negative gauge factors up to 20 and 40 in magnitude, respectively. The high performance and flexible nature of the sensors, combined with the capability of scalable manufacturing processes, exhibits promising application potentials.

The mechanism of carbon nanotube (CNT) alignment during stretching was examined by the in situ characterization of carbon nanotube networks (CNTNs) under tensile strains using X-ray and Raman scattering techniques. A method of quantifying the inhomogeneous alignment of macroscopic CNTNs is explored based on bulk property measurements of their electrical anisotropy and X-ray diffraction diagrams. The results show that the process of stretch-induced alignment of CNTNs included straightening the waviness of the long nanotube ropes, as well as the self-assembling and denser packing of the nanotubes. For samples at a strain of 40%, the fraction of aligned nanotubes was as high as 0.85. The aligned fraction of CNTs serves as an important parameter for the quality control of the alignment process and numerical simulations of structure–property relationships of CNTNs and their composites.

Carbon nanotubes (CNTs) show great promise to improve composite electrical and thermal conductivity due to their exceptional high intrinsic conductance performance. In this research, long multi-walled carbon nanotubes (long-MWCNTs) and its thin sheet of entangled nanotubes were used to make composites to achieve higher electrical and thermal conductivity. Compared to short-MWCNT sheet/epoxy composites, at room temperature, long-MWCNT samples showed improved thermal conductivity up to 55 W/mK. The temperature dependence of thermal conductivity was in agreement with κ ∝ T n ( n = 1.9–2.3) below 150 K and saturated around room temperature due to Umklapp scattering. Samples with the improved CNT degree of alignment by mechanically stretching can enhance the room temperature thermal conductivity to over 100 W/mK. However, functionalization of CNTs to improve the interfacial bonding resulted in damaging the CNT walls and decreasing the electrical and thermal conductivity of the composites.

Carbon/carbon composites consisting of single-walled carbon nanotube (SWCNT) buckypaper (BP) and mesophase pitch resin have been produced through impregnation of BP with pitch using toluene as a solvent. Drying, stabilization and carbonization processes were performed sequentially, and repeated to increase the pitch content. Voids in the carbon/carbon composite samples decreased with increasing impregnation process cycles. Electrical conductivity and density of the composites increased with carbonization by two to three times that of pristine BP. These results indicate that discontinuity and intertube contact barriers of SWCNTs in the BP are partially overcome by the carbonization process of pitch. The temperature dependence of the Raman shift shows that mechanical strain is increased since carbonized pitch matrix surrounds the nanotubes.

Multi-walled carbon nanotube (MWNT)-sheet-reinforced bismaleimide (BMI) resin nanocomposites with high concentrations (∼60 wt%) of aligned MWNTs are successfully fabricated. Applying simple mechanical stretching and prepregging (pre-resin impregnation) processes on initially randomly dispersed, commercially available sheets of millimeter-long MWNTs leads to substantial alignment enhancement, good dispersion, and high packing density of nanotubes in the resultant nanocomposites. The tensile strength and Young's modulus of the nanocomposites reaches 2 088 MPa and 169 GPa, respectively, which are very high experimental results and comparable to the state-of-the-art unidirectional IM7 carbon-fiber-reinforced composites for high-performance structural applications. The nanocomposites demonstrate unprecedentedly high electrical conductivity of 5 500 S cm−1 along the alignment direction. Such unique integration of high mechanical properties and electrical conductance opens the door for developing polymeric composite conductors and eventually structural composites with multifunctionalities. New fracture morphology and failure modes due to self-assembly and spreading of MWNT bundles are also observed.

Combinations of nanoscale and microscale silver particles were used to construct heterogeneously conductive paths in graphite composites for improving through-thickness thermal conductivity. Composites with four different pitch-based graphite fibers were studied and compared to realize the improved thermal conductivity with adequate mechanical performance. The test panels of EWC300X graphite fibers and sliver fillers achieved a through-thickness thermal conductivity of ~10.0 W/m·K at 25 °C. The rheological behavior of Epon862/curing agent resin and silver flakes filled Epon862/curing agent resin was studied for the standardization and quality control of large panel fabrication. By adding 40wt% silver flakes in the resin system, the viscosity greatly increased. The lowest viscosity of 40% silver flakes filled Epon862/curing agent resin system occurred at 65 °C; therefore, the curing process was modified accordingly to ensure resin/fiber wetting and reduction of voids. The hybrid composites with IM7 and pitch-based carbon fiber were also made in order to optimize the composite performance with a balanced thermal conductivity and mechanical properties. The failure mechanisms of laminated composites with and without silver fillers were investigated. The IM7/EWC300X hybrid composite with a density of 2.31 g/cm showed a thermal conductivity of 5.29 W/ m·K and adequate mechanical properties for structural applications.

