Formation Of Carbon Nanotubes Network Research Articles

Dual-mode Janus Structural-Color films toward an integrated sensing device with synergistic optical and electrical outputs

Real-time sensing technologies have shown significant importance for environmental management, chemical monitoring, and industrial production. Current chemical sensors with high sensing performances toward gases or liquids have been realized individually; however, a simultaneous detection of targets in the both forms along with a low detection limit and a wide detection range remains a big challenge. Here, we present a novel switchable dual-mode Janus-structured sensor with integrated structural colors and electrical conductivity for chemical detection. The photonic layer formed by the self-assembly of cellulose photonic crystals shows highly selective responses to high-concentration targets via the color-changing channel, while the electrically conductive layer generated by the formation of carbon nanotube networks demonstrates target perceptions at low concentration and even ppm levels. Notably, the integrated sensor is further designed as a wearable sensing system for chemical monitoring in different application scenarios. These findings offer new thinking to design switchable multi-mode sensors for environmental and industrial surveillance.

Carbon nanotube reinforced biobased poly(urethane urea) covalent adaptable network composite

AbstractA solvent‐free approach was developed to incorporate carbon nanotube (CNT) into castor oil derived poly(urethane urea) (PPU) covalent adaptable network (CAN) based on dynamic piperidine‐urea bonds to fabricate CNT reinforced PPU composites. The approach includes two steps i.e., pre‐polymerization of castor with isophorone diisocyanate in flask and subsequent chain‐extension with 1,3‐bis(4‐piperidinyl)propane (PIP) in the presence of CNT in an internal mixer. The effect on CNT content of the morphology, mechanical property, stress relaxation, and reprocessability of the PPU/CNT composites was investigated in detail. The results demonstrated that CNT dispersed well in the PPU networks due to the applied strong shear force which facilitated the dispersion of CNT in the PPU matrix before cross‐linking. The well‐dispersed CNT reinforced the mechanical properties of PPU significantly and the Young's modulus (E) of the composites were enhanced significantly when the content of CNT was ≥6 wt% due to the formation of CNT network in the PPU matrix. When the content of CNT was 10 wt%, the E of PPU‐10%CNT was 927.59 ± 149.05 MPa, which was improved by ~60% compared to PPU. The reprocessability of the PPU network was remained although the stress relaxation rate was reduced with incorporation and increasing content of CNT. In addition, the PPU/CNT composites could be degraded chemically to recycle CNT through reaction of the dynamic piperidine‐urea bonds with additional PIP.

Carbon nanotube network formation and configuration/morphology on reinforcing and conductive performance of polymer-based nanocomposites

The present research work shows that carbon nanotube network formation and configuration/morphology play significant role in mechanical and conductive performance of polymer-based nanocomposites. Using several experimental techniques and theoretical approaches, the main entities of the established networks with aligned and random morphology were identified, the underlying mechanisms of network formation and the nature of its building blocks were revealed, and the corresponding network effects on the mechanical and electrical performances of such nanocomposites using single-walled carbon nanotubes in low- and high-density polyethylene matrices were thoroughly studied. Results show that the formed network is in majority constructed from nanotube bundles, considered as stiff rod-like Brownian particles, geometrically entangled at the critical volume fraction φcv (geometrical percolation threshold). The geometrical percolation threshold was found to be independent of the network morphology (aligned of injection molded samples and random of compression molded samples). Moreover, the mechanical and rheological properties were found to be independent of the nanocomposite morphology. However, the electrical performance of such materials was surprisingly different, strongly dependent on the morphology (aligned versus random as defined above). The random configuration significantly decreases the transition of the nanocomposites from isolator to conductor. The electrical percolation threshold, φcve, was found to be by one order of magnitude lower than the corresponding geometrical threshold, thus profoundly improving the conductivity of such nanocomposites. This key discovery enables the production of not only highly reinforced and conductive nanocomposites, but also the ones with remarkable conductivity and flexibility with minimal addition carbon nanotubes.

