Low Concentrations Of Carbon Nanotubes Research Articles

Synergistic toughening on CFRP via in-depth stitched CNTs

Efficient toughening of the interlaminar fracture toughness for CFRP composites without sacrificing in-plane mechanical performance remained an unresolved challenge. Here, we present a synergistic toughening strategy by construction of hierarchical architecture, within which carbon nanotubes (CNTs) in-depth stitched into the nano-channel between neighboring carbon fibers at the interlaminar region. Mode I fracture test revealed that, even at low concentration of CNTs (0.3 wt%), a considerable improvement (50%) on mode I fracture energy GIc of composites can be realized, from 1098.6 to 1647.8 J/m2 which is nearly two times greater than that of most aerospace CFRP laminates (∼500 J/m2). The excellent fracture toughness is predominantly attributed to the desired hierarchical architecture, which simultaneously triggered the intrinsic toughening by CNTs nano-bridging and extrinsic toughening mechanisms due to carbon fiber bridging, as was demonstrated by SEM images of fracture surfaces and further verified by finite element simulations. These findings offer significant guidelines for designing CFRP composites with high fracture toughness by application of low content CNTs using cost-effective resin mixing process.

Performance and limitation of mineral oil-based carbon nanotubes nanofluid in transformer application

Transformer oil-based carbon nanotube (CNT) nanofluids which have unique dielectric behaviour, is effective as the posterity insulation fluids that can boost the performance of the transformer as they proposed inspiring, distinctive behaviour compared to existing transformer oil which is widely used in practice namely mineral oils. With this motivation, the effect of AC breakdown voltages for two sonication duration (30 min and 120 min) techniques were applied in producing nanofluids, two different diameter sizes of CNTs (<8 nm and >20 nm) and five different weight concentrations (0.01 g/L to 0.2 g/L) are investigated. The results indicate CNT with a longer sonication process, a smaller diameter and low concentrations of CNT provides the highest breakdown values that gave a huge potential impact on the conventional transformer oil. The Weibull and Normal distributions functions are used in this paper to obtain a successful forecast of the lowest, average, and highest possibility of breakdown rates (1%, 50% and 90%). It figures out that, CNT nanofluid can reach the greatest breakdown efficiency as good insulating oil at 0.01 g/L concentration. To understand the characterization of CNT nanofluids samples in detail, Raman spectroscopy analysis, storage modulus, viscosity and heat flow of mineral oil have been evaluated accordingly as a function of increasing temperature.

Ultra-permeable CNTs/PES membranes with a very low CNTs content and high H2/N2 and CH4/N2 selectivity for clean energy extraction applications

Polysulfone (PES) membranes are among the rare membranes that are capable of double filtration: a pre-filter layer captures agglomerates and a thin-dense layer is responsible for the main separation process. This work aims to enhance the permeability and H2/N2 and CH4/N2 of the dense layer of PES by mixing it with low concentrations of carbon nanotubes (CNTs: 0.01–0.03 wt.%) using solution casting and doctor blade techniques. The pore topology, microstructure, chemical, thermal, and mechanical properties of the synthesized CNTs/PES membranes were investigated using FTIR, XRD, TGA, and a universal testing machine, while permeability of single CO2, H2, N2, and CH4 permeability of the CNTs/PES membranes were tested under different temperatures (20–60 °C) and pressures (1–6 bar). Also, the effect of added CNTs, separation temperature, and pressure on the gas separation mechanism were investigated. The results showed that adding of CNTs contributed to increase in porosity from 81.7% (PES) to 88.4% (CNTs/PES) and decrease in pore sizes from 84 nm (PES) to 50 nm (CNTS/PES). Meanwhile, the thermal and mechanical analysis showed that CNTs/PES membranes had higher thermal stability and somewhat lower strength compared with neat membranes. Also, the permeability measurements showed a big increase when only 0.01 wt.% of CNTs had been added, where H2, CH4, N2, CO2 permeabilities were increased up to 28,553, 11,358, 7540, 6720 Barrer, respectively, vs 10.4, 4.6, 13.7, and 12.3 Barrer in case of PES membranes. In addition, CO2/N2, CH4/N2, and H2/N2 selectivity of CNTs/PES membranes were enhanced by 29%, 396%, and 426%, respectively, as a result of pores refining and increasing of free space in the prepared CNTs/PES membranes. According to these results, CNTs/PES membranes with small loading of CNTs have a tremendous ability to deal with separation of H2/N2 and CH4/N2, what make them promising candidates for clean energy extraction applications.

