Length Of Carbon Nanotubes Research Articles

Finite element modeling of the effective creep compliance of carbon nanotube-polymer nanocomposites: A critical microstructure-level investigation

A micromechanical model implemented with the finite element method (FEM) using the representative volume element (RVE) approach is developed to investigate the creep compliance behavior of carbon nanotube (CNT)-polymer nanocomposites. Effects of the CNT waviness, orientation, volume fraction and aspect ratio on the creep compliance of the nanocomposite are examined. Contribution of the interphase region formed due to the interaction between CNTs and matrix to the nanocomposite creep behavior is considered. The nanocomposite creep behavior is affiliate on the waviness and orientation of CNTs. Increasing the volume fraction and length of CNT and interphase thickness improves the nanocomposite creep performance.

Investigating the Electromechanical Sensitivity of Carbon-Nanotube-Coated Microfibers.

The piezoresistance of carbon nanotube (CNT)-coated microfibers is examined using diametric compression. Diverse CNT forest morphologies were studied by changing the CNT length, diameter, and areal density via synthesis time and fiber surface treatment prior to CNT synthesis. Large-diameter (30-60 nm) and relatively low-density CNTs were synthesized on as-received glass fibers. Small-diameter (5-30 nm) and-high density CNTs were synthesized on glass fibers coated with 10 nm of alumina. The CNT length was controlled by adjusting synthesis time. Electromechanical compression was performed by measuring the electrical resistance in the axial direction during diametric compression. Gauge factors exceeding three were measured for small-diameter (<25 μm) coated fibers, corresponding to as much as 35% resistance change per micrometer of compression. The gauge factor for high-density, small-diameter CNT forests was generally greater than those for low-density, large-diameter forests. A finite element simulation shows that the piezoresistive response originates from both the contact resistance and intrinsic resistance of the forest itself. The change in contact and intrinsic resistance are balanced for relatively short CNT forests, while the response is dominated by CNT electrode contact resistance for taller CNT forests. These results are expected to guide the design of piezoresistive flow and tactile sensors.

Investigating the influence of pull-out speed on the interfacial properties and the pull-out behavior of CNT/polymer nanocomposites

Carbon nanotubes (CNTs) improve the fracture toughness of nanocomposites and composite materials by bridging the crack growth path using the pull-out mechanism. Achieving the ability to estimate the dynamic fracture toughness of these materials, the effects of pull-out speed on the pull-out behavior and the critical interfacial shear stress (CISS) between CNT and epoxy resin are investigated in this paper. Employing the molecular dynamics (MD) simulation, the optimized unit cells consisting of epoxy resin containing a single-walled CNT are prepared. Appropriate boundary conditions are applied to the ensembles and the pull-out simulation is performed for the pull-out speeds in the range of 0.0015–0.0050 Å/fs resembling medium-rate loading. The effect of the pull-out speed on the maximum pull-out force is studied. It is revealed that by increasing the pull-out speed, the maximum pull-out force decreases. The corresponding average pull-out force–displacement curves are independent of the pull-out speed and are approximately similar. Based on these simulations, the pull-out speed affects the pull-out energy in the specified speed range. The variations of the pull-out energy and the CISS relative to the changes in the CNT length and diameter are also partially affected by the pull-out speed.

A nanoscroll robustly formed by self-folding a γ-graphyne ribbon on a CNT

Graphynes are one-atom-thick carbon allotropes that are similar to graphene. Direct synthesis of the nanoscrolls from graphynes still faces challenges. This study introduces a physical approach for producing a γ-graphyne nanoscroll (GNS) from a graphyne ribbon. The molecular dynamics method is used to verify the feasibility and the robustness of the physical method by simulating the process of fabricating γ-GNSs from γ-graph-n-yne (acetylenic repeats n = 1–4 in this study) ribbons on a carbon nanotube (CNT). The competition between the bending energy required by the ribbon and the local van der Waals (vdW) interaction energy dominates the self-folding process. Both energies decrease with an increase in n and the ambient temperature. Only when the vdW interaction energy is higher than the bending energy, the self-folding process can be triggered. Hence, either the radius or the length of CNT has a minimum, over which the ribbon can fold into a scroll. The minima are higher at a relatively low temperature between 100 K and 300 K. The initial relative locations of the CNT to the free edge of the ribbon influences the successful formation of the nanoscrolls. The conclusions obtained are helpful for fabricating high-quality γ-GNSs.

