Structural transformation of vapor grown carbon nanofibers studied by HRTEM

Vapor grown carbon nanofibers have been extensively manufactured and investigated in recent years. In this study commercially available vapor grown carbon nanofibers subjected to different processing and post processing conditions were studied employing high resolution TEM images. The analysis showed that the fibers consist primarily of conical nanofibers, but can contain a significant amount of bamboo nanofibers. Most conical nanofibers were found to consist of an ordered inner layer and a disordered outer layer, with the cone angle distribution of the inner layers indicating that these cannot have a stacked cone structure but are compatible with a cone-helix structure. Fibers that have been heat treated to temperatures above 1,500 °C undergo a structural transformation with the ordered inner layers changing from a cone-helix structure to a highly ordered multiwall stacked cone structure. The bamboo nanofibers were found to have a tapered multiwall nanotube structure for the wall and a multishell fullerene structure for the cap of each segment, surrounded by a disordered outer layer. When these fibers are heat treated the disordered outer layers transform to an ordered multiwall nanotube structure and merge with the wall of each segment. The end caps of each segment transform from a smooth multiwall fullerene structure to one consisting of disjointed graphene planes. A reaction-diffusion mechanism is proposed to explain the growth and structure of the bamboo nanofibers.     

Carbon nanotubes grown on electrospun polyacrylonitrile-based carbon nanofibers via chemical vapor deposition

Carbon nanotubes (CNTs) grown on electrospun polyacrylonitrile-based carbon nanofibers (CNFs) via chemical vapor deposition were studied in this paper. Analyses of Raman spectra and X-ray diffraction patterns revealed that incorporation of CNTs could improve the crystalline and structure integrity of the obtained CNFs/CNTs composite. About 7.4 wt% of CNTs were grown on the electrospun CNFs confirmed by thermal gravimetric analysis. The electrochemical results showed that the surface activity and the cycle retention of the CNFs/CNTs composite were enhanced due to its three-dimensional nanostructure, enhanced pore distribution, and good conductivity. The CNFs/CNTs composite offers a great potential for high-performance lithium-ion batteries as the electrode.     

化学气相沉积法制备定向碳纳米管阵列

以二茂铁和二甲苯分别作为催化剂和碳源，采用一种无模板的化学气相沉积法，使用单温炉设备，成功地制备了高度定向的碳纳米管阵列.分别用扫描电子显微镜、透射电子显微镜和电子能量散射谱、拉曼光谱对碳纳米管阵列进行形貌观察和表征, 并研究了不同工艺参数对碳纳米管阵列形貌的影响.结果表明：在生长温度为800℃，催化剂浓度为0.02g/mL，抛光硅片上容易获得高质量的定向碳纳米管阵列，在此优化条件下生长的定向碳纳米管的平均生长速率可达25μm/min.     

Growth and doping of bulk GaN by hydride vapor phase epitaxy

Doping is essential in the growth of bulk GaN substrates,which could help control the electrical properties to meet the requirements of various types of GaN-based devices.The progresses in the growth of undoped,Si-doped,Ge-doped,Fedoped,and highly pure GaN by hydride vapor phase epitaxy(HVPE) are reviewed in this article.The growth technology and precursors of each type of doping are introduced.Besides,the influence of doping on the optical and electrical properties of GaN are presented in detail.Furthermore,the problems caused by doping,as well as the methods to solve them are also discussed.At last,highly pure GaN is briefly introduced,which points out a new way to realize high-purity semi-insulating(HPSI) GaN.     

等离子体增强热丝CVD生长碳纳米尖端的研究

用CH₄，NH₃和H₂为反应气体，利用等离子体增强热丝化学气相沉积系统在不同偏压电流的条件下制备了碳纳米尖端，并用扫描电子显微镜和显微Raman光谱仪对碳纳米尖端进行了研究.结果表明碳纳米尖端是石墨结构，随着偏压电流的增大，碳纳米尖端的顶角减小，生长速率增大.结合有关等离子体和溅射的理论，分析讨论了碳纳米尖端的形成和碳纳米尖端的生长随偏压电流的变化. 关键词： 碳纳米尖端 等离子体 化学气相沉积     

NMR study of the vapor phase contribution to diffusion in partially filled silica glasses with nanometer and micrometer pores

The contribution of the vapor phase to molecular diffusion in porous silica glasses with nanometer (Vycor) and micrometer (VitraPor#5) pores partially filled with water (polar) or cyclohexane (nonpolar) was investigated with the aid of field-gradient NMR diffusometry. Due to the vapor phase, the effective diffusion coefficient of cyclohexane filling micrometer pores (VitraPor#5) increased up to 10 times relative to the value in bulk liquid upon reduction of the pore space filling factor. On the other hand, the effective diffusion coefficient of water first decreases and then increases when the liquid content is reduced. The dependence of the effective diffusion coefficient on the pore filling factor is strongly related to the pore dimension. A general two-phase exchange model is presented that is well accounting for all experimental diffusion features.     

