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.     

The conformal silicon deposition on carbon nanotubes as enabled by hydrogenated carbon coatings for synthesis of carbon/silicon core/shell heterostructure photodiodes

One-dimensional heterostructures, based on functionalities of dissimilar materials within a monolithic structure, are promising building blocks for different applications. Herein, utilizing surface decoration of multiwalled carbon nanotubes (MWCNTs) with hydrogenated graphitic carbon layers (HGCLs), the realization of a vertically aligned MWCNT/amorphous-silicon (a-Si) core/shell heterostructure is reported. The proposed method enables the formation of conformal, continuous, and all-around silicon deposition on the carbon nanotube and eliminates any signature of line-of-sight deposition problem, even for thicknesses as low as a few nanometers. Precise elaboration using comparative Raman analysis reveals that the HGCLs play a major role in the construction of such structures. Evidence of direct binding between Si and C, a missing remarkable feature in previous reports, has been observed in high-resolution transmission electron microscope images and X-ray photoelectron spectroscopy. Monitoring time evolution during the formation of the silicon shell declares a diffusive mechanism for the deposition of Si on the surface of MWCNTs. Furthermore, the electro-optical proficiency of the MWCNT/a-Si heterostructure was studied by fabrication of a photodiode. Unlike previous attempts, a naturally formed Schottky junction at the high-quality a-Si/nanotube interface is exploited for charge separation in this photodiode, which provides a sensitivity of >10⁷% in the reverse saturation current for a wavelength of λ = 405 nm.     

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 preparation of cation-functionalized multi-wall carbon nanotube/sulfonated polyurethane composites

Covalent functionalization of multi-wall carbon nanotubes (MWCNTs) with minimal alteration to the MWCNT surface is important to achieve homogenously dispersed carbon nanotubes while maintaining their unique mechanical and electrical properties. Carboxylic acid derivatized MWCNTs (MWCNT-COOH) were covalently functionalized with 3,3′-iminobis(N,N-dimethylpropylamine) (DMPA). Upon subsequent quaternization of DMPA, dendritic ammonium cation-functionalized MWCNTs (MWCNT-DMPA⁺) were formed, where two ammonium cations were incorporated per amide site. Thermogravimetric analysis and X-ray photoelectron spectroscopy demonstrated successful covalent functionalization and formation of the surface-bound ammonium salt. Raman spectroscopy and atomic force microscopy indicated the absence of an appreciable decrease in the MWCNT aspect ratio. Compared with pristine MWCNTs and MWCNT-COOH, MWCNT-DMPA⁺ exhibited enhanced dispersibility in N,N-dimethylformamide (DMF) as observed with UV–Visible spectroscopy and transmission electron microscopy (TEM). In addition, blending the cation-bound MWCNT-DMPA⁺ with anion-bound sulfonated polyurethane in DMF generated novel composites with a nanotube content ranging from 0.5 to 5 wt.%. Characterization of the composite films using both field emission scanning electron microscopy and TEM revealed that MWCNT-DMPA⁺ exhibited uniform dispersion in sulfonated polyurethane matrices even at 5 wt.%. Tensile analysis showed that the modulus of the sulfonated polyurethane matrix linearly increased with MWCNT-DMPA⁺ content.     

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.     

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.     

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.     

Multiwalled carbon nanotube/cement composites with exceptional electromagnetic interference shielding properties

Multi-walled carbon nanotube (MWCNT)/portland cement(PC) composites have been fabricated to evaluate their electromagnetic interference (EMI) shielding effectiveness (SE). The results show that they can be used for the shielding of EMI in the microwave range. The incorporation of 15 wt.% MWCNTs in the PC matrix produces a SE more than 27 dB in X-band (8.2–12.4 GHz), and this SE is found to be dominated by absorption. Furthermore, the structural analysis, surface morphology and surface interaction of MWCNTs with PC matrix have been explored using XRD, SEM and X-ray photoelectron spectroscopy technique.     

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.     

