Low Concentrations Of Carbon Nanotubes Research Articles

Structure and Properties of Y2O3-Doped Al2O3-MWCNT Nanocomposites Prepared by Pressureless Sintering and Hot-Pressing

This study describes the combined effects of multi-walled carbon nanotubes (CNTs) additions and Y2O3 doping on the microstructures and mechanical properties of Al2O3-CNT nanocomposites fabricated by pressureless and hot-press sintering processes. A uniform dispersion of CNTs within the Al2O3 matrix was successfully attained via a combined approach using surfactant, sonication, and adequate period of incubation. Small amounts (1 wt.%) of Y2O3, as dopants, significantly affected the densification and properties of pressureless sintered monolithic Al2O3 and its nanocomposites at low CNT concentrations (<1 wt.%); however, they hardly showed any improvement at higher CNT contents. As opposed to the pressureless sintering, pressures applied during high temperature sintering in combination with the Y2O3 doping contributed in generating a homogenous microstructure and improved the densities (7 and 15%) and microhardness (11 and 12%) of Al2O3 reinforced with higher CNT contents (2 and 5 wt.%), respectively. Adding on, hot-pressed Y2O3-doped Al2O3 reinforced with 2 and 5 wt.% CNTs showed higher hardness (19 and 70%), flexural strength (10 and 5%), and fracture toughness (26 and 11%), respectively, compared to similar but CNT-free samples. These results showed that pressure-assisted sintering and Y2O3 are promising for the fabrication of CNT-reinforced Al2O3 nanocomposites, especially at higher CNT concentrations.

Ablative and mechanical evaluation of CNT/phenolic composites by thermal and microstructural analyses

Highly ablation resistant carbon nanotube (CNT)/phenolic composites were fabricated by the addition of low concentrations of CNTs. Tensile and compressive mechanical properties as well as ablation resistance were significantly improved by the addition of only 0.1 and 0.3wt% of uniformly dispersed CNTs. An oxygen–kerosene-flame torch and a scanning electron microscope (SEM) were used to evaluate the ablative properties and microstructures. Thermal gravimetric analysis (TGA) revealed that the ablation rate was lower for the 0.3wt% CNT/phenolic composites than for neat phenolic or the composite with 0.1wt% CNTs. Ablation mechanisms for all three materials were investigated using TGA in conjunction with microstructural studies using a SEM. The microstructural studies revealed that CNTs acted as an ablation resistant phase at high temperatures, and that the uniformity of the CNT dispersion played an important role in this ablation resistance.

Effect of carbon nanotubes on the curing and thermomechanical behavior of epoxy/carbon nanotubes composites

The processing of carbon nanotube based nanocomposites is one of the fastest growing areas in materials research due to the potential of significantly changing material properties even at low carbon nanotube concentrations. The aim of our work is to study the curing and thermomechanical behavior of carbon nanotube/epoxy nanocomposites that are critical from an application standpoint. Multiwall carbon nanotubes–epoxy composites are prepared by solvent evaporation based on a commercially available epoxy system and functionalized multiwalled carbon nanotubes. Three weight ratio configurations are considered (0.1, 0.5, and 1.0 wt%) and compared to both the neat epoxy to investigate the nano‐enrichment effect. We focus here on the modification of the curing behavior of the epoxy polymer in the presence of carbon nanotubes. It has been observed that introducing the multiwall carbon nanotubes delays the polymerization process as revealed by the modification of the activation energy obtained by differential scanning calorimetry. The viscoelastic response of the nanocomposites was studied from the measurements of storage modulus and the loss factor using dynamic mechanical analysis to evaluate the effect of the interface in each matrix/carbon nanotube system with changing matrix mobility. These measurements provide indications about the increase in the storage modulus of the composites, shift in the glass transition temperature due to the restriction of polymer chain movement by carbon nanotubes. POLYM. COMPOS., 35:441–449, 2014. © 2013 Society of Plastics Engineers

Evaluation of the Mechanical Properties of PMMA Reinforced with Carbon Nanotubes - Experiments and Modeling