Influence of carbon nanofibers on electrochemical properties of carbon nanofibers/glass fibers composites

Conducting glass fiber fabrics (GFs) with double-scale roughness were fabricated by growing carbon nanofibers (CNFs) on its surfaces. The homogeneous growth of CNFs was achieved by the decomposition of C₂H₂ on glass fibers surfaces coated with Fe-doped mesoporous silica films at different C₂H₂ flow rates. The chemical composition and surface structure of the GFs before and after CNFs growth were examined by electron dispersive X-ray spectrometry (EDX), N₂ full isotherms, and scanning electron microscopy (SEM). The electrical properties of the GFs were examined using a four-probe volume resistivity tester. The CNFs with a mean diameter of 50 nm grew uniformly and densely on the glass fiber surfaces. The CNFs/GFs fabrics surface exhibited excellent electrochemical properties due to the CNFs. The specific capacitance of the GFs ranged from 0.2 to 4 F/g at 1 A/g in a 1 M H₂SO₄ aqueous solution.     

Growth of carbon nanotubes by Fe-catalyzed chemical vapor processes on silicon-based substrates

In this paper, a site-selective catalytic chemical vapor deposition synthesis of carbon nanotubes on silicon-based substrates has been developed in order to get horizontally oriented nanotubes for field effect transistors and other electronic devices. Properly micro-fabricated silicon oxide and polysilicon structures have been used as substrates. Iron nanoparticles have been obtained both from a thin Fe film evaporated by e-gun and from iron nitrate solutions accurately dispersed on the substrates. Single-walled nanotubes with diameters as small as 1 nm, bridging polysilicon and silicon dioxide “pillars”, have been grown. The morphology and structure of CNTs have been characterized by SEM, AFM and Raman spectroscopy.     

Growth of carbon nanofibers on aligned zinc oxide nanorods and their field emission properties

Carbon nanofibers were grown by electrodeposition technique onto aligned zinc oxide (ZnO) nanorods deposited by hybrid wet chemical route on glass substrates. X-ray diffraction traces indicated very strong peak for reflections from (0 0 2) planes of ZnO. The Raman spectra were dominated by the presence of G band at about 1597 cm⁻¹ corresponding to the E_2g tangential stretching mode of an ordered graphitic structure with sp² hybridization and a D band at about 1350 cm⁻¹ originating from disordered carbon. Fourier transformed infrared studies indicated the presence of a distinct characteristic absorption peak at ∼511 cm⁻¹ for Zn-O stretching mode. Photoluminescence spectra indicated band edge luminescence of ZnO at ∼3.146 eV along with a low intensity peak at ∼0.877 eV arising out of carbon nanofibers. Field emission properties of these films and their dependence on the CNF coverage on ZnO nanorods are reported here. The average field enhancement factor as determined from the slope of the FN plot was found to vary between 1 × 10³ and 3 × 10³. Both the values of turn-on field and threshold field for CNF/ZnO were lower than pure ZnO nanorods.     

Growth and morphology of carbon nanostructures on nickel oxide nanoparticles in catalytic chemical vapor deposition

The present study explores the conditions favorable for the growth of cylindrical carbon nanostructures such as multi-walled carbon nanotube (MWCNT) and carbon nanofiber by catalytic chemical vapor deposition (CCVD) method using nickel oxide-based catalyst nanoparticles of different average sizes as well as different levels of doping by copper oxide. The role of doping and the average size have been related to the observed melting behavior of nanoparticles of nickel oxide by thermal and diffraction analysis, and the importance of melting has been highlighted in the context of growth of cylindrical nanostructures. In the reducing environment prevailing in the CCVD chamber due to decomposition of flowing acetylene gas at elevated temperature, there is extensive reduction of oxide nanoparticles. Lack of melting and faster flow of carbon-bearing gases favor the formation of a carbon deposit cover over the catalyst nanoparticles giving rise to the formation of nanobeads. Melting allows rapid diffusion of carbon from the surface to inside catalyst particles, and reduced flow of gas lowers the rate of carbon deposit, both creating conditions favorable for the formation of cylindrical nanostructures, which grows around the catalyst particles. Smaller particle size and lower doping favor growth of MWCNT, while growth of fiber is commonly observed on larger particles having relatively higher level of doping.     