Cycle and rate performance of chemically modified super-aligned carbon nanotube electrodes for lithium ion batteries

Super-aligned multi-walled carbon nanotubes (MWCNTs), which had been produced in large-scale, were oxidized by H₂O₂ and HNO₃. The surface defects and oxygen-containing functional groups introduced during the oxidizing process were characterized by Raman spectroscopy and X-ray photoelectron spectroscopy. The surface modification of MWCNTs improved the electrochemical properties. As a result, H₂O₂-treated and HNO₃-treated MWCNTs displayed reversible capacities of 364 mA h/g and 391 mA h/g, respectively, after 80 galvanostatic cycles, corresponding to 143% and 154% improvements compared with pristine MWCNTs. The rate capability was also increased. At a current density of 3500 mA/g, H₂O₂-treated and HNO₃-treated MWCNTs exhibited reversible capacities of 66 mA h/g and 156 mA h/g, respectively. In contrast, pristine MWCNTs were only able to deliver 27 mA h/g at this current density.     

Bomb calorimetry as a bulk characterization tool for carbon nanostructures

Multiwalled carbon nanotubes (MWCNTs) consisting of coaxial graphene cylinders (cylindrical MWCNTs), cones (herringbone MWCNTs) or carbon fibers were combusted in an isothermal bomb calorimeter. Their standard enthalpies of formation were determined to be 16.56 ± 2.76 kJ mol⁻¹(C – per carbon mol) for carbon fibers, 21.70 ± 1.32 kJ mol⁻¹(C) for herringbone MWCNTs and 8.60 ± 0.52 kJ mol⁻¹(C) for cylindrical ones. All materials were characterized by transmission electron microscopy, scanning electron microscopy, Raman spectroscopy, thermogravimetry, and elemental analysis. A linear correlation between the standard enthalpies of formation and D/G and G′/G Raman bands ratio (D – band is centered at 1350 cm⁻¹, G – 1585 cm⁻¹, G′ – 2700 cm⁻¹) demonstrates the applicability of bomb calorimetry for characterization of the “defectiveness” of the bulk carbon material in the sense Raman spectroscopy is widely used nowadays. Also, we show that the calorimetry may be used to estimate the oxygen content in the bulk carbon nanomaterials, as there is a linear correlation between the oxygen content (both total content and in carboxyl groups separately) and the standard enthalpies of formation for herringbone nanotubes oxidized by nitric acid.     

Synthesis,characterization and thermal decomposition behaviour of triphenylphosphine-linked multiwalled carbon nanotubes

Triphenylphosphine-linked multiwalled carbon nanotubes (Tpp-MWCNT) were prepared in aprotic media and under anhydrous conditions by treating bromo-arylated-MWCNT with potassium diphenylphosphine (Tpp-MWCNT (1)), or with chlorodiphenylphosphine and butyllithium (Tpp-MWCNT (2)). Tpp-MWCNT were characterised by various techniques, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). The presence of the expected elements (C and P) is evident in the XPS spectra. Furthermore XPS results showed the presence of phosphorus with average concentrations of 0.7% and 2.6% in Tpp-MWCNT (1) and Tpp-MWCNT (2), respectively. TGA results revealed the following thermal decomposition order: Tpp-MWCNT (2) < Tpp-MWCNT (1) < purified MWCNT, thus suggesting that the thermal stability of the MWCNT decreases with increasing amount of triphenylphosphine moieties attached to their surface. Moreover TGA results demonstrate that Tpp-MWCNTs have improved flame retardant behaviour since they produce 4–5 times more char than purified MWCNT.     

The influence of nanoscale defects on the fracture of multi-walled carbon nanotubes under tensile loading

