Inherent sensing and interfacial evaluation of carbon nanofiber and nanotube/epoxy composites using electrical resistance measurement and micromechanical technique

Inherent sensing of load, micro-damage and stress transferring effects were evaluated for carbon nanotube (CNT) and carbon nanofiber (CNF)/epoxy composites (with various added contents) by an electro-micromechanical technique, using the four-point probe method. Carbon black (CB)/epoxy composites, with conventional nanosize material added, were used for the comparison with CNT and CNF composites. Subsequent fracture of the carbon fiber in the dual matrix composites (DMC) was detected by acoustic emission (AE) and by the change in electrical resistance, ΔR due to electrical contacts of neighboring CNMs. Stress/strain sensing of the nanocomposites was detected by an electro-pullout test under uniform cyclic loading/subsequent unloading. CNT/epoxy composites showed the best sensitivity to fiber fracture, matrix deformation and stress/strain sensing, whereas CB/epoxy composite exhibited poorer sensitivity. From the apparent modulus (as a result of matrix modulus and interfacial adhesion), the stress transferring effects reinforced by CNT was highest among three CNMs. The thermodynamic work of adhesion, W_a as found by dynamic contact angle measurements of the CNT/epoxy composite as a function of added CNT content was correlated and found to be consistent with the apparent mechanical modulus. Uniform dispersion and interfacial adhesion appear to be key factors for improving both sensing and mechanical performance of nanocomposite. Thermally treated-CNF composites exhibited a slightly higher apparent modulus, whereas higher testing temperatures appeared to lower the apparent modulus.     

Electrical resistance-based strain sensing in carbon nanotube/polymer composites under tension: Analytical modeling and experiments

This paper presents an analytical and experimental study on the strain sensing behavior of carbon nanotube (CNT)-based polymer composites. Tensile tests were conducted on CNT/polycarbonate composites and the responses in the electrical resistance were measured during the tests. An analytical model incorporating the electrical tunneling effect due to the matrix material between CNTs was also developed to describe the electrical resistance change as a result of mechanical deformation. The model deals with the inter-nanotube matrix deformation at the micro/nanoscale due to the macroscale deformation of the nanocomposites. A comparison of the analytical predictions and the experimental data showed that the proposed model captures the sensing behavior. In addition, the effect of the micro/nanoscale structures on the strain induced resistance change was discussed to provide useful information for designing CNT-based polymer composites with high strain sensing capability.     

Mechanical strength improvement of polypropylene threads modified by PVA/CNT composite coatings

Poly (vinyl alcohol)/carbon nanotube (PVA/CNT) composite was coated on the surface of polypropylene thread for toughness enhancement. Multiwall carbon nanotubes (MWNTs) were treated in acid and alkali to get water-soluble nanotubes, and then embedded into poly (vinyl alcohol) (PVA) matrix, resulting in polymer-carbon composite with homogeneous nanotube dispersion. The stress-strain measurements show that the tensile strength and toughness of the PVA/CNT coated thread increased by 117% and 560%, respectively. These results are supportive of good interfacial bonding between the carbon nanotubes (CNTs) and polymer matrix.     

Characterization of carbon nanotube 3D-structures infused with low viscosity epoxy resin system

This work presents the use of carbon nanotube (CNT) skeletons and the resin infusion process as a path towards the production of polymer composites with high and well dispersed nanotube content. A general purpose low viscosity epoxy resin was used as matrix in the reported process assessment. Thin CNT papers, called skeletons, were initially produced to obtain CNT networks. The impregnation was made by infiltrating the non-diluted resin through the carbon nanotube structure. The results show the proposed processing approach as one capable of producing well dispersed nanocomposites with high CNT loading (more than 15 wt% CNT by composite weight), which are important for developing high performance structures based on carbon nanotubes with good thermal and electrical conductivity. The absolute mechanical performance was lower than expected, and discussed in light of manufacturing problems detected by microscopy observations under scanning electron microscopy (SEM).     

The effect of carbon nanotube dispersion on CO gas sensing characteristics of polyaniline gas sensor

Polyaniline is one of the most promising conducting polymers for gas sensing applications due to its relatively high stability and n or p type doping capability. However, the conventionally doped polyaniline still exhibits relatively high resistivity, which causes difficulty in gas sensing measurement. In this work, the effect of carbon nanotube (CNT) dispersion on CO gas sensing characteristics of polyaniline gas sensor is studied. The carbon nanotube was synthesized by Chemical Vapor Deposition (CVD) using acetylene and argon gases at 600 degrees C. The Maleic acid doped Emeradine based polyaniline was synthesized by chemical polymerization of aniline. CNT was then added and dispersed in the solution by ultrasonication and deposited on to interdigitated AI electrode by solvent casting. The sensors were tested for CO sensing at room temperature with CO concentrations in the range of 100-1000 ppm. It was found that the gas sensing characteristics of polyaniline based gas sensor were considerably improved with the inclusion of CNT in polyaniline. The sensitivity was increased and response/recovery times were reduced by more than the factor of 2. The results, therefore, suggest that the inclusion of CNT in MA-doped polyaniline is a promising method for achieving a conductive polymer gas sensor with good sensitivity, fast response, low-concentration detection and room-operating-temperature capability.     

