Cathode Erosion Rate Research Articles

Molybdenum erosion in iodine plasma at hollow cathode conditions

In this study, the authors investigate the erosive effects of iodine plasma on molybdenum. By exposing molybdenum samples to three different test cases of iodine vapor, iodine plasma, and argon plasma, the plasma enhancement of erosion was characterized. A roughly exponential behavior with respect to temperature was found in iodine plasma at hollow cathode conditions and erosion rates as high as 70.1 ± 7.7 μm/h. Despite an oxygen content of <1%, evidence of oxidation was found in all test cases. Behavior consistent with preferential oxidation based on grain orientation was found in the iodine test cases.

Cathode erosion in a gas arc discharge at the threshold currents

The results of experimental studies of the erosion rate of high-purity tungsten cathodes after pulsed arc discharges in a pure oil-free ultrahigh vacuum and in gases of various purities are reported. The erosion rate in high-purity argon does not change compared to the rate of erosion of electrodes in a pure oil-free vacuum. The rate of erosion in "technical" argon is 10-20% less than the rate of erosion of electrodes in an oil-free vacuum. The rate of erosion in "technical" nitrogen is 15-35% less than the rate of erosion of electrodes in vacuum. Particle sizes formed in gas arcs are smaller than those formed in vacuum arcs. Keywords: arc in gas, erosion rate, ecton mechanism, particles, electron microscopy

Эрозия катода в газовом дуговом разряде в области пороговых токов

The results of experimental studies of the erosion rate of high-purity tungsten cathodes after pulsed arc discharges in a pure oil-free ultrahigh vacuum and in gases of various purities are reported. The erosion rate in high-purity argon does not change compared to the rate of erosion of electrodes in a pure oil-free vacuum. The rate of erosion in "technical" argon is 10% less than the rate of erosion of electrodes in an oil-free vacuum. The rate of erosion in "technical" nitrogen is 15-35% less than the rate of erosion of electrodes in vacuum. Particle sizes formed in gas arcs are smaller than those formed in vacuum arcs.

Erosion rates of diffuse and constricted magnetron discharges in helium over aluminium, gallium, molybdenum, and tantalum

Magnetically-enhanced discharge plasmas in light monatomic gases such as helium may have applicability in future electric power grid switches. For practical use of a gas discharge switch in power conversion the net cathode erosion rate must be sufficiently low to enable stable operation over an extended life. We have found that magnetically-enhanced He plasmas at pressures of 200–800 mTorr can operate in one of several distinct modes that are characterized by the plasma geometry (diffuse, constricted) and motion (stationary, orbiting, slow). The erosion rates of various cathode materials (aluminium, gallium, molybdenum, and tantalum) were measured in both diffuse and constricted, magnetized He plasmas. The constricted orbiting (CO) mode exhibited a lower erosion rate than either the diffuse stationary or constricted slow modes, even when comparing loss rates against a cathode material that was more liable to sputter (e.g. aluminium versus molybdenum). However, the erosion rate for operation in the CO mode was still found to be significantly higher than required for practical use in a gas discharge switch.

X-ray radiography of aluminum cathodes eroded in high-current vacuum arcs

The aim of the study presented in this paper was to characterize quantitatively the erosion of aluminum cathodes in high-current vacuum arcs. The experimental setup comprised two current generators. The first one, capable of generating a current of amplitude up to 350 kA, was used to produce a plasma jet, that is, the object to be investigated. The second generator was used to produce a source of probe radiation for imaging the object under investigation in soft x rays of energy ħν ≈ 0.5–3 keV. The findings of the study are based on experimental data obtained by electrophysical and radiographic methods. It has been shown that the cathode erosion rate in a high-current vacuum arc is a function of the charge passed through the cathode.

Influence of Ambient Gas Pressure on Cathode Erosion Rate in a Vacuum Arc

A simple formula to calculate the cathode erosion rate due to an arc burning in ambient gas is proposed. The relatively well-known parameters are used. The formula gives the correct order of magnitude of the erosion rate and correctly describes the erosion rate dependence on the ambient pressure.

Comparison and Evaluation of Electrode Erosion Under High-Pulsed Current Discharges in Air and Water Mediums

In order to analyze the electrode erosion rate and erosion mechanism under high-pulsed current discharge in water and air, the erosion characteristics of the tungsten–copper (W–Cu) electrode under high-pulsed current were comparatively studied in the experiments. Based on the weight loss measurement, the cathode erosion rate, the anode erosion rate, and the total erosion rate of the electrodes discharged in water and air are obtained. Then the eroded surfaces after 100 shots are analyzed by electron transfer dissociation and energy dispersive X-ray spectroscopy, and concentrations of the metal ions after discharge in the water are measured and analyzed. Results show that the erosion of W–Cu electrode in water is more serious than that in air. Macromorphology of the eroded electrodes in water indicates that there are obvious arc spots caused by more focused arc and higher current density, leading to more serious erosion. Micromorphology shows that the electrode erosion in water is mainly caused by the evaporation of the metal, while the jet of the molten metal in air is the major erosion form. Apart from physical erosion caused by metal ablation and evaporation, the complex electrochemistry reaction, which is induced by the high-temperature arc in water, is one of the significant causes resulting in more serious erosion in water.

