Multi-walled Carbon Nanotubes Network Research Articles

Does the Processing Method Resulting in Different States of an Interconnected Network of Multiwalled Carbon Nanotubes in Polymeric Blend Nanocomposites Affect EMI Shielding Properties?

Electromagnetic interference (EMI), an unwanted phenomenon, often affects the reliability of precise electronic circuitry. To prevent this, an effective shielding is prerequisite to protect the electronic devices. In this study, an attempt was made to understand how processing of polymeric blend nanocomposites involving multiwalled carbon nanotubes (MWCNTs) affects the evolving interconnected network structure of MWCNTs and eventually their EMI shielding properties. Thereby, the overall blend morphology and especially the connectivity of the polycarbonate (PC) component, in which the MWCNTs tend to migrate, as well as the perfectness of their migration, and the state of nanotube dispersion are considered. For this purpose, blends of varying composition of PC and poly(methyl methacrylate) were chosen as a model system as they show a phase diagram with lower critical solution temperature type of characteristic. Such blends were processed in two different ways: solution mixing (from the homogeneous state) and melt mixing (in the biphasic state). In both the processes, MWCNTs (3 wt %) were mixed into the blends, and the evolved structures (after phase separation induced by annealing in solution-mixed blends) and the quenched structures (as the blends exit the extruder) were systematically studied using transmission electron microscopy (TEM). Both the set of blends were subjected to the same thermal history, however, under different conditions such as under quiescent conditions (in the case of solution mixing) and under shear (in the case of melt mixing). The electrical volume conductivity and the evolved morphologies of these blend nanocomposites were evaluated and correlated with the measured EMI shielding behavior. The results indicated that irrespective of the type of processing, the MWCNTs localized in the PC component; driven by thermodynamic factors and depending on the blend composition, sea-island, cocontinuous, and phase-inverted structures evolved. Interestingly, the better interconnected network structures of MWCNTs observed using TEM in the solution-mixed samples together with larger nanotube lengths resulted in higher EMI shielding properties (−27 dB at 18 GHz) even if slightly higher electrical volume conductivities were observed in melt-mixed samples. Moreover, the shielding was absorption-driven, facilitated by the dense network of MWCNTs in the PC component of the blends, at any given concentration of nanotubes. Taken together, this study highlights the effects of different blend nanocomposite preparation methods (solution and melt) and the developed morphology and nanotube network structure in MWCNT filled blend nanocomposites on the EMI shielding behavior.

Fabrication of open-cell thermoelectric polymer nanocomposites by template-assisted multi-walled carbon nanotubes coating

Abstract In this work, open-cell polyvinylidene fluoride (PVDF) templates were employed to assist the formation of multi-walled carbon nanotubes (MWCNT) network in polymer matrices. The continuous network of MWCNT coating and the macro-porosity helped to simultaneously promote the electrical conductivity and suppress the effective thermal conductivity of organic thermoelectric (TE) materials, and thereby enhance their TE conversion efficiencies. In-situ polymerization of polypyrrole during the template-assisted nanotube coating process also promoted the Seebeck coefficient of the polymer nanocomposites by two-fold, while suppressing their electrical conductivity. An optimum ZT value of 1.4 × 10−5 was achieved for PVDF/MWCNT nanocomposite foams with loaded 24.9 wt% MWCNT. A series of parametric experiments were performed in this study to investigate the effects of open-cell morphology, nanotubes content, and polypyrrole-nanotube interfaces on TE properties of organic materials.

Excellent Electromagnetic Interference Shielding and High Electrical Conductivity of Compatibilized Polycarbonate/Polypropylene Carbon Nanotube Blend Nanocomposites

Herein, we have fabricated a series of immiscible blend nanocomposites based on polycarbonate/polypropylene (PC/PP) by uniformly dispersing multiwall carbon nanotubes (MWCNTs) via a melt mixing technique. The effect of polypropylene-grafted maleic anhydride (PP-g-MA) as a compatibilizer has played a pivotal role in reducing the interfacial tension and thereby enhancing the attenuation performance of PC/PP blends. The substantial effect of compatibilization on the dispersion of MWCNTs has resulted in higher conductivity values of 0.33 S cm–1, which is a major requirement for designing EM shields. Nevertheless, both the uncompatibilized and compatibilized blend nanocomposites have shown viscoelastic phase separation processes. The percolation networks of MWCNTs in compatibilized PC/PP blends were clearly observed using a high resolution transmission electron microscope. There was also a remarkable 5-fold decrease in the percolation threshold and 2.5 fold increase in total shielding effectiveness (54.78 dB) ...