Shear-Induced Carbon Nanotube Migration and Morphological Development in Polylactide/Poly(vinylidene fluoride) Blend Nanocomposites and Their Impact on Dielectric Constants and Rheological Properties

Polymer composites featuring high dielectric constants are known to be useful materials for the development of modern transducers, piezo- and thermal-sensors, and energy storage devices. In this study, blend nanocomposites of carbon nanotube (CNT)-containing (0.25, 0.75, and 1.0 wt %) polylactide (PLA)/poly(vinylidene fluoride) (PVDF) (70/30 w/w) are developed by melt-mixing PVDF-CNT composite with PLA in a twin-screw extruder. The novelty of this process lies in the fact that the CNTs can migrate from the PVDF to the interface between PVDF and PLA, which could potentially improve the dielectric constant (eᵣ) and elastic moduli G′of the composites. For purposes of comparison, the PLA/PVDF/CNT composite is processed using melt extrusion of all three components together. Furthermore, the effects of CNT network formation are understood using high (L) and low (S) CNT aspect ratios. The dielectric and rheological properties of the composite are measured under different shearing and annealing conditions using a dielectro-rheological device. The results show that the dielectric properties are temperature sensitive, exhibiting higher values at higher temperatures, and that L-CNTs form stronger networks than S-CNTs in nanocomposites. Moreover, the CNTs can migrate toward the interface and coarsen the morphologies of the composite when annealing at a temperature of 200 °C. Such phenomena enhance the eᵣ of the PLA/(PVDF + 0.75 wt % L-CNT), from eᵣ ≈ 22 × 10³ at t = 0 to eᵣ ≈ 3 × 10⁴ after 2 h of annealing at 1 kHz. In contrast, under constant shear rates the eᵣ value of the same composite reduces from eᵣ ≈ 22 × 10³ to eᵣ ≈ 8.2 × 10³ after 1 h. However, under the specific measurement conditions of a 0.001 s–¹ shear rate applied for 300 s, there is a resulting overshoot in both the viscosity and dielectric constant. Such an observation indicates the suitability of these composites for potential application in highly sensitive strain sensors.

Enhanced Thermal Buffering of Phase Change Materials by the Intramicrocapsule Sub per Mille CNT Dopant.

Microencapsulation of a carbon nanotube (CNT)-loaded paraffin phase change material, PCM in a poly(melamine-formaldehyde) shell, and the respective CNT-PCM gypsum composites is explored. Although a very low level (0.001-0.1 wt %) of intramicrocapsule loading of CNT dopant does not change the thermal conductivity of the solid, it increases the measured effusivity and thermal buffering performance during phase transition. The observed effusivity of 0.05 wt % CNT-doped PCM reaches 4000 W s-0.5 m-2 K-1, which is higher than the reported effusivity of alumina and alumina bricks and an order of magnitude larger than the solid, CNT-free PCM powder. The CNT dopant (0.015 wt %) in a 30 wt % PCM-plaster composite improved the effusivity by 60% compared to the CNT-free composite, whereas the addition of the same amount of CNTs to the bulk of the plaster does not improve either the effusivity or the thermal buffering performance of the composite. The thermal enhancement is ascribed to a CNT network formation within the paraffin core.

Novel natural rubber composites based on silver nanoparticles and carbon nanotubes hybrid filler

