Dispersion and aspect ratio of carbon nanotubes in aqueous suspension and their relationship with electrical resistivity of carbon nanotube filled polymer composites

Multiwalled carbon nanotube (MWCNT)/acrylonitrile butadiene styrene (ABS) composites were prepared by a processing method using solvent–nonsolvent precipitation. Size distributions of MWCNT agglomerates in aqueous suspension were investigated in order to predict aspect ratio of nanotubes by evaluating the effects of sonication time, MWCNT content, and surfactant. Aspect ratios of MWCNTs were predicted on the basis of the size distribution measurements for MWCNT agglomerates. Sonication time or applied sonic energy has a strong effect on the size distribution of MWCNT agglomerates. Compared with simple shear mixing method, it was shown that this processing method is more suitable for the MWCNT/ABS composites. An electrical percolation threshold was observed for the weight fraction of MWCNTs in the range of about 0.5–1.0 wt.%. Shorter MWCNTs are more suitable to induce fine dispersion, but lead to higher percolation threshold weight fraction. It was illustrated that fine dispersion can overcome the handicap of short length or low aspect ratio of MWCNTs.     

A study on rheological behavior of MWCNTs/epoxy composites

Rheological behaviors of multiwalled carbon nanotubes (MWCNTs)/epoxy composites with various MWCNT contents were investigated by using a time sweep and frequency sweep experiment with oscillatory rheometry. The functional groups on the acid-treated MWCNTs were investigated by fourier transfer-infrared spectroscopy (FT-IR). The composites containing acid-treated MWCNTs exhibited faster gel time than pure epoxy resins. The storage (G′) and loss (G″) moduli of the composites showed solid-like behavior owing to interaction between the MWCNTs and the epoxy resins. The 1.0 wt% MWCNT composites had the highest crosslinking activation energy (E_c) due to good dispersion and strong interfacial bonding. These results imply that three-dimensional crosslinking might take place among the hydroxyl group in epoxy resins and the carbonyl or hydroxyl group in acid-treated MWCNTs.     

Probing the carbon nanotube-surfactant interaction for the preparation of composites

Adsorption isotherms of four different surfactants, sodium dodecyl sulfate (SDS), sodium dodecyl benzyl sulfonate, benzethonium chloride and Triton X-100 were measured on multi-wall carbon nanotubes (MWCNT) in water. With the surfactant SDS, the isotherms were also measured on single-wall carbon nanotubes (SWCNT) as well as on MWCNT under various ionic strength and temperature conditions. The nature of the polar head had only little influence on adsorption which was mainly driven by hydrophobic interactions. However, the outcome of the dispersion experiment was dependent on the purity of the carbon nanotubes. Using these results, it was possible to prepare concentrated colloidaly stable dispersions of MWCNTs in water (c = 32 g/L). Conducting MWCNT/polymer composite films could then readily be prepared by simple formulation of the MWCNTs with a polymeric dispersion.     

The production of carbon nanotube/epoxy composites with a very high dielectric constant and low dielectric loss by microwave curing

Multiwalled carbon nanotube (MWCNT)/epoxy (EP) composites were developed using microwave curing (m-MWCNT/EP). They have a very high dielectric constant and low dielectric loss. For comparison, composites based on the same components were also prepared by thermal curing (t-MWCNT/EP). Results show that the two types of composites have greatly different dielectric properties. With the same content of MWCNTs, m-MWCNT/EP composites show a much higher dielectric constant and lower dielectric loss than t-MWCNT/EP composites. Specifically, the dielectric constant and loss at 100 Hz of m-MWCNT/EP composite with 0.04 vol% MWCNTs are about 2.5 and 0.05 times the corresponding value of t-MWCNT/EP composites, respectively, because of their different structures. Compared with t-MWCNT/EP composites, the nanotubes in m-MWCNT/EP composites not only have a better dispersion in the matrix, but also align in a direction. An equivalent circuit model was set up to evaluate the influence of dispersion and spatial distribution of MWCNTs on the dielectric properties. It shows that it is possible to control the dispersion and spatial distribution of carbon nanotubes using a different curing technique to obtain high performance composites with unexpected dielectric properties, especially those with very high dielectric constant and low dielectric loss.     

