Well-dispersed Carbon Nanotubes Research Articles

Computational micromechanics modeling of piezoresistivity in carbon nanotube–polymer nanocomposites

The macroscale piezoresistive response, i.e. the change in electrical resistivity under the application of strain, of carbon nanotube–polymer nanocomposites has been observed to lead to gauge factors which are much larger than the gauge factors of commonly used strain gauges. Whereas most strain gauges rely on geometric effects, the gauge factors of carbon nanotube–polymer nanocomposites are the result of a combination of nanoscale mechanisms, namely electrical tunneling (electron hopping) and carbon nanotube inherent piezoresistivity, which can lead to substantial differences between the nanocomposite resistivity at zero strain and the resistivity under an applied strain. This paper focuses on modeling the piezoresistive effect of carbon nanotube–polymer nanocomposites by using computational micromechanics techniques based on finite element analysis. For nanocomposites with aligned carbon nanotubes, an electromechanically coupled code is developed for nominal well-dispersed carbon nanotube representative volume elements (RVEs) and for non well-dispersed cases in the aligned and transverse directions. The microscale mechanisms that may have a substantial influence on the overall piezoresistivity of the nanocomposites, i.e. the electrical tunneling effect, and the coupled effect of the electrical tunneling effect and the inherent piezoresistivity of the carbon nanotube, are included in microscale RVEs in order to understand their influence on macroscale piezoresistive response in terms of both the normalized change in effective resistivity and the corresponding effective gauge factor under applied strain. It is found that in the transverse directions, the electrical tunneling effect is the dominant mechanism, and in order for the inherent carbon nanotube piezoresistivity to have a noticeable coupling effect or influence, the local volume fraction of the carbon nanotube should be sufficiently high or the height of barrier of the polymer matrix should be sufficiently low. It is also found that in the axial direction, although the electrical tunneling effect is still the dominant mechanism, the inherent piezoresistivity of the carbon nanotube may have a substantial contribution to the overall axial piezoresistive response.

Enhancing the effectiveness of silicone thermal grease by the addition of functionalized carbon nanotubes

Functionalized carbon nanotubes (CNTs) were introduced into silicone grease to accompany the subsistent metallic oxide particles (micron-sized Al2O3, submicron-sized ZnO) with the aim of enhancing the thermal contact conductance of the composite grease as thermal interface materials (TIMs). The well-dispersed CNTs located among the metallic oxide particles to construct a three dimensional network structure and cooperated with them to form a highly efficient thermal transferring path. The functionalization of CNTs played a key role in achieving a good dispersion of CNTs in silicone grease matrix. The carboxylated CNTs were observed to show better dispersion in silicone grease and weaker reaction with oxide particles than pristine CNTs and amino-functionalized CNTs. Thus the thermal impedance of the silicone grease could be further decreased by 35% (as low as 0.18cm2K/W) with the addition of 2wt.% carboxylated CNTs. Finally, such CNT-modified silicone grease was used to enhance the performance of high-power light emitting diode and showed the prospective applications in TIMs.

Influence of Trace Amount of Well-Dispersed Carbon Nanotubes on Structural Development and Tensile Properties of Polypropylene

Functionalized multiwalled carbon nanotubes (F-MWCNTs) were individually dispersed in a commercial polypropylene (PP) matrix using our well-established α-zirconium phosphate (ZrP) nanoplatelet-assisted dispersion approach. The F-MWCNTs remained uniformly dispersed in PP after melt mixing and injection-molding and were found to remarkably enhance modulus and strength at only 0.1 wt % loading. The individual F-MWCNTs were mostly oriented and extended in the flow direction and were shown to be integrated within the crystalline structure of the matrix. The change in mechanical properties is attributed to both the modification in crystal structure due to MWCNT-induced nucleation and the direct reinforcement of crystalline lamellae and amorphous regions by the MWCNTs. We propose that the compatibilized F-MWCNTs exhibit sufficient interfacial interaction with the PP matrix to anchor lamellae stacks and resist interlamellar slip. The F-MWCNTs also reinforce amorphous domains between crystallites and behave as “su...