Unique erosion features of hafnium cathode in atmospheric pressure arcs of air, nitrogen and oxygen

Experimental investigation of cathode erosion in atmospheric pressure hafnium-electrode plasma torches is reported under different plasma environments along with the results of numerical simulation. Air, nitrogen and oxygen are the plasma gases considered. Distinct differences in the erosion features in different plasmas are brought out. Cathode images exhibiting a degree of erosion and measured erosion rates are presented in detail as a function of time of arc operation and arc current. Physical erosion rates are determined using high precision balance. The changes in the surface microstructures are investigated through scanning electron microscopy (SEM). Evolution of cathode chemistry is determined using energy dispersive x-ray spectroscopy (EDX). Numerical simulation with proper consideration of the plasma effects is performed for all the plasma gases. The important role of electromagnetic body forces in shaping the flow field and the distribution of pressure in the region is explored. It is shown that the mutual interaction between fluid dynamic and electromagnetic body forces may self-consistently evolve a situation of an extremely low cathode erosion rate.

Electrode erosion and lifetime performance of a triggered corona-stabilized switch in SF6at a repetition rate of 1 kHZ

This paper describes the work undertaken to investigate the electrode erosion and lifetime performance of an electrically triggered, corona stabilized (TCS) switch. The switch, which had a rod-plane geometry, was filled with SF6 at a pressure of 0.8 bar (1.0bar = 100kPa) absolute and was operated at a pulse repetition frequency (PRF) of 1kHz. The erosion rates of anode and cathode electrode materials such as elkonite (a copper tungsten composite), brass, aluminum and two types of stainless steel was measured, and their surfaces were studied optically, photographed and chemically analyzed. The erosion rate of the anode (rod) electrode materials ranged from 21.39 x 10-6 cm-3C-1 for elkonite to 60.4 x 10-6 cm-3C--1 for brass, whilst a cathode (trigger disc) erosion rate as high as 51.23 x 10-6cm-3C--1 for aluminum was measured. The lifetime of the TCS switch was experimentally determined for each of the electrode material tested, and was found to be inversely proportional to the erosion rate of the rod electrode. The electrode surface morphology and the gas-electrode compound distribution have also been evaluated to address the erosion mechanism in effect. Additionally, the influence of rod electrode surface conditions and gas deterioration has been investigated in respect to the performance and operational behavior of the TCS switch.

Physics of Cathode Phenomena in a Vacuum Arc With Respect to a Plasma Thruster Application

The local cathode heating, vaporization, electron emission, cathode sheath, and so on were considered taking into account the kinetics of mass flow at the vaporized cathode surface. A self-consistent model was developed and simulation was conducted for given spot current $I$ to obtain the cathode erosion rate $G$ and jet velocity $V$ determined the force on the cathode. The time-dependent cathode and plasma characteristics were calculated for Cu, Ag, Al, and W cathodes. The primary plasma needed to initiate the spot was taken into account. Various primary plasma parameters were defined by different spot ignition time $\tau$ at which then the spot can be developed. The model shows that the time-dependent range of $G$ agrees well with the measured data and the jet velocity $V$ calculated from the experimental data of cathode force also close to that velocity obtained experimentally in a literature. The trust performance was obtained larger for W cathode than that for Cu and Ag cathodes and this and other results were discussed.

Cu film deposition using a vacuum arc with a black-body electrode assembly

A vacuum arc with black body electrode configuration is used to deposit thin Cu films on glass substrates. In this configuration, a cup shaped anode, a refractory insulator, and a water cooled cathode form an enclosed volume from which dense plasma of cathode material was extracted through different types of apertures (one hole, frontal or radial shower-head) in the anode. All materials emitted from the cathode (including macroparticles) were re-evaporated from the hot anode producing in the enclosed volume a dense plasma which extracted through the anode aperture. The Cu deposition rate with the frontal shower anode was about 3.6μm/min at a distance Ls=80mm from the anode for current 200A. Deposition with the radial shower anode produced a film which was azimuthally equal. The cathode erosion rate with this electrode configuration was a factor of 2–3 lower than that in a conventional cathodic arc indicating higher cathode material utilization.

