Hollow cathode and hybrid plasma processing

Generation and features of the radio frequency (rf) hollow cathode discharge (HCD) and its transition into the hollow cathode arc (HCA) are described. Rf linear hollow cathodes for generation of plasma over large areas and suitable for further scale-up are presented. Examples of surface processing and coating by PVD, both by HCD and HCA, are given. The hybrid reactor, combining hollow cathode and microwave plasmas, integrates features of both and provides more options to control plasma characteristics and consequently properties of deposited films. The rf hollow cathodes can be operated in both, PVD and PE CVD regimes, depending on process parameters. These regimes can even be combined within one process. New concepts of fused hollow cathode (FHC), microwave antenna (MWA) and Hybrid hollow electrode activated discharge (H-HEAD) cold atmospheric plasma sources are introduced. The FHC with its modular concept can be used for gas conversion, cleaning and for surface treatment of temperature-sensitive materials at ambient atmosphere. The H-HEAD cold atmospheric plasma source, capable of generating plasma plumes more than 15 cm long, enables treatment of 3-d and complex geometry objects even at low gas flows.     

Measurement of total energy flux density at a substrate during TiOx thin film deposition by using a plasma jet system

The total energy flux density delivered to an electrically isolated substrate in a low-pressure pulsed DC hollow cathode plasma jet sputtering system during TiO₂ thin film deposition has been quantified. The plasma source was operated in constant average current mode and in a mixture of argon and oxygen or only in pure argon working gas. A titanium nozzle served as the hollow cathode. The total energy flux density measurements were made using a planar calorimeter probe. The main results from the calorimeter probe showed clearly that the total energy flux density at the electrically isolated substrate decreases significantly with duty cycle from 100% (DC mode) to 10% at a given pulsing frequency 2.5 kHz. A local maximum at duty cycle 60% for only pure argon operation has been observed. In addition, the voltage waveforms on the hollow cathode and before the ballast resistor have been saved for pulsed DC measurements for both pure argon and argon + oxygen mixture. A similar transient phenomenon on the cathode voltage and discharge current as observed recently in mid-frequency pulsed DC magnetron discharge has been discovered in the hollow cathode plasma jet sputtering system. We can conclude from these preliminary measurements that the main asset of the pulsed DC hollow cathode plasma jet discharge as distinct from the DC driving of the same plasma system lies in the possibility to reduce or to increase energy influx on the floating substrate within the change of duty cycle.     

Anwendung der Hohlkatodenplasmaquelle in der Vakuumtechnik

Various effective and simple constructed plasma sources can be developed by using the hollow cathode effect. The improve of the plasma efficiency is caused by the penetration of two or more glow regions of different parts of the cathode. The equatation p × d ∽ 1 mbar cm is a rule of thumb for the appearance of the hollow cathode effect. The hollow cathode arc discharge is caused by an additional thermal electron emission besides the effectes of the hollow cathode glow discharge. All this effects are the reason for the higher degree of ionization for a hollow cathode source in comparison with the most other plasma sources. Application of the hollow cathode glow discharge for surface finish are technics like plasmacleaning, electron beam ablation or gas flow sputtering. Latters are new technics which allow the development of new sources for the coating technology. The application of the hollow cathode are discharge is possible by using the thermal energy generated by the electron beam (electron beam heating, electron beam melting, and electron beam evaporation) and by using the different possibilities of coating technologies whith the hollow cathode arc source (plasma enhanced evaporation, plasma enhanced reactive PVD and plasma CVD).     

Effect of the electrode material on the atmospheric plasma conversion of NO in air mixtures

Non-thermal atmospheric pressure plasma is widely used for conversion of hazardous gases. Results from different laboratories confirm importance of energy non-equilibrium in the plasma where dominant energy carriers are electrons and a dominant chemistry is based on formation and interactions of radicals. Because of rather high electric fields required for generation and sustaining of air discharges at atmospheric pressure many plasma systems were found rather to create a lot of NO instead of removing it. A widely supported way to clean NO and NO₂ from air mixtures is a plasma assisted catalytic reduction where the cold plasma is combined with the solid-state catalyst. In an ideal case the plasma acts as an oxidation catalyst where an atomic oxygen from air oxidizes NO to NO₂ and the solid-state catalysts are then capable to convert all NO₂ to N₂ and O₂. In most cases it is also necessary to involve auxiliary gases, e.g., propylene, to make the process efficient enough. This work introduces an original cold plasma system based on atmospheric hollow cathodes generated by a nanopulse DC power with controllable voltage and pulse frequency. The system was optimized in both the geometry and the applied power. However, the material of electrodes was found to be the most important factor affecting the plasma performance and consequently the chemical kinetics. A 100% conversion of NO to NO₂ was achieved with a graphite electrode, without using any auxiliary gas and without catalyst. Plasma performance and conversion efficiency are compared for several electrode materials.     

