Investigation of electro-mechanical behavior of carbon nanotube yarns during tensile loading

Carbon nanotube (CNT) yarns were evaluated for sensor applications by measuring electrical properties during uniaxial tension loading. Mechanical properties (tenacity and failure strain) and electrical properties (resistivity and gauge factor) were investigated and statistical distributions for these properties were obtained. Cyclic loading test results showed that permanent strain after unloading exists and that the resistance at zero load increases linearly with permanent strain. Furthermore, the relative resistance change during loading was found to be linear with strain. Although mechanical properties of CNT yarns exhibited a significant statistical variation, the resistance was found to have much less statistical variation making them good candidates as sensors for structural health monitoring in composites.     

Insights into carbon nanotube growth using an automated gravimetric apparatus

The development of an automated chemical vapour deposition micro-reactor based on a thermogravimetric analyser is presented. This apparatus was used to investigate carbon nanotube (CNT) growth. Gas flow and reaction modelling highlighted problems with current systems and demonstrated why experimental results are currently apparatus specific. Modelling also indicated that the large-scale production of chiral selective CNTs cannot be achieved in current systems using hydrocarbons. The apparatus provided unprecedented insight into the synthesis of CNTs. Catalyst reduction does not reach completion, probably due to alumina stabilisation of iron oxides, and any discrete pre-reduction step is detrimental to final carbon yield. Any CNT nucleation period was solely attributable to carbon source or H₂ supply restrictions, and growth and final yield were highly dependent on deposition rates within the initial period. The apparatus high throughput capabilities were used for a two-stage optimisation protocol. An optimum carbon yield of >40 g_C/g_Fe·h was found at 700 °C with 20% C₂H₄ and 80% H₂, comparable with the highest reported literature values. A new method to selectively accelerate, slow, or even stop, CNT growth rates using ‘carrier’ gas modulation is also demonstrated.     

Investigation of cure kinetics in epoxy/multiwalled carbon nanotube nanocomposites

With the increased interest in thermoset resin nanocomposites, it is important to understand the effects of the material on nanoscale characteristics. In this study, a curing reaction of an epoxy resin, which contained 0.25, 0.50, or 1.00 wt % of multiwalled carbon nanotubes (MWCNTs), at different heating rates was monitored by differential scanning calorimetry; cure kinetics were also evaluated to establish a relationship between crosslinking (network formation) and mechanical properties. MWCNT concentrations above 0.25 wt % favored crosslinking formation and decreased the activation energy (E_a) in the curing reaction. Examination of the kinetic mechanism suggests that the MWCNT locally restricted the spatial volume and favored the formation of nodular morphology in the resin, especially for high MWCNT concentrations. The MWCNT exhibited some entanglement in the matrix, which hindered a more pronounced effect on the mechanical properties. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39857.     

Anisotropic carbon nanotube papers fabricated from multiwalled carbon nanotube webs

We fabricated large-scale anisotropic carbon nanotube (CNT) paper sheets by stacking long-lasting multiwalled CNT (MWCNT) webs without using binder materials. The MWCNTs are highly aligned in the webs and they retain their alignment in the fabricated paper. Although MWCNTs are just connected by van der Waals force, tensile strength is as strong as 75.6 MPa. In addition, resistivity and thermal conductivity is as good as 2.5 × 10⁻³ Ω cm and 70 W/m K, respectively. The present high anisotropy ratios of 7.3 in resistivity and of 8.1 in thermal conductivity are due to the high alignment of the ultra-long MWCNTs which have lengths of millimeters. High-speed web drawing with a draw speed of over 10 m/s enables very rapid fabrication. The material properties of CNT structures can be measured by conventional methods for macroscopic samples rather than methods designed for nanomaterials. CNT web technology will enable CNTs to be used in new applications.     

