Oriented Carbon Nanotubes Research Articles

Determination of tadalafil in pharmaceutical samples by vertically oriented multi-walled carbon nanotube electrochemical sensing device

Abstract In this work, an electrochemical sensor based on vertically oriented carbon nanotubes (voMWCNT), electrically wired via connection upon a copper plate, for the detection of tadalafil (TD) is reported. The voMWCNT is easily prepared via microwave plasma reactor with no additional post-treatment. The voMWCNT was characterized by scanning and transmission electron microscopy in addition to electrochemical characterization with cyclic voltammetry. Once TD is of easy access and used in the treatment of erectile dysfunction and related diseases, the quality control of this drug is of high importance. Using linear sweep voltammetry, the detection of TD was found to be possible from 0.85 to 8.9 μmol L−1, with an R2 of 0.992 with a limit of detection found to correspond to 78 nmol L−1. The quantification of TD in different pharmaceutical drugs, as well as the device stability, indicates that the voMWCNT has potential for the routine sensing of TD.

Printed hydrogel nanocomposites: fine-tuning nanostructure for anisotropic mechanical and conductive properties

Additive manufacturing of composites offers a potential for a new level of control over a material’s structure at the microscale. The focus of this work is a 2-hydroxyethyl methacrylate (HEMA)–based gelation system with orderly distributed carbon nanotubes (CNTs). CNTs undergo shear-induced alignment during printing process, and retain their orientation after the polymerisation of HEMA monomers, thereby, forming a nanocomposite with anisotropic mechanical and electrical properties. It is characterised with an intensive programme of mechanical tests including quasistatic uniaxial stretching, and dynamic cyclic loadings, as well as its four-terminal sensing of conductive characteristics. A coupling effect of mechanical and electrical properties is also studied. The experimental findings are discussed in detail and demonstrate that the orientation of CNTs affects both the mechanical and electrical conductive properties of the nanocomposites in terms of its ultimate strength, resistivity, and a piezoresistive coefficient. Understanding of anisotropic electromechanical properties of printed PHEMA-CNT hydrogel nanocomposite will ultimately underpin the development of smart soft materials for diverse applications, such as biomimetic nucleus pulposus or flexible electronics.

Electromagnetic resonance analysis of asymmetric carbon nanotube dimers for sensing applications

In this work, we study the electromagnetic scattering characteristics of asymmetric carbon nanotube (CNT) dimers with rigorous computational experiments. We show that the configurational asymmetry in the CNT dimer assembly creates a unique field distribution in the vicinity of the dimer, which in turn generates two distinct resonances representing the bonding and anti-bonding modes. The sensitivity of these two modes towards CNT lengths, orientations, and shapes, is studied. We also show the ability of asymmetric CNT dimer for the contactless detection of nanoparticles (NP). The presence of a NP in the vicinity of the CNT dimer perturbs the dimer’s field distribution and causes unequal shifts in the bonding and anti-bonding resonances depending on the NP location, material, size and shape. By studying the differences in these resonance shifts, we show that the relative location and orientation of the NP can be reconstructed. The computational experiments performed in this work have the potential to guide the use of asymmetric CNT dimers for novel sensing applications.

Longitudinal modeling and properties tailoring of functionally graded carbon nanotube reinforced composite beams: A novel approach

Carbon Nanotubes (CNTs) are considered a promising reinforcement for engineering advanced structures, such as aerospace applications. It became inevitable to tailor structures not only by conventional geometric but also by mechanical properties tailoring, to acquire the maximum load-carrying capacity. Therefore, this research presented a novel Finite Element (FE) modeling of Axially Functionally Graded CNT Reinforced Composite (AFG-CNTRC) beams, based on Timoshenko beam theory. However, an obstacle is raised due to properties discontinuity produced between consecutive elements in axial gradation. A solution is proposed based on the ladder-pattern concept. It defined a locally specific value at each element, satisfying the overall required distribution. It followed by a convergence test, static, and dynamic analysis with comprehensive parametric studies. The obtained results showed that the model is convergent, accurate, free of shear-locking, and suitable for axial gradation. Moreover, this study introduced an advanced technique for axial properties tailoring, utilizing privileges of CNT and aggregation. The longitudinal tailoring depends on both the CNT orientation angle and gradation index, which played a crucial role in response control and adjusting beam stiffness together with strength. This approach could solve enormous engineering problems, especially at critical cross-sections design and constraint response, without affecting the total weight.