Optical properties of photonic crystals filled with iodine vapor

Properties of electromagnetic waves in the photonic crystal with pores filled with iodine vapor are considered. The conditions of the permittivity resonance appearance in the visible region of the spectrum, caused by the electronic transition in iodine molecules, are determined. For the resonant photonic crystal under study, dispersion curves, group velocities of electromagnetic waves, and reflection spectra are calculated for various globule diameters.     

Catalytic production of carbon nanotubes by vapor phase growth method using tungsten-containing organic compound

We report the production of carbon nanotubes of high purity by a vapor phase growth method using a catalytic reaction of a tungsten-containing organic compound (C₁₄H₁₀O₇W) and acetylene mixture. The products were multi-walled carbon nanotubes with hollow cores. Transmission electron microscopy investigation revealed that the graphitic layers at the inner region were highly crystallized but the graphitic layers at the outer region had a wavy structure. We have demonstrated the effective production of carbon nanotubes using a tungsten-based catalyst. PACS 81.07.De; 61.46.+w; 68.37.-d     

Geometry of the vapor phase in explosive near-wall boiling-up

Methods for calculating the geometric characteristics of the vapor phase in explosive wall boiling-up processes on a metal wall are analyzed. A monotonic growth of superheat in the liquid above the equilibrium evaporation temperature is specified. We show that the choice of the model for bubble interaction has a profound influence on the geometric characteristics which define the value of the heat flux. Computer simulation was employed to obtain the dependence of dry area on time in two interaction models. We have found that, for a model with instantaneous bubble coalescence, the dry area can be evaluated by the Kolmogorov formula using a correction factor for the most probable triple interaction. An approximation of the distribution length of wetting line over the lifetime of wetting-line segments is obtained. The possibility of using the obtained data for calculation of rapid condensation is analyzed.     

Alkali vapor phase doping of polyacetylene

We have discovered a vapor phase doping process of polyacetylene by alkali metals. The existence of a threshold temperature and nucleation characterize the doping as an intercalation process. The transport and optical properties, as obtained by dc conductivity and photoabsorption studies, imply a high intrinsic conductivity.     

Room-temperature growth of carbon nanofibers on plastic substrates

A very simple method to synthesize densely distributed carbon nanofibers (CNFs) on flexible plastic substrates at room temperature with no catalyst is demonstrated. Carbon film was deposited onto polyimide, poly-ethylene-terephthalate (PET) films, Si plates and a Ni mesh, which were then sputtered with obliquely incident Ar⁺ ions at 3 keV at room temperature. Linear-shaped CNFs oriented in the incidence direction of the ion beam grew on the sputtered substrates, as confirmed by scanning (SEM) and transmission electron microscopes (TEM). CNF growth on a PET substrate, which is a non-heat-tolerant plastic, has never been reported so far. CNFs thus grown were characterized as amorphous without a hollow structure. The diameter of CNFs was almost identical (20-30 nm) despite a large difference in CNF length (0.1-4 μm). In addition, the CNF-tipped cones were demonstrated to act successfully as a template to fabricate one dimensional (1-D) zinc oxide (ZnO) nanostructures on a PET substrate. Thus, it was believed that the ion-irradiation technique would open up a new approach to fabricate any kinds of 1-D nanomaterials on flexible substrates at room temperature.     

Kinetics of water vapor diffusion in activated carbon

We describe an experimental method for studying rapid processes of water vapor sorption by fine-dispersed and porous materials. The concentration of gas-phase water molecules is detected during adsorption by a laser-diode spectrometer. The kinetic pressure curves are recorded in a time window of 10^?1 to 10³ s and are analyzed using analogy of the diffusion flow with the electric current in a branched RC circuit. The proposed model establishes the relation between the kinetics curves being measured and the structural parameters of the medium.     

纳米碳纤维红外消光数值计算

为探讨纳米碳纤维作为烟幕粒子的红外消光特性,基于电磁场理论建立了细直的纳米碳纤维感应电流积分方程,并利用矩量法进行了求解,导出了纳米碳纤维散射场及吸收、散射和消光截面的计算式.通过与变分法的计算结果相比较,表明了该方法的有效性.利用该方法数值分析了纳米碳纤维红外消光截面与入射场波长、入射角、纤维长度和半径的关系,计算结果为纳米碳纤维用于红外烟幕干扰提供了理论依据.