Abstract Body

REBaCuO (RE: a rare earth element) bulk superconductors can trap magnetic fields as large as several tesla after magnetization process, in which electro-magnetically induced currents flow quasi-permanently inside its material.

The most, commonly used magnetization technique, field-cooled magnetization (FCM), requires three stages: cooling, magnetization, and demagnetization. During the magnetization, a large magnetic strain due to Lorentz force may cause mechanical fracture of the bulk magnets because of that brittle ceramic material nature. Thus, to realize a compact bulk magnet with higher trapped fields over 5 T, it is essential to perform an external reinforcement support, and hence, induced magnetic stress should be reduced as low as the mechanical strength of the bulk throughout the entire process. Experimental studies of the mechanical behaviors of an EuBaCuO ring bulk reinforced by a metal ring have been reported, in which a compressive strain, which is applied under cooling, and a magnetic tensive strain, which is caused during magnetization, were consistently measured using strain gauges directly attached on the bulk [1]. However, for assessment of the mechanical instability, it is necessary to clarify such the mechanical behavior and the reinforcement effect on the bulk in the entire FCM process including demagnetization.

In this work, we have measured a mechanical strain of an EuBaCuO cylindrical bulk (64 mm OD, 28 mm ID, 20 mm height (H), made by NIPPON STEEL Corporation, Japan) which is externally reinforced by a 5 mm thick Al alloy ring, under the entire FCM process before and after magnetization. Strain gauges were adhered at four points on the top and bottom surface of the bulk, and then, attached to the stage of a cryogen-free 10 K refrigerator. The FCM sequence in this experiment was performed in the following three stages:

Stage 1(cooling): The bulk was cooled from room temperature, 300 K to 100 K, then cooled to a magnetizing temperature of 50 K under the external magnetic field of 5 T.

Stage 2(magnetization): The external magnetic field was ramped down to zero at a rate of -0.222 T/min.

Stage 3(demagnetization): The bulk was heated up to 100 K linearly at +0.1 K/min., in which the trapped magnetic field was removed down to zero.

Figure 1 shows the experimental results of a mechanical strain ε_θ on the bottom surface of the ring bulk under the magnetization and demagnetization process followed by cooling process. Supplemental measured values of the magnetic field and the temperatures at each point, (Ⅰ) to (Ⅳ) were also shown in table. At point (Ⅰ), just after the cooling process before the magnetization, a minimum compressive strain of -0.219 % was obtained, which indicates the bulk shrunk due to compressive stress. In the following point (Ⅱ) during magnetization, the bulk expanded due to Lorentz force, resulting in a maximum peak value of -0.185 %. During the demagnetization, ε_θ started to recover because the trapped field was gradually reduced at point (Ⅲ), and then attains a secondary minimum value of -0.207 % at 72.2 K at point (Ⅳ). Comparing between (Ⅰ) and (Ⅳ), it is confirmed that the reinforcement effect of the Al alloy ring under cooling process cannot be maintained until the trapped magnetic field was removed. Therefore, in demagnetization process, it is nessesary to consider the decrease in reinforcement effect. In the presentation, the mechanical behavior and the instability of superconducting bulk would be discussed.

References

1) Sora Namba, Hiroyuki Fujishiro, Tomoyuki Naito, Mark D Ainslie, and Kai Y Huang, Supercond. Sci. Technol. 32 (2019) 125011