Commentary

Recent Developments in Practical High. T c Superconductors

Chandra S. Pande

Shortly after the discovery of high­critical-transition temperature (high­TJ superconduc­tors, it was realized that the critical cur­rent OJ in the poly­crystalline form of these materials is much smaller than

in single crystals. This was in contrast to behavior in low-Te superconductors (e.g., Nb3Sn or NbTi), where the reverse was true. The conclusion was obvious that the random arrangement of grains or some intrinsic properties of grain boundaries (weak link) prevents Ie from reaching the value obtainable in single crystals.

These conclusions were put on a quan­titative footing by a series of pioneering experiments at IBM using bicrystal films of Y-Ba-Cu-O (YBCO) aimed at identi­fying the factors responsible for low Je in polycrystalline materials.! It was con­clusively shown that Ie across the bi­crystal boundary was directly related to the misorientation of the boundary. These experiments implied that an al­most perfect grain alignment or texture is necessary, and high-angle grain boundaries would normally degrade Je' A drop in Ie by a factor of more than ten for a misorientation of only a few de­grees in the boundary appears surpris­ing since these small-angle boundaries can be thought of as walls of dislocations that are fairly widely separated, the separation being given by the well­known Read-Shockley formula. How­ever, such a drop at least for small misorientation boundaries is now un­derstood to some degree and can be shown to be related to the stress field associated with the dislocations forming the boundary,2 coupled with the fact that the coherence length (roughly a charac­teristic distance of electron-electron pair­ing) is relatively small (-1 nm in high-Te materials compared to -10 nm in low-Te materials). In the case of high-angle boundaries, the situation is more com­plex and is not fully understood since these behave like Josephson junctions.!

Since low Ie in polycrystalline materi­als seem to have their origin in the in­trinsic property of the grain boundary itself, the problem is reduced to produc-

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ing materials having high texture. The developments in this endeavor have been promising. As early as 1988, Jin and coworkers3 produced material prepared by directional solidification with a Ie of 17,000 AI cm2. The next year, Salama and coworkers,4 using a liquid phase pro­cessing technique, achieved Ie in excess of 18,500 AI cm2•

Much improved and simpler methods of producing texture in these materials, especially in (BiPb)2Sr2Cu30!O has been developed in recent years by the pow­der-in-tube technique (see the article by Balachandran et al.). It is hoped that Ie will soon routinely exceed 105 AI cm2 (at liquid nitrogen temperature), normally the value needed by the commercial pro­ducers of superconducting magnets.

Two new developments in this direc­tion are noteworthy, both using YBCO. A team at Los Alamos National Labora­tory (LANL) has developed a process of producing a textured coating of YBCO in a flexible nickel alloy tape.s This is done by growing the superconductor on an aligned zirconia template produced by an ion-beam-assisted deposition (IBAD) process, which inhibits the growth of misaligned crystals.

The IBAD process employs two argon ion guns. The first gun is aimed at a sta­bilized-zirconia target and sputters at­oms from this target onto a nickel sub­strate, forming cubic zirconia crystals on the nickel substrate. The second argon gun bombards these crystals. Certain planes in the zirconia crystals on the sub­strate "channel" argon atoms; so they continue to grow. However, even a small misorientation from the channeling di­rection is enough to cause increased in­teraction with the incoming ions, "etch­ing" away the misaligned crystal. Thus, eventually cubic zirconia textured in two orthogonal directions and containing virtually no large-angle boundaries are produced. It is then relatively easy to produce a epitaxial layer of YBCO, say by pulsed laser deposition, on the zirco­nia template. By refining this basic tech­nique, LANL researchers have produced superior conductors with Je above lOS AI cm2 in fields above 4 Tesla at liquid ni­trogen temperature-a remarkable feat. However, a detailed comparison of this process with the commercial powder-in­tube process is not as one sided as it ap­pears, since the powder-in-tube process

is simpler and cheaper and the engineer­ing currents as distinct from Ie may still be comparable.

A competing technique for producing high-Je wires has been developed by Goyal and coauthors at the Oak Ridge National Laboratory (the article by Goyal briefly mentions the process). The performance of conductors that are pro­duced by these techniques are compa­rable and leads one to hope that practi­cal high-Te superconductor wires will soon be available.

These reports of practical supercon­ductors with high Ies have begun to re­focus the need to understand the mecha­nism that limits Je in these materials, es­pecially at high magnetic fields and at liquid nitrogen temperature, since a key problem in the application of high-Te superconductors is the drastic drop in Ie under these conditions. This is the so­called flux-creep problem. This problem is possibly due to a lack of a sufficiently high density of lattice defects, which act as pinning agents for the flux lines that are entering the material on the introduc­tion of a magnetic field and must be held (pinned) so as to not produce an energy loss by their motion. It will be most ben­eficial if the recent breakthrough in pro­ducing high Ie has also resulted in a dras­tic reduction of the flux creep problem.

A major recent development toward the commercialization of high-Te super­conductors is the use of these materials for viable superconducting motors. Gubser reports on the U.s. Navy's efforts toward that goal. In the fall of 1995, this motor achieved a record of 124 kW at 4.2 K and 91 kW at 28 K. An indepen­dent effort involves a joint project be­tween the U.S. Department of Energy, American Superconductor Corporation, and Reliance Electric. They have already demonstrated a 149 kW motor6 operat­ing at 1,800 rpm with the coils at a tem­perature of 27 K.

References 1. D. Dimos et aI., Phys. Rev. Lett. 61 (1988), p. 219. 2. C.s. Pande and RA. Masumura, Mater. Sci. and Eng., B32 (1995), p. 247; D. Agassi, C.s. Pande, and RA. Masumura, Phys. Rev., B52 (1995), p. 16237. 3.5. Jin et aI.. Phys. Rev., B37 (1988), p. 7850. 4. K. Salama et aI., App/. Phys. Lett., 54 (1989), p. 2352. 5. XD. Wu et aI., App. Phy. Lett., 67 (1995), [. 2397. 6. R&D Magazine, 38 (7) (1996), p. 14.

Chandra S. Pande is section head at the Naval Research Laboratory in Washington, D.C., and the JOM Advisor from the TMS Superconducting Materials Committee.

JOM • October 1996