Fabrication of spherical cellulose/carbon tubes hybrid adsorbent anchored with welan gum polysaccharide and its potential in adsorbing methylene blue

A high performance adsorbent was performed by emulsification of carbon nanotubes (CNTs) entrapped cellulose solution, and followed by grafting welan gum onto the support as tentacle-type polysaccharide ligands. The morphology and mechanical properties of the adsorbent were characterized with optical microscopy, scanning electron microscopy, and flow hydrodynamics. The well-dispersed CNTs throughout the support as nanofillers were proved to effectively enhance both the structure stability and the mechanical strength of support. Moreover, the tentacle-type adsorbent was further investigated carefully for its efficiency in adsorbing methylene blue from water in a batch system. It demonstrated that higher pH and longer adsorption time contributed to larger adsorption capacity since the adsorbent provided more adsorption sites available for the dye adsorption at such conditions. Fitting the isotherm data to Langmuir and Sips models gave the conclusion that the adsorption behavior matched best with Sips model, proving a multilayer adsorption at lower concentration and monolayer adsorption at higher concentration. The equilibrium adsorption capacity onto the adsorbent was determined to be 302.1mgg−1 at pH 6.0 from Sips model. Combined with the advantages of mechanical strength and adsorption performance, the novel adsorbent would be considered to a top-priority adsorbent for separation field.

Inhalation Toxicity Assessment of Carbon-Based Nanoparticles

Although the demand for nanomaterials has grown, researchers have not conclusively determined the effects of nanomaterials on the human body. To understand the effects of nanomaterials on occupational health, we need to estimate the respiratory toxicity of nanomaterials through inhalation studies, intratracheal instillation studies, and pharyngeal aspiration studies. The discrepancies observed among these studies tend to result from differences in the physiochemical properties of nanomaterials, such as aggregation and dispersion. Therefore, in all toxicity studies, identification of the physicochemical properties of nanomaterials is essential. This Account reviews the inhalation toxicity of manufactured nanomaterials and compares them with inhalation and intratracheal instillation studies of well-characterized fullerene and carbon nanotubes. In many reports, pulmonary inflammation and injury served as pulmonary endpoints for the inhalation toxicity. To assess pulmonary inflammation, we examined neutrophil and macrophage infiltration in the alveolar and/or interstitial space, and the expression of the neutrophil and/or monocyte chemokines. We also reported the release of lactate dehydrogenase (LDH) and alkaline phosphatase (ALP) in the bronchoalveolar lavage fluid (BALF), the expression of oxidative stress-related genes characteristic of lung injury, and the presence of granulomatous lesion and pulmonary fibrosis. In the inhalation and intratracheal instillation studies of well-characterized fullerenes, exposure to fullerene did not induce pulmonary inflammation or transient inflammation. By contrast, in an inhalation study, a high concentration of multiwall carbon nanotubes (MWCNTs) and single-wall carbon nanotubes (SWCNTs) induced neutrophil inflammation or granulomatous formations in the lung, and intratracheal instillation of MWCNTs and SWCNTs induced persistent inflammation in the lung. Among the physicochemical properties of carbon nanotubes, the increased surface area is associated with inflammatory activity as measured by the increase in the rate of neutrophils measured in bronchoalveolar lavage fluid. Metal impurities such as iron and nickel enhanced the pulmonary toxicity of carbon nanotubes, and SWCNTs that included an amorphous carbon induced multifocal granulomas in the lung while purer SWCNTs did not. The aggregation state also affects pulmonary response: Exposure to well-dispersed carbon nanotubes led to the thickening of the alveolar wall and fewer granulomatous lesions in the lung, while agglomerated carbon nanotubes produced granulomatous inflammation. The values of the acceptable exposure concentration in some countries were based on the data of subacute and subchronic inhalation and intratracheal instillation studies of well-characterized fullerene and carbon nanotubes. In Japan, the acceptable exposure concentration of fullerene is 0.39 mg/m³. In Europe, the proposal concentration is 44.4 μg/m³ for acute toxicity and 0.27 μg/m³ for chronic toxicity. The proposal acceptable exposure concentrations of carbon nanotubes are 0.03, 0.05, and 0.007 mg/m³ in Japan, Europe, and the United States, respectively.

Compression and tension stress-sensing of carbon nanotube-reinforced cement

This paper is aimed at characterising the self-sensing capability of carbon nanotube-reinforced cement, as enabled by piezoresistivity. Carbon nanotubes/cement composites with two different levels of carbon nanotube content (0·2% and 0·3% by weight of cement) are considered in the study and their pressure-sensitivity properties under both tension and compression are examined. The primary focus of the work is to explore the effects of two parameters including type of loading (tension or compression) and carbon nanotube concentration on the composite's piezoresistive behaviours. The experimental results show profound piezoresistive behaviours observed from composite with well-dispersed carbon nanotubes of only 0·3 wt%. It is also confirmed that tensile loading increases electrical resistance of the composite, while the opposite occurs upon compression. Furthermore, the composite's pressure sensitivity to changes in either the compressive or tensile stresses increases with the concentration of carbon nanotubes, as confirmed by the consistent changes in electrical resistance in response to both compressive and tensile stresses.