Cathode erosion of graphite and Cu/C materials in airarcs

Cathode erosion of graphite and Cu/C was studied in direct current arcs, which were ignited between two electrodes comprised of two kinds of carbon materials and a tungsten anode in air. The arced zones on the cathode surface were investigated by a scanning electron microscope. Also, the cathode erosion rates of the investigated materials were measured. The results show that two distinct zone can be seen on both cathodes. The eroded area was located at the zone just opposite to the anode and surrounded by a white zone. The arced surface on the Cu/C containing 9.3 % Cu is rougher than that of the pure graphite. Many particles with various sizes distributed on the Cu/C. The vaporization of Cu can lower the surface temperature and reduce the cathode erosion. Therefore, the cathode erosion rate of the Cu/C is lower than that of the pure graphite.

CHARACTERISTICS OF ARC EROSION FOR Cu-C COMPOSITE MATERIALS IN AIR

Arc erosion of Cu - C composites was investigated in air. The eroded zones on the cathode surface were observed by a scanning electron microscope, and the cathode erosion rates were measured. It can be found that the main mass loss results from ejection of copper and oxidization of carbon during discharges. The degree of erosion increases with the copper content. Meanwhile, the arc erosion rate is larger for the Cu - C materials with higher copper content. A large number of bubble-like particles appear on the electrode surface. The formation of those particles is closely related to the melting and ejection of copper and material transfer between the electrodes.

Modeling of spark erosion rate in microwire-EDM

Micro-wire electro-discharge machining (WEDM) has been identified as a micromachining process for the fabrication of components of size down to the micrometer level. This process is derived from WEDM, and the principle of both the process is similar, yet due to significant scaling down of the micro-WEDM process, lots of modifications in the circuit design, wire diameter, stresses developed, and energy levels are needed. Hence, specific analysis and modeling of micro-WEDM process is required to understand its capability and limitations. Herein, a comprehensive mathematical model has been developed to incorporate plasma features, moving heat source characteristics, multi-spark phenomenon, and wire vibrational effect to predict the cathode erosion rate for a single- and multi-spark in micro-WEDM. The erosion rate evaluated by the model shows dependence on variables like voltage, wire diameter, number of sparks, and wire vibration but is independent of the wire velocity. The experimental validation of the model shows that the trends predicted by the model are logical and match fairly well with the experimental trends.

Effect of Cathode Microstructure on Arc Velocity and Erosion Rate of Cold-Sprayed Copper Cathodes in a Magnetically Rotated Atmospheric Pressure Arc

Atmospheric pressure arc velocity and erosion measurements were performed on cold-sprayed cathodes in a continuously running arc system. Ultrahigh purity (99.99% pure) argon was used as plasma forming gas. An external magnetic field of 0.10 T was used to rotate the arc, which was operated at a constant power of 6 kW (40 V). Cathodes having microstructures with mean grain sizes, ranging from 1.12 to 3.06 μm, were produced using cold spraying (CS) and annealing methods. CS cathodes were tested in their as-sprayed state and annealed state. Annealed CS coatings with near equi-axed grains of 2.29 μm average size gave 60% higher steady-state arc velocities and up to 50% lower erosion rates than massive copper cathodes having 20-23 μm average grain size. An effect of cathode microstructure on arc velocity and on arc erosion rates was observed.

Effect of cathode microstructure on arc velocity and erosion rate of cold cathodes in magnetically rotated atmospheric pressure arcs

Atmospheric pressure arc velocity and arc erosion measurements were performed on thermal sprayed copper cathodes having grain size distributions from submicrometre to a few micrometres in a continuous running arc system. Ultrahigh purity (UHP), 99.99% pure, argon and a mixture of UHP argon with 10% (volumetric) extra dry air were used as plasma forming gases. An external magnetic field of 0.10 T was used to rotate the arc which was operated at a constant power setting of 6 kW (150 A and 40 V). Cathodes having different microstructures were formed using high velocity oxygen fuel (HVOF) spraying and vacuum plasma spraying (VPS) methods with different annealing times. HVOF sprayed cathodes were tested for arc erosion and arc velocity in their as-sprayed state and also after annealing them at 300 and 600 °C for 1 and 8 h durations in an inert argon atmosphere. VPS sprayed coatings were tested as-sprayed. The effect of the initial coating thickness on erosion rates was investigated. Annealing HVOF coatings at 600 °C for 8 h produced near equi-axed grains with a 2–3 µm average size. These coatings showed 60% higher steady state arc velocities and up to 68% lower erosion rates compared with massive copper cathodes having a 20–23 µm average grain size. VPS coatings having a 0.9–1.5 µm average grain size gave up to 70% lower erosion rates when compared with massive copper cathodes. The results show a clear effect of the cathode microstructure on the arc velocity and the erosion rate.