The effective mechanical properties of polymethyl-methacrylate (PMMA) reinforced with carbon nanotubes (CNTs) were evaluated by means of two approaches: experiments and a micromechanical model. With various concentrations of CNTs, two specimen fabrication processes were examined: hot pressing (HP) and injection molding (IM). Experiments included a series of uniaxial tensile tests guided by an ASTM standard. Using displacement control, tests were carried out while images were taken of the gage area. The in-plane displacement fields were evaluated by means of Digital Image Correlation (DIC). A MATLAB program was then used to calculate strains, create stress-strain and strain-force curves and determine Young’s modulus E, Poisson’s ratio \(\nu \), the ultimate tensile stress \(\sigma _{uts}\) and the strain to failure \(\varepsilon _{f}\). In addition, simulations were carried out using a micromechanical model (High-Fidelity Generalized Method of Cells or HFGMC). A Repeating Unit Cell (RUC) consisting of one CNT and PMMA surrounding it was modeled and analyzed in order to determine the effective mechanical properties of the composite. This method allows for imperfect bonding between the phases which is controlled by two parameters. These damage parameters decrease the stress-strain response of the material. However, the increase of the volume fraction increases the composite response. These two conflicting effects appear to provide the observed decrease in Young’s modulus for low volume fractions as discussed. The effects of CNT concentration, geometry and orientation, as well as the interface between the phases, were examined. It was seen from the experimental results, for HP specimens, that for low concentrations of CNTs, E initially decreases and then increases significantly as the weight fraction increases. This behavior of E was quantitatively predicted by the HFGMC model. For IM specimens, Young’s modulus is nearly constant for low weight fractions of CNTs and then increases with weight fraction.

Influence of multi-walled carbon nanotubes concentration on the properties of nanocomposites with poly(o-ethoxyaniline)

The influence of multi-walled carbon nanotubes (MWNTs) concentration on the physical and chemical properties of poly(o-ethoxyaniline) (POEA) nanocomposite films was investigated. We synthesized the POEA-MWNTs nanocomposites by in situ chemical oxidative polymerization at different monomer/MWNTs weight ratios under standardized conditions and thin films were fabricated by Langmuir–Shaefer (LS) method. In all cases, the synthesized nanocomposites showed enhanced physical and chemical properties and morphology with respect to pure POEA films. Experimental data showed the different concentration of MWNTs in the medium of reaction affected the final oxidation ratio of the polymer chains and highlighted that the conducting polymer chains simply wrapped up around MWNTs with no strong interaction. The presence of a very low concentration of carbon nanotubes (CNTs) inside the polymer matrix strongly enhanced the polymer chains growing process by tangling around the surface of MWNTs with a major degree of freedom and higher state of protonation.

Activation of the phospholipase C signaling pathway in nerve growth factor-treated neurons by carbon nanotubes

Low concentrations of carbon nanotubes (CNTs) promoted the number of nerve growth factor (NGF)-treated neurons with neurite outgrowth by activating extracellular signal-regulated kinase (ERK), even when MEK inhibitor was added to the neuron culture medium. We speculated that CNTs may activate ERK through the phospholipase C (PLC) signaling pathway independent of the Ras/Raf/MEK cascade involved in the ERK signaling pathway. CNTs enhanced phosphorylation of PLC-γ1 in NGF-treated neurons but failed to increase the number and length of neurites of NGF-treated neurons with neurite outgrowth when a PLC inhibitor, an inositol triphosphate receptor (IP3R) inhibitor, or an inhibitor of protein kinase C (PKC) in the PLC signaling pathway were added to the neuron culture medium. Furthermore, intracellular Ca++ levels of cells treated with CNTs+NGF were higher than those of cells treated with NGF alone. Although the combination of CNTs and NGF increased the concentration of phosphorylated ERK (p-ERK) in MEK inhibitor-treated neurons, CNTs did not induce phosphorylation of ERK in PLC inhibitor-treated neurons. These data suggest that PKC in the PLC signaling pathway may activate ERK independent of the Ras/Raf/MEK cascade. In summary, we identified a role of PLC signaling in mediating neurite outgrowth of NGF-treated neurons in the presence of CNTs.

