Growth of carbon nanotubes on carbon fibers using the combustion flame oxy-acetylene method

Multi-walled carbon nanotubes (MWCNTs) were directly grown on carbon fibers (CFs) using the combustion flame oxy-acetylene method. Ferrocene deposited on the fiber surface acts as a catalyst for carbon nanotubes (CNTs) growth. The effects of ferrocene concentration on the morphology of the CNTs coating were investigated. Growth temperature ranges from 500 to 650 °C at atmospheric pressure, while growth surface is a continuous 10 × 1000 mm² tape. CNTs are produced with a dense entanglement, covering the CFs uniformly. Tube outer diameters are in the range of 20–40 nm. Tube length is quite long (about 4–5 μm) and uniform. Particularly, growth times are very short: about 0.3–0.6 s. Growth morphology and other characteristics of the as-grown tubes were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Energy dispersive X-ray (EDX) and by Raman spectroscopy.     

The electrical conductivity of carbon nanotube/carbon black/polypropylene composites prepared through multistage stretching extrusion

We reported the design of a grape-cluster-like conductive network in a polypropylene (PP) matrix, where oriented multi-walled carbon nanotubes (MWCNTs) served as branches and provided charge transport over large distances while grape-like carbon black (CB) aggregates enriched around MWCNTs and linked these conductive tubes through charge transport over small distances. The key for construction of this grape-cluster-like conductive network was the extension and orientation of MWCNTs, which was achieved in this work by multistage stretching extrusion with an assembly of laminating-multiplying elements (LMEs, which divide and recombine polymer melts). The highest efficient grape-cluster-like conductive network was obtained at a CB:MWCNT weight ratio of 6. The experimental results showed that this novel grape-cluster-like conductive network provided a low percolation threshold for PP/CB/MWCNT composites due to the synergistic effect of CB and oriented MWCNTs. When the combined CB and MWCNT content was about 6.9 vol%, the electrical resistivity of PP/CB/MWCNT composites prepared by multistage stretching extrusion with 6 LMEs decreased to only 0.63 Ω cm.     

Thermal and electrical properties of carbon nanotubes based polysulfone nanocomposites

Carbon nanotubes (CNTs)-reinforced polysulfone (PSU) nanocomposites were prepared through solution mixing of PSU and different weight percent of multi-walled carbon nanotubes (MWCNTs). Thermal properties of nanocomposites were characterized using thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA studies revealed an increase in thermal stability of the PSU/MWCNTs nanocomposites, which is due to the hindrance of the nanodispered carbon nanotubes to the thermal transfer in nanocomposites and also due to higher thermal stability of CNTs. Morphological properties of nanocomposites were characterized by high resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscope (FESEM). The influence of CNTs loading on electrical properties of PSU/MWCNTs nanocomposites was studied by the measurement of AC and DC resistivity. Dielectric study of nanocomposites was carried out at different frequencies (10 Hz–1 MHz) by using LCR meter. An increase in dielectric constant and dielectric loss was observed with increase in CNTs content, which is due to the interfacial polarization between conducting CNTs and PSU.     

Enhanced electrical conductivity of nylon 6 composite using polyaniline-coated multi-walled carbon nanotubes as additives

Aggregation in polymer composites is one of the major obstacles in the carbon nanotubes (CNTs) applications. Authentic CNTs are known to have very good electrical conductivity and mechanical strengths. Surface functionalization can avoid aggregation and help dispersion of CNTs, but reduces CNT’s electrical conductivities and mechanical strengths dramatically. It needs a good way to resolve the above dilemma situation; i.e., poor dispersion-good conductivity vs. good dispersion-poor conductivity. Herein, we demonstrate that in-situ polymerized polyaniline (PANI)-coated CNTs have good polymer matrix compatibility, and are superior electrically conductive fillers to nylon 6 composites. In this report, multi-walled CNTs (MWCNTs) were surface-modified with poly(acrylic acids) (PAA), followed by further coating with PANI. The electrical conductivity of (PANI-MWCNTs)-nylon 6 composite thin film was increased from 10⁻¹² to 7.3 × 10⁻⁵ S/cm in the presence of 1 wt% PANI-coated MWCNTs prepared by physical mixing of PANI and PAA-grafted MWCNTs. When in-situ polymerized PANI-coated MWCNTs were added, the electrical conductivity of MWCNTs-nylon 6 composite was further enhanced by 3 orders to be 3.4 × 10⁻² S/cm at the same 1 wt% loading of MWCNTs. Both Fourier-transformed infrared and uv-visible absorption spectra indicate that there exist very strong site-specific charge transfer interactions between the quinoid rings of PANI and MWCNTs, which results in the superior electrical conductivity of MWCNT-nylon 6 composite.     