Size dependent free vibration and buckling of multilayered carbon nanotubes reinforced composite nanoplates in thermal environment

In this contribution, we analyze buckling and free vibration of simply-supported cross-ply/angle-ply single-walled carbon nanotubes reinforced composite CNTRC laminated nanoplates in thermal environment. The nonlocal strain gradient theory within the third-order shear deformation plate theory are adopted to capture the size-dependent effects of CNTRC laminated nanoplates. Several types of reinforcement material distributions, a uniform distribution and functionally graded distributions, are inspected. Material properties are temperature-dependent and performed using Touloukian principal. The influence of CNTs reinforcement patterns, composite structure, nonlocal parameter, length scale parameter, carbon nanotubes orientation, and the geometric parameters on buckling loads and natural frequencies is examined.

Directional Response of Randomly Dispersed Carbon Nanotube Strain Sensors.

Tests on a double lap bonded joint, with transverse strips of randomly oriented carbon nanotubes (CNT) sprayed onto an epoxy adhesive film, showed a positive increment in electrical resistance under tensile load, even though the transverse strains were negative. Other experiments included in this work involved placing longitudinal and transversal CNT sensors in a tensile loaded aluminum plate, and, as reported by other authors, the results confirm that the resistance change is not only dependent on the strains oriented with the electrode line, while the other strain components also influence the response. This behavior is quite different to that of conventional strain gages which have a near zero sensitivity to strains not aligned to the sensor direction. The dependence of the electrical response on all the strain components makes it quite difficult, possibly unfeasible, to experimentally determine the individual strain components with this kind of sensors; however, the manufacturing of aligned CNT sensors could deal with this issue.

Micromechanical modeling and characterization of elastic behavior of carbon nanotube‐reinforced polymer nanocomposites: A combined numerical approach and experimental verification

AbstractIn this study, a combined analytical and finite element‐based micromechanical modeling is performed to characterize the elastic properties of carbon nanotube (CNT) reinforced polymer nanocomposites and validated with experimental work. First, coordinates of armchair and zigzag type CNTs are generated using MATLAB based on the geometric structure of the CNTs and imported to the computational software ANSYS to model and characterize the elastic properties of the individual single‐walled CNT (SWCNT) and multi‐walled CNT by applying various loading and boundary conditions. The present developed finite element method (FEM) model of CNTs is validated with the available literature in terms of elastic properties. Subsequently, the equivalent elastic properties of CNT reinforced epoxy nanocomposites are determined through representative volume element (RVE) model by finite element simulations and Mori‐Tanaka homogenization techniques by replacing CNTs with equivalent fibers as microinclusions. The equivalent elastic properties of nanocomposite obtained by the FEM and analytical model are compared and validated with the experimental results. Further, the detailed parametric study is performed to investigate the influence of tube chirality, volume fraction, and orientation of CNT in terms of the elastic properties of the nanocomposite. It was observed that the armchair type CNT reinforced nanocomposites are stiffer than the zigzag type SWCNT reinforced nanocomposites in terms of elastic moduli. Further, it was noticed that the tube chirality and the number of walls of CNTs have significantly influenced the elastic behavior of nanocomposites. It can be concluded that the presented combined model provides an efficient methodology and comprehensive understanding to analyze the elastic behavior of CNTs and CNT reinforced nanocomposites. So, the presented combined numerical and experimental study could serve as a guideline in micromechanical modeling and characterization of elastic behavior of CNT‐reinforced polymer nanocomposites.