The effect of a doubly modified carbon nanotube derivative on the microstructure of epoxy resin

The surface of multi wall carbon nanotubes (MWCNTs) was first covalently functionalized with oleyl amine and then non-covalently wrapped with polycarbosilane (PCS). The hybrid functional groups were chosen to introduce different features in the MWCNTs properties. For covalent functionalization a long chain unsaturated aliphatic amine was used to simultaneously achieve the dissociation of MWCNT bundles along with the dispersion and interaction with the host matrix using the amide functionality and double bond. On the other hand, a thermally stable polymer was selected which can interact with both resin and glass fabric to promote interfacial adhesion. This hybrid doubly modified MWCNT is thus possesses duel advantages in glass fiber based epoxy composite. The pristine, covalent, noncovalent and covalent-noncovalent doubly modified MWCNT systems were used to study the viscoelastic behavior and interactions of functionalized MWCNTs in the matrix above and below the glass transition temperature of the matrix. The PCS coating on the MWCNTs is amorphous and thermally insulating whereas the nanotube is highly graphitized and thermally conducting. This contrasting behavior provides us to insight into the temperature dependant resin microstructure and curing thermodynamics of epoxy resin in the presence of MWCNTs.     

The effect of electro-degradation processing on microstructure of polyaniline/single-wall carbon nanotube composite films

Mesostructured polyaniline/single-wall carbon nanotube (PAni/SWCNT) composite film has been prepared through electrochemical polymerization/degradation processing. The microstructures of the films are observed before and after electro-degradation. Initial twisty SWCNT bundles are broken down and linked by the polymerization of PAni. Both crystalline and disordered PAni regions coexist in the PAni/SWCNT composite nanowires. The disordered regions are gradually dissolved while the crystalline regions are basically preserved after electro-degradation. The formation mechanism of the composite with special construction has been proposed. In addition, cyclic voltammetry measurements demonstrate that the electroactive performance of PAni/SWCNT composite is enhanced after electro-degradation. It is found that the specific capacitance of electro-degraded composite reaches up to 848.7 F/g, more than twice over the untreated film, which is ascribed to its profitable charge accessible interface and increased available crystalline PAni regions.     

Influence of microstructure on the capacitive performance of polyaniline/carbon nanotube array composite electrodes

A series of polyaniline/carbon nanotube array (PANI/CNTA) composite electrodes are prepared by electrodeposition of PANI onto CNTA electrodes by 100-500 cyclic voltammetry (CV) cycles, with the aim to investigate the influence of microstructure on the capacitive performance of PANI/CNTA composites. The morphology of PANI/CNTA composites varies remarkably with the CV cycles of electrodeposition. The optimum condition is obtained for the PANI/CNTA composite prepared by 100 CV cycles, corresponding to the highest specific capacitance, best rate performance, and longest cycle life, which are much better than that of activated carbon fiber cloth, the PANI electrodeposited on stainless steel substrate, and CNTA electrode. The forming process of the microstructure and its influence on the capacitive performance of PANI/CNTA composites are presented in this paper.     

Investigation on carbon nanotube oxide for anhydrous proton exchange membranes application

The widespread participation of polymers in the membrane preparation has been considering to be critical for the development of proton exchange membranes (PEMs). For the polymers without functional groups to conduct protons, the introduction of proton conduction carriers with the formation of composite membranes is an effective strategy to prepare PEMs with the outstanding proton conductivity. However, there remains a potential risk of the components leaking from composite membranes due to the lack of the interaction force. Here, the composite of carbon nanotube oxide (OCNT) assembling with cadmium telluride (CdTe) and 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆) was introduced into the system of phosphoric acid (PA) doping poly(vinylidene fluoride) (PVDF) with the formation of PVDF/OCNT-CdTe-bmimPF₆/85%PA membranes. PA molecules are anchored by the inorganics of OCNT-CdTe-bmimPF₆ and are stabilized in membranes. The high and stable proton conductivity values at the elevated temperature are obtained comparing the reported PVDF/bmimPF₆/PA membranes. Specifically, the proton conductivity value reached 1.28 × 10⁻¹ S/cm at 160 °C and the value is stable 1.70 × 10⁻² S/cm at 120 °C lasting for 350 h. The fine stability in components could make the membranes extricate from the predicament of proton conductivity decline exceeding 120 °C under anhydrous conditions in PVDF/bmimPF₆/PA membranes. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48833.     

