Properties of Second Generation High Temperature Superconductor

Transcription

1 WJP, PHY 381 (2014) Wabash Journal of Physics v2.1, p.1 Properties of Second Generation High Temperature Superconductor J. Qi, D. T. Tran, A. Wadlington, B. Badger, B. Hayhurst, and M. J. Madsen Department of Physics, Wabash College, Crawfordsville, IN (Dated: December 15, 2014) In this paper we reported the on-axis magnetic field of two superconducting loops of wire, one soldered and one connected by tape. Our measurements indicate that the magnetic field, created by flux pinning in the type-ii superconducting wire, is consistent with a magnetic quadrupole. Also, the magnetic field for the soldered and tape-connected wire show the same on-axis field maps. This is evidence that the soldered loop does not make a complete superconducting current loop.

2 WJP, PHY 381 (2014) Wabash Journal of Physics v2.1, p.2 I. INTRODUCTION The science about superconductor is always a promising research area. With the recently development in the type-ii superconductor, we now have the superconducting material whose critical temperature is much higher than the original type I superconductor. Researches about pulse-field magnetization [2][3][4] indicate that a current could be induced through a type-ii superconducting loop through flux pinning. However, there is not much research we find that investigates the structure of induced current inside the superconducting loop. In this experiment, we examine the structure of the induced current in a soldered superconducting loop. The z-component of magnetic field on the axis of the loop is mapped and compared with classical wire models. II. THEORETICAL MODEL Using Lenz s Law, if there is a nonzero magnetic flux goes through the closed loop, we can induce a current in the loop by expel the magnetic flux through the loop. If we do that while the wire loop is superconducting, because theoretically there is no electrical resistance in the loop, the induced current in the loop will stay forever. Flux pinning also has effect on the magnetic flux since the loop is a thin Type II superconductor. It is more clearly shown when we took out the magnet out from the center of the loop when it is in its superconducting state. The loop was pinned in space with the magnet, leading to the fact that the removal of the magnet from the center of the loop involves keeping the loop from levitating with the magnet out of the liquid nitrogen chamber[1]. Because the superconducting is in circular shape around the center. We came up with two models for the induced current: a simply circular current (Fig.(1)(a)) and two circular current with currents in opposite directions (Fig.(1)(b)). The z-components of magnetic field on the axis of those two current configurations are easy two calculate. For configuration (a), and for configuration (b), B z = µ 0 2πR 2 I, (1) 4π (z 2 + R 2 ) 3/2 B z = µ 0 4π 2πR 2 I ( h + z) 2 + R 2 ) 3/2 µ 0 4π 2πR 2 I. (2) ((h + z) 2 + R 2 ) 3/2

3 WJP, PHY 381 (2014) Wabash Journal of Physics v2.1, p.3 where z is position in the z direction, r is the radius of the current loop, h is half of the separation between two current loops and I is the current in the loop. With Eq.(1) and Eq.(2), the z-component magnetic field maps are as Fig.(1)(c). It is obviously that the shape of the field maps are different. The field map for the single circular current is an even function around the center, but the field map for the double circular currents is an odd function around the center. FIG. 1. (a) Model for a single circular loop around a center, where I is the current, r is the radius of the circular loop; (b) Model for a double circular with two parts 2h apart and carrying same currents in opposite direction; (c) The expected graph for the z-component of magnetic field on the center axis of two configuration: the blue one is from setup (a) and the red one is from the setup (b) III. EXPERIMENTS PROCEDURE To compare the field map with our model, we first built two configurations of classical wires (Fig.(2)) according to our model. By putting current in the wire loops by power supply, we measure the on-axis z-component magnetic field map for both configurations. we put the two configurations of classical wires in the same conditions and created two on-axis field maps. Those magnetic field maps will be used to compare with those from superconducting loops. The next step for our experiment is to build the loop of superconducting wires. For the first one, we soldered the two ends of wire together to make a loop (Fig.(3)(b)). For

4 WJP, PHY 381 (2014) Wabash Journal of Physics v2.1, p.4 FIG. 2. Two configuration of wires from according to our model. (a) is a single circular wire loop with 10 turns and (b) is a double circular wire loop with currents in opposite directions, 5 turns. We built those two loop using magnetic wire. The currents going through both wires are 1A. the second, we use nonconducting tape to put the two ends of superconductor together (Fig.(3)(c)). However, for investigating quadruple field behaviour later, we let the two ends of superconductor do not touch each other(fig.(3)). In this way, we built circular shaped open loops of superconductor for three different width (1 cm, 0.6 cm, and 0.3cm). FIG. 3. The setup for constructing the superconducting wire loop with solder or tape to connect two ends. Part (b) use soldering to connect two ends of superconducting wire together, and part (c) use tape to connect but separate two ends. Then we put the superconducting wire and a strong magnet in a container and then use liquid nitrogen to cool the system below T c. Since the temperature of the liquid nitrogen is significantly below T c, we just need to wait for about less than a minute to make sure the wire become superconducting. After the wire has reached its superconduting state, we

