Electrical instabilities in stationary contacts: Al/plated-brass junctions

The electrical stability of stationary point contacts in oxygen gas and in ultrahigh vacuum is examined. The localized junctions were formed by the contact of two hemispherical surfaces consisting, respectively, of aluminum and 70 Cu/30 Zn brass plated with either In, Sn or Zn. The contact-spot temperature was determined by the magnitude of a direct electrical current. The contact spots were found to grow rapidly in vacuum, irrespective of contact temperature, but eroded in the presence of oxygen gas. This erosion usually led to catastrophic failure. Junctions generated with Sn- and In platings generally 'self-healed' (i.e. developed and maintained a low contact resistance) after contact melting, whereas contacts formed with Zn platings did not reveal self-heal characteristics. Possible causes for contact degradation in oxygen gas are discussed.<
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