Synthesis of aligned carbon nanotubes

Vertically aligned carbon nanotubes (ACNTs) are bundles of carbon nanotubes oriented perpendicular to a substrate, and horizontally aligned CNTs are parallel to the substrate. Their dense and orderly arrangement, along with outstanding physical and chemical properties, enables ACNTs to be used in various fields. The methods of synthesising ACNTs can be classified into single-step and double-step techniques. Thermal pyrolysis and flame synthesis are the common single-step methods, and both are relatively simple. The double-step methods, including catalyst coating and chemical vapour deposition, provide more control over the catalyst morphology. This review explores different methods used for ACNT growth, the process parameters that determine the morphology of ACNTs and the applications of structured ACNTs.     

Anisotropy in tribological performances of long aligned carbon nanotubes/polymer composites

A new continuously aligned carbon nanotubes (ACNTs) reinforced epoxy composite was fabricated by wetting as-grown arrays of ACNTs. The uniform distribution of ACNTs in the epoxy composites was confirmed by high magnification scanning electron microscopy observations. Owing to its load-transfer-favored structure, 1.1 vol.% ACNTs achieved orders of magnitude higher improvement in the wear resistance of epoxy composite compared to randomly dispersed CNTs in epoxy reported before. The research demonstrated for the first time that the use of ACNTs alone can achieve significant improvement in the wear resistance of epoxy for applications, and the specific wear rate under normal direction was stable at ∼10⁻⁶ mm³/Nm with the sliding conditions varying from 0.4 to 2.5 MPa m/s. Further, the experimental results showed that the wear performance of ACNT/epoxy composite is highly dependent on the orientation of the ACNTs relative to the sliding direction. In particular, it was found that ACNTs achieved the highest improvement in wear resistance under normal orientation whereas the least improvement was observed under parallel direction. The results are different from that of carbon fiber reinforced epoxy composites, indicating different wear mechanisms. Based on microscopic observations, new ACNTs related wear mechanisms were further proposed and discussed.     

How to achieve high electrical conductivity in aligned carbon nanotube polymer composites

Carbon nanotube reinforced polymer composites may provide a unique option for the aviation industry due to their high strength-to-weight ratio and multifunctionality. Specifically their electrical conductivity and consequent shielding capabilities can be strongly enhanced by featuring vertically aligned nanotube arrays in the polymer composites. We report here a detailed study of the electrical transport mechanisms within aligned carbon nanotube reinforced polymer composites. The experimental part of our investigation relies on extensive use of both macroscopic and high spatial resolution experimental techniques by which we shed light on the factors dominating the electrical transport, namely the contact resistance which depends on the wetting properties of CNT–metal interface, and the resistance at point-junctions which scale with the size of interconnecting tubes. Our modeling effort well describes our experimental observations and reveals the key parameters to achieve high nanocomposite intrinsic electrical conductivity and to reduce its interfacial contact resistance.     

Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review

Thermally conductive polymer composites offer new possibilities for replacing metal parts in several applications, including power electronics, electric motors and generators, heat exchangers, etc., thanks to the polymer advantages such as light weight, corrosion resistance and ease of processing. Current interest to improve the thermal conductivity of polymers is focused on the selective addition of nanofillers with high thermal conductivity. Unusually high thermal conductivity makes carbon nanotube (CNT) the best promising candidate material for thermally conductive composites. However, the thermal conductivities of polymer/CNT nanocomposites are relatively low compared with expectations from the intrinsic thermal conductivity of CNTs. The challenge primarily comes from the large interfacial thermal resistance between the CNT and the surrounding polymer matrix, which hinders the transfer of phonon dominating heat conduction in polymer and CNT.This article reviews the status of worldwide research in the thermal conductivity of CNTs and their polymer nanocomposites. The dependence of thermal conductivity of nanotubes on the atomic structure, the tube size, the morphology, the defect and the purification is reviewed. The roles of particle/polymer and particle/particle interfaces on the thermal conductivity of polymer/CNT nanocomposites are discussed in detail, as well as the relationship between the thermal conductivity and the micro- and nano-structure of the composites.     

