Multi-walled Carbon Nanotubes Clusters Research Articles

Effect of Injection Molding Conditions on Crystalline Structure and Electrical Resistivity of PP/MWCNT Nanocomposites.

Polypropylene (PP) / multi-walled carbon nanotube (MWCNT) nanocomposites were prepared by melt-mixing and used to manufacture samples by injection molding. The effect of processing conditions on the crystallinity and electrical resistivity was studied. Accordingly, samples were produced varying the mold temperature and injection rate, and the DC electrical resistivity was measured. The morphology of MWCNT clusters was studied by optical and electron microscopy, while X-ray diffraction was used to study the role of the crystalline structure of PP. As a result, an anisotropic electrical behavior induced by the process was observed, which is further influenced by the injection molding processing condition. It was demonstrated that a reduction of electrical resistivity can be obtained by increasing mold temperature and injection rate, which was associated to the formation of the γ-phase and the related inter-cluster morphology of the MWCNT conductive network.

Interface Analysis of Spark Plasma Sintered Carbon Nanotube Reinforced Ti6Al4V

This study investigated the dispersion and interface reactions in a multi-walled carbon nanotube (MWCNTs) reinforced Ti6Al4V composite consolidated by the spark plasma sintering technique. The 1 wt.% MWCNTs were dispersed in Ti6Al4V by high-energy ball milling, and the milled composite powder was consolidated at 1000°C. The other parameters adopted during the sintering process were applied pressure (50 MPa), heating rate (100°C/min) and 5 min holding time, respectively. Thin lamellae from the synthesized bulk Ti6Al4V/1 wt.% MWCNT composite were prepared and characterized by transmission Kikuchi diffraction (TKD), scanning electron microscopy and high-resolution scanning transmission electron microscopy. Large agglomerations of MWCNTs in the order of 50–100 µm were distributed in the composite, and the TKD analysis on the sample retrieved from the MWCNT/Ti6Al4V interface had a polycrystalline layer of titanium carbide formed between the Ti6Al4V matrix particles and the MWCNT clusters.

Length-dependent broadband electric properties of PMMA composites filled with carbon nanotubes

Broadband dielectric/electric properties of multiwalled carbon nanotubes (MWCNT) polymethylmetacrylate composites were studied in a wide temperature range (25–400 K). It was established that the percolation threshold is lower in composites with the shortest MWCNT, due to the better distribution of the MWCNT in the polymer matrix. The value of the dielectric permittivity and the electrical conductivity below the percolation threshold is also higher in composites with the shortest MWCNT. Below the percolation threshold the dielectric dispersion in composites is mainly related with the β relaxation in the polymer matrix. The length of the MWCNT and its concentration have also the huge influence on polymer matrix molecular dynamics, the relaxation time is shortest in composites with the higher MWCNT concentration and in composites with the shortest MWCNT. At low temperatures the conductivity of composites above the percolation threshold is governed by an electron tunneling between MWCNT clusters. At higher temperatures the electrical transport occurs also due to the finite electrical conductivity of the polymer matrix.

In English

The effect of acid treatment on the structure and properties of suspensions of multiwalled carbon nanotubes (MWCNTs) in glycerol has been investigated. The concentrations of MWCNTs were within 0–2% by weight. Suspensions were treated during 5–60 min at T = 343 K by a mixture of concentrated nitric and sulphuric acids with the volume ratio of 1:3. The degree of acid treatment influence on the structure of suspension was analyzed using methods of thermal analysis and infrared spectroscopy. Oxidation of MWCNTs could affect significantly their thermal stability and degree of destruction. The changes in the structure of MWCNT clusters in glycerol, electrical conductivity and percolation behaviour of the systems are discussed. The acid treatment is shown to cause significant decrease in conductivity of suspension and enhancement of its temperature dependence. Reduction of the length of MWCNTs after acid treatment is reported.

On the Motion of Carbon Nanotube Clusters near Optical Fiber Tips: Thermophoresis, Radiative Pressure, and Convection Effects.