Effect of the electrode material on the atmospheric plasma conversion of NO in air mixtures

Non-thermal atmospheric pressure plasma is widely used for conversion of hazardous gases. Results from different laboratories confirm importance of energy non-equilibrium in the plasma where dominant energy carriers are electrons and a dominant chemistry is based on formation and interactions of radicals. Because of rather high electric fields required for generation and sustaining of air discharges at atmospheric pressure many plasma systems were found rather to create a lot of NO instead of removing it. A widely supported way to clean NO and NO₂ from air mixtures is a plasma assisted catalytic reduction where the cold plasma is combined with the solid-state catalyst. In an ideal case the plasma acts as an oxidation catalyst where an atomic oxygen from air oxidizes NO to NO₂ and the solid-state catalysts are then capable to convert all NO₂ to N₂ and O₂. In most cases it is also necessary to involve auxiliary gases, e.g., propylene, to make the process efficient enough. This work introduces an original cold plasma system based on atmospheric hollow cathodes generated by a nanopulse DC power with controllable voltage and pulse frequency. The system was optimized in both the geometry and the applied power. However, the material of electrodes was found to be the most important factor affecting the plasma performance and consequently the chemical kinetics. A 100% conversion of NO to NO₂ was achieved with a graphite electrode, without using any auxiliary gas and without catalyst. Plasma performance and conversion efficiency are compared for several electrode materials.     

Products on electrodes in an argon-methane magnetically rotating arc at atmospheric pressure

The arc discharge method in a hydrocarbon atmosphere has a promising application in the nanocarbon synthesis field due to its high yield. In this paper, products on electrodes in an argon-methane magnetically rotating arc at atmospheric pressure are investigated. Different nanocarbons are obtained on the cathode and anode surfaces, respectively. Material analysis indicates that the products on the cathode are mainly carbon tubes. Some carbon tubes are well-aligned in one direction, and the others have a coil-like structure. The products on the anode are a mixture of graphene nano-flakes and aggregate hollow carbon particles. The hollow carbon particles exhibit a chain-like structure with internal space to be connected. To the authors’ knowledge, this is possibly the first time that aggregate hollow carbon particles with a chain-like structure have been observed in the absence of any catalyst. Based on the configuration of arc plasma, the formation mechanism of electrode products is briefly proposed.     

空心阴极等离子体放电结构的优化设计

 低温等离子体表面处理可以有效克服液相处理法存在的环境污染、耗能大和成本高的缺点,对材料表面进行清洗、活化和接枝处理,而设计一个合理的低温等离子体放电结构能够较好地改进表面处理的质量．通过建立CRFHCP空心阴极等离子体放电的数学模型,分析影响低温等离子体放电的关键因素,并设计了不同电极配置方式、样品位置和不同远区空心阴极结构的众多方案．通过接触角测定、表面能计算、SEM照片等方式,对不同方案下处理前后的疏水性PP薄膜形态结构进行表征分析．实验结果发现：远区径向喷嘴式空心阴极等离子体放电结构处理的薄膜表面比其他处理方式具有较小的接触角和较大的表面能．这表明采用优化设计的CRFHCP空心阴极等离子体放电结构,可以较其他结构更为有效地改善材料表面的亲水性能．     

Plasma cathode for a broad-beam electron accelerator

We propose a grid-stabilized plasma cathode based on a slit-contracted low-pressure glow discharge with hollow anode. The area of the plasma cathode is one order of magnitude higher than that in systems where electrons are extracted immediately from plasma in the cathode cavity. Conditions for the discharge initiation, the current switching to the hollow anode, and the obtaining of uniform emission from the plasma cathode are determined. At an accelerating voltage of 160 kV, an electron beam with a 1000 × 180 mm cross section, a total current of several amperes, and a current pulse duration of up to 10⁻³ s was obtained. The plasma cathode operates under technical vacuum conditions (air, 0.1 Pa) and ensures high stability and reproducibility of the beam current pulses.     