Vertical direction current–voltage (I–V) characteristics of Polypyrrole (PPy) nanotube (diameter of 120 nm) deposited on Au were measured at room temperature using a metal-coated tapping-mode atomic-force-microscope tip. Linear I–V characteristics are observed and resistance from the slope is decreased, as the contact force is increased. Using the Hertz model, the electrical resistivity, ρ, is estimated to be ρ ∼1 Ωcm consistent with that of bulk PPy film. In comparison, we report the I–V characteristics of PPy nanotube (diameter of 250 nm) in lateral electrodes configuration. To align the PPy nanotube, we applied the ac electric field across the lateral electrodes patterned on the SiO2 substrate.

This work presents a new method for fabricating thermal devices, such as heat sinks, using a 3D printing technique and lightweight composite ink. The method focuses on formulating composite inks with desired properties and direct ink writing for manufacturing. The ink undergoes two phases: phase one uses low viscosity epoxy to provide viscoelastic properties and phase two provides the fillers consisting of carbon fiber and graphite nanoplatelets to provide high thermal conductivity and structural properties. By combining these functional materials, 3D structures with a high thermal conductivity (≈2 W m−1 K−1) are printed for thermal management applications with the storage modulus of 3000 MPa and a density only 1.24 g cm−3. The results show that by carefully tailoring functional properties of the ink, net-shape multifunctional structures can be directly printed for thermal management device applications, such as heat sinks.

Ethylene-propylene-diene elastomer (EPDM), which is grafted with an isocyanate-containing monomer (HI), was blended with polyethyleneterephthalate (PET) and its morphological, thermal, rheological, and mechanical properties were studied. HI was incorporated onto EPDM backbone through a solution graft reaction. When the PET was blended with HI-grafted EPDM (EPDM-g-HI), the morphologies of dispersed phases showed considerable differences in the aspects of particle size and interfacial adhesion compared with those of a PET/EPDM blend. DSC analysis showed that, when blends are cooled slowly, the PET phase in PET/EPDM-g-HI is somewhat amorphous compared with that in the PET/EPDM blend. The increase in complex viscosity, storage modulus and impact strength of PET/EPDM-g-HI blends enabled us to ensure that the compatibility between PET and EPDM improved through functionalization of EPDM with the isocyanate moiety. These results are mainly due to graft PET-EPDM copolymer in situ formed through the chemical reaction between the isocyanate group of EPDM-g-HI and hydroxyl or carboxyl end groups of PET. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2227–2233, 2000

Poly(urethane acrylate) (PUA)/poly(methylmethacrylate) (PMMA) core–shell composite particles were prepared by two-stage emulsion polymerization. The sizes of composite particles could be varied from 25 to 210 nm by introducing polyoxyethylene (POE) groups to the urethane acrylate molecular backbone. Core–shell morphology was identified by investigating the polarity of the surface of the core and shell polymer particles and by measuring the contact angle of the composite particles. A composite particle prepared with relatively small particles (about 20 nm) did not show the core/shell morphology, because the high polar surface of the core polymer particle and the low-stage ratio of the core to the shell cause the formation of a core/shell two-stage latex to be more thermodynamically unstable. The fracture toughness of rubber-toughened PMMA containing PUA/PMMA composite particles increased as the particle sizes decreased and the shell thickness of the composite particles increased. In particular, when the average size of the composite particle was about 43 nm and the stage ratio was 50/50, the fracture toughness of the rubber-toughened PMMA increased more than three times compared with that of pure PMMA. Furthermore, the transparency of toughened PMMA could be maintained up to 91% in the visible spectra range. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2291–2302, 1998

Polypyrrole (PPy) nanotubes were synthesized using the pores of track-etched polycarbonate membrane as a template. Its size depends on the pore diameter of template, range from 50 to 200 nm. Direct I–V measurements of PPy nanotube (diameter of 120 nm) deposited on Au were done using a metal-coated tapping-mode atomic-force-microscope tip. Linear I–V characteristics are observed, and the resistance is decreased as the contact force is increased. Using the Hertz model, the elastic modulus E and electrical resistivity ρ are estimated to be E∼1 GPa and ρ∼1 Ωcm. These values are consistent with those obtained in bulk PPy film.

Buckypapers (BPs) are free standing thin sheets made of carbon nanotubes, such as single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs) or their mixtures. In this research, through in situ electrical resistance measurements, we studied the electrical conductance changes of carbon nanotube networks (NTNs) in various BP samples from complete immersion to evaporation using different chemical solvents. BP samples demonstrated a 20–30% decrease in conductance upon the immersion and almost full recovery after the drying process. We found that by pre-stressing, BP samples demonstrated highly reproducible patterns of conductance changes corresponding to solvent quantity. This feature can be potentially used for sensor applications to simultaneously detect both the occurrence and the amount of organic solvent leakage.