Development of Direct-printed Tactile Sensors for Gripper Control through Contact and Slip Detection

This work demonstrates the use of printed tactile sensors for detection of contact location and slip in a robot gripper. Research and development of robots for behaviors similar to those of humans are being conducted by many institutions. For these robot systems, flexible tactile sensors imitating human tactile senses have been developed and applied to robots. The sensors used in this work were fabricated through a direct-print process using a multi-walled carbon nanotube (MWCNT)/polymer composite. These sensors are a resistance type and were characterized by detecting changes in resistance of MWCNT networks within the composite in response to external forces. With tactile sensors attached to gripper fingers, signals generated when the gripper grasped objects were analyzed and the resulting information was used for robot gripper control.

A built-in sensor with carbon nanotubes coated by Ag clusters for deformation monitoring of glass fibre/epoxy composites

A multiwalled carbon nanotube network embedded in a polyurethane membrane was integrated into a glass fibre reinforced epoxy composite by means of vacuum infusion to become a part of the composite and has been serving for a strain self-sensing functionality. Besides the pristine nanotubes also nanotubes with Ag nanoparticles attached to their surfaces were used to increase strain sensing. Moreover, the design of the carbon nanotube/polyurethane sensor allowed formation of network micro-sized cracks which increased its reversible electrical resistance resulted in an enhancement of strain sensing. The resistance sensitivity, quantified by a gauge factor, increased more than hundredfold in case of a pre-strained sensor with Ag decorated nanotubes in comparison with the sensor with pristine nanotubes.

Enhanced Conductivity for Carbon Nanotube Based Materials through Supramolecular Hierarchical Self‐Assembly

AbstractThe currently limited comprehension of hierarchical control over out‐of‐equilibrium (dynamic) self‐assembly processes in nanoscience and nanotechnology has limited the exploitation of multicomponent systems in the design of new nanostructured functional materials. In this study's contribution, molecular building blocks with tailored nanoscale anisotropic supramolecular self‐assembly behavior enable the creation of mesoscale percolation networks of multiwalled carbon nanotubes through collinear interconnections at the microscale. This strategy affords polymeric composites with tunable properties at the macroscale, where the organization mechanism is regulated by dynamic self‐assembly at 4 hierarchical levels of auto‐organization. Such multilevel self‐assembly system reduces up to eightfolds the nanotube concentration required for percolation and enhances conductivity up to 6 orders of magnitude against blanks, thus yielding anisotropically semiconducting and conducting materials. The approach is based on casting‐from‐solution, thus simplifying preparative steps when compared to state‐of‐the‐art electron carrier counterparts such as single‐walled carbon nanotube‐, graphene‐, or indium‐tin‐oxide‐based technologies. Finally, promising material transparency levels can be reached across the visible and near‐infrared regimes for compositions above the percolation threshold, which provides new opportunities beyond the current spectral restrictions in commercial transparent conductors.

Poisson effect enhances compression force sensing with oxidized carbon nanotube network/polyurethane sensor

Poisson effect when a sensor made of multi-walled carbon nanotube network embedded in the elastic polyurethane was compressed in one direction and expanded in the other two directions perpendicular to the direction of compression several fold enhanced the sensing owing to nano-sized cracks of the network. The composite sensitivity was further multiplicatively enhanced by KMnO4 oxidation of carbon nanotubes. As an example of the composite use as a compression sensor, a pressure on the shoe sole was monitored as well as a ball bullet impact on the sensor.

2D end-to-end carbon nanotube conductive networks in polymer nanocomposites: a conceptual design to dramatically enhance the sensitivities of strain sensors.

