Achieving uniform field emission from carbon nanotube composite cold cathode with different carbon nanotube contents: Effects of conductance and plasma treatment

A carbon nanotube (CNT) composite cold cathode was studied for field emission display application. The CNT composite cold cathode was composed of CNTs and silicon dioxide binder. Field emission from CNT composite cold cathode with different CNT contents was studied. It was found that with increase in CNT contents, the threshold field decreased. The conductance of the composite cathode was measured and with increasing CNT content, there was a critical CNT content where the conductance increased several orders of magnitude. Plasma etching using SF₆ as the etchant was adopted to treat the cathode. Improvement in emission uniformity was achieved. It was also found that after post-treatment the threshold field of the cathode decreased. The morphology of the etched cathode was analyzed and the improvement of uniformity and lowering of the threshold field was attributed to the exposure of CNTs after etching.     

Improving the emission characteristics of a carbon nanotube cathode in an aging process

Aging of the field emission performance of the printed carbon nanotubes (CNTs) cathode is studied. A continuous increase of the field emission current as well as the density of field emission sites under a constant voltage is observed. It is revealed that the resistant heating may play an important role in the activation of the potential emitters during the aging process. A technique of activating the printed CNT cathode with an aging process is suggested. F-N curves before and after the aging processes are analyzed and it is revealed that the effective emission area increases during the aging process.     

分散剂对电泳沉积制备碳纳米管薄膜形貌和场发射性能的影响

采用丙酮、异丙醇、丙酮/无水乙醇混合溶液3种不同的分散剂,利用电泳沉积法在硅基底上制备了碳纳米管(CNTs)薄膜;采用超景深光学显微镜和电子显微镜观察了不同薄膜的表面形貌,并在高真空中对碳纳米管薄膜阴极进行了场发射特性测试。结果表明:以异丙醇作分散剂制备的CNTs薄膜表面均匀连续,场发射性能较好,开启电场和阈值电场分别为0.188V.μm-1和2.8V.μm-1。     

Measurements of the self-sustained enhancement of field emission by carbon fiber microemitters

Two types of self-sustained enhancement in field emission by carbon fibers are described. In the first, the field is increased until the emission current switches from zero to between 1 and 10 microA. Next the field is reduced, but not so far that the current would drop. Then the current remains for several hours to several days, with transient increases from the 10 microA to between 14 and 22 microA. It is believed that the transients are caused by the activation of new microtips on the fiber surface. These effects were noted when the carbon fiber tip was mounted in a closed glass vacuum bulb pumped by barium getters, and also in a vacuum system using the combination of a molecular drag pump and ion pumps. The second type of enhancement occurs under ultrahigh vacuum conditions, during in situ thermal treatment of the carbon fiber tip while the emission current is about 2.5 microA. A specially built cathode assembly enables heating the tip to approximately 725 degrees C. After continuous heating at 570 degrees C for 20 to 35 h, the current suddenly increases to between 13 and 25 microA. This enhancement is reversible if the emitted current is kept at the newly increased value for at least 30 min. The current-voltage characteristics at several temperatures were recorded and analyzed. Similar field-forming phenomena were previously observed with Molybdenum and ZnO-W tips.     

碳纳米管场致发射特性的研究

详细论述了碳纳米管的场致发射特性，包括开启电场、阈值电场、发射电流密度、发射电流的稳定性及场发射电子能量分布等；阐述了场发射的机制；分析了发射特性与其几何结构、吸附态及尖端缺陷等因素的关系；并简要介绍了碳纳米管场发射特性在平板显示领域中的实际应用。     

The application of carbon nanotubes in high-efficiency low power consumption field-emission luminescent tube

In this report, details are given of our recent experimental study of using carbon nanotubes (CNTs) as cathode of the luminescent tubes. The CNT film is synthesized by thermal chemical vapor deposition. Two kinds of luminescent tubes that have different gaps have been fabricated. The luminescent tube with 0.1mm gap has a low threshold voltage of about 780V and high emission current of 300 microA when the gate voltage is 970V. The DC power consumption of the luminescent tube with a 0.1mm gap is approximately 1.12W. The results show that the CNTs are very good emission sources and suitable for application in the luminescent tube.     

Field electron emission and nanostructural correlations for diamond and related materials

Results are presented on a complex study of field electron emission (FEE) and structural correlations for nanocrystalline diamond and nitride films. It was found that all the samples studied showed similar dependences of the Fowler-Nordheim work function and effective emitting area on the threshold emission field. Besides it was generally observed that FEE occurred at nanosized regions on the boundary of high and low conducting areas, and peaks of the emission intensity were associated with a lowered surface electron potential. Based on the experimental data, the following mechanism of low-FEE from the materials studied can be supposed. Electrons are transferred from the conducting channel into a vacuum through the low-dimensional region where the emission probability is high due to the quantum well effect. A physical model of the electron escape from a quantum well was analyzed. As follows from the estimations, the quantum size effect, being combined with a moderate field enhancement (the field enhancement factor beta=10-100), allows us to explain the observed variation in the energetic parameters for the samples studied. The function of the insulating grains is mainly to support the conducting channels in the sample body. Also, the grains can fulfill an additional function of the heat sink.     

