Oriented and exfoliated single wall carbon nanotubes in polyacrylonitrile

Polyacrylonitrile (PAN)/single wall carbon nanotubes (SWNT) fibers were gel spun at 0, 0.5, and 1 wt% SWNT content to a draw ratio of 51. Structure, morphology, and mechanical and dynamic mechanical properties of these fibers have been studied. PAN/SWNT composite exhibited much higher electron beam radiation resistance than PAN. As a result, PAN lattice images could be easily observed in the composite fiber by high resolution transmission electron microscopy. The PAN/SWNT composite fiber also exhibited higher solvent resistance than the control PAN fiber. UV-vis spectroscopy of highly drawn fiber exhibited van Hove transitions, suggesting SWNT exfoliation upon drawing. SWNT exfoliation was also confirmed by high resolution transmission electron microscopy (HRTEM). At 1 wt% SWNT loading, fiber storage modulus (at 1 Hz) increased by 13.9, 6.6, and 0.2 GPa at −75, 25, and 150 °C, respectively. This suggests that the load transfer ability and hence interfacial strength is increasing with decreasing temperature, even below the polymer's γ transition temperature.     

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.     

Carbon nanotube dispersion and exfoliation in polypropylene and structure and properties of the resulting composites

Nitric acid treated single and multi wall carbon nanotubes (SWNT and MWNT) have been dispersed in polypropylene using maleic anhydride grafted polypropylene (MA-g-PP) and butanol/xylene solvent mixture. SWNT exfoliation was characterized by Raman and UV-vis-NIR spectroscopies. Evidence for hydrogen bonding between maleic anhydride grafted polypropylene and nitric acid treated nanotubes was obtained using infrared spectroscopy. Polypropylene/carbon nanotube composites were melt-spun into fibers. Dynamic mechanical studies show that for fibers containing 0.1 wt% SWNT, storage modulus increased by 5 GPa at −140 °C and by about 1 GPa at 100 °C, suggesting temperature dependent interfacial strength. The crystallization behavior has been monitored using differential scanning calorimetry and optical microscopy. Control fibers exhibited 27% shrinkage at 160 °C, while the shrinkage in the composite fibers was less than 5%. Fibers heat-treated to 170 °C show very narrow polypropylene melting peak (peak width about 1 °C).     

Structure changes of single-wall carbon nanotubes and single-wall carbon nanohorns caused by heat treatment

Raman spectra and transmission electron microscope images showed that diameter enlargement of HiPco, a kind of single-wall carbon nanotube, accompanied by tube-wall corrugation was caused by heat treatment (HT) at 1000 to 1700 °C. Further enlargement accompanied by straightening of the tube walls and incorporation of carbon fragments within the tubes became obvious after HT at 1800 to 1900 °C. The transformation of some single-wall carbon nanotubes into multi-wall nanotubes was observed after HT at 2000 °C, and most single-wall tubes were transformed into multi-wall ones by HT at 2400 °C. What influence the Fe contained in the HiPco tubes had on these structure changes was unclear; similar changes were observed in single-wall carbon nanohorns that did not contain any metal. This indicates that thermally induced changes in the structure of single-wall carbon nanotubes can occur without a metal catalyst. Heat treatment increased the integrity of the nanotube-papers, and this increase may have been due to tube-tube interconnections created by HT.     

Stabilization and carbonization of gel spun polyacrylonitrile/single wall carbon nanotube composite fibers

Gel spun polyacrylonitrile (PAN) and PAN/single wall carbon nanotube (SWNT) composite fibers have been stabilized in air and subsequently carbonized in argon at 1100 °C. Differential scanning calorimetry (DSC) and infrared spectroscopy suggests that the presence of single wall carbon nanotube affects PAN stabilization. Carbonized PAN/SWNT fibers exhibited 10-30 nm diameter fibrils embedded in brittle carbon matrix, while the control PAN carbonized under the same conditions exhibited brittle fracture with no fibrils. High resolution transmission electron microscopy and Raman spectroscopy suggest the existence of well developed graphitic regions in carbonized PAN/SWNT and mostly disordered carbon in carbonized PAN. Tensile modulus and strength of the carbonized fibers were as high as 250 N/tex and 1.8 N/tex for the composite fibers and 168 N/tex and 1.1 N/tex for the control PAN based carbon fibers, respectively. The addition of 1 wt% carbon nanotubes enhanced the carbon fiber modulus by 49% and strength by 64%.     