Globally Aligned, Wafer-Scale Deposition of Carbon Nanotubes Via Interfacial Assembly

Floating evaporative self-assembly (FESA) is a solution-based technique for fabricating highly-aligned semiconducting carbon nanotube films.1,2 Field-effect transistors (FETs) based on carbon nanotubes aligned via FESA have exhibited current densities exceeding that of GaAs and Si, demonstrating the promise of this technique.3 However, the global uniformity of FESA films is poor, as the films are composed of stripes of aligned and randomly oriented carbon nanotubes. Additionally, this technique is inherently difficult to scale. In the work presented here, we demonstrate a scalable technique to overcome the shortcomings of FESA and to achieve globally uniform films of aligned carbon nanotubes at desirable packing densities for FETs.In our technique, carbon nanotubes are continuously aligned across target substrates by lifting the substrates through a flowing nanotube ink/water interface. The ink/water interface collects and confines the nanotubes, which is vital to achieving a high degree of alignment. Polarized Raman spectroscopy mapping is used to quantify the degree of alignment in the resulting aligned nanotube films. The full width at half maximum (FWHM) of the alignment distribution within the films is 6.0° measured across a 1 mm2 area. We control the aligned carbon nanotube packing density from 0.2 to 59 µm-1 by varying the ink concentration from 0.1 to 100 µg mL-1. Using optimized conditions, we demonstrate the scalability of this technique by aligning carbon nanotubes across a 100 mm wide wafer. Charge transport measurements are performed using FETs with the aligned carbon nanotube films as the active channels, and the performance is compared to FETs based on FESA films. 1. Jinkins, K., R. et al. Nanotube Alignment Mechanism in Floating Evaporative Self-Assembly. Langmuir 33, 13407–13414 (2017).2. Joo, Y., Brady, G. J., Arnold, M. S. & Gopalan, P. Dose-Controlled, Floating Evaporative Self-assembly and Alignment of Semiconducting Carbon Nanotubes from Organic Solvents. Langmuir 30, 3460–3466 (2014).3. Brady, G. J. et al. Quasi-ballistic carbon nanotube array transistors with current density exceeding Si and GaAs. Sci. Adv. 2, (2016).

Effect of carbon nanotube alignment on nanocomposite sensing performance

The objective of this study is to derive a numerical model of carbon nanotube (CNT)-based thin films that accurately reflect their electrical and electromechanical performance as observed through experimental tests. Although nanocomposites based on CNTs dispersed in polymer matrices have been studied extensively, their nanocomposite properties vary depending on CNT orientations. This study aimed to explain how differences in nanocomposite behavior could be revealed by numerical models considering different CNT alignment conditions. First, a percolation-based thin film model was generated by randomly dispersing CNT elements in a predefined two-dimensional domain. The degree of CNT alignment in the film was controlled by limiting the CNT elements’ maximum angle they make with respect to the film’s longitudinal axis. Then, numerical simulations on CNT-based film models were conducted. Second, multi-walled carbon nanotube (MWCNT)-epoxy films were prepared via drop casting. Alternating current was applied to the MWCNT-epoxy mixture before curing to prepare films with different degrees of CNT alignment. The electrical and electromechanical properties of these specimens were characterized, and the results were compared with simulations. Good agreement between experiments and simulations was observed.

Conception of Nanosized Membrane Reactors for Hydrogenation with Accumulated Hydrogen

Results are summarized from studies to develop the concept of next-generation nanosized membrane reactors (NMRs) in which hydrogenation reactions are run in pores of varying diameter using hydrogen preadsorbed in mono- and multilayer oriented carbon nanotubes with graphene walls (OCNTGs) formed on the inner surface of pores of ceramic membranes. A comparative analysis is performed of the morphology of such carbon nanostructures as graphenes, OCNTGs, and pyrocarbon nanocrystallites (PNCs), and morphology is studied via X-ray diffraction analysis, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy (TEM). It is shown that only OCNTGs are capable of accumulating and storing hydrogen. The mechanism of hydrogen adsorption and the laws governing adsorption in this nanostructure are discussed. Results from the noncatalytic hydrogenation of decene-1 and naphthalene in NMRs with hydrogen adsorbed in OCNTGs are analyzed, and the kinetics and activation energies for these reactions are determined. The effect of the substrate on which graphene nanostructures are synthesized is estimated through a comparative analysis of the mechanisms of hydrogen adsorption in OCNTGs and cylindrical and planar graphene nanostructures (CPGNSes) formed on zeolites. The concept of NRMs is formulated, the directions of developing the concept are discussed. These include devising ways to create of catalytically active metal-containing sites in the NMR structure and the effect of the steric factor of the nanoreactor configuration on the selective hydrogenation of different organic substrates.