Theoretical considerations on electron optical brightness for thermionic,field and T-F emissions

General theoretical considerations are made on electron optical brightness. The relation between the electron optical brightness and the energy distribution function of electrons emitted from the cathode surface is studied. A general expression, which is valid in various electron emission regions ranging from thermionic to field emission, is derived for the electron optical brightness for a general energy distribution at the emitting cathode. The result is then specialized to the case of the axial brightness, that is, the brightness on the axis in the axial direction. The axial brightness is found to be related to the tangential energy distribution function at zero tangential energy. The axial brightness is numerically evaluated over a wide range of the cathode temperature and field strength as well as work function, including the thermionic and field emission cases. The simplified analytical expressions of the axial brightness both for Schottky and field emission are also given together with the limits of their applicabilities. The useful numerical data for emission parameters necessary to evaluate the axial brightness are given. The theoretical values of the axial brightness are compared with the experimental ones in the case of the field emission gun.     

催化剂颗粒对自持放电型碳纳米管气体传感器电极的影响

根据高电场作用下，气体的自持暗放电曲线不同区分气体的方法灵敏度高，不易产生交叉敏感，但是工作电压高，用催化热解法在硅基上生长出垂直于基底的碳纳米管阵列，利用碳纳米管的尖端效应，使得高电场集中在一个微小的区域内（纳米级），从而使得气体导电所需的工作电压大幅度下降，催化热解法生长出的碳纳米管顶端含有催化剂颗粒，当其紧密排列时，形成屏蔽层，减弱了碳纳米管的表面场强，这里，提出三步纯化法，去除了碳纳米管阵列顶端的催化剂颗粒，极大地增强了碳纳米管的表面场强。     

Field-dependent electron emission patterns from individual SWCNTs simulated with a multi-scale algorithm

The electron distribution of open-ended single-walled carbon nanotubes (CNTs) with chirality indexes (7,0) and (5,5) in field emission conditions was calculated via a multi-scaled algorithm. The field emission images were produced numerically. It was found that the emission patterns change with the applied macroscopic field. Especially, the symmetry of the emission pattern of the (7,0) carbon nanotube is breaking in the lower field but the breaking is less obvious in the higher field. The magnification factor increases with the applied macroscopic field.     

Modeling the electron field emission from carbon nanotube films.

A theoretical framework for the electron field emission from carbon nanotubes (CNTs) is discussed. Using the tunneling theory, the influence of the detailed electron energy dispersion is proven to be of little importance for the electron field emission. By means of numerical computations in a simplified model, the influence of the environment on the local field on a CNT is discussed for an aligned CNT film. In a simple triangular model for the potential energy barrier at the tube end, a tunneling probability was obtained. A statistical model was developed for the structural and functional parameters of aligned CNT films. Practical CNT films of excellent alignment, obtained directly on a tungsten wire by plasma-enhanced chemical vapor deposition, were analyzed by this statistical model. Their distribution in the enhancement factors was thus deduced. An indirect method to get the average electrical parameters of the film using only a limited amount of experimental data was thus established.     

Field enhancement factor and field emission from a hemi-ellipsoidal metallic needle

We present an exact solution for the electrostatic field between a metallic hemi-ellipsoidal needle on a plate (as a cathode) and a flat anode. The basic idea is to replace the cathode by a linearly charged thread in a uniform electric field and to use a set of “image” charges to reproduce the anode. We calculate the field enhancement factor on the needle surface and ponderomotive force acting on the needle. Using the Fowler–Nordheim theory we obtain an exact analytical formula for the total current.     

The effect of amorphous carbon layer on the field emission characteristics of carbon nanotube film

Carbon nanotube (CNT) has excellent field emission characteristics and could play as a good cold cathode in the application of vacuum electronic devices. However, the practical application faces a big obstacle regarding current fluctuation and deterioration of the CNT cathode. In this research, the formation of amorphous carbon (ac) layer between the CNT film and the substrate, and the effect of the existence of this layer on field emission stability of the CNT film are studied. The formation of the ac layer could be controlled by adjustment of growth temperature and hydrocarbon flow rate. The field emission character and current stability of the CNT film without ac layer are better than those of the CNT film with ac layer. The results attribute to the ac layer a thermal disequilibrium state under high current level. Moreover, adhesion capacity of the CNT film without ac layer is also better than that with the ac layer. It is concluded that the ac layer between the CNT film and substrate is a key factor in the stability of field emission characteristics and should be eliminated before applications.     

Multi‐walled carbon nanotubes decorated by platinum catalyst nanoparticles—Examination and microanalysis using scanning and transmission electron microscopies

Carbon nanotubes (CNTs) decorated with platinum (Pt) nanoparticles (NPs) have been characterized using a cold field‐emission scanning electron microscope (SEM) and a high resolution field‐emission transmission electron microscope (TEM). With this particular composite material, the complementary nature of the two instruments was demonstrated. Although the long CNTs were found to be mostly bent and defective in some parts, the nucleation of Pt occurred randomly and uniformly covered the CNTs. The NPs displayed a large variation in size, were sometimes defective with twins and stacking faults, and were found to be faceted with the presence of surface steps. The shape and size of the NPs and the presence of defects may have significant consequences on the activity of the Pt catalyst material. Also, thin layers of platinum oxide were identified on the surface of some NPs.     

