Growth termination mechanism of vertically aligned centimeter long carbon nanotube arrays

The mechanism of abrupt growth termination for growing vertically aligned centimeter long CNT arrays in chemical vapor deposition process was studied. We found that the growth length increases linearly with increasing growth time and the CNT growth terminates abruptly for all the experimental conditions applied. We investigated the change of particle size distribution, particle number density and the catalyst composition on the surface of the substrate with the exposure time and temperature in the reactor. For this study X-ray Photoelectron Spectroscopy (XPS), High Resolution Transmission Electron Microscopy (HRTEM) and Atomic Force Microscopy (AFM) were employed. Focused Ion Beam (FIB) was used for preparation of the STEM samples. The experimental results demonstrated that the formed catalyst nanoparticles undergo compositional and morphological changes during the heating step of the substrate, when the temperature of the reactor is rising from ambient to the growth temperature, and during annealing at the set growth temperature. Most of the changes in the catalyst composition and morphology were related to metal diffusion into neighboring substrate layers during heating up to the growth temperature. Based on the results, a new scenario of abrupt termination mechanism for growing centimeter long CNT arrays using an iron–gadolinium binary catalyst was proposed.     

The reinforcing mechanism study of carbon nanotube in the NR matrix

The reinforcement mechanism of CNT in rubber matrix is an important and interesting subject. In this paper, carboxyled multi-walled carbon nanotubes (CNT) were used to prepare natural rubber (NR) nanocomposites (CNT/NR). CNT/NR composites were formed by mechanical blending method and the properties such as tensile strength, tensile modulus, tear strength, elongation at break and hardness were studied. The results of Mechanical property show that the physical property of NR filled with 9 phr CNT (CNT/NR-9) is similar to that with 30 phr carbon black (CB) (CB/NR-30). In addition, the dispersion and interaction between CNT and NR were also studied by scanning electron microscope, transmission electron microscope, dynamic mechanical testing system and bound-rubber analysis. According to the results, CNT is considered to exhibit random orientations and form “string bag” structure in rubber matrix and the reinforcement mechanism of CNT is different to that of CB.     

活性炭/碳纳米管复合电极电吸附机理研究

按活性炭∶碳纳米管∶PVDF=7.2∶0.8∶2质量比制备活性炭/碳纳米管复合电极,并对其构成的EST模块进行吸附动力学、等温吸附和电迁移拟合研究,分析其吸附机理。结果表明,准一级反应动力学模型和Langmuir模型能够很好的拟合、描述活性炭/碳纳米管复合电极在EST脱盐过程中的吸附机理,说明电吸附速率只与一种反应物的浓度有关且是单离子层吸附,EST吸附过程中离子的电迁移率随迁移时间的变化趋势可以用指数形式的方程很好的拟合。     

On the mechanism of piezoresistivity of carbon nanotube polymer composites

Carbon nanotube (CNT) polymer composites exhibit strong nonlinear and asymmetric piezoresistivity about zero strain in tensile and compressive strain states. The existing models explain the characteristic qualitatively but not quantitatively. This paper attempts to understand the mechanisms of this piezoresistivity by developing a new 3-dimensional percolation CNT network model, where the effect of CNT deformation (wall indentation and tube bending) is considered for the first time. The predicted electrical conductivity and piezoresistivity agree with experiments quantitatively, which reveals that the CNT deformation is a dominant mechanism for the nonlinearity and asymmetry of piezoresistivity of CNT-polymer composites. Parametric studies have been conducted to show the effects of morphology and electrical properties of CNTs, work functions and Poisson's ratio of polymer on the piezoresistivity of CNT-polymer composites for future application in nanosensing composites.     

火柴棒状纳米碳管的制备及其生长机理

以钨酸钠为钨源,氯化钠为诱导剂,通过水热法制备了三氧化钨(WO₃)纳米棒,再以葡萄糖为碳源,经再次水热反应对WO₃表面进行碳包覆,然后在氢气和甲烷混合气氛中反应一段时间获得了具有火柴棒状结构的纳米碳管。采用X射线衍射分析、场发射扫描电子显微镜、透射电子显微镜和X射线能量散射谱等手段对样品的晶型、形貌、微结构和表面化学元素进行了表征与分析。结果表明,样品由纳米碳管和碳化钨(WC)构成。其中,纳米碳管为火柴棒状,长度0.5～1.0 μm,直径100 nm左右;WC颗粒位于纳米碳管内部,其大小决定了火柴棒状纳米碳管的内径。这充分说明WC在碳管的生长过程中充当催化剂的作用。     

Understanding the scattering mechanism of single-walled carbon nanotube based gas sensors

In the interaction between gas molecules with single-walled carbon nanotube (SWCNT) we show that as a result of collisions the gas scattering contributes with an important background signal and should be considered in SWCNT-based gas sensors. Experimental evidence of the collision-induced tube wall deformation is demonstrated using in situ X-ray absorption near-edge structure spectroscopy. Results support the occurrence of the scattering process and show how gas collisions may affect the electronic structure of SWCNTs.     

