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.     

Novel method of flat cold cathode formation from carbon-nitrogen nanofibers

A novel method of high-efficiency cold cathode formation is developed. The technique is based on the growth of nitrogenated carbon nanofibers in a high-pressure apparatus on a graphite substrate. An average nitrogen concentration up to 13% was achieved. The turn-on and threshold fields for such cathodes are substantially lower than those for cathodes based on other carbon materials. A special method of substrate preparation provides strong adhesion of carbon-nitrogen nanomaterial and its durability during long-term cathode operation. It is shown that due to high uniformity, emission efficiency and time reliability, the field emission cathodes based on carbon-nitrogen nanofibers (CNNs) are very promising for high-brightness flat indicators and displays.     

Stabilization of carbon-fiber cold field-emission cathodes with a dielectric coating

A comprehensive investigation has been carried out to determine the source of an inherent temporal instability in the spatial distribution and the electron emission current obtained from field-emitting carbon fiber tips. These instability effects were successfully overcome by coating the tip with a sub-micron layer of dielectric epoxy resin coating. The influence of the coating thickness was studied and an optimum thickness of 0.2-0.3 microm that produced high emission stability was found. A large reduction in the intensity fluctuations of the emission image, at this coating thickness is demonstrated by using chart recorder traces in addition to slow scans of an optically monitored screen signal.The current-voltage (I-V) characteristics were obtained at a threshold field that is a few times lower than that of the uncoated tip. At low emission current levels linear F-N plots were obtained with a slope value lower than that of the uncoated emitter. The spatial distribution consisted of a very bright spot without any internal structure. The total energy distribution of the emitted electrons demonstrated a non-metallic behavior. The spectra obtained consisted of a single peak for low currents and a double peak for higher currents. The electron energy was measured relative to the Fermi level of tungsten and a spectral shift was shown to be a function of the current. Experiments have shown that the coated tips are not affected by the variations of pressure conditions down to 10(-6) mbar.These results suggest that a resin coated fiber tip offers superior performance to tungsten as a cold field emission electron source. Numerous improvements in the performance are underway. This includes a variety of polymeric coatings and more emissive carbon fibers.     

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.     

On the application of quantum transport theory to electron sources

Electron sources (e.g., field emitter arrays, wide band-gap (WBG) semiconductor materials and coatings, carbon nanotubes, etc.) seek to exploit ballistic transport within the vacuum after emission from microfabricated structures. Regardless of kind, all sources strive to minimize the barrier to electron emission by engineering material properties (work function/electron affinity) or physical geometry (field enhancement) of the cathode. The unique capabilities of cold cathodes, such as instant ON/OFF performance, high brightness, high current density, large transconductance to capacitance ratio, cold emission, small size and/or low voltage operation characteristics, commend their use in several advanced devices when physical size, weight, power consumption, beam current, and pulse repletion frequency are important, e.g., RF power amplifier such as traveling wave tubes (TWTs) for radar and communications, electrodynamic tethers for satellite deboost/reboost, and electric propulsion systems such as Hall thrusters for small satellites. The theoretical program described herein is directed towards models to evaluate emission current from electron sources (in particular, emission from WBG and Spindt-type field emitter) in order to assess their utility, capabilities and performance characteristics. Modeling efforts particularly include: band bending, non-linear and resonant (Poole-Frenkel) potentials, the extension of one-dimensional theory to multi-dimensional structures, and emission site statistics due to variations in geometry and the presence of adsorbates. Two particular methodologies, namely, the modified Airy approach and metal-semiconductor statistical hyperbolic/ellipsoidal model, are described in detail in their present stage of development.     

Grating spectrometer system for beam emission spectroscopy diagnostics using high-energy negative-ion-based neutral beam injection on LHD

A beam emission spectroscopy (BES) system was developed for density gradient and fluctuation diagnostics in the Large Helical Device (LHD). In order to cover the large Doppler shift of the Hα beam emission because of the high-energy negative-ion-based neutral beam atom (acceleration voltage V(acc)=90-170?kV) and the large motional Stark splitting due to the large v×B field (magnetic field B=3.0?T), a grating spectrometer was used instead of a conventional interference filter system. The reciprocal linear dispersion is about 2 nm/mm, which is sufficient to cover the motional Stark effect spectra using an optical fiber with a diameter of 1 mm.     