Influence of wall number and surface functionalization of carbon nanotubes on their antioxidant behavior in high density polyethylene

Carbon nanotubes (CNTs) are extensively incorporated as reinforcement into polymeric materials due to their extraordinary properties. The antioxidant ability of CNTs in high density polyethylene (HDPE) was studied. Single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), and hydroxylated multi-walled carbon nanotubes (MWCNTs-OH) were involved to investigate the influence of wall number and surface functionalization of CNTs on their antioxidant behavior in HDPE. Based on measurements of the oxidation induction temperature and oxidation induction time of CNT/HDPE composites, it is found that the antioxidant ability of the three kinds of CNTs is in the following order: MWCNTs-OH > MWCNTs > SWCNTs. The antioxidant ability and mechanism of CNTs are further examined by electron spin resonance spectra and Raman spectra. It is observed that the antioxidant behavior of CNTs obeys a free radical scavenging mechanism. The order of the radical scavenging efficiency and the defect concentration for CNTs are in good agreement with that of their antioxidant ability in HDPE. With more walls and surface hydroxyl groups, the CNTs have more structural defects and exhibit higher antioxidant ability. The study raises the possibility that CNTs can improve antioxidant properties as well as mechanical properties of polymer matrix.     

Synthesis and processing of PMMA carbon nanotube nanocomposite foams

Poly(methyl methacrylate) (PMMA) multi-walled carbon nanotubes (MWCNTs) nanocomposites were synthesized by several methods using both pristine and surface functionalized carbon nanotubes (CNTs). Fourier transform infrared (FTIR) spectroscopy was used to characterize the presence and types of functional groups in functionalized MWCNTs, while the dispersion of MWCNTs in PMMA was characterized using scanning electron microscopy (SEM). The prepared nanocomposites were foamed using carbon dioxide (CO₂) as the foaming agent. The cell morphology was observed by SEM, and the cell size and cell density were calculated via image analysis. It was found that both the synthesis methods and CNTs surface functionalization affect the MWCNTs dispersion in the polymer matrix, which in turn profoundly influences the cell nucleation mechanism and cell morphology. The MWCNTs are efficient heterogeneous nucleation agents leading to increased cell density at low particle concentrations. A mixed mode of nucleation mechanism was observed in nanocomposite foams in which polymer rich and particle rich region co-exist due to insufficient particle dispersion. This leads to a bimodal cell size distribution. Uniform dispersion of MWCNTs can be achieved via synergistic combination of improving synthesis methodology and CNTs surface functionalization. Foams from these nanocomposites exhibit single modal cell size distribution and remarkably increased cell density and reduced cell size. An increase in cell density of ∼70 times and reduction of cell size of ∼80% was observed in nanocomposite foam with 1% MWCNTs.     

Influence of carbon nanotubes on thermal and electrical conductivity of zirconia-based composite

Carbon nanotubes (CNTs) are widely used in ceramic-matrix composites (CMC) as a filler. An individual carbon nanotube exhibits extremely high thermal conductivity, however, the influence of CNTs on the thermal conductivity of CMCs is moderate. In contrast, even a small quantity of CNTs significantly increases the electrical conductivity of CMCs. The present paper studies this contradictory influence for ZrO₂-CNTs composites with 3, 5, 10 and 20 vol% multi-wall carbon nanotubes (MWCNTs). Their thermal and electrical conductivity was studied by the laser flash method and electrochemical impedance spectroscopy. The analysis reveals that the moderate influence of MWCNTs on the thermal conductivity of composites originates from the similar thermal conductivity of MWCNTs in a bundle and zirconia. On the other hand, the substantial difference in the electrical conductivity of MWCNTs and zirconia leads to an exponential increase in the electrical conductivity of the ZrO₂-CNTs composite even with small additions of nanotubes.     