Abriebschutzbeschichtung von Kunststoffen durch plasmaaktivierte Hochratebedampfung. Abrasion Resistant Coatings on Plastics by Plasma Activated High Rate Deposition

Transparent plastics, e.g. PMMA or Polycarbonate, have low weight, optical clarity and processing benefits of a thermoplastic. A great disadvantage of such plastics is the lack of abrasion resistance. Therefore the plastics have to be protected for practical application. SiO₂ layers perform very well as clear abrasion resistant layers. The typical layer thickness ranges from 1 μm to 6 μm depending on the required abrasion resistance. To get an abrasion resistance comparable to float glass a layer thickness of about 6 μm for SiO₂ layers is needed. The layers are deposited by the HAD‐process (hollow cathode activated deposition). There the coating material is evaporated by an electron beam and is deposited on the substrate by adding reactive gases (e.g. O₂, N₂O or organic monomers) at the presence of an intensive plasma. A special hollow cathode, adapted to this process, is used as plasma source. The typical deposition rates are 300 nm/s to 600 nm/s for the deposition of SiO₂.     

A Vapor-Air Discharge with a Porous Electrolytic Cathode at Atmospheric Pressure

An experimental investigation is performed for the first time of the structures and electrical and thermal characteristics of a vapor-air discharge between a porous (solid cylindrical body and hollow cylinder) electrolytic cathode and a solid anode in the range of current from 0.2 to 8 A with an interelectrode spacing of 2 to 200 mm for electrolytes of different compositions and concentrations with vertical and horizontal orientations of the plasma column in space at atmospheric pressure. An electrode of a new type is developed, namely, a porous electrolytic cathode (PEC), which makes it possible to produce cone-shaped, multichannel, and mixed discharges. The moist, boiling, and film modes of PEC operation are revealed. It is found that the heat loss on a PEC depends on the mode of its operation. The minimal heat loss is observed in the moist cathode mode, in which the electrolyte is delivered to the cathode working surface in the form of vapor only. In so doing, an almost complete regeneration occurs of heat delivered to the cathode from discharge plasma. It is found that the characteristics of a vapor-air discharge between a PEC and electrolytic anode depend significantly on the composition and concentration of the PEC electrolyte. The discharge voltage fluctuations and the nonuniform pattern of distribution of electric field intensity are revealed. The results of experimental investigation of a vapor-air discharge with a PEC are generalized in the form of an empirical formula.     

Generation of a homogeneous plasma in a glow discharge with a hollow anode and a wide-aperture hollow cathode

The principles for designing electrode systems for sources of ion beams of large cross section on the basis of a glow discharge are considered, and a system with combined magnetic and electrostatic confinement of the fast electrons in a wide-aperture hollow cathode and the generation of an ion-emitting plasma in the anode cavity is proposed. It is shown that the system investigated generates a plasma with a nearly homogeneous distribution of the ion emission current density at low gas pressures and can be effectively used to obtain ion beams over a broad range of energies. Pis’ma Zh. Tekh. Fiz. 25, 83–88 (June 26, 1999)     

Dual‐Function Electrocatalytic and Macroporous Hollow‐Fiber Cathode for Converting Waste Streams to Valuable Resources Using Microbial Electrochemical Systems

Dual‐function electrocatalytic and macroporous hollow‐fiber cathodes are recently proposed as promising advanced material for maximizing the conversion of waste streams such as wastewater and waste CO₂ to valuable resources (e.g., clean freshwater, energy, value‐added chemicals) in microbial electrochemical systems. The first part of this progress report reviews recent developments in this type of cathode architecture for the simultaneous recovery of clean freshwater and energy from wastewater. Critical insights are provided on suitable materials for fabricating these cathodes, as well as addressing some challenges in the fabrication process with proposed strategies to overcome them. The second and complementary part of the progress report highlights how the unique features of this cathode architecture can solve one of the intrinsic bottlenecks (gas–liquid mass transfer limitation) in the application of microbial electrochemical systems for CO₂ reduction to value‐added products. Strategies to further improve the availability of CO₂ to microbial catalysts on the cathode are proposed. The importance of understanding microbe–cathode interactions, as well as electron transfer mechanisms at the cathode–cell and cell–cell interface to better design dual‐function macroporous hollow‐fiber cathodes, is critically discussed with insights on how the choice of material is important in facilitating direct electron transfer versus mediated electron transfer.     

Vapor-air discharges between electrolytic cathode and metal anode at atmospheric pressure

Experimental investigation is performed of the structures and electrical characteristics of vapor-air discharges between a metal (solid, hollow, pointed) anode and an electrolytic cathode at atmospheric pressure. Singular features are revealed of free vapor-air discharges with an electrolytic cathode and their relative transition. Analysis and generalization of the experimental results enable one to reveal the basic physical processes which define the possible mechanism of maintaining a vapor-air discharge with an electrolytic cathode.Translated from Teplofizika Vysokikh Temperatur, Vol. 43, No. 1, 2005, pp. 005–010. Original Russian Text Copyright © 2005 by A. F. Gaisin and E. E. Son.     