Transient Cathode Spot Operation at a Microprotrusion in a Vacuum Arc

A physical model and system of equations was formulated to describe transient cathode spot evolution at a single protrusion. The model considered the self-consistent processes including protrusion heating, kinetics of cathode vaporization, plasma generation, and current continuity. The time dependencies of spot current, current density, and protrusion size were calculated, taking into account the heat flux from the plasma (surface heat source) and Joule energy (volume source). The plasma density, temperature, degree of vapor ionization, electron current fraction, and cathode erosion rate were calculated as functions of the protrusion size, which reduced in time during the spot operation. It was shown that the spot operation on the protrusion was limited by the disappearance of the protrusion due to cathode erosion. The protrusion disappearance time determined the duration of the transient arc spot at a single protrusion. It was calculated that 1-5 mum protrusions reduce their size significantly due to evaporation in 0.5-10 mus, respectively. The smaller time of protrusion destruction occurs due to it relatively high heating, which was mostly by the heat flux from the surrounding plasma.

Experimental study on copper cathode erosion rate and rotational velocity of magnetically driven arcs in a well-type cathode non-transferred plasma torch operating in air

The cathode erosion rate, arc root velocity and output power of a well-type cathode (WTC), non-transferred plasma torch operating in air are studied experimentally in this paper. An external solenoid to generate a magnetically driven arc and a circular swirler to produce a vortex flow structure are equipped in the studied torch system, which is designed to reduce the erosion rate at the cathode. A least square technique is applied to correlate the system parameters, i.e. current, axial magnetic field and mass flow rate, with the cathode erosion rate, arc root velocity and system power output. In the studied WTC torch system, the cathode erosion has a major thermal erosion component and a minor component due to the ion-bombardment effect. The cathode erosion increases with the increase of current due to the enhancement in both Joule heating and ion bombardment. The axial magnetic field can significantly reduce the cathode erosion by reducing the thermal loading of cathode materials at the arc root and improving the heat transfer to gas near the cathode. But, the rise in the mass flow rate leads to the deterioration of erosion, since the ion-bombardment effect prevails over the convective cooling at the cathode. The most dominant system parameter to influence the arc root velocity is the axial magnetic field, which is mainly contributed to the magnetic force driving the arc. The growth in current has a negative impact on increasing the arc root velocity, because the friction force acting at the spot due to a severe molten condition becomes the dominant component counteracting the magnetic force. The mass flow rate also suppresses the arc root velocity, as a result of which the arc root moves in the direction against that of the swirled working gas. All system parameters such as current, magnetic field and gas flow rate increase with the increase in the torch output power. The experimental evidences suggest that the axial magnetic field is the most important parameter to operate the straight-polarity WTC plasma torch at high output power with a limited cathode erosion rate. This emphasizes the importance of an external magnetic field on a WTC torch system for reducing the erosion at the cathode.

Organic layer-coated metal nanoparticles prepared by a combined arc evaporation/condensation and plasma polymerization process

A process for the synthesis of organic layer-coated metal nanoparticles is presented. Metal nanoparticles are synthesized by the low-pressure pulsed arc evaporation of a metal cathode surface, followed by the in-flight deposition of a thin organic layer by capacitively coupled radio-frequency plasma polymerization from a gaseous hydrocarbon monomer. The stability of the pulsed arc system and the cathode erosion rate are discussed. The effects of operating conditions such as power, reactor pressure and inert gas flow rate on the average size of produced bare copper nanoparticles and the properties of the organic coating are studied. A ‘characteristic map’ for the in-flight plasma polymerization from the C2H6 monomer of the organic layer onto the Cu nanoparticles is developed. The produced copper nanoparticles are characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). The FE-SEM and TEM images reveal coated copper nanoparticles with the diameter of the metal core ranging from a few to 50 nm and the thickness of the organic layer ranging from 3 to 10 nm. The XPS results show that the plasma polymer film is chemically adsorbed onto the surface of the copper nanoparticle.

ジルコニウム陰極を用いたアーク風洞による原子状酸素気流の生成

An arc-heater using a zirconium cathode was developed as an atomic-oxygen generator. With pre-mixed argon-oxygen injection, stable discharge was maintained for more than three hours, and the cathode erosion rate was as low as 1.7x10-5 g/s. The plume temperature and velocity distributions were measured and chemical composition and total specific enthalpy was evaluated by laser absorption spectroscopy. As a result, in the center area of the plume, the degree of dissociation in oxygen was found almost 100% with the total specific enthalpy of 4.5±0.1 MJ/kg.