Lifetime of the thermoemission cathodes of nitrogen plasma generators

Implementation of plasma technologies widely introduced into modern industrial processes requires plasma generators with a high efficiency, great economy, and long lifetime. The last requirement is caused by the necessity of an increase in the lifetime of the most easily worn plasmatron elements, i.e., the electrodes and, primarily, their cathode units. Thermoemission cathodes made of tungsten, the most refractory metal with the melting point T _m = 3695 K, are widely exploited in high-current (I = 300–1000 A) plasmatrons operating in oxygen-free media (inert gases like nitrogen and hydrogen). By the middle of the 1980s, due to the discovery and application of the phenomenon of the recirculation of electrode material in the cathode zone, erosion of the cooled thermocathode was successfully reduced to G = 10^–10 g/C. The present study performed by means of a plasmatron with a self-adjusting arc length concerns important problems, such as measurement of the emission current, determination of cathode material erosion, and analysis of means of enhancement of the thermoemission cathode lifetime. Through the investigations performed and the resource tests of the above-mentioned plasmatron type with currents of 300–500 A, with nitrogen as the working gas, the requirements for the cathode and the plasmatron operating regimes providing for its low erosion losses, G ≤ 10^–10 g/C, at a cathode surface temperature close to its melting point are determined. The experimentally obtained densities of the electron emission currents exceed, by an order of magnitude, those calculated according to the Richardson–Dushman theory with the Schottky correction and with account for the photoemission on the cathode under the action of the resonance emission generated by the positive column of the arc. In that connection, attention is paid to the mechanism of the anomaly electron emission proposed by S.V. Lebedev and caused by occurrence of the Frenkel defect accompanied by crystal lattice deformation, Fermi energy increase, and the respective decrease in the electron-from-metal work function. The obtained estimates of the Frenkel defect concentration in tungsten at the premelting temperatures are a cogent argument in favor of the anomaly electron emission concept.     

Metallurgical characterization of arc-conditioned carbon coating on copper electrodes

During arc discharge between two parallel electrodes, the electrode surface is eroded by the arc. Carbon coating of the electrode surface often results in a decreased erosion. The surface structure of the arc-conditioned carbon-coated copper electrode was metallurgically characterized by the use of (1) scanning electron microscopy and (2) Auger in-depth profiling. Two different types of a.c. conditioning with varying pulse characteristics were studied.It was found that the peak current density, the pulse duration and the total number of pulses were the most important parameters in determining the metallurgical structure of the conditioned electrode surface. The vapor arcs which were produced by pulses with a high current density and short pulse duration were most effective in compacting the carbon coating and promoting good adhesion between the carbon and the copper electrode. The compaction and the adhesion are effected by a high ionic beam pressure on the cathode reaching up to 1000 atm.A lower degree of carbon penetration was effected when surface melting was produced by thermionic arcs which have a lower current density and longer pulse duration. In this case, carbon adhesion is due to the mechanical entrapment of carbon powder in the thin molten copper layer produced on the electrode surface by the arcs.     

Step Model of Erosion of Electrodes. I. Application to Arc Spots on the Cathodes of Electric‐Arc Heaters

A simple model of erosion of cold electrodes which considers the step motion of arc spots is proposed. It is assumed that the macroscopic destruction of an electrode begins with fusion of the electrode in the spot. The basic thermal characteristics of the arc spot, the thermophysical properties of the electrode material, and the regime of the electric arc are taken into account. The notion of the effective erosion enthalpy which summarizes all the elementary processes leading to the erosion of the electrode is used. The model has successfully been applied to arc spots on the cathodes of electricarc heaters. A comparison with the earlier model of continuous motion of the spot has been made.     

Erosion of a Copper Cathode in a Nonstationary Arc Spot. I. Experimental Investigation

A study has been made of the dependence of the erosion of a copper cathode of an electricarc heater on the basic operating parameters: the arc current, the velocity of movement of an arc spot, and the temperature of the electrode surface. The experiments were conducted in a coaxial electricarc heater with a magnetic movement of the arc in an air medium for an arc current of 95–480 A and a magnetic field of 0.005–0.417 T. The duration of the experiment, the axial velocity of a plasmaforming gas, and the flow rate of a cooling water were held constant. It has been shown that there are two different erosion regimes: the microerosion regime characterized by a weak dependence on the current and the macroerosion regime with a strong dependence on the current; the transition from one regime to the other is realized upon the attainment of the critical value of the current, dependent on the magnetic field and the thermal regime of the electrode. The existence of a velocity interval in which the specific erosion is minimum has been shown, which confirms the predictions of the earlier thermal model of erosion of cold electrodes.     