Interfacial shearing strength and reinforcing mechanisms of an epoxy composite reinforced using a carbon nanotube/carbon fiber hybrid

The performance of a composite material system depends critically on the interfacial characteristics of the reinforcement and the matrix material. In this study, the interfacial shearing strength (IFSS) of a composite with an epoxy matrix and a novel carbon nanotube/carbon fiber (CNT/CF) multi-scale reinforcement was determined by single fiber-microdroplet tensile test, and the interfacial reinforcing mechanisms of the composite were discussed. Results show that the IFSS of the epoxy composite reinforced by CNT/CF is as high as 106.55 MPa, which is 150% higher than that of the as-received T300 fiber composite. And the main interfacial reinforcing mechanisms of this novel composite could be interpreted as chemical bonding, Van der Waals binding, mechanical interlocking, and surface wetting.     

Detection of 10 nM ammonium ions in 35 per thousand NaCl solution by carbon nanotube based sensors

We fabricated carbon nanotube (CNT) based chemical sensors for marine applications by photolithography process, where the electrodes were insulated by photoresist exposing only the carbon nanotube sensing section (2 microm gap width) for detection of ammonium ions (NH4+) in 35 per thousand NaCI solution used as artificial seawater environment. The I-V curve of the CNT sensor was measured by sweeping the source-drain voltage from -3 to 3 V and the on/off ratio of the CNT sensor was measured to be 20 when the gate voltage was swept from -5 to 5 V and from these results the CNTs were found to appear as a p-type semiconductor. All of the cocktail solutions prepared for experiment were measured to have -pH 6 which implied 99.9% of NH4+ remained ionized. We successfully detected 10, 100, 1000 nM (0.18, 1.8, 18 ppb) concentration of NH4+ in 35 per thousand NaCI solutions by using the CNT sensor.     

Interface tailoring to enhance mechanical properties of carbon nanotube reinforced magnesium composites

This study highlights the use of a metallic coating of nanoscale thickness on carbon nanotube to enhance the interfacial characteristics in carbon nanotube reinforced magnesium (Mg) composites. Comparisons between two reinforcements were targeted: (a) pristine carbon nanotubes (CNTs) and (b) nickel-coated carbon nanotubes (Ni–CNTs). It is demonstrated that clustering adversely affects the bonding of pristine CNTs with Mg particles. However, the presence of nickel coating on the CNT results in the formation of Mg₂Ni intermetallics at the interface which improved the adhesion between Mg/Ni–CNT particulates. The presence of grain size refinement and improved dispersion of the Ni–CNT reinforcements in the Mg matrix were also observed. These result in simultaneous enhancements of the micro-hardness, ultimate tensile strength and 0.2% yield strength by 41%, 39% and 64% respectively for the Mg/Ni–CNT composites in comparison with that of the monolithic Mg.     

Self-sensing and dispersive evaluation of single carbon fiber/carbon nanotube (CNT)-epoxy composites using electro-micromechanical technique and nondestructive acoustic emission

Self-sensing and interfacial evaluation were investigated with different dispersion solvents for single carbon fiber/carbon nanotube (CNT)-epoxy composites by electro-micromechanical technique and acoustic emission (AE) under loading/subsequent unloading. The optimized dispersion procedure was set up to obtain improved mechanical and electrical properties. Apparent modulus and electrical contact resistivity for CNT-epoxy composites were correlated with different dispersion solvents for CNT. CNT-epoxy composites using good dispersion solvents exhibited a higher apparent modulus because of better stress transferring effects due to the relatively uniform dispersion of CNT in epoxy and enhanced interfacial adhesion between CNT and the epoxy matrix. However, good solvents exhibited a higher apparent modulus but lower thermodynamic work of adhesion, W_a for single carbon microfiber/CNT-epoxy composite. It is attributed to the fact that hydrophobic behavior with high advanced contact angle was observed for CNT-epoxy in the good solvent, which might not be compatible well with the carbon microfiber. Damage sensing was also detected simultaneously using AE combined with electrical resistance measurement. Electrical resistivity increased stepwise with progressing fiber fracture due to the decrease in electrical contact by the CNT.     

