Speaker
Description
A robust technology for producing superconducting joints between Coated Conductors (CCs) is crucial for those applications in which closed-loops coils working in persistent current mode are required, such as Nuclear Magnetic Resonance (NMR). The preparation of these joints involves applying both temperature and a transverse compressive load simultaneously to produce the bonding of two adjacent REB2Cu3O7-x (REBCO, RE = rare earth) layers. We carried out experiments to simulate this process, subjecting samples from commercial CCs to various combinations of pressure (50 to 80 MPa) and temperature (600 to 850 °C). The goal was to understand the impact of thermomechanical cycles on the critical current (Ic) of the CCs and find out the upper limit for the achievable current in a superconducting joint between CCs. Across the studied parameter range, we observed a degradation in Ic performance. For example, the average reduction in Ic, measured at 77 K in self-field, is approximately 45% for samples subjected to simultaneous heating at 820°C and pressing at 60-70 MPa compared to samples heated at the same temperature without applied pressure. In this work, tests have been performed on CCs with different RE elements in the REBCO layer and varying not only the temperature and external pressure, but also the time at maximum temperature, the heating ramp and the oxygen partial pressure. Electrical transport measurements combined with microstructural analysis by means of TEM, SEM, and EDX served to determine a possible correlation between Ic degradation and variations in the REBCO microstructure caused by changes in these parameters. Our analyses suggest that the concurrent application of temperature and pressure eases the decomposition of the REBCO phase. EDX data indicates that decomposition products correspond to a melting process of the REBCO occurring at lower temperatures, accelerated by the presence of external pressure and as expected, taking place earlier when the RE in the REBCO compound leads to a lower peritectic point. The larger time at maximum temperature, the shorter the heating ramps or the lower oxygen partial pressure also speed up the decomposition of the REBCO phase. The observed early decomposition occurs in localized areas of the REBCO layer, reducing the available cross-section for current flow and thus decreasing the critical current.
| Submitters Country | Switzerland |
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