Thermal Chemical Vapor Deposition of Superblack Randomly Oriented Carbon Nanotube Coatings

Chemical Vapor Deposition In article number 1900704 by Hameeda Jagalur Basheer, Kamal Baba, and Naoufal Bahlawane, a superblack coating process is reported using thermal chemical vapor deposition at temperatures below 500 °C. Ethanol vapor is converted to uniform films of randomly oriented carbon nanotubes via the action of cobalt catalyst and MgO promoter, both are formed in situ. The superblack coatings feature a substantial light absorption property irrespective of the incidence angle. The displayed photos show a difficulty to distinguish the edges of the coated substrates. A systematic investigation is provided to determine the impact of the growth parameters on the kinetics and characteristics of the obtained films.

Enhancing Strength and Toughness of Hierarchical Composites through Optimization of Position and Orientation of Nanotubes: A Computational Study

Hierarchical composites that combine microscopic fibers and carbon nanotubes (CNTs) offer opportunities to further improve mechanical properties. Motivated by the experimental evidence that the spatial distribution of CNTs has a significant effect on the strength and toughness of these composites, we developed a novel modelling tool to help us explore mechanisms of strengthening and toughening in an efficient way. The spatial position and orientation of CNTs are chosen as design variables and their optimization is performed on the example of a unidirectional fiber-reinforced composite (FRC) subjected to transverse tensile loading. The model relies on the use of genetic algorithm and finite element method. Our modelling results show that the CNT network with an optimized morphology suppresses stress concentrations in the matrix near the fibers. The optimized morphology is shown to activate a new strengthening and toughening mechanism—diffusion of damage at micro-scale. It allows substantial increase in the consumption of the strain energy by matrix cracking, delocalization of damage, and with it, improvement of the strength and toughness. When the network morphology of 1.0 wt% of CNTs is optimized, the strength and toughness are increased by 49% and 65%, respectively, compared to the pristine FRC. The same amount of homogenously distributed CNTs in the composite leads to only 2% of the strength increase accompanied by a 13% decrease in toughness. The work emphasizes the importance of optimizing spatial position and orientation of CNTs for the strength and toughness improvements of composites.

Inhibition of α-chymotrypsin by pristine single-wall carbon nanotubes: Clogging up the active site

The preferred spatial orientation of single-wall carbon nanotubes (SWCNTs) in their interaction with enzymes determines their behavior either as nano-supports or as inhibitors. α -chymotrypsin (α-CT) is considered a serine protease model for studying nanomaterial/proteases interactions. The interaction of α-CT with pristine single-wall carbon nanotubes is still unknown. Here α-CT/SWCNT hybrids are synthesized and characterized. Spectroscopic, microscopic and kinetic measurements, coupled to molecular dynamics simulations, provide a detailed description of the interaction between α-CT and SWCNTs. The SWCNT binding pocket was unambiguously identified. A perfect match is observed with the crevice structure of the α-CT substrate binding pocket. The activity of α-CT, upon SWCNT binding, is dramatically reduced, as expected by the interaction of the SWCNT in the active site of the protein. π-π stacking between aromatic residues and the conjugated surface of SWCNT governs α-CT/SWCNT interactions. An important role in the bonding appears also for purely hydrophobic residues and with residues able to establish surfactant-like interactions. The secondary structure of α-CT and the catalytic triad structure are not perturbed by the complex formation, on the contrary the volume of the substrate binding pocket is strongly reduced by SWCNT binding because SWCNT occupies the α-CT substrate binding site, clogging the active site.

Representative Volume Element for Mechanical Properties of Carbon Nanotube Nanocomposites Using Stochastic Finite Element Analysis

AbstractThe aim of this study is to present a representative volume element (RVE) for nanocomposites with different microstructural features using a stochastic finite element approach. To that end, computer-simulated microstructures of nanocomposites were generated to include a variety of uncertainty present in geometry, orientation, and distribution of carbon nanotubes. Microstructures were converted into finite element models based on an image-based approach for the determination of elastic properties. For each microstructure type, 50 realizations of synthetic microstructures were generated to capture the variability as well as the average values. Computer-simulated microstructures were generated at different length scales to determine the change in mechanical properties as a function of length scale. A representative volume element is defined at a length scale beyond which no change in variability is observed. The results show that there is no universal RVE applicable to all properties and microstructures; however, the RVE size is highly dependent on microstructural features. Microstructures with agglomeration tend to require larger RVE. Similarly, random microstructures require larger RVE when compared with aligned microstructures.