We analyze the motion of multiwalled carbon nanotubes clusters in water or ethanol upon irradiation with a 975 and 1550 nm laser beam guided by an optical fiber. Upon measuring the velocities of the nanotube clusters in and out of the laser beam cone, we were able to identify thermophoresis, convection and radiation pressure as the main driving forces that determine the equilibrium position of the dispersion at low optical powers: while thermophoresis and convection pull the clusters toward the laser beam axis (negative Soret coefficient), radiation pressure pushes the clusters away from the fiber tip. A theoretical solution for the thermophoretic velocity, which considers interfacial motion and a repulsive potential interaction between the nanotubes and the solvent (hydrophobic interaction), shows that the main mechanism implicated in this type of thermophoresis is the thermal expansion of the fluid, and that the clusters migrate to hotter regions with a characteristic thermal diffusion coefficient D(T) of 9 × 10(-7) cm(2) K(-1) s(-1). We further show that the characteristic length associated with thermophoresis is not that of the nanotube clusters size, O(1) μm, but that corresponding to the microstructure of the clusters, O(1) nm. We finally discuss the role of the formation of gas-liquid interfaces (microbubbles) at high optical powers on the deposition of carbon nanotubes on the optical fiber end faces.

Release Kinetics of Multiwalled Carbon Nanotubes Deposited on Silica Surfaces: Quartz Crystal Microbalance with Dissipation (QCM-D) Measurements and Modeling

Understanding the kinetics of the release of carbon nanotubes (CNTs) from naturally occurring surfaces is crucial for the prediction of the environmental fate and transport of CNTs. In this study, the release kinetics of multiwalled CNTs (MWNTs) from silica surfaces was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). MWNTs were first deposited on silica surfaces under favorable deposition conditions (1.50 mM CaCl2 and pH 7.1) and the deposited MWNTs were then rinsed at different electrolyte solutions to induce the release of MWNTs from the primary energy minimum. The kinetics of MWNT release was shown to be first order with respect to the deposited MWNTs when complete release took place. A model that accounts for the releasable and unreleasable components of MWNTs was used to fit the experimental data in order to derive the release rate coefficients. When the CaCl2 concentration in the eluent was decreased, a larger fraction of deposited MWNTs was released and the release rate coefficient of the releasable MWNTs also increased. The rise in the surface charges of both MWNTs and silica surfaces with the drop in CaCl2 concentration likely resulted in the decrease in the height of the energy barrier, thus facilitating the release of the nanotubes. Moreover, when the initial surface concentrations of deposited MWNTs were over 1000 ng/cm(2), the release rate coefficient was lower than expected. The reduced release kinetics was likely due to the formation of large surface-bound MWNT clusters which had considerably lower diffusion coefficients than dispersed MWNTs or MWNT aggregates.

Regulation of dispersion of carbon nanotubes in binary water+1-Cyclohexyl-2-pyrrolidone mixtures

The microstructure and electrical conductivity of suspensions of multi-walled carbon nanotubes (MWCNTs) in binary water+1-Cyclohexyl-2-pyrrolidone (CHP) liquid mixtures were studied in the temperature interval of 253–318K, in the heating and cooling cycles. The concentration of MWCNTs was varied in the interval between 0 and 1wt% and the content of water in a binary mixture X=[water]/([CHP]+[water]) was varied within 0–1.0. The experimental data have shown that dispersing quality of MWCNTs in a mixture of good (CHP) and bad (water) solvents may be finely regulated by adjustment of composition of the CHP+water mixtures. The aggregation ability of MWCNTs in dependence on X was discussed. The surface of MWCNT clusters was highly tortuous, its fractal dimension df increased with increase of X, approaching ≈1.9 at X→1. It was concluded that the surface tension is not suitable characteristic for prediction of dispersion ability in the mixture of good and bad solvents. The electrical conductivity data evidenced the presence of a fuzzy-type percolation with multiple thresholds in the systems under investigation. This behavior was explained by formation of different percolation networks in dependence of MWCNT concentration.

Grain growth analysis of multiwalled carbon nanotube-reinforced bulk Ni composites

This work focuses on the influence of multiwalled carbon nanotube (MWCNT) concentration in the thermal evolution of the microstructure of MWCNT/Ni composites produced by pressureless sintering. The composites were prepared by a colloidal mixing process using 1.0, 2.0, 3.0 and 5.0wt.%. The grain growth is analysed by X-ray diffraction (XRD) and the final microstructure is studied by electron backscattered diffraction (EBSD). The grain growth is observed by analysing the thermal evolution of the width of the XRD reflections. The grain growth rates are influenced by the high thermal conductivity of the nanotubes up to 3.0wt.%, where the agglomeration of CNTs during sintering affects the growth kinetics. The distribution of grain sizes correlates the MWCNTs concentration in the matrix to the Zener pinning effect. The mean grain sizes were 47.6, 22.4, 4.9, 4.9 and 5.3μm for pure Ni, 1.0, 2.0, 3.0 and 5.0wt.%, respectively. An empirical reinforcement limit is settled at 2.0wt.% beyond which, no further microstructural refinement is observed. At higher concentrations, agglomeration occurs and densification is hindered, resulting in large voids filled with MWCNT clusters. Our results highlight the use of blending processes for the manufacturing of metal/CNT composites according to empirical reinforcing limits.