Electrical Properties of Transformer Oil with the Addition of Extremely Low Concentrations of Carbon Nanotubes

Electrical properties of transformer oil with additives of carbon nano-tubes are studied at the extremely low concentrations of the nano-tubes. Electrical strength of the mixtures is more than an order of magnitude lower than that of pure oil. Strengthening of the large gap is registered after several breakdowns in contrast to the small gap. The formation of a carbon bridge between flat electrodes is registered optically. This bridge is destroyed by an electrical discharge in the chain of bubbles generated on the bridge. Experiments with helium bubbles floating up in the mixture of carbon nano-tubes with transformer oil are performed at the deficiency of initiating electrons. Additives of carbon nano-tubes stimulate the partial discharges in the bubbles. Also, at lower voltages, the development of intense micro-flows in the liquid dielectric is observed that can be the result of the injection of free electrons from the nano-tubes.

Reinforcing cement with pristine and functionalized carbon nanotubes: experimental and simulation studies

ABSTRACT In recent years, the addition of carbon nanotubes to construction materials has attracted considerable interest, due to the improvement of mechanical, electrical, and thermal properties of cement. The incorporation of carbon nanotubes into a cement matrix causes an increase of several mechanical properties of up to 170% even with low carbon nanotubes concentrations. The objective of this study is to analyze the influence of the type of functionalization and number of walls of carbon nanotubes on the interaction between these nanostructures and a cement surface and thus, on the improvement of their mechanical properties. Thus, single-walled and double-walled carbon nanotubes were used to investigate the influence of the number of walls. The effect of carbon nanotube functionalization was studied using carbon nanotubes functionalized with carboxyl and carboxylate groups. The experimental results demonstrate that the incorporation of carbon nanotubes into the cement matrix improves the mechanical properties of the resulting material. Functionalized carbon nanotubes perform better than pristine carbon nanotubes. Electrostatic attractions play a central role in establishing strong interactions between the carbon nanotubes and the cement surface. The presence of neutral polar groups on the carbon nanotube surface also improves this interaction. The number of walls seems to be less important.

Suspended multiwalled, acid-functionalized carbon nanotubes promote aggregation of the opportunistic pathogen Pseudomonas aeruginosa.

The increasing prevalence of carbon nanotubes (CNTs) as components of new functional materials has the unintended consequence of causing increases in CNT concentrations in aqueous environments. Aqueous systems are reservoirs for bacteria, including human and animal pathogens, that can form biofilms. At high concentrations, CNTs have been shown to display biocidal effects; however, at low concentrations, the interaction between CNTs and bacteria is more complicated, and antimicrobial action is highly dependent upon the properties of the CNTs in suspension. Here, impact of low concentrations of multiwalled CNTs (MWCNTs) on the biofilm-forming opportunistic human pathogen Pseudomonas aeruginosa is studied. Using phase contrast and confocal microscopy, flow cytometry, and antibiotic tolerance assays, it is found that sub-lethal concentrations (2 mg/L) of MWCNTs promote aggregation of P. aeruginosa into multicellular clusters. However, the antibiotic tolerance of these “young” bacterial-CNT aggregates is similar to that of CNT-free cultures. Overall, our results indicate that the co-occurrence of MWCNTs and P. aeruginosa in aqueous systems, which promotes the increased number and size of bacterial aggregates, could increase the dose to which humans or animals are exposed.