Field emission,developments and possibilities

The overall performance of any electron microscope is to a great extent determined by the electron source. For example, the current acceptance of the STEM concept is due to the stimulation provided by field emission sources, which made it possible to image single atoms in the STEM. The field emission source remains the electron source with the highest brightness and the lowest energy spread. As a consequence, considerable research and engineering work has been and is still being conducted in a number of important areas. These areas include: (i) the mechanisms which determine the beam current stability; (ii) the electron-electron interactions which lead to beam spreading and an increase of the energy spread; (iii) various types of emitters such as bare tungsten tips, oxygen processed tips, zirconium coated tips, carbon emitters, carbon coated tungsten emitters and even solid state emitters; (iv) the lifetime limiting factors, e.g. inclusions, ion bombardment and flashovers effects; (v) different types of optical systems for field emission guns, e.g. choice of magnetic and/or electrostatic lenses at high voltage potential in UHV systems, and special magnetic lenses just below the anode; (vi) field emission guns operating at higher kV's, up to 1·6 MeV. Because analytical work in the smallest possible volumes is a growing area of scientific interest, the demand for field emission sources which are reliable, easy to operate and stable will continue.     

Studying the effect of adsorbed molecules on the operation of a diode with an explosive-emission cathode

The results of an experimental study of the effect of an adsorbed gas and surface contaminations of an explosive-emission cathode on the operation of a diode during generation of a high-current electron beam of nanosecond duration are presented. The effect of contaminations was revealed from the change in the rate of expansion of the planar-diode cathode plasma for cathodes of different designs manufactured from different materials and different initial anode-cathode gaps. The plasma velocity was calculated from the experimental perveance of the diode with a resolution of 0.2 ns. Experiments were performed on a ТЭУ-500 pulsed electron accelerator (350–450 kV, 100 ns, and 250 J/pulse) in a mode of matching the diode impedance to the output impedance of the nanosecond generator. It has been found that the velocity of cathode plasma is constant for 70–90 ns after applying voltages to different cathodes at different anode-cathode gaps. The velocities were 2.0 ± 0.5 cm/ μs for carbon cathodes (of different diameters), 3 ± 0.5 cm/μs for multispike tungsten cathodes, and 4.0 ± 0.5 cm/μs for copper (solid or multispike) cathodes. An appreciable dependence of the plasma velocity on the cathode material shows an insignificant influence of the adsorbed gas and cathode surface contaminations on the expansion velocity of the explosive-emission plasma in a planar diode during generation of the electron beam (10–15 ns after a voltage is applied).     

Field emission from individual multiwalled carbon nanotubes prepared in an electron microscope

Individual multiwalled carbon nanotube field emitters were prepared in a scanning electron microscope. The angular current density, energy spectra, and the emission stability of the field-emitted electrons were measured. An estimate of the electron source brightness was extracted from the measurements. The results show that carbon nanotubes are promising candidates to replace existing sources in high-resolution electron beam instruments.     

Influence of molecular weight on performance properties of polyethersulphone and its composites with carbon fabric

The effect of molecular weight (MW) of a polymer on the wettability of fibers and its influence on the performance properties need to be addressed in detail. Specialty polymer, viz. polyethersulphone (PES), with varying MW was selected as a matrix material to develop the composites with carbon fabric (CF). Since carbon fiber is inert towards the matrix, cold remote nitrogen–oxygen plasma (CRNOP) treatment was employed to improve its chemical reactivity, by incorporating functional groups to promote the fiber–matrix adhesion. Evaluation of mechanical and sliding wear properties of polymers and composites led to the conclusion that the CRNOP treatment was beneficial to enhance performance properties. The MW and MFI have inverse relation. MW proved to be a controlling parameter for pristine polymers while melt flow index (MFI) was the decisive parameter for the performance of composites. Perforations and increased roughness on the treated carbon fiber, as observed by the field emission scanning electron microscopy (FESEM), were responsible for the improved fiber–matrix adhesion and hence performance properties.     