The electrical conductivity of carbon nanotube/carbon black/polypropylene composites prepared through multistage stretching extrusion

We reported the design of a grape-cluster-like conductive network in a polypropylene (PP) matrix, where oriented multi-walled carbon nanotubes (MWCNTs) served as branches and provided charge transport over large distances while grape-like carbon black (CB) aggregates enriched around MWCNTs and linked these conductive tubes through charge transport over small distances. The key for construction of this grape-cluster-like conductive network was the extension and orientation of MWCNTs, which was achieved in this work by multistage stretching extrusion with an assembly of laminating-multiplying elements (LMEs, which divide and recombine polymer melts). The highest efficient grape-cluster-like conductive network was obtained at a CB:MWCNT weight ratio of 6. The experimental results showed that this novel grape-cluster-like conductive network provided a low percolation threshold for PP/CB/MWCNT composites due to the synergistic effect of CB and oriented MWCNTs. When the combined CB and MWCNT content was about 6.9 vol%, the electrical resistivity of PP/CB/MWCNT composites prepared by multistage stretching extrusion with 6 LMEs decreased to only 0.63 Ω cm.     

Fabrication and origin of high-k carbon nanotube/epoxy composites with low dielectric loss through layer-by-layer casting technique

The distribution of polarized space charges and their relaxation behavior in high dielectric constant electric conductor/polymer composites are main factors that determine the frequency-dependent dielectric constant and dielectric loss. However, few reports focus on this motif. We present here the dielectric performance and mechanism of a unique kind of composites with multi-layers (coded as [MWCNT/EP]_x, where x refers to the number of layers), fabricated by using layer-by-layer casting technique. Each composite layer with same thickness was composed of multi-walled carbon nanotubes (MWCNTs) and epoxy (EP) resin. When the loading of MWCNTs is 0.5 wt%, the four-layer [MWCNT0.5/EP]₄ material shows the highest dielectric constant (465 at 1 Hz) and low dielectric loss tangent (0.7 at 1 Hz), about 4 and 2.1 × 10⁻² times the values of traditional MWCNT0.5/EP composite, respectively. By investigating the space charge polarization (SCP), Debye polarization and dielectric moduli in [MWCNT/EP]_x materials, the complex relationships and the origin among dielectric constant, dielectric loss, frequency and the content of filler were clearly elucidated. The SCP within each layer is different from that between layers. The greatly improved dielectric properties of [MWCNT/EP]_x materials are believed to be the reinforced SCP and blocked transport of carriers between every two layers.     

New approach for distribution of carbon nanotubes in alumina matrix

Alumina–carbon nanotubes composites were studied with respect to obtain the homogeneous distribution of nanotubes within the alumina matrix. Disaggregation and uniform dispersion of carbon nanotubes in alumina matrix are crucial requirements for improvement fracture toughness and also electrical conductivity of these composites. New approach comprises functionalisation MWCNTs by acid treatment, stabilisation of alumina/MWCNT dispersion with subsequent freezing has been used, which resulted in formation of granulated homogenous mixture. The ceramic composites were prepared by hot pressing at 1550 °C using these mixtures. Microstructural analysis as well as electrical conductivity measurements has been used for observation of distribution of nanotubes within composites. Electrical conductivity, as an indicator of homogeneity of conductive network distribution, increases from 6 to 1140 S/m when compared the conventional process and approach presented in this work at the same volume fraction of MWCNTs 10 vol.%.     

Thermal conductivity of a graphene oxide–carbon nanotube hybrid/epoxy composite

Thermally conductive graphene oxide (GO)–multi-wall carbon nanotube (MWCNT)/epoxy composite materials were fabricated by epoxy wetting. The polar functionality on the GO surface allowed the permeation of the epoxy resin due to a secondary interaction between them, which allowed the fabrication of a composite containing a high concentration of this hybrid filler. The thermal transport properties of the composites were maximized at 50 wt.% of filler due to fixed pore volume fraction in filtrated GO cake. When the total amount of filler was fixed 50 wt.% while changing the amount of MWCNTs, a maximum thermal conductivity was obtained with the addition of about 0.36 wt.% of MWCNTs in the filler. Measured thermal conductivity was higher than the predicted value based on the by Maxwell–Garnett (M–G) approximation and decreased for MWCNT concentrations above 0.4%. The increased thermal conductivity was due to the formation of 3-D heat conduction paths by the addition of MWCNTs. Too high a MWCNT concentration led to increased phonon scattering, which in turn led to decreased thermal conductivity. The measured storage modulus was higher than that of the solvent mixed composite because of the insufficient interface between the large amount of filler and the epoxy.     