Structural transformation of double-walled carbon nanotube bundles into multi-walled carbon nanotubes induced by current treatment

In this paper, double-walled carbon nanotube (DWNT) bundles can be transformed into multi-walled carbon nanotubes (MWNTs) by electric current treatment in vacuum. The morphology of the transformed MWNTs shows an obvious dependence on power density of the current, which changes from partly transformed MWNTs to collapse MWNTs with the increase of power density. The process of this structural transformation follows the tendency of forming more stable structures, and can be ascribed to the sequential coalescence of DWNTs inside the bundles.     

Investigation on sensitivity of a polymer/carbon nanotube composite strain sensor

Extensive numerical simulation and experimental measurements have been conducted to understand the effects of processing parameters and material properties on sensor sensitivity in polymer/carbon nanotube (CNT) composite sensors. The numerical simulation was based on an improved three-dimensional statistical resistor network model incorporating the tunneling effect between the neighbouring nanotubes, and a fiber reorientation model. The behaviors of a sensor subjected to both tensile and compressive strains were investigated. Both numerical and experimental results indicate that a higher tunneling resistance or higher ratio of the tunneling resistance to the total resistance of the sensor leads to a higher sensor sensitivity. Processing conditions and material properties, such as weight fraction, diameter and conductivity of CNTs, curing temperature, mixing rate and barrier height of polymer matrix all play a role in determining the sensor sensitivity.     

Temperature dependence of oxygen diffusion into polymer/carbon nanotube composite films

This study examines the transport properties of polystyrene (PS)/multiwalled carbon nanotube (MWNT) composite films taking into consideration both MWNT composition and temperature, via fluorescence technique. Three different (3, 15, and 40 wt%) MWNT content films were prepared from PS/MWNT mixtures by annealing them at 170°C, above the glass transition temperature of PS for 10 min. The diffusivity of the PS/MWNT composite was determined by performing oxygen (O₂) diffusion measurements within a temperature range of 24 to 70°C for each film and pyrene (P) was used as the fluorescent probe. The diffusion coefficients (D) of oxygen were determined by the fluorescence quenching method assuming Fickian transport. Results indicated that D values are strongly dependent on both temperature and the MWNT content in the film and it was also observed that D coefficients obey Arrhenius behavior, from which diffusion energies were produced and increased along with increases of MWNT content. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Synergetic carbon nanotube growth

We report an effect in vertically-aligned carbon nanotube growth in which small catalyst features in proximity to large features show an enhanced growth rate. We apply this so-called “synergetic growth” effect in micrometer-scale patterns to produce vertical-like growth horizontally. This approach enables carbon nanotube integration into device applications requiring a fair degree of carbon nanotube alignment. The synergetic growth effect corroborates growth mechanisms describing carbon nanotube growth as a more complex process than elemental carbon supersaturating metal catalyst.     

Investigation on morphology and properties of melt compounded polyoxymethylene/carbon nanotube composites

Polyoxymethylene nanocomposites containing different contents of carbon nanotubes were produced by a two‐step melt compounding process using a twin‐screw extruder. The dispersion quality, thermal and mechanical properties, and the creep as well as the tribological behaviors of the nanocomposites were investigated. Morphological investigations show that the masterbatch dilution process significantly improves the dispersion quality of carbon nanotubes within polyoxymethylene matrix, and as a consequence, enhanced mechanical properties and creep resistance are gained. Furthermore, to predict the long‐term property based on the short‐term experimental data, the time–temperature superposition principle and Findley model were used. Master curves with extended time scale are constructed using time–temperature superposition principle to horizontally shift the short‐term experimental data. The simulated results confirm the reinforced creep resistance by incorporation of the carbon nanotubes into the polymer matrix even at extended long time scale. By contrast, the tribological performance of polyoxymethylene was remarkably impaired after adding carbon nanotubes. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42639.     