DC and AC conductivity in epoxy resin/multiwall carbon nanotubes percolative system

This study attempts to investigate how the inclusion of Multiwalled Carbon‐NanoTubes (MWCNT) influences the DC and AC conductivity response of standard high performance epoxy systems. Towards this direction, the highly electrical conductive fillers were homogenously dispersed at various weight contents (in the range of 0.1% up to 1.0%) using a well established shear mixing protocol. The DC and AC conductivity of the prepared nanocomposites was measured. AC conductivity was examined in the frequency range from 101 to 106 HZ at ambient temperature. An enhancement of conductivity, in accordance to percolation theory, was evidenced increasing the weight content of the conductive nano‐filler. The AC conductivity was found to be frequency dependent beyond a critical frequency that increased with the nano‐filler content. It is proposed that the critical frequency follows also a percolation type law with the weight fraction of the nanotubes. A deeper analysis of all aforementioned observations highlights the multiparametric reliance of the macroscopic electrical response on the properties of the nanotubes, their electrical network topology, and their interactions with the surrounding polymer and CNTs. POLYM. COMPOS., 31:1874–1880, 2010. © 2010 Society of Plastics Engineers.     

The production of horizontally aligned single-walled carbon nanotubes

The current progress on the production of aligned single-walled carbon nanotubes (SWCNTs), particularly the horizontally aligned ones, is reviewed. There are two main categories for the alignment of SWCNTs: the post synthesis assembly and the in situ growth approaches. The post synthesis assembly approach mainly involves dispersing SWCNTs in solutions and aligning SWCNTs using spin-coating, Langmuir–Blodgett assembly, mechanical shearing, or blown bubble film techniques. The in situ growth approach produces aligned SWCNTs directly during their growth using controlled chemical vapor deposition and arc discharge techniques. The latter approach has the advantage of avoiding the defects generated during the post treatment methods, and may also be combined with other growth controls such as structure selectivity of SWCNTs and direct device patterning for scale up applications.     

Effects of carbon nanotubes and interphase properties on the interfacial conductivity and electrical conductivity of polymer nanocomposites

L_c is the minimum length of carbon nanotubes (CNTs) required for efficient transfer of filler conductivity to polymer matrix in polymer CNT nanocomposites (PCNTs). In this work, L_c is correlated with the dimensions of the CNTs and the interphase thickness. Subsequently, the interfacial conductivity as well as the effective length and concentration of CNTs are expressed by CNT and interphase properties. Moreover, a simple model for the tunneling conductivity of PCNTs is developed with these effective terms. The impacts of all parameters on L_c, the interfacial conductivity, the fraction of CNTs in the networks and the conductivity of the PCNT are explained and justified. In addition, the predictions of the percolation threshold and conductivity are compared with the experimental results of several samples. The desirable values of interfacial conductivity are achieved by thin, short and super‐conductive CNTs, high waviness and a thick interphase. However, thin and long CNTs, low waviness, a thick interphase, poor tunneling resistivity due to the polymer matrix and a short tunneling distance advantageously affect the conductivity of PCNTs, because they produce large conductive networks. The predictions also show good agreement with the experimental measurements of percolation threshold and conductivity, which confirms the developed equations. © 2020 Society of Chemical Industry     

Thermal conductivity of an aligned carbon nanotube array

The thermal conductivity of vertically aligned CNT film arrays prepared using carbon vapor deposition (CVD) on a glass substrate was examined. A modern light flash device was first used to measure the effective thermal conductivity of the combined sample consisting of a sandwiched CNT film array thermal interface material between two glass plates. The results were used in an effective thermal resistance model to estimate the thermal conductivity of the CNT film. The value obtained was higher than that found in the literature and clearly demonstrated the advantage of using aligned CNTs and thus utilizing their maximum axial thermal conductivity. It was concluded that the quality of the CNT array produced using CVD directly contributed to the high thermal conductivity value obtained for the CNT film.     

