Interconnected multi-walled carbon nanotubes reinforced polymer-matrix composites

A modified method for interconnecting multi-walled carbon nanotubes (MWCNTs) was put forward. And interconnected MWCNTs by reaction of acyl chloride and amino groups were obtained. Scanning electron microscopy shows that hetero-junctions of MWCNTs with different morphologies were formed. Then specimens of pristine MWCNTs, chemically functionalized MWCNTs and interconnected MWCNTs reinforced epoxy resin composites were fabricated by cast moulding. Tensile properties and fracture surfaces of the specimens were investigated. The results show that, compared with pristine MWCNTs and chemically functionalized MWCNTs, the chemically interconnected MWCNTs improved the fracture strain and therefore the toughness of the composites significantly.     

Fabrication and effective thermal conductivity of multi-walled carbon nanotubes reinforced Cu matrix composites for heat sink applications

A novel particles-compositing method was used for the first time to disperse different contents of multi-walled carbon nanotubes (CNTs) in micron sized copper powders, which were subsequently consolidated into CNT/Cu composites by spark plasma sintering (SPS). Microstructural observations showed that the homogeneous distribution of CNTs and dense composites could be obtained for 0–10 vol.% CNT contents. The CNT clusters were appeared in the powder mixture with 15 vol.% CNTs, which resulted in an insufficient densification of the composites. The effective thermal conductivity of the composites was analyzed both theoretically and experimentally. The addition of CNTs showed no enhancement in overall thermal conductivity of the composites due to the interface thermal resistance associated with the low phase contrast of CNT to copper and the random tube orientation. Besides, the composite containing 15 vol.% CNTs led to a rather low thermal conductivity due possiblely to the combined effect of unfavorable factors induced by the presence of CNT clusters, i.e. large porosity, lower effective conductivity of CNT clusters themselves and reduction of SPS cleaning effect. The CNT/Cu composites may be a promising thermal management material for heat sink applications.     

Structure-property relationship in polyethylene reinforced by polyethylene-grafted multi-walled carbon nanotubes

Polyethylene-grafted multiwalled carbon nanotubes (PE-g-MWNT) were used to reinforce polyethylene (PE). The nanocomposites possessed not only improved stiffness and strength, but also increased ductility and toughness. The effects on the structure and morphology of composites due to pristine multiwalled carbon nanotubes (MWNT) and PE-g-MWNT were studied and compared using small angle X-ray scattering (SAXS), wide angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The SAXS long period, crystalline layer thickness and crystallinity of polymer lamellar stacks were found to decrease significantly in MWNT composites, while the decreases were much smaller in PE-g-MWNT composites. PE-g-MWNT allowed a more efficient and unhindered crystallization at a lamellar level, while MWNT disrupted the order of lamellar stacks, probably because of their tendency to aggregate. The SAXS crystallinity and the mechanical properties of the composites showed similar trends as a function of MWNT content. This suggested that the improvement of the interfacial strength between polymer and carbon nanotubes was a result of synergistic effects of better dispersion of the filler, better stress transfer, due to the grafting of polymer and MWNT, and the nucleation of a crystalline phase around MWNT. The latter effect was confirmed by measurements of kinetics of non-isothermal crystallization.     

Thermo-physical properties of epoxy nanocomposites reinforced with amino-functionalized multi-walled carbon nanotubes

The modification of multi-walled carbon nanotubes (MWNTs) with amine groups was investigated by FTIR, Raman spectroscopy and XPS after such steps as carboxylation, acylation and amidation. Nanotube-reinforced epoxy polymer composites were prepared by mixing amino-functionalized MWNTs with epoxy resin and curing agent. DSC, TGA, SEM and flexural test were used to investigate the thermal and mechanical properties of the composites. The results showed that amino-functionalized MWNTs could enhance the interfacial adhesion between the nanotubes and the matrix, thus greatly improve the thermal and mechanical properties of the resin epoxy bulk material.     

