Contact resistivity due to oxide layers between two REBCO tapes

In a no-insulation (NI) REBCO magnet, the turn-to-turn contact resistivity (ρ _c) determines its quench self-protection capability, charging delay time and the energy loss during field ramps. Therefore it is critically important to be able to control a range of ρ _c values suitable for various NI magnet coils. In this work, we investigate two possibilities to control ρ _c: by controlling the oxide layer of the copper surface of REBCO tapes; and by controlling the oxide layer in stainless steel co-wind tapes. We used a commercial oxidizing agent Ebonol^® C to treat the copper surface of REBCO tapes. The copper oxide layer was characterized by cross-sectional transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). The oxide layer formed in Ebonol^® C at 98 °C for 1 min is Cu₂O of 0.5–1 μm. The ρ _c between two oxidized REBCO is in the order of 35 mΩ cm² at 4.2 K which decreases to 10 mΩ cm² after 30 000 contact pressure cycles. The ρ _c increases but only by 5% at 77 K. We also investigated the effect of oxidation of stainless steel co-wind tape on ρ _c. The native oxides on 316 stainless steel tape as well as those heated in air at 200 °C–600 °C were examined by TEM and XPS. The native oxides layer is about 3 nm thick. After heating at 300 °C for 8 min and 600 °C for 1 min, its thickness increases to about 10 and 30 nm respectively. For the stainless steel tapes with about 10 nm surface oxides, pressure cycling for 30 000 cycles decreases ρ _c by almost 4 orders of magnitude. Whereas at 77 K, it only decreases by a factor of 3. For a surface with 30 nm oxide, the ρ _c decreases moderately with load cycles. The results suggest that for an oxidized stainless steel to achieve stable ρ _c over large number of load cycles a relatively thick oxide film is needed. (paper)