High-current electron emission characteristics of cathodes based on diamond films

Using different gas source, four types of diamond thin films were prepared on silicon substrate by microwave plasma chemical vapor deposition (MPCVD) technology, and characterized in detail through scanning electron microscopy (SEM), Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. High-current pulsed emission characteristics, tested with a 2 MeV line-inducing injector, showed that all of CVD diamond films had high emission current density (> 70 A/cm²) and [100] textured B-doped microcrystalline diamond film possessed the largest emission current density of 115.1 A/cm². No obvious bright light and luminescent zones from side view CCD images indicated a possible pure field-emission mechanism of these diamond cathodes. Simultaneously, large decrease in the electron emission capability, above 15%, could be observed after several pulsed measurements, but this decrease could be completely recovered through the treatment of surface re-hydrogenation for emitted diamond cathodes, suggesting that emission performance of CVD diamond cathodes was closely relevant to hydrogen coverage ratio. The present data indicated that as-deposited CVD diamond films could be a potential candidate as cold cathode for the application in high-current electron emission field.     

Electron field emission from semiconducting nanowires

In this paper we report on investigations of field emission (FE) properties of semiconducting (SiC, ZnO) one-dimensional (1D) nanostructures – nanowire/nanorod arrays, and fabrication of low-voltage field emission display (FED) devices based on these 1D nanomaterials. SiC nanowires were grown on Ni-coated Si substrates using a thermal metal-organic chemical vapor deposition (MOCVD) technique, and ZnO nanostructures were grown on gold-coated Si substrates by a thermal CVD method. Electron field emission properties of SiC and ZnO nanostructures were examined in plane geometry using a flat phosphor screen. The interrelation between the FE characteristics (emission thresholds, current density, surface uniformity, etc.) and microstructure and surface morphology of the produced 1D nanostructures was established. Diode-type FED devices (flat vacuum lamps) with SiC-nanowire-based cathodes were developed and fabricated. The FEDs are characterized by low threshold and operating electric fields – lower 2 V/μm and 5 V/μm, respectively, high current density and brightness, and stable performance of the nanowire-based cathodes.     

High current field emission from individual non-linear resistor ballasted carbon nanotube cluster array

Field emission (FE) electron sources based on carbon nanotubes (CNTs) have the potential to serve as cold cathodes for various vacuum microelectronic and nanoelectronic devices. Emission currents are extremely sensitive to variation in emitter geometry and local surface states, both of which are difficult to synthesize uniformly when fabricating a CNT field emission array (FEA). Such non-uniformities cause unstable emission, limiting the current output. Here, we propose a method for simulating and fabricating a high performance CNT-FEA with emission units that are individually connected to a single crystalline silicon pillar (SP), which acts as an non-linear ballast resistor. Results showed that the driving field for this CNT-FEA was greatly reduced relative to CNT-FEAs on a flat silicon substrate. This improvement was due to the high aspect ratio of the CNT clusters combined with SPs. The FE behavior demonstrated that the emission current was limited by the non-linear resistors (NLRs). Emitted currents density over 1.65 A/cm² at a low extraction field of 5.8 V/μm were produced by a 1 mm² emmiting area. The proposed technology may be used to fabricate cathodes capable of reliable, uniform, and high current emission.     

Thermionic emission properties and the work function determination of arrays of conical carbon nanotubes

Thermionic emission properties of conical carbon nanotubes (CCNTs) grown on platinum wires and planar graphite foils were investigated. The work function (Φ) values extracted from the thermionic emission data range from 4.1 to 4.7 eV. The range of Φ values is attributed to the morphological characteristics, such as tip radius, aspect ratio, density, and wall structure of CCNTs. The observed lower values for Φ are significantly smaller than that of multi-walled carbon nanotubes (MWNTs). The reduced Φ values are attributed to field penetration effect as a result of the local field enhancement from these structures having high aspect ratio and an excellent field enhancement factor. The high amplification of the external field at the apex of the nanostructures is capable of reducing both the barrier height and the width, in turn contributing to the improved emission current at lower temperatures. The ultraviolet photoemission spectroscopy data of CCNTs grown on Pt wires are in reasonable agreement with the thermionic emission data. The conical carbon nanotubes may be potential candidates for thermionic cathodes with superior performance over conventional cathodes.     