真空微电子压力传感器的研制

提出了一种带过载保护功能的真空微电子压力传感器.对传感器的压力敏感膜尺寸、阴阳极间距等结构参数进行了分析计算;针对过载保护的问题,在结构上设计了过载保护环,实现了真空微电子压力传感器的过载自保护功能.采用硅的干、湿法结合的腐蚀、氧化锐化和真空键合等工艺技术,成功地研制出传感器实验样品.对传感器实验样品的参数进行了测试分析,其场致发射阴极锥尖阵列密度达24000个/mm²,起始发射电压为0.5～1V,反向电压≥25V,当正向电压为5V时,单尖发射电流为0.2nA,压力灵敏度为0.1μA/KPa.     

Damages of screen-printed carbon nanotube cold cathode during the field emission process

The field emission properties of the screen-printed carbon nanotube (CNT) composite cathode have close relationship with its microstructure. In this study, carbon nanotube composite cold cathode with ZnO nano-particles as binding material was prepared using screen-printing method. Electric field cycles were used to post-treat the carbon nanotube composite cold cathode. During the process of electric field cycle treatment, obvious heat-induced damages were observed from the cathode. Scanning electron microscope and transmission electron microscope were employed to analyze the morphology and microstructure of the cathode. The possible mechanisms responsible for damages were discussed.     

Some comments on models for field enhancement

To estimate the apex field-enhancement factor gamma(a)associated with a pointed protrusion on a flat planar surface, the simple physical models of a 'floating sphere at emitter-plane potential' and a 'hemisphere on a post' are often discussed. The corresponding mathematical expressions have the form: gamma(a)=m+h/rho, where rho is the sphere or hemisphere radius, h is its 'height above the emitter plane', and m is a constant variously taken as 0, 2 or 3. Recent numerical simulations for the 'hemisphere on a post' model, reported elsewhere by two of us (CJE and GV) and by Kokkaris, Modinos and Xanthakis, have shown that all of these simple formulae significantly overpredict gamma(a) if h/rho is large. This article first reexamines the basis of these simple formulae and confirms that they are less secure than is sometimes thought. The formulae reported elsewhere as fits to the numerical results are then quoted and compared with the simple formulae, and with the known exact analytical result for the 'hemi-ellipsoid on a plane' model. Discrepancies can be rationalised. Some general conclusions are drawn.     

多台矿用潜水电泵并列布置吸入条件的数值模拟分析

采用计算流体力学的方法,对流量分别为275、550、725、1100、1450m3/h的井内2台潜水泵同时工作时的入口外部流场进行了数值模拟,根据速度场的分布以及涡流情况来确定不同流量时两泵工作时的吸入条件。数值模拟结果表明,平行安装与错位安装时的2台泵吸水管过滤网中心线之间的距离、错位安装时吸水管过滤网的高度差要求随着流量的增大而增大,而吸水管滤网下缘与井底的距离、吸水管滤网中心线与矿井内壁的距离随着流量的增大变化较小。     

A series of tufted carbon fiber cathodes designed for different high power microwave sources

We report the fabrication technique of tufted carbon fiber cathodes for different microwave sources. Three carbon fiber cathodes were constructed, including a planar cathode, an annular cathode, and a cylindrical cathode for radial emission. Experimental investigations on these cathodes were performed in a reflex triode virtual cathode oscillator (vircator), a backward wave oscillator (BWO), and a magnetically insulated transmission line oscillator (MILO), respectively. The pulse duration of microwave emission from the reflex triode vircator was lengthened by using the planar carbon fiber cathode. In the BWO with the annular carbon fiber cathode, the uniform electron beam with a kA/cm(2) current density was observed. In addition, carbon fiber has great promise as field emitter for MILOs. These results show that the carbon fiber cathodes can be utilized for electron emission in high power diodes with different structures.     

A source of gas, vapor, metal, and carbon ions based on a low-pressure hollow-cathode discharge

A compact source of gas, vapor, metal, and carbon ions based on a cold-hollow-cathode reflective discharge has been developed, in which a 6-mm-diameter flat target (Cu, Mo, W, C) is installed on the bottom of the cold cathode insulated from it. The density of the ion flow from cathode plasma reaches 100 mA/cm² at an accelerating voltage of up to 10 kV and a discharge current of 0.2-0.5 A. Vapors produced during ion sputtering of the target are ionized in the cathode and anode cavities. A beam containing ions of the plasma-producing gas and vapor is extracted throug h the channel in the reflector cathode. A fraction of the vapor of the sputtered target, the flow of which is sufficient for growing layers at a rate of ∼0.03 nm/s at a distance of 10 cm from the emission channel under the action of an ion beam, is extracted together with ions. The fraction of metal ions in the extracted beam is 0.05-0.10. The total current of the ion beam is 20-30 mA.