High-strength Carbon Nanotube Research Articles

Microstructural behavior of CNT-PDMS thin-films for multifunctional systems

Heterogeneous ribbed and non-ribbed carbon nanotube (CNT)-PDMS thin-film systems manufactured by large-scale rolling exhibit large-strain and high strain-rate characteristics with favorable surface behaviors, such as superhydrophobicity and drag reduction. However, it is not well understood how the multi-phase microstructure and material properties of non-ribbed thin-films are related to the surface material behavior and fracture. Hence, the objective of this investigation is to characterize the large-strain mechanical behavior and the microstructure of various CNT-PDMS compositions to understand how the CNT loading, agglomeration, distribution, and orientation affect the mechanical behavior and fracture of CNT-PDMS unribbed systems. Non-ribbed thin tensile testing specimens were fabricated for neat PDMS and CNT-PDMS with different weight CNT distributions to understand non-ribbed behavior. The ultimate strain, strength, and global stress–strain behavior were obtained by uniaxial mechanical testing. Scanning electron microscopy (SEM) of the fracture surface was also obtained for each sample to analyze the microstructure and relate the damage mode to the different weight distributions. Based on these experimental measurements and observations, large-strain, hyperelastic and hyper-viscoelastic material models were used to characterize the material behavior. The hyper-viscoelastic material model was shown to provide the most accurate material description of the thin-film behavior of the viscoelastic PDMS with the high-strength CNTs.

Shear Behavior of High-Strength and Lightweight Cementitious Composites Containing Hollow Glass Microspheres and Carbon Nanotubes

In this study, an experimental program was conducted to investigate the shear behavior of beams made of high-strength and lightweight cementitious composites (HS-LWCCs) containing hollow glass microspheres and carbon nanotubes. The compressive strength and dry density of the HS-LWCCs were 87.8 MPa and1.52 t/m3, respectively. To investigate their shear behavior, HS-LWCC beams with longitudinal rebars were fabricated. In this test program, the longitudinal and shear reinforcement ratios were considered as the test variables. The HS-LWCC beams were compared with ordinary high-strength concrete (HSC) beams with a compressive strength of 89.3 MPa to determine their differences; the beams had the same reinforcement configuration. The test results indicated that the initial stiffness and shear capacity of the HS-LWCC beams were lower than those of the HSC beams. These results suggested that the low shear resistance of the HS-LWCC beams led to brittle failure. This was attributed to the beams’ low elastic modulus under compression and the absence of a coarse aggregate. Furthermore, the difference in the shear capacity of the HSC and HS-LWCC beams slightly decreased as the shear reinforcement ratio increased. The diagonal compression strut angle and diagonal crack angle of the HS-LWCC beams with shear reinforcement were more inclined than those of the HSC beams. This indicated that the lower shear resistance of the HS-LWCCs could be more effectively compensated for when shear reinforcement is provided and the diagonal crack angle is more inclined. The ultimate shear capacities measured in the tests were compared with various shear design provisions, including those of ACI-318, EC2, and CSA A23.3. This comparison showed that the current shear design provisions considerably overestimate the contribution of concrete to the shear capacity of HS-LWCC beams.

Lamellar structure of high-strength direct-spun CNT yarns: Implications for interface failure

A previously unreported nanoscale lamella structure in a direct spun high strength carbon nanotube (CNT) yarn was identified and characterized. The thickness of the lamellae is in the order of 100 nm. The lamella structure is present in the entire cross-section. It is hypothesized that the lamella structure originates from a single thin sheet (felt) during the manufacturing process, between steps of CNT aerogel and CNT roving. The existence of this lamella structure has direct implications on the yarn-resin interface behavior of derived composites and, subsequently, the mechanical performance of CNT yarn composites. Peridynamic simulations of four distinct yarn pullout experiments captured the observed failure behavior influenced by microstructure.

Kinetic Modulation of Carbon Nanotube Growth in Direct Spinning for High-Strength Carbon Nanotube Fibers.