Fabrication of novel sol-gel silica coatings reinforced with multi-walled carbon nanotubes

Novel sol-gel silica coatings reinforced with multi-walled carbon nanotubes (MWCNTs) have been prepared using the organic sol-gel route and the dip-coating technique on magnesium alloy substrates. Homogeneous and dense composite coatings with good reinforcement dispersion were fabricated using low temperature and atmospheric pressure fabrication conditions. The presence of nanotubes caused a substantial enhancement of silica coating fracture toughness on coatings deposited on grounded substrates but it was not as effective on polished substrates because of the low adhesion of the coating to the substrate. Bridging phenomena caused by the MWCNTs was observed, indicating that an effective load transfer between the silica matrix and the nanotube reinforcement was also achieved.     

Mechanical and thermal properties of epoxy nanocomposites reinforced with amino-functionalized multi-walled carbon nanotubes

The functionalized multi-walled carbon nanotubes (MWNTs) with amino groups were prepared after such steps as oxidation, the addition of carboxyalkyl radicals, acylation and amidation. Besides oxidated-MWNTs/epoxy nanocomposites, amino-functionalized MWNTs/epoxy nanocomposites, in which MWNTs with amino groups acted as a curing agent and covalently attached into the epoxy matrix, were fabricated. Subsequently, the effects of MWNT content on the mechanical and thermal properties for the two systems were investigated. It is found that both the tensile strength and impact strength enhance with the increase of MWNT addition, and the most significant improvement of the tensile strength (+51%) and especially impact strength (+93%) is obtained with amine-treated MWNTs at an 1.5 wt.% content. Moreover, the thermal stability of the nanocomposites also distinctly improves. The improvement of the properties of the amine-treated MWNTs system is more remarkable than those of o-MWNTs system. The reasons for these changes were discussed.     

Macrodispersion of multi-walled carbon nanotubes for conductive films

Understanding of the effect of the multi-walled carbon nanotube (MWCNT) dispersion process on physical properties of MWCNT film is crucial in process optimization of MWCNT film-based products. In the present work, the electrical conduction property of MWCNT films according to various conditions in MWCNT dispersion is investigated. Spectroscopic analysis of dispersed MWCNTs show that the electrical resistance of the MWCNT conductive film is affected by an increase in the electrical contacts between adjacent CNTs due to CNT debundling and physical damage caused by ultrasonic processing. Based on the two conflicting parameters, dispersion guidelines for highly conductive MWCNT film are presented.     

Electrical characterization of multi-walled carbon nanotubes

Electrical characteristics of multi-walled carbon nanotubes (MWNTs) grown by chemical vapor deposition have been investigated as a function of the bias voltage, nanotubes length and temperature, in 2 and 4 terminal configurations. Nanotubes were deposited over metal electrodes using ac dielectrophoresis method. For better contacts between the nanotubes and electrodes, Ni and Pd films were deposited by an electroless deposition technique. Differential conductance was found to rise considerably with bias, and this effect was more pronounced for Ni. Using 2 and 4 terminal configurations, electrical resistance measurements for individual MWNTs were performed, and the results were interpreted using the model of nanotube as a resistive transmission line, where current at low bias flows mainly through the two outermost shells.     

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.     

Stress transfer in multi-walled carbon nanotubes

The interaction and load transfer between the multiple shells of a multi-walled carbon nanotube (MWNT) are the subject of intense research by both analysts and experimentalists. Observations of both lubricated sliding and adhesion between individual shells in MWNT have been observed. While the atomic interactions due to simple separation have been successfully modeled by the Lennard-Jones interaction potential for graphene structures, modeling of the shearing deformation mode has been problematic. In the present work, the authors utilize two approaches in continuum mechanics to examine the shearing transfer between shells in a MWNT subjected to extensional and torsional loading wherein the load is transferred through the outermost shell to interior shells. The first approach follows the earlier developments of the authors wherein imperfect bonding between the shells is governed by a shearing transfer efficiency that varies between perfect bonding and zero shearing traction. The second approach utilizes a classical shear lag model to study the shearing transfer between the shells. A comparison between the shear lag and shear transfer models shows the equivalence of the two approaches for two-shell MWNT and numerical solutions are presented for the shear lag model for multiple layers beyond two. Agreement between the two models for multi-shells is demonstrated by varying an adjustable parameter that depends solely on the MWNT geometry. The simplicity of the shear transfer model as compared to the shear lag model constitutes an important advantage. The fundamental discrepancy between the two models lies in the fact that length dependence is inherent to the shear lag analysis, while according to the shear transfer model, stress transfer does not depend explicitly on length.     