Reinforcement of epoxy resins with multi-walled carbon nanotubes for enhancing cryogenic mechanical properties

Epoxy resins are widely applied in cryogenic engineering and their cryogenic mechanical properties as important parameters have to be improved to meet the high requirements by cryogenic engineering applications. Carbon nanotubes (CNTs) are regarded as exceptional reinforcements for polymers. However, poor carbon nanotube (CNT)–polymer interfacial bonding leads to the unexpected low reinforcing efficiency. This paper presents a study on the cryogenic mechanical properties of multi-walled carbon nanotube reinforced epoxy nanocomposites, which are prepared by adding multi-walled carbon nanotubes (MWCNTs) to diglycidyl ether of bisphenol-F epoxy via the ultrasonic technique. When the temperature decreases from room temperature to liquid nitrogen temperature (77 K), a strong CNT–epoxy interfacial bonding is observed due to the thermal contraction of epoxy matrix because of the big differences in thermal expansion coefficients of epoxy and MWCNTs, resulting in a higher reinforcing efficiency. Moreover, synthetic sequence leads to selective dispersion of MWCNTs in the brittle primary phase but not in the soft second phase in the two phase epoxy matrix. Consequently, the cryogenic tensile strength, Young's modulus, failure strain and impact strength at 77 K are all enhanced by the addition of MWCNTs at appropriate contents. The results suggest that CNTs are promising reinforcements for enhancing the cryogenic mechanical properties of epoxy resins that have potential applications in cryogenic engineering areas.     

Can the properties of carbon nanotubes influence their internalization by living cells?

Carbon nanotubes (CNTs) are widely used for biomedical applications as intracellular transporters of (bio)molecules, due to their high propensity to cross cell membranes. However, there is a clear discrepancy in the literature about their uptake mechanism, which should be related to the differences existing in the nanotube materials, as well as the experimental procedures. Despite the fact that there are some studies on the influence of the CNT surface chemistry, the role of the properties of non-functionalized CNTs in cellular uptake has not been much investigated to a great extent. In this work, different kinds of multi-wall CNTs (MWCNTs) are produced and fully characterized, in terms of diameter, length, metal impurity, carbon soot and surface chemistry. These MWCNT samples are tested in vitro, and the cellular uptake is indirectly evaluated by using standard fluorescent probes and confirmed by TEM images. Our assays demonstrate that nanotube length clearly influences their uptake and shorter (sub-1 μm) MWCNTs are easier to be internalized through an energy-independent pathway. The results of this investigation may be useful for the design of promising CNT-based vectors for cell therapy.     

Pseudocapacitive properties of carbon nanotube/manganese oxide electrode deposited by electrophoretic deposition

Nanocrystalline manganese oxide powder prepared by spray pyrolysis (SP) was mixed with multi-walled carbon nanotubes (MWCNTs) in a suspension and subsequently co-coated onto graphite substrates via an electrophoretic deposition technique (EPD). The deposited coatings with or without CNTs mix were examined by X-ray diffractometry (XRD); the coatings were examined by cycling voltammetry (CV) and charge/discharge tests. Structural investigation by XRD revealed that the as-deposited Mn-oxide powder exhibited Mn₃O₄ phase with nanocrystalline structure. The CNTs/Mn-oxide coatings exhibited a porous structure with some hollow or fractured Mn-oxide particles inserted and possessed a high surface area for the electrochemical redox reactions. CV examination showed that the porous CNTs/Mn-oxide coatings exhibited the high specific capacitance of 260 F/g which was higher than the value (234 F/g) of Mn-oxide coatings without CNTs mix at a scan rate of 25 mV/s. After 500 cycles of CV tests, the specific capacitance decreased to 228 F/g, showing a comparatively good efficiency of 88%. The high CV cycling stability of CNTs/Mn-oxide mixture is due to the fact that the CNTs network existent in between the Mn-oxide particles inhibits the structural breakdown of Mn-oxide particles during the repetitive insertion–expulsion of cations from aqueous electrolyte.     

Evaluation of mechanical properties of untwisted carbon nanotube yarn for application to composite materials

Carbon nanotubes (CNTs) with superior mechanical properties have been of interest as reinforcement for polymer composites. However, the length of individual CNTs is limited. As a solution, yarns spun by twisting together multi-walled carbon nanotubes (MWCNTs) have been reported. In this study, untwisted CNT yarns were prepared by a non-conventional method drawing CNTs through a die. The MWCNTs in these yarns are held together by strong van der Waals forces that arise due to the interactions on the long and smooth surfaces of the MWCNTs. Here, mechanical properties of untwisted CNT yarn were studied by tensile tests. The strength of the CNT yarn was increased by increasing the apparent density of the yarn. The CNT yarns showed high tensile strength of 1 GPa and elastic modulus of 79 GPa at a yarn diameter of 35 μm. The interfacial shear strength between the CNT yarn and epoxy resin was studied by the microdroplet method, and it was very low. The wettability of the CNT yarn was affected by a type of curing agent. A unidirectional composite of epoxy resin and CNT yarn was prepared by the pultrusion molding method. Mechanical properties of the unidirectional composite were affected by the type of curing agent.     