Formation of polymer/carbon nanotubes nano-hybrid shish-kebab via non-isothermal crystallization

Multi-walled carbon nanotubes (MWNTs) periodically decorated with polyethylene (PE) lamellar crystals had been prepared using the non-isothermal crystallization method. The morphology and structure of polyethylene attached to MWNTs were investigated by means of transmission electron microscopy (TEM). A nano-hybrid shish-kebab (NHSK) structure was observed wherein the average diameter of PE lamellar crystals varies from 30 to 150 nm with average periodicity of 35-80 nm. The TEM images of samples obtained at 125 °C showed that MWNTs were first wrapped by a homogeneous coating of PE with few subglobules, then PE chains epitaxially grew from the subglobule and formed lamellar crystals perpendicular to the carbon nanotube axis. It is suggested that the homogeneous coating plays a key role in the formation of NHSK structures. And the formation process was discussed based on the intermediate state images of samples obtained at 95 °C. While NHSK structures cannot be formed by using polypropylene (PP). This may attribute to the zigzagged conformation of PP chains on the surface of MWNTs, which hinders the formation of homogeneous coating of PP on it.     

High temperature rearrangement of disordered nanoporous carbon at the interface with single wall carbon nanotubes

Composites of nanoporous carbon and single wall carbon nanotubes were heat treated in vacuum at temperatures ranging from 1200 to 2000 °C. The resultant interface between the two allotropes of carbon was characterized using high resolution transmission electron microscopy and Raman spectroscopy. At the interface between the nanoporous carbon and the nanotube, the nanotube served as a template for ordering and orientation of the normally disordered nanoporous carbon along the nanotube axis during high temperature treatment. When annealed at 2000 °C, the nanoporous carbon transformed to graphitic nanoribbon which in turn crushed the nanotube to form a nanoscale carbon “bulb”. This result is interesting since at these temperatures, the native nanoporous carbon is well known to resist ordering and is therefore referred to as being a “non-graphitizing” carbon. That the nanotube should act as a template for the incipient graphitization suggests that bonding and strength for load transfer may be developed at these interfaces.     

Synthesis of close-packed multi-walled carbon nanotube bundles using Mo as catalyst

A simple mixture of porous magnesium oxide and commercial molybdenum oxide shows high efficiency for the synthesis of carbon nanotubes through the catalytic decomposition of methane at 900 °C. Field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, and transmission electron microscopy (TEM) were used to characterize the products. The results indicate that close-packed multi-walled carbon nanotube (MWCNT) bundles were synthesized and the carbon nanotubes restricted within the bundles were about 5-7 nm in diameter. A growth mechanism for the bundles was suggested based on the FE-SEM images of bundles produced using different reaction times, and the X-ray diffractions of the raw products grown at the initial stage. Raman spectroscopy and FE-SEM results on the bundles grown using different methane flow rates confirmed the growth mechanism of close-packed MWCNT bundles.     

Tin dioxide/carbon nanotube composites with high uniform SnO2 loading as anode materials for lithium ion batteries

SnO₂/multi-walled carbon nanotube (MWCNT) composites were prepared by the solvothermal method and subsequent heat treatment at 360 °C. The samples were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Results on the higher SnO₂ content composite sample indicate that a uniform layer of SnO₂ nanocrystals with crystal size around 5 nm was deposited on the surface of the carbon nanotubes. The composite demonstrates a reversible lithium storage capacity of 709.9 mAh g⁻¹ at the first cycle and excellent cyclic retention up to 100 cycles as anode for lithium ion batteries.     

Preparation and characterization of one-dimensional SiC-CNT composite nanotubes

One-dimensional silicon carbide-carbon nanotube composite was fabricated using a direct solid reaction process between pure silicon powders and carbon nanotubes at temperatures ranging from 1300 to 1400 °C. Scanning electron microscopy, X-ray diffraction, Raman scattering, and cathodoluminescence techniques were used to characterize their structures. The as-prepared composite products have shown pronounced luminescent properties with two emission bands centered at 2.3 eV and 2.9 eV by room-temperature cathodoluminescence.     

Halloysite nanotubes as a novel β-nucleating agent for isotactic polypropylene

The nucleating ability of halloysite nanotubes (HNTs) towards isotactic polypropylene (iPP) was investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), polarized optical microscopy (POM) and scanning electron microscopy (SEM). HNTs are identified to have dual nucleating ability for α-iPP and β-iPP under appropriate kinetics conditions. The formation of β-iPP is dependent on the HNTs loading in the iPP/HNTs composites. The composite with 20 phr of HNTs is found to have the highest content of β-iPP. Under non-isothermal crystallization the content of β-iPP increases with decreasing of the cooling rate. The maximum β-crystal content is obtained at cooling rate of 2.5 °C/min. The supermolecular structure of the β-iPP is identified as β-hedrites with flower-cup-like and axialite-like arrangements of the lamellae. Under isothermal crystallization the β-crystal can be formed in the temperature range of 115-140 °C. Outside the temperature range, no β-iPP can be observed. The content of β-crystal reaches the maximum value at crystallization temperature of 135 °C. The formation of the β-iPP in the composites is correlated to the unique surface characteristics of the HNTs.     