Multiwalled carbon nanotubes–BaTiO3/silica composites with high complex permittivity and improved electromagnetic interference shielding at elevated temperature

Composites with silica matrix and mixed filler of multiwalled carbon nanotubes (MWCNTs) and BaTiO₃ powder were fabricated. Excellent uniform dispersion of MWCNTs can be obtained using a two-step mixing method. Both of the real and imaginary parts of complex permittivity increased with increasing MWCNT content and measured temperature. The electromagnetic interference (EMI) shielding results showed that the absorption mechanism is the main contribution to the total EMI shielding effectiveness (SE). Compared with the EMI SE resulting from reflection, the absorption showed more dependence on the MWCNT content, measured temperature and frequency. The total EMI SE is greater than 20 dB at 25 °C and 50 dB at 600 °C in the whole frequency range of 12.4–18 GHz with a 1.5 mm composite thickness, which suggests that the MWCNT–BaTiO₃/silica composites could be good candidates for the EMI shielding materials in the measured frequency and temperature region.     

The effect of milling time on the sintering kinetics of Si3N4 based nanocomposites

Multi-walled carbon nanotube (MWCNT) reinforced silicon nitride composites have been prepared by hot isostatic pressing at 20 MPa and gas pressure sintering at 2 MPa. To assure a good dispersion of the MWCNTs a highly efficient attritor milling was employed in the preparation process of the powder mixtures. The morphological and micro-structural evolution of the powder particles during the high-energy milling was monitored.We have found that the milling time has a complex influence on the structure and mechanical properties of the resulting nanocomposites through affecting both the dispersion and degradation of the nanoscale filler as well as the phase transformations of the ceramic host.     

Self-assembled natural rubber/multi-walled carbon nanotube composites using latex compounding techniques

Functionalization of multi-walled carbon nanotubes (MWCNTs) plays an important role in eliminating nanotube aggregation for reinforcing polymeric materials. We prepared a new class of natural rubber (NR)/MWCNT composites by using latex compounding and self-assembly technique. The MWCNTs were functionalized with mixed acids (H2SO₄/HNO₃ = 3:1, volume ratio) and then assembled with poly (diallyldimethylammonium chloride) and latex particles. The Fourier transform infrared spectroscopy, transmission electron microscopy, and scanning electron microscopy were used to investigate the assembling mechanism between latex particles and MWCNTs. It is found that MWCNTs are homogenously dispersed in the natural rubber (NR) latex as individual nanotubes since strong self-aggregation of MWCNTs has been greatly depressed with their surface functionalization. The well-dispersed MWCNTs produce a remarkable increase in the tensile strength of NR even when the amount of MWCNTs is only 1 wt.%. Dynamic mechanical analysis shows that the glass transition temperature of composites is higher and the inner-thermogenesis and thermal stability of NR/MWCNT composites are better, when compared to those of the pure NR. The marked improvement in these properties is largely due to the strong interfacial adhesion between the NR phase and MWCNTs. Functionalization of MWCNTs represents a potentially powerful technology for significant reinforcement of natural rubber materials.     

Electrical conductivity of carbon nanotube/poly(vinylidene fluoride) composites prepared by high-speed mechanical mixing

Electrically conductive multi-walled nanotube (MWCNT)/poly(vinylidene fluoride) (PVDF) composites with a segregated structure were prepared by high-speed mechanical mixing method. It was found that MWCNTs were uniformly dispersed on polymer particle surfaces. At the MWCNTs composition of 0.1 vol.%, the composites exhibited a dramatic enhancement in electrical conductivity by 11 orders of magnitude. A low percolation threshold was achieved at the CNT concentration of 0.078 vol.%. The mechanical mixing method presented can be adapted to other CNT/polymer composites with a segregated structure.     