Electrical conductivity and thermal stability of polypropylene containing well-dispersed multi-walled carbon nanotubes disentangled with exfoliated nanoplatelets

The morphology, crystallization behavior, electrical conductivity, and thermal stability of polypropylene (PP) modified with disentangled multi-walled carbon nanotubes (MWCNTs) is reported. Slightly oxidized MWCNT clusters were disentangled in solution by mild sonication in the presence of exfoliated α-zirconium phosphate nanoplatelets. The disentangled MWCNTs were isolated using acid-induced coagulation to precipitate the nanoplatelets, and were subsequently reacted with octadecylamine. The recovered functionalized MWCNTs (F-MWCNTs) are disentangled and easily dispersed in a commercial PP matrix, and serve as more efficient nucleating agents than the untreated MWCNTs. The PP/F-MWCNT composites exhibit an extremely low percolation-like transition in electrical conductivity, which is attributed to the preservation of a random dispersion of disentangled F-MWCNTs upon cooling from the melt. The thermal stability of PP in air is also substantially enhanced at loadings below the percolation threshold due to the tremendous interfacial area between the polymer chains and the free radical scavenging F-MWCNTs. The present approach provides an efficient and potentially scalable route for commercial production of conductive semi-crystalline thermoplastics. The method may be adapted to uniformly disperse MWCNTs in other polymer matrices by appropriate selection of surface functionality.

The thermal flash technique: The inconsequential effect of contact resistance and the characterization of carbon nanotube clusters

This article presents a comprehensive mathematical treatment of the theory behind the thermal flash technique used to measure the thermal diffusivity of nanostructures. Analytical expressions predicting the temperature and its rate of change for various combinations of sample length and diffusivity confirmed that the presence of contact resistance between the heat sink/source or within a cluster of materials does not influence the measurement. Measurements on multi-walled carbon nanotube clusters provide further experimental evidence supporting the claim that contact resistance is inconsequential to this technique and yield a thermal conductivity of 2665 W/m K, which corresponds to an isolated nanotube and not the overall cluster.

Platinum nanoparticle-decorated carbon nanotube clusters on screen-printed gold nanofilm electrode for enhanced electrocatalytic reduction of hydrogen peroxide

In the present study the electrocatalytic behavior of platinum nanoparticle-decorated multi-walled carbon nanotube clusters vertically aligned on a screen-printed gold nanofilm electrode substrate (SPGFE/MWCNTC/PtNP) toward hydrogen peroxide was intensively investigated. Oriented multi-walled carbon nanotube clusters (MWCNTC) were firstly well-organized onto the screen-printed gold film electrode (SPGFE) surface by covalent immobilization, and then platinum nanoparticles (PtNPs) dispersed on the MWCNTC structure were in situ synthesized by electrochemically reducing chloroplatinic acid. It was demonstrated that the MWCNTC/PtNP hybrid structure on the SPGFE substrate could significantly enhance the electrochemical reduction of hydrogen peroxide in neutral solutions. Compared with platinum nanoparticles loading on disordered multi-walled carbon nanotubes (SPGFE/MWCNT/PtNP), the proposed SPGFE/MWCNTC/PtNP exhibited more sensitive responses for hydrogen peroxide reduction. The fabricated electrode provided linear current responses for hydrogen peroxide in the concentration range from 5 to 2000μM with a detection limit of 1.23μM. The non-enzymatic sensor also offered good reproducibility and high selectivity for hydrogen peroxide detection. The carbon nanotube cluster/platinum nanoparticle hybrid system holds great promise as a scaffold to fabricate enzyme biosensors.

Electrical conductivity and fracture behavior of epoxy/polyamide‐12/multiwalled carbon nanotube composites

AbstractUntreated multiwalled carbon nanotubes (MWCNTs) were incorporated into a brittle epoxy matrix at 1 wt% by solvent evaporation and showed significant gains in tensile strength, strain at break, and electrical conductivity. However, only fractional improvement in fracture toughness was observed. To improve fracture toughness, 10 μm polyamide‐12 (PA) preformed particles were added at 20 wt% loading. The PA particles were observed to aid in the dispersion of MWCNTs, reducing most of the large MWCNT clusters to submicron sizes. The epoxy/PA/MWCNT ternary composite exhibits an increased electrical percolation threshold, but nearly identical electrical scaling behavior as the epoxy/MWCNT composite. The similarity in percolation parameters suggest conduction occurs by efficient electron transport through MWCNT clusters, rather than pathways composed of individual tubes. The ternary composite also shows significant improvements in strain at break and fracture toughness due to synergistic interaction between filler phases. The usefulness of this ternary composite approach in vacuum‐assisted resin transfer molding for advanced composite applications is discussed. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers

