Wavy Carbon Nanotubes Research Articles

A thermomechanical shear lag analysis of short fuzzy fiber reinforced composite containing wavy carbon nanotubes

Abstract A novel three-phase shear lag model is derived to study the load transfer characteristics of the short fuzzy fiber reinforced composite (SFFRC) subjected to the thermomechanical loading. The distinctive feature of the SFFRC is that the short carbon fiber reinforcements coated with radially aligned carbon nanotubes (CNTs) are uniformly interlaced in the polymer matrix. The main novelty of the shear lag model derived in this study is that the interactions between the representative volume elements (RVEs) of the SFFRC, the application of the radial and the thermal loads on the RVE, and the radial as well as the axial deformations of different orthotropic constituent phases of the SFFRC have been taken into account. Particular emphasis has been placed on investigating the effect of waviness of CNTs on the load transfer characteristics of the SFFRC when the wavy CNTs are coplanar with either of the two mutually orthogonal planes. In the absence and the presence of the applied radial and thermal loads on the RVE, the shear lag analysis revealed that if the wavy CNTs are coplanar with the axial plane of the carbon fiber such that the amplitudes of the CNTs are parallel to the length of the carbon fiber then the load transfer characteristics of the SFFRC are significantly improved over those of the composite with and without the straight CNTs. The limiting value of the effective aspect ratio of the carbon fiber is also found for the efficient load transfer to the carbon fiber.

Waviness reduces effective modulus of carbon nanotube forests by several orders of magnitude

Waviness is invariably present in vertically-aligned Carbon Nanotubes (CNTs) regardless of how controlled the fabrication process is. This study, using experiments and models, shows that such inherent waviness is the main mechanism by which the effective modulus of CNTs is reduced by several orders of magnitude. At this time, most studies have shown that the compliant mechanical response of the CNT forests under compressive loading is due to bending and buckling of CNTs as well as the variation of CNT density throughout the forest height. Subjecting CNT forests to tensile loads as well as to compressive loads, it is shown here that the high compliance of CNT forests is due to the inherent waviness of individual CNTs, and not necessarily due to bending and buckling of CNTs. The experimental findings are also supported through analytical models and numerical models that show that the CNT wavy geometry causes the CNTs to have 4–5 orders of magnitude greater compliance than a straight CNT.

Effective mechanical properties of nanocomposites reinforced with wavy carbon nanotubes

Carbon nanotubes (CNTs) possess exceptional mechanical properties and are therefore suitable reinforcements for composite materials. Nanotube efficiency in reinforcing the matrix depends on the CNT alignment, volume fraction and configuration, as well as matrix properties. In this investigation, finite element method (FEM) is used to investigate the effects of nanotube waviness ratio, volume fraction, and matrix modulus on properties of CNT based polymer nanocomposites. Nanocomposite mechanical properties are evaluated using a 3D nanoscale representative volume element. Models consisting of CNTs with different waviness ratios are created to investigate the effects of nanotube configuration on nanocomposite mechanical properties. Next, the effect of nanotube volume fraction on nanocomposite moduli of elasticity is investigated. Finally, the effects of matrix modulus are investigated by analysing models consisting of matrices with different moduli. The results of this investigation are compared with those found in the literature and good agreement is observed.

Axial buckling behavior of wavy carbon nanotubes: A molecular mechanics study

In this study, we investigate the axial compressive stress–strain relationships and deformation processes of wavy single-walled carbon nanotubes (SWCNTs) with multiple Stone–Wales defects using molecular mechanics simulations with the adaptive intermolecular reactive empirical bond-order potential. The wavelength to amplitude ratios of the wavy SWCNTs modeled in this study are 50, 100, or 200. The wavy models have point-symmetric or asymmetric waveforms with respect to the tube axial center. It is found that the symmetric wavy models exhibit a buckling point during axial compression due to bifurcation of the deformation path and experience nearly the same buckling stresses as their pristine straight counterparts. Our simulations show that, regardless of the waveform, wavelength, and amplitude, the axial compressive strength of wavy SWCNTs is in good agreement with the Euler buckling stress.