Enhanced angular current intensity from Schottky emitters

Even though the Schottky emitter is a high‐brightness source of choice for electron beam systems, its angular current intensity is substantially lower than that of thermionic cathodes, rendering the emitter impractical for applications that require high beam current. In this study, two strategies were attempted to enhance its angular intensity, and their experimental results are reported. The first scheme is to employ a higher extraction field for increasing the brightness. However, the tip shape transformation was found to induce undesirably elevated emission from the facet edges at high fields. The second scheme exploits the fact that the angular intensity is proportional to the square of the electron gun focal length [ Fujita, S. & Shimoyama, H. (2005) Theory of cathode trajectory characterization by canonical mapping transformation. J. Electron Microsc. 54 , 331–343], which can be increased by scaling‐up the emitter tip radius. A high angular current intensity (J_Ω∼ 1.5 mA sr⁻¹) was obtained from a scaled‐up emitter. Preliminary performance tests were conducted on an electron probe‐forming column by substituting the new emitter for the original tungsten filament gun. The beam current up to a few microamperes was achieved with submicron spatial resolution.     

Visualization of individual emission sites on flat broad-area field emission cathodes

We introduce a novel scanning projection field emission microscope (SPFEM) designed to study flat broad-area field emission cathodes. The instrument merges capabilities of measuring the electron field emission current from an individual emitting site and genuine projection of electrons onto a luminescent screen. This is achieved by an optimized shape of the anode probe having a 0.04 mm aperture which generates an uniform macroscopic electric field across the investigated area of the cathode. This fact also enables presentation of the relation between the current density and the applied electric field. The magnification of the electron-optical system alone was calculated by computational modeling for some cathode-probe distances and for some voltages. The unique SPFEM performance is demonstrated on smooth sulfur-doped nanodiamond films synthesized on molybdenum substrates.     

基于斯特封的往复密封摩擦磨损界面性能实验研究

斯特封是常用的往复密封件,其中斯特封的PTFE圈性能及活塞杆表面材料在往复密封过程中起着重要作用。搭建往复密封实验台,取4组添加碳纤维PTFE的密封圈分别与镀Cr膜活塞杆和镀DLC膜活塞杆进行往复密封台架实验,实验后获取使用过的4组密封圈作为实验样本,并取1个全新未使用的添加碳纤维PTFE的密封圈作为参考样本。通过三维白光干涉表面形貌仪、场发射环境扫描电子显微镜和冷场发射高分辨扫描电子显微镜分别对实验样本的密封唇进行表面形貌、表面磨损和磨损表面元素进行测定。通过实验测定,得出镀膜材料脱落形成磨粒导致密封圈表面磨损。还对密封圈的加工方法和活塞杆镀膜材料的选择提出了建议。     

Revealing the 1 nm/s extensibility of nanoscale amorphous carbon in a scanning electron microscope

In an ultra‐high vacuum scanning electron microscope, the edged branches of amorphous carbon film (～10 nm thickness) can be continuously extended with an eye‐identifying speed (on the order of ～1 nm/s) under electron beam. Such unusual mobility of amorphous carbon may be associated with deformation promoted by the electric field, which resulted from an inner secondary electron potential difference from the main trunk of carbon film to the tip end of branches under electron beam. This result demonstrates importance of applying electrical effects to modify properties of carbon materials. It may have positive implications to explore some amorphous carbon as electron field emission device. SCANNING 35: 261‐264, 2013. © 2012 Wiley Periodicals, Inc.     

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.     

Application of response surface methodology in the optimization of laser treatment in buckypaper lighting for field emission displays

Carbon nanotube field emission backlight (CNT-BLU) is promising to replace traditional backlighting devices in liquid crystal display (LCD) industry. This study reports a laser irradiation process to enhance field emission properties of buckypaper, a thin sheet of high-loading carbon nanotube network. The scanning laser treated the selected region of buckypaper to activate CNT emitters. The improvement of phosphorescence luminance intensity, uniformity, and the reduction of turn-on field were achieved by adjusting machining parameters of laser power, laser lens motion speed, laser resolution, laser beam size, and pattern orientation. Design of experiment and response surface methodology provided ways to rapidly search the feasible laser parameter setting for processing buckypaper field emitters and improving field emission properties within fewer experimental runs. 2^5?1 Fractional fracotrial design presented the initial models of five repsponses. In addition, the face-centered central composite design is applied since the 2^5?1 factional factorial design showed curvature significance. It assisted to give the scientifical insight of the following conclusions. High-energy laser treatment damages and burns the CNTs into carbon oxide materials; furthermore, it loses the effective CNTs. Low-energy laser treatment performs CNT activation and produced low field emission performance. In this study, we succeeded to apply statistical analysis methods to understand the physics and mechanics of laser-activated buckypaper field emission and, furthermore, improve, optimize, and demonstrate performance by material selection, process development, and characterization.     