Effect of oxygen concentration and system geometry on the current-voltage relations during reactive sputter deposition of titanium dioxide thin films

Current-voltage relations at different magnetron sputtering systems and gas mixtures were studied during reactive sputter deposition of titanium dioxide thin films. The main goal of this work was to investigate the influence of reactive gas mixture (Ar + O₂) and system geometry on the electrical characteristics of the discharge. The geometries utilized were the conventional magnetron sputtering, hollow cathode magnetron sputtering and triode magnetron sputtering. A change in the system geometry leads to a change in the electric field distribution, which alters the working range of the discharge voltage and magnetron efficiency. It is noticed that the discharge voltage at constant current can be reduced when the geometry is altered from conventional magnetron to hollow cathode magnetron or triode magnetron, at the same time the magnetron efficiency is increased when hollow cathode magnetron or triode magnetron are used instead of conventional magnetron sputtering.     

LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> microspheres: Synthesis,characterization and use as a cathode in lithium ion batteries

Solid and hollow microspheres of LiMn₂O₄ have been synthesized by lithiating MnCO₃ solid microspheres and MnO₂ hollow microspheres, respectively. The LiMn₂O₄ solid microspheres and hollow microspheres had a similar size of about 1.5 ?m, and the shell thickness of the hollow microspheres was only 100 nm. When used as a cathode material in lithium ion batteries, the hollow microspheres exhibited better rate capability than the solid microspheres. However, the tap density of the LiMn₂O₄ solid microspheres (1.0 g/cm³) was about four times that of the hollow microspheres (0.27 g/cm³). The results show that controlling the particle size of LiMn₂O₄ is very important in terms of its practical application as a cathode material, and LiMn₂O₄ with moderate particle size may afford acceptable values of both rate capability and tap density.     

Anomalous microwave emission from a stationary plasma thruster

The results of ground radio engineering tests on the thruster modules of a combined power unit for Yamal-100 spacecraft are presented. A pulsed electromagnetic radiation component was detected, which is explained by nonstationary processes of the electron emission from plasma of a hollow compensator cathode.     

High-current electron sources based on gaseous discharges

We review the operation of a ferroelectric plasma source (FPS) with enhanced plasma density and of high-current hollow cathode (HC) and hollow anode (HA) discharges. Different schemes (arc sources, magnetron and FPS) were used for ignition and sustaining the HC and HA discharges with current amplitude ?4 kA.These discharges are characterized by a positive anode potential with respect to the hollow electrode walls and the plasma density and temperature inside the hollow electrode cavity reach ∼3×10¹⁹m⁻³ and ?12 eV, respectively. It was shown that the incorporation of the FPS in the HA and HC enables one to develop a compact high-current electron source. The characteristics of an electron diode with FPS, HA and HC were studied under an accelerating voltage ?250 kV and ∼400 ns pulse duration. It was shown that these sources allow the generation of an electron beam with a cross-sectional area of 0.01 m² and amplitude up to 1-2 kA.     

电池隔膜表面空心阴极等离子体接枝聚合研究

应用自制的空心阴极等离子体装置,引发丙烯酸在丙纶表面的接枝聚合,研究了等离子体接枝聚合作用机理,分析了等离子体接枝聚合各参数(放电功率、气体流量、丙烯酸蒸气流量、样品位置等)对聚合速率的影响。通过红外光谱、扫描电镜等对丙纶接枝聚合膜表面的化学组成和形态结构等进行了表征分析,证明了亲水基团的引入,改善了丙纶隔膜的亲水性能。     

Modeling of the chemical composition of DC atmospheric pressure air discharge plasma in contact with aqueous solutions of sodium dodecylbenzenesulfonate

The results of calculation of the chemical composition of plasma of the DC atmospheric pressure discharge in air in contact with aqueous solutions of sulfonol (C₁₂H₂₅C₆H₄SO₃Na) in the concentration range of 0–10 g/L at a discharge current of 40 mA are presented. At modeling, the combined solution of Boltzmann equation for the electron gas, equations of vibrational kinetics for ground states of N₂, O₂, H₂O, and NO molecules, equations of chemical kinetics and plasma conductivity equation were used. It was shown that sodium atoms appearing as a result of transfer processes from the liquid cathode affect the chemical composition of the plasma and the characteristics of the electron gas.     

Specific features of wide-aperture hollow cathode discharge in helium

The characteristics of wide-aperture discharge with a hollow cathode in helium have been studied under the conditions of predominating electron generation via photoemission from the cold cathode. In this regime, currents exceeding by an order of magnitude those for the conventional abnormal discharge have been obtained. The current-voltage characteristics are independent of the helium pressures in a range of 5–15 Torr, which is a distinctive feature of the photoemission dominated discharge with electron beam in the system under consideration.