AbstractHybrid filler of carbon nanotubes (CNT) and silver nanoparticles (AgNP) was prepared and then mixed with natural rubber (NR) with CNT:AgNP ratio = 100:1. The AgNP was first synthesized by reacting silver nitrate with sodium citrate as a reducing agent in an aqueous solution. Then, the UV‐vis spectrometry and transmission emission microscopy were used to follow the reaction forming AgNP and the optimum reaction time was found at about 20 minutes. The latex mixing of NR and CNT‐AgNP hybrid filler was eventually performed. It was also found that the CNT‐AgNP hybrid filler enhanced various properties of the NR composites, including torque difference, storage modulus, and initial modulus (from temperature scanning stress relaxation test) as compared to the conventional NR‐CNT composite. Furthermore, higher electrical conductivity with comparatively low percolation threshold of the hybrid composites was found as compared to the NR‐CNT composite. This might be due to decoration of AgNP particles on CNT surfaces or between CNT bundles caused hindering CNT agglomeration and facilitating electron tunneling. Also, CNT network formation the NR hybrid composite was stronger than in the NR‐CNT composite. Furthermore, strong infinite CNT network formation increased thermal diffusion in the NR‐CNT/AgNP hybrid composite.

Largely improved thermal conductivity of HDPE/expanded graphite/carbon nanotubes ternary composites via filler network-network synergy

Utilizing the synergistic effect of various fillers is an efficient strategy to enhance the thermal conductivity of polymer composites, in which the key is to modulate their dispersion and network formation in polymer matrix. In this work, expanded graphite (EG) was individually added into high density polyethylene (HDPE) to fabricate first the binary composites through melt blending. The electrical conductivity of the prepared composites was measured to determine the percolation threshold for HDPE/EG composites. Then HDPE/EG composites with three compositions, representing below percolation, just percolation and above percolation, respectively, were chosen as matrix and melt mixed with carbon nanotubes (CNTs) to make HDPE/EG/CNTs ternary composites. It was found that adding CNTs results in a linear increase of thermal conductivity for HDPE/EG composites with composition below percolation, along the line by adding the same amount of EG. While a jump of thermal conductivity was observed by adding CNTs for HDPE/EG composites with composition just and above percolation. The electrical conductivity and rheology property were measured and SEM experiment was carried out to explore the filler dispersion and their network formation in HDPE matrix. All the results suggested a possible location CNTs in EG filler network for HDPE/EG composites with composition just and above percolation. Thus the formation of CNTs network within EG network is attributed to the main reason for the largely enhanced thermal property. This work endows a new enlightenment to fabricate the composites with a great thermal conductivity.

Electromagnetic, Morphological, and Electrical Characterization of POMA/Carbon Nanotubes-Based Composites

This study involves the preparation of conducting composites based on poly(o-methoxyaniline) (POMA) and carbon nanotubes (CNT) and the evaluation of them as radar absorbing materials (RAM), in the frequency range of 8.2–12.4 GHz (X-band). The composites were obtained by synthesis in situ of POMA in the presence of CNT (0.1 and 0.5 wt% in relation to the o-methoxyaniline monomer). The resulting samples—POMA/CNT-0.1 wt% and POMA/CNT-0.5 wt%—were incorporated in an epoxy resin matrix in the proportion of 1 and 10 wt%. FT-IR analyses show that the POMA was successfully synthesized on the CNT surface. SEM analyses show that the synthesized POMA recovered all CNT surface. Electrical conductivity measurements show that the CNT contributed to increase the conductivity of POMA/CNT composites (1.5–6.7 S·cm−1) in relation to the neat POMA (5.4 × 10−1 S·cm−1). The electromagnetic characterization involved the measurements of complex parameters of electrical permittivity (ε) and magnetic permeability (µ), using a waveguide in the X-band. From these experimental data reflection loss (RL) simulations were performed for specimens with different thicknesses. The complex parameters show that the CNT in the composites increased ε and µ. These results are attributed to the CNT network formation into the composites. Simulated RL curves of neat POMA and POMA/CNT in epoxy resin show the preponderant influence of POMA on all RL curves. This behavior is attributed to the efficient CNT recovering by POMA. RL results show that the composite based on 10 wt% of POMA/CNT-0.5 wt% in epoxy resin (9 mm thickness) presents the best RL results (≈87% of attenuation at 12.4 GHz).

