Using High-Temperature Superconductors for Levitation Applications

Using High-Temperature Superconductors for Levitation Applications

INTRODUCTION

The image of a permanent magnet (PM) stably levitated over a bulk high-temperature superconductor has becomea major symbol of high-temperature superconducting (HTS) technology that often evokes visions of applications such as high-speed maglev vehicles and en- ergy-efficient flywheel energy storage. The advantages of noncontacting sur- faces without an active feedback system, the ability to operate in a vacuum, and the potential for extremely low rotational drag should, in many applications, out- weigh the inconvenience of refrigerat- ing the superconductor.

The phenomena associated with the strongly stable passive levitation of the PM/high-temperature superconductor system is not attainable with any other method, and its exploration can provide a tactile and visual indication of many of the basic phenomena associated with high-temperature superconductors, in- cluding persistent superconducting cur- rents and flux pinning. If the PM is pushed up, down, or sideways, or tilted, there is a restoring force that returns the PM to its initial position. The forces can also be highly hysteretic. It is possible to change the equilibrium position of the PM into almost any orientation or to move the center of mass of the PM to a new equilibrium position if pushed hard enough. If the PM is a cylinder with a relatively symmetric magnetic field, it readily rotates about its axis of symme- try. Such behavior suggests that the high-temperature uperconductor could be used in the construction of a super- conducting bearing.

The first stable levitation involving a superconductor was reported in 1945. 1,2 This was followed by investigations of gyroscopes and motors employing bear- ings that incorporated low-temperaturesuperconductors cooled with liquid he-lium. 3 Major interest in superconduct- ing levitation then focused on the use of low-temperature superconducting (LTS) magnets to provide levitational and guid- ance forces for vehicles moving at high speed at low heights above ground-sup- ported guideways. 3–5 Interest in super- conducting levitation increased dramati- cally after the discovery of the first high- temperature superconductor, 6 when ba-sic levitation demonstrations could be done in simple styrofoam cups contain-ing liquid nitrogen. Several reviews of the early experiments on HTS levitation are available 3,7–10.

Although some investigations have been made of the use of wires and thin films in HTS levitation, most of the present efforts involve the use of bulk high-temperature superconductors. Un- like superconducting wire applications, in which the supercurrent must pass from grain to grain along a length that encompasses many grains, a distinction of levitation applications is that the su-percurrent need only circulate within individual grains.

The present material of choice for su-perconducting levitation is Y-Ba-Cu-O (YBCO) and its RE-Ba-Cu-O (where RE denotes rare-earth elements Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, and La) analogs, because they exhibit a high magnetic-irreversibility field Hirr at liq-uid-nitrogen temperatures and many have the ability to grow large grains. Hirr marks a phase transition between the region where magnetic flux is solidly pinned in the superconductor and the region where flux may move. Sometimes the curve is said to denote the boundary between the region where flux is frozen and the region where flux is melted. Compared to some of the members of the bismuth, thallium, or mercury HTS families, YBCO has a relatively low criti-cal temperature of 92 K, but its irrevers-ibility curve is one of the highest at 77 K and lower temperatures. For stable levi-tation, it is important that the flux be frozen in the superconductor; otherwise, the PM slowly loses levitation height.

The levitational force is proportional to the mean magnetization of the high-temperature superconductor. The mag-netization of a bulk high-temperature superconductor is proportional to the product of the critical-current density and the grain diameter. Large grain di- ameters are important to achieve suffi- ciently large magnetizations for useful levitation forces. In bulk YBCO materi- als, the grains grow to diameters of sev- eral centimeters when made by a melt- texturing process11 (Figure 1). In the present state of the art, the upper limit of the grain diameter produced by this pro- cess is ≈10 cm. The ability to produce
good-quality YBCO thin films is also limited to this size. If techniques to grow large grains for other HTS materials with high irreversibility curves were devel- oped, these materials would also be of interest for levitation applications.

Many excellent reviews12–15 and col- lected papers16,17 cover the detailed fab- rication issues of melt-textured high-tem- perature superconductors. In addition to the levitation applications described here, melt-textured high-temperature superconductors can be used in other applications, such as new forms of elec- tric motors 18.

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