Investigation of the flickering of La2O3 and ThO2 doped tungsten cathodes

Abstract

Short-arc lamps are equipped with tungsten electrodes due to their ability to withstand a high thermal load during operation. Nominal currents of more than one hundred amperes lead to a cathode tip temperature near the melting point of tungsten. To reduce the electrode temperature and, thereby, to increase the maintenance of such lamps, ThO2 or tentatively La2O3 are added to the electrode material. They generate a reduced work function by establishing a monolayer of emitter atoms on the tungsten surface. Emitter enrichments on the lateral surface of doped cathodes are formed. They are traced back to transport mechanisms of emitter oxides in the interior of the electrode and on the electrode surface in dependence of the electrode temperature and to the redeposition of vaporized and ionized emitter atoms onto the cathode tip by the electric field in front. The investigation is undertaken by means of glow discharge mass spectrometry, scanning electron microscope images, energy dispersive x-ray spectroscopy, and through measurements of the optical surface emissivity. The effect of emitter enrichments on the stability of the arc attachment is presented by means of temporally resolved electrode temperature measurements and by measurements of the luminous flux from the cathode-near plasma. They show that the emitter enrichments on the lateral surface of the cathode are attractive for the arc attachment if the emitter at the cathode tip is depleted. In this case, it moves along the lateral surface from the cathode tip to sections of the cathode with a reduced work function. It induces a temporary variation of the cathode tip temperature and of the light intensity from the cathode-near plasma, a so-called flickering. In particular, in case of lanthanated cathodes, strong flickering is observed.

The Relation Between Electrode Lifetime and Additive Consumption During TIG Welding

“…3 shows the computational results of the temperature at 0.1 mm axially from the electrode tip when the adsorption energy of thorium was set to be 7.1 eV, 7.4 eV, 7.478 eV and 7.5 eV. Here, the adsorption energy was set within the range of the actual values obtained by the experiment 6,7) . When the temperature at 0.1 mm from a Welding Letters Vol.37 No.4 (2019) central axis of the electrode tip exceeds 3653 K which is the melting point of tungsten, the moment is defined as the electrode lifetime.…”

Investigation of erosion mechanism of tungsten-based cathode in Ar–N2 DC arc

“…Since frequent electrode exchange greatly increases the cost of manufacturing nanoparticles, extending the life of the electrodes is an important issue in the industrial application of DC arc. Past work has attempted to reduce cathode erosion in a rare gas atmosphere using tungsten added with a metal oxide having a low work function [7][8][9][10][11]. The major mechanism of cathode erosion in a rare gas atmosphere can be considered to be cathode evaporation, and a quantified model has been proposed [12].…”

The cataphoretic emitter effect exhibited in high intensity discharge lamp electrodes

“…The Ba-compound is dissociated at elevated temperature supplying Ba atoms, which diffuse along the surface to the electrode tip [11,13]. A recent development is the introduction, not only in high intensity discharge lamps but also in arc welding, of tungsten electrodes which are doped with La 2 O 3 ; tungsten electrodes doped with Ce 2 O 3 have also been tested [14][15][16]. These electrodes show, principally, a behavior quite similar to thoriated electrodes.…”