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.     

Photocatalytic properties of mesoporous TiO2 nanocomposites modified with carbon nanotubes and copper

The photocatalytic activity of mesoporous TiO₂ modified by the addition of carbon nanotubes (CNTs) and Cu is reported. Nanocomposites of carbon nanotubes (CNTs) containing varying amounts of Cu were formed by treatment with Cu²⁺ then reduced to Cu⁰ using NaBH₄ as the reducing agent. The mesoporous TiO₂, synthesized by a sol-gel method from titanium isopropoxide, was combined with the CNT/Cu nanocomposites to form the photocatalysts which were characterized by XRD, SEM, TEM, FTIR, XPS and BET surface area analysis. The photocatalytic properties of the mesoporous TiO₂ composites were studied by measuring the degradation of methyl orange (MO) which was optimal in the sample containing 20 wt% of the Cu-CNT nanocomposite. The degradation efficiency for MO was a synergistic effect of photo-degradation of TiO₂ and may be due to improvement of the electrical conductivity of the system by the presence of the CNT/Cu networks, since the photodegradation of MO and the photocatalytic activity of the photoactive systems increased with increasing copper content.     

CVD synthesis of coal-gas-derived carbon nanotubes and nanocapsules containing magnetic iron carbide and oxide

Iron carbide-oxide filled carbon nanotubes and nanocapsules (CNCs) are separately synthesized by catalytic chemical vapor deposition of coal gas at 950 °C with ferrocene as catalyst. The products are examined using transmission electron microscopy and XRD techniques, showing that nanosized iron carbide-oxide particles are encapsulated by well ordered carbon layers. Magnetic moment measurement reveals that these carbon encapsulated iron carbide-oxides exhibit large magnetic coercivity at room temperature. It has been found that the filled CNCs are corrosion-proof and stable in hydrochloric acid. The effect and interaction between different gaseous components in the coal-gas during the formation of magnetic iron carbide-oxide filled carbon nanostructures are discussed.     

Hollow nickel microspheres covered with oriented carbon nanotubes and its magnetic property

A composite structure consisting of hollow nickel microspheres coated with oriented carbon nanotubes (CNTs) was synthesized via chemical vapor deposition at 800 °C. The hollow microspheres were composed of discrete Ni nanoparticles which acted as the catalyst. FESEM images showed that the CNTs grew on the surface of the Ni spheres. HRTEM, Raman and XRD analyses showed that the CNTs were highly graphitized. Magnetic measurements demonstrated that these composite structures exhibited enhanced ferromagnetic behaviour compared with hollow Ni spheres and bulk Ni.     

Multiwalled carbon nanotubes and nanofibers grafted with polyetherketones in mild and viscous polymeric acid

Direct covalent attachment of amorphous and semicrystalline polyetherketones onto the surface of either an as-received multi-walled carbon nanotube (MWNT) or a vapor-grown carbon nanofiber (VGCNF) in polyphosphoric acid (PPA) with optimized P₂O₅ content resulted in uniform grafting of polyetherketones to these carbon nanoscale materials. Soxhlet extraction experiment, the spectra from FT-IR spectroscopy and the clear images from high-resolution transmission electron microscopy showed that the covalent attachment is effective in uniformly coating the PEK grafts on the surfaces of both MWNT and VGCNF. Additionally, a drastic increase in solution viscosity due to the formation of giant molecules was monitored during polymerization. As such, the resulting nanocomposites were easily fabricated via a simple compression molding technique. The alignment possibility of MWNT and VGCNF grafted with semicrystalline PEK in these thermoplastic nanocomposites via solution fiber spinning was also demonstrated.     