Buckypapers (BPs) are thin films made up of carbon nanomaterials, such as single-walled carbon nanotubes (SWCNTs) or mixtures of SWCNTs with multi-walled carbon nanotubes (MWCNTs) or vapor-grown carbon nanofibers (VGCNFs). In this research, BPs were exposed to high electrical current densities under different environments, and the effects on nanotube and BP breakdown were observed. In ambient conditions, SWCNT BP breakdown happened at around 430 °C with a flash of light. Mixed BPs of SWCNTs/MWCNTs and SWCNTs/VGCNFs showed higher ignition temperatures of over 500 °C. The results were compared to those from thermogravimetric analysis. In a vacuum, current-driven thermal heating from the samples can generate temperatures greater than 2000 °C. The breakdown current density increased to more than three times that in open air. The breakdown current density of a BP sample increased proportionally to its conductivity. A finite-element model based on Joule heating and heat convection was used to explain this relationship. Further experiments also proved that the high current-carrying capacity of microscale nanotube array samples improved to 10(6) A cm(-2) due to increased heat dissipation through the substrate.

Mechanical properties of suspended quasi-one-dimensional polymer nanostructures were investigated using atomic force microscopy (AFM). A recently developed new acid-free etch method combined with electron beam lithography was used to fabricate suspended polypyrrole (PPy) nanotubes and helical polyacetylene (HPA) nanofibres. The elastic modulus of each suspended structure was obtained by AFM force-distance measurements. The estimated modulus value of the PPy nanotube (HPA nanofibre) was 0.96 GPa (0.5 GPa). Using this acid-free method, all-organic flexible NEMS devices can be fabricated in the future.

Preformed carbon nanotube thin films (10-20 microm), or buckypapers (BPs), consist of dense and entangled nanotube networks, which demonstrate high electrical conductivity and provide potential lightweight electromagnetic interference (EMI) solutions for composite structures. Nanocomposite laminates consisting of various proportions of single-walled and multi-walled carbon nanotubes, having different conductivity, and with different stacking structures, were studied. Single-layer BP composites showed shielding effectiveness (SE) of 20-60 dB, depending on the BP conductivity within a 2-18 GHz frequency range. The effects on EMI SE performance of composite laminate structures made with BPs of different conductivity values and epoxy or polyethylene insulating layer stacking sequences were studied. The results were also compared against the predictions from a modified EMI SE model. The predicted trends of SE value and frequency dependence were consistent with the experimental results, revealing that adjusting the number of BP layers and appropriate arrangement of the BP conducting layers and insulators can increase the EMI SE from 45 dB to close to 100 dB owing to the utilization of the double-shielding effect.

Single-walled carbon nanotube (SWNT)/polymer composites are widely studied because of their potential for high mechanical performance and multifunctional applications. In order to realize highly ordered multilayer nanostructures, we combined the layer-by-layer (LBL) assembly method with magnetic force-induced alignment to fabricate SWNT/poly(ethylamine) (PEI) multilayer composites. The SWNTs were functionalized with the anionic surfactant sodium dodecylbenzenesulfonate (NaDDBS) to realize negative charge at pH>7, while the PEI is positively charged at pH

Using metal-coated atomic force microscope (AFM) tips and single-walled carbon nanotube (SWNT) BuckyPaper films, the authors have successfully explored the feasibility of electro-machining (EM) at the nanometer scale. Highly ordered pyrolytic graphite (HOPG) and thin metal films were used as a substrate (workpiece) and metal-coated (Cr/Au) Si AFM tips and BuckyPaper films were used as electrodes. A negative voltage pulse was applied to the AFM tip to fabricate holes as small as 30 nm in diameter on the HOPG surface. Using SWNT BuckyPaper films, the submicron holes were fabricated on a metal surface, demonstrating that SWNTs can work as electrodes.Copyright © 2006 by ASME

Sub-100 nm holes were made on a highly oriented pyrolytic graphite (HOPG) surface using a metal-coated atomic force microscope (AFM) tip and carbon nanotube. HOPG was used as a substrate (work piece) and a metal-coated (10 nm Cr/30 nm Au) Si AFM tip served as the other electrode. A negative voltage pulse was applied to the AFM tip to fabricate holes as small as 10 nm in diameter on the HOPG surface with a depth of 0.34 nm, which corresponds to a single layer of graphene. We also explored an individual multi-walled carbon nanotube (MWNT) attached to the AFM tip for nanoscale machining. Unlike the pyramidal shape of the AFM tip, the high aspect ratio of a carbon nanotube can make it possible to form deeper holes at even smaller surface diameter. The hole-formation mechanism is related to the chemical reaction of graphite with adsorbed water and tunneling electrons from the tip to substrate.