Voltage conditioning effect on CuCr contacts

Abstract

A dc field emission scanning microscope (FESM) was used to characterize the electron emission current of CuCr25 sintered contacts. Field emission scans at 20-50 MV/m using a truncated cone anode of 300 μm in diameter localized emitting defects with a positioning accuracy of ~150 μm in the measured area (~25 mm2) of the contact. The strongest activation of emitters was noticed at 30-40 MV/m resulting in 15 emitters at 50 MV/m. Afterwards, the I(E)-characteristic of the 10 dominant emission sites was measured locally. Two different types of emission could be observed: a normal Fowler-Nordheim-like emission where the current depends only on the applied voltage, and an abnormal emission where the current shows a strong hysteresis between the rising voltage branch and the decreasing voltage branch. The abnormal behavior can lead to an emission current increase once a critical electric field is exceeded. Possible surface effects influencing the emission current are either open pores or dielectric inclusions at the grain boundaries. The field emission enhancement factor β of these sites vary from 115 to 300 similar to values reported before. The number density of emitters is well above 0.5/mm2. In sealed vacuum interrupters only one emitter is active at a specific moment in time, shadowing the other emitters.