Cathode Surface Temperature Research Articles

Cathode Surface State and Cathode Temperature Distribution After Current Zero of Different AMF-Contacts

The objective of this paper is to compare the state of cathode surface of different axial magnetic field (AMF) contact systems (CSs) during arcing and after arc extinction. The arcs were ignited by opening of electrodes and were fed by sine half-wave current pulses with a duration of 10 ms. Investigations were performed in the range of arc currents up to the levels of guaranteed failure. The measurements were done by a high-speed Phantom M310 video camera. The cathode surface state during arcing was studied using a set of neutral density filters. It was discovered that melting of a part of the cathode surface during arc burning takes place in high-current vacuum arcs. A changing relief in the form of hillocks and hollows forms on the melted surface. We found out that there are two components in droplet erosion of the cathode, when its surface is partially melted: in addition to droplets formed directly in the cathode spot (which can be called droplets of the first type), the liquid part of the surface also emits droplets (which can be called droplets of the second type) significantly different in their sizes and velocities. The formation mechanisms of droplets of the first and the second type are different. The cathode surface temperature distribution immediately after current zero was obtained by comparing the results of cathode surface filming to the results of calibrated band-lamp filming in identical conditions. Reflection of anode radiation from cathode surface was analyzed by mathematical simulation. The filming was carried out using an interference filter. We found that the distribution of the cathode surface temperature is substantially nonuniform. The axial region is the most heated. We conclude that in all investigated CSs with the same amplitude value of the current, the temperature of the most heated parts of the cathode surface is significantly lower than the temperature of the most heated parts of the anode surface. Analysis of obtained results shows that switching failure can be provoked by the cathode and by the anode processes depending on CS design.

Arc-cathode attachment in GMA welding

A model for arc-cathode attachment in gas metal arc welding is presented. It considers the cathodic heating in the case of a non-refractory iron cathode, with the assumption of the lowering of the local plasma temperature, due to cold metal vapor. It takes the plasma bulk temperature as well as the cathode drop voltage as free parameters and it gives a maximum of heat flux and current density for a cathode surface temperature below boiling temperature. The applicability as a heat source description in a weld pool simulation has been shown, and the temperature field of the cathode was calculated, giving rise to heat flux and current density distributions, which differ significantly from the converntionally used axisymmetric Gaussian distribution. The maximum cathode surface temperature was lower than boiling temperature, which is in agreement with the observeration.

Research on Characteristics of freezing and thawing process controlled by Foundation temperature field

This research introduces the soil buildings’ foundation of geological structure in Liaodong Peninsula and the building foundation type. It shows that there is a cosine changing of the distribution of the temperature field on an annual cycle, and a temperature changing lag exists for the different depth of foundation. Based on the law of frozen depth of foundation, the frozen procedures can be divided into four stages. Based on the observation data, the cathode surface temperature is positively correlated with both the frost heave quantity and the frozen depth. In addition, to reduce the observation cost and the measure difficulty, the frost heave quantity can be obtained through the observation of the frozen depth.

Influence of Gas Diffusion Media Compression on Open-Cathode Fuel Cells

Open-cathode fuel cell power production increases significantly when gas diffusion media (GDM) compressive stress is increased to values greater than those recommended for forced-convection fuel cells by mitigating common shortcomings related to cell thermal and water management. Increasing GDM compression from 20% to 37% reduces cell contact resistances by an order of magnitude. This decreases ohmic resistance by 32% and increases current density by 31% at 0.6 V when GDM compression increases from 20% to 40%. Though the effect of GDM compression on contact and ohmic resistances is significant, higher compression also improves thermal management and cell hydration, which are quantified by decreasing cathode surface temperatures and lower charge transfer resistances. A disadvantage of increasing GDM compression is the onset of a rising mass transport resistance at the highest levels of GDM compression (37–40%). However, this effect is of secondary importance compared to the benefits of higher GDM compression on enhancing cell hydration and reducing ohmic resistance as higher GDM compression results in the greatest power production in all operating conditions tested.

