Synthesis and characterization of carbon nanotubes on carbon microfibers by floating catalyst method

In this paper, carbon nanotubes were synthesized on carbon microfibers by floating catalyst method with the pretreatment of carbon microfibers at the temperature of 1023 K, using C₂H₂ as carbon source and N₂ as carrier gas. The morphology and microstructure of carbon nanotubes were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The composition of carbon nanotubes was determined by energy dispersive X-ray spectroscopy (EDX). The results showed that the surface of treated carbon microfibers was thickly covered by carbon nanotubes with diameters of about 50 nm. EDX image indicated that the composition of carbon nanotubes was carbon. In comparison with the sample grown on untreated carbon microfibers surface, it was found that after carbon microfibers were boiled in the solution of sulfur acid and nitric acid (VH₂SO₄:VHNO₃ = 1:3) and immersed in the solution of iron nitrate and xylene, carbon nanotubes with uniform density can be grown on carbon microfibers surface. Based on the results, we concluded that the pretreatment of carbon microfibers had great effect on the growth of carbon nanotubes by floating catalyst method.     

Sonochemical preparation of carbon nanosheet from carbon black

Preparation of carbon nanosheet via ultrasound irradiation of carbon black under ambient conditions was reported for the first time. The structure of resulting carbon nanosheet was characterized by TEM, HRTEM, EDX and AFM. The experimental results showed that the carbon nanosheet is composed of ordered graphite carbon layers with a thickness of several nanometers.     

Dimensional changes of carbon fiber fabrics reinforced phenolic resin-based carbon/carbon composites during pyrolysis

The dimensional changes of two-dimensional phenolic resin-based carbon/carbon composites during pyrolysis were investigated. The carbon/carbon composites were obtained by pyrolyzing the carbon fiber/phenolic resin composites that were fabricated using a vacuum bag hot pressing technique. Length, width and thickness of the rectangular composite samples were measured carefully before and after pyrolysis. The effects of heat treatment temperature and fiber pre-heat treatment on the dimensional changes of carbon/carbon composites were investigated. The measurement results indicated that different behavior in dimensional changes could be obtained for carbon/carbon composites with different fiber–matrix bonding in the interface.     

Gasification of porous carbon particle by carbon dioxide

The diffusive-kinetic model of porous carbon particles gasification is developed. The model considers the processes of heat and mass transfer both inside the porous particle and above it. Analysis of the model shows that heat and mass transfer have an influence to the gasification process to a marked degree. Gasification of carbon particle by carbon dioxide is impossible if particle temperature is lower about 850 K because concentration of carbon dioxide at the particle surface becomes lower than its equilibrium concentration. The rate of the carbon particle gasification is determined as a function of the porous particle internal surface area for different pressures and furnace temperatures.     

Monte Carlo simulation of carbon monoxide,carbon dioxide and methane adsorption on activated carbon

In this study, the adsorption capacity of pure and activated carbon with regard to carbon monoxide (CO), carbon dioxide (CO₂) and methane (CH₄) gases at 298?K and pressure from 0.01 up to 2.0?MPa has been investigated computationally. Computational work refers to Monte Carlo (MC) simulation of each adsorbed gas on a graphite model with varying density of activation sites. The Grand Canonical Monte Carlo (GCMC) simulation technique was employed to obtain the uptake of each adsorbed gas by considering a graphite model of parallel sheets activated by carboxyl and hydroxyl groups, as observed experimentally. The simulation adsorption data for these gases within the examined carbon pore material are presented and discussed in terms of the adsorbate fluid molecular characteristics and corresponding interactions between adsorbate species and adsorbent material. We found that the simulated adsorption uptake of the examined graphite model under these conditions with regard to the aforementioned fluids increases in the order CO?<?CH₄?<?CO₂.     

X射线荧光光谱研究粮食中生命有机碳与碳化学循环

采用X射线荧光光谱(XRF)方法测定六种粮食:稻谷(大米)、小麦(面粉)、黄豆、小米、高粱和玉米中生命有机碳含量和化学元素组成,建立一种新方法测定它们的蛋白质含量.粮食中生命有机碳平均含量约为44%.黄豆中的蛋白质含量最高(42.74%),小米蛋白质含量28.56%,麦子蛋白质含量27.57%,玉米蛋白质含量24.99...     

Growth of carbon nanotubes from ring carbon clusters

The possibility of growing single-wall carbon nanotubes from ring carbon clusters that appear at a certain stage of cooling carbon vapor is discussed. Such a technique could allow one to grow single-wall nanotubes without introducing a macroscopic amount of a catalyst and to retain nanotubes open during their growth. An analysis performed using semiempirical quantum-chemical methods shows that, when catalyst atoms interact with the edge of an already formed nanotube surface, the bonds of these atoms with carbon tend to occupy positions normal to the generatrix of the nanotube. This situation is natural for transition-metal atoms, since they favor the destruction of pentagonal cycles at the edge of the surface. The destruction mechanism consists in the fact that pentagons incorporate carbon atoms from the outside and become hexagons. The dependence of this tendency on the type of catalyst atom is considered.     