Wire Melt Electrospun Polymer Nanocomposite Fibers as Radio Frequency Responsive Heaters

We have demonstrated noncontact heating of melt electrospun polymer fibers by using radio-frequency (RF) fields which heat carbon nanotube (CNT) receptors inside the fibers. RF radiation is attractive as it allows for noncontact heating of polymers with low concentrations of CNTs. We observed that the heating rate scales with the CNT loading even below the bulk electrical percolation threshold, suggesting that individual CNTs can serve as RF receptors/heat sources. This capability eliminates the requirement for a percolated network of CNTs inside a fiber as a means to enable heating. We also showed that a strong radial temperature gradient will develop within the fibers. For a 2 μm diameter fiber, the temperature of the core is 10–15 °C higher than the surface. Hence, the temperature of the core can surpass the melting temperature inside the fiber without altering the morphology of the fibers (i.e., without fusing between fibers). These electrospun fibers that can be stimulated through RF energy can be us...

A physics investigation for influence of carbon nanotube agglomeration on thermal properties of composites

In the present study, a micromechanics model is developed to investigate the thermal properties of nanocomposites with agglomerated fibers. This paper mainly analyzes the influence of carbon nanotube (CNT) agglomeration on the thermal expansion coefficients and thermal conductivity of epoxy nanocomposites based on the micromechanical theory proposed. The agglomeration of CNTs in epoxy matrix is divided into two parts: the regions with high concentrations of CNTs; the regions with low concentrations of CNTs. The analytical expressions are derived according to the micromechanics method, and the two aggregation parameters are introduced to analyze the effect of CNTs agglomerated. The influences of high and low CNT agglomerations on the thermal expansion coefficients and thermal conductivity of composites are investigated, respectively. It is shown that the non-uniform dispersion of CNTs in matrix has a significant influence on the thermal expansion coefficients and thermal conductivity of nanocomposites.

Recycled polyethylene terephthalate/carbon nanotube composites with improved processability and performance

Recycled poly(ethylene terephthalate) (PET) nanocomposites containing multiwalled carbon nanotubes (CNTs) were prepared through melt compounding via masterbatch dilution method. The masterbatch and the nanocomposites were processed in a twin-screw extruder. The rheological, morphological, thermal and mechanical properties of the PET–CNT nanocomposites have been investigated. Incorporation of CNTs into recycled PET at low concentration (0.25 wt%) significantly increases the viscosity. The storage modulus and loss modulus of nanocomposites were also increased with increasing amount of CNTs. This effect was more pronounced at lower frequencies. The incorporated CNTs in recycled PET increase the degree of crystallinity and crystallization temperature through heterogenous nucleation. Thermal stability and glass transition temperature of PET–CNT nanocomposites were slightly higher than the reference recycled PET. The tensile strength and modulus of PET–CNT nanocomposites increased even at low concentrations of CNTs. Morphological investigation through scanning electron microscopy indicated homogeneous dispersion of CNTs at lower concentrations. At higher concentrations, the CNTs tend to agglomerate due to nanotube–nanotube interactions.

Hierarchical self-entangled carbon nanotube tube networks

Three-dimensional (3D) assemblies based on carbon nanomaterials still lag behind their individual one-dimensional building blocks in terms of mechanical and electrical properties. Here we demonstrate a simple strategy for the fabrication of an open porous 3D self-organized double-hierarchical carbon nanotube tube structure with properties advantageous to those existing so far. Even though no additional crosslinking exists between the individual nanotubes, a high reinforcement effect in compression and tensile characteristics is achieved by the formation of self-entangled carbon nanotube (CNT) networks in all three dimensions, employing the CNTs in their high tensile properties. Additionally, the tubular structure causes a self-enhancing effect in conductivity when employed in a 3D stretchable conductor, together with a high conductivity at low CNT concentrations. This strategy allows for an easy combination of different kinds of low-dimensional nanomaterials in a tube-shaped 3D structure, enabling the fabrication of multifunctional inorganic-carbon-polymer hybrid 3D materials.

