Carbon nanotube electron source technology

The carbon nanotube embodies a unique combination of properties which make it potentially an extraordinary field emission electron source. These properties include small tip radii (and small source size), high electrical conductivity, high melting point, and resistance to electromigration under an applied electric field. Here, carbon nanotube electron point sources are shown to be remarkably stable, with high brightness, low energy spread, and low noise. These are favorable attributes of an electron source to be used in an electron-optical system. By combining wafer-scale carbon nanotube growth technology with microfabrication techniques, it is possible to mass produce high-performance emitter arrays that can deliver high current beams at high frequencies. Author’s Note: This article is intended to be an overview of carbon nanotube electron source technology, concentrating on results by the author and his collaborators. It is not intended to be a full review of the field.     

结构钢残余应力的电磁法测量优化研究

采用万能试验机、金相显微镜、X射线衍射仪及电磁应力仪，研究了电磁法测残余应力灵敏系数与层深之间的关系， 对相应工件不同层深残余应力进行测量，并提出了使用线性拟合法取得更合适的灵敏系数，最后使用X射线衍射法进行了验证。结果表明：灵敏系数随着层深增加，先增大后减小；工件近表层残余应力分布与深层有着显著区别，在0.6 mm以上更深层处的残余应力差异较小；使用线性拟合法取得的灵敏系数更加契合实际灵敏系数，能一定程度上减小电磁法测量误差。     

Preparation of titanium dioxide nanotube arrays on titanium mesh by anodization in (NH4)2SO4/NH4F electrolyte

The self‐organized titanium dioxide (TiO₂) nanotube arrays on titanium mesh were prepared by electrochemical anodization with the neutral electrolyte containing ammonium sulfate and ammonium fluoride in a two‐electrode electrochemical cell. The effects of the fluoride ion concentration, the anodic potential, and the oxidation time on the formation of the titanium dioxide nanostructures on titanium mesh with complex geometry were investigated. The anodized titanium mesh was characterized by field emission scanning electron microscope and in situ high temperature X‐ray diffraction. The results show that the titanium dioxide nanotube arrays are grown in a radially outward direction around the titanium wire. The optimized anodization condition for preparing titanium dioxide nanotube arrays with superior architecture on the titanium mesh is 0.5 wt% of ammonium fluoride, 20 V of applied potential, and 20 min of oxidation time. The amorphous titanium dioxide nanotubes on titanium mesh turn to anatase phase at 400 °C and further to rutile phase at 650 °C.     

Electrochemical synthesis of polypyrrole films over each of well-aligned carbon nanotubes

Polypyrrole (PPy) films were uniformly electropolymerized over each carbon nanotube of the well-aligned carbon nanotube arrays. For comparison, PPy films were also coated on flat metallic titanium (Ti) and platinum (Pt) substrates by the same technique. The synthesis and the redox performance of the PPy films were conducted by cyclic voltammetry (CV). The structural characterization including the thickness and uniformity of the PPy films was carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is observed that the coating of the PPy film over carbon nanotubes is much faster than that on flat Ti/Pt surface. Furthermore, the redox performance of the PPy-coated carbon nanotube electrodes over flat Ti/Pt electrodes was significantly improved due to the high accessible surface area of the carbon nanotubes in the aligned arrays, especially in large film formation charge (Q_film). It is very promising that the electrode developed in this study could be used as high performance electrode in rechargeable batteries.     

Application of spherical Ni（OH）2/CNTs composite electrode in asymmetric supercapacitor

1 Introduction Electrochemical capacitors (hereafter ECs) have greater power density than usual batteries and can be deeply discharged without any deleterious effect on life time[1]. Activated carbon(AC) with various modifications is the electrode materia…     

Simulations of the dielectric constant of bonding materials and field emission properties of CNT cathodes

The effect of the dielectric constant (k) of bonding materials in a screen-printed carbon nanotube (CNT) cathode on the field enhancement factor was investigated for high-efficiency CNT cathodes using the ANSYS software. The values obtained by a simulation study were compared to the experimental results obtained for screen-printed CNT cathodes. The field enhancement factor increased as the dielectric constant decreased, reaching a maximum value at a dielectric constant of 1, the value for a vacuum. The findings indicate that the larger sheet resistance of the bonding materials, after the firing process, can be attributed to the larger emission current of the CNT cathode. From these results, it was concluded that the best bonding materials for screen-printed CNT cathodes should have a low dielectric constant and a high sheet resistance. This finding can be used as criteria for selecting bonding materials for use in CNT pastes for highly efficient CNT cathodes.     