Safety Evaluation of Engineered Nanomaterials for Health Risk Assessment: An Experimental Tiered Testing Approach Using Pristine and Functionalized Carbon Nanotubes

Increasing application of engineered nanomaterials within occupational, environmental, and consumer settings has raised the levels of public concern regarding possible adverse effects on human health. We applied a tiered testing strategy including (i) a first in vitro stage to investigate general toxicity endpoints, followed by (ii) a focused in vivo experiment. Cytotoxicity of laboratory-made functionalized multiwalled carbon nanotubes (CNTs) (i.e., MW-COOH and MW-NH2), compared to pristine MWCNTs, carbon black, and silica, has been assessed in human A549 pneumocytes by MTT assay and calcein/propidium iodide (PI) staining. Purity and physicochemical properties of the test nanomaterials were also determined. Subsequently, pulmonary toxic effects were assessed in rats, 16 days after MWCNTs i.t. administration (1 mg/kg b.w.), investigating lung histopathology and monitoring several markers of lung toxicity, inflammation, and fibrosis. In vitro data: calcein/PI test indicated no cell viability loss after all CNTs treatment; MTT assay showed false positive cytotoxic response, occurring not dose dependently at exceedingly low CNT concentrations (1 μg/mL). In vivo results demonstrated a general pulmonary toxicity coupled with inflammatory response, without overt signs of fibrosis and granuloma formation, irrespective of nanotube functionalization. This multitiered approach contributed to clarifying the CNT toxicity mechanisms improving the overall understanding of the possible adverse outcomes resulting from CNT exposure.

Mechanical and interfacial evaluation of CNT/polypropylene composites and monitoring of damage using electrical resistance measurements

Carbon nanotube (CNT)/polypropylene (PP) composites were compounded using a solvent dispersion method to more uniformly disperse the filler. A twin screw extruder was then used to manufacture specimens. The effect of low CNT concentrations on the mechanical and interfacial properties of PP were investigated using tensile and impact tests as well as a pull-out test of a microdroplet of the composite, on a single fiber. Low concentrations of CNT resulted in small, but significant increases, in Young’s modulus, impact strength and interfacial adhesion . The increase in these mechanical properties is attributed to good reinforcing effects of the CNT filler. Measurement of the change in electrical resistance during bending and fatigue loading was used to monitor internal damage in the CNT/PP composite specimens. For CNT/PP composites with low CNT concentrations, these resistance measurements provide useful insight into internal failure, during cyclic loading.

Fabrication and electro-optic properties of a MWCNT driven novel electroluminescent lamp

We present a novel, cost-effective and facile technique, wherein multi-walled carbon nanotubes (CNTs) were used to transform a photoluminescent material to exhibit stable and efficient electroluminescence (EL) at low voltages. As a case study, a commercially available ZnS:Cu phosphor (P-22G having a quantum yield of 65 ± 5%) was combined with a very low (∼0.01 wt%) concentration of CNTs dispersed in ethanol and its alternating current driven electroluminescence (AC-EL) is demonstrated. The role of CNTs has been understood as a local electric field enhancer and facilitator in the hot carrier injection inside the ZnS crystal to produce EL in the hybrid material. The mechanism of EL is discussed using an internal field emission model, intra-CNT impact excitation and the recombination of electrons and holes through the impurity states.

In situ thermoresistive characterization of multifunctional composites of carbon nanotubes

The thermoresistive behavior of nanocomposites with electrically conductive networks of carbon nanotubes (CNTs) in a vinyl ester polymer matrix were characterized using an in situ electrical resistance measurement during thermomechanical analysis. A series of specimens above the electrical percolation threshold ranging from 0.10 to 1.00 wt% CNT were studied. The results show the thermoresistive behavior is strongly dependent on the CNT concentration, thermal expansion and polymer segmental motion. At higher concentrations of carbon nanotubes the temperature coefficient of resistance (TCR) varied with temperature and could either be positive or negative while specimens with low CNT concentrations near the percolation threshold showed a near-zero but negative TCR. Since the nanocomposite bulk resistivity of the specimens is dominated by the tunneling resistance at CNT/CNT junctions the nanotubes effectively act as a network of sensors that can detect thermal transitions and other thermochemical changes in situ.