With impressive individual properties, carbon nanotubes (CNTs) show great potential in constructing high-performance fibers. However, the tensile strength of as-prepared carbon nanotube fibers (CNTFs) by floating catalyst chemical vapor deposition (FCCVD) is plagued by the weak intertube interaction between the essential CNTs. Here, we developed a chlorine (Cl)/water (H2O)-assisted length furtherance FCCVD (CALF-FCCVD) method to modulate the intertube interaction of CNTs and enhance the mechanical strength of macroscopic fibers. The CNTs acquired by the CALF-FCCVD method show an improvement of 731% in length compared to that by the conventional iron-based FCCVD system. Moreover, CNTFs prepared by CALF-FCCVD spinning exhibit a high tensile strength of 5.27 ± 0.27 GPa (4.62 ± 0.24 N/tex) and reach up to 5.61 GPa (4.92 N/tex), which outperforms most previously reported results. Experimental measurements and density functional theory calculations show that Cl and H2O play a crucial role in the furtherance of CNT growth. Cl released from the decomposition of methylene dichloride greatly accelerates the growth of the CNTs; H2O can remove amorphous carbon on the floating catalysts to extend their lifetime, which further modulates the growth kinetics and improves the purity of the as-prepared fibers. Our design of the CALF-FCCVD platform offers a powerful way to tune CNT growth kinetics in direct spinning toward high-strength CNTFs.

Optimization of Si-containing and SiO based Anodes with Single-Walled Carbon Nanotubes for High Energy Density Applications

Lithium-ion batteries require a high energy density when being used in applications such as electric vehicles or portable electronics. This can be achieved on a large scale by improving packaging and implementation, or on a material scale by selecting more energy dense electrode active material. Silicon can be used as a replacement for graphite in negative electrodes if the detrimental volume expansions can be contained. These volume expansions cause continuous mechanical degradation capacity loss leading to short lifetimes that do not meet industry standards. These high-capacity high volume expansion materials such as silicon and SiO must be used in conjunction with more stable electrode materials like graphite to reduce the mechanical degradation caused by volume change. Single-walled carbon nanotubes are shown to be a simple yet effective drop in addition to improve electrical connectivity and increase capacity retention in these silicon-based composite negative electrodes. This added particle interconnectivity from the high tensile strength carbon nanotubes allows for the use of simple binders such as CMC/SBR to create composite electrodes with competitive performance without the use of expensive polymers or complex nanostructures.

Advance processing of cluster-free CNT reinforced 6082 Al matrix nanocomposites: influence of mechanical milling and cryomilling

ABSTRACT In the present study, lightweight high-strength carbon nanotubes (CNTs) reinforced aluminium-based composite was fabricated through powder metallurgical processing. The effect of planetary milling and cryogenic assisted milling on the structure, morphology, mechanical properties, phase composition, and thermal stability of CNTs reinforced Al6082 Al nanocomposites powders was investigated using X-ray diffraction (XRD), scanning electron microscope (SEM)- Energy dispersive spectroscopy (EDS), transmission electron microscope (TEM), and indentation techniques. The CNTs in the Aluminium (AL) matrix were distributed uniformly after 50 h of milling. Further, the cryomilling of 50 h milled Aluminium matrix composites (AMC) decreases agglomeration even more and was successful in cleaving the nanotubes as well as minimising entanglements. The broadening of diffraction peaks suggests a reduction in the crystallite size (from ~36 to ~20 nm) and an increase in induced strain. Milled powders were consolidated through the hot-press and spark plasma sintering technique. The mechanical properties of these AMCs were evaluated through indentation for hardness (>250 HV) and compressive testing techniques. SPSed samples reported better properties in comparison to hot-pressed samples. It is discovered that post-CM of Mechanical Milling (MM) powder aids in overcoming issues related to the agglomeration of Al/CNT composites, and the goal to fabricate tangle-free CNT reinforced composite was achieved. Moreover, it showed enhanced mechanical properties with a maximum yield strength of 210 MPa and good synergy between strength and ductility.

High-Strength Carbon Nanotube Fibers from Purity Control by Atomized Catalytic Pyrolysis and Alignment Improvement by Continuous Large Prestraining.

Transferring the high strength of individual carbon nanotubes (CNTs) to macroscopic fibers is still a major technical challenge. In this study, CNT fibers are wound from a hollow cylindrical assembly. In particular, atomized catalytic pyrolysis is utilized to produce the fiber and control its purity. The pristine fiber is then continuously prestrained to have a highly aligned structure for subsequent full densification. Experimental measurements show that the final fiber possesses a high tensile strength (8.0 GPa), specific strength (5.54 N tex-1 (tex: the weight (g) of a fiber of 1 km long)), Young's modulus (350 GPa), and elongation at break (4%). Such an excellent combination is superior to that of any other existing fiber and attributed to the efficient stress transfer among the highly aligned and packed CNTs. Our study provides a new strategy involving atomized catalysis for developing superstrong CNT assemblies such as fibers and films for practical applications.

Controllable Preparation and Strengthening Strategies towards High-Strength Carbon Nanotube Fibers.