Understanding the effect of functionalization in CNT-epoxy nanocomposite from molecular level

The excellent physical, mechanical, thermal and electrical properties of carbon nanotubes (CNTs) make them the promising reinforcements for polymer materials. The resulting nanocomposites have been applied in various applications, including but not limited to aerospace components and nanoelectronics. The mechanical performance of CNT reinforced nanocomposites is significantly affected by the interfacial interactions between CNT and polymer matrices. Enhancing the CNT-polymer interface from molecular level, such as chemical functionalization on CNTs, opens new perspectives for improving the mechanical properties of nanocomposites. Although there is a risk that functionalization may destroy the perfect surface of CNTs during fabrication, such chemical treatment can provide desired CNT dispersion state and orientation, resulting in significant improvement in mechanical properties of polymer nanocomposites. In this work, epoxy nanocomposite systems with pristine and functionalized CNTs are modeled respectively. Molecular dynamics simulations are applied to investigate the interfacial characteristics and the mechanical responses of these CNT-polymer nanocomposites. The results reveal that the functionalization can enhance the interfacial shear stress and the Young's modulus of CNT-epoxy nanocomposites. The functional groups not only provide stronger interfacial adhesion with the epoxy matrix, but also act as the sites of mechanical interlocking during deformation. The understanding of the reinforcing mechanism from atomistic level provide physical new insights to material design for advanced nanocomposites.

Enhancing flutter stability in nanocomposite thin panels by harnessing CNT/polymer dissipation

Numerical and analytical investigations into the aeroelastic response of thin nanocomposite panels are discussed. The mechanical data of the nanocomposite plates with different weight fractions of randomly oriented carbon nanotubes (CNT) are obtained via an extensive experimental campaign based on dynamic mechanical analysis (DMA). Panel flutter induced by high supersonic flows is studied through both linear and nonlinear dynamic analyses aimed to investigate the effect of the CNT weight fraction onto the flutter and post-flutter condition. To this end, multiple scales perturbation analysis is carried out to characterize the flutter boundaries and the type of Hopf bifurcation at flutter. It is shown that a few percent CNT weight fraction, while entailing negligible weight penalty, can considerably increase the flutter speed as well as reduce the limit cycle oscillations amplitude.

Nanofiber-guided orientation of electrospun carbon nanotubes and fabrication of aligned CNT electrodes for biodevice applications

Carbon nanotubes (CNTs) have quite large specific surface areas and high electron transfer rates. They are thus anticipated to be valuable for the construction of high-power biodevices, such as biosensors and biofuel cells. However, CNTs tend to aggregate due to van der Waals forces, which generates electrical resistance. Optimal use of their properties cannot be achieved simply by incorporating CNTs into electrodes. A technique for orienting CNTs on electrode surfaces is thus urgently needed. In this study, we aimed to develop a CNT alignment method to maximize the intrinsic conductive and structural properties of CNTs. We used poly(ethylene-co-vinyl acetate) (PEVA) nanofibers to guide CNT orientation during electrospinning. A mixed solution of PEVA and CNT was spun onto the surfaces of Au substrates. Following electrospinning, combustion at 400 °C pyrolyzed the PEVA nanofibers, leaving only CNTs on the substrates. After combustion, the morphology and chemical state of the CNTs were evaluated with scanning electron microscopy (SEM), field-emission SEM, and Raman spectroscopy. We then immobilized enzyme onto modified the CNTs and evaluated their electrochemical performance. Examination of the spun CNT-PEVA core-sheath nanofibers following combustion confirmed that the CNTs were oriented in the direction of the fibers. Impedance measurements of the enzyme-immobilized electrodes showed that the charge transfer resistance was lower with the aligned CNTs than with randomly oriented CNTs. The enzymatic electrochemical reaction was evaluated in cyclic voltammetry tests. We found that the CNT alignment on the electrode surface increased the current in enzyme-substrate redox reaction.