Assembly of modified ferritin proteins on carbon nanotubes and its electrocatalytic activity for oxygen reduction

Highly effective dispersions of carbon nanotubes (CNTs) can be made using a commercially available buffer solution. Buffer solutions of 3-(N-morpholino)-propanesulfonic acid (MOPS), which consists of a cyclic ring with nitrogen and oxygen heteroatoms, a charged group, and an alkyl chain greatly enhance the dispersibility and stability of CNTs in aqueous solutions. Additionally, the ability of biomolecules, especially cationized Pt-cored ferritins, to adhere onto the well-dispersed CNTs in the aqueous buffer solution is also improved. This was accomplished without the use of surfactant molecules, which are detrimental to the electrical, mechanical, and other physical properties of the resulting products. The assembled Pt-cored ferritin proteins on the CNTs were used as an electrocatalyst for oxygen reduction.

A simple approach to prepare Al/CNT composite: Spread–Dispersion (SD) method

In carbon nanotube (CNT) reinforced metal matrix composites (MMCs), the good dispersion of CNTs in the matrix as well as the processing problems are the major challenges inhibiting the development of these composites. In this study, well-dispersed CNTs reinforced aluminum (Al) matrix nanocomposite was fabricated by a novel Spread–Dispersion (SD) method. Specimens with ultra-fine grain size down to 20 nm were obtained. The tensile strength of the CNT nanocomposite was 66% greater than the base matrix with a minor decrease in ductility. Such enhancement was analyzed on the basis of segregation and uniform distribution of clustered CNTs, disappearance of the CNT-free zones, eliminated porosity, stronger Al/CNT bonding and the retention of CNT graphitic structure.

Characterization and Spark Plasma Sintering of Mechanically Milled Aluminum-Carbon Nanotube (CNT) Composite Powders

Carbon nanotube (CNT) reinforced metallic composites have been recently gaining a significant amount of scientific interest. The promise of transmitting part of the remarkable and outstanding properties of CNTs to metals has spurred efforts to design and manufacture metallic composites reinforced with CNTs. Out of the metallic matrices investigated, Al has received significant attention. The stability of these powders at high temperatures is important to investigate, since they would ultimately be consolidated at elevated temperatures into useful products, and service temperature limits will be important. In this article, we discuss the detailed characterization of SPEX milled Al-CNT powders (well-dispersed CNTs in an Al matrix at 2.5 and 5.0 wt% loading), their heat treatment and consolidation using spark plasma sintering.

Electrophoretic deposition of carbon nanotubes and bioactive glass particles for bioactive composite coatings

The production of bioactive coatings consisting of 45S5 Bioglass ® and mutli-walled carbon nanotubes (CNTs) by electrophoretic deposition (EPD) was investigated. In addition to pure Bioglass ® coatings, the co-deposition and sequential deposition of Bioglass ® particles (size <5 μm) and CNTs on stainless steel substrates were carried out in order to fabricate bioactive, nanostructured composite layers. The optimal experimental conditions were determined using well-dispersed suspensions by means of a trial-and-error approach by varying the relevant EPD parameters: applied voltage and deposition time. SEM images demonstrated the successful fabrication of Bioglass ®/CNT composites by revealing their morphology and topography. The co-deposition of Bioglass ® particles and CNTs resulted in homogenous and dense coatings exhibiting the presence of well-dispersed CNTs placed in-between micron-sized Bioglass ® particles. This network of high-strength CNTs embedded in the glass layer could act as reinforcing element leading to higher mechanical stability of the coatings. The coatings obtained by sequential deposition offered a two-dimensional nanostructured fibrous mesh of CNTs covering the Bioglass ® layer thus providing a controlled (ordered) nano-topographical surface. This surface nanostructure has the potential to promote the attachment and growth of osteoblast cells and to benefit the formation of bone-like nanosized hydroxyapaptite crystals in contact with body fluids.

