Controlling bubble density in MWNT/polymer nanocomposite foams by MWNT surface modification

Polymer nanocomposite foams are promising substitutes for polymeric foams. Carbon nanotube/polymer nanocomposite foams possess high strength, low density, and can be made conductive. Creating polymer foams with controlled foam morphology is of great importance for controlling foam properties. The foam morphology is influenced by the foaming conditions and filler properties. For carbon nanotube/polymer composite foams, dispersion state and aspect ratio of the carbon nanotubes have been shown to influence the bubble density and bubble size. In the current study, the influence of carbon nanotube surface chemistry on the bubble density of multi-walled carbon nanotube/poly(methyl methacrylate), MWNT/PMMA, nanocomposite foams was investigated. The surface of the MWNTs with controlled aspect ratio was covalently modified with glycidyl phenyl ether (GPE). Surface modified MWNT/PMMA nanocomposite foams were produced using a supercritical carbon dioxide foaming process. At constant MWNT concentration, the bubble density of polymer nanocomposite foams filled with GPE surface modified MWNT was found to be several times higher than that of polymer nanocomposite foams filled with nitric acid treated MWNT. After the MWNTs were modified with GPE, the surface chemistry of the MWNT became the dominant factor in determining the bubble density while the MWNT aspect ratio became less influential.     

Enhanced field emission from aligned multistage carbon nanotube emitter arrays

In this work we report on the synthesis and field emission properties of carbon nanotube multistage emitter arrays grown on porous silicon by catalytic thermal chemical vapor deposition. The vertically oriented multistage array structures consisted of SWNTs and thin MWNTs grown on MWNTs, confirmed by TEM and Raman analysis. Higher field emission current ～32 times and low threshold field ～1.5 times were obtained for these structures in comparison to only MWNT arrays. The enhanced field emission results for these multistage emitters are a consequence of higher field concentration, which is ～3 times more than MWNTs.     

熔融纺丝方法制备纳米碳管/聚丙烯复合纤维及其拉伸性能

采用传统的熔融纺丝技术大量制备了定向性良好的纳米碳管／聚丙烯复合纤维。扫描电镜观察证实了纳米碳管在纤维里的定向性以及分散性都得到了较大的改善。通过拉伸实验测试了纳米碳管／聚丙烯复合纤维的力学性能，采用weibull统计分析发现纳米碳管的添加显著提高了复合纤维的拉伸强度，当添加纳米碳管的质量分数达到3％时，纤维强度最高，达到61MPa，超过聚丙烯纤维强度120％。复合纤维拉伸断口的形貌特征也证实了纳米碳管添加对复合纤维拉伸性能影响存在临界现象。     

Morphological characterization and modelling of electrical conductivity of multi-walled carbon nanotube/poly(p-phenylene sulfide) nanocomposites obtained by twin screw extrusion

In this study, poly(p-phenylene sulfide) based nanocomposites containing multi-walled carbon nanotubes (MWNTs) were produced by dilution of a 15 wt.% MWNT/PPS masterbatch via twin screw extrusion process. The electrical conductivities of the nanocomposites were measured and percolation threshold was observed below 0.77 vol.% MWNTs. The state of dispersion and distribution quality of MWNTs was analyzed on macro- and nanoscale through transmission light and scanning electron microscopy (SEM). A good deagglomeration of primary macroagglomerates and a homogenous MWNT distribution on nanoscale was found. The dependence of conductivity on MWNT concentration was estimated using statistical percolation theory which matches the experimental data quite well. A new empirical equation was set up to fit the electrical conductivity using quantitative values of visible percolating MWNTs which were detected by charge contrast imaging in SEM.     