Electric-field induced elastic stretching of multiwalled carbon nanotube clusters: a realistic model

The oscillating electric-field induced stretching phenomenon of multiwalled carbon nanotube (MWCNT) clusters in liquid crystal medium demonstrates distinct threshold behaviour under optical microscopic investigation. The optimum field required for the initiation of MWCNT cluster stretching is found to depend on their length in the field-off state. The phenomenon has been explained in light of a classical theoretical model assuming MWCNT agglomerates as a single electric dipole. The spring constant and induced charge obtained by fitting the formulated theoretical model show good agreement with previous reports, hence establish the proposed dipolar reorientation mechanism of MWCNT clusters induced by the electric field.

Carbon nanotube clusters as universal bacterial adsorbents and magnetic separation agents

The magnetic susceptibility and high bacterial affinity of carbon nanotube (CNT) clusters highlight their great potential as a magnetic bio-separation agent. This article reports the CNT clusters' capability as "universal" bacterial adsorbents and magnetic separation agents by designing and testing a multiwalled carbon nanotube (MWNT) cluster-based process for bacterial capturing and separation. The reaction system consisted of large clusters of MWNTs for bacterial capture and an external magnet for bio-separation. The designed system was tested and optimized using Escherichia coli as a model bacterium, and further generalized by testing the process with other representative strains of both gram-positive and gram-negative bacteria. For all strains tested, bacterial adsorption to MWNT clusters occurred spontaneously, and the estimated MWNT clusters' adsorption capacities were nearly the same regardless of the types of strains. The bacteria-bound MWNT clusters also responded almost instantaneously to the magnetic field by a rare-earth magnet (0.68 Tesla), and completely separated from the bulk aqueous phase and retained in the system. The results clearly demonstrate their excellent potential as highly effective "universal" bacterial adsorbents for the spontaneous adsorption of any types of bacteria to the clusters and as paramagnetic complexes for the rapid and highly effective magnetic separations.

Nanoelectrode ensembles based on semi-interpenetrating network of carbon nanotubes

A method for preparing nanoelectrode ensembles based on semi-interpenetrating network (SIN) of multi-walled carbon nanotubes (MWNTs) on gold electrode through phase-separation method is initially proposed. Individual nanoelectrode owns irregular three-dimensional MWNTs networks, which is denoted as SIN-MWNTs. On the as-prepared SIN-MWNTs nanoelectrode ensembles, the assembled MWNTs clusters in nanoscale serve as individual nanoelectrode and the electroinactive lipid networks located on the top of alkanethiol monolayer are used as a shielding layer. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), tapping-mode atomic force microscopy (TM-AFM) and scanning electron microscopy (SEM) were used to characterize the as-prepared SIN-MWNT nanoelectrode ensembles. Experimental results indicate that the well-defined nanoelectrode ensembles were prepared through self-assembly technology. Meantime, sigmoid curves in a wide scanning range can be obtained in CV experiments. This study may pave the way for the construction of truly nanoscopic nanoelectrode arrays by bottom-up strategy.

Carbon nanostructures in opposed-flow methane oxy-flames

Carbon nanostructures formed in the opposed-flow flames of methane- and oxygen-enriched air are studied experimentally using thermophoretic sampling technique and high-resolution transmission electron microscopy (TEM). Reconstructed evolution of soot particles along the burner centerline shows the existence of two characteristic layers. The narrow layer of polydisperse precursor particles is located on the oxidizer side of the stagnation plane. The precursor particles undergo carbonization and agglomeration as they are driven to the stagnation plane forming the layer of mature soot aggregates on the fuel side of the precursor layer. High-resolution TEM imaging performed on precursors and mature soot particles reveals the presence of highly organized carbon nanostructures formed inside the tarlike amorphous condensate by carbonization process. Two characteristic structures are observed. They resemble shapes of carbon onions and carbon nanopolyhedral particles. Multiwalled carbon nanotubes (MWNTs) appeared to be present in the collected samples along with other carbon particulates. TEM imaging reveals incidence of isolated MWNTs, MWNT clusters, and clusters of MWNT with soot and nanopolyhedral particles. The nanotube growth by elongation of partially carbonized nanopolyhedral particles is considered as a possible mechanism of noncatalytic MWNT formation.