Assessment of dispersion in carbon nanotube reinforced composites using differential scanning calorimetry

For assessing the state of dispersion, which plays an important role in improving the mechanical properties of carbon nanotube (CNT) reinforced composite material, the relationship between the state of particle distribution and the change in the cure kinetics was experimentally investigated by the use of differential scanning calorimetry. It was found that the addition of CNTs to epoxy resin reduced the total heat of reaction. This may be because CNTs act as obstacles to the cross-linking reaction. The decrease was found to depend on the CNT concentration as well as the state of dispersion. To obtain a correlation between the decrease in the total heat of reaction and the quantity of well-dispersed particles, the mass of aggregated particles was measured by dynamic light scattering. The results showed that there is a correlation between the concentration of well-dispersed CNTs and the total heat of reaction. Any departure from this correlation could be interpreted as poor dispersion and the magnitude of the difference may serve as an indicator of the overall state of dispersion.

Simultaneous alignment and micropatterning of carbon nanotubes using modulated magnetic field

We report simultaneous alignment and micropatterning of carbon nanotubes (CNTs) using a high magnetic field. It is important to prepare well-dispersed CNTs for alignment and patterning because CNT aggregation obstructs alignment. In magnetic field, highly anisotropic CNTs rotate in the direction stabilized in energy. Owing to their diamagnetic nature, CNTs suspended in a liquid medium are trapped in a weak magnetic field generated by a field modulator; meanwhile, they align to the applied strong magnetic field. The alignment has been achieved not only in polymers but also in ceramic and silicone composites.

Fabrication and properties of aligned multiwalled carbon nanotube-reinforced epoxy composites

A method to fabricate continuous and aligned multiwalled carbon nanotube (CNT)/epoxy composites is presented in this paper. CNT/epoxy composites were made by infiltrating an epoxy resin into a stack of continuous and aligned multiwalled CNT sheets that were drawn from super-aligned CNT arrays. By controlling the amount and alignment of the continuous multiwalled CNT sheets, a CNT/epoxy composite with high content of well-dispersed CNTs can be obtained. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) results show that the thermal stability of these CNT/epoxy composites was not affected by the addition of CNTs. The mechanical properties and electrical properties of the CNT/epoxy composites were dramatically improved compared to pure epoxy, suggesting that the CNT/epoxy composites can serve as multifunctional materials with combined mechanical and physical properties.

Role of structure and morphology in the elastic modulus of carbon nanotube composites

The nature of nanoscale reinforcements in the carbon nanotube composites indicates nanocomposite properties are heavily dependent on the micro/nano-structure and morphology. Macroscopic parameter-based properties estimation may lead to deviation as large as 30%. In this paper, a modified shear-lag model, combined with probability statistical theory and composites morphology, is established to investigate the elastic properties of single wall carbon nanotubes (SWNTs)-reinforced polymer composites. The computational results indicated that elastic modulus of nanocomposite was remarkably dependent on the micro/nano-structure, including diameter, length, and orientation of the dispersed SWNTs. Microstructure-dependent shape factor and orientation effect factor played a key role on achieving high-performance nanocomposites. Elastic modulus of nanocomposite with well-dispersed carbon nanotubes was more susceptible to the orientation. Similarly, nanocomposite modulus was more subject to the dispersion influence when SWNTs were well-aligned. The maximal modulus was located in the zone of small rope diameters and small orientation angles when adequate interfacial bonding was provided. The computational results were also compared with experimental outcome and demonstrated good consistence.

Synthesis of carbon nanotubes and nanospheres with controlled morphology using differentcatalyst precursors

Carbon nanotubes and carbon nanospheres have been selectively synthesized in apolypropylene matrix by simply altering the catalyst precursor. When a mixture offerrocene and cobalt (II) acetate was added to the polypropylene, well-dispersed carbonnanotubes were produced. When only cobalt (II) acetate was added, however, carbonnanospheres were obtained. In a third case, when the added catalyst precursor wasferrocene, no graphitic carbon was formed.

On the influence of the processing conditions on the performance of electrically conductive carbon nanotube/polymer nanocomposites

We prepared multiwalled carbon nanotube/polystyrene (MWCNT/PS) nanocomposites using a latex-based process, the main step of which consists of directly mixing an aqueous suspension of exfoliated MWCNTs and a PS latex, both stabilized by an anionic surfactant. After freeze drying and compression molding homogeneous polymer films with well-dispersed carbon nanotubes were produced as evidenced by scanning electron microscopy. Conductivity measurements performed on our nanocomposite films show that they have a low percolation threshold and exhibit high levels of electrical conductivity above this threshold. We observe that both these properties are influenced by the applied processing conditions, i.e., temperature and time, and provide a plausible explanation based on the diffusive motion of the MWNTs in the polymer melt during the compression molding stage.