Detection of Individual Molecules and Ions by Carbon Nanotube‐Based Differential Resistive Pulse Sensor

This paper presents a new method of sensing single molecules and cations by a carbon nanotube (CNT)‐based differential resistive pulse sensing (RPS) technique on a nanofluidic chip. A mathematical model for multichannel RPS systems is developed to evaluate the CNT‐based RPS signals. Individual cations, rhodamine B dye molecules, and ssDNAs are detected successfully with high resolution and high signal‐to‐noise ratio. Differentiating ssDNAs with 15 and 30 nucleotides are achieved. The experimental results also show that translocation of negatively charged ssDNAs through a CNT decreases the electrical resistance of the CNT channel, while translocation of positively charged cations and rhodamine B molecules increases the electrical resistance of the CNT. The CNT‐based nanofluidic device developed in this work provides a new avenue for single‐molecule/ion detection and offers a potential strategy for DNA sequencing.     

An improved shear-lag model for carbon nanotube reinforced polymer composites

An improved shear-lag model has been proposed for assessing the interface characteristics of carbon nanotube (CNT) reinforced polymer–matrix composites (PMCs). Instead of considering any possible chemical bonding at the CNT/matrix interface, this study focuses on stress transferring mechanism of nanotube arising from the combined effects of mechanical interlocking, Poisson’s contraction, thermal mismatch and van der Waals interactions. Analytical solutions are derived for axial and interfacial shear stresses and parametric study has also been conducted to obtain the effect of key composite parameters. This enhanced model is then used to understand true stress transferring mechanism of CNT reinforced polymer composites.     

Microstructure and properties of polyurethane nanocomposites reinforced with methylene-bis-ortho-chloroanilline-grafted multi-walled carbon nanotubes

Methylene-bis-ortho-chloroanilline (MOCA), an excellent cross-linker widely used to prepare cured polyurethane (PU) elastomers with high performance, was used to modify a multi-walled carbon nanotube. PU/carbon nanotube (CNT) nanocomposites were prepared by incorporation of the MOCA-grafted CNT into PU matrix. Fourier transform infrared spectra have shown that the modified CNTs have been linked with PU matrix. The microstructure of composites was investigated by Field-Emission Scanning Electron Microscopy. The results of Dynamic Mechanical Thermal Analysis and Differential Scanning Calorimetry have investigated the grafted CNTs as cross-linker in the cured composites. The studies on the thermal and mechanical properties of the composites have indicated that the storage modulus and tensile strength, as well as glass transition temperature and thermal stability are significantly increased with increasing CNT content.     

Micromechanics modeling of the electrical conductivity of carbon nanotube (CNT)–polymer nanocomposites

A mixed micromechanics model was developed to predict the overall electrical conductivity of carbon nanotube (CNT)–polymer nanocomposites. Two electrical conductivity mechanisms, electron hopping and conductive networks, were incorporated into the model by introducing an interphase layer and considering the effective aspect ratio of CNTs. It was found that the modeling results agree well with the experimental data for both single-wall carbon nanotube and multi-wall carbon nanotube based nanocomposites. Simulation results suggest that both electron hopping and conductive networks contribute to the electrical conductivity of the nanocomposites, while conductive networks become dominant as CNT volume fraction increases. It was also indicated that the sizes of CNTs have significant effects on the percolation threshold and the overall electrical conductivity of the nanocomposites. This developed model is expected to provide a more accurate prediction on the electrical conductivity of CNT–polymer nanocomposites and useful guidelines for the design and optimization of conductive polymer nanocomposites.     

A Versatile Biomolecular Charge-Based Sensor Using Oxide-Gated Carbon Nanotube Transistor Arrays

Label-free deoxyribonucleic acid (DNA) hybridization detection using carbon nanotube transistor (CNT) arrays is demonstrated. The present scheme is distinguished from other CNT sensing methods as it uses a gate oxide overlayer on top of the carbon nanotubes, which function solely as charge sensors but are not participants in the chemical binding process. Because it involves DNA probe attachment on the gate oxide rather than on the CNT, this approach allows the use of conventional DNA functionalization and bioassay protocols, and is less prone to false positives. The signal sought is a few tens of millivolts in threshold voltage shift due to the increase of surface charges after target hybridization. The hybridization detection is shown to be highly specific and sensitive to a minimum concentration of about 30 nM of 61-mer DNA. Despite differences in the transistor properties due to the spread in the CNT parameters during fabrication, the yields are very high and the sensing characteristics are uniformly consistent in nearly all transistors.     