The effect of nanotubes waviness on mechanical properties of CNT/SMP composites

In this paper, the effects of inclusion waviness and its distribution to the effective carbon nanotubes (CNTs)/shape memory polymer (SMP) composites stiffness is investigated. A micro mechanical model is developed to take into account the waviness effect of nanotubes, which are assumed as solid short fibers with a circular cross-section and exhibiting bow shape waviness. The mechanical properties of the composite are significantly increased by reinforcing with CNTs and the desired mechanical properties in addition to proper shape recovery behavior are obtained. The improvements in modulus are remarkably less than the micromechanical prediction assuming straight nanotubes. It is shown that the lack of fiber waviness distribution assumption was found to lead to inaccurate composite stiffness prediction. Since the effects of CNT waviness and aggregation in the composite frequently encountered in real engineering situation, in this study the effects of these defects on the overall properties of the CNT/SMP composite are simultaneously examined. The results indicate that both the waviness and aggregation can drastically reduce the stiffening effect of the CNTs and are two important factors influencing the reinforcing efficiency.

Understanding the effect of CNT characteristics on the tensile modulus of CNT reinforced polypropylene using finite element analysis

The focus of this study is to understand the reinforcing efficiency of carbon nanotubes (CNT) in polymers through finite element modeling. The novelty of our work is that the probability distribution functions of CNT diameter, orientation, dispersion and waviness, determined through image analysis, are incorporated in the finite element model allowing thus for fundamental understanding of how the CNT characteristics affect the tensile modulus of CNT reinforced polypropylene (PP) composites. The presence of interphase, confirmed by atomic force microscopy, is also accounted for in our model. The image analysis approach utilizes scanning electron microscopy images of the CNT/PP composites made by melt mixing and injection molding. Model predictions are compared with the data obtained experimentally according to ASTM D638. A good agreement between the model predictions and experimental data is observed.

Vibration analysis of functionally graded nanocomposite cylinders reinforced by wavy carbon nanotube based on mesh-free method

This work reports the effect of nanotube aspect ratio on the free vibration characteristics of a functionally graded nanocomposite cylinders reinforced by wavy single-walled carbon nanotubes (CNTs) based on mesh-free method. In this simulation, an axisymmetric model is used and axisymmetric natural frequencies of CNT reinforced composite cylinders are presented. The material properties of functionally graded CNT reinforced composites are assumed to be graded in the thickness direction and are estimated by a micromechanical model. The effect of the waviness of the CNTs and its parameters are studied. In the mesh-free analysis, moving least squares shape functions are used for approximation of displacement field in the weak form of motion equation, and the transformation method is used for the imposition of essential boundary conditions. It is observed that the waviness significantly reduces the effective reinforcement of the nanocomposites. The effective moduli and frequency response are very sensitive to the waviness but this sensitivity decreases with the increase of the waviness. The validity of the Young's modulus and the frequency response were assessed by a comparison with available literature data, providing a good agreement.

Effect of carbon nanotube geometry upon tunneling assisted electrical network in nanocomposites

This paper examines the effect of carbon nanotube (CNT) geometry upon the electrical properties of the corresponding functionalized nanocomposites. Specifically, Monte Carlo (MC) simulations are conducted to evaluate the effect of CNT length non-uniformity and waviness upon tunneling. Three aspects of the work are considered. The first is concerned with the application of periodic boundary condition that ensures periodic connectivity of the percolating paths via the use of an improved connective percolating network recognition scheme. The second is concerned with the determination of the electrical conductivity of the percolated system rather than the critical percolation threshold for varied CNT geometries using Weibull distribution to statistically account for the geometry variations. The third is concerned with the validation of our MC simulations. Our results reveal that (i) the CNT geometry, as determined by CNT length variability and waviness, plays a more dominant role in percolation threshold rather than tunneling barrier height and (ii) higher CNT loading beyond a critical percolation significantly influences the role of tunneling barrier height upon the electrical conductivity.