Multiwalled carbon nanotubes functionalized with PS via emulsion polymerization

This study demonstrated the in-situ functionalization with polymers of multi-walled carbon nanotubes (MWNTs) via emulsion polymerization. Polystyrene-functionalized MWNTs were prepared in an aqueous solution containing styrene monomer, non-ionic surfactant and a cationic coupling agent ([2-(methacryloyloxy)ethyl]trimethylammonium chloride (MATMAC)). This process produced an interesting morphology in which the MWNTs, consisting of bead-string shapes or MWNTs embedded in the beads, when polymer beads were sufficiently large, produced nanohybrid material. This morphology was attributed to the interaction between the cationic coupling agent and the nanotube surface which induced polymerization within the hemimicellar or hemicylindrical structures of surfactant micelles on the surface of the nanotubes. In a solution containing MATMAC alone without surfactant, carbon nanotubes (CNTs) were not well-dispersed, and in a solution containing only surfactant without MATMAC, polymeric beads were synthesized in isolation from CNTs and continued to exist separately. The incorporation of MATMAC and surfactant together enabled large amounts of CNTs (× 0.05 wt%) to be well-dispersed in water and very effectively encapsulated by polymer chains. This method could be applied to other well-dispersed CNT solutions containing amphiphilic molecules, regardless of the type (i.e., anionic, cationic or nonionic). In this way, the solubility and dispersion of nanotubes could be increased in a solvent or polymer matrix. By enhancing the interfacial adhesion, this method might also contribute to the improved dispersion of nanotubes in a polymer matrix and thus the creation of superior polymer nanocomposites.

On the Crucial Role of Wetting in the Preparation of Conductive Polystyrene−Carbon Nanotube Composites

Polystyrene-single-wall carbon nanotube (PS-SWNT) nanocomposites were prepared by directly mixing aqueous suspensions of exfoliated SWNTs and PS latex particles. After freeze-drying and compression molding, homogeneous polymer films were obtained with well-dispersed carbon nanotubes, as evidenced by scanning electron microscopy imaging. The nanocomposite films display a low percolation threshold and high levels of electrical conductivity. Simultaneously, a considerable increase in the glass-transition temperature of PS is achieved, provided that a sufficient amount of low-molar-mass PS is present in the matrix material. It is suggested that a certain extent of molar mass segregation occurs in the samples, with shorter PS chains preferentially adsorbed onto the nanotube surface. The latter wetting mechanism is indispensable for obtaining favorable electrical and thermal properties.

Synthesis and lubricating performance of a carbon nanotube seeded miniemulsion

A novel carbon nanotube (CNT)/polystyrene (PS) hybrid miniemulsion was synthesized by an in situ process. The structure of composites in the prepared miniemulsion was characterized by scanning and transmission electron microscopes. The lubricating property of this emulsion as an additive in water-based fluid was investigated by a four-ball tribo-tester. Results confirmed that the anti-wear performance and load-carrying capacity of the base stock were therefore significantly raised and the friction coefficient decreased. This was attributed to the excellent mechanical properties of CNTs and potentially attributed to the well-dispersed CNT composties acting as micro roller bearings between sliding surfaces.

Polyaniline/Carbon Nanotube Composite Cathode for Rechargeable Lithium Polymer Batteries Assembled with Gel Polymer Electrolyte

A nanocomposite of polyaniline (PANI) and multiwalled carbon nanotube (CNT) was prepared by the in situ chemical polymerization of aniline in well-dispersed CNT solution. The PANI/CNT composite was employed as an active cathode material in rechargeable lithium polymer cells. Gel polymer electrolyte based on the porous poly(vinylidenefluoride-co-hexafluoropropylene) membrane was used as an electrolyte material in assembling the cells. The cell delivered a maximum discharge capacity of at the 80th cycle with an average coulombic efficiency of 98%. To investigate the effect and role of CNT in the above composite, two different kinds of cathodes were prepared by mechanically mixing the pure polyaniline with CNT or super-p carbon separately. The electrochemical performances of these two cathode materials were also evaluated in the lithium polymer cells.