Cathodic arc plasma deposited TiAlSiN thin films using an Al-15 at.% Si cathode

Thin films of TiAlSiN were deposited on SKD 11 tool steel substrates using two cathodes, of Ti and Al-15 at.% Si, in a cathodic arc plasma deposition system. The influence of AlSi cathode arc current and substrate bias voltage on the mechanical and structural properties of the films was investigated. The TiAlSiN films had a multilayered structure in which nanocrystalline cubic TiN layers alternated with nanocrystalline hexagonal AlSiN layers. The hardness of the films decreased with the increase of the AlSi cathode arc current. The hardness of the films also decreased as the bias voltage was raised from − 50 V to − 200 V. The maximum hardness of 43 GPa was observed at the films deposited at the pressure 0.4 Pa, Ti cathode arc current 55 A, Al cathode arc current 35 A, temperature 250 °C and bias voltage of − 50 V.     

Effect of high content nano-thoria addition on the properties of tungsten electrode

By hot swaging, the nano-composite W-4.5%ThO₂ cathode was fabricated. A comparative investigation has been made on the operation characteristics for a conventional W-2%ThO₂ cathode, a conventional W-4%ThO₂ cathode and a nano-composite W-4.5%ThO₂ cathode. The results showed that the arc starting and arc voltage-current characteristic of the nano-composite W-4.5%ThO₂ electrode was the best among the three cathodes. These operating characteristics depended on the content and the size of thoria. The anti-erosion ability of the nano-composite W-4.5%ThO₂ cathode was slightly better than that of conventional W-4%ThO₂ cathode, but was obviously improved as compared to W-2%ThO₂ cathode, indicating that the content of thoria governs the resistance to arc erosion of the electrodes.     

Influence of the tip effect of a carbon nanostructure on low current electrical arc initiation

During their setting off, circuit breakers and vacuum switch devices are accompanied by an electric arc whose physical and chemical properties are governed by emissive current sites at the cathode surface called cathode spots. Assuming the continuity of the current on the cathode surfaces, the cathode spots contribute to supply the electrical arc by electron emission and material ejection. Thus they cause erosion on the contact electrodes inducing a dysfunction of contactors. The apparition of these cathode spots is due essentially to dust particles and surface irregularities at different scales. This experimental work represents a contribution to the understanding of the electrical arc behaviour by an investigation at submicronic scale on the influences of nanostructured carbon tips on the electric arc initiation and the cathode spots birth.     

Dependences of the Thermophysical Parameters of Activated Thermal-Emission Cathodes on the Current in a Stable Arc Discharge

The influence of electrical current (in the range 20–150 A) on the thermophysical characteristics of activated thermal cathodes (W + 2% ThO₂) in a freely glowing low-pressure argon arc is investigated. For the first time, the dependences of key cathode parameters, such as the heat flux at the electrode, the voltage equivalent of the heat, the specific erosion, and the distributions of the temperature and work function over the extension length of the cathode from the holder, are obtained. A nonmonotonic dependence of the specific erosion on the electrical current with a minimal level at 50–60 A is noted.     

Laser Cladding of Layered Ti/Nb/Cu Cathode with Homogeneous Arc Ablation Behaviors

Arc ablation threatens the cathode operating time and restricts the development of high-power arc heaters. Surface modification is an effective strategy in improving cathode ablation resistance without reducing matrix conductivities. Herein, Nb layer and Ti layer are laser clad on Cu matrix to decrease the arc ablation of Cu cathode. The total thickness of laser-clad Nb/Ti layer reaches 1850 μm. The Nb layer restrains Cu from diluting into surface cladding and no detrimental Ti–Cu intermetallic is formed. The surface Ti content is as high as 98.34 at%, guaranteeing the arc discharge homogeneity. The arc ablation behaviors of Ti/Nb/Cu cathodes are investigated in air atmosphere. The layered cathode discharges and ablates homogeneously. The arc discharge center is shallow with no appearance of deep pits or craters. The maximum ablation depth (72.1 μm) after 30 s discharging is ≈33.4% lower than that of Cu cathode. Besides, the cathode ablation rate, 1.61 μg C⁻¹, is ≈27.5% lower than Cu cathode. The improved arc ablation resistance is interpreted in the protective effect of refractory TiO₂ layer formed during air arc discharging.     