The mechanical properties of MWNT/PMMA nanocomposites fabricated by modified injection molding

This paper established the procedure to fabricate MWNT/PMMA nanocomposite by using both the injection molding and film casting processes. The combined fabrication process could remove demerits while maintaining the merits of each process. Tensile strength of the MWNT/PMMA nanocomposite increased more than 15% and tensile stiffness also increased about 17.5%, compared to the pure PMMA. It was confirmed that this combined fabrication process efficiently dispersed MWNTs in the PMMA matrix, and also maintained the well-dispersed state more effectively. SEM images of the fractural surface show that the degree of dispersion was improved. In addition, a surfactant was used to disperse MWNTs more efficiently, and its effect on mechanical properties was also investigated.     

Masterbatch-based multi-walled carbon nanotube filled polypropylene nanocomposites: Assessment of rheological and mechanical properties

Polypropylene (PP)/multi-wall carbon nanotubes (MWNTs) nanocomposites were prepared by diluting a PP/MWNT masterbatch by melt compounding with a twin screw extruder and prepared nanocomposites were characterized for their rheological, mechanical and morphological properties in terms of MWNT loading. The rheological results showed that the materials experience a fluid–solid transition at the composition of 2 wt.%, beyond which a continuous MWNT network forms throughout the matrix and in turn promotes the reinforcement. The tensile modulus and yield stress of the nanocomposites are substantially increased relative to the neat polypropylene. Nanotube reinforcement thus enhanced the yield stress, while reducing the ductility. The same behavior is observed in flexural tests. Charpy impact resistance of the notched samples increases slightly by the addition of MWNT, while impact resistance for the un-notched samples decreases with the addition of MWNTs. Finally, optimum in mechanical properties was observed at 2 wt.% MWNTs, which is near the rheological percolation threshold. From transmission electron microscopic (TEM) and scanning electron microscopy (SEM) images, it was observed that nanotubes are distributed reasonably uniformly indicating a good dispersion of nanotubes in the PP matrix. These results reveal that, preparation of nanocomposites from masterbatch dilution is an excellent method to obtain well-dispersed CNTs, while limiting the handling difficulties in plastics processing industrial workshops.     

Making carbon nanotube electron sources of defined lengths and with closed caps

A method is reported to make an electron source consisting of an individual multi-walled carbon nanotube (MWNT) mounted on a tungsten support tip, and cut to length using localized electron beam irradiation in a scanning electron microscope. The apex of the MWNT was transformed into a closed cap with at least one fullerene-like layer via an annealing process involving simultaneous heating and the extraction of an emission current of ～ 1 mA. The electron emission occurred at localized emission sites. The electron emission showed Fowler-Nordheim behavior, was highly stable with time, and exhibited a low energy spread. The structure of the caps of two MWNTs was studied with transmission electron microscopy before and after the cap closure.     

利用笼形聚倍半硅氧烷增强多壁碳纳米管在水溶液中的分散性

以笼形聚倍半硅氧烷(POSS)为物理分散剂,利用POSS与多壁碳纳米管(MWNTs)间较强的相互作用,在水溶液中对MWNTs进行分散。借助紫外吸光度对POSS分散的MWNTs浓度和稳定性进行了表征,测定了分散液的Zeta电位和离心后上清液中固体物质的质量分数,并用透射电镜观察了POSS与MWNTs间的结合情况。结果表明,POSS可以显著提高MWNTs在水溶液中的分散性,且提高程度与其有机官能团R的结构有关。在所采用的五种POSS中,八异丁基笼形聚倍半硅氧烷(POSSC)的分散效果最好。在此基础上,优化得到POSSC分散MWNTs的最佳条件,即MWNTs浓度为30mg/L,POSSC和MWNTs的质量比为1。此时,POSSC可较好地吸附在MWNTs表面对其进行分散,得到具有良好稳定性的MWNTs水分散液。该分散方法简单高效且不破坏MWNTs的完整结构,所得分散液可用于复合材料的制备。     