Diameter-controlled and nitrogen-doped vertically aligned single-walled carbon nanotubes

Diameter controlled and vertically aligned single-walled carbon nanotubes were synthesized from pure and mixed ethanol/acetonitrile feedstock. With increasing acetonitrile concentration in the feedstock, nitrogen incorporation into the sp² carbon network increased until saturating at approximately one atomic percent. The incorporation of nitrogen correlates with a significant diameter reduction from a mean diameter of 2.1 nm down to 0.7 nm. Heteroatom-mediated diameter control is independent of catalyst preparation and represents a versatile tool for the direct synthesis of tailored single-walled carbon nanotubes.     

Shear-induced preferential alignment of carbon nanotubes resulted in anisotropic electrical conductivity of polymer composites

Multiwalled carbon nanotubes were used as filler to furan resin in the aim of producing an electrically conducting polymer composite that may be useful for electrode applications. The orientation of the nanotubes is controlled to prepare a composite with fillers unidirectionally oriented, which may result in higher electrical conductivity at one direction and at lower nanotube loading. Using the doctor blade technique, composite films were prepared and the alignment and its effect on the electrical conductivity of the composite were investigated. It was found that the doctor blade technique induced preferential alignment of the nanotubes in composite and a higher degree of alignment is achieved in composites with lower contents of nanotubes. Also, for low contents of nanotubes, the electrical conductivity of the composite with preferentially aligned nanotubes was up to a million times higher in the direction of alignment compared to that of the composite with randomly oriented nanotubes; however, at higher contents of nanotubes, this effect was diminished. The preferential alignment of the nanotubes also caused anisotropic electrical conductivity. The alignment and distribution is thought to create more junctions between nanotubes that resulted into the formation of more conducting channels in the polymer matrix parallel to blading direction.     

AC conductivity of carbon fiber–polymer matrix composites at the percolation threshold

This paper reports results on experimental investigation of the conductivity behavior of carbon fiber filled polymer composites at the percolation threshold. Two types of carbon fiber‐epoxy matrix composites have been studied and comparison of the measured data has been made. These two types of composites differ in the surface modification of carbon fibers (in one case the surface of carbon fibers is covered with polymer beads using the microencapsulation technology, in the other their surface stayed unmodified). Experimental data reveal that surface modification of carbon fibers influences greatly the DC conductivity (percolation threshold moves to higher concentrations) but does not influence the AC electrical properties. From the frequency dependence of conductivity upon fiber concentration it becomes clear that it is not possible to predict the high frequency conductivity (electromagnetic interference shielding properties) based on the DC conductivity. Percolation behavior of conductivity as a function of conductive filler concentration is typical only for DC or low frequency AC conductivity. The percolation threshold gradually vanishes for high frequencies of electromagnetic field. The temperature dependence of electrical properties has also been studied. Composites with concentration near the percolation threshold show the switch‐off effect (at the specific temperature the DC conductivity drops by several orders of magnitude). This switch‐off effect does not occur for high frequency AC conductivity.     

Structural and morphological control of aligned nitrogen-doped carbon nanotubes

Nitrogen-doped carbon nanotubes (CN_x-NTs) were prepared using a floating catalyst chemical vapor deposition method. Melamine precursor was employed to effectively control nitrogen content within the CN_x-NTs and modulate their structure. X-ray photoelectron spectroscopy (XPS) analysis of the nitrogen bonding demonstrates the nitrogen-incorporation profile according to the precursor amount, which indicates the correlation between the nitrogen concentration and morphology of nanotubes. With the increase of melamine amount, the growth rate of nanotubes increases significantly, and the inner structure of CN_x-NTs displayed a regular morphology transition from straight and smooth walls (0 at.% nitrogen) to cone-stacked shapes or bamboo-like structure (1.5%), then to corrugated structures (3.1% and above). Both XPS and CHN group results indicate that the nitrogen concentration of CN_x-NTs remained almost constant even after exposing them to air for 5 months, revealing superior nitrogen stability in CNTs. Raman analysis shows that the intensity ratio of D to G bands (I_D/I_G) of nanotubes increases with the melamine amount and position of G-band undergoes a down-shift due to increasing nitrogen doping. The aligned CN_x-NTs with modulated morphology, controlled nitrogen concentration and superior stability may find potential applications in developing various nanodevices such as fuel cells and nanoenergetic functional components.     