Polymeric nanocomposite films from functionalized vs suspended single-walled carbon nanotubes

The reported work was to demonstrate that the defect-derived photoluminescence in functionalized single-walled carbon nanotubes could be exploited in probing the dispersion of these nanotubes in polymeric nanocomposites because the luminescence emissions are sensitive to the degree of nanotube bundling and surface modification. The polyimide-SWNT nanocomposite thin films obtained from nanotubes with and without functionalization were compared. The spectroscopic results suggest that despite a similar visual appearance in the two kinds of films, the nanotube dispersion must be significantly better in the film with functionalized nanotubes, as reflected by the strong photoluminescence. In fact, the nanotubes embedded in polymer matrix that can be readily characterized by Raman spectroscopy are non-luminescent, while those that are difficult for Raman are strongly luminescent. Therefore, Raman and photoluminescence serve as complementary tools in the investigation of nanocomposites concerning the nanotube dispersion-related properties.     

Investigation on flame retardancy and rheological and thermomechanical characterisation of multiwall carbon nanotube reinforced nanocomposites

Abstract

In the present work, the influence of multiwalled carbon nanotubes (MWCNTs) on the flame retardancy and rheological, thermal and mechanical properties of polybutilen terephthalate (PBT) and polypropylene (PP) matrixes has been investigated. The carbon nanotube content in the thermoplastic materials was 2 and 5?wt‐%. The nanocomposites were obtained by diluting a masterbatch containing 20?wt‐% nanotubes using a twin‐screw extruder and the thermal properties were analysed by differential scanning calorimetry and thermogravimetric analysis; thermomechanical properties were determined by dynamic mechanical thermal analysis and the rheological behaviour was studied by a Thermo Haake Microcompounder. The results concerning the flame retardancy show that the MWCNTs are not equally effective as flame retardants in PP and PBT. The ignition time is increased only for PBT whereas the extinguishing time is decreased for PP and PBT. The reinforcement of the thermoplastics with multiwall carbon nanotubes is improved regarding the mechanical and thermal properties of the nanocomposites compared to pristine materials and the behaviour of thermoplastic nanocomposites regarding fire retardancy depends on the nature of the polymeric matrix.     

Facile synthesis and electrochemical capacitance of composites of polypyrrole/multi-walled carbon nanotubes

Composites of polypyrrole (PPy) and multi-walled carbon nanotubes (MWCNTs) were synthesized by a facile method involving one-step electrochemical deposition from a thin-layer of ionic liquid solution attached on a glassy carbon electrode. The morphology of the composites was characterized by field emission scanning electron microscopy, and the capacitance properties were investigated by cyclic voltammetry (CV). The charge-discharge behavior of the composites prepared in this work was examined by chronopotentiometry at a constant current density for multi-cycle scans. The results show that the PPy/MWCNT composites have a porous 3D nanostructure, with high specific capacitance (SC) of 890 F/g (for the mass of the PPy in the composites) calculated from CV at 2 mV/s in 1.0 M KCl. The stability of the composites in 1.0 M KCl electrolyte was also examined by multi-cycle CV and only 9% decrease of SC value was observed for the 1000 cycles.     

Easy synthesis of carbon nanotubes with polypyrrole nanotubes as the carbon precursor

An easy synthesis route for carbon nanotubes with polypyrrole nanotubes as a carbon precursor has been developed. Polypyrrole nanotubes were fabricated via a reactive self-degraded template method. Carbon nanotubes were further obtained by pyrolysis of the polypyrrole nanotube at 900 °C under a nitrogen atmosphere. The resultant carbon nanotube structure was found to be amorphous carbon on the basis of XRD, Raman spectra and high-resolution transmission electron microscopy (HRTEM) studies.     

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.     

Constructing polymer brushes on multiwalled carbon nanotubes by in situ reversible addition fragmentation chain transfer polymerization

A fascinating nanoobject, diblock polymer brushes with a hard core of multiwalled carbon nanotubes (MWNTs) and a relatively soft shell of poly(methylmethacrylate)-block-polystyrene (PMMA-b-PS), was easily constructed by in situ reversible addition fragmentation chain transfer polymerization (RAFT) of methylmethacrylate followed by styrene (St) on the modified convex surfaces of MWNTs (MWNT-PMMA). The structure and morphology of the hybrid nanomaterials were characterized by FTIR, TEM, SEM, NMR, DSC and TGA. The results showed that both styrene and acrylate type monomers can be easily initiated and then propagated on the MWNT sidewalls via the in situ RAFT approach, and the length of the PS blocks increases with increasing St:MWNT-PMMA weight feed ratio.     