High intensity, pulsed electron beam emission from carbon nanotube cathodes

High intensity electron emission cathodes based on carbon nanotube films have been successfully fabricated by use of a screen printing method. The emission properties of the cathode were investigated in single-pulse and double-pulse modes. The high intensity emission from the cathode is obtained and the highest emission current density reaches 267 A/cm² at an electric field of 15.4 V/μm in double-pulse mode. Emission images of the cathode surface prove that the plasma layer forms on the cathode surface, and the production mechanism of the high-current electron beams is explosive electron emission. This carbon nanotube cathode appears to be suitable for high-power microwave device applications.     

Homogenization of the current density in polymer electrolyte fuel cells by in-plane cathode catalyst gradients

Polymer electrolyte fuel cells (PE fuel cells) working with air at low stoichiometries (<2.0) and standard electrochemical components show a high degree of inhomogeneity in the current density distribution over the active area. An inhomogeneous current density distribution leads to a non-uniform utilization of the active area, which could negatively affect the time of life of the cells. Furthermore, it is also believed to lower cell performance. In this work, the homogenization of the current density, realized by means of tailored cathodes with along-the-air-channel redistributed catalyst loadings, is investigated. The air stoichiometry range for which a homogenization of the current density is achieved depends upon the gradient with which the catalyst is redistributed along the air channel. A gentle increasing catalyst loading profile homogenizes the current density at relatively higher air stoichiometries, while a steeper profile is suited better for lower air stoichiometries. The results show that a homogenization of the current density by means of redistributed catalyst loading has negative effects on cell performance. Model calculations corroborate the experimental findings on homogenization of the current density and deliver an explanation for the decrease in cell performance.     

无铅阴极的相变致电子发射研究

研究了最新无铅阴极钛酸铋钠(Na1 2Bi1.2TiO3，NBT)基铁电陶瓷的电子发射性能和电子发射的机理。采用常规陶瓷工艺制备了发射阴极样品，并在阴极上施加高达15kV的激励脉冲，获得了20A／cm^2的最大发射电流密度。在施加激励电压的同时观察到3个发射电流脉冲峰值，该现象不同于其他的阴极。通过测试NBT基铁电陶瓷的相变性能，修正了得到公认的快极化反转致电子发射理论，提出相变致电子发射理论公式，合理解释了该新型无铅NBT基阴极的电子发射3个峰现象，说明了电子发射不仅可以由快极化反转产生，也可以由相变导致的极化变化产生。     

Enhanced field electron emission properties of hierarchically structured MWCNT-based cold cathodes

Hierarchically structured MWCNT (h-MWCNT)-based cold cathodes were successfully achieved by means of a relatively simple and highly effective approach consisting of the appropriate combination of KOH-based pyramidal texturing of Si (100) substrates and PECVD growth of vertically aligned MWCNTs. By controlling the aspect ratio (AR) of the Si pyramids, we were able to tune the field electron emission (FEE) properties of the h-MWCNT cathodes. Indeed, when the AR is increased from 0 (flat Si) to 0.6, not only the emitted current density was found to increase exponentially, but more importantly its associated threshold field (TF) was reduced from 3.52 V/μm to reach a value as low as 1.95 V/μm. The analysis of the J-E emission curves in the light of the conventional Fowler-Nordheim model revealed the existence of two distinct low-field (LF) and high-field (HF) FEE regimes. In both regimes, the hierarchical structuring was found to increase significantly the associated β_LF and β_HF field enhancement factors of the h-MWCNT cathodes (by a factor of 1.7 and 2.2, respectively). Pyramidal texturing of the cathodes is believed to favor vacuum space charge effects, which could be invoked to account for the significant enhancement of the FEE, particularly in the HF regime where a β_HF as high as 6,980 was obtained for the highest AR value of 0.6.     

Effects of hydrogen flow rate on the growth and field electron emission characteristics of diamond thin films synthesized through graphite etching

Diamond film deposition on silicon was explored using our newly developed graphite etching technology in a microwave plasma reactor. The effects of hydrogen flow rate on the growth rate, morphology and field electron emission properties of the synthesized diamond were investigated systematically. The growth rate and nucleation density of diamond films increased significantly with the decrease of hydrogen flow rate. Nanocrystalline diamond films were obtained with a low hydrogen flow rate (i.e. 3 sccm or lower). Diamond quality is improved and the growth rates are much higher in this graphite etching process, compared to conventional H₂ + 1% CH₄ gas mixture. The results suggest that diamond growth is enhanced by activated hydrocarbon radicals formed through in-situ etching of graphite by atomic hydrogen. The turn-on electric field decreased and the emission current increased with the decrease of the hydrogen flow rate. The enhanced field electron emission property of the diamond films synthesized at lower hydrogen flow rate is attributed to the decreased diamond grain size because the small diamond grains increase the electron conduction channels which facilitate the electron transport inside diamond films.     