A comparison of reinforcement efficiency of various types of carbon nanotubes in polyacrylonitrile fiber

Polyacrylonitrile (PAN)/carbon nanotubes (CNTs) composite fibers were spun from solutions in dimethyl acetamide (DMAc), using single wall (SWNTs), double wall (DWNTs), multi wall (MWNTs) carbon nanotubes, and vapor grown carbon nanofibers (VGCNFs). In each case, CNT content was 5 wt% with respect to the polymer. Structure, morphology, and properties of the composite fibers have been characterized using X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopy, tensile tests, dynamic mechanical tests, as well as thermal shrinkage. While all nanotubes contributed to property improvements, maximum increase in modulus (75%) and reduction in thermal shrinkage (up to 50%) was observed in the SWNT containing composites, and the maximum improvement in tensile strength (70%), strain to failure (110%), and work of rupture (230%) was observed in the MWNTs containing composites. PAN orientation is higher in the composite fiber (orientation factor up to 0.62) than in the control PAN fiber (orientation factor 0.52), and the PAN crystallite size in the composite fiber is up to 35% larger than in the control PAN (3.7 nm), while the overall PAN crystallinity diminished slightly. Nanotube orientation in the composite fibers is significantly higher (0.98 for SWNTs, 0.88 for DWNTs, and 0.91 for MWNTs and VGCNFs) than the PAN orientation (0.52-0.62). Improvement in low strain properties (modulus and shrinkage) was attributed to PAN interaction with the nanotube, while the improvement in high strain properties (tensile strength, elongation to break, and work of rupture) at least in part is attributed to the nanotube length. Property improvements have been analyzed in terms of nanotube surface area and orientation.     

Growth of multiwalled carbon nanotubes during the initial stages of aerosol-assisted CCVD

We report a study of the initial stages of growth of aligned multiwalled carbon nanotubes (MWNT) synthesised by catalytic chemical vapour deposition (CCVD) of liquid aerosol obtained from toluene/ferrocene solution. A special experimental procedure has been developed to stop the process after short durations (30 s to 2 min). Two different pyrolysis temperatures are considered: 800 and 850 °C. Both scanning and transmission electron microscopy (SEM, TEM) coupled to energy-dispersive X-ray (EDX) analyses are used in order to determine the location of catalyst particles and to examine their chemical nature, morphology and size distribution when nanotubes start to grow. During the early stages (30 s), we observe the formation of a layer of catalyst particles on silicon substrates before the growth of nanotubes. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements indicate the occurrence of iron oxide (γ-Fe₂O₃ or Fe₃O₄). In addition, XPS analysis reveals the formation of graphite-like carbon, demonstrating that iron oxide particles catalyse the decomposition of toluene vapour. SEM and TEM observations show that these particles are most often located at the nanotube root, suggesting a base growth mechanism responsible for the formation of aligned nanotube when prolonging growth time (2 min).     

The thermal and mechanical properties of a polyurethane/multi-walled carbon nanotube composite

A polyurethane/multi-walled carbon nanotube elastomer composite was synthesized. The microstructure of the composite was examined by field-emission scanning electron microscopy and transmission electron microscopy. The thermal and mechanical properties of the composite were characterized by dynamic mechanical thermal analysis, thermogravimetric analysis and tensile testing. The chemical linkage of carbon nanotubes with polyurethane matrix was confirmed by Fourier transform infrared spectra. The study on the structure of the composite showed that carbon nanotubes could be dispersed in the polymer matrix well apart from a few of clusters. The results from thermal analysis indicated that the glass transition temperature of the composite was increased by about 10 °C and its thermal stability was obviously improved, in comparison with pure polyurethane. The investigation on the mechanical properties showed that the modulus and tensile strength could be obviously increased by adding 2 wt% (by weight) CNT to the matrix.     

Preparation and band gap energies of ZnO nanotubes, nanorods and spherical nanostructures

Nanostructured zinc oxides were prepared with no catalysts or substrates used. The advantages of this method were a direct and a relatively easy way of getting large amounts of different morphologies of nanostructured ZnO. Another advantage was the formation of the nanotubes at a relatively lower temperature than most other methods. This presented a promising way for commercialization of the ZnO nanomaterials. The materials were characterized using simultaneous thermogravimetric analysis, X-Ray diffraction and transmission electron microscopy. Structures such as nanotubes, nanorods and spherically shaped crystals were formed at certain annealing temperatures. It was found that the ZnO nanotubes grew in the direction of the c-axis. For comparisons of size and morphology, the ZnO precursors were annealed at 300 °C, 400 °C and 700 °C. The lattice parameter of the ZnO nanotube was smaller than conventional micron sized materials. It was also found that the band gap energies of the ZnO nanomaterials were dependent on the morphology of the nanostructures.     