Rheological behaviors and viscosity prediction model of cementitious composites with various carbon nanotubes

This study aims to understand the effects and mechanisms of length, diameter, and functional group of carbon nanotubes (CNTs) on rheological behaviors of cementitious composites. The experimental results show that the addition of CNTs decreases the flow index and increases the critical shear rate of cementitious composites. CNTs with a sub-micrometer length and small diameter endow cementitious composites with high yield stresses and minimum viscosities. Influenced by the high water absorption of hydroxylic groups, the minimum viscosity of cementitious composites with hydroxyl functionalized CNTs is larger than that of composites with pristine carbon nanotubes (p-CNTs). By contrast, the yield stress and minimum viscosity of cementitious composites with carboxyl functionalized CNTs are smaller than that of cementitious composites with p-CNTs at most contents due to the high dispersion induced by carboxyl groups. The effect mechanisms of CNTs on rheological behaviors can be attributed to adsorption effect and entanglement effect, which are closely related to length, diameter and functionalization groups of CNTs. The established minimum viscosity prediction model considering the influence of CNT physicochemical features can provide guidance for regulating the workability and hardened performance of CNTs modified cementitious composites.

Continuous Growth of Carbon Nanotubes on Carbon Fiber Surface by Chemical Vapor Deposition Catalyzed by Cobalt with Thiourea

The continuous preparation of carbon nanotubes (CNTs)/carbon fiber (CF) multiscale reinforcements was realized via one-step chemical vapor deposition (CVD) method. In order to alleviate the damage of CF caused by catalyst etching at high temperature, catalytic promoter was adopted to improve the activity of the catalyst, thus reducing the CVD temperature. By adjusting the cobalt-based catalyst system, the effects of thiourea concentration on the morphology, microstructure and tensile strength of CNTs/CF samples were systematically investigated. It was found that the length and quantity of CNTs were added as the thiourea concentration increased, and the graphitization degree of the sample increased at first because of the well-grown CNTs, and then decreased due to the formation of amorphous carbon. Moreover, the tensile strength of CNTs/CF multiscale reinforcements was improved, which derived from the enhanced defect repair function of synthesized CNTs. Remarkably, CNTs/CF-0.02 exhibited a high single-filament tensile strength value (up to 4.51 GPa), which was about 6.6% higher than that of Desized cf Besides, the crystal structure and composition of reduced catalysts were analyzed, confirming the successful sulfur doping into cobalt particles. Therefore, the work offers a facile, economical, and efficient route for manufacturing CNTs/CF multi-scale reinforcements at comparatively low temperature.

A refined model for the effective tensile stiffness of Carbon NanoTube fibers

The paper presents a solid-mechanics based method for the determination of the macroscopic tensile properties of fibers composed of monodispersed Carbon NanoTubes (CNTs), whose length is much lower than the fiber length, arranged in a cross-sectional square lattice. In the longitudinal direction, each CNT is offset, with respect to the neighboring ones, of a given quantity. The interaction between the CNTs caps is negligible, while the model takes into account the coupling occurring on their lateral surfaces, thanks to van der Waals forces and cross-links, modeled as distributed springs. One of the main improvements with respect to previous studies is that the CNTs are here modeled as deformable elastic bars, with given axial stiffness.Under the assumption that, due to the periodicity of the CNTs arrangement, each CNT is subjected to the same loading state, it is demonstrated that the axial strain/stress fields are governed by a delayed-advanced differential equation, that is here solved by means of finite difference technique. This allows to evaluate the total axial force on the fiber, and, consequently, its effective tensile stiffness, strongly dependent on the length of the constituent CNTs, their offset and their axial compliance. Comparisons with literature data confirm the accuracy of the proposed approach.

On interfacial stress transmission of carbon nanotubes/alumina composites

The interface of carbon nanotube (CNTs)/alumina ceramic composites has a very important effect on their mechanical properties. In this study, an appropriate theoretical cell model was established to study the interfacial stress transmission in CNT/alumina composites. The stress transfer equation is derived as follows: The tensile stress of the CNTs and interfacial shear stress were simulated under an axial tension load, and the relationship between the stress transmission and the effective length of the CNTs was analyzed. The theoretical results were compared with the finite element method (FEM) results, and the results were found to be in good agreement. This study can provide a theoretical basis for adjusting the appropriate length of CNTs and interfacial interactions under different tension loads.