Carbon nanotubes–SU8 composite for flexible conductive inkjet printable applications

We have developed a conducting composite based on multi-walled carbon nanotubes and bisphenol A novolak epoxy resin (SU8) for inkjet printing purposes. We could deposit regular conducting stripes and arrays of 150 μm spots on glass and on flexible polyethylene terephthalate (PET) films. The UV and temperature processed structures show very good adhesion and reasonably good resistivity with a low concentration of carbon nanotubes. Our composite material for inkjet deposition is completely compatible with current MEMS fabrication processes.

Acid-treated SWCNT/polyurethane nanoweb as a stretchable and transparent Conductor

To obtain carbon-nanotube (CNT) conductors with high conductance and transmittance, well-connected conducting paths must be formed with a relatively small number of CNTs. In this study, we employed electrospun polyurethane (PU) nanofibers to guide the deposition of CNTs at desired locations, which makes it possible to acquire high conductivity at low concentrations of CNTs. PU has an affinity for CNTs and inherently elastic characteristics; thus, it can be applied as a stretchable scaffold for CNT deposition. A sample fabricated by nine dipping-and-washing cycles in acid-treated single-walled CNT (a-SWCNT) solution and distilled water, followed by doping, exhibited a sheet resistance of 424 Ω sq−1 at 63%T. When we measured the resistance of a-SWCNT/PU conductors under a wide range of strains (up to 100%), their change in resistance was reduced as more stretching/releasing cycles were applied. In addition, these variations in resistance according to the number of stretching cycles could be significantly reduced when dipping and doping processes were performed using a pre-stretched PU nanoweb. After more than six stretching cycles, the resistance under a tensile strain of up to 100% remained nearly stable at a value of 8.15 kΩ. Our results demonstrate an effective way of preparing a transparent and stretchable conductor with stable performance during repetitive stretching and releasing.

Carbon-nanotube sensitized nematic elastomer composites for IR-visible photo-actuation

Liquid crystal elastomers (LCEs) are of considerable interest for their potential as actuators, due to the ability of aligned monodomain LCEs to reversibly change their bulk dimensions in response to a phase change. Many LCEs reported in the literature will contract in one dimension in response to a temperature change or irradiation with ultraviolet light. For practical applications, photo-actuation is a more useful technology than thermal actuation due to the achievable speed and localization of the response. The use of UV light sources to ‘switch’ the materials is impractical, however, due to considerations of both safety and cost. Sensitization of LCEs to light of higher wavelengths may be achieved by mixing a small concentration of carbon nanotubes (CNTs) into the polymer matrix; CNTs will absorb light over a large range of wavelengths, and convert it into local heat, thus triggering the required phase changes in the LCE. In this article we demonstrate that a low concentration of multi-walled CNTs does not affect the thermal response of the polymers but does significantly enhance their response to infra-red (IR) and visible light.

Carbon Nanotubes and Carbon Nanofibers for Enhancing the Mechanical Properties of Nanocomposite Cementitious Materials

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are quickly becoming two of the most promising nanomaterials because of their unique mechanical properties. The size and aspect ratio of CNFs and CNTs mean that they can be distributed on a much finer scale than commonly used microreinforcing fibers. As a result, microcracks are interrupted much more quickly during propagation in a nano- reinforced matrix, producing much smaller crack widths at the point of first contact between the moving crack front and the reinforcement. In this study, untreated CNTs and CNFs are added to cement matrix composites in concentrations of 0.1 and 0.2% by weight of cement. The nanofilaments are dispersed by using an ultrasonic mixer and then cast into molds. Each specimen is tested in a custom-made three-point flexural test fixture to record its mechanical properties; namely, the Young's modulus, flexural strength, ultimate strain capacity, and fracture toughness, at 7, 14, and 28 days. A scanning electron microscope (SEM) is used to discern the difference between crack bridging and fiber pullout. Test results show that the strength, ductility, and fracture toughness can be improved with the addition of low concentrations of either CNTs or CNFs. DOI: 10.1061/(ASCE)MT.1943-5533.0000266. © 2011 American Society of Civil Engineers.