Fluffy carbon nanotubes produced by shearing vertically aligned carbon nanotube arrays

Fluffy carbon nanotubes (CNTs), which are cotton-like macroscopic structures, are obtained by simple high-speed shearing of vertically aligned CNT (VACNT) arrays. The fluffy CNTs are composed of CNT bundles with a diameter of several micrometers, and have an extremely low apparent density of 3-10 g/L. A requisite for their formation is the alignment of CNTs in the initial array. The shear between the rotor and the arrays tears the arrays along the axial direction and this results in their dispersion into low density fluffy CNTs.     

Electrical conductivity of well-exfoliated single-walled carbon nanotubes

Aqueous suspensions of single-walled carbon nanotubes (SWCNTs) with controlled degree of exfoliation were used to prepare conductive thin films. Controlled exfoliation was achieved by physical separation of SWCNT bundles using our previously established nanoplatelet-dispersion method. Thin film networks of individual SWCNTs produced with this approach exhibit universal conduction behavior indicative of an isotropic network of random resistors with nearly monodisperse bond conductance distribution. Networks made of partially exfoliated SWCNTs experience a significant shift in percolation threshold because of effective local alignment of individual SWCNTs into bundles. Bundling increases the conductivity of the SWCNTs at higher concentration because of low contact resistance electron transport between metallic SWCNTs. The most significant impact of bundling is the development of non-universal electrical scaling. These findings suggest that while individually exfoliated SWCNTs should be of substantial importance for electrical devices requiring small increases in electrical conductivity at low concentration, adequate control of bundling may enable or enhance performance for applications requiring higher conductivity.     

Covering vertically aligned carbon nanotubes with a multiferroic compound

This work highlights the possible use of vertically-aligned multiwall carbon nanotubes (VA-MWCNTs) as bottom electrodes for microelectronics, for example for memory applications. As a proof of concept BiFeO₃ (BFO) films were fabricated in-situ deposited on the surface of VA-MWCNTs by RF (radio frequency) magnetron sputtering. For in situ deposition temperature of 400 °C and deposition time up to 2 h, BFO films cover the MWCNTs and no damage occurs either in the film or MWCNTs. In spite of the macroscopic lossy polarization behaviour, the ferroelectric nature, domain structure and switching of these conformal BFO films was verified by piezo force microscopy. G type antiferromagnetic ordering with weak ferromagnetic ordering loop was proved for BFO films on VA-MWCNTs having a coercive field of 700 Oe.     

Polymers with aligned carbon nanotubes: Active composite materials

We review the current state of the polymer-carbon nanotube composites field. The article first covers key points in dispersion and stabilization of nanotubes in a polymer matrix, with particular attention paid to ultrasonic cavitation and shear mixing. We then focus on the emerging trends in nanocomposite actuators, in particular, photo-stimulated mechanical response. The magnitude and even the direction of this actuation critically depend on the degree of tube alignment in the matrix; in this context, we discuss the affine model predicting the upper bound of orientational order of nanotubes, induced by an imposed strain. We review how photo-actuation in nanocomposites depend on nanotube concentration, alignment and entanglement, and examine possible mechanisms that could lead to this effect. Finally, we discuss properties of pure carbon nanotube networks, in form of mats or fibers. These systems have no polymer matrix, yet demonstrate pronounced viscoelasticity and also the same photomechanical actuation as seen in polymer-based composites.     

Electrical conductivity of interphase zone in polymer nanocomposites by carbon nanotubes properties and interphase depth

In this work, carbon nanotubes (CNT) properties and interphase depth define the interphase conductivity in polymer CNT nanocomposites (PCNT). In addition, the operative CNT length and volume portion are linked to the conductivity transportation between CNT and insulated polymer medium to propose a simple model for conductivity. The significances of various terms on the interphase conductivity and conductivity of PCNT are justified and the model's predictions are examined using the experimental outputs of certain examples. Thin CNT and dense interphase obtain the extraordinary conductivity transportation, while CNT length and conductivity are ineffective. Moreover, thin, small, and high-conductive CNT as well as dense interphase introduce the high interphase conductivity. The estimations of conductivity appropriately follow the experimental data authorizing the established model. This model is capable to substitute the conventional models owing to the assumption of innovative nanocomposite's terms.