Modeling carbon nanotube reinforced composite materials with the cylindrical method of cells

Micromechanics modeling, utilizing a cylindrical method of cells (CMOC) model, is employed to obtain the effective mechanical properties of an elastic transversely isotropic, isothermal material system consisting of a hollow carbon nanotube (CNT) embedded in an isotropic polymeric material matrix. It is shown that weak interfacial bonding between the CNT and polymeric matrix, which is characteristic of this type of material system, can be modeled with the CMOC. Numerical solutions of the effective independent material constants are obtained, based upon appropriate values of the properties of the carbon nanotube and epoxy matrix. The numerical results are presented graphically and compared with corresponding classical closed‐form solutions. Copyright © 2015 John Wiley & Sons, Ltd.     

An assessment of the science and technology of carbon nanotube-based fibers and composites

This paper examines the recent advancements in the science and technology of carbon nanotube (CNT)-based fibers and composites. The assessment is made according to the hierarchical structural levels of CNTs used in composites, ranging from 1-D to 2-D to 3-D. At the 1-D level, fibers composed of pure CNTs or CNTs embedded in a polymeric matrix produced by various techniques are reviewed. At the 2-D level, the focuses are on CNT-modified advanced fibers, CNT-modified interlaminar surfaces and highly oriented CNTs in planar form. At the 3-D level, we examine the mechanical and physical properties CNT/polymer composites, CNT-based damage sensing, and textile assemblies of CNTs. The opportunities and challenges in basic research at these hierarchical levels have been discussed.     

Synthesis and properties of poly(hexamethylene terephthalate)/multiwall carbon nanotubes nanocomposites

Poly(hexamethylene terephthalate) (PHT)/carbon nanotubes (CNT) nanocomposites containing 1% and 3% (w/w) of filler were prepared by two procedures: in situ ring-opening polymerization of hexamethylene terephthalate cyclic oligomers in the presence of CNT and melt blending of PHT/CNT mixtures. Arc discharge multiwalled carbon nanotubes, both pristine (MWCNT) and hydroxyl functionalized (MWCNT-OH), were used. The objective was to evaluate the effect of preparation procedure, nanotube side-wall functionalization and amount of nanotube loaded on properties of PHT. All nanocomposites showed an efficient distribution of the carbon nanotubes within the PHT matrix but interfacial adhesion and reinforcement effect was dependent on both functionalization and nanotubes loading. Significant differences in thermal stability and mechanical properties ascribable to functionalization and processing were observed among the prepared nanocomposites. All the prepared nanocomposites showed enhanced crystallizability due to CNT nucleating effects although changes in melting and glass transition temperatures were not significant.     

碳纳米管增强金属基复合材料的研究进展

碳纳米管增强金属基复合材料由于高的比强度、比模量以及优异的热、电性能在航空航天领域具有很好的应用潜力,本文在分析大量文献的基础上,评述该类材料的制备技术和界面研究进展,对其典型性能进行归纳,指出碳纳米管的分散技术以及碳管、基体之间的界面特性应该是今后本领域的重点研究方向。     

碳纳米管/聚合物复合材料界面结合性能的研究进展

碳纳米管(CNT)优异的力学性能使其成为复合材料优选的增强体。CNT/聚合物复合材料的力学性能主要受其界面结合性能的影响。综述了CNT/聚合物复合材料界面结合性能的研究方法和研究现状。对CNT/聚合物复合材料界面结合性能的研究,实验上采用微观表征技术、拉曼光谱分析技术和纳米力学拔出法,分子模拟方法则是通过对CNT施加位移或外力模拟CNT从聚合物基体中的抽拔过程。概述了聚合物的类型、晶态结构以及CNT的手性、功能化处理等因素对CNT/聚合物复合材料界面结合性能的影响,并展望了CNT/聚合物复合材料界面结合性能未来研究的重点方向。     

A simple approach to prepare Al/CNT composite: Spread–Dispersion (SD) method

In carbon nanotube (CNT) reinforced metal matrix composites (MMCs), the good dispersion of CNTs in the matrix as well as the processing problems are the major challenges inhibiting the development of these composites. In this study, well-dispersed CNTs reinforced aluminum (Al) matrix nanocomposite was fabricated by a novel Spread–Dispersion (SD) method. Specimens with ultra-fine grain size down to 20 nm were obtained. The tensile strength of the CNT nanocomposite was 66% greater than the base matrix with a minor decrease in ductility. Such enhancement was analyzed on the basis of segregation and uniform distribution of clustered CNTs, disappearance of the CNT-free zones, eliminated porosity, stronger Al/CNT bonding and the retention of CNT graphitic structure.