Effect of Work Function and Ionization Potential on the Reignition Voltage of Arcs Between Ag/Metal Oxide Contacts

Ac arcs between electrodes with a low boiling point, e.g., Ag/CdO, exhibit an immediate recovery of a thin space charge layer in front of the new cathode after current zero. At arc currents above 200 A the dielectric breakdown voltage of this layer (immediate recovery voltage) lies, nearly independently of the arc current, in the region of a few hundred volts. Recent papers have investigated the reigniti0n voltage .of Ag/CdO with alkali additives. Further results of immediate recovery voltage measurements for Ag/CdO and Ag/SnO2 with and without various additives of alkali or second metal oxide components are presented. It is obvious that the immediate recovery voltage is governed by a similar mechanism as the minimum spark breakdown potential according to the Townsend theory, or the cathode fall of a .glow discharge. Based on this fact, and under simplifying assumptions, the dependence of the minimum sparking potentials of the contact materials under consideration is estimated using calculated effective work functions and ionization potentials of the composite materials. The breakdown voltage changes nearly linearly with the effective ionization potential Of the metal vapor, while the work function seems to he of minor influence. The calculated tendency is in good agreement with the measured values. The calculated values of the work functions and ionization potentials are compared with minimum arc voltages at contact bounce. This minimum voltage is expected to be equal to the sum of the work function and the ionization potential. The agreement in tendency supports the above estimations.