A self-entanglement mechanism for continuous pulling of carbon nanotube yarns

We report an in situ electron microscopy study of pulling mechanism of super-aligned carbon nanotube (CNT) arrays grown by chemical vapor deposition. The formation of entangled structures at the ends of CNT bundles during pulling is very critical for the pulling process. By cutting the top and bottom layers of both pullable and not pullable CNT arrays, we confirm that the structures at the top or bottom surfaces of the as-grown CNT arrays are not the dominant reason to ensure the continuous pulling process. We have observed the formation of entangled structures when the pulling process approaches the bottom and top ends of the CNT arrays. These entangled structures are responsible for maintaining the continuity of pulling CNTs.     

The effect of nanotube alignment on stress graphitization of carbon/carbon nanotube composites

Carbon nanotubes, when used as filler in a glass-like carbon matrix, has been reported to induce stress graphitization in the matrix. The effects on stress graphitization of the amount of carbon nanotube loading and nanotube orientation in the composite were investigated through microscopy and X-ray diffraction analyses. Results showed that an increase in nanotube content and nanotube alignment could increase the extent of formation of anisotropic regions, thereby hastening stress graphitization. It was shown that the distance between nanotubes could affect the formation of the anisotropic structures, such that they could develop in a circumferential manner around the nanotubes when the nanotubes are situated far from each other or develop continuous regions between nanotubes when they are closer together. The development of these microstructures and its relationship to the residual stresses that accumulate in the material during heat treatment is discussed here.     

Stretching-dominated deformation mechanism in a super square carbon nanotube network

Connecting straight carbon nanotube (CNT) arms with different CNT junctions, 2D CNT covalent networks can be theoretically constructed. The equivalent material properties of two kinds of 2D CNT network, the super square and super hexagon, are compared using molecular structure mechanics. Under uniaxial tensile loading in the principle direction, the super square could be regarded as a stretching-dominated network, while the super hexagon as a bending-dominated network. It is found that the super square has a higher stiffness than the super hexagon network. Similar to the super hexagon network, the aspect ratio of the CNT arm plays an important role in the properties of the super square network. Both the in-plane stiffness and Poisson’s ratio of the super square are proportional to the reciprocal of the tube aspect ratio.     

Microscopic mechanism of reinforcement in single-wall carbon nanotube/polypropylene nanocomposite

We analyzed mechanical properties and structure of polypropylene fibers with different concentrations of single-wall carbon nanotubes (SWNTs) and draw down ratios (DDR). Tensile tests show a three times increase in the Young's modulus with addition of only 1 wt% SWNT, and much diminished increase of modulus with further increase in SWNT concentration. Microscopic study of the mechanism of reinforcement by SWNT included Raman spectroscopy and wide-angle X-ray diffraction (WAXD). The results show linear transfer of the applied stress from the polymer matrix to SWNT. Analysis of WAXD data demonstrates formation of a β-crystal phase in polypropylene matrix under the strain.     

Anisotropic carbon nanotube papers fabricated from multiwalled carbon nanotube webs

We fabricated large-scale anisotropic carbon nanotube (CNT) paper sheets by stacking long-lasting multiwalled CNT (MWCNT) webs without using binder materials. The MWCNTs are highly aligned in the webs and they retain their alignment in the fabricated paper. Although MWCNTs are just connected by van der Waals force, tensile strength is as strong as 75.6 MPa. In addition, resistivity and thermal conductivity is as good as 2.5 × 10⁻³ Ω cm and 70 W/m K, respectively. The present high anisotropy ratios of 7.3 in resistivity and of 8.1 in thermal conductivity are due to the high alignment of the ultra-long MWCNTs which have lengths of millimeters. High-speed web drawing with a draw speed of over 10 m/s enables very rapid fabrication. The material properties of CNT structures can be measured by conventional methods for macroscopic samples rather than methods designed for nanomaterials. CNT web technology will enable CNTs to be used in new applications.     

多孔树脂及碳纳米管对环丙沙星的吸附行为与机理

以环丙沙星为研究对象,研究了多孔树脂(超高交联树脂MN-202,氨基修饰树脂MN-150,大孔树脂XAD-4)和多壁羧基碳纳米管C-MWCNT对环丙沙星的吸附行为和机理。结果表明:吸附剂的吸附量随溶液pH值的升高呈先增加后减小的变化趋势。3种树脂的吸附能力随溶液离子强度的增强而提高,而碳纳米管则呈相反趋势,这是因为疏水作用和静电引力对两类吸附剂的贡献不同。环丙沙星在4种吸附剂上的吸附动力学都符合准二级动力学模型,吸附等温线符合Langmuir模型。升高温度可以增强MN-202和MN-150的吸附能力,可能是微孔填充受温度影响;温度对XAD-4吸附能力的负相关作用是由于疏水作用,是主要吸附机理。对于C-MWCNT,温度对它的影响很小,这是由于疏水作用、静电作用、π-π键和氢键的共同作用。     