Optimized network of multi-walled carbon nanotubes for chemical sensing

This work reports the design of a resistive gas sensor based on 2D mats of multi-walled carbon nanotubes (MWCNTs) grown by aerosol-assisted chemical vapour deposition. The sensor sensitivity was optimized using chlorine as analyte by tuning both CNT network morphology and CNT electronic properties. Optimized devices, operating at room temperature, have been calibrated over a large range of concentration and are shown to be sensitive down to 27 ppb of chlorine. The as-grown MWCNT response is compared with responses of 2000?°C annealed CNTs, as well as of nitrogen-doped CNTs and CNTs functionalized with polyethyleneimine (PEI). Under chlorine exposure, the resistance decrease of as-grown and annealed CNTs is attributed to charge transfer from chlorine to CNTs and demonstrates their p-type semiconductor behaviour. XPS analysis of CNTs exposed to chlorine shows the presence of chloride species that confirms electron charge transfer from chlorine to CNTs. By contrast, the resistance of nitrogen-doped and PEI functionalized CNTs exposed to chlorine increases, in agreement with their n-type semiconductor nature. The best response is obtained using annealed CNTs and is attributed to their higher degree of crystallinity.     

Hydrogen sensing properties of multi-walled carbon nanotubes

The hydrogen sensing properties of multi-walled carbon nanotubes (MWNTs) synthesized by a hot filament CVD process are reported in this paper. The MWNTs were synthesized by a hot filament assisted chemical vapor deposition method using cobalt oxide nanoparticles as the catalyst on SiO₂ surfaces. The MWNTs were characterized with Raman spectroscopy and scanning electron microscopy. Two-terminal test devices were fabricated by depositing a layer of MWNTs between prefabricated gold electrodes on SiO₂ surfaces. The diameter of these MWNTs was in the 5–8 nm range. The sensitivity of carbon nanotubes was measured for different gas concentrations at different temperatures. It was found that the MWNTs were sensitive to H₂ in low temperature regions of 140–350 °C and had a maximum sensitivity (80%) at 230 °C. No sensitivity was observed at a temperature lower than 140 °C or higher than 400 °C. Though bare MWNTs are not sensitive to H₂ at room temperature, they exhibited very good sensing characteristics in the 140–300 °C range.     

Dispersion of functionalized multi-walled carbon nanotubes in multi-walled carbon nanotubes/liquid crystal nanocomposites and their thermal properties

A novel liquid crystal functionalized multi-walled carbon nanotubes (LC-MWNTs)/2-methyl-N,N′-bis(4′-methoxy benzoyloxy)-terephthalamide liquid crystal (LC) nanocomposite (LC-MWNTs/LC) was prepared via solution blend. The dispersion and thermal property of the nanocomposites with different loadings of LC-MWNTs (0.1-1 wt.%) were investigated using SEM, TGA and DSC. The results show that the dispersion of LC-MWNTs in LC matrix is more homogeneous than purified MWNTs. The decomposition temperature of nanocomposites exhibits obvious decrease at first and then increase with increasing concentration of LC-MWNTs, which is lower than that of LC for 0.1-0.4 wt.% LC-MWNT loadings and higher than that of LC for 0.5-1 wt.% LC-MWNT loadings. The addition of LC-MWNTs has little effects on the texture of smectic mesophase. These results illustrate the LC-MWNTs/LC nanocomposites, which have lower melting point and higher decomposition temperature than those of LC by adding adequate amount of LC-MWNTs, show a wide temperature range of mesophase and high thermostability. The increased mesophase temperature region of LC materials will be beneficial to their practical applications.     