Electron spectrometer in adjustable triode configuration for photo-induced field emission measurements

We have constructed a new ultrahigh vacuum apparatus with a triode configuration for the systematic investigation of photo-induced field emission (PFE) from metallic or semiconducting cathodes. These are exposed to electric fields up to 400 MV∕m and laser irradiation by means of hole or mesh gates. Cathodes and gates are in situ exchangeable and adjustable with high precision to ensure a homogeneous extraction of electrons which are partially transmitted to the fixed electron spectrometer. Its hemispherical sector analyzer provides an energy resolution limit of 8 meV. The commissioning of the measurement system has been performed with a tungsten needle. Its temperature showed up in the high-energy tail of the electron spectrum, while its work function was derived from the spectral low-energy part combined with the integral current-voltage curve. First PFE measurements on B-doped Si-tip arrays yielded a small field emission current increase under green laser illumination. A shift and splitting of the energy spectra was observed which revealed different emission regimes as well as the photosensitivity of the cathode due to carrier excitation into the conduction band. For the full exploitation of the PFE system, a tunable laser over a wide eV-range is required.     

Low background cold cathode ion source for molecular beam detection

An extractor gauge type electron bombardment ion source using carbon fiber bundles as field electron emitters is described. The cold cathode permits operation of the ionizer within a liquid He cooled cryopump. The high pumping speed for all molecules (except helium) together with its low background pressure make this ion source a very promising detector for crossed molecular beam scattering experiments.     

Measurement of the D alpha spectrum produced by fast ions in DIII-D

Fast ions are produced by neutral beam injection and ion cyclotron heating in toroidal magnetic fusion devices. As deuterium fast ions orbit around the device and pass through a neutral beam, some deuterons neutralize and emit D(alpha) light. For a favorable viewing geometry, the emission is Doppler shifted away from other bright interfering signals. In the 2005 campaign, we built a two channel charge-coupled device based diagnostic to measure the fast-ion velocity distribution and spatial profile under a wide variety of operating conditions. Fast-ion data are acquired with a time resolution of approximately 1 ms, spatial resolution of approximately 5 cm, and energy resolution of approximately 10 keV. Background subtraction and fitting techniques eliminate various contaminants in the spectrum. Neutral particle and neutron diagnostics corroborate the D(alpha) measurement. Examples of fast-ion slowing down and pitch angle scattering in quiescent plasma and fast-ion acceleration by high harmonic ion cyclotron heating are presented.     

Free vibration analysis of functionally graded CNT-reinforced nanocomposite beam using Eshelby-Mori-Tanaka approach

This work deals with the effect of agglomeration and distribution of carbon nanotube on the free vibration characteristics of a functionally graded nanocomposite beams reinforced by single-walled carbon nanotubes (SWCNTs) by employing an equivalent fiber based on the Eshelby-Mori-Tanaka approach. Different SWCNTs distributions in the thickness directions are introduced to improve fundamental natural frequency of polymer composite beam. The micromechanics models used in the study include a two parameter model of agglomeration. An embedded carbon nanotube in a polymer matrix and its surrounding inter-phase is replaced with an equivalent fiber for predicting the mechanical properties of the carbon nanotube/polymer composite. The system of equations of motion is derived by using the principle of virtual work under the assumptions of the Euler-Bernoulli beam theory. The finite element method is employed to obtain a numerical approximation of the motion equation. Numerical results are presented in both tabular and graphical forms to figure out the effects of nanotube agglomeration, CNTs distribution and boundary conditions on the dynamic characteristics of the beam. The above mentioned effects play very important role on the dynamic behavior of the beam.