Electrically conductive and super-tough polyamide-based nanocomposites

Polymer nanocomposites can exhibit superior multi-functional properties if they possess phase separated morphology at the nanoscale. Despite the huge potential of these materials, there are several fundamental issues including the ultimate microstructures, which need to be resolved to tailor different physical and mechanical properties required for specific applications. A ‘ternary nanocomposites’ approach is adopted to prepare electrically conductive and super-tough¹ (in terms of notched impact energy) hybrid polymer nanocomposites (polyamide 6/carbon nanotube/elastomer) that possess better properties than either of the constituent binary polymer nanocomposites (polyamide 6/carbon nanotubes and polyamide 6/elastomer). The individual roles of the nano-fillers involved in achieving multi-functionality are emphasized. The level of property enhancements of ternary nanocomposites depends essentially on the microstructure inducing a volume exclusion effect and the capability of fillers to activate the plastic deformation mechanisms in the matrix.     

In situ preparation and continuous fiber spinning of poly(p-phenylene benzobisoxazole) composites with oligo-hydroxyamide-functionalized multi-walled carbon nanotubes

A graft-from approach has been performed to achieve covalent functionalization of multi-walled carbon nanotubes (MWNTs) with oligo-hydroxyamide (oHA). Pristine MWNT was first oxidized to MWNT-COOH and then functionalized to MWNT-COCl by acyl chloride. MWNT-COCl was copolymerized with oHA to produce oHA-grafted MWNTs (MWNT-oHA). The thickness of the oHA shell in MWNT-oHA is about 7.5 nm. MWNT-oHA has a remarkable solubility in polar solvents and a good thermal stability because characteristic dehydrative ring closure occurs upon heating and forms a thermally more stable benzoxazole component. MWNT-oHA has been further covalently incorporated with a rigid-rod polymer matrix, poly(p-phenylene benzobisoxazole) (PBO), through in situ polymerization. Continuous PBO–MWNT composite fibers with different MWNT compositions have been fabricated using dry-jet wet-spinning technique. The structure and morphology of PBO–MWNT composite fibers have been characterized and their mechanical, thermal, conducting properties have been investigated. The tensile modulus, tensile strength, and thermal stability of PBO–MWNT composite fibers have been improved because of a good dispersion and high alignment of MWNTs in PBO as well as enhanced interfacial interaction between these two components. Furthermore, increased conductivity has been discovered in the PBO–MWNT composite films and the inner core of the composite fibers; however, not on the outer surface. The phenomena can be interpreted using percolation model together with the heterogeneous fiber morphology and nanotube distribution over the cross-section of the fiber.     

Semi-continuous production of multiwalled carbon nanotubes using magnetic field assisted arc furnace

A high-temperature arc furnace with an applied external magnetic field has been used to grow carbon nanotubes. The magnetic field was able to spread and stabilize the plasma enabling the use of larger electrodes than could be used successfully with no magnetic field. By having a stable plasma across the entire anode surface, larger amounts of carbon black were able to be transformed into carbon nanotubes. In addition, a multiple-pronged anode was designed. The use of the pronged anode created a semi-continuous process which allowed for the amount of nanotubes produced per run to increase.     

