Third generation superconducting wires
The vast publicity of the discovery of high temperature superconductors in 1987 led many to the conclusion that we were about to enter a superconducting age. People may still remember the excitements about the huge business potentials and significant impacts on civilization. However, after 15 years, people are disappointed at the limited commercial applications. The key for many large-scale applications is the development of commercially viable superconducting wires. The idea of third generation superconducting wires may bring the dreams of a superconducting age into reality.
High temperature superconducting wires are classified by the texture formation mechanism. The first generation superconducting wires depend on the easy cleavage of the double Bi-O layer of the Bi2212 phases for the texture formation. Epitaxial growth of the thin film process is the texture formation mechanism for the second generation superconducting wires, or coated conductors. A unique “roller-skate” powder structure provides a novel texture formation mechanism for the third generation superconducting wires. The texture formation mechanism is applicable to all layered compounds of high temperature superconductors, and hence the term “third generation superconducting wires” is used.
The crystal structure of Bi2212 is shown on the left. The structure contains perovskite-derived slabs delimited by a BiO layer with the NaCl structure. The relatively weak BiO layer provides the easy cleavage of this phase. This is responsible for the easy formation of texture during deformation for the preparation of the first generation superconducting wires by the powder-in-tube process.
This phenomenon also appears in other materials with a similar structural feature of a weak layer. A well-known example would be graphite with the structure shown on the right. Graphite has stacks of planar layers within which the nearest neighbor distance is 1.42 Ĺ, but the layers are widely separated by a distance of 3.35 Ĺ. The weak van der Waals forces between the layers accounts for the ready cleavage of graphite and its use as a lubricant.
YBaCuO also has a layered structure as shown on the left. The crystal structure of YBaCuO consists of three similar perovskite blocks. Unlike the Bi2212 phase, it does not have a weak layer. Therefore, it does not have the easy cleavage available to the Bi2212 phase, and it is difficult to develop strong texture of the YBaCuO phase during deformation in the powder-in-tube process for the preparation of superconducting wires. The crystal structure is anisotropic with the length of c axis being about three times greater than the length of a or b axis. There is a natural tendency to form plate-like crystals during crystal growth.
Professor J. Akimitsu (Aoyama Gakuin University) announced the discovery of superconductivity in MgB2 at 39 K on January 10, 2001, at a symposium on "Transition Metal Oxides" in Sendai, Japan. The compound consists of alternative layers of magnesium and boron. The crystal structure is also anisotropic, but to a less degree in comparison with high Tc oxide superconductors. The absence of weak link is a great news for wire processing[3,4]. It has been shown in the literature  that there is a need for uniaxial texture in order to obtain better superconducting performance than that of Nb3Sn superconducting wires. However, there is no easy-cleavage layer as in the case of Bi2212, and previous results in the literature did not show texture formation for normal powder-in-tube deformation process for this material[5-10].
Now we consider the roller-skate powder structure for the third generation superconducting wires. Here we consider an aligned part of the powder, and not aligned part can be considered as part of another imaginative grain and texture will form eventually in the whole powder during shear deformation due to the self-alignment nature. The difference in thickness of the roller powder due to the scattering nature of a powder process will not affect the discussion of the basic mechanism. In a large scale, the plate-like particle (skate board) is like the strongly connected layer structure in Bi2212 and graphite. The roller powder provides the easy sliding and cleavage between the layers in a similar way as in the Bi2212 weak layer and in graphite. Therefore, the novel roller-skate powder structure artificially mimics the structure in Bi2212 and graphite. The force between the layers (skate board) in the new powder structure would be very weak. In this aspect, the new powder structure would have better texture behavior than the Bi2212 phase since the forces between the layers are weaker than the forces between the BiO layers. As plate-like powders can be prepared for all the above layered-compounds due to their anisotropy by well-known techniques, the proposed powder structure can be used for the preparation of textured composites of these materials.
The texture mechanism is independent of the superconducting material and the sheath material. Therefore, the method can be applied to composite wires of all layered high temperature superconductors: MgB2, Bi-based superconductors, rare earth 123 superconductors, Hg-based superconductors, and Tl-based superconductors. Since the industry has mature experience with the powder-in-tube method, commercially viable third generation superconducting wires will be soon developed for a variety of industrial applications.
The first and most straightforward example of the third generation superconducting wires would be MgB2 superconducting wires. Paul Grant has shown the benefits of the low cost of MgB2  and S. Patnaik et al.  have calculated that uniaxial texture is necessary for superior properties over that of Nb3Sn. Since MgB2 does not have the weak-link problem, the roller-skate powder structure will lead to low-cost high-performance superconducting wires prepared by the powder-in-tube method. It is expected that the third generation MgB2 superconducting wires will be the workhorse superconducting wires for low temperature applications.
The idea of the third generation superconducting wires will help to improve the processing condition and product performance of Bi2223 and Bi2212 wires. The experiences gained in the development of the first generation superconducting wires are valuable knowledge for the processing of the third generation superconducting wires. The third generation Bi-based superconducting wires will find their applications in high magnetic fields at low temperatures and in low magnetic fields at high temperatures.
The third generation rare earth 123 superconducting wires are the most desirable superconducting wires for applications at high temperatures around 77 K. Fundamentally, there is not much difference between the processing of the third generation 123 superconducting wires and the processing of the third generation Bi2223 superconducting wires. We face the same processing challenges of texture formation and weak-link problem. The difference is more experience with the processing of the Bi2223 superconducting wires by the powder-in-tube process. Therefore, more processing optimization and strict processing control are necessary for the development of the third generation rare earth 123 superconducting wires.
The concept of the third generation superconducting wires provides a blueprint for a series of commercially viable high temperature superconducting wires. If the discovery of the high temperature superconductors in 1987 has given us a view into the future of the superconducting age, the idea of the third generation superconducting wires may finally make the door to the superconducting age wide open.
1. David C Larbalestier, The Road to Conductors of High Temperature Superconductors:10 Years Do Make a Difference!, IEEE TRANS. ON APPLIED SUPERCONDUCTIVITY VOL. 7, NO.2, 1997, 90-97
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