We report the mechanical behavior of a unique type of multi-walled carbon nanotube (MWCNT) and an acid-treated version of this MWCNT type that have nanoscale defects on their surfaces from the acid treatment. These defects, from scanning electron microscope (SEM) and transmission electron microscope (TEM) imaging have a ‘channel-like’ appearance, as if a ring of material was cut away from the MWCNT around the circumference. The mechanical properties of individual MWCNTs have been experimentally shown to strongly depend on their structure and structural disorder can drastically reduce the mechanical properties. Tensile-loading experiments using a nanomanipulator tool operated inside a SEM revealed that the tensile strengths of 10 pristine MWCNTs ranged from ～ 2 to ～ 48 GPa (mean 20 GPa). For 10 acid-treated MWCNTs with channel-like defects, tensile strengths ranged from ～ 1 to ～ 18 GPa (mean 6 GPa, thus roughly 70% lower than those of the pristine MWCNTs). Microstructural observations revealed that the fracture of the acid-treated MWCNTs occurred at a channel-like defect region in 8 of the 10 samples. This indicates that the channel-like defects associated with the acid etching are typically going to be the weakest points in the acid-treated MWCNT structure and that stress concentration is present at the defect region.     

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.     

Catalytic hydrogen transfer of ketones over atomic layer deposited highly-dispersed platinum nanoparticles supported on multi-walled carbon nanotubes

Hydrogen transfer of ketones, catalyzed by highly-dispersed platinum nanoparticles supported on multi-walled carbon nanotubes (MWCNTs), was studied. Pt nanoparticles were deposited on gram quantities of non-functionalized MWCNTs by atomic layer deposition (ALD) at 300 °C, using (methylcyclopentadienyl) trimethylplatinum and oxygen as precursors. TEM analysis showed that ~ 1.4 nm Pt nanoparticles were highly dispersed on MWCNTs. The heterogeneous hydrogen-transfer reactions of acetophenone indicated that an acetophenone conversion of 100% and a 1-phenylethanol selectivity of 99.0% could be obtained with a ketone to Pt mass ratio of 24,690 and a ketone to KOH mass ratio of 22 at 150 °C for 5 h. The selectivity of the Pt/MWCNT catalyst was higher than that of the commercial Pt/C catalyst, due to the highly-dispersed, uniform Pt nanoparticles and the unique porous structures of the Pt/MWCNT catalyst. The high stability of the Pt/MWCNT catalyst was demonstrated by reutilization of the catalyst. The high reactivity and selectivity of this catalyst for hydrogen transfer reduction were also demonstrated for other ketones.     

Germanium coated vertically-aligned multiwall carbon nanotubes as lithium-ion battery anodes

The performance of polycrystalline and amorphous germanium (Ge) as anode active materials for lithium-ion batteries was studied systematically. Polycrystalline Ge on vertically-aligned multiwall carbon nanotube (MWCNT) arrays (MWCNT/c-Ge) and amorphous Ge on the arrays (MWCNT/a-Ge) were fabricated using a low pressure chemical vapor deposition system and a radio frequency sputtering system, respectively. The vertically-aligned MWCNT arrays were used as a platform to minimize pulverization problem. The MWCNT/a-Ge had a specific capacity of 1096.1 mA hg⁻¹ at the rate of 162.4 mA g⁻¹ at the 100th cycle. In comparison, the MWCNT/c-Ge only showed a specific capacity of 730.2 mA hg⁻¹ at the rate of 162.4 mA g⁻¹ at the 100th cycle. The MWCNT/a-Ge sample showed better performances as the MWCNT/a-Ge skipped the electrochemically-driven solid-state amorphization of crystalline Ge during the first lithiation.     

High performance electrochemical H2O2 sensor based on MWCNT thin films fabricated by novel electron beam evaporation

High quality multi-walled carbon nanotubes (MWCNT) were grown by electron beam evaporation (EBE) under a high vacuum of 10⁻⁶ mtorr. The influence of deposition thickness on the orientation, morphology and vibrational bands of MWCNT films fabricated on tantalum (Ta) substrate was discussed. XRD patterns of the film revealed the presence of (002) preferential plane of carbon. Raman spectral analysis show the G-band Raman feature corresponding to high frequency E₂g of first order mode, suggesting that CNTs were composed of crystalline carbon. SEM image of 200 nm thick MWCNT film shows well shaped homogenous fine nanotubes of length ~300 nm and diameter ~70 nm with high purity. The electrochemical performance of the MWCNTs/Ta electrodes was studied by cyclic voltammetry. The sensor prepared with optimum thickness can detect H₂O₂ in the wide range covering 5 µM to 0.025 mM, with the detection limit as low as 0.09 µM. The results demonstrate that the fabrication of MWCNTs/Ta electrode by EBE is a very interesting and useful approach, likely to be a focus of upcoming research efforts in electrochemical sensing.     

