CONSIDERATIONS ON REACTION AND EVAPORATION MECHANISM IN OXIDE-CONTAINING GRAPHITE ELECTRODES DURING ARC EXCITATION

The thermochemical behaviour of various elements of the periodic system in pressed graphite electrodes under a d.c. arc-excitation was systematically studied. Use was made mainly of the oxides of those elements (5 % wt. metal content), which at elevated temperatures tend to form carbides. By means of thermodynamic calculations indications were obtained of the possible thermochemical reaction mechanism in the electrodes. The reaction products were identified and their distributions in the anodes were ascertained by X-ray diffraction analysis. The X-ray diffraction data were frequently in good agreement with the results of the thermodynamic calculations. They also showed, however, that the entire evaporation mechanism can only be discussed on the basis of the reaction steps as determined from the kinetics. For this reason experiments were also carried out with radioactive iron (59Fe-nuclide). From measurements of the evaporation rate and the activity distribution in the electrodes an insight was obtained into the evaporation behaviour. With the aid of the radioactive iron it was possible to demonstrate the influence of the cathode shape and the form of the gas discharge chamber on the evaporation of the material. © 1968.