Coupling boundary condition for high-intensity electric arc attached on a non-homogeneous refractory cathode

The boundary coupling high-intensity electric arc and refractory cathode is characterized by three sub-layers: the cathode sheath, the Knudsen layer and the pre-sheath. A self-consistent coupling boundary condition accounting for these three sub-layers is presented; its novel property is to take into account a non-uniform distribution of electron emitters on the surface of the refractory cathode. This non-uniformity is due to cathode non-homogeneity induced by arcing. The computational model is applied to a one-dimensional test case to evaluate the validity of different modeling assumptions. It is also applied coupling a thoriated tungsten cathode with an argon plasma (assumed to be in local thermal equilibrium) to compare the calculation results with uniform and non-uniform distribution of the electron emitters to experimental measurements. The results show that the non-uniformity of the electron emitters’ distribution has a significant effect on the calculated properties. It leads to good agreement with the cathode surface temperature, and with the plasma temperature in the hottest region. Some differences are observed in colder plasma regions, where deviation from local thermal equilibrium is known to occur.

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 Tm = 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.

Real-time Monitoring for Freezing and Thawing Process of Subgrade in Seasonal Frozen Regions

This research shows that there is a cosine changing of the distribution of the temperature field on an annual cycle, and a temperature changing lag exists for the different depth of subgrade. Based on the law of frozen depth of subgrade, the frozen procedures can be divided into four stages. Based on the observation data, the cathode surface temperature is positively correlated with both the frost heave quantity and the frozen depth. In addition, to reduce the observation cost and the measure difficulty, the frost heave quantity can be obtained through the observation of the frozen depth.

Recent Advances in Electrical Resistance Preheating of Aluminum Reduction Cells

There are two mainpreheating methods that are used nowadays for aluminum reduction cells. One is based on electrical resistance preheating with a thin bed of small coke and/or graphite particles between the anodes and the cathode carbon blocks. The other is flame preheating, where two or more gas or oil burners are used. Electrical resistance preheating is the oldest method, but is still frequently used by different aluminum producers. Many improvements have been made to this method by different companies over the last decade. In this paper, important points pertaining to the preparation and preheating of these cells, as well as measurements made during the preheating process and evaluation of the performance of the preheating, are illustrated. The preheating times of these cells were found to be between 36 h and 96 h for cell currents between 176 kA and 406 kA, while the resistance bed thickness was between 13 mm and 60 mm. The average cathode surface temperature at the end of the preheating was usually between 800°C and 950°C. The effect of the preheating methods on cell life is unclear and no quantifiable conclusions can be drawn. Some works carried out in the mathematical modeling area are also discussed. It is concluded that there is a need for more studies with real situations for preheated cells on the basis of actual measurements. The expected development in electrical resistance preheating of aluminum reduction cells is also summarized.

Diffuse and spot mode of cathode arc attachments in an atmospheric magnetically rotating argon arc

A model including the cathode, near-cathode region, and arc column was constructed. Specifically, a thermal perturbation layer at the arc fringe was calculated in order to couple sheath/presheath modelling with typical arc column modelling. Comparative investigation of two modes of attachment of a dc (100, 150, 200 A) atmospheric-pressure arc in argon to a thermionic cathode made of pure tungsten was conducted. Computational data revealed that there exists two modes of arc discharge: the spot mode, which has an obvious cathode surface temperature peak in the arc attachment centre; and the diffuse mode, which has a flat cathode surface temperature distribution and a larger arc attachment area. The modelling results of the arc attachment agree with previous experimental observations for the diffuse mode. A further 3D simulation is obviously needed to investigate the non-axisymmetrical features, especially for the spot mode.