Y-junction single-wall carbon nanotube electronics

This article presents the synthesis of a Y-junction single-wall carbon nanotube (SWNT) and its electrical characterization. The formation of Y-junction branches is found to be dependent on the catalyst composition, which can be correlated to the Gibbs free energy of metal carbide formation. Radial breathing mode peaks in Raman spectra show that the sample has both metallic and semiconducting nanotubes, and their portion is varied with type of catalyst and growth temperature. The localized gating effect exhibited by the Y-SWNT suggests the resemblance of its electrical characteristics with ambipolar field effect transistor. The temperature dependence of the I–V characteristics reveals that the conduction mechanism in the Y-SWNT is governed by thermionic emission at temperatures above 100 K and by tunneling at T < 100 K.     

Carbon nanotube/epoxy resin composite: Correlation between state of nanotube dispersion and Zener tunneling parameters

Carbon nanotube/epoxy resin composites with different levels of nanotube dispersion in the polymer matrix were prepared. The effects of nanotube dispersion on tunneling conductive behavior of such composites were investigated. The composites with homogeneous nanotube dispersion were found to exhibit larger static electrical conductivity and smaller percolation threshold than those with poorer nanotube dispersion. In addition, uniformly dispersed nanotubes induced strong Zener effect under the application of an electric field. The static conductivity and Zener tunneling parameters were shown to be good indicators for the state of nanotube dispersion.     

银金修饰TiO2纳米管阵列光阳极的制备及其光伏性质

采用阳极氧化法,以金属钛箔为原料制备了TiO2纳米管阵结构染料敏化纳米晶太阳能电池(DSSC)光阳极,并用电沉积、光沉积和真空蒸发法对其表面分别进行金属Ag和Au修饰。对金属修饰前后的光阳极用扫描电子显微镜(SEM)和X射线衍射分析仪(XRD)表征发现:阳极氧化法制备的TiO2纳米管阵列结构规整有序;经过化学沉积后,其表面均覆盖有一层致密的金属粒子薄膜。测量光电流密度-电压(J-V)曲线和电化学阻抗谱(EIS)发现:相比纯TiO2纳米管阵列,经过Ag修饰后的TiO2阵列其短路电流密度和开路电压最大可以分别提高到前者的4.9和1.7倍,并且其电化学阻抗出现显著降低。研究表明:金属Ag修饰改善了TiO2纳米管之间的连通性,提高了TiO2光阳极中电子的传输效率,并通过等离子共振激发出光电子,同时与TiO2相互作用,使其在可见光区域也能够促进电子-空穴对的分离。     

电沉积钴磷基碳纳米管复合镀层

对电弧放电法制备的碳纳米管进行混酸氧化后,在含有锡和钯的溶液中进行敏化、活化预处理,使碳纳米管的表面形成密集的活性中心;采用电沉积法制备碳纳米管钴磷基复合镀层。研究比较了电沉积钴磷层和钴磷基碳纳米管复合镀层的物理特性和电化学性质。结果表明,碳纳米管的加入明显提高了钴磷基复合镀层的结合力和耐腐蚀性能。     

Self-aligned TiO2 nanotube arrays produced by air-cathode as electrode

Air-cathodes were used to produce TiO₂ nanotube arrays. The effects of pH, voltage and degradation of air-cathode in tailoring the morphologies of TiO₂ nanotube arrays were investigated. Preliminary results show that TiO₂ nanotubes could be formed and are comparable to those produced by platinum electrodes under similar conditions. The lengths and diameters of TiO₂ nanotube arrays obtained are in the range of 1.0-2.2 μm and 40-150 nm, respectively. It is found that the rate of formation of the nanotubes is closely related to the pH of the solution. Air-cathodes are found to have relative low values of mass loss rates.     