Thermoresistive mechanisms of carbon nanotube/polymer composites

The mechanisms governing thermoresistivity of carbon nanotube (CNT)/polymer composites are theoretically and experimentally investigated. Two modeling approaches are proposed to this aim considering a broad range of CNT concentrations (0.5–50wt%). In the first model, thermal expansion of the polymer composite is predicted using a finite element model; the resulting CNT-to-CNT separation distance feeds a classical tunneling model to predict the dependence of the electrical resistance with temperature. The second approach uses the general effective medium considering the dilution of the CNT volume fraction due to the thermal expansion of the polymer. Both models predict that the electrical resistance increases with increased temperature (i.e. a positive temperature coefficient of resistance, TCR) for all investigated CNT concentrations, with higher TCRs for lower CNT concentrations. Comparison between modeling outcomes and experimental data suggests that polymer thermal expansion (and tunneling) play a dominant role for low CNT concentrations (≤ 10wt%) heated above room temperature. On the other hand, for composites at high CNT concentrations (50wt%) or for freezing temperatures (−110°C), a negative TCR was experimentally obtained, suggesting that for those conditions the CNT intrinsic thermoresistivity and the electronic conduction between CNTs by thermal activation may play a paramount role.

Thermal and Rheological Properties of Industrial Mineral Gear Oil and Paraffinic Oil/CNTs Nanolubricants

Carbon nanotubes (CNTs) are used as a nanoadditive due to their unique properties and for improving the lubricant’s properties. In this research, the effect of CNTs in different concentrations (0.1, 0.5, 1 and 2 wt%) on kinematic viscosity, pour point, flash point and thermal conductivity coefficient as four quality parameters which are effective in the functionality of oil is evaluated and compared against the standard Mobil gear 627 and paraffinic mineral oils. The samples were tested according to ASTM D-445, ASTM D-97 and ASTM D-92 standards, respectively. The results indicated that the thermal conductivity, flash point, pour point and kinematic viscosity of nanolubricants with low concentration of CNTs improved with respect to the base oil. Dispersion of CNTs as additive has improved the lubricant properties and operational characteristics of Mobil gear 627 and paraffinic mineral oils.

The multiple roles of a dispersant in nanocomposite systems

The loading of individual carbon nanotubes (CNTs) in matrices provides state-of-the-art nanocomposite reinforcement at low CNT concentrations. The dispersion of the CNTs in the matrix is often assisted by surfactants (termed also dispersants), which de-bundle the CNT agglomerates. Despite the critical role of the dispersants in this process, their effect on the properties of the nanocomposite has not been sufficiently studied. In the current study, we elucidate the influence of freeze-dried dispersant-CNT on the performance of polymeric and cementitious nanocomposites. We show that the dispersant (Pluronic) is crucial for exfoliating the CNTs in both types of nanocomposite, whereas it also acts as a plasticizer – thus lowering the mixture's effective viscosity – in cementitious matrices. We identify the optimal nanomaterial concentration (ONC) in each nanocomposite system, namely, the critical filler concentration in which optimal mechanical properties are obtained, and above which they degrade dramatically due to the formation of air voids that can initiate mechanical failure. Importantly, the effect of the dispersant on the nanocomposite system – and, accordingly, the origin of the resulting air voids – is different between polymeric and cementitious matrices. In polymers, a high filler concentration increases the viscosity, and, consequently, entraps air voids in the matrix during mixing. In cement, by contrast, the dispersant stabilizes the air and leads to the formation of foam in the matrix. Taken together, our findings indicate that the behavior of the dispersant throughout the composite preparation process – both in the fresh mixture and in the hardened state – should be considered when employing dispersants for use in applicative composite systems.