Evaluation of the effect of diameter of multi-walled carbon nanotube on the tensile strength using sonication-induced fragmentation

Ultrasonication is a simple way to cut carbon nanotubes in suspension. The length of fragmented carbon nanotubes is made shorter by iterating ultrasonication but approaches a critical aspect ratio. The critical aspect ratio is determined by the tensile strength of the carbon nanotube, the viscosity of the solvent, and physical properties of the cavitation bubbles produced by ultrasonication. Thus, it is possible to evaluate the tensile strength of carbon nanotubes from the critical aspect ratio. In this study, the tensile strengths of multi-walled carbon nanotubes with different diameters were evaluated using sonication-induced fragmentation. The results confirmed the strong dependency of tensile strength on diameter.

Highly efficient synthesis of benzyl benzoate directly from self-coupling of benzyl alcohol in water

Benzyl benzoate (BB), an important ester, still demands for green synthesis routes. In this work, by size regulation and proper functionalization of carbon nanotubes (CNTs) as the support of gold catalyst, this ester can be synthesized very efficiently directly from benzyl alcohol via oxidative coupling. More attractively, the reaction is performed using water as green solvent and molecular oxygen as green oxidant. Simultaneously, very high selectivity to BB can be obtained near full conversion within very short reaction time (just 0.5 h), while the low-value benzoic acid byproduct is negligible. This is very different from many reported gold catalysts that yield much benzoic acid and/or benzaldehyde in water. The results show that, besides the size of CNTs (length and diameter), the functionalization of CNTs is also critical for improving both conversion and the selectivity to BB. In addition, the reaction mechanism for forming BB ester is put forward as well.

Effect of terahertz electromagnetic field on single-file water transport through a carbon nanotube.

With the advancement in terahertz technology, the terahertz electromagnetic field has been proven to be an effective strategy to tune the nanofluidic transport. In this study, we utilize molecular dynamics simulations to systematically analyze the transport of single-file water through a carbon nanotube (CNT) under terahertz electromagnetic fields, focusing on the CNT length, field strength, polarization direction and frequency. Strikingly, with the increase in field strength, the water flow exhibits a transition from normal to super permeation states because of the resonance effect, and the threshold field shifts to low values for long CNTs. The field component parallel to the CNT axis contributes to the resonance effect and increasing water flow, but the vertical component maintains the structure of the single-file water chain and even impedes the water flow. As a result, for a continuous change of field direction, the water flow changes from super permeation to normal states. With the increase in field frequency, the water flow also changes from super permeation to normal or even frozen states, where a higher frequency is required to trigger the super permeation states for lower field strength. Our results provide a comprehensive insight into the effect of terahertz electromagnetic field on the transport of single-file water chains and should have great implications for designing novel nanofluidic devices.

The Impact of Carbon Nanotube Length and Diameter on their Global Alignment by Dead-End Filtration.

Dead-end filtration has proven to effectively prepare macroscopically (3.8 cm2 ) aligned thin films from solutionbased single-wall carbon nanotubes (SWCNTs). However, to make this technique broadly applicable, the role of SWCNT length and diameter must be understood. To date, most groups report the alignment of unsorted, large diameter (≈1.4 nm) SWCNTs, but systematic studies on their small diameter are rare (≈0.78 nm). In this work, films with an area of A = 3.81 cm2 and a thickness of ≈40 nm are prepared from length-sorted fractions comprising of small and large diameter SWCNTs, respectively. The alignment is characterized by cross-polarized microscopy, scanning electron microscopy, absorption and Raman spectroscopy. For the longest fractions (Lavg = 952 nm ± 431 nm, Δ = 1.58 and Lavg = 667 nm ± 246 nm, Δ = 1.55), the 2D order parameter, S2D, values of ≈0.6 and ≈0.76 are reported for the small and large diameter SWCNTs over an area of A = 625 µm2 , respectively. A comparison of Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory calculations with the aligned domain size is then used to propose a law identifying the required length of a carbon nanotube with a given diameter and zeta potential.

Effects of solid-state interfacial reaction on the mechanical properties of carbon nanotubes reinforced aluminum matrix composites with heterogeneous structure

To overcome strength-ductility trade-off in carbon nanotubes (CNTs) reinforced Al matrix composites, we developed a novel and controllable strategy via a combination of different length of CNTs and fine/coarse-grained matrices. Microstructure analysis showed that Al4C3 nanorods about 100 nm were distributed in the fine grain zone (FGZ), while CNTs of micron length were distributed in the coarse grain zone (CGZ). Different sizes of reinforcements play different strengthening roles, and the bimodal grains can effectively coordinate deformation, thereby achieving a good combination of strength and ductility. Heat treatment does not affect the microstructure of the FGZ, but significantly affect the CNT/Al interface bonding in the CGZ. Heat treatment at an appropriate temperature can further improve the strength of the composite by improving the interface bonding. When the heat treatment temperature is too high, the coarsening of Al4C3 in CGZ will deteriorate the mechanical properties of the composite.

