Contact resistivity due to oxide layers between two REBCO tapes

Abstract

In a no-insulation (NI) REBCO magnet, the turn-to-turn contact resistivity ( c ) determines its quench selfprotection capability, charging delay time and the energy loss during field ramps. Therefore it is critically important to be able to control a range of  c values suitable for various NI magnet coils. 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 Cu 2 O of about 0.5 m. The  c between two oxidized REBCO is in the order of 35 m-cm 2 at 4.2 K which decreases slowly with contact pressure cycles. The  c increases but only slightly at 77 K. We also investigated the effect of oxidation of stainless steel co-wind tape on  c . The native oxides on 316 stainless steel tape as well as those heated in air at 200 -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  c by almost 4 orders of magnitude. Whereas at 77 K, it only changes slightly. For a surface with 30 nm oxide, the  c decreases moderately with load cycles. The results suggest that for an oxidized stainless steel to achieve stable  c over large number of load cycles a relatively thick oxide film is needed.

Quench transient current and quench propagation limit in pancake wound REBCO coils as a function of contact resistance, critical current, and coil size

“…There has begun to be active attempts to adjust and control the value of the contact resistance, through the application of resistive films on the conductor, including copper oxide, and creation of oxide layers on steel co-wind by heat treatment in air [20,21]. It is not the purpose of the present work to examine the engineering of the value of Rs.…”

Effects of Wax Impregnation on Contact Resistivity Between REBCO Tapes

“…The same is true for other pressure contact situations in insulated coils, such as coils wound with multiple REBCO tapes or with a cowind copper tape for stabilization. However, wax impregnation does not provide robust mechanical support to mitigate the issue of remarkable Rct reduction under cyclic pressure due to wearing [5]. A contact of REBCO/stainless-steel which was vacuum impregnated by paraffin experienced remarkable Rct reduction after 30,000 pressure cycles at 4.2 K. It is similar to the case without wax impregnation.…”

“…In fact, our preliminary experiment in one sample showed no significant increase in the contact resistivity by applying wet Stycast L28 epoxy between REBCO and stainless-steel tape and cured under contact pressure. In addition, this sample was tested under cyclic pressure of 2.5 -25 MPa up to 30,000 cycles at 4.2 K. The reduction in Rct was only about a factor of 2 compared with a factor of about 1000 in a sample without epoxy [5]. In the case of epoxy impregnated REBCO NI coil, however, a method mitigating the issue of REBCO degradation by epoxy [15] must be developed.…”

An experimental study to investigate magnetic field and winding force-dependent contact resistance of NI REBCO coil

“…The fundamental study to measure the property of the contact resistance started, focusing on the nature that the contact between winding turns induces the resistance. As a result, contact pressure, the asperity ratio of the electroplated surface, and temperature dependencies have been investigated and formulated at the tape level [48][49][50]. However, the practical properties of the contact resistance at the coil level have been discussed insufficiently, even though NI REBCO magnets genuinely aim to generate high fields.…”