Physics of Spotless Mode of Current Transfer to Cathodes of Metal Vapor Arcs

A fresh attempt is made to clarify the physics of the diffuse, or spotless, mode of current transfer that may occur on cathodes of vacuum arcs if the average cathode surface temperature is high enough, about $2000 \operatorname {K}$ . It is shown that in the case of chromium cathode the usual mechanism of current transfer to arc cathodes cannot sustain current densities of the order of $10^{5}$ – $10^{6}~\operatorname {A}{\cdot }\operatorname {m}^{-2}$ observed in the experiment, the reason being that the electrical power deposited into electron gas in the near-cathode space-charge sheath is insufficient. It is hypothesized that the electrical power is supplied to the electron gas primarily in the bulk plasma, rather than in the sheath, and a high level of electron energy in the vicinity of the sheath edge is sustained by electron heat conduction from the bulk plasma. Estimates of the current of ions diffusing to the sheath edge from the quasi-neutral plasma gave values comparable with the experimental current density, which supports the above hypothesis. On the contrary, the spotless attachment of vacuum arcs to gadolinium cathodes may be interpreted as a manifestation of the usual mechanism of current transfer to arc cathodes. Results given for gadolinium cathodes by a model of near-cathode layers in vacuum arcs conform to available experimental information.

Influence of Convective Heat Transfer by Air Flow on Local Current/Temperatures along Microtubular Solid Oxide Fuel Cells In-situ Identified by Electrodesegmentation Method for Co- and Counter-flow Configurations

Introduction Major advantages of solid oxide fuel cells (SOFC), such as employing non-precious metals as catalyst, tolerating various fuels, and releasing high quality waste heat, are virtues of high operation temperature (873-1273 K). On the other hand, thermo-mechanical stresses, as the primary problem of the SOFC systems develop due mainly to the heat treatment and operation at high temperatures. For both planar and tubular designs, a number of numerical studies addressing the thermo-mechanical stresses have been published; the thermal expansion coefficient mismatch, residual stresses, and the spatial surface temperature variations during the operation, have been stated to be the primary reasons [1-3]. To estimate the stresses stemming particularly from the spatial temperature variations, researches have been relying on the local temperatures computed numerically with thermo-electrochemical models [1,2,4,5,6]. However, as repeatedly stated, due to the difficulty in conducting experiments at high temperatures, the models were hardly validated in terms of surface temperatures. We thus measured the local current and temperatures along a microtubular SOFC with the electrode-segmentation method. Given the surplus heat generated during the cell operation is commonly removed by excess air flow method, and this method likely affects the temperature distribution along the cells, we conducted experiments for co- and counter-flow configurations under various air flow rates. Experimental Setup The tubular anode substrate of the cell was made of NiO/YSZ (65:35 wt %). Upon coating with the 8YSZ electrolyte colloid by dip-coating, we sintered the tube at 1693 K for two hours. With a special mask designed for the segmentation of the cell, we brush-coated the La0.7Sr0.3MnO3/YSZ cathode slurry (10:3 wt %) on the electrolyte surface by a cotton swab and subsequently sintered at 1323 K for two hours. Eventually, we connected the silver wires and thermocouples to the regarding segments for local current/voltage, and temperature measurements. As depicted in Fig. 1, we fed fuel and air to the anode and cathode, respectively, in the co-flow configuration. For counter-flow configuration, we exchanged the fuel inlet and outlet, retaining inlet and outlet of the air flow. Note that we keep the segment labels as on in Fig. 1 throughout the discussion, despite the switch of the flow configuration. Though the inlet air temperature was 298 K, the fuel flow was pre-heated before entering the cell. Prior to fuel supply, we sustained the cathode surface temperatures of the segments at 1073 K by an electric furnace. Results and Discussion The temperature distribution profile along the cell alters with the gas flow configuration. With the rising air flow rate at 0.6 V in the co-flow configuration, the midstream and downstream segment temperatures arise, while the upstream segment temperature decreases with a relatively steeper slope. This entails a larger maximum temperature difference along the cell. We observe the same trend in the local currents, however, the slope of the current variations in all the segments are rather small and resemble each other. With the increasing air flow rate, at 0.6V in the counter-flow configuration, all the segment temperatures drop down, where the slope of the upstream segment is relatively higher than the other segments. As a result, the maximum temperature along the cell becomes larger. The midstream and downstream segment currents increase with rather small slopes, whereas the upstream segment current remains nearly constant. Conclusions The in-situ identified current/temperature distribution profiles for co- and counter –flow configurations differ from each other considerably. With the increasing air flow rate, the maximum temperature difference along the cell grows in both flow configurations; where the total current output variation is rather small. The counter-flow configuration exhibits larger maximum temperature difference along the cell comparing to the co-flow case. With these findings, we can deduce larger thermo-mechanical stresses at high air flow rates in the counter-flow configuration; which we will explore numerically on the basis of local temperatures in-situ measured here.