Electrochemical lithium storage of sodium titanate nanotubes and nanorods

Layered hydrated sodium titanate nanotubes are synthesized via a hydrothermal reaction in alkaline solution. The as-prepared nanotubes are calcined at different temperatures (300-600 °C) in air. The microstructure of obtained samples is characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It is observed that the calcined products maintain their parent tubular morphologies below 500 °C. After calcinations at 600 °C, the hollow tubular morphology could completely be converted to the short solid nanorod morphology. In the meanwhile, the monoclinic sodium hexatitanate as a main phase is formed in nanorods, coexisted with sodium trititanate as a residual phase. The electrochemical lithium storage of obtained samples is studied by galvanostatic method and cyclic voltammetry. It is demonstrated that the nanotubes calcined at 500 °C have relatively large reversible capacity, good reversibility and excellent high rate discharge capability. The lithium intercalation process is shown to have pseudocapacitive feature caused by their layered structure and open lithium insertion tunnels, which is in favor of the high rate charge/discharge capability of sodium titanate nanotubes.     

Solid-phase synthesis of multi-walled carbon nanotubes from butadiynyl-ferrocene-containing compounds

Multi-walled carbon nanotubes (MWCNTs) have been synthesized from novel butadiynyl-ferrocene-containing compounds. The formation of the MWCNTs occurs in the solid phase at ambient pressure in a typical high-temperature furnace. Heat treatment of the various compounds to temperatures up to 1300 °C under atmospheric pressure resulted in the decomposition of the ferrocene units and the formation of Fe nanoparticles in the polymeric-to-carbon nanoparticle-to-carbon nanotube compositions. The Fe atoms, clusters, and/or nanoparticles are the key to the formation of the carbon nanotubes. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy studies show the presence of a large quantity of MWCNTs in the carbonaceous solid residue.     

Oriented growth of single-walled carbon nanotubes on a MgO(0 0 1) surface

Oriented growth of carbon nanotubes lying on a single-crystal MgO(0 0 1) surface is reported. The nanotubes were grown at 900 °C by chemical vapour deposition using a mixture of carbon monoxide and hydrogen, with Co catalyst nanoparticles formed by self-assembling on MgO under ultra high vacuum conditions. Field emission scanning electron microscopy measurements reveal that the nanotubes grow parallel to the MgO(0 0 1) plane and preferentially along the two perpendicular directions, [1 1 0] and . The interactions between oxygen atoms of the MgO substrate and carbon nanotubes would drive the directional growth. The Raman spectra support that most nanotubes with a diameter ranging from 1.5 to 4 nm, observed by transmission electron microscopy and atomic force microscopy, are single-walled. The observed directional growth requires a very good surface quality of the MgO substrate. Interestingly, rough areas damaged by water vapour prevent the formation of nanoparticles necessary to the nucleation of nanotubes.     

Conversion of carbon nanotubes to diamond by spark plasma sintering

The diamond phase has been converted directly from carbon nanotubes by spark plasma sintering (SPS), at 1500 °C under 80 MPa pressure, without any catalyst being involved. Well-crystallized diamond crystals, with particle sizes ranging from 300 nm to 10 μm were obtained. After sintering at 1200 °C, the tips of the carbon nanotubes were found to be open and the conversion from carbon nanotubes to diamond started. The mechanism for carbon nanotube to diamond conversion in SPS may be described as that from carbon nanotubes to an intermediate phase of carbon nano-onion, and then to diamond. It is believed that the plasmas generated by the low-voltage, vacuum spark, via a pulsed DC in the SPS process, played a critical role in the low pressure diamond formation. This SPS process provides an alternative approach to diamond synthesis.     

Amorphous carbon nanostructures from chlorination of ferrocene

The chlorination of ferrocene at different temperature conditions yields several carbon nanostructures, which were studied by means of transmission and scanning electron microscopies. Amorphous carbon nanotubes (α-CNTs) up to 10 μm long with thick walls and ∼15 nm of internal diameter were observed in a sample treated at 200 °C during 30 min. They consisted on ∼90% of carbon, while the remaining 10% consists on iron and chlorine. At this temperature, amorphous carbon bags and open-ended branches were also found. When chlorinating ferrocene at the same temperature but with longer reaction time (180 min), no α-CNTs were formed. At higher temperature (300 °C, 30 min), amorphous carbon bags were found, with lower content of residual chlorine and iron, and presenting thinner walls. In the sample treated at even higher temperature (900 °C, 30 min) the carbon nanobags (wall thickness ∼12 nm) were almost spherical and more graphitic, and without impurities.