Highly Sensitive Detection and Quantitative Analysis of Ultra Low Concentration of Carbon Nanotubes by Absorptiometric Method

Highly Sensitive Detection and Quantitative Analysis of Ultra Low Concentration of Carbon Nanotubes by Absorptiometric Method

Effects of water-soluble functionalized multi-walled carbon nanotubes examined by different cytotoxicity methods in human astrocyte D384 and lung A549 cells

The widespread projected use of functionalized carbon nanotubes (CNTs) makes it important to understand their potential harmful effects. Two cell culture systems, human A549 pneumocytes and D384 astrocytoma cells, were used to assess cytotoxicity of multi-walled CNTs (MWCNTs) with varying degrees of functionalization. Laboratory-made highly functionalized hf-MW-NH 2 and less functionalized CNTs (MW-COOH and MW-NH 2) were tested in comparison with pristine MWCNTs, carbon black (CB) and silica (SiO 2) by MTT assay and calcein/propidium iodide (PI) staining. Purity and physicochemical properties of the test nanomaterials were also determined. In both MTT and calcein/PI assays, highly functionalized CNTs (hf-MW-NH 2) caused moderate loss of cell viability at doses ≥100 μg/ml being apparently less cytotoxic than SiO 2. In preparations treated with CB or the other nanotube types (pristine MWCNTs, MW-COOH and the less functionalized amino-substituted MW-NH 2) the calcein/PI test indicated no loss of cell viability, whereas MTT assay apparently showed apparent cytotoxic response, occurring not dose-dependently at exceedingly low CNT concentrations (1 μg/ml). The latter nanomaterials were difficult to disperse showing higher aggregate ranges and tendency to agglomerate in bundle-like form in cell cultures. In contrast, hf-MW-NH 2 were water soluble and easily dispersible in medium; they presented lower aggregate size range as well as considerably lower length to diameter ratios and low tendency to form aggregates compared to the other CNTs tested. The MTT data may reflect a false positive cytotoxicity signal possibly due to non-specific CNT interaction with cell culture components. Thus, these properties obtained by chemical functionalization, such as water solubility, high dispersibility and low agglomeration tendency were relevant factors in modulating cytotoxicity. This study indicates that properties obtained by chemical functionalization, such as water solubility, high dispersibility and low agglomeration tendency are relevant factors in modulating cytotoxicity of CNTs.

Colloidal poly(styrene-co-butyl acrylate)/multi-walled carbon nanotubes nanocomposite by heterocoagulation in aqueous media

Abstract Colloidal nanocomposite particles were prepared by heterocoagulation of carbon nanotubes (CNTs) dispersion stabilized by cationic hexadecyltrimethylammonium bromide (CTAB) and negatively charged poly(styrene-co-butyl acrylate), P(St-co-BA), latex prepared by conventional emulsion polymerization in aqueous medium with the aid of a flocculant, polyaluminium chloride (PAC), at elevated temperature above the T g of P(St-co-BA). The hybrid nanocomposite particles were prepared by a simple procedure consisting of mixing two dispersions, particle growth, and stabilization steps. The effects of the surface properties of CNTs, molecular weight of latex polymer, types of flocculants and its optimum concentration were investigated. At a well-controlled condition, the spherical or potato-like particles in the size range of 10–100 μm were achieved. After film formation of the nanocomposite particles, it was able to confirm that CNTs were uniformly distributed without agglomeration in the matrix, resulting in the percolation at a low concentration of CNTs.