Improved processing of carbon nanotube/magnesium alloy composites

Carbon nanotubes (CNTs) are promising reinforcements for light weight and high strength composites due to their exceptional properties. However, until now, the main obstacle is to obtain a homogenous dispersion of the CNTs in the desired material matrix. Quite a few methods have been studied to help improving the dispersion of CNTs in a polymer matrix. But not much research has been conducted on how to disperse CNTs in metal matrices. In this study, a two-step process was applied. In the first stage, a block copolymer was used as a dispersion agent to pre-disperse multiwall carbon nanotubes (MWNTs) on Mg alloy chips. Then the chips with the well dispersed MWNTs on their surface were melted and at the same time vigorously stirred. The molten MWNT Mg alloy composites were poured into a cylindrical mould to solidify quickly. For the pre-dispersion step, the microstructures of the Mg alloy chips were studied under SEM. MWNTs were quite successfully dispersed on the surfaces of the Mg alloy chips. The mechanical properties of the MWNT/Mg composites were measured by compression testing. The compression at failure, the compressive yield strength and ultimate compressive strength have all been improved significantly up to 36% by only adding 0.1 wt% MWNTs to the Mg alloy. In order to predict the potential yield strengths of the MWNT reinforced Mg alloy composites, the contributions by load transfer, Orowan strengthening and thermal mismatch were added up.     

Crystallization behavior of poly (vinylidene fluoride)/multi-walled carbon nanotubes nanocomposites

The non-isothermal and isothermal crystallization of poly (vinylidene fluoride) (PVDF)/multiple-walled carbon nanotubes (MWNTs) composites containing pristine (MWNT1) and carboxyl group (–COOH) functionalized MWNT (MWNT2) were investigated. The effects of MWNT on the crystallization behavior of PVDF were dependent on the dispersion state of MWNT. Pristine MWNT could increase the nucleation due to better dispersion, and thus, PVDF/MWNT1 composites exhibited higher crystallization peak temperatures (T _cps) and crystallinities (X _cs) compared with PVDF/MWNT2 composites. Meanwhile, the formation of MWNT network confined the growth of crystals. For the isothermal crystallization, MWNT acted as nucleating agents, and the crystallization rate constant k was increased with the addition of MWNT. Besides, the half crystallization time, t _0.5, was remarkably shortened with the increase of MWNT content, especially for the pristine MWNT.     

Syndiotactic polystyrene/multi-walled carbon nanotube nanocomposites: Polymorphism,thermal properties,electrical conductivity,and rheological properties

Syndiotactic polystyrene (sPS)/multi-walled carbon nanotube (MWNT) nanocomposites were prepared using a melt mixing technique. SEM images demonstrated the fine dispersion of MWNTs through the sPS matrix. DSC results illustrated that the MWNTs facilitated the nucleation of sPS (up to ca. 12.2 °C increase), but retarded the subsequent crystals growth. Based on the Avrami analysis, the dimension of sPS crystals growth in the composites decreased because of the effects of extensive nucleation and the formation of a nanoconfined/constrained environment from the MWNTs. XRD data confirmed that the presence of MWNTs facilitated the formation of β-form sPS crystals. The thermal stability of sPS improved to approximately 31 °C at 3 phr of MWNT loading. A rheological percolation threshold between an MWNT loading of 0.5 and 1 phr was determined by measuring the rheological properties of the samples. The incorporation of MWNTs reduced the electrical resistivity of sPS by 10 orders of magnitude.     

Joining of multiwall carbon nanotubes for the end-contact configuration by applying electric current

Multiwall carbon nanotubes (MWNTs) were manipulated in a transmission electron microscope, and the joining of two capped MWNT tips was performed by applying an electric current to the tips. The inosculation process was observed in situ while simultaneous current and bias voltage measurements were performed. Under application of a current of 95.9 μA across the contact region between the two MWNT tips, the formation of a tubular structure occurred only at the outermost wall layers of the MWNTs. At a higher current of 126.9 μA, the tubular structure collapsed without forming bonds between the inner wall layers of the two MWNTs. These results suggest that it is difficult to join the inner wall layers at the tips of MWNTs by only the control of the electric current.     