Modulation of water transport in carbon nanotubes by local charges

Carbon nanotubes (CNTs) enable extremely fast water transport that make them strong candidates for constructing artificial nanochannels and membranes. However, water transport properties in CNTs with defects along with local charges are rarely studied. In this work, with molecular dynamics simulations, it is found that water transport in CNTs can be greatly modulated by local charges at the CNT middle. The flux modulation factors show an increasing trend with CNT diameters and are slightly influenced by the axial positions of local charges and the CNT lengths. Analysis suggests that water transport is hindered by the confinement-enhanced concentration oscillations and bipolar orientations of water molecules induced by local charges. It is also interesting that the water flux of CNT (6, 6) with high local charges has a reversal behavior due to the overscreening effect. Simultaneously, ions are also impeded due to the water structuring and bipolarizations, which can improve the salt rejection of wide CNTs. In general, introducing local charges on CNT (10, 10) can reject 100% ions while maintaining a high water permeability of ∼90 L/cm2/day/Mpa in the reverse osmosis desalination. This study promotes the understanding of water transport in nanochannels with defects and guides the design of CNTs for high-performance desalination membranes.

Does Size Matter? The Case of Piezoresistive Properties of Carbon Nanotubes/Elastomer Nanocomposite Synthesized through Mechanochemistry.

The growing interest in piezoresistive sensors has favored the development of numerous approaches and materials for their fabrication. Within this framework, carbon nanotubes (CNTs) are often employed. However, CNTs are a heterogeneous material with different morphological characteristics in terms of length and diameter, and, so far, experimental studies have not usually considered the effect of these parameters on the final sensor performances. Here, we observe how, by simply changing the CNTs length in a solvent-free mechanochemistry fabrication method, different porous 3D elastomeric nanocomposites with different electrical and mechanical properties can be obtained. In particular, the use of longer carbon nanotubes allows the synthesis of porous nanocomposites with better mechanical stability and conductivity, and with a nine-times-lower limit of detection (namely 0.2 Pa) when used as a piezoresistive sensor. Moreover, the material prepared with longer carbon nanotubes evidenced a faster recovery of its shape and electrical properties during press/release cycles, thus allowing faster response at different pressures. These results provide evidence as to how CNTs length can be a key aspect in obtaining piezoresistive sensors with better properties.

Grasping the little things: Modeling and simulation of the electromechanical behavior of individual carbon nanotubes and nanotweezers

Nanotweezers are electromechanical nanosized devices with versatile applications. Due to the challenges for experiments on the nanoscale, the support of models and simulations is crucial to get a better insight into the operational mode of nanotweezers. In the current work, armchair and zigzag individual carbon nanotubes (CNTs) as well as nanotweezers are investigated with regard to their behavior under various prescribed electrostatic loads. The applied model is a combination of the model by Cox and Hill for the geometry, the modified molecular structural mechanics (MSM) model for the covalent bonds and the charge-dipole-model by Mayer for the electric field. Compared to the models based on the solution of the Schrdinger equation, the computational cost of the MSM model is much lower. A weak coupling is given between the mechanical and the electrical field through the application of the Coulomb forces on the mechanical model. The finite element software ABAQUS was used for the numerical simulation. It was shown that (i) the resulting strains and closing gaps are nearly identical for armchair and zigzag CNTs with the same diameter and that (ii) an increase in CNT length leads to greater strains for individual CNT and narrower closing gaps between the arms of the nanotweezers.

Modelling the effects of carbon nanotube length non-uniformity and waviness on the electrical behavior of polymer composites

Carbon nanotube (CNT) length non-uniformity and waviness exist inevitably in practical CNT/polymer composites (CPCs) and are two major factors that have significant influences on the composite electrical behavior. However, these two factors have been rarely considered simultaneously in previous modellings. In this study, a numerical model is presented to predict the electrical behavior of CPCs by simultaneously taking into account the two factors besides other ones considered in existing modellings. The length non-uniformity of CNTs is represented by the Weibull distribution and each wavy CNT is modeled by a piecewise sequence of several line segments. Besides, the tunneling distance is reasonably defined to calculate the tunneling resistance between adjacent CNTs. The prediction results reveal that both the CNT length non-uniformity and waviness have significant effects on the electrical behavior of CPCs. It is shown that the percolation threshold evidently decreases with either increasing the CNT length non-uniformity or decreasing the CNT waviness degree. Furthermore, the predictions agree well with existing experimental data, indicating the validity of the present numerical model in predicting the electrical conductivity of CPCs. In particular, it should be emphasized that compared with the previous models without simultaneously considering the effects of these two factors, the present model provides better predictions of the electrical behavior of CPCs.