A growth mark method for studying growth mechanism of carbon nanotube arrays

A novel and simple growth mark method was developed to make marks during the growth process of carbon nanotube arrays. These marks can be read out under scanning electron microscope or optical microscope. Based on this method, the growth rates at different temperatures and under different acetylene partial pressures were measured, from which the activation energy and the order of reaction were determined. Based on our experimental results, the growth of carbon nanotube arrays in our experimental condition could not be diffusion-limited. The measured activation energy could possibly be attributed to the heterogeneous decomposition of acetylene over the catalyst particle. Furthermore the marked array with special segmental structure may be found some applications in the future.     

碳纳米管复合材料的最新研究进展

介绍了碳纳米管的历史和结构特点,然后重点阐述碳纳米管复合材料的制备方法及在各个领域的最新应用情况,最后对碳纳米管复合材料未来的研究方向进行展望。     

碳纳米管复合材料的最新研究进展

介绍了碳纳米管的历史和结构特点,然后重点阐述碳纳米管复合材料的制备方法及在各个领域的最新应用情况,最后对碳纳米管复合材料未来的研究方向进行展望。     

The termination mechanism for radical oligomerization of methacrylonitrile

Summary Decomposition of azobisisobutyronitrile in benzene containing small amounts of methacrylonitrile produces a mixture of oligomers of methacrylonitrile. Analysis by gas chromatography-mass spectrometry indicates that combination is the preponderant (>90%) termination mechanism.     

The growth of carbon nanotube stacks in the kinetics-controlled regime

A CNT stack formation technique is used for the pseudo in-situ monitoring of CNT growth in the kinetics-controlled regime. CNT stacks are fabricated by water-assisted selective etching and the cyclic introduction of ethylene into the chemical vapor deposition (CVD) reactor. By varying the growth temperature and the ethylene flow rate within growth cycles, the reaction was shown to be first order with an activation energy of 201.2 kJ/mol. This technique can be implemented to monitor the initial stage of CNT growth by deceasing the ethylene flow time in growth cycles. Within one minute, the CNT growth reaches steady state. The formation of CNT stacks could be tailored to investigate the effects of growth parameters, such as pressure, temperature, and carbon source flow rate, on the CNT growth.     

The role of nitrogen in carbon nanotube formation

To examine the role of nitrogen, Co- and Ni-coated substrates were pretreated with three different gas compositions to compare the pretreated catalyst surfaces; the Fe, Co and Ni foils were subjected to carbon nanotube (CNT) growth experiments with CH₄/H₂ and CH₄/N₂ as source gases; the catalyst pretreatment plus the CNT growth experiments on Co- and Ni-coated Si substrates were carried out using both microwave plasma chemical vapor deposition and electron cyclotron resonance chemical vapor deposition (ECR-CVD) under different nitrogen-containing gases. The results show that the role of nitrogen may be summarized as follows: by comparing with hydrogen plasma, the bombardment energy of nitrogen plasma is greater. Therefore, the presence of nitrogen during CNT growth can keep the front catalyst surface clean and active to prolong surface passivation to enhance carbon bulk diffusion. The higher temperature due to higher bombardment energy of nitrogen plasma can promote agglomeration effects during catalyst pretreatment and the initial stage of CNT growth to produce larger size nano-particles. The presence of nitrogen is a favorable condition for formation of the bamboo-like CNTs, but not a necessary condition. Another favorable condition for formation of the bamboo-like CNTs is to deposit CNTs by ECR-CVD.     

Synergetic carbon nanotube growth

We report an effect in vertically-aligned carbon nanotube growth in which small catalyst features in proximity to large features show an enhanced growth rate. We apply this so-called “synergetic growth” effect in micrometer-scale patterns to produce vertical-like growth horizontally. This approach enables carbon nanotube integration into device applications requiring a fair degree of carbon nanotube alignment. The synergetic growth effect corroborates growth mechanisms describing carbon nanotube growth as a more complex process than elemental carbon supersaturating metal catalyst.     

Connection of macro-sized double-walled carbon nanotube strands by bandaging with double-walled carbon nanotube films

Based on their special shrinkage characteristics, double-walled carbon nanotube (DWCNT) films were used as bandages to bind the overlapped ends of macro-sized (centimeters long) DWCNT strands for connection to get structures of random length. Tensile tests indicated that the joints made in this way had relatively high tensile strength with a maximum value of 311 MPa corresponding to that of the original strands. The equivalent contact resistance of the joints was very small. And the connected strands showed better electronic properties in our investigation on the temperature dependence of resistivity and the same remarkable current capacity, in contrast to the original ones. This technique may offer a promising potential for the future extensive use of macro-sized CNTs in many fields, such as electrical cables and wires.