Characterization of conductive multiwall carbon nanotube/polystyrene composites prepared by latex technology

Conductive multiwall carbon nanotube/polystyrene (MWCNT/PS) composites are prepared based on latex technology. MWCNTs are first dispersed in aqueous solution of sodium dodecyl sulfate (SDS) driven by sonication and then mixed with different amounts of PS latex. From these mixtures MWCNT/PS composites were prepared by freeze-drying and compression molding. The dispersion of MWCNTs in aqueous SDS solution and in the PS matrix is monitored by UV–vis, transmission electron microscopy, electron tomography and scanning electron microscopy. When applying adequate preparation conditions, MWCNTs are well dispersed and homogeneously incorporated in the PS matrix. The percolation threshold for conduction is about 1.5 wt% of MWCNTs in the composites, and a maximum conductivity of about 1 S m⁻¹ can be achieved. The approach presented can be adapted to other MWCNT/polymer latex systems.     

Stabilization and dispersion of PtRu and Pt nanoparticles on multiwalled carbon nanotubes using phosphomolybdic acid,and the use of the resulting materials in a direct methanol fuel cell

PtRu and Pt nanoparticles were deposited on the surface of multiwalled carbon nanotubes (MWCNTs) with the assistance of phosphomolybdic acid (PMo) by a one-pot hydrothermal reduction strategy. Transmission electron microscopy shows a high-density PtRu (or Pt) nanoparticles uniformly dispersed on the surface of the MWCNTs with an average diameter of 1.8 nm for PtRu nanoparticles and 2.4 nm for Pt nanoparticles. Moreover, the as-prepared PMo/PtRu/MWCNT and PMo/Pt/MWCNT electrocatalysts are highly electroactive for the electrochemical oxidation of methanol. Cyclic voltammograms show a high electrochemical surface area (ESA) and a large current density for methanol oxidation at the modified electrode by PMo/PtRu/MWCNT and PMo/Pt/MWCNT electrocatalysts. Electrochemical impedance spectroscopy reveals a high CO tolerance for PMo/PtRu/MWCNT and PMo/Pt/MWCNT electrocatalysts in the electrochemical catalysis of methanol oxidation. For comparison, PtRu/MWCNT and Pt/MWCNT electrocatalysts were prepared in control experiments without PMo. The results demonstrate that PtRu and Pt nanoparticles deposited on MWCNTs in the presence of PMo were superior to those on MWCNTs without PMo in several respects including: (1) a smaller size and a higher dispersion; (2) a higher ESA; (3) a larger current density for methanol oxidation; (4) a higher tolerance for CO poisoning.     

Excellent electromagnetic interference shielding and mechanical properties of high loading carbon-nanotubes/polymer composites designed using melt recirculation equipped twin-screw extruder

This paper describes for the first time a facile, scalable and commercially viable melt blending approach involving use of twin-screw extruder with melt recirculation provision, for uniform dispersion of up to 4.6 vol% multiwall carbon nanotubes (MWCNTs) within polypropylene random copolymer (PPCP). Morphological characterization of PPCP/MWCNT nanoscale composites (NCs) was done using scanning electron microscopy and transmission electron microscopy, which show good dispersion of MWCNTs in the PPCP matrix even at high loadings and confirm the formation of true NCs. The improved dispersion leads to the formation of electrically conducting three dimensional networks of MWCNTs within PPCP matrix at very low percolation threshold (∼0.19 vol%). The attainment of dc conductivity value of ∼10⁻³ S/cm, tensile strength of ∼42 MPa and good thermal stability for 4.6 vol% MWCNTs loading NC along with electromagnetic interference (EMI) shielding effectiveness (SE) value of −47 dB (>99.99% attenuation), demonstrate its potential for making light weight, mechanically strong and thermally stable EMI shields. These NCs also display specific SE value of ∼−51 dB cm³/g which is highest among unfoamed polymer NCs.     

Electrically conducting polyolefin composites containing electric field-aligned multiwall carbon nanotube structures: The effects of process parameters and filler loading

The characteristics of network formation of multiwall carbon nanotubes (MWCNTs) inside ethylene–octene copolymer (EOC) melts under an alternating current (AC) electric field and the resulting electrical conductivity improvements are studied by combining dynamic and steady state resistivity measurements. Fine MWCNT dispersion during melt compounding of the samples is accomplished by means of a novel non-specific, non-covalent functionalization method. It is found that the electrified composite films exhibit nanotube assembly into columnar structures parallel to the electric field, accompanied by dramatic increases in electrical conductivity up to eight orders of magnitude. Experimentally acquired resistivity data are used to derive correlations between the characteristic insulator-to-conductor transition times of the composites and process parameters, such as electric field strength (E), polymer viscosity (η) and nanotube volume fraction (ϕ). Finally, a criterion for the selection of (η, E, C) conditions that enable MWCNT assembly under an electric field controlled regime (i.e., minimal Brownian motion-driven aggregation effects) is developed. The correlations presented herein not only provide insights in the MWCNT assembly process, but can also guide the experimental design in future studies on electrified composites or assist in the selection of process parameters in composites manufacturing.     