Cooling strategy for effective automotive power trains: 3D thermal modeling and multi-faceted approach for integrating thermoelectric modules into proton exchange membrane fuel cell stack

The 3D Thermal modeling utilizes a Finite Differencing heat alteration method augmented with empirical boundary conditions is employed to develop 3D thermal model for the integration of thermoelectric modules with proton exchange membrane fuel cell stack. Hardware-in-Loop was designed under pre-defined drive cycle to obtain fuel cell performance parameters along with anode and cathode gas flow-rates and surface temperatures. The fuel cell model is used to conjugate the experimental boundary conditions with the Finite Differencing code, which implemented heat generation across the stack to depict the chemical composition process. The structural and temporal temperature contours obtained from this model are in compliance with the actual recordings obtained from the infrared detector and thermocouples. The model is harmonized with thermo-electric modules with a modeling strategy, which enables optimize better temporal profile across the stack. This study presents the improvement of a 3D thermal model for proton exchange membrane fuel cell stack along with the interfaced thermo-electric module. The model provided a virtual environment using a model-based design approach to assist the design engineers to manipulate the design correction earlier in the process and eliminate the need for costly and time consuming prototypes.

Research on Cathode Chronopotentiograms and Interelectrode Resistance during Vacuum Boiling Electroforming

Studies have showed that the electrolyte vacuum boiling electroforming process can significantly improve surface quality and microstructures of the deposited layer. To explore the mechanisms that produce the above-mentioned favorable improvements, this paper focused on analyzing the change characteristics of cathode potential and interelectrode resistance carried out under different process conditions. Research results showed that, reducing vacuum degree or increasing cathode surface temperature result in an intense fluctuation in the cathode chronopotentiometry and interelectrode resistance; decreasing the vacuum degree causes the whole cathode chronopotentiometry to shift negatively, but situation is opposite when the cathode surface temperature rises.

Evaluation of Cathode Heater Assembly for 42 GHz, 200 kW Gyrotron

AbstractIn this paper, the evaluation of cathode-heater assembly of magnetron injection gun (MIG) for 42 GHz, 200 kW gyrotron is presented. The cathode-heater assembly is purchased from M/S SEMICON.The cathode-heater assembly is experimentally studied in three different conditions; in a belljar system, during vacuum processing of MIG and during MIG testing to ensure the required rise of cathode surface temperature for pre-set heater power.

Kinetic Modeling of Initiation of Explosion Center on Cathode Under Dense Plasma

A kinetic 1-D spherically symmetric model of heating of microprotrusion (with a curvature radius of 1 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula> ) on the cathode under dense plasma is developed. The kinetic model is a model of 1D3V PIC/DSMC type. The model takes into account the main types of elastic and inelastic collisions of particles in the plasma as well as evaporation and thermofield electron emission from the cathode. The surrounding plasma parameters in the model are fixed on the boundary placed at a certain distance from the microprotrusion. The applied voltage drop is also fixed on the same boundary. The further evolutions of the plasma, current, and cathode temperature are calculated self-consistently taking into account the influence of the plasma generated during the process. The calculations have shown that, under the certain condition, thermal runaway is developed and the cathode surface temperature reaches the critical one. However, this instability is not caused by microprotrusion Joule heating because the current density is comparatively small <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$(&lt;10^{8}\ \hbox{A}/\hbox{cm}^{2})$</tex></formula> at that moment.