Carbon nanotubes (CNTs) with superior mechanical properties are expected to play a role in the next generation of critical engineering mechanical materials. Crucial advances have been made in CNTs, as it has been reported that the tensile strength of defect-free CNTs and carbon nanotube bundles can approach the theoretical limit. However, the tensile strength of macro carbon nanotube fibers (CNTFs) is far lower than the theoretical level. Although some reviews have summarized the development of such fiber materials, few of them have focused on the controllable preparation and performance optimization of high-strength CNTFs at different scales. Therefore, in this review, we will analyze the characteristics and latest challenges of multiscale CNTFs in preparation and strength optimization. First, the structure and preparation of CNTs are introduced. Then, the preparation methods and tensile strength characteristics of CNTFs at different scales are discussed. Based on the analysis of tensile fracture, we summarize some typical strategies for optimizing tensile performance around defect and tube–tube interaction control. Finally, we introduce some emerging applications for CNTFs in mechanics. This review aims to provide insights and prospects for the controllable preparation of CNTFs with ultra-high tensile strength for emerging cutting-edge applications.

Progress and perspective on high-strength and multifunctional carbon nanotube fibers

Progress and perspective on high-strength and multifunctional carbon nanotube fibers

High-strength and toughness carbon nanotube fiber/resin composites by controllable wet-stretching and stepped pressing

Carbon nanotubes (CNTs) have been used as reinforcements for various composites due to their excellent mechanical properties. However, CNTs have widely suffered from heterogeneous distribution and poor alignment in the composites. To tackle these issues, we herein report an effective strategy towards preparing highly orientated CNT/epoxy (EP) composites from CNT fibers. That is, CNT fibers were continuously soaked and stretched in an EP resin solution and winded into a film on a drum. During this process, the EP resin was infiltrated and the CNTs were aligned simultaneously, leading to a highly uniform and orientated composite. Stepped pressing before formal curing was further utilized to improve the distribution of the EP resin, the densification and thus the internal bonding strength of the composite. The final CNT/EP composite showed a high tensile strength (3.5 GPa) and toughness (80.2 J cm−3), appearing superior to those for previous CNT and carbon fiber reinforced composites.

Lithium Deposition-Induced Fracture of Carbon Nanotubes and Its Implication to Solid-State Batteries.

The increasing demand for safe and dense energy storage has shifted research focus from liquid electrolyte-based Li-ion batteries toward solid-state batteries (SSBs). However, the application of SSBs is impeded by uncontrollable Li dendrite growth and short circuiting, the mechanism of which remains elusive. Herein, we conceptualize a scheme to visualize Li deposition in the confined space inside carbon nanotubes (CNTs) to mimic Li deposition dynamics inside solid electrolyte (SE) cracks, where the high-strength CNT walls mimic the mechanically strong SEs. We observed that the deposited Li propagates as a creeping solid in the CNTs, presenting an effective pathway for stress relaxation. When the stress-relaxation pathway is blocked, the Li deposition-induced stress reaches the gigapascal level and causes CNT fracture. Mechanics analysis suggests that interfacial lithiophilicity critically governs Li deposition dynamics and stress relaxation. Our study offers critical strategies for suppressing Li dendritic growth and constructing high-energy-density, electrochemically and mechanically robust SSBs.

High-strength carbon nanotube fibers with near 100% purity acquired via isothermal vacuum annealing

The Fe and carbonaceous particles are the main impurities in a carbon nanotube fiber (CNTF) fabricated by the floating catalyst chemical vapor deposition (FCCVD) method, which have an adverse effect on various applications, leading to strong demand for purification process to obtain high-purity CNTFs. However, the commonly used purification methods combining oxidation and acid refluxing cannot remove the impurities thoroughly, and will introduce undesired damage onto CNT wall at the same time. Herein, we propose an efficient purification strategy involving an isothermal annealing process under high vacuum to achieve mass preparation of CNTFs with high purity and mechanical/electrical properties. The Fe and carbonaceous impurities can be almost completely removed after vacuum annealing at 1800 °C. The purified CNTFs exhibit enhanced oxidation resistance due to the removal of Fe catalyst particles. The diffusion and evaporation of Fe atoms are most likely responsible for the removal of Fe particles enclosed with graphitic shells. The purified CNTF shows a high specific tensile strength of 1.82 ± 0.04 N/tex due to the high alignment of the CNT bundles within fibers, which can retain 80% of the value for pristine CNTF. The purified CNTF exhibits similar trends of electrical conductivity as the specific tensile strength and has a high conductivity of 3.65 ± 0.11 × 105 S/m. The large-scale preparation of high-purity CNTFs with high-performance will promote the real application of CNTFs.