The application of the glow discharge to material processing

The mechanism and characteristics of the glow discharge are outlined. Energetic electrons and ions, which are generated in the cathode fall region, may be focused or guided to form a beam by geometrical arrangements of the discharge electrodes and by magnetic fields. Two practical arrangements are described: a spherical hollow cathode device, which produces a hot zone at its centre; and a hollow anode device, which produces wellcollimated electron and ion beams. Glow discharge beam devices work at rough vacuum pressures, in the range 10^–2 to 1 mm Hg, and are particularly suited to the processing of glass and oxide ceramics since, owing to the presence of plasma, no electrical charging of target insulating materials takes place. Limitations are set on the operating conditions of glow discharges by the heat input to the electrodes, by erosion due to sputtering, and by the glow-to-arc transition. Proper engineering, however, should allow cathode current densities of the order of 1 A/cm² when operated at an anode voltage of the order of 10 kV, and focusing should realise electron beam power densities of up to the order of 1 MW/cm², with efficiency of about 50%. Thus, the glow discharge may have a wide range of applications to material processing, from etching by sputtering at low power to processing by the most intense heating. New methods of forming and fabricating oxide bodies based on condensation of vapour and on powder deposition are discussed.     

Experimental study of the transition to high-current regime of discharge with a hollow self-heated titanium cathode in nitrogen

It is established that rapid formation of a nitride layer on the surface of a self-heated hollow titanium cathode in arc discharge in nitrogen leads to an increase in the thermal stability and emissive properties of the cathode. Optimum regimes of cathode training that ensures rapid Ti → TiN transition over the entire cathode wall thickness in the active zone are determined, which allows the operation temperature to be increased above 2200 K at a discharge current of up to 50 A for the cathode with an inner diameter of 8 mm and a wall thickness of 1 mm. Results of measurements of the current-voltage characteristics of discharge, thermal regimes of operation, and chemical composition of the cathode are presented. The rates of erosion of the cathode operating in nitrogen and argon have been determined.     

4.4 Co-sputtered optical films

We have investigated the co-sputtering of two dielectric materials with indices of refraction as widely different as possible with the aim of obtaining both homogeneous films with any intermediate index of refraction and inhomogeneous films with predetermined profiles. An rf sputtering module will be described which has been especially designed, with two separate cathodes and two independently tunable rf generators. The substrates are placed on a circular anode rotating underneath the two cathodes. So far mainly CeO₂, TiO₂ and SiO₂ targets have been used. The deposition rate from each cathode and the total film thickness are determined by means of two quartz thickness monitors, sputtering compatible. Values obtained for the refractive index and optical thickness will be reported, as well as repeatability, mechanical and chemical characteristics, reliability and high power optical radiation resistance. Finally, results obtained on optical components of practical interest will be discussed.     

Effect of deposition conditions on the characteristics of ZnO-SnO2 thin films deposited by filtered vacuum arc

ZnO-SnO₂ thin films were deposited on microscope glass substrates by filtered vacuum arc deposition system. The effects of deposition conditions on film characteristics were studied using cathodes prepared with three different ratios of atomic concentrations of Zn to Sn. The micro and the macro properties of the films were investigated as a function of cathode composition, arc current, background oxygen deposition pressure, and deposition time. X-ray diffraction analysis indicated that deposited films were amorphous, independent of the cathode composition. The atomic concentration ratio of Zn to Sn in the film as determined by XPS analysis were 33.9%: 10.6%, 43.9%: 3.8%, 44.7%: 4.7% for 50%: 50%, 70%: 30% and 90%: 10% Zn-Sn alloy cathodes, respectively. Film transmission in the visible was 70 to 90%, affected by interference effects. The maximal and minimal values of the refractive index n and the absorption coefficient k in the visible were 2.11 to 1.94 and 0.07 to 0.001, respectively. The optical band gap was in the range of 3.13 to 3.59 eV. All films were highly resistive independent of deposition conditions used.     

Cathode spots on the surface of nanostructured W alloys

The tracks of cathode spots produced after arcing on the surfaces of nanostructured W alloys were investigated using a scanning electron microscope (SEM). For nanostructured W–2%ThO₂ cathodes, the distribution of cathode spots was dispersed and their geometrical properties were completely controlled by the shape and size of ThO₂. For nanostructured W–20%Cu cathodes, the spots were also separated on Cu-rich phase. The results showed that cathode spots on the surface of nanostructured W alloys were quite different from those on the surface of commercial W alloys with coarse grain size, and the microstructure of the cathode materials had significant influence on the cathode spots propagation. Also, space-charge regions induced by internal electric field at the interface which responded to the effect of microstructure on cathode spots were suggested.     