Au负载N掺杂TiO2纳米管阵列的制备及其性能

TiO₂ 纳米管阵列较大的禁带宽度是导致其光催化效率较低的重要原因,采用磁控溅射、阳极氧化以及气氛退火相结合的方法对 TNAs 改性后制备了 Au 负载 N 掺杂 TiO₂ 纳米管阵列(Au@ N-TNAs),然后以甲基橙为目标污染物, 进一步分析了 Au@ N-TNAs 在不同 Au 负载量时光降解效率的变化情况。 采用 SEM、XRD、TEM 和 X 射线光电子能谱 (XPS)等对 Au 和 N 在 Au@ N-TNAs 中的存在形式进行表征和分析,发现 Au 主要是负载在 TiO₂ 纳米管阵列上,而 N 元素则是以掺杂的方式进入 TiO₂ 纳米管阵列的晶格中。 此外,在光降解试验中发现通过 Au 负载与 N 掺杂相结合的方法对 TiO₂ 纳米管阵列进行复合改性后,TiO₂ 纳米管阵列的光催化效率得到显著提升,其中 20s-Au@ N-TNAs 具有最佳的光降解效率。 但 Ti-N 薄膜中间的 Au 层太厚时会影响阳极氧化过程中 TiO₂ 纳米管阵列的生长,而且过量的 Au 在退火处理时很难及时地扩散均匀,进而使得改性后的 TiO₂ 纳米管阵列(40s-Au@ N-TNAs)的光催化效率明显降低。     

Bi_(3.15)Nd_(0.85)Ti_3O_(12)纳米管阵列的制备与微结构表征

采用溶胶凝胶法,在孔径为200 nm的阳极氧化铝模板中制备了Bi_3.15Nd_0.85Ti_3O_(12)纳米管阵列.通过XRD、SEM、TEM、HRTEM、SAED和Raman光谱测试手段对纳米管阵列的物相、微结构和声子振动特性进行了表征.研究表明,所合成BNdT纳米管为钙钛矿相多晶结构,纳米管外径约为200 nm,管壁厚约10 nm,管径和长度与所用AAO模板尺寸一致.Raman光谱分析表明,Nd离子取代了类钙钛矿层中A位的Bi离子.     

Field emission enhancement of ZnO nanorod arrays with hafnium nitride coating

Vertically well-aligned single crystal ZnO nanorod arrays were synthesized and enhanced field electron emission was achieved with hafnium nitride (HfN_x) coating under proper sputtering condition. HfN_x films with various composition have been coated on ZnO nanorod arrays using a reactive direct current (DC) magnetron sputtering system. Morphology and crystal configuration of the ZnO nanorod arrays were investigated by scanning electron microscopy and X-ray diffraction. The field emission properties of the coated and uncoated ZnO nanorod arrays were characterized. The as-grown ZnO nanorod arrays showed a turn-on electric field of 6.60 V μm^− 1 at a current density of 10 μA cm^− 2 and an emission current density of 1 mA cm^− 2 under the field of 9.32 V μm^− 1. While the turn-on electric field of the coated ZnO nanorod arrays sharply decreased to 2.42 V μm^− 1, an emission current density of 1 mA cm^− 2 under the field of only 4.30 V μm^− 1 can be obtained. A method to accurately measure the work function of the coated films was demonstrated.     

氧化参数对TiO_2纳米管结构及双室光电化学池产氢性能的影响

采用乙二醇电解液,在不同氧化电压、氧化时间条件下通过阳极氧化纯钛片制备了一系列TiO_2纳米管阵列薄膜。使用场发射扫描电镜(FESEM)表征TiO_2纳米管的表面、断面形貌,探讨氧化时间及氧化电压对纳米管生长速率的影响。同时通过电化学方法测试TiO_2纳米管的光电化学性能,以无外加电压下双室光电化学池中的产氢量考察其光催化活性。结果表明,相比延长氧化时间,提高氧化电压更容易获得高长/径比的TiO_2纳米管阵列,同时可显著提高TiO_2纳米管的光电流、光电转换效率及产氢量。     

