Joining of single-core ex-situ MgB2/Fe wires by termination architecture

High-temperature superconductors, such as Magnesium diboride (MgB2), are enabling materials for building state-of-the-art high-field magnets, which will provide new findings in many fields of physics. One of the hurdles in the development of superconducting magnets is a “persistent switch”. Persistent switch is a joint in superconducting magnet, which after cooling down to cryogenic temperatures lowers its electrical resistant to very low or zero and short-circuits the superconducting magnet. Thanks to that, the superconducting magnet can operate in persistent mode – magnetic field generated by the magnet persists without noticeable degradation even after it is disconnected from a power source. In order to address this problem, Tibor Melišek and colleagues proposed a specific MgB2/Fe wire joint technology, to develop joints with the required low electrical resistance. This joint architecture is made by placing MgB2/Fe wires into a steel cup into which is poured MgB2 powder with specific particle size. Afterwards, the joint is pressed by two split dies and the resulting joints are annealed for 30 minutes (Fig. 1).

Figure 1: The sketch of a termination joint architecture

The main goal of the study is to investigate the impact of the particle size on the electrical properties of the joint. The measured parameter was critical current Ic joint of the joint in comparison to the critical current Ic wire of the MgB2/Fe wires. In the first step of the joint preparation, commercially acquired MgB2 powder is sieved through sieve plates, to acquire powders with 15 µm, 20-25 µm, 35 µm size particles. Then, joints from the sieved and the original not sieved powder, are prepared. The properties of the joints were measured at two temperatures – 4.2 K and 20 K. The best performing joint with a ratio of 80% Ic joint/Ic wire was made from the 20-25 µm powder and was measured at 20 K and a 1.5 T external magnetic field (Fig. 2). The results show that it is possible to create a joint of superconducting MgB2/Fe wires by a simple method that has low enough electrical resistance to be used as a persistent switch for future state-of-the-art magnets.

Figure 2: Critical current of a joint made from 20-25 µm particles powder in a external magnetic field compared to the critical current of a MgB2/Fe wire

Authors: Tibor Melišek, Dušan Berek, Marek Búran, Michal Bennár, Pavol Kováč

Link: https://www.sciencedirect.com/science/article/pii/S0011227524000778