A Micromechanics Model for the Electrical Conductivity of Nanotube-Polymer Nanocomposites

The introduction of carbon nanotubes (CNTs) into nonconducting polymers has been observed to yield orders of magnitude increases in conductivity at very low concentrations of CNTs. These low percolation concentrations have been attributed to both the formation of conductive networks of CNTs within the polymer and to a nanoscale effect associated with the ability of electrons to transfer from one CNT to another known as electron hopping. In the present work, a micromechanics model is developed to assess the impact of the effects of electron hopping and the formation of conductive networks on the electrical conductivity of CNT-polymer nanocomposites. The micromechanics model uses the composite cylinders model as a nanoscale representative volume element where the effects of electron hopping are introduced in the form of a continuum interphase layer, resulting in a distinct percolation concentration associated with electron hopping. Changes in the aspect ratio of the nanoscale representative volume element are used to reflect the changes in nanocomposite conductivity associated with the formation of conductive networks due to the formation of nanotube bundles. The model results are compared with experimental data in the literature for both single- and multi-walled CNT nanocomposites where it is observed that the model developed is able to qualitatively explain the relative impact of electron hopping and nanotube bundling on the nanocomposite conductivity and percolation concentrations.

Modified and unmodified multiwalled carbon nanotubes in high performance solution-styrene–butadiene and butadiene rubber blends

The outstanding properties of carbon nanotubes have generated scientific and technical interests in the development of nanotube-reinforced polymer composites. Therefore, we investigated a novel mixing approach for achieving a good dispersion of multiwalled carbon nanotubes (CNTs) in a rubber blend. In this approach the CNTs were incorporated into a 50:50 blend of solution-styrene–butadiene rubber and butadiene rubber. First, the CNTs were predispersed in ethanol and then this CNT–alcohol suspension was mixed with the rubber blend at elevated temperature. The rubber nanocomposites prepared by such method exhibit significantly enhanced physical properties already at very low nanotube concentrations. Additionally, we have analysed the dielectric and thermal properties of the compound. The high aspect ratio of the carbon nanotubes enabled the formation of a conductive percolating network in these composites at concentrations below 2wt.%. In contrast to the electrical conduction behaviour, the thermal conductivity of the composites has not been influenced significantly by the presence of carbon nanotubes. Dynamic mechanical analysis indicates that the incorporation of CNTs affects the glass transition behaviour by reducing the height of the tanδ peak considerably. Above the glass transition temperature the storage modulus has been increased after incorporation of a small amount of CNTs. Finally, the ‘Payne effect’, an indication of filler–filler interactions, was observed at very low concentrations of CNT in the rubber matrix.

Influence of Carbon Nanotube Aspect Ratio on Normal Stress Differences in Isotactic Polypropylene Nanocomposite Melts

We consider the impact of varying the aspect ratio A of carbon nanotubes (CNT) on the rheological properties of isotactic polypropylene/CNT nanocomposites. Specifically, we focus on multiwall CNT having a relatively low aspect ratio (A in the range from 22 to 45), since previous work (Kharchenko et al., Nature Mater. 2004) has emphasized the rheological properties of relatively high aspect ratio multiwall CNT nanocomposites (A from 300 to 400). Correspondingly, we find that the formation of a nonequilibrium CNT network structure occurs at a higher CNT concentration in our low A CNT nanocomposites, as theoretically expected. The proposed mechanism for the large apparent negative normal stress differences (ΔN) described by Kharchenko et al. relies on the capacity of the “struts” of the CNT network to rotate about their impingement junctions, much like the links of a deformed chain link fence. This model implies the absence of appreciable negative ΔN at low CNT concentrations where the network does not yet exist and also for short CNT and at high concentrations of CNT where the mesh size of the network becomes too small to accommodate appreciable rotational distortion. In conformity with this simple mechanical model, we observe only a positive apparent ΔN in our CNT/iPP nanocomposites, even well above the CNT gelation concentration. This striking change in the rheology of CNT nanocomposites with a change in A has been further confirmed in die-swell measurements, where a large die swell has been seen in the short CNT nanocomposites, rather than the die-shrinkage found before for the large A or the highly “entangled” CNT network counterpart.