New generation wearable devices require mechanically compliant strain sensors with a high sensitivity in a full detecting range. Herein, novel 2D end-to-end contact conductive networks of multi-walled carbon nanotubes (MWCNTs) were designed and realized in an ethylene-α-octene block copolymer (OBC) matrix. The prepared strain sensor showed a high gauge factor (GF) of 248 even at a small strain (5%) and a linear resistance response throughout the whole strain range. The sensors also exhibited very good stretchability up to 300% and high cycling durability. This novel design solved the intrinsic problem of sensors based on carbon nanotube bundles, i.e., a long sliding phase before the disconnection of CNTs in a cost-effective and scalable way. This study rationalizes the 2D end-to-end contact concept to improve the sensitivity of the existing sensors and has great potential to be used in a wide variety of polymer based sensors.

Performance optimization of freestanding MWCNT-LiFePO4 sheets as cathodes for improved specific capacity of lithium-ion batteries.

The typical lithium-ion-battery positive electrode of “lithium-iron phosphate (LiFePO4) on aluminum foil” contains a relatively large amount of inactive materials of 29 wt% (22 wt% aluminum foil + 7 wt% polymeric binder and graphitic conductor) which limits its maximum specific capacity to 120.7 mA h g−1 (71 wt% LiFePO4) instead of 170 mA h g−1 (100 wt% LiFePO4). We replaced the aluminum current-collector with a multi-walled carbon nanotube (MWCNT) network. We optimized the specific capacity of the “freestanding MWCNT-LiFePO4” positive electrode. Through the optimization of our unique surface-engineered tape-cast fabrication method, we demonstrated the amount of LiFePO4 active materials can be as high as 90 wt% with a small amount of inactive material of 10 wt% MWCNTs. This translated to a maximum specific capacity of 153 mA h g−1 instead of 120.7 mA h g−1, which is a significant 26.7% gain in specific capacity compared to conventional cathode design. Experimental data of the freestanding MWCNT-LiFePO4 at a low discharge rate of 17 mA g−1 show an excellent specific capacity of 144.9 mA h g−1 which is close to its maximum specific capacity of 153 mA h g−1. Furthermore, the freestanding MWCNT-LiFePO4 has an excellent specific capacity of 126.7 mA h g−1 after 100 cycles at a relatively high discharge rate of 170 mA g−1 rate.

Sensitivity Enhancement of Benzene Sensor Using Ethyl Cellulose-Coated Surface-Functionalized Carbon Nanotubes

A hybrid sensor based on the integration of functionalized multiwalled carbon nanotubes (MWCNTs) with ethyl cellulose (EC) was fabricated for sensitivity enhancement of benzene detection. To functionalize the surface of MWCNTs, MWCNTs were treated with hydrochloric acid for 60 min (A60-MWCNTs), while other MWCNTs were treated with oxygen plasma for 30, 60, 90, and 120 min (P30-MWCNTs, P60-MWCNTs, P90-MWCNTs, and P120-MWCNTs, resp.). Pristine MWCNTs, A-MWCNTs, and P-MWCNTs were dispersed in 1,2-dichloroethane, then dropped onto a printed circuit board consisting of Cu/Au electrodes used as the sensor platform. Next, EC was separately spin coated on the pristine MWCNTs, A-MWCNTs, and P-MWCNTs (EC/MWCNTs, EC/A-MWCNTs, and EC/P-MWCNTs, resp.). All sensors responded to benzene vapor at room temperature by increasing their electrical resistance which was sensitive to benzene vapor. The EC/P90-MWCNTs enabled an approximately 11-fold improvement in benzene detection compared to EC/MWCNTs. The sensitivity of all sensors would be attributed to the swelling of EC, resulting in the loosening of the MWCNT network after benzene vapor exposure. The differences of the sensing responses of the EC/MWCNTs, EC/A-MWCNTs, and EC/P-MWCNTs would be ascribed to the differences in crystallinity and functionalization of MWCNT sidewalls, suggesting that acid and oxygen plasma treatments of MWCNTs would be promising techniques for the improvement of benzene detection.