Fabrication of Highly Stretchable Conductors via Morphological Control of Carbon Nanotube Network

Stretchable conductors, which can keep their excellent electrical conductivity while highly stretched, have been investigated extensively due to their wide range of applications in flexible and stretchable electronics, wearable displays, etc.; however, their preparation is often complicated and expensive. Herein, an efficient method to prepare high performance stretchable conductors through morphological control of conductive networks formed with carbon nanotubes (CNTs) in an elastomer matrix is reported. It is observed that an interface-mediated method could be used to align randomly oriented filler during stretching and to induce buckling of CNTs during relaxation. Further morphological studies indicate the possible formation of a wavy CNT structure induced by cyclic pre-straining. Subsequent thermal annealing is observed to collapse the oriented network and improve the local contacts between conductive networks. Through such a simple procedure, a conductivity of nearly 1000 S m(-1) and a stretchability of 200% can be achieved for composites containing 20 wt% CNTs. CNTs are observed to buckle over a large area in polymer bulk, and the combination of pre-straining and thermal annealing modifies the conductive network in the elastomer matrix. As a general method, this could be used for easy fabrication of high-performance stretchable conductors for arbitrary-shaped objects on a large scale.

Effects of CNT waviness on the effective elastic responses of CNT-reinforced polymer composites

In this study, the effects of fiber waviness on the effective elastic responses of CNT–polymer composites are investigated based on the framework of micromechanics and homogenization. By taking advantage of an ad hoc Eshelby tensor, the load-transfer capability of wavy carbon nanotube (CNT) embedded in the polymer matrix is accounted for. Further, the effective elastic responses of composites are simulated by using the multi-phase Mori–Tanaka method to study the influence of randomly oriented wavy CNT. It is demonstrated that the proposed micromechanics-based closed form solution is effective to tackle the underlying problem. The present predictions and the comparisons with the available experimental data indicate that the CNT waviness leads to the degradation of effective responses of composites. Finally, in addition to the effect of CNT waviness, the significance of CNT interface is briefly discussed based on the experimental observations.

Effect of Carbon Nanotube Waviness on the Elastic Properties of the Fuzzy Fiber Reinforced Composites

A fuzzy fiber reinforced composite (FFRC) reinforced with wavy zig-zag single-walled carbon nanotubes (CNTs) and carbon fibers is analyzed in this study. The distinct constructional feature of this composite is that the wavy CNTs are radially grown on the surface of carbon fibers. To study the effect of the waviness of CNTs on the elastic properties of the FFRC, analytical models based on the mechanics of materials (MOM) approach is derived. Effective elastic properties of the FFRC incorporating the wavy CNTs estimated by the MOM approach have been compared with those predicted by the Mori–Tanaka (MT) method. The values of the effective elastic properties of this composite are estimated in the presence of an interphase between the CNT and the polymer matrix which models the nonbonded van dar Waals interaction between the CNT and the polymer matrix. The effect of waviness of CNTs on the effective properties of the FFRC is investigated when the wavy CNTs are coplanar with two mutually orthogonal planes. The results demonstrate that the axial effective elastic properties of the FFRC containing wavy CNTs can be improved over those of the FFRC with straight CNTs.

1533 波状カーボンナノチューブの圧縮座屈特性に及ぼす波形の影響

1533 波状カーボンナノチューブの圧縮座屈特性に及ぼす波形の影響

Influence of carbon nanotube waviness on the stiffness reduction of CNT/polymer composites

The degree to which the non-straight shape of carbon nanotube (CNT) affects the mechanical properties of CNT-based composites is studied. The effective scale of CNT curvature is determined using top-down scanning and then appropriate representative volume element encompassing of several straight and/or non-straight CNTs is defined. A multi-scale modeling relying on bottom-up approach is employed and CNT curvature is taken into account as a random parameter to capture any arbitrary non-straight shapes of CNTs. Comparing obtained results from developed modeling with available experimental data, the developed modeling procedure is validated. The results demonstrate that non-straight shape of CNT considerably reduces its efficiency to reinforce polymer matrix. It is observed that low contents of CNT enhance Young’s modulus of polymer more efficiently due to the lower level of CNT waviness and agglomeration. The results exhibit non-linear variations in the Young’s modulus of CNT-based composites versus CNT contents which was approved by experimental observations.