The synergetic relationship between the length and orientation of carbon nanotubes in direct spinning of high-strength carbon nanotube fibers

Floating catalytic chemical vapor deposition (FCCVD) is an attractive method for synthesizing carbon nanotube (CNT) fibers of high strength, toughness and electrical conductivity. During the process of CNT fibers synthesis, the assembly structure, such as the CNT orientation, CNT length, and the inter-tube particle impurity inside the CNT fibers, are key factors to determine the mechanical properties of CNT fibers. However, the regulation mechanism of the assembly structure during the process of CNT fiber synthesis is still unclear. Herein, in order to obtain high-strength CNT fibers, we demonstrated the synergistic relationship between the orientation and length of CNT inside the CNT fibers during the FCCVD growth process by adjusting the winding rate (5–30 m/min) and the length of the high-temperature reaction zone (660–1060 mm). It also revealed that the particle impurities inside the fibers could greatly reduce mechanical strength of CNT fibers. By optimizing the assembly structure of CNT fibers, the stable and continuous synthesis was realized, with the specific tensile strength of 3.1 N/tex (~ 3.5 GPa) without any post-treatment process.

Synthesis of carbon nanotube reinforced Al matrix composite coatings via cold spray deposition

For the fabrication of high-strength carbon nanotube (CNT) reinforced Al matrix composites, the uniform dispersion, strong interface bonding and high structural integrity of CNTs have been regarded as the three most important issues. In this work, two distinct approaches, namely high shear dispersion (HSD) and shift-speed ball milling (SSBM), were applied to disperse CNTs (1.5 wt%) into pure Al powders. These two kinds of CNT/Al composite powders as well as the pure Al powder (as a comparison) were deposited onto stainless steel plates by cold spraying using different processing parameters. The velocity and the temperature of the particle prior to impact was simulated by the commercial code of Fluent. The deposition efficiency, microstructure evolution, as well as the distribution and structural integrity of CNTs in the composite coatings produced from different starting powders were comparatively investigated in terms of X-ray diffraction (XRD), Raman spectra, and electron back scanning diffraction (EBSD). According to the XRD and Raman analysis, no new phases such as oxides or brittle Al4C3 were detectable in both CNT/Al composite coatings. Some structural damages of CNTs were found in both composite coatings, especially the one fabricated from HSD composite powder. The dispersion of CNTs onto Al particle surfaces by HSD approach did not achieve a significant strengthening effect on the composite coatings, but adversely affected the metallic bonding of the particles and the substrate. The microhardness of the CNT/Al composite coating produced from SSBM powders reached around ~115 HV0.1, showing a significant improvement compared to the pure Al coating mainly due to the grain refinement and CNTs strengthening.

Enhanced Mechanical Properties of Carbon Nanotube/Aluminum Composites Fabricated by Powder Metallurgical and Repeated Hot-Rolling Techniques

This research aimed to fabricate lightweight and high-strength carbon nanotube (CNT)/aluminum (Al) composites by powder metallurgical and repeated hot-rolling techniques. The fabrication was conducted in three steps: (1) CNT dispersion, (2) preparation of CNT/Al compacts by powder metallurgical slurry methods, and (3) strengthening and refining of CNT/Al composites by repeated hot rolling. The processes of dispersion of CNTs were carried out with dimethylacetamide as a solvent and potassium carbonate as a dispersing agent, which is an inorganic salt, under ultrasonic sonication conditions. Effect of sonication time on dispersion states and mechanical properties was also examined.

Highly stretchable CNT Fiber/PAAm hydrogel composite simultaneously serving as strain sensor and supercapacitor

The integration of different functional components into one device is attracting increasing attention. In this work, a high strength carbon nanotube (CNT) fiber/polyacrylamide (PAAm) hydrogel composite is developed by embedding quasi-sinusoidal shaped CNT fibers in PAAm hydrogel. Combination of ionic conductive hydrogel and wavy CNT fibers yields a highly stretchable and wearable device with integration of strain sensing component and electrochemical energy storage component. On the one hand, the CNT fiber/PAAm hydrogel (CFPH) composite shows an excellent strain sensing performance with a stretch-ability up to 100% in a wide frequency of 0.1–10 Hz. As a result, it is effective as stretchable and wearable strain sensor to monitor a range of small to large scale human activities. On the other hand, with two parallel quasi-sinusoidal shaped CNT fibers as electrodes and one ionic conductive PAAm hydrogel as electrolyte and separator simultaneously, the CFPH composite serves as a stretchable all-solid-state supercapacitor with an areal capacitance of 10.6 mF cm−2. The electrochemical performance of the CFPH composite under both static and dynamic loading is very stable, exhibiting a capacitance retention of 90.0% after 3000 charge-discharge cycles with dynamic stretching applied simultaneously. The developed dual-mode CNT fiber/PAAm hydrogel composite is promising to serve as two functional components in one device.