Copolyester nanocomposites based on carbon nanotubes: reinforcement effect of carbon nanotubes on viscoelastic and dielectric properties of nanocomposites

Nanocomposites based on an amorphous copolyester, poly(ethylene glycol‐co‐cyclohexane‐1,4‐dimethanol terephthalate) and carbon nanotubes were fabricated using a simple melt processing technique. The reinforcement effect of carbon nanotubes in the copolyester was investigated experimentally using different approaches based on dynamic mechanical analysis, rheology and dielectric analysis. The nanocomposites show a mechanical reinforcement effect with significant increase in the stiffness especially in the rubbery regime with increasing nanotube content. An increase in T_g and a decrease in damping are seen, which are derived from the presence of a percolating superstructure of the filler. Rheological experiments show an increase in storage modulus up to four orders of magnitude. Viscolelastic characterization shows that the percolation threshold is at 3 wt% of nanotubes. Dielectric relaxation spectroscopy confirms the presence of this percolating structure. We conclude that the responses of both rheological and electrical properties are different, although both are related to the formation of a percolating network superstructure of the filler. Copyright © 2007 Society of Chemical Industry     

Spectroscopic evidence for the bulk polymerization of N-vinyl carbazole in the presence of single-walled carbon nanotubes

The bulk polymerization reaction of N-vinylcarbazole (VK) at 70 °C in the presence of single-walled carbon nanotubes (SWNTs) leads to a new composite, whose optical properties were studied by photoluminescence (PL), surface enhanced Raman scattering (SERS) and Fourier transform infrared (FTIR) spectroscopies. A dramatic reduction of the poly(N-vinylcarbazole) (PVK) PL efficiency and a change in the vibrational structure of the PL spectrum of this polymer were observed by adding SWNTs to the synthesis mixture. Steric hindrance effects were evidenced both in SERS spectra of the VK when it interacts mechanico-chemically with SWNTs and in FTIR spectra of the un-doped PVK/SWNTs' composites. Cyclic voltammetry was used to demonstrate the doping process of PVK in PVK/SWNTs' composite.     

Supercritical carbon dioxide-assisted silanization of multi-walled carbon nanotubes and their effect on the thermo-mechanical properties of epoxy nanocomposites

Supercritical carbon dioxide was employed as the solvent for the functionalization of multi-walled carbon nanotubes (MWCNTs) with an epoxy-capped silane. The silanization protocol was found to be a suitable green alternative to traditional routes that rely on organic solvents for grafting nearly monolayers of silane molecules onto the nanotube surfaces. The addition of silanized MWCNTs to a model epoxy markedly increased its T_g, and measurements of the network cooperativity length scale linked this change to a reduction in polymer segment mobility. Composites filled with low loading levels of both pristine and silanized MWCNTs exhibited significantly higher strain at break and toughness than the neat epoxy, and the greatest improvements were observed at low loading levels. SEM analysis of the composite fracture surfaces revealed that nanotube pullout was the primary failure mechanism in epoxy loaded with pristine MWCNTs while crack bridging predominated in composites containing silanized MWCNTs as the result of strong interfacial bonding with the matrix. The elevated T_g and toughness achieved with small additions of silanized MWCNTs promise to extend the engineering applications of the epoxy resin.     

Fex-Co1-x bimetallic catalysts for highly efficient growth of carbon nanotubes on carbon fibers

This paper aims to develop a catalyst for highly efficient growth of carbon nanotubes (CNTs) on carbon fibers (CFs) with improved tensile strength by chemical vapor deposition (CVD). The effect of the composition of Fe_x-Co_1-x catalysts on the morphology of the synthesized CNTs and the tensile properties of the CFs at different stages of the CVD process were explored. It was found that the bimetallic catalysts possessed better catalytic effect than monometallic catalysts at low temperature. The detailed study of the tensile properties of CFs at different stages of CVD process manifested that the composition of catalysts affected the catalytic activity. When the atomic fraction of Fe was 50%, the bimetallic catalyst among Fe_x-Co_1-x family was considered to hold the highest catalytic efficiency due to the least damage to the fibers and the ability to generate more carbon atoms to repair and reinforce CFs. The corresponding tensile strength of the final CNTs-grafted CFs was 11.53% higher than that of the desized CFs.