Characterization of carbon nanotube dispersion and filler network formation in melted polyol for nanocomposite materials

Different grades of carbon nanotubes (CNTs) were dispersed in poly(tetramethylene ether glycol) (PTMEG) without any solvent in the presence of a block copolymer dispersing agent by ultrasonication at a temperature well above the melting point of the PTMEG. The state of CNT dispersion at different length scales was evaluated by using optical microscopy, UV–Vis spectroscopy, rheological measurements, differential scanning calorimetry, thermogravimetric analysis and sedimentation tests. Optical microscopy can be used to characterize the state of dispersion and CNT network formation on a micrometer scale, whereas UV–Vis provides useful information about the dispersion of individual CNTs at nanoscale. Furthermore, the strength and quality of the network formed can be assessed using different modes of rheological measurements. Dispersion at nanoscale also affects the crystallization behavior of PTMEG, while network formation strongly influences suspension thermal stability and sedimentation behavior. The results obtained using different methods indicate equivalence between UV–Vis spectroscopy, rotational rheometry and PTMEG crystallization behavior for probing the CNT state of dispersion at nanoscale, while an analogy in detection of CNT network formation and strength is observed between oscillatory rheometry, thermogravimetric analysis and sedimentation tests.

A continuum model with a percolation threshold and tunneling-assisted interfacial conductivity for carbon nanotube-based nanocomposites

A continuum model that possesses several desirable features of the electrical conduction process in carbon-nanotube (CNT) based nanocomposites is developed. Three basic elements are included: (i) percolation threshold, (ii) interface effects, and (iii) tunneling-assisted interfacial conductivity. We approach the first one through the selection of an effective medium theory. We approach the second one by the introduction of a diminishing layer of interface with an interfacial conductivity to build a "thinly coated" CNT. The third one is introduced through the observation that interface conductivity can be enhanced by electron tunneling which in turn can be facilitated with the formation of CNT networks. We treat this last issue in a continuum fashion by taking the network formation as a statistical process that can be represented by Cauchy's probability density function. The outcome is a simple and yet widely useful model that can simultaneously capture all these fundamental characteristics. It is demonstrated that, without considering the interface effect, the predicted conductivity would be too high, and that, without accounting for the additional contribution from the tunneling-assisted interfacial conductivity, the predicted conductivity beyond the percolation threshold would be too low. It is with the consideration of all three elements that the theory can fully account for the experimentally measured data. We further use the developed model to demonstrate that, despite the anisotropy of the intrinsic CNT conductivity, it is its axial component along the CNT direction that dominates the overall conductivity. This theory is also proved that, even with a totally insulating matrix, it is still capable of delivering non-zero conductivity beyond the percolation threshold.

High performance of CNT-interconnects by the multi-layer structure

In this work, we propose two carbon nanotube (CNT) network fabrication processes, the normal spin rate coating (NR) and the slow spin rate coating (SR), and two interconnect structures, the single layer structure (SL) and the double layer structure (DL), to construct CNT-interconnects. We demonstrate and compare the performance of the CNT-interconnects with four kinds of process combinations: NR/SL, NR/DL, SR/SL and SR/DL. Generally, in the midst of these four combinations, the DL samples have higher conductive probabilities and less conductance variations, while SL/SR samples have the higher average conductance under the same amount of the CNT solution for CNT network formation. In addition, the phase transition phenomena occurred in the size dependent average conductance of CNT-interconnects are characterized and investigated by percolation theory. With the elongation of CNT-interconnects, the relationships between the average conductance and the square number would shift from linear region, power region to percolation region. Moreover, the results show that the resistance from the additional layer of Al2O3 in the double layer interconnect structure would influence the phase transition in the conductance of CNT-interconnects as well.