5 WJP, PHY 381 (2014) Wabash Journal of Physics v2.1, p.5 removed the magnet so there would be a current induced in the loop (Fig.4). FIG. 4. Process to induce the current in the superconducting wire by magnet: a) Put the magnet at the center of the superconductor wire and put them together into liquid nitrogen; b) Remove the magnet, but hold the superconducting wire at its original position; c) After removing the magnet, there will be current induced in the superconducting wire, and we can use a magnetic probe to measure the magnetic field of this current. Then, we use a magnetic probe to test the persistence of the current (Fig.4). To achieve this, we measure the magnetic field from the current in the loop. As we measured the magnetic field for a sufficiently long time (roughly more than 10 3 seconds), we saw the current will stay for sufficient time for our later measurement of field maps. After trying with three width of superconducting loops and measure the magnetic field, we found that the 1cm width wire give a most significant and persistent reading for the magnetic field. Thus, for the following experiments, we just used the 1cm loop. As we are interested in is the structure of the current in the superconducting loop we then measure the on axis z-component magnetic field maps of the superconducting current wire. To achieve this, we build to an apparatus (Fig.(5)) to measure the z component of magnetic field along the axis the current loop. We measured the magnetic field, ranging from 5cm to 5cm, from the center of the current loop. With those data, we built a field map for the superconducting current loop: the change of z component of magnetic field versus the change in z direction distance from the center. We used this process to generate

6 WJP, PHY 381 (2014) Wabash Journal of Physics v2.1, p.6 FIG. 5. Apparatus for measuring the z component of magnetic field of the current loop with different position on z axis. Part (a) is the top view and part (b) is the side view of the apparatus. While measuring the z-component magnetic field on the axis of the superconducting wire, we put the wire loop inside the liquid nitrogen area and put insert a magnetic probe in the center hole. The magnetic probe is labelled with 0.5cm marks, such that we can record the position of the probe. By changing the position of magnetic probe, we can get the z-component of magnetic field at different positions field maps for both the soldered and the taped loop. IV. DATA AND DISCUSSION Using two configurations for the classical wires, we created two field maps for those wires. The shape of the field maps of those wires are as expected by our model (Fig. (1)). Using our data for the z component of magnetic field at different positions on the axis and above the superconducting loop, we got the field map for the soldered superconducting wire loop (Fig.(7)). The shape of the field map shows a add function about z position. Compared with field maps from the classical wire configurations, our superconducting loops data agrees best with the double circular loop (Fig.(6)(b)) also. Thus, we suspect the structure of the current in the superconducting loop is similar to the model (b) in Fig.(1). We also got the same field map for the taped superconducting wire loop (Fig.(7)). The shape of the field map shows a add function about z position. Even this is not a closed wire

7 WJP, PHY 381 (2014) Wabash Journal of Physics v2.1, p.7 FIG. 6. The field map for the z component of magnetic field versus positions on z direction for the two classical wires configurations. The red one is for the single circular wire. The Blue one is for the double circular wire. loop, its field map shows strong similarity with that of the soldered wire. Compared with field maps from the classical wire configurations, our superconducting loops data also agrees best with the double circular loop (Fig.(6)(b)) also. Thus, the current in both soldered and taped wire loops are most possible magnetic quadruples. This indicates that the soldered superconducting wire loops is not a closed loop. V. CONCLUSION AND FUTURE WORK In our experiment, the field map for the z-component of magnetic field shows a quadruple behaviour for both soldered and taped wire. we suspected that the current induced in the magnetic field is most likely an Eddy current. As we compared the field maps for superconducting wire with two different configurations of classical wires we build, the data shows a similarity to the configuration with double current with opposite directions. This indicates the soldered wire does not make a closed superconducting loop. A direction for the future work is to use a 3D magnetic field probe to create a 3D vector field map for magnetic

8 WJP, PHY 381 (2014) Wabash Journal of Physics v2.1, p.8 FIG. 7. The field map for the z component of magnetic field versus positions on z direction for the 1cm width soldered superconducting loop. The shape of the field map shows a odd function behaviour about z. field, which will be more straightforward for comparison. [1] Cardwell, D. A., and N. Hari Babu. Improved Magnetic Flux Pinning in Bluk (RE)BCO Superconductore. AIP, 3 Mar [2] Mizutani, U., T. Oka, Y. Itoh, Y. Yanagi, M. Yoshikawa, and H. Ikuta. Pulsed-field Magnetization Applied to High-Tc Superconductors. Applied Superconductivity (1998): [3] Ohsaki, Hiroyuki, Tatsuya Shimosaki, and Naoyuki Nozawa. Pulse Field Magnetization of a Ring-shaped Bulk Superconductor. Superconductor Science and Technology 15.5 (2002): [4] Sander, M. Novel Pulsed Magnetization Process for Cryo-permanent Magnets. Physica C: Superconductivity (2003): ScienceDirect. Web. 28 Sept. 2014

9 WJP, PHY 381 (2014) Wabash Journal of Physics v2.1, p.9 FIG. 8. The field map for the z component of magnetic field versus positions on z direction for the 1cm width taped superconducting loop. The shape of the field map shows a odd function behaviour about z.