Anisotropic mechanical properties and strengthening mechanism in superaligned carbon nanotubes-reinforced aluminum

High-strength carbon nanotubes (CNTs) enhance the mechanical properties in metal matrix composites; however, their extremely high aspect ratio leads to the anisotropy of mechanical properties. This underlying issue has not yet been clarified owing to the complicated multiple strengthening mechanisms. Herein, we report the anisotropic mechanical properties of a CNT-reinforced aluminum composite and strengthening mechanisms. The uniaxial alignment of CNTs and control of alignment angles were achieved via a mechanical pulling method using a vertically grown CNT forest. As a result, the modulus and strengths decreased in proportion to the misorientation angle. Owing to the superaligned CNTs, the experimental tensile strength in the iso-strain state of the Al-0.15 vol% CNT composite (improved by 20.1%) was near the theoretical value (21.8%), and the strengthening efficiency of the composite was ∼1000. On the other hand, there was a significant deviation between the experimental result and theoretical value in the iso-stress state of the composites. This unusual anisotropic tendency was demonstrated by the strengthening effect of the CNT bridges, which tied the aligned CNTs together, in line with the interconnecting model. The anisotropic mechanical properties corroborate well with our predicted model from calculation by the failure criterion theory with the interconnecting model.

Fabrication of high-strength carbon nanotube bundles using iron oxides co-assisted chemical vapor deposition

We report the fabrication of centimeter-long, defect-free, and parallel-aligned single-wall carbon nanotubes (SWCNTs) using a laboratory-designed two-step catalytic chemical vapor deposition method. Our in situ mass spectral analysis reveals that the initial stainless-derived iron oxide block decomposes methane into acetylene and ethylene, subsequently promoting the growth of high-purity SWCNTs on the second pigment-derived iron oxide fine powders dispersed on the Si/SiO2 substrate. Raman spectral imaging shows the negligible defect-induced D-band in the resulting sample, indicating the highly crystalline structure of SWCNTs. By using a gas-flow-type mechanical test system, an SWCNT bundle containing 20 components of individual SWCNTs exhibits the tensile strength over 50 GPa. Since the catalysts used in this study are widely available on the industrial scale, our iron oxide coassisted method will lead to the scalable production of high-strength SWCNT bundles.

Functionalized carbon nanotube films by thiol-ene click reaction

Preparation of carbon nanotube (CNT) films with both high strength and high elongation is a key challenge to wide application of CNT. A simple method has been investigated to design and prepare high strength and elongation CNT films. This was achieved using a thiol-ene click reaction between the double bond groups of the CNT films and the thiol groups of mercaptans. The effects of mercaptans contents in the functionalized CNT film, click reaction time and the ratio between mercaptans and CNT film on their mechanical strength have been researched. The tensile strength of the functionalized CNT film was 560% higher than that of the prime CNT film. The improvement and functionalized mechanism of CNT film were studied by scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and Fourier transform infrared spectroscopy. Compared with prime carbon nanotube films, the inter-tube interaction of the functionalized CNT films has been changed from weaker van der Waals force to stronger chemical bond, and the cross-link network of mercaptans chain connected CNTs also enhances the load transfer among the CNTs and reduces the distance of the interfacial slippage, which contributes to the improved mechanical properties. This method about improving strength of CNT films will supply a new strategy for preparing high strength CNT film.

Strengthening and toughening mechanisms of CNTs/Mg-6Zn composites via friction stir processing

High strength and toughness carbon nanotubes (CNTs) reinforced Mg-6Zn composites were fabricated by stirring casting integrated with friction stir processing (FSP). The strengthening mechanisms of the CNTs/Mg-6Zn composites were expounded by the characterization of the microstructural evolution and the mechanical properties. The singly dispersed CNTs formed compact bonding with the matrix, which contributed to the grain refinement and the mechanical properties enhancement of the Mg-6Zn matrix. The strengthening contributions are based on the grain refinement, load transfer and Orowan looping mechanisms. The yield strength, ultimate tensile strength, elongation of the FSPed CNTs/Mg-6Zn composites reached 171 MPa, 330 MPa and 15%, which were 144%, 156% and 87% higher than those of the as-cast pristine Mg-6Zn alloy. The fabrication route is proved to be effective to develop innovative CNTs-reinforced metal matrix composites with exceptional mechanical properties.