Multifunctional role of an ionic liquid in melt-blended poly(methyl methacrylate)/ multi-walled carbon nanotube nanocomposites

Poly(methyl methacrylate) (PMMA)/multi-walled carbon nanotube (MWNT) nanocomposites with a homogeneous dispersion of MWNTs have been fabricated by a simple melt-mixing method in the presence of a room-temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6]. It has been found that the ionic liquid provided multiple functions in the preparation of the nanocomposites: (1)?the ionic liquid acts as an effective compatibilizer to significantly improve the dispersion of MWNTs in the PMMA matrix; (2)?the ionic liquid is an efficient plasticizer for the prepared nanocomposites, inducing a drastically reduced glass transition temperature in the PMMA matrix; (3)?the ionic liquid acts as a processing aid for the melt processing of PMMA/MWNT nanocomposites; (4)?the ionic liquid increases the electrical conductivity as a dopant. This strategy is free of solvents and compatible with industrial applications, which opens new avenues for the larger-scale fabrication of polymer/carbon nanotube nanocomposites.     

Field emission from multi-walled carbon nanotubes with various fillers

Pastes for field emission test were prepared by 3-roll milling of multi-walled carbon nanotubes (MWNTs) and UV-sensitive binder solution. The effects of four filler additives, namely two types of indium tin oxide (ITO) powder, glass frit and Ag on the field emission properties of the screen-printed paste were investigated and compared to those without filler using a diode-type configuration. The paste formulation was shown to be adequate for fine patterning using a UV-lithography technique. MWNT pastes containing any type of filler showed better emission properties than the paste without filler, thereby confirming the importance of the filler. The MWNT paste with 1 wt.% glass frit showed the best results with the lowest turn-on field of 1.75 V/μm at 1 μA/cm², highest emission current density of 78 μA/cm² at 5 V/μm, and β-factor of 17,000 approximately, which are satisfactory for practical application.     

多壁碳纳米管的化学修饰及其增强环氧树脂复合材料的性能

研究原生多壁碳纳米管和己二胺修饰的多壁碳纳米管分别对环氧树脂的增强作用。用SEM、FT-IR及XPS对修饰前后的碳纳米管进行的表征表明,所用的方法可以在碳纳米管的表面接上己二胺。研究发现,修饰后的碳纳米管比原生碳纳米管对环氧树脂有更明显的增强作用。修饰后的碳管含量为2%时,拉伸强度、断裂伸长率和冲击强度比纯环氧树脂分别提高79.7%、160.4%和188.2%。     

Influence of multi-walled carbon nanotubes on electrochemical performance of transparent graphene electrodes

In this work, carbon-carbon nanocomposites as transparent electrodes were prepared by a chemical reduction of graphite oxide (GO) and multi-walled carbon nanotubes (MWNTs). The electric, optical, and electrochemical properties of graphene-MWNT nanocomposites (G-MCs) were investigated as a function of the MWNT content. It was found that chemically bonded G-MCs were successfully formed with a reduction of the functional groups of the GO and acid-treated MWNTs, resulting in the conjugation of 1D MWNTs onto a 2D graphene surface. The electrical conductivity of the graphene was significantly enhanced by introducing the MWNTs. In addition, the G-MCs showed improved current density and high efficiency compared with graphene alone. This indicated that the improved electrochemical performance of the G-MCs can be attributed to the increase in the activity and electrical conductivity enhanced by π-π interaction between graphene and MWNTs.     

Engineering Multiwalled Carbon Nanotubes Inside a Transmission Electron Microscope Using Nanorobotic Manipulation

This paper provides a review of recent experimental techniques developed for shell engineering individual multiwalled carbon nanotubes (MWNTs). Basic processes for the nanorobotic manipulation of MWNTs inside a transmission electron microscope are investigated. MWNTs, bamboo-structured carbon nanotubes (CNTs), Cu-filled CNTs, and CNTs with quantum dots attached are used as test structures for manipulation. Picking is realized using van der Waals forces, “sticky” probes, electron-beam-induced deposition, and electric breakdown. Cap opening and shell shortening are presented using field emission current. Controlled peeling and thinning of the shells of MWNTs are achieved by electric breakdown, and changes in MWNT structures are correlated with electrical measurements. These processes are fundamental for the characterization of nanoscale materials, the structuring of nanosized building blocks, and the prototyping of nanoelectromechanical systems.      