Characterization and hydrogen storage in multi-walled carbon nanotubes grown by aerosol-assisted CVD method

The characterization and hydrogen storage capacity of multi-walled carbon nanotubes (MWCNTs) have been studied in the present work. MWCNTs with high purity and bulk yield were achieved from a mixture of camphor/alcohol on a Ni/zeolite support by aerosol-assisted chemical vapor deposition (AACVD). The morphology, surface quality and structure of MWCNTs were characterized by transmission electron microscopy (TEM). Crystallinity and defects of the MWCNTs were studied by Raman spectroscopy and thermo gravimetric analysis (TGA). Hydrogen storage properties of MWCNTs were investigated using a quartz crystal microbalance (QCM). Values between 1.2 and 2.0 wt.% of adsorbed H₂ were reached depending on the exposure pressure. The results also showed that the remaining zeolite present in the as-prepared MWCNT powder adsorbs hydrogen, allowing better adsorption performance of the CNT12 and CNT13 samples. The hydrogen adsorption behavior of CNTs is significantly affected by their structural and morphological characteristics.     

Increasing the interfacial strength in carbon fiber/epoxy composites by controlling the orientation and length of carbon nanotubes grown on the fibers

Multi-walled carbon nanotubes (MWCNTs) were grafted onto carbon fibers (CFs) using an injection chemical vapor deposition method. The orientation and length (16.6–108.6 μm) of the MWCNTs were controlled by the surface treatment of the CFs and the growth time, respectively. The interface between the MWCNTs and the CFs indicated the grafted CNTs were immobilized by embedding catalyst on CFs. Two orders of magnitude increase in the specific surface areas of CFs was obtained by grafting the MWCNT. MWCNT–CF hybrids exhibited good wettability with the epoxy resin due to the surface roughness and capillary action. Single-fiber composite fragmentation tests revealed an remarkable improvement of interfacial shear strength (IFSS) controlled by the orientation and length of MWCNTs. MWCNTs with an perpendicular alignment and long length showed a high IFSS in epoxy composites due to better wettability and a large contact interface between the hybrids and the resin. Hybrids with an optimum length (47.2 μm) of aligned MWCNTs showed a dramatic improvement of IFSS up to 175% compared to that of pristine CFs.     

Hard and tough carbon nanotube-reinforced zirconia-toughened alumina composites prepared by spark plasma sintering

It is demonstrated that 0.1 wt% of multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs) added to zirconia toughened alumina (ZTA) composites is enough to obtain high hardness and fracture toughness at indentation loads of 1, 5, and 10 kg. ZTA composites with 0.01 and 0.1 wt% of MWCNTs or SWCNTs were densified by spark plasma sintering (SPS) at 1520 °C resulting in a higher hardness and comparable fracture toughness to the ZTA matrix material. The observed toughening mechanisms include crack deflection, pullout of CNTs as well as bridged cracks leading to improved fracture toughness without evidence of transformation toughening of the ZrO₂ phase. Scanning electron microscopy showed that MWCNTs rupture by a sword-in-sheath mechanism in the tensile direction contributing to an additional increase in fracture toughness.     

Differentiation of chemical reaction activity of various carbon nanotubes using redox potential: Classification by physical and chemical structures

The present study systematically examined the kinetics of a hydroxyl radical scavenging reaction of various carbon nanotubes (CNTs) including double-walled and multi-walled carbon nanotubes (DWCNTs and MWCNTs), and carbon nano peapods (AuCl₃@DWCNT). The theoretical model that we recently proposed based on the redox potential of CNTs was used to analyze the experimental results. The reaction kinetics for DWCNTs and thin MWCNTs agreed well with the theoretical model and was consistent with each other. On the other hand, thin and thick MWCNTs behaved differently, which was consistent with the theory. Additionally, surface morphology of CNTs substantially influenced the reaction kinetics, while the doped particles in the center hollow parts of CNTs (AuCl₃@DWCNT) shifted the redox potential in a different direction. These findings make it possible to predict the chemical and biological reactivity of CNTs based on the structural and chemical nature and their influence on the redox potential.     