Predicting mechanical properties of carbon nanotube-reinforced cementitious nanocomposites using interpretable ensemble learning models

Compressive and flexural strength are important characteristics that indicate the efficiency of utilizing carbon nanotube (CNT) in cementitious nanocomposites. Preparing numerous test samples and conducting mechanical tests at different ages is costly and time-consuming, so a predictive model appears essential. Due to the complexity of recent building materials, experimental and statistical models do not function well for such complex materials, so machine learning techniques were adopted in this study. From the results of experiments performed in the literature, separate comprehensive data sets for compressive and flexural strength were collected, and water to cement ratio (W/C), CNT type, CNT content, CNT length, CNT diameter, sand to cement ratio for mortars (S/C), surfactant type, dispersion method, curing days (age) and compressive or flexural strengths of the control sample (C0 or F0) considered as input variables. Four ensemble learning-based algorithms, including random forest, AdaBoost, Gradient Boost, and XGBoost, were developed alongside a standalone model, the decision tree. To assess the sensitivity of input variables, four methodologies, including the Gini importance, permutation importance, F score, and mean of relative Shapely additive explanation (SHAP) values, were obtained, and compressive or flexural strengths of the control sample and CNT content identified overall as the most influential variables. The results revealed that XGBoostprovides more reliable and accurate results for compressive and flexural characteristics and better maps the relationships between input variables. In addition, SHAP analysis was performed to explain the contribution of each input according to its value on the output of XGBoost.

Anomalous water transport in narrow-diameter carbon nanotubes

Carbon nanotubes (CNTs) mimicking the structure of aquaporins support fast water transport, making them strong candidates for building next-generation high-performance membranes for water treatment. The diffusion and transport behavior of water through CNTs or nanoporous graphene can be fundamentally different from those of bulk water through a macroscopic tube. To date, the nanotube-length-dependent physical transport behavior of water is still largely unexplored. Herein, on the basis of molecular dynamics simulations, we show that the flow rate of water through 0.83-nm-diameter (6,6) and 0.96-nm-diameter (7,7) CNTs exhibits anomalous transport behavior, whereby the flow rate increases markedly first and then either slowly decreases or changes slightly as the CNT length l increases. The critical range of l for the flow-rate transition is 0.37 to 0.5 nm. This anomalous water transport behavior is attributed to the l-dependent mechanical stability of the transient hydrogen-bonding chain that connects water molecules inside and outside the CNTs and bypasses the CNT orifice. The results unveil a microscopic mechanism governing water transport through subnanometer tubes, which has important implications for nanofluidic manipulation.

Nanoporous Membranes of Densely Packed Carbon Nanotubes Formed by Lipid-Mediated Self-Assembly.

Nanofiltration technology faces the competing challenges of achieving high fluid flux through uniformly narrow pores of a mechanically and chemically stable filter. Supported dense-packed 2D-crystals of single-walled carbon nanotube (CNT) porins with ∼1 nm wide pores could, in principle, meet these challenges. However, such CNT membranes cannot currently be synthesized at high pore density. Here, we use computer simulations to explore lipid-mediated self-assembly as a route toward densely packed CNT membranes, motivated by the analogy to membrane-protein 2D crystallization. In large-scale coarse-grained molecular dynamics (MD) simulations, we find that CNTs in lipid membranes readily self-assemble into large clusters. Lipids trapped between the CNTs lubricate CNT repacking upon collisions of diffusing clusters, thereby facilitating the formation of large ordered structures. Cluster diffusion follows the Saffman-Delbrück law and its generalization by Hughes, Pailthorpe, and White. On longer time scales, we expect the formation of close-packed CNT structures by depletion of the intervening shared annular lipid shell, depending on the relative strength of CNT-CNT and CNT-lipid interactions. Our simulations identify CNT length, diameter, and end functionalization as major factors for the self-assembly of CNT membranes.