Role of rheology in tunning thermal conductivity of polyamide 12/polyamide 6 composites with a segregated multiwalled carbon nanotube network

Effect of multiwalled carbon nanotubes in thermal conductivity of an immiscible blend of polyamides, 50/50 (wt%/wt%) polyamide 12/polyamide 6, was analyzed as function of nanofiller amount and temperature. Effect of the molding temperature in the structure of conductive network was investigated by rheology. Data show that 5 vol% multiwalled carbon nanotubes caused an increase of 41% in thermal diffusivity and 78% in thermal conductivity respect to polyamide blend values. Thermal conductivity improvement could be described by percolation theory, with a low threshold composition (φc = 0.09 vol% carbon nanotube). Fitting parameters obtained from Agari’s adjustment model show that polyamides structure is not affected by carbon nanotubes and the nanofillers can easily form conductive paths in the polyamide 12/polyamide 6 matrix. The temperature increase facilitates nanofiller dispersion causing the formation of a denser carbon nanotube network and rising the thermal diffusivity of carbon nanotube composites with low percolation level, as was proved on annealed samples at 255℃.

Ultralow-Threshold and Lightweight Biodegradable Porous PLA/MWCNT with Segregated Conductive Networks for High-Performance Thermal Insulation and Electromagnetic Interference Shielding Applications.

Lightweight, biodegradable, thermally insulating, and electrically conductive materials play a vital role in achieving the sustainable development of our society. The fabrication of such multifunctional materials is currently very challenging. Here, we report a general, facile, and eco-friendly way for the large-scale fabrication of ultralow-threshold and biodegradable porous polylactic acid (PLA)/multiwalled carbon nanotube (MWCNT) for high-performance thermal insulation and electromagnetic interference (EMI) shielding applications. Thanks to the unique structure of the microporous PLA matrix embedded by conductive 3D MWCNT networks, the lightweight porous PLA/MWCNT with a density of 0.045 g/cm3 possesses a percolation threshold of 0.00094 vol %, which, to our knowledge, is the minimum value reported so far. Furthermore, the material exhibits excellent thermal insulation performance with a thermal conductivity of 27.5 mW·m-1·K-1, which is much lower than the best value of common thermal insulation materials. Moreover, it also shows outstanding EMI shielding performance characterized by its high shielding effectiveness (SE) values and absorption-dominated shielding feature. More importantly, its specific EMI SE is as high as 1010 dB·cm3·g-1, which is superior to those of other shielding materials reported so far. Thus, this novel multifunctional material and its general fabrication methodology provide a promising way to meet the growing demand for high-performance multifunctional materials in sustainable development.

Feasibility of multi-walled carbon nanotube/epoxy thermoset-based strain sensors for sensing in structural applications

Embedded sensors are being explored to non-intrusively monitor the loading experienced by a part either during the manufacturing process or in subsequent service. With the advancement of additive manufacturing techniques, it may be possible to print sensors directly into a part fabricated from metals, polymers, or composites. There is even more benefit if the sensor is also an integral part of the assembly. Single-walled carbon nanotubes and multi-walled carbon nanotubes are used to improve the stiffness and strength of polymeric resins. But they also offer an additional benefit of reversible piezo-resistive behavior. The objective of this study was to explore the fidelity of a signal extracted from nanocomposite samples of multi-walled carbon nanotube/epoxy resin under uniaxial loading, as a function of increasing content of multi-walled carbon nanotube. Data obtained from the multi-walled carbon nanotube networks formed correlated with the strain as a function of the mechanical loading. The gauge factors calculated for the multi-walled carbon nanotube/epoxy samples ranged from 2.651 to 5.064 as compared to the metallic strain gauge factor of 2.125 used to calibrate the samples. Under repeated mechanical loading, the initial zero-strain condition was recovered within 5 min after unloading. In samples with 5 wt% of multi-walled carbon nanotubes, the electrical conductivity and gauge factor plateaued. Furthermore, increases in the w/w% of multi-walled carbon nanotube resulted in degradation of the mechanical properties of the multi-walled carbon nanotube/epoxy resin.