Coupled Effects of Film Thickness and Filler Length on Conductivity and Strain Sensitivity of Carbon Nanotube/Polymer Composite Thin Films

The coupled effects of varying composite film thicknesses and filler lengths on the conductivity and strain sensitivity of carbon nanotube (CNT)/polymer composite films are investigated through modeling and experiments. Change in average intertube distance is calculated statistically through the Monte Carlo simulations for samples with different CNT concentrations and film thicknesses for a given filler aspect ratio. The composite conductivity is then estimated from the intertube distance with a semi-analytical model based on a tunneling current. The dependence of conductivity on mechanical strain is investigated for varying film thickness for strain sensor applications. A partial alignment of CNTs introduced at film thicknesses less than the CNT length is observed to have a significant influence on the composite conductivity and strain sensitivity, specially at low CNT concentrations. The modeling results can explain the observed experimental results of conductivity for CNT composites, which illustrate a unique dip in conduction with increasing thickness. These results are important for understanding the composite characteristics with different filler orientations and film thicknesses for a given filler length, and useful for the design optimization of high performance composite electronic films for applications in electronic skin and sensors.

Improvement of ablation resistance of phenolic composites reinforced with low concentrations of carbon nanotubes

Carbon nanotubes (CNTs) were incorporated into a phenolic resin in order to improve its ablation resistance. The CNT-reinforced phenol composites showed enhanced ablation resistance compared to neat phenolic resin, even with a low concentration of CNTs (0.1 wt%). The proposed mechanism of the CNTs for the CNT/Phenolic resin composite's enhanced ablation resistance is that the CNTs functioning as crystal growth nuclei for graphitization of carbonized phenolic resin. The graphitic structures formed around the CNTs improved the composite's resistance to high temperature ablation. X-ray diffraction (XRD) and Raman spectroscopy, scanning electron microscopy (SEM), and Transmission electron microscopy (TEM) were employed to identify the proposed CNT reinforcement mechanism for the enhanced ablative resistance of the CNT/Phenolic resin composites.

Effect of low concentrations of carbon nanotubes on electric dipole relaxation in a polyurethane elastomer

The effect of small (up to 0.018 wt %) additions of single-walled carbon nanotubes (SWNTs) on the complex electric modulus M*= M′ - jM″ and the spectrum of the relaxation times G(τ) of a cross-linked polyurethane elastomer containing ∼10 vol % of polyamide-6 dispersed in the polyurethane matrix and incompatible with it was studied. The measurements were conducted in the range of electric field frequencies 10−3–105 Hz at temperatures from 133 to 413 K. Based on the shape analysis of the M″(M′) diagrams, the contributions of electric conductivity and dielectric relaxation to complex dielectric permittivity ɛ* = ɛ′ - jɛ″ were separated and the effect of additions on α and β relaxation for both polyurethane and polyamide phases was analyzed in accordance with the peculiarities of phase-separated systems. The introduction of SWNTs in the composite affected the dielectric properties of the material; the maximum effect was observed at concentrations of 0.002–0.008 wt %; at higher SWNT concentrations, the scatter of data increased and did not allow us to evaluate the effect. The effect of SWNTs on G(τ) in the main phase was opposite to that in the polyamide phase. In the temperature range of α relaxation of the polyurethane phase, the relaxation times increased after the introduction of SWNTs evidently because of the decrease in the free volume that determines the α relaxation times of polyurethane. In contrast, for the polyamide phase in the range of α relaxation, the relaxation times decreased after the introduction of SWNTs. The results agree with the literature data on the effect of ultrasmall SWNT concentrations on the physicomechanical characteristics of the polyurethane elastomer and its electric conductivity.