Direct evidence for lip-lip interactions in multi-walled carbon nanotubes

The stability of open edged multi-walled carbon nanotubes has been investigated by using in situ high resolution transmission electron microscopy at elevated temperatures. Formation of inter-shell structures was experimentally observed for the first time and attributed to a robust interaction between adjacent concentric shells (so-called lip-lip interaction). The fl uctuating behavior of the inter-shell structures suggests a mechanism by which the carbon atoms can pass in or out through the inter-shell edges during carbon nanotube growth or shrinkage processes. This article is published with open access at Springerlink.com     

Fabrication and properties of WC–10Co cemented carbide reinforced by multi-walled carbon nanotubes

High-velocity parting-off has been applied to 80 mm bars of pearlitic 100CrMn6, resulting in shear localisation and white-etching bands in a severely deformed region below the fracture surface. Electron microscopy showed that going from the bulk material towards the fracture surface the grains become elongated and refined. The region below the fracture surface can be divided into three subzones: 50–100 μm below the surface grains are elongated, cementite lamellae are distorted, break up and the lamellar spacing decreases. <50 μm below the fracture surface the microstructure becomes a mix of cementite lamellae and carbides in a ferrite matrix. Within the white-etching band the microstructure consists of equiaxed ferrite refined to a grain size of 50–150 nm. Several twinned regions caused by the deformation can be observed. Selected area electron diffraction and low angle convergent beam electron diffraction indicate nanocrystalline cementite dispersed in the ferrite matrix.     

Thermally activated reactions of multi-walled carbon nanotubes reinforced aluminum matrix composite during the thermal spray consolidation

The interfacial phenomena of multi-walled carbon nanotubes (MWCNTs) embedded in an Al matrix during high-velocity oxygen fuel (HVOF) spraying were investigated. The high thermal energy supplied from the high temperature gas flow of HVOF spraying activated the interfacial reactions of MWCNTs with oxygen and Al, such as MWCNT oxidation and aluminum carbide formation. Interfacial reactions deteriorated the contribution of the MWCNTs to the coating properties by destroying the intrinsic structure of the MWCNTs. In this study, the interfacial reaction mechanisms of the MWCNTs are discussed based on microstructural and thermodynamic analysis.     

Highly efficient luminescence from hybrid structures of ZnO/multi-walled carbon nanotubes for high performance display applications

We report an interesting observation on strong enhancement in green luminescence from hybrid ZnO/multi-walled carbon nanotubes (MWCNTs). The hybrid structures were synthesized via a high temperature sintering method. The strong green emission at 510 nm has been attributed to surface defects of ZnO, originating from interactions between ZnO and the MWCNT surface, which has been confirmed by high resolution transmission electron microscopy and x-ray photoelectron spectroscopy. Furthermore, the two-dimensional (2D) layer of this hybrid material shows a high degree of homogeneity and 82% transparency. Time resolved emission spectroscopy measurement shows a photoluminescence decay time in microseconds, which is suitable for making optoelectronic devices.     

Influence of ozone on N-doped multi-walled carbon nanotubes

Nitrogen-doped multi-walled carbon nanotubes (N-MWCNT) were selectively grown on oxidised silicon substrate by means of catalytic chemical vapour deposition with decomposition of acetonitrile in the presence of ferrocene (FeCp₂) at 900°C. The synthesised N-MWCNT film was initially treated with continuous ozone flow for various time periods and characterised by means of scanning electron microscopy and Raman spectroscopy. The electrochemical response of ozone-treated N-MWCNT film towards ferrocyanide/ferricyanide, [Fe(CN)₆]^3?/4? redox couple was investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy. The results were compared with those obtained for untreated N-MWCNT film. The findings reveal the occurrence of structural defects on surface of ozone-treated N-MWCNT film due to oxidation of nanotubes. Furthermore, the handling of N-MWCNT film with ozone increases the barrier for interfacial electron transfer and slows down the kinetics of redox reaction occurring on this particular electrode.     

多壁纳米碳管的制备与表面改性处理

以Co-MCM-41作催化剂,采用化学气相沉积(CVD)法催化热解无水乙醇制备纳米碳管(CNTs),然后将纳米碳管在120℃下用浓硝酸回流,进行纯化及表面酸氧化改性处理。通过XRD、FT-IR、TEM、N2吸附-脱附和Raman光谱等分析手段对酸处理前后的纳米碳管进行了表征。结果表明制备出品质较好、管径均匀、管壁较厚、顶端开口的多壁纳米碳管。浓硝酸氧化处理后在纳米碳管的表面存在羧基和羟基等官能团。