Field emission of carbon nanotubes and electronic structure of carbon nanopeapods

We calculate the electron emission from nanostructures under an applied electric field using the ab initio pseudopotential method. The transition rates of the electrons are calculated by integrating the time-dependent Schrödinger equation for the states initially inside the emitter. A localized basis set is used for obtaining the eigenstates before emission and the potential that drives the field emission. The calculated electron-tunneling graph is quite linear in the short-time region, giving the transition rate within the simulation time. We have applied this new method to the field emission of carbon nanotubes as a test. Then we calculate the electronic structure of the fullerenes encapsulated inside the carbon nanotubes, the so-called carbon “nanopeapods”. The fullerenes may or may not contain metallic atoms such as gadolinium or potassium. There is an interesting effect of strain when the diameter of the carbon nanotube is smaller than that of the inserted fullerene plus twice the van der Waals distance.     

Comparison of carbon fiber and cesium iodide-coated carbon fibercathodes

Presents results of an experimental comparison of a bare carbon fiber cathode and the same cathode when coated with cesium iodide salt (CsI). An annular cathode was constructed by arranging carbon fibers in an annular tuft pattern. The cathode was then operated as a bare carbon fiber cathode and in a configuration with a CsI coating. The cathode was tested at electric field strengths ranging from 50 kV/cm to 265 kV/cm at anode-cathode (A-K) gaps of 3.175 cm. The applied voltage had a 1-μs duration and the modulator was operated at up to 1 Hz repetition rate. The system had a low base pressure (<1.0×10^-7 torr). The article reports on results concerning the conditioning of the cathodes, the shot-to-shot reproducibility of the cathodes and the pressure evolution of the diode under 1 Hz operation. We also report on the impedance evolution of each of the diodes     

导电炭黑微观结构及其应用性能

以乙炔炭黑、副产炭黑和炉法导电炭黑为研究对象,较为系统地分析了与导电炭黑应用性能密切相关的微晶结构、表面性质、颗粒形貌和粒度分布特征,探讨了导电炭黑微观结构对应用性能的影响。研究结果表明,导电炭黑的微观结构决定了其应用性能,而决定应用性能的关键在于其与基体材料的匹配性。副产炭黑在橡胶和塑料中导电性能较好,乙炔炭黑在锂离子电极材料中的导电性能较好。     

High-field electron emission of carbon nanotubes grown on carbon fibers

Carbon nanotubes with uniform density were synthesized on carbon fiber substrate by the floating catalyst method. The morphology and microstructure were characterized by scanning electron microscopy and Raman spectroscopy. The results of field emission showed that the emission current density of carbon nanotubes/carbon fibers was 10 μA/cm² and 1 mA/cm² at the field of 1.25 and 2.25 V/μm, respectively, and the emission current density could be 10 and 81.2 mA/cm² with the field of 4.5 and 7 V/μm, respectively. Using uniform and sparse density distribution of carbon nanotubes on carbon fiber substrate, the tip predominance of carbon nanotubes can be exerted, and simultaneously the effect of screening between adjacent carbon nanotubes on field emission performance can also be effectively decreased. Therefore, the carbon nanotubes/carbon fibers composite should be a good candidate for a cold cathode material.     

导电炭黑微观结构及其应用性能

以乙炔炭黑、副产炭黑和炉法导电炭黑为研究对象,较为系统地分析了与导电炭黑应用性能密切相关的微晶结构、表面性质、颗粒形貌和粒度分布特征,探讨了导电炭黑微观结构对应用性能的影响。研究结果表明,导电炭黑的微观结构决定了其应用性能,而决定应用性能的关键在于其与基体材料的匹配性。副产炭黑在橡胶和塑料中导电性能较好,乙炔炭黑在锂离子电极材料中的导电性能较好。     

初始温度及碳源对碳纳米管气相爆轰法合成的影响

改变初始温度以及分别使用甲烷和乙炔气体作碳源时气相爆轰合成碳纳米管,研究了初始温度与不同碳源对碳纳米管的影响。利用X射线衍射(XRD)、透射电镜(TEM)、拉曼(Raman)光谱等对碳纳米管进行表征。结果表明,随着初始温度的升高,所合成的碳纳米管的产量减少且石墨化程度降低,但管壁会变得光滑且管径有所增加。当使用乙炔时,所合成的产物中没有碳纳米管,而是合成了石墨化程度较高的无定形碳,随着催化剂量的增加,产物中碳包覆颗粒增多且包覆层清晰可见,但存在结构缺陷。当初始温度在110~130 ℃时,使用甲烷气体运用气相爆轰的手段是合成碳纳米管的较佳方案。     