Influences of Carbon Nanotube Networking on the Conductive, Crystallization, and Thermal Expansion Behaviors of PA610-Based Nanocomposites

The electrical, crystallization and thermal expansion behaviors of polyamide 610 (PA610)/multi-walled carbon nanotube (CNT) nanocomposites prepared by melt mixing were investigated. Electron microscopy (Scanning Electron Microscopy and Transmission Electron Microscopy) revealed that a good dispersion of CNT and CNT network was obtained in the PA610 matrix. Addition of CNT to PA610 matrix led to polymer nanocomposites exhibiting higher electrical conductivity and lower thermal expansion. The network of CNT in the PA610 matrix, which can be tuned by the loading of CNT and the melt isothermal treatment, was found to play an important role in reducing thermal expansion and achieving higher conductivity. Furthermore, it was shown that significant reduction in thermal expansion in PA610/CNT nanocomposites was due to both thermally insensitive CNT and formation of CNT network.

Effect of Modified and Nonmodified Carbon Nanotubes on the Rheological Behavior of High Density Polyethylene Nanocomposite

This paper reports the results of studies on the rheological behavior of nanocomposites of high density polyethylene (HDPE) with pristine multiwall carbon nanotubes (CNT) as well as phenol and 1-octadecanol (C18) functionalized CNT at 1, 2, 3, 4, 5, and 7 wt% loading. The viscosity reduction at 1 wt% CNT follows the order, pristine CNT < phenol functionalized CNT < C18 functionalized CNT. As the filler loading increases from 1 to 2, 3, and 4 wt%, neat HDPE and filled HDPE systems show similar moduli and viscosity, particularly in the low frequency region. As the filler loading increases further to 5 and 7 wt%, the viscosity and moduli become greater than the neat HDPE. The storage modulus, tan, and the Cole-Cole plots show that CNT network formation occurs at higher CNT loading. The critical CNT loading or the rheological percolation threshold, where network formation occurs is found to be strongly dependant on the functionalization of CNT. For pristine CNT, the rheological percolation threshold is around 4 wt%, but for functionalized CNT it is around 7 wt%. The surface morphologies of CNT and functionalized CNT at 1 wt% loading showed good dispersion while at 7 wt% loading, dispersion was also achieved, but there are few regions with agglomeration of CNT.

Architecting Three-Dimensional Networks in Carbon Nanotube Buckypapers for Thermal Interface Materials

Carbon nanotube (CNT) buckypaper, which has large specific surface area and tunable network structures, shows great potential in the application of heat dissipation for high power electronic devices. In this article, we report that the heat conduction in a buckypaper depends greatly on CNT network formation, in which CNT structures, lengths, and orientations are important issues. The buckypaper composed of multiwalled CNTs with large diameter (around 50 nm) and suitable length (1–10 μm) shows lower thermal impedance compared with those made by longer CNTs with smaller diameter. The thermal impedance of such buckypapers can be reduced to 0.27 cm 2 ·K/W, lower than that of commercialized graphite foil and thermal grease. Thus, the buckypaper may serve as a promising candidate for advanced thermal interface materials. Detailed structural characterization indicates that the three-dimensional networks of buckypapers, with CNT orientations perpendicular to the surfaces, result in both the reduction of thermal contact resistance and the enhancement of heat conduction along the thickness.

Network Formation and Electrical Conduction in Carbon Nanotube Modified Polydimethylsiloxane

ABSTRACTThe focus of this research is on network formation and electrical conduction in carbon nanotube polydimethylsiloxane nanocomposites. Carbon nanotube network formation prior to and during polymerization was monitored by means of simultaneous electrical and rheological characterization. Processing induced network formation at filler concentrations below statistical percolation was observed, and both the electrical resistivity and the voltage dependence of the sample resistance were found to increase with the degree of polymerization, indicating carbon nanotube separation during polymer cure. Electron tunnelling through insulating polymer layers was identified as the main conduction mechanism. Information about the final network structure and further details about electrical conduction were obtained from the piezoresistive response of the material. Electron tunnelling was found to be dominant at filler concentrations close to the percolation threshold. With increasing filler concentration a densification of the carbon nanotube network was observed, and the resistance behaviour at high filler content was better described by the behaviour of a parallel circuit.