Critical buckling load of honeycomb sandwich panels reinforced by three-phase orthotropic skins enhanced by carbon nanotubes

The buckling analyses illustrated in this research aim to provide useful results for the design and application of sandwich plates with a honeycomb core and three-phase orthotropic skins, which are reinforced by both carbon nanotubes (CNTs) and straight oriented fibers. A multiscale approach is developed to this aim, which is based on the Eshelby-Mori-Tanaka scheme and the Hahn homogenization technique. The outcomes are presented in terms of critical buckling loads for different boundary conditions, lamination schemes, fiber orientation and mass fraction of CNTs, in order to prove that all these elements represent fundamental design parameters in the analysis, manufacturing and behavior of these sandwich plates. The theoretical framework is based on the Reissner-Mindlin theory for laminated plates and on the von Kármán hypothesis as far as the nonlinear terms are concerned. The kinematic model includes the Murakami’s function to deal with such peculiar mechanical configurations, which is required to capture the zig-zag effect due to the different mechanical properties of the core and the external skins. The numerical approach and the theoretical methodology are validated by means of the comparison with the experimental and the theoretical results available in the literature.

Redox-enhanced solid-state supercapacitor based on hydroquinone-containing gel electrolyte/ carbon nanotube arrays

By introducing redox active substances into electrolyte, the energy density can be effectively increased without reducing the power density. Considering the influence of ionic conductivity and environmental safety, we introduce the redox small molecule hydroquinone (HQ) into the PVA/H<sub>3</sub>PO<sub>4</sub> gel electrolyte, which then will recombine with the carbon nanotube arrays (ACNT) possessing high specific surface area and vertical orientation structure. The symmetrical “sandwich” type redox-enhanced solid-state super capacitor is then designed and prepared. We systematically study the effects of oriented structure and pore space on the electrochemical properties of the ACNT@PVA@HQ device and charge storage mechanism. With the addition of hydroquinone (0.1%, mol%), the specific capacitance of ACNT@PVA@HQ composite device increases 6.4 times compared with that of the ACNT@PVA, and maintains the extremely high rate performance and cyclic stability. When the current density increases 10 times, the specific capacitance of the device still possesses 85% of the original value. The energy storage mechanism is mainly ascribed to a diffusion-control behavior at a low scan rate while it will change into a capacitive behavior at a high scan speed. Furthermore, we prepare highly densified oriented carbon nanotube arrays (DACNT) by solvent evaporation, enhancing the mechanical stability of carbon nanotube arrays and improving the specific capacitance and energy density of the devices. Compared with the specific capacitance of ACNT and random carbon nanotube (CCNT), that of DACNT@PVA@HQ device under the current density of 11.1 mA·cm<sup>–2</sup> increases up to 385 mF·cm<sup>–2</sup> (1674 mF·cm<sup>–3</sup>), which is 6.6 times higher than that of the CCNT@PVA@HQ device and 18 times higher than that of the ACNT@PVA device. The maximum energy density can finally reach as high as 0.06 mW·h·cm<sup>–2</sup> (0.26 mW·h·cm<sup>–3</sup>), which is much better than those of many other reported CNTs-based devices. The oriented structure of the arrays effectively shortens the ion migration path of the device, achieving a good rate performance and lower internal resistance. This new type of redox-enhanced solid-state supercapacitor not only has excellent electrochemical energy storage properties, but also meets the requirements for environmental protection and safety. This design provides a new idea for developing the new energy devices in the future, which has a good prospect in practical applications.

Micro milling of polymeric micro injected specimens with randomly oriented carbon nanotube fillers

The interest in the application of carbon nanotube (CNT) composites is recently increasing in several industrial sectors. The main reason for this growing attention is the reinforcing effect of the CNTs. However, the composite use is limited by technological issues concerning the manufacturing processes when small features are required. A multistage process chain could exploit the advantages of suitable processes to enhance the control of the filler orientation. This paper investigates the feasibility of milling micro features on micro injected specimens of POM/CNT and LCP/CNT composites. Design of experiment is used to study a suitable experimental design to investigate the influence of the material and the process parameters on the machinability and the feature geometry. POM-based composites showed a better machinability and allowed a fabrication of more accurate features, while LCP showed high cutting forces and the presence of diffused burrs, preventing the fabrication of very small features.