Design of bio-based conductive and fast crystallizing nanocomposites with controllable distribution of multiwalled carbon nanotubes via interfacial stereocomplexation

In this study, a novel route was proposed to fabricate entirely bio-based conductive nanocomposites with a low percolation threshold of conductivity (ϕc) and enhanced crystallization and mechanical performances. In order to achieve this objective, poly(d-lactide)/poly(hydroxyalkanoate) (PDLA/PHA) nanocomposites with multiwalled carbon nanotubes (MWCNTs) and MWCNTs-g-PLLA (prepared via ring opening polymerization of l-lactide with grafting degree of 17%) were designed and fabricated. The grafting of PLLA facilitates the selective distribution of MWCNTs in the sole PDLA phase via interfacial stereocomplexation, which in combination with the volume exclusion effect of PHA leads to a lower value of ϕc (ϕc = 0.32 wt%) compared to that of the unmodified MWCNTs (ϕc = 0.56 wt%). The localization of MWCNTs-g-PLLA in the PDLA phase was directly confirmed by scanning electron microscopy, transmission electron microscopy and supported by crystallization analysis by differential scanning calorimetry, X-ray diffraction, and polarizing optical microscopy. The presence of MWCNTs, in particular, the MWCNTs-g-PLLA significantly enhanced the crystallization rate of PDLA. Furthermore, MWCNTs networks in the nanocomposites were monitored by rheological analysis when the MWCNTs and MWCNTs-g-PLLA loading reached a critical value. The MWCNTs networks not only resulted in conductivity, but also strong reinforcement of the nanocomposites. For example, tensile strength of the PDLA/PHA blends increased from 32 to 50 MPa after the formation of MWCNTs network and leveled off at higher MWCNTs loadings. The bio-based conductive nanocomposites reported in this study exhibit significant potential in antistatic films and (semi-)conducting materials.

Microsphere-Assisted Robust Epidermal Strain Gauge for Static and Dynamic Gesture Recognition.

A novel and robust epidermal strain gauge by using 3D microsphere arrays to immobilize, connect, and protect a multiwalled carbon nanotubes (MWNTs) pathway is presented. During the solvent deposition process, MWNTs sedimentate, self-assemble, and wrap onto surface of polystyrene (PS) microspheres to construct conductive networks, which further obtain excellent stretchability of 100% by combining with commercially used elastomer. Benefiting from its 3D conductive pathway defined by microspheres, immobilized MWNT (I-MWNT) network can be directly used in practical occasions without further packaging and is proved by tape tests to be capable of defend mechanical damage effectively from external environment. By parameter optimization, the strain sensor with 3 µm PS spheres obtains stable resistive responses for more than 1000 times, and maintains its gauge factor (GF) of 1.35. This thin-film conductive membrane built by this effective construction method can be easily attached onto fingers of both robot and human, and is demonstrated in sensitive epidermal strain sensing and recognizing different hand gestures effectively, in static and dynamic modes, respectively.

Mechanical properties and reinforcing mechanisms of cementitious composites with different types of multiwalled carbon nanotubes

In this study, reinforcement effect of 12 types of multiwalled carbon nanotubes (MWCNTs) on mechanical properties of cementitious composites was investigated Research results showed that among pristine MWCNTs with different diameters and lengths, the short MWCNTs with large diameter present the best reinforcing effect on strength of composites. Functionalization of MWCNTs is beneficial for enhancing strength of composites. Moreover, hydroxyl-functionalized MWCNTs feature a better reinforcement effect compared to carboxyl-functionalized MWCNTs. The best relative/absolute enhancements of 79%/74 MPa and 64.4%/5.6 MPa in compressive and flexural strength of composites are achieved by incorporating 0.5% of nickel-coated MWCNTs. XRD analyses revealed that the incorporation of MWCNTs decreases the orientation of CH in matrix, which is consistent with SEM observations. TG analyses showed that MWCNTs inhibit hydration of composites due to their absorption effect. However, extensive MWCNT networks improve microstructure of matrix and hinder the crack development under loading through fiber bridging and pull-out.