Effect of the aspect ratio and waviness of carbon nanotubes on the vibrational behavior of functionally graded nanocomposite cylindrical panels

AbstractIn this study, based on the three‐dimensional theory of elasticity, free vibration characteristics of nanocomposite cylindrical panels reinforced by single‐walled carbon nanotubes (CNTs) are considered. The carbon nanotube‐reinforced (CNTRC) cylindrical panel has smooth variation of CNT fraction in the radial direction, and the material properties are estimated by the extended rule of mixture. In this work, the classical theory concerning the mechanical efficiency of a matrix embedding finite length fibers has been modified by introducing the tube‐to‐tube random contact, which explicitly accounts for the progressive reduction of the tubes' effective aspect ratio as the filler content increases. Symmetric and asymmetric volume fraction profiles are provided in this work for comparison. Suitable displacement functions that identically satisfy the boundary conditions at the simply supported edges are used to reduce the equilibrium equations to a set of coupled ordinary differential equation with variable coefficients, which can be solved by a generalized differential quadrature method. The results show that the kind of distribution and volume fraction of CNT have a significant effect on normalized natural frequency.POLYM. COMPOS., 33:2036–2044, 2012. © 2012 Society of Plastics Engineers

Load-transfer efficiency and mechanical reliability of carbon nanotube fibers under low strain rates

For practical applications, long term performance of carbon nanotube (CNT) fibers is essential, as in these cases CNT fibers are expected to work under cyclic loading or under a certain stress/strain for a long time. In this paper, the mechanical behavior of CNT fibers, including stress relaxation, load transfer and failure process, are investigated under low strain rates which are close to practical situations. Static mechanical test, electrical measurement and Raman spectroscopy are used to in situ monitor the load transfer and failure process. Transmission electron microscopy is utilized to characterize the structures and morphologies of CNTs inside the fibers. Based on the experimental results, it is believed that there is a close relationship between CNT morphology and fiber performance. Nonuniform load bearing, straightening of wavy CNTs and serious sliding exist in the fibers and are main factors that degrade fiber’s reliability.

Defining the lower and upper limit of the effective modulus of CNT/polypropylene composites through integration of modeling and experiments

The focus of this study is to evaluate the effective modulus of carbon nanotube (CNT) reinforced polypropylene (PP) composites based on numerical techniques that account for CNTs non-uniform distribution, agglomeration, waviness and alignment with respect to the applied load direction. A three-dimensional (3D) finite element model with multiple CNTs embedded within the PP matrix in presence of CNT/PP interphase is built and subjected to tensile loading. The thickness and stiffness of CNT/PP interphase, dispersion and distribution of CNTs within the PP matrix, their alignment with respect to the applied load and their waviness are characterized using atomic force microscopy and scanning electron microscopy of the CNT/PP composites made by melt mixing and injection molding. The modulus predicted numerically is in good agreement with that obtained experimentally, according to ASTM D638. The proposed elaborated numerical model can provide the lower and upper limit of the modulus and guide the manufacturing of CNT/PP composites with engineered properties in a scalable and cost effective way.