Biocompatible contactless electrically responsive hydrogel‐based force maker

We present a biocompatible contactless gel actuator based on poly(methacrylic acid) with magnesium counterions (MgPMA) that, being placed in sodium chloride solution, is able to generate a high value of active mechanical force in a DC electric field. Force generation of the gel goes simultaneously with its swelling due to the osmotic pressure provided by the ionic exchange in the MgPMA polyelectrolyte matrix induced by the electric current. An increase in the charge density of the polymeric network enhances the mechanical stiffness of the gel, which provides negative feedback to force generation. The extent of such influence depends on the networking density of the gel matrix. As a result, soft MgPMA gels with low networking density show higher level of force generation than dense gels with high networking density. It is shown that cylindrical gel samples of ca 8 mm in length and ca 11 mm in diameter immersed in NaCl solution some distance apart from platinum electrodes are able to exert a force of up to 0.4 N in a DC electric field for a constant length of sample with a rate of ca 1.0 mN per second on average.     

中国高品质阴极炭块的工业生产及应用

综述了近几年中国高品质阴极炭块的生产技术及现状,研究了石墨含量对阴极炭块各项理化指标的影响,随着石墨含量的增加,阴极炭块的导电、导热性提高,抗电解质侵蚀性能明显增强,耐压强度有所降低。成型技术对石墨化阴极炭块的各向异性有明显的影响,振动成型生产的石墨化阴极炭块的各向异性不明显。总结了高石墨质阴极炭块、石墨化阴极炭块、硼化钛可湿润阴极炭块在大型预焙铝电解槽上的工业试验结果,高品质阴极炭块使大型槽的生产更平稳,能够提高电流效率、降低电耗和延长电解槽寿命。并初步分析了其技术经济性。     

外加电场对静电流化床中颗粒运动与床层粘壁的调控机制

对流化床反应器中的颗粒运动行为进行调控可达到强化反应器性能的目的。通过冷模实验研究了直流/交流电场对静电流化床中颗粒运动的影响规律与影响机制,建立了通过外加电场调控静电流化床中床层粘壁的方法。结果表明,在低场强条件下,库仑力主导外加直流电场对颗粒运动的影响,由床层壁面指向床层中心的外加直流电场使得颗粒运动强度和轴向颗粒运动分率降低,而由床层中心指向床层壁面的外加直流电场则作用相反;在高场强条件下,极化力主导外加直流电场对颗粒运动的影响,使得颗粒运动强度减弱。在外加交流电场中,无库仑力存在时,极化力仍在高电场强度下使得颗粒运动强度减弱,但当库仑力存在时,电场强度和方向的周期性改变使得颗粒发生周期性摆动,颗粒运动强度增强。在本文的实验条件下,外加交流电场是一种控制床层粘壁的良好方法。2.5 kV/cm、50 Hz的正弦交流电场使得床层粘壁下降76%。研究结果可为聚烯烃流化床反应器的安全运行和过程强化提供指导。     

In situ oxidative degradation of formaldehyde with hydrogen peroxide electrogenerated on the modified graphites

The cathodic reduction of oxygen and the degradation of formaldehyde with hydrogen peroxide electrogenerated in situ was studied at graphite and modified graphite as cathodes. The peak current of the cathodic reduction of oxygen increased significantly when anodically treated graphite and graphite modified with 2,3,7,8,12,13,17,18-octaethyl-21 H,23H-porphyrine cobalt (Co(IIOTP) were used as cathodes, respectively. The current density for the oxidative degradation of formaldehyde was significantly increased when graphite was replaced by anodized graphite as the cathode. Using anodized graphite as cathode, the degradation fractions of the formaldehyde were 99.6 and 98.3% in divided and undivided cells, respectively.     