Improved fracture toughness and integrated damage sensing capability by spray coated CNTs on carbon fibre prepreg

The development of a hierarchically engineered micro-nano hybrid composite system is described. A spray coating technique has been utilized as an effective way to deposit carbon nanotubes (CNTs) onto carbon fibre prepregs with good control of network formation and the potential for localization. Compared to more traditional approaches of introducing CNTs into epoxy matrices for enhancing composite properties, this technique has benefits in terms of its simplicity and versatility, as well as the potential for industrial scale-up. The effectiveness of the technique is demonstrated by the extremely low CNT loading (0.047wt.%) needed to significantly increase the Mode-I fracture toughness of the carbon fibre laminates by about 50%, which is so far the largest reported improvement for such extremely low concentrations of non-functionalized CNTs. In-situ damage sensing has also been presented for the monitoring of structural health of these nano-engineered composite laminates upon loading, and a systematic analysis of sensing signals is performed.

Heat-resistant unfired phosphate ceramics with carbon nanotubes for electromagnetic application

Composite materials, containing low concentrations of carbon nanotubes (CNTs) of three different diameters and heat-resistant phosphate ceramic as a matrix were prepared by cold-pressing method. Their dielectric properties were studied at room temperature in a wide frequency range (20 Hz–1 MHz). It was found experimentally and proved theoretically via modeling of the composites as a random capacitor–resistor–diode network that electrical percolation concentration depends significantly on the diameter of the nanoinclusions. The main conclusion is that the best candidate providing both the lower percolation threshold and high absolute values of ac conductivity is thinner carbon nanotubes (in our particular case, 8–15 nm).

The effect of surface functionalization of carbon nanotubes on properties of natural rubber/carbon nanotube composites

The objective of this study was to prepare natural rubber composites filled with carbon nanotubes (CNTs) that show an electrical percolation threshold at very low CNT concentrations. Therefore, two methods of surface functionalization of CNTs were investigated to enable an improved dispersion of CNTs and chemical interaction between CNTs and rubber matrix. On one hand, the CNTs have been functionalized ex situ by acid treatment and silanization reaction with bis(triethoxysilylpropyl) tetrasulfide before mixing with the rubber and otherwise in situ functionalization was directly carried out during the processing of the composites in the internal mixer. The grafting of silane molecules onto CNT surface was established by Fourier transform infrared spectroscopy and scanning electron microscopy. Tensile tests revealed the outstanding properties of composites prepared by in situ silanization method. The in situ silanization led to a better dispersion of the CNTs and the formation of chemical linkages between CNT surface and rubber and this became manifest in higher reinforcement of the rubber, higher crosslink densities, and a lower electrical percolation threshold. It was also shown that the in situ silanization is retarding the vulcanization reaction. POLYM. COMPOS., 36:2113–2122, 2015. © 2014 Society of Plastics Engineer

Evaluation of cardiovascular toxicity of carbon nanotubes functionalized with sodium hyaluronate in oral regenerative medicine.

It has been demonstrated that carbon nanotubes (CNTs) associated with sodium hyaluronate (HY-CNTs) accelerate bone repair in the tooth sockets of rats. Before clinical application of HY-CNTs, it is important to assess their biocompatibility. Moreover, cardiac toxicity may be caused by the translocation of these particles to the blood stream. The aim of this study was to evaluate possible changes in cardiovascular function in male Wistar rats whose tooth sockets were treated with either CNTs or HY-CNTs (100 μg/mL, 0.1 mL). Blood pressure and heart rate were monitored in conscious rats 7 days after treatment. Cardiac function was evaluated using the Langendorff perfusion technique. The data showed no changes in blood pressure or heart rate in rats treated with either CNTs or HY-CNTs, and no significant changes in cardiac function were found in any of the groups. To confirm these findings, experiments were conducted in rats injected intraperitoneally with a high concentration of either CNTs or HY-CNTs (0.75 mg/kg). The same parameters were analyzed and similar results were observed. The results obtained 7 days following injection indicate that the administration of low concentrations of CNTs or HY-CNTs directly into tooth sockets did not cause any significant change in cardiovascular function in the rats. The present findings support the possibility of using these biocomposites in humans.