Wavy Ribbons of Carbon Nanotubes for Stretchable Conductors

AbstractWavy ribbons of carbon nanotubes (CNTs) are embedded in elastomeric substrates to fabricate stretchable conductors that exhibit excellent performance in terms of high stretchability and small resistance change. A CNT ribbon with a thin layer of sputtered Au/Pd film is transferred onto a prestrained poly(dimethylsiloxane) (PDMS) substrate and buckled out‐of‐plane upon release of the prestrain. Embedded in PDMS, the wavy CNT ribbon is able to accommodate large stretching (up to the prestrain) with little change in resistance. For a prestrain of 100%, the resistance increases only about 4.1% when the wavy CNT ribbon is stretched to the prestrain. A simple stretchable circuit consisting of a light‐emitting diode and two wavy ribbons is demonstrated and shows constant response on significant twisting, folding, or stretching. Fabricated with a simple buckling approach, the wavy CNT‐ribbon‐based stretchable conductors (e.g., interconnects and electrodes) could play an important role in stretchable electronics, sensors, photovoltaics, and energy storage.

Vibration of wavy single-walled carbon nanotubes based on nonlocal Euler Bernoulli and Timoshenko models

The transverse vibration of a single-walled carbon nanotube (SWCNT) with light waviness along its axis is modeled by the nonlocal Euler-Bernoulli and Timoshenko beam theory. Unlike the Euler-Bernoulli beam model (EBM), the effects of transverse shear deformation and rotary inertia are considered within the framework of the Timoshenko beam model (TBM). The surrounding elastic medium is described as both Winkler-type and Pasternak-type foundation models. The governing equations are derived using Hamilton’s principle, and the Galerkin method is applied to solve these equations. According to this study, the results indicate that the frequency calculated by TBM is lower than that obtained by EBM. Detailed results show that the importance of transverse shear deformation and rotary inertia become more significant for stocky SWCNTs with clamped-clamped boundary conditions. Moreover, the influences of the amplitude of waviness, nonlocal parameter, medium constants, boundary conditions and aspect ratio are analyzed and discussed. It is shown that waviness in the curved SWCNT causes an obvious increase in the natural frequency in comparison with the straight SWCNT, especially for a compliant medium, pinned-pinned boundary condition, short SWCNT and large nonlocal coefficient.

Nonlinear Dynamic Analysis of Single-Walled Carbon Nanotube Based Mass Sensor

In previous studies, experimentally measured resonance frequencies of carbon nanotubes have been used along with classical beam theory for straight beams. However, it is found that these carbon nanotubes are not straight, and that they have some significant surface deviation associated with them. This paper deals with the nonlinear vibration analysis of a wavy single-walled carbon nanotube based mass sensor, which is doubly clamped at a source and a drain. Nonlinear oscillations of a single-walled carbon nanotube excited harmonically near its primary resonance are considered. The carbon nanotube is excited by the addition of an excitation force. The modeling is carried out using the elastic continuum beam theory concept, which involves stretching of the central plane and phenomenological damping. This model takes into account the existence of waviness in carbon nanotubes. The equation of motion involves two nonlinear terms due to the curved geometry and the stretching of the central plane. The dynamic response of the carbon nanotube based mass sensor is analyzed in the context of the time response, Poincaré maps, and fast Fourier transformation diagrams. The results show the appearance of instability and chaos in the dynamic response as the mass on carbon nanotube is changed. Period doubling and mechanism of intermittency have been observed as the routes to chaos. The appearance of regions of periodic, subharmonic, and chaotic behavior is observed to be strongly dependent on mass and the geometric imperfections of carbon nanotube. Poincaré maps and frequency spectra are used to elucidate and to illustrate the diversity of the system behavior.

Mechanical and electrical property improvement in CNT/Nylon composites through drawing and stretching

The excellent mechanical properties of carbon nanotubes (CNTs) make them the ideal reinforcements for high performance composites. The misalignment and waviness of CNTs within composites are two major issues that limit the reinforcing efficiency. We report an effective method to increase the strength and stiffness of high volume fraction, aligned CNT composites by reducing CNT waviness using a drawing and stretching approach. Stretching the composites after fabrication improved the ultimate strength by 50%, 150%, and 190% corresponding to stretch ratios of 2%, 4% and 7%, respectively. Improvement of the electrical conductivities exhibited a similar trend. These results demonstrate the importance of straightening and aligning CNTs in improving the composite strength and electrical conductivity.