Surface and sub-surface modifications of copper electrodes exposed to electric high-field conditioning at cryogenic temperatures

It has been stipulated that the conditioning of metal surfaces under a high electric field is dominated by material hardening. To test this, we subjected three pairs of copper electrodes to high-voltage conditioning at distinct temperatures (300, 30, and 10 K) until each reached its saturation field. The sets conditioned at colder temperatures showed a significant increase (36%) in the field-holding capability with respect to the room-temperature sample. The samples were then investigated with high-resolution microscopy, characterizing the breakdown (BD) spots on the anode and cathode according to their morphology, and with STEM, analyzing the changes in the subsurface regions. A self-shielding mechanism is offered to explain the observation of a central protected region within the anode spot. Unusual BD features were found on the cold-conditioned cathode surfaces, with very shallow craters of a star-like shape. The number of atypical spots increased with decreasing temperatures, reaching 26% and 53% of the total number of spots at 30 and 10 K, respectively, and its form was suggested to be related to the different dynamics of the evolution of the spots due to thermal diffusivity variations. Sub-surface analysis showed clear structural changes in high-field areas, with stronger effects in the cold-conditioned sample. These suggested considerable sub-yield plastic activity during conditioning, leading to the initial observation of the formation of dislocation-denuded zones close to the surface. Thus, while conditioning is a result of sub-surface plastic evolution, it is not necessarily a result of surface hardening.