Synergistic improvement of thermal conductivity of thermoplastic composites with mixed boron nitride and multi-walled carbon nanotube fillers

The thermal conductivity of composites with a polyphenylene sulfide (PPS) matrix and a mixture of boron nitride (BN) power and multi-wall carbon nanotube (MWCNT) fillers was investigated. Synergistic improvement in thermal conductivity of the composite was observed due to the generation of three-dimensional thermal transfer pathways between the BN and MWCNT. The improvement strongly depended on surface treatment of the MWCNTs, such as hydrogen peroxide and acid treatments. The thermal conductivity of the composite was affected by the interaction and interfacial thermal resistance between the PPS matrix and the MWCNTs. The maximum thermal conductivity achieved was 1.74 W/m K for a composite that was pelletizable, injection moldable, and thermally conductive with low electrical conductivity and good mechanical properties.     

Two-layer materials of polyethylene and a carbon nanotube/cyanate ester composite with high dielectric constant and extremely low dielectric loss

Double-layer materials with one layer being a polyethylene (PE) film and the other layer a multi-wall carbon nanotube (MWCNT)/cyanate ester (CE) resin composite, PE-MWCNT/CE, were prepared. They have high dielectric constant and extremely low dielectric loss. For comparison, MWCNT/CE composites with different contents of MWCNTs were also prepared. Results show that the two kinds of materials have greatly different dielectric properties. With the same content of MWCNTs, the PE-MWCNT/CE material shows a higher dielectric constant and much lower dielectric loss than the MWCNT/CE composite. More specifically, the dielectric constant and loss tangent at 10 Hz of the PE-MWCNT/CE material with 0.5 wt.% MWCNTs are respectively 168 and 0.006, about 1.4 and 2.5 × 10⁻⁵ times the values of the corresponding MWCNT/CE composite. The nature behinds these interesting data was detected from the space charge polarization effect and equivalent circuits. The mechanism for the unique dielectric behavior of the PE-MWCNT/CE materials is that the presence of PE film not only reinforces the space charge polarization, but also subdues the leakage current. On the other hand, based on the discussion on the Cole–Cole plots, an effective method is developed to accurately calculate the relaxation time of space charge polarization in electric conductor/polymer materials.     

Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries

CuO mesocrystal entangled with multi-wall carbon nanotube (MWCNT) composites are synthesized through a facile scalable precipitation and a followed oriented aggregation process. When evaluated as anode materials for lithium ion batteries, the CuO-MWCNT composites exhibit high areal capacity and good cycling stability (1.11 mA h cm⁻² after 400 cycles at the current density of 0.39 mA cm⁻²). The excellent electrochemical performance can be ascribed to the synergy effect of the unique structure of defect-rich CuO mesocrystals and the flexible conductive MWCNTs. The assembled architecture of CuO mesocrystals can favor the Li-ion transport and accommodate the volume change effectively, as well as possess the structural and chemical stability of bulk materials, while the entangled MWCNTs can maintain the structural and electrical integrity of the electrode during the cycles.     

Slip casting of nanozirconia/MWCNT composites using a heterocoagulation process

The addition of carbon nanotubes (CNTs) is expected to increase the fracture toughness of ceramic matrix composites, but an uniform dispersion of the nanotubes in the matrix is essential. This is a complex issue in aqueous medium because of the nanotubes hydrophobicity. In this work, the stability and rheological behaviour of nanozirconia concentrated suspensions, from 20 to 33 vol% solids, with and without multiwall carbon nanotubes (MWCNTs) was optimised in order to obtain homogeneous green samples by slip casting. The manufacture of nanozirconia/MWCNTs composites was performed using a heterocoagulation process in which the CNTs were homogeneously coated by the ceramic particles through strong electrostatic attractive forces between the two phases and further consolidation by a slip casting route. After sintering, the effect of the MWCNT on the hardness and fracture toughness of the nanostructured zirconia samples was evaluated.