Efficient usage of highly dispersed Pt on carbon nanotubes for electrode catalysts of polymer electrolyte fuel cells

A Pt-deposited carbon nanotube (CNT) shows higher performance than a commercial Pt-deposited carbon black (CB) with reducing 60% Pt load per electrode area in polymer electrolyte fuel cells (PEFCs) below 500 mA/cm². K₂PtCl₄ and H₂PtCl₆·6(H₂O) are used for the Pt deposition onto multi-walled CNTs (MWCNTs), which are produced by the catalytic decomposition of hydrocarbons. The electric power densities produced using the Pt/CNT electrodes are greater than that of the Pt/CB by a factor of two to four on the basis of the Pt load per power. CNTs are thus found to be a good support of Pt particles for PEFC electrodes. TEM images show 2–4-nm Pt nanoparticles dispersed on the CNT surfaces. These high performances are considered to be due to the efficient formation of the triple-phase boundaries of gas–electrode–electrolyte. The mechanisms of Pt deposition are discussed for these Pt-deposited CNTs.     

Disruption of model cell membranes by carbon nanotubes

Carbon nanotubes (CNTs) have one of the highest production volumes among carbonaceous engineered nanoparticles (ENPs) worldwide and are have potential uses in applications including biomedicine, nanocomposites, and energy conversion. However, CNTs possible widespread usage and associated likelihood for biological exposures have driven concerns regarding their nanotoxicity and ecological impact. In this work, we probe the responses of planar suspended lipid bilayer membranes, used as model cell membranes, to functionalized multi-walled carbon nanotubes (MWCNT), CdSe/ZnS quantum dots, and a control organic compound, melittin, using an electrophysiological measurement platform. The electrophysiological measurements show that MWCNTs in a concentration range of 1.6–12 ppm disrupt lipid membranes by inducing significant transmembrane current fluxes, which suggest that MWCNTs insert and traverse the lipid bilayer membrane, forming transmembrane carbon nanotubes channels that allow the transport of ions. This paper demonstrates a direct measurement of ion migration across lipid bilayers induced by CNTs. Electrophysiological measurements can provide unique insights into the lipid bilayer–ENPs interactions and have the potential to serve as a preliminary screening tool for nanotoxicity.     

Slipping of carbon nanotubes in a rubber matrix

The interactions of carbon nanotubes (CNTs) and carbon black (CB) with rubber matrices are of great interest. Although both belong to the carbon filler family, their interactions are different. In this study the adhesion of CNTs, if any, with natural rubber (NR) was examined. Scanning electron microscopy examinations made on cryogenically fractured surfaces of a crosslinked NR sample containing 7% by weight of CNTs showed that the CNT bundles emerged from the side surface (narrowed by Poisson's ratio) and slowly slid back in when the deformation was removed. The protruded lengths were many times larger than the nanotube bundle diameters. This extensive slipping out of CNTs from the rubber matrix suggests that interfacial interactions between CNTs and NR are quite weak. In contrast, relatively strong interactions were found between CB and rubber, indicated by the large amount of bound rubber formation. Reinforcement of rubber by CNTs is therefore attributed to the large aspect ratio of CNT bundles. Physical entanglement with rubber molecules is then able to generate effective load transfer, replacing the strong adhesion found with CB. Copyright © 2010 Society of Chemical Industry     

Synthesis of bamboo-like carbon nanotubes on a copper foil by catalytic chemical vapor deposition from ethanol

Bamboo-like carbon nanotubes (CNTs) were synthesized on a copper foil by catalytic chemical vapor deposition (CVD) from ethanol. The effects of temperature (700–1000 °C) and duration (5–60 min) on the growth of CNTs were investigated. Morphology and structure of the CNTs were characterized by scanning and transmission electron microscopy and Raman spectroscopy. The yield and size of the CNTs increased with temperature. Those prepared at 700 °C had a copper droplet tip and those at 800–900 °C had a copper nanoparticle inside. An amorphous carbon film consisted of a porous and non-porous layer was observed on the surface of the copper substrate, and the CNTs were really grown from this carbon film. The thickness of the carbon film increased from 187 to 900 nm when the duration increased from 5 to 60 min. It was also found that the copper foil became porous after ethanol CVD treatment. The growth mechanism of the CNTs, carbon film and motion of copper catalyst were discussed. It is proposed that a carbon film first deposited on the top surface of the copper foil while the top surface of the copper foil partially melted and migrated across the carbon film, where CNTs formed.