Investigation into the antibacterial property of carbon films

Here we reported the antibacterial characteristics of carbon film and the effect of film composition on its antibacterial performance. Two kinds of carbon films, amorphous hydrogenated carbon film and hydrogen-free carbon film deposited on medical stainless steel using microwave plasma were characterized using Raman Spectra, FT-IR (Fourier transform-infrared spectroscopy), AFM (atomic force microscopy) methods and investigated on their antibacterial property. By the colony-counting method, it was found that the H-free carbon film coating and a-C:H film coating reduced the numbers of E. coli colonies down to about 15% and 33% of those in the original SUS substrate, respectively. The present results suggested that the antibacterial property was strongly related with the chemical inertness of carbon film.     

Direct conversion of highly aromatic phthalonitrile thermosetting resins into carbon nanotube containing solids

A method for the in situ synthesis of multi-walled carbon nanotubes in a bulk solid composition from the pyrolysis of a mixture formulated from Co₂(CO)₈ and various high temperature phthalonitrile resins is presented. Carbon nanotube containing compositions in various forms (solid, fiber, and film) can be readily fabricated by this method utilizing slight modifications to the standard resin processing techniques. The formation of carbon nanotubes occurs during the carbonization process at ambient pressure. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy studies show the presence of a copious amount of multi-walled carbon nanotubes in the carbonaceous composition.     

The effect of nanotube alignment on stress graphitization of carbon/carbon nanotube composites

Carbon nanotubes, when used as filler in a glass-like carbon matrix, has been reported to induce stress graphitization in the matrix. The effects on stress graphitization of the amount of carbon nanotube loading and nanotube orientation in the composite were investigated through microscopy and X-ray diffraction analyses. Results showed that an increase in nanotube content and nanotube alignment could increase the extent of formation of anisotropic regions, thereby hastening stress graphitization. It was shown that the distance between nanotubes could affect the formation of the anisotropic structures, such that they could develop in a circumferential manner around the nanotubes when the nanotubes are situated far from each other or develop continuous regions between nanotubes when they are closer together. The development of these microstructures and its relationship to the residual stresses that accumulate in the material during heat treatment is discussed here.     

Connection of macro-sized double-walled carbon nanotube strands by bandaging with double-walled carbon nanotube films

Based on their special shrinkage characteristics, double-walled carbon nanotube (DWCNT) films were used as bandages to bind the overlapped ends of macro-sized (centimeters long) DWCNT strands for connection to get structures of random length. Tensile tests indicated that the joints made in this way had relatively high tensile strength with a maximum value of 311 MPa corresponding to that of the original strands. The equivalent contact resistance of the joints was very small. And the connected strands showed better electronic properties in our investigation on the temperature dependence of resistivity and the same remarkable current capacity, in contrast to the original ones. This technique may offer a promising potential for the future extensive use of macro-sized CNTs in many fields, such as electrical cables and wires.     

Physical properties of carbon nanotube sheets drawn from nanotube arrays

We report mechanical, thermal, and electrical properties of novel sheet materials composed of multiwalled carbon nanotubes, drawn from a CNT array. At low loading there is some slippage of CNTs but at higher loading tensile strength σ₀ = 7.9 MPa and Young’s modulus E = 310 MPa. The room-temperature thermal conductivity of the CNT sheet was 2.5 ± 0.5 W m^?1 K^?1, giving a thermal conductivity to density ratio of κ/ρ = 65 W m^?1 K^?1 g^?1 cm³. The heat capacity shows 1D behavior for T > 40 K, and 2D or 3D behavior at lower temperatures. The room-temperature specific heat was 0.83 J g^?1 K^?1. The iV curves above 10 K have Ohmic behavior while the iV curve at T = 2 K is non-Ohmic, and a model to explain both ranges is presented. Negative magnetoresistance was found, increasing in magnitude with decreasing temperature (?15% at T = 2 K and B = 9 T). The tensile strength, Young’s modulus and electrical conductivity of the CNT sheet are low, in comparison with other CNT materials, likely due to defects. Thermal conductivity is dominantly phononic but interfacial resistance between MWCNTs prevents the thermal conductivity from being higher.