Fabrication of multi-emitter array of CNT for enhancement of current density

We studied and compared field emission properties of two kinds of emitters of randomly oriented multi-wall carbon nanotubes (MWNTs), viz. continuous film emitter (CFE) and multi-emitter array (MEA). The CFE has a continuous film of MWNTs while the MEA consists of many equidistant small circular emitters. Both types of emitters were prepared by dispersing MWNTs over a titanium (Ti) film (for CFEs) or Ti circular islands (for MEAs) deposited on tantalum (Ta) followed by rooting of MWNTs into the Ti film or the Ti islands at high temperature. Emission properties of both types of emitters were analyzed with changing their emission areas. In case of the CFEs, current density decreased with an increase in emission area whereas consistent current densities were achieved from MEAs with different emission areas. In other words, the total emission current was achieved in proportion to the emission area in the case of MEAs. Additionally a high current density of 22 A/cm² was achieved at an electric field of 8 V/μm from MEAs, which was far better than that obtained from CFEs. The high current density in MEAs was attributed to edge effect, in which higher emission current is achieved from the edge of film emitter. The results indicate that the field emission characteristics can be greatly improved if a cathode contains many small equidistant circular emitters instead of a continuous film. The outstanding stability of the CFE and the MEA has been demonstrated for 2100 and 1007 h, respectively.     

Light-scattering and dispersion behavior of multiwalled carbon nanotubes

Elliptically polarized light-scattering measurements were performed to investigate the dispersion behavior of multiwalled carbon nanotubes (MWNT). Xylene- and pyridine-derived MWNT powders were dispersed in water and ethanol in separate optic cells and allowed to sit undisturbed over a two-week time period after probe sonication. Continuous light-scattering measurements taken between scattering angles of 10-170 deg and repeated over several days showed that the nanotubes formed fractal-like networks. The pyridine-derived MWNTs showed greater dispersion variation over time, tending to aggregate and clump much faster than the xylene-derived tubes. The water suspensions appeared much more stable than the ethanol suspensions, which transformed into nonfractal morphology after a few hours. We relate the dispersion stability to size and fringe patterns on the outer surface of the nanotubes. Measured values of fractal dimension were distinctly lower than those in previous studies of single-walled carbon nanotubes. Profiles of both diagonal and off-diagonal scattering matrix elements are presented.     

Effect of dispersion condition of multi-walled carbon nanotube (MWNT) on bonding properties of solderable isotropic conductive adhesives (ICAs)

Carbon nanotubes have been considered as reinforcements in composite materials because of their exceptional mechanical, electrical, and thermal properties. In this paper, the effect of dispersion conditions of multi-walled carbon nanotubes (MWNTs) on bonding properties of solderable isotropic conductive adhesives (ICA) was investigated. Two types of ICAs, untreated pristine MWNT-filled ICAs and acid-treated MWNT (a-MWNT)-filled ICAs were formulated with 1 wt% MWNTs and 83 wt% low-melting-point alloy (LMPA) fillers. X-ray photoelectron spectroscopy analysis was conducted to characterize the surface chemical states of pristine and a-MWNTs and verify the effectiveness of a-MWNTs. The fracture surface of the polymer matrix and solderable ICAs with a-MWNTs showed good dispersion conditions through field-emission scanning electron microscope. After the interconnection process, the a-MWNT-filled solderable ICA showed uniform dispersion of MWNTs in the polymer matrix and formed a stable metallurgical conduction path because of the good rheology-coalescence-wetting behavior of LMPA. Alternatively, pristine MWNT-filled ICA showed poor dispersion and an unstable conduction path formed by aggregated MWNTs.