New voltammetric method useful for water insoluble or weakly soluble compounds: application to pKa determination of hydroxyl coumarin derivatives

Here, we reveal a different way of doing the voltammetric experiments that considers the electroactive species packaged in the electrodic phase instead of dissolved in solution. In this way, it is possible to obtain voltammograms of insoluble species. In this work, the method is exemplified by obtaining voltammograms for weakly soluble coumarins but it could be extrapolated to other weakly soluble compounds. We have studied a comprehensive series of 3-acetyl-hydroxycoumarins derivatives which are insoluble in aqueous medium but capable of being trapped in a three-dimensional multi-walled carbon nanotubes (MWCNT) network. Consequently, an electrodic phase composed of an MWCNT modified glassy carbon electrode (GCE) containing the coumarin derivative is prepared. The voltammetric experiment is performed with the above electrodic phase and an aqueous medium as the solution phase. All the coumarin derivatives show one anodic peak due to the oxidation of the hydroxyl group in the phenyl ring. The oxidation peaks follow a one-electron, one-proton irreversible, pH-dependent process for all monohydroxylated compounds. The Ep values are closely dependent of the substituent effect being the 7,8-cum derivative more easily oxidized due to both the electron donor effect of neighboring hydroxyls groups and hydrogen bonding interaction between them. On the other hand, the hydroxyl of the 7-cum derivative is the most difficult to oxidize due to the electron-attracting effect of the lactone carbonyl group at position 2 and acetyl carbonyl at position 3 which is conjugated with OH at 7 positions. From the breaks in the graphs Ep versus pH, we estimate the voltammetric pKa values for all the studied coumarin derivatives.

Preparation of ZnO nanoribbon–MWCNT composite film and its application as antimicrobial bandage, antibacterial filter and thermal IR camouflage material

A zinc oxide nanoribbon (ZnO NR)–multiwall carbon nanotube (MWCNT) composite film was prepared by filtration technique. The film was characterized by X-ray diffraction spectroscopy, scanning electron microscopy (SEM), Raman spectroscopy, infrared (IR) spectroscopy and reflectance spectroscopy. The SEM images showed ZnO NRs trapped in the porous MWCNT network. This composite film displayed a strong antimicrobial property and porous structure, which has potential application as an antimicrobial bandage material. The composite film successfully removed the Escherichia coli bacteria from water and destroyed the bacteria retained on its surface due to the antibacterial action of ZnO NRs. The absorption of thermal IR radiation by the composite film was studied by thermography, which can be useful in IR camouflage applications.

Constructing conductive multi-walled carbon nanotubes network inside hexagonal boron nitride network in polymer composites for significantly improved dielectric property and thermal conductivity

With the rapid development of high performance capacitors for energy storage, materials with both giant dielectric constant and low dielectric loss are urgently needed. Adding conductive filler could largely enhance the dielectric constant of polymer matrix but also greatly increase the dielectric loss. In this study, we provide a new strategy by using hybrid fillers to construct a segregated double network, where conductive multi-walled carbon nanotubes (MWCNT) network is wrapped by insulating hexagonal boron nitride (h-BN) network to destroy the continuity of embedded MWCNT network. To do this, pre-fabricated micron-sized PS/MWCNT particles were completely coated by h-BN through π-π interaction. As a result, the MWCNT network inside h-BN network provides good conductivity while h-BN network provides the isolation effect but do not increase the distance between two adjacent MWCNT agglomerations, which together can maintain the high dielectric constant and decrease the dielectric loss. Therefore, a high dielectric constant of 123 is achieved while a relatively low dielectric loss is also kept as 0.36. More importantly, this special structure of segregated double network also leads to obviously enhanced thermal conductivity which is 2.23-fold of that of the composites with randomly dispersed hybrid fillers. This high thermal conductivity is ascribed to the high synergistic efficiency between segregated h-BN network and dense MWCNT network. We believe that these good comprehensive performances promise this structure to offer a unique and effective way to prepare high-performance dielectric materials with not only high dielectric constant and low dielectric loss, but also good capability of heat dissipation.

Self assembled transparent conducting network of multi-walled carbon nanotubes formed by evaporation

We develop a novel bubble-array technique to create a network pattern of multi-walled carbon nanotubes (MWCNT) on glass, through evaporation of a thin layer of the suspension of MWCNT. Spherical bubbles of approximately equal diameter (∼25μm) are created at the air-fluid interface. On complete evaporation of the solvent (1,2 dicloroethane), MWCNT self assemble to form a connected and roughly polygonal network. The glass surface thus becomes electrically conducting, while the gaps in the network allow considerable transmission of light in the visible range. The concentration of the MWCNT suspension has been varied to optimize the balance between sheet conductance and transmittance.