初始温度及碳源对碳纳米管气相爆轰法合成的影响

改变初始温度以及分别使用甲烷和乙炔气体作碳源时气相爆轰合成碳纳米管,研究了初始温度与不同碳源对碳纳米管的影响。利用X射线衍射(XRD)、透射电镜(TEM)、拉曼(Raman)光谱等对碳纳米管进行表征。结果表明,随着初始温度的升高,所合成的碳纳米管的产量减少且石墨化程度降低,但管壁会变得光滑且管径有所增加。当使用乙炔时,所合成的产物中没有碳纳米管,而是合成了石墨化程度较高的无定形碳,随着催化剂量的增加,产物中碳包覆颗粒增多且包覆层清晰可见,但存在结构缺陷。当初始温度在110~130 ℃时,使用甲烷气体运用气相爆轰的手段是合成碳纳米管的较佳方案。     

Optical Kerr effect exhibited by carbon nanotubes and carbon/metal nanohybrid materials

Structural modification of carbon nanotubes in combination with metallic nanoparticles is reported. An enhancement in the nonlinear optical refraction of multi-wall carbon nanotubes by the incorporation of platinum nanoparticles was observed. Comparative results were analyzed taking into account the participation of single-wall carbon nanotubes that originate a decrease in the nonlinear optical response of the multi-wall carbon nanotubes integrating a thin film. A Nd:YAG laser system featuring 532 nm wavelength with 4 ns pulse duration in a two-wave mixing experiment was employed for exploring the studied optical nonlinearities of the samples. The contribution of optical processes to mechanical characteristics dependent on high optical irradiance in carbon nanotubes was described. A variation in the mass density associated to the optically irradiated tubes allowed us to calculate the change in Young's modulus in a thin film configuration. The estimation of an opto-mechanical phenomenon was based on the evaluation of the nonlinearity of index responsible for the optical Kerr effect. According to Raman and optical evaluations, the inclusion of metallic nanoparticles in carbon structures results in a modification of surface that also gives origin to noticeable optical Kerr nonlinearities. Potential applications for developing laser-induced controlled opto-mechanical nanohybrid systems can be contemplated.     

Microstructure and wear property of carbon nanotube carburizing carbon steel by laser surface remelting

Carbon nanotube was used to carburize the surface of medium carbon steel and mild steel, respectively, by means of laser surface remelting. The slurry of carbon nanotube of ethanol was coated on the surface of the materials prior to laser irradiation. Microstructures, microhardness and wear property of the surface layers treated by different laser performance parameters were studied. Graphite coating was also used for carburizing. The results showed that both carbon nanotube and graphite were dissolved in the surface molten layer, leading a carburized hardening layer on the surface of the substrate. However, different microstructures formed in the carburizing layers, depending mainly on the type of carburization materials, carbon nanotube or graphite. The carbon nanotube hardening layer exhibits a little higher hardness than the graphite hardening layer. The carburized layer greatly increases the wear resistance of the base material.     

Fabrication and structure: a study of aligned carbon nanotube/carbon nanocomposites

Aligned carbon nanotube/carbon (Acnt/C) nanocomposites have been fabricated by densifying an Acnt preform with chemical vapour infiltration technology. Microstructure observations show that pyrocarbon in Acnt/C was mainly rough lamella type while pyrocarbon in carbon fiber reinforced carbon matrix (C/C) composites was typically smooth lamella type in spite of the same process. The thermal conductivity of these Acnt/C nanocomposites is about 4 times that of C/C composites. Their electrical conductivity of Acnt/C nanocomposites was anisotropic, i.e. approximately 1.61×10⁴ Ω⁻¹ m⁻¹ and 5.68×10³ Ω⁻¹ m⁻¹ in the direction parallel and vertical to the aligned carbon nanotubes, respectively.     

Amorphous carbon

The properties of various types of amorphous carbon and hydrogenated amorphous carbon are reviewed with particular emphasis on the effect of atomic structure on the electronic structure. It is shown how the proportion of sp³ and sp² sites not only defines the short-range order but also a substantial medium-range order. Medium-range order is particularly important in amorphous carbon because it is the source of its optical gap, whereas short-range order is usually sufficient to guarantee a gap in other amorphous semiconductors. The review discusses the following properties: short-range order and the radial distribution function, the infrared and Raman spectra, mechanical strength, the electronic structure, photoemission spectra, optical properties, electron energy-loss spectra, core-level excitation spectra, electrical conductivity, electronic defects and the electronic doping of hydrogenated amorphous carbon.     

基于硬度回归法测定退火碳钢的含碳量

测量了已知退火碳钢的硬度,揭示了含碳量与硬度线性相关的特性.利用最小二乘法进行回归分析,得到硬度与含碳量的线性方程,并以此实现对未知碳钢的估计.另通过金相组织观察法和杠杆定律再次对未知碳钢含碳量进行估计,结果发现与硬度测量法估计的含碳量基本一致.比较两种方法,硬度测量法更客观,更准确.并与标准金相图谱比对,进一步验证了实验结论.     

Methane conversion into hydrogen and carbon nanostructures

Methane pyrolysis in an ac electric arc is implemented: CH4 - C+2H2. The synthesized nanostructure carbon material is analyzed using a transmission electron microscope. It is shown that a catalyst enables carbon conversion into carbon nanotubes.