Thin film deposition using a plasma source with a hot refractory anode vacuum arc

Vacuum arc generated plasma was used to deposit metallic Al, Zn, and Sn coatings on glass substrates. An arc mode with a refractory anode and an expendable cathode (the “hot refractory anode vacuum arc”), overcomes macroparticle (MP) contamination experienced in other arc modes. I = 100–225 A arcs were sustained between a water-cooled coating source cathode and an anode, which was heated by the arc, separated from each other by a 10-mm gap, for times up to 150 s. The distance from the arc axis to the substrate (L) was 80–165 mm. Film thickness was measured with a profilometer. It was found that the deposition rate increased with time to a peak, and then decreased to a steady-state value. The peak occurred earlier when using short anode (9 mm long), e.g., with the Al cathode, L = 110 mm, and I = 200 A, the peak was at t _p = 15 s after arc ignition while with the long anode t _p = 45 s. t _p decreased with I, from 45 s with I = 100 A, to 10 s with I = 225 A with the short anode. The peak is believed to appear due to initial condensation of cathode material (including MPs) on the cold anode, and its subsequent evaporation as the anode heated. In the later HRAVA steady state, a balance between condensation and evaporation on the anode is established. The deposition rate peak was significant with low melting temperature Al and Zn cathodes, which produce many MPs, and negligible with Cu and Ti cathodes.     

不同TiB2颗粒粒径的TiB2/Cu复合材料耐电弧侵蚀行为

采用放电等离子烧结法（SPS）制备了不同TiB₂颗粒粒径的3wt% TiB₂/Cu复合材料,研究了3wt% TiB₂/Cu复合材料致密度、导电率、硬度和耐电弧侵蚀性能随TiB₂颗粒粒径的变化规律,重点分析了不同TiB₂颗粒粒径的3wt% TiB₂/Cu复合材料耐电弧侵蚀行为。结果表明:3wt% TiB₂/Cu复合材料致密度和硬度随TiB₂颗粒粒径的增大而略有降低;TiB₂颗粒粒径越小,TiB₂/Cu复合材料的综合性能越好。随着TiB₂颗粒粒径的增大,3wt% TiB₂/Cu复合材料耐蚀稳定性降低,3wt% TiB₂/Cu阴极材料的损耗量明显增加;当TiB₂颗粒粒径为10 μm时,3wt% TiB₂/Cu复合材料的耐电弧侵蚀性能最佳。电弧蚀形貌观察表明:不同TiB₂颗粒粒径的3wt% TiB₂/Cu复合材料经电弧侵蚀后,3wt% TiB₂/Cu复合材料均由阴极向阳极发生转移;随着TiB₂颗粒粒径的增大,阴极质量损耗逐渐增加,触头表面电弧侵蚀面积增加;而在Cu基体中引入较小的TiB₂颗粒,有利于减弱电接触实验过程中TiB₂/Cu复合材料的喷溅现象。      

Numerical Investigation of the Effect of the Cathode Geometry on the Characteristics of an Electric Arc

The effect of the cathode geometry on the characteristics of an electric arc is treated. It is found that the characteristics of plasma in discharges with cathodes of different geometry (cone, sphere, cylinder) are almost the same on condition of equality of the areas of current-conducting surfaces. It is assumed that a ring-shaped arc discharge may be realized in the case of axisymmetric arc spot on the external side surface of a cylindrical cathode.__________Translated from Teplofizika Vysokikh Temperatur, Vol. 43, No. 3, 2005, pp. 359–366.Original Russian Text Copyright © 2005 by R. M. Urusov and T. E. Urusova.     

Substrate temperature effects on the tin oxide films prepared by spray pyrolysis

Indium-doped tin oxide films were prepared by the spray pyrolysis technique at different substrate temperatures ranging from 400–525 C. Texture coefficients for (200) and (112) reflections of tetragonal SnO₂ were calculated. The surface morphology of the prepared films was revealed by using scanning electron microscopy. A dendrite structure was observed in the films deposited at a substrate temperature of 525 C. The obtained specific resistances were correlated with those obtained from X-ray diffraction analysis and scanning electron microscopy. A study of the effect of film thickness on the plane of preferred orientation was carried out.     

Effect of composition on the polarization and ohmic resistances of LSM/YSZ composite cathodes in solid oxide fuel cell

\(\hbox {La}_{0.8}\hbox {Sr}_{0.2}\hbox {MnO}_{3-\delta }\) (LSM)/8 mol% yttria-stabilized \(\hbox {ZrO}_{2}\) (YSZ) (LSM/YSZ) composite cathodes with varying composition are studied for both polarization and ohmic resistance by electrochemical impedance spectroscopy. It was found that total resistance and polarization resistance are lowest for the composite with 60 wt% of LSM (LSM60/YSZ40). However, the ohmic resistance was highest for the same composition and amounted to 60% of the total resistance value. Compositional dependence of resistances has been explained based on the variations of the triple phase boundaries and width of the \(\hbox {O}^{2-}\) ion migration path with the composition of the electrode. Based on the observed area specific ohmic resistance values for the composite cathodes, it is proposed to verify the advantages of LSM/YSZ over LSM cathode in anode-supported solid oxide fuel cell with thin electrolyte.