Damage monitoring in fiber-reinforced composites under fatigue loading using carbon nanotube networks

The formation of carbon nanotube networks around the structural reinforcement in fiber composites has enabled in situ monitoring of matrix damage accumulation. Real-time monitoring of damage development under fatigue loading was studied. The electrical response of the fatigue specimens change synchronously with the applied fatigue loading and enable a quantitative measure of the damage state. The fatigue response of the nanotube network was examined and the damage accumulation validated using microscopic technique. Various damage stages in composite cross-ply laminates under fatigue loading can be clearly detected by adopting the quantitative parameter, damaged resistance change. The sensitivity of the technique to the onset and accumulation of damage may enable future life prediction methodologies.

Electrochemical Performance of Nanocrystalline SnO<SUB>2</SUB>-Carbon Nanotube Composites as Anode in Lithium-Ion Cells

SnO2-carbon nanotube composites were prepared by chemical treatment of tin chloride salt mixed with carbon nanotubes, followed by heat-treatment at high temperature. Nanosize SnO2 particles were formed and embedded in a carbon nanotube matrix. TEM and HRTEM observation confirmed the homogeneous distribution of SnO2 nanoparticles. SnO2-carbon nanotube anodes demonstrated high lithium storage capacity and stable cyclability, which could be attributed to the nanosize SnO2 crystals and the formation of carbon nanotube networks in the electrode.

Role of multi-wall carbon nanotube network in composites to crystallization of isotactic polypropylene matrix

The composites (iPP/CNTs) made of isotactic polypropylene (iPP) and multi-wall carbon nanotubes (CNTs) were prepared by solution blending. To improve compatibility between CNTs and iPP and to enhance dispersion of CNTs in iPP matrix, CNTs were chemically modified by grafting alkyl chains. The chemically modified CNTs had about 6wt% grafted alkyl chains. Rheological measurements indicated that CNTs caused gelation in iPP/CNTs due to CNT network formation and the critical gelation CNT concentration was about 7.4wt%, which was considered to be high due to the low CNT aspect ratio in this study. Crystallization behaviors of iPP/CNTs were studied by using optical microscopy (OM) and differential scanning calorimetry (DSC). Radial growth rates of spherulites during isothermal crystallization of iPP/CNTs with CNT concentrations less than 2.0wt% measured by using OM showed decreasing trends with increasing CNT concentration. Avrami analysis of the exothermic heat flow curves during isothermal crystallization of iPP/CNTs measured by DSC indicated that crystallization rates were accelerated when CNT concentrations were lower than the critical gelation concentration, because CNTs mainly functioned as nucleating agents for crystallization, while crystallization rates did not change obviously when CNT concentrations were higher than the critical gelation concentration, because CNT network could form and mainly functioned to provide restriction to mobility and diffusion of iPP chains to crystal growth fronts.

Tube−Tube and Tube−Surface Interactions in Straight Suspended Carbon Nanotube Structures

An investigation concerning the tautness of suspended carbon nanotubes (CNTs) grown using the chemical vapor deposition (CVD) method is presented. The suspended nanotubes were analyzed with both a transmission electron microscope (TEM) and a high-resolution scanning electron microscope (HR-SEM). The HR-SEM and TEM investigations revealed that the interaction between CNTs among themselves as well as with the surface on which they are grown is a primary cause for the tautness of suspended tubes. Specifically, the tube-tube and tube-surface dynamics cause adjoining tubes to create a "zipper-effect", thereby straightening and tightening them. Suspended CNTs cling to each other and to as much of the surface as possible and thus minimize their total energy, creating taut, suspended structures. This effect can be so strong so as to force wide tubes to buckle, with no other external force involved. The implications of this study include all forms of alignment processes of nanotubes using the CVD method. The results presented here provide the groundwork for the capability of fine-tuning the control of CNT network formation using substrate mechanical features.