Macroscopic self-standing SWCNT fibres as efficient electron emitters with very high emission current for robust cold cathodes

A novel of self-standing nanotube-based cold cathode is described. The electron emitter is a single macroscopic fibre spun from neat single wall carbon nanotubes and consists of an ensemble of nanotube bundles held together by van der Waals forces. Field emission measurements carried out using two different types of apparatus demonstrated the long working life of the realised cathode. The system is able to emit at very high current densities, up to 13 A/cm², and shows very low values of both turn on and threshold field, 0.12 V/μm and 0.21 V/μm, respectively. Such easy to handle self-standing electron sources assure good performances and represent an enabling technology for a scalable production of cold cathodes.     

强电流铁电阴极材料的研究进展

铁电阴极材料是一种在脉冲电压或脉冲激光激励下从铁电材料表面获得脉冲强度流电子发射的新型功能材料,本文介绍了铁电阴极的主要特点,并对电子发射机理,材料种类和发射实验进行了讨论,最后还概述了铁电阴极材料的应用前景及实用化过程中尚待研究的问题。     

Field emission properties of diamond and carbon nanotubes

Both diamond and carbon nanotubes are efficient field emitters. Both materials can emit electrons at very low electric fields (3–7 V/μm for a current density of 10 mA/cm²). Moreover, nanotube emitters are capable of delivering very high emission currents densities, with current density routinely exceeding 1 A/cm². Improvements of emission uniformity are critical in realizing the potential of these carbon materials in enabling practically useful cold cathode devices.     

Flash Sintering of Nanocrystalline Zinc Oxide and its Influence on Microstructure and Defect Formation

We report the sintering behavior of nanocrystalline zinc oxide under external AC electric field between 0 and 160 V/cm. In situ acquisition of density by means of laser dilatometry, evaluation of specimen temperature, real‐time measurement of electric field and current help analyze this peculiar behavior. Field strength and blocking electrodes significantly affect densification and microstructure, which was evaluated in the vicinity of the flash event and for the fully sintered material. High current densities flow through the sample at high electric fields, entailing a sudden increment of the temperature estimated to several hundreds of K and an exaggerated grain growth. In contrast, low current density flows through the sample at lower electric fields, which guarantees normal grain growth and highest final density. Macroscopic photoluminescence measurements give insights into the development of the defect structure. Electric fields are expected to enhance defect mobility, explaining the high densification rates observed during the sintering process.     

Study on the characteristics of ferroelectric electron emission

Electron emission from ferroelectric cathodes is commonly suggested as an electron source for different applications due to its special characteristics such as high current density, easy treatment and operation. In this experimental research, the electron emission properties of ferroelectric cathodes made of lanthanum-doped lead zirconate–titanate (PLZT) ceramic are studied. The electron beam spot shape was recorded via proximity-imaging techniques.     

Highly conductive vertically aligned molybdenum nanowalls and their field emission property

ABSTRACT: We report that vertically aligned molybdenum (Mo) nanowalls can grow on various substrates by simple thermal vapor deposition. Individual nanowalls have a typical thickness of about 50 nm and very good conductivity with a typical average value of about 1.97 [MULTIPLICATION SIGN] 104 [OHM SIGN][MINUS SIGN]1 cm[MINUS SIGN]1, i.e., only an order of magnitude less than the value of bulk Mo. The formation process is characterized in detail, and it is found that Mo nanowalls grow from nanorods through nanotrees. The atomic arrangement, lattice mismatch relationship, and competition growth are all believed to contribute to the growth mechanism. The field emission performance is attractive, typically with a very low fluctuation of about approximately 1.18% at a high current density level of 10 mA/cm2, and a sustainably stable very large current density of approximately 57.5 mA/cm2 was recorded. These indicate that the Mo nanowall is a potential candidate as a cold cathode for application in vacuum electron devices, which demand both a high current and high current density.     

Radial AlN nanotips on carbon fibers as flexible electron emitters

A facile catalyst-free approach using a simple thermal transport method has been developed to fabricate high-density AlN nanotips on flexible carbon cloth at large scales for use as field emission (FE) emitters. The AlN nanotips exhibit good performance as flexible cold-cathode electron emitters, with a very low turn-on electric field of 1.1–2.3 V μm⁻¹, a low threshold electric field of 1.5–2.5 V μm⁻¹, and a high emission current density. The excellent field emission properties of the AlN nanotips are attributed to the large field enhancement factor of 6895 as well as the combined effect of the tip profile of the AlN nanostructures and the excellent electron transport path of the conductive carbon cloth substrate.