一维W纳米材料的场发射性能及其可控制备的研究进展

一维W纳米材料是一种新型的场发射材料,具有优异的场发射性能。介绍场致电子发射的基本原理及性能评价标准;讨论一维W纳米材料的6种制备方法:气相沉积法、电子束诱导法、蚀刻法、溅射法、自组装法和模板法;分析不同制备方法对场发射性能的影响;指出电场增强因子是决定一维W纳米材料场发射性能优劣的首要因素,设计制造出结构可靠、场发射性能优异、成品率高以及寿命长的一维W纳米材料器件是未来的研究重点。     

An Energy-Efficient Electrochemical Method for CuO-TiO2 Nanotube Array Preparation with Visible-Light Responses

A rapid and energy-efficient method was presented for preparing CuO-TiO₂ nanotube arrays.TiO₂ nanotube arrays were first prepared by anodic oxidation using titanium anode and platinum cathode.Then,the formed TiO₂ nanotube arrays and Pt were used as cathode and anode,respectively,for subsequent formation of CuO-TiO₂ nanotube arrays,through an electrochemical process in a solution of 0.1 mol/L CuSO₄.The morphology and composition of the CuO-TiO₂ nanotube arrays were characterized using field-emission scanning electron microscopy,X-ray photoelectron spectroscopy（XPS）,X-ray diffraction（XRD）,and UV-Vis diffusion reflection spectroscopy（UV-Vis DRS）.XPS and XRD analyses suggested that the Cu element in the nanotubes existed in CuO form,and its content changed along with the voltage during the second electrochemical process.The photocatalytic activities of the CuO-TiO₂ nanotube arrays were evaluated by the degradation of a model dye,rhodamine B.The results showed that Cu incorporation aroused wide visible-light adsorption and improved the photocatalytic efficiency of TiO₂nanotube arrays significantly under visible-light irradiation.The stability of the CuO-TiO₂ nanotube arrays was also detected.     

非晶TiO2 纳米管形貌对其作为超级电容器电化学性能的影响

采用改进的两步电化学阳极氧化和电化学氢化法制备了不同管径、长度和壁厚的氢化无定型TiO₂纳米管阵列（H@am-TNAs）。结果表明,电化学氢化对TiO₂纳米管阵列的结构影响不大。经过电化学氢化后,纳米管在100 mV·s^-1时的比电容为4.05 mF·cm^-2,比未氢化的管长和管径相同的TiO₂纳米管的比电容大20倍。纳米管的比电容不仅与管长有关,还受管径的影响。通过指数函数拟合,纳米管的长径比呈线性关系。面积电容/长径比达到0.056,几乎相当于锐钛矿相TiO₂纳米管。阳极化处理后的纳米管具有最小的电荷转移阻力和最佳的离子扩散/输运动力学,具有最高的面积容量。此外,为了研究H@am-TNAs纳米管的电化学性能的润湿性,相同的H@am-TNAs电极在C-V和C-P测试前,在电解液中浸泡不同时间,结果表明,比电容随着浸泡时间的增加而减小。     

Internal field emission and conductivity relaxation in carbon nanofiber filled polymer system

Conducting polymer nanocomposites based on high density polyethylene (HDPE) and carbon nanofiber (CNF) were fabricated by melt compounding. The conductivity of the nanocomposites was found to follow the percolation behavior. Effect of electric field on the electrical conduction behavior of such composites was investigated. The results revealed two competing processes in the composite: internal field emission and electrical conduction relaxation. The former is dominant at lower filler concentrations or under low electric field, while the latter is pronounced under the application of strong electric field. Further, the relaxation time is nearly independent on the electric field strength but decreased with increasing temperature.     

The role of defects in the design of space elevator cable: From nanotube to megatube

Researchers are claiming that the feasibility of space elevator cable is now realistic, thanks to carbon nanotube technology, proposing its realization within a decade. However, the current view of basing the design of the megacable on the theoretical strength of a single carbon nanotube is naïve, as has recently been emphasized. In this paper the role of thermodynamically unavoidable atomistic defects with different size and shape is quantified on brittle fracture, fatigue and elasticity, for nanotubes and nanotube bundles. Nonasymptotic regimes, elastic plasticity, rough cracks, finite domains and size effects are also discussed. The results are compared with atomistic simulations and nanotensile tests of carbon nanotubes. Key simple formulas for the design of a flaw-tolerant space elevator megacable are reported, suggesting that it would need a taper ratio (for uniform stress) of about two orders of magnitude larger than currently proposed.