Cathode diameter and operating parameter effects on hafnium cathode evaporation for oxygen plasma cutting arc

Evaporation of hafnium cathode in a plasma cutting arc torch was investigated numerically and experimentally to assess the effects of cathode diameter and operating parameters such as gas pressure, gas flow rate and arc current. A numerical model was developed for arc plasma with consideration of hafnium cathode evaporation. Using this model, the surface cathode temperature and the amount of cathode mass loss caused by evaporation were predicted for different operating parameters. Simulated results show that the cathode surface temperature decreases at the hafnium cathode centre with increasing gas flow rate and cathode diameter. However, an increase in gas pressure elevates the surface cathode temperature in the radial range of 0.35 mm. The total amount of mass loss of hafnium cathode evaporation was predicted to be enhanced with increasing gas flow rate, gas pressure and cathode diameter as well. Comparison of the calculated results with experimentally obtained results shows good agreement in terms of the average surface cathode temperature, with variation in gas pressure and cathode diameter.

Assessment of the Power Balance at a Copper Cathode Submitted to an Electric Arc by Surface Temperature Measurements and Numerical Modelling

The aim of this work is to improve the understanding of the heating of copper cathodes under the action of a nonstationary electric arc in air. An experimental method is proposed for the measurement of the surface temperature distribution just after a very fast arc-controlled arc extinction. The arc current intensity is about 60-70 A, and the arc duration is in the range of 2.5-5 ms. Different kinds of surface temperature distributions have been observed depending on the arc root behavior. The measurement of the temperature decrease after the arc extinction was used to estimate the cathode surface temperature just at the moment of arc extinction. Two-dimensional thermal modeling taking into account phase changes is used to estimate the power balance at the cathode surface and to propose a second estimation of the surface temperature at the point of arc extinction. The comparisons between experimental results and thermal modeling lead in the present experimental conditions to a volt equivalent at the cathode in the range 6.7-10.7 V, to a surface power density in the range0.6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> - 2.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and to a maximum surface temperature in the range 850°C-1300 °C .

Spectroscopic measurement of temperatures in pulsed TIG welding arcs

Time resolved plasma temperatures in a pulsed tungsten-inert-gas (TIG) welding arc have been measured using optical emission spectroscopy. The peak and base pulse-averaged plasma temperatures both decrease with time after the arc ignition, and the plasma temperature decreases during the peak pulse period and increases during the base pulse period when the arc reaches the steady state. The decrease in the plasma temperature is associated with the increase in the cathode surface temperature and the decrease in the arc voltage and vice versa. The importance of the cathode surface temperature on the arc properties has been discussed.

Simulation of Filament Heater for Uniform Emission from Dispenser Cathode

This paper presents the design study of toroid shape filament heater for dispenser cathode.The filament heater will be used in cathode assembly of 200 kW 42 GHz gyrotron. A 3 D model of cathode assembly is designed using electromagnetic and thermal simulation software, ANSYS. The simulations are performed for optimizing the input filament heater power with respect to cathode surface temperature. The parametric study shows that the input power and cathode surface temperature depends strongly on the potting material, diameter of filament, number of turns, position and height of the filament heater with respect to cathode pellet. The design analyses are also carried out for two different filament heater materials i.e. tungsten and molybdenum. Further, the thermal, structural and transient analyses are also carried out to study the mechanical strength of the filament heater. It is concluded that the input heater power should be greater than 200 W to achieve cathode surface temperature greater than 1,000°C.

Time resolved characterization of a free-burning argon arc after ignition by optical emission spectroscopy

Time resolved properties of a free-burning argon arc after ignition have been characterized using optical spectroscopic method. After ignition, when the arc current keeps constant, the plasma temperature decreases with time at any position of the arc. The decrease of the plasma temperature is associated with the increase of the arc cathode surface temperature. It is suggested that the variation of the cathode surface temperature, which changes the current density distribution over the cathode surface, leads to the decrease of the plasma temperature in the free-burning arc after ignition.