Direct electron transfer in a mediator-free glucose oxidase-based carbon nanotube-coated biosensor

A novel biosensor was prepared by immobilizing glucose oxidase on multi-walled carbon nanotube (MWCNT)-coated electrospun gold fibers. Homogeneous coating of the electrospun gold fibers by MWCNTs was achieved by electrophoretic deposition at 20 V (40 V cm^?1), a deposition time of 30 s and a solution concentration of 0.25 mg mL^?1. Scanning electron microscopy confirmed the complete coverage of MWCNTs on the fiber surface. The carboxylated MWCNTs on the gold fibers provided an anchor for covalent immobilization of glucose oxidase (GOX). GOX covalently coupled to conductive carbon nanotubes demonstrated direct electron transfer between the enzyme and the electrode surface without the need for a redox active mediator. Electrochemical characterization of the fabricated sensor by cyclic voltammetry revealed that the immobilized GOX exhibited a surface-confined reversible two-electron and two-proton reaction, with an electron transfer rate constant, k_s, of 1.12 s^?1 and a surface coverage of 1.1 × 10^?12 mol cm^?2. The sensor produced a linear response to glucose concentration up to 30.0 mM with a sensitivity of 0.47 μA mM^?1 cm^?2 and a detection limit of 4 μM.     

A new polystyrene-based ionomer/MWCNT nanocomposite for wearable skin temperature sensors

The present work outlines the fabrication and testing of a novel skin temperature sensor based on exfoliated and undamaged multi-walled carbon nanotubes (MWCNTs) dispersed in a poly(vinylbenzyl chloride) derivative with triethylamine (PVBC_Et₃N). The dispersions were prepared by sonicating MWCNT/PVBC_Et₃N mixtures in dimethylformamide for 5 min and the quantification of the MWCNTs dispersed was evaluated by UV–vis spectroscopy investigations and thermogravimetric analyses.The investigations demonstrated the realization of MWCNT/PVBC_Et₃N sensors with a resistance sensitivity to temperature close to −0.004 K⁻¹, an absolute value that is comparable to the highest values found in metals. The temperature dependence of the resistance was also found very reproducible in the range 20–40 °C, thus suggesting the possibility of using the MWCNT/PVBC_Et₃N system for the fabrication of small wearable temperature sensors for the monitoring of chronic wounds.     

Surface modification of poly-l-lactic acid fibrous scaffolds by a molecular-designed multi-walled carbon nanotube multilayer for enhancing cell interactions

We presented a molecular-designed multi-walled carbon nanotube (MWCNT) layer-by-layer (LbL) multilayer on poly-l-lactic acid (PLLA) electrospun fibers for engineering cell/CNT interfaces. A stable, positively charged monolayer was created on the fiber surface by the aminolysis reaction of poly(ethylene imine) (PEI) with PLLA, followed by alternate deposition in negatively charged MWCNT and positively charged chitosan (CS). Thermogravimetric analysis indicated a sustained growth of the MWCNT during the self-assembly process. The interactions between MWCNT and polycation crucially affected the specific structure and properties of the MWCNT multilayer. MWCNT/PEI electrostatic interactions reduced the gap between MWCNTs and improved the π → π¹ transitions. However, the CS chains tended to be more serpentine than the chains of PEI molecules, which might have hindered the π → π¹ transitions. On the other hand, the electrostatic interactions might have enhanced the disorder grade of the MWCNT structure, as indicated by Raman analysis. The scaffolds maintained their fibrous and porous structure after MWCNT multilayer modification and supported fibroblast growth. The MWCNT multilayer induced cell migration toward the interior of the scaffolds. Therefore, we created a simple yet efficient method of building a CNT multilayer on three-dimensional (3D) fibrous scaffolds for enhancing cell-matrix interactions.