Contact Resistivity Due to Oxide Layers Between Two REBCO Tapes

In a no-insulation (NI) REBCO magnet, the turn-to-turn contact resistivity (rho(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 rho(c) values suitable for various NI magnet coils. In this work, we investigate two possibilities to control rho(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 (R) 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 (R) C at 98 degrees C for 1 min is Cu2O of 0.5-1 mu m. The rho(c) between two oxidized REBCO is in the order of 35 m Omega cm(2) at 4.2 K which decreases to 10 m Omega cm(2) after 30 000 contact pressure cycles. The rho(c) increases but only by 5% at 77 K. We also investigated the effect of oxidation of stainless steel co-wind tape on rho(c). The native oxides on 316 stainless steel tape as well as those heated in air at 200 degrees C-600 degrees C were examined by TEM and XPS. The native oxides layer is about 3 nm thick. After heating at 300 degrees C for 8 min and 600 degrees 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 rho(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 rho(c) decreases moderately with load cycles. The results suggest that for an oxidized stainless steel to achieve stable rho(c) over large number of load cycles a relatively thick oxide film is needed.