PRESENT TECHNICAL STATUS

Performance of HTS conductor materials has been improving continually since the discovery of HTS materials in 1986 and the development of suitable conductor configurations in the following two years. Maeda at NRIM discovered Bi-2212 in 1988; that same year, Vacuumschmelze in Germany developed the oxide powder-in-tube process for Bi-2212. Sumitomo Electric in Japan was one of the earliest organizations to apply this same process to Bi-2223. Many other laboratories around the world began working on these two materials, and in the United States, both ASC and IGC are now producing high quality Bi-2223 conductors for a variety of applications such as motors, magnets, and transmission lines.

Several highlights from the WTEC panel's site visits in Japan and Germany illustrate some of the significant progress in this field.

Bi-2223/Ag Conductors

Bi-2223/Ag-sheath tape has been manufactured by Sumitomo Electric Corporation for a number of years. Sumitomo makes 61-filament Bi-2223/Ag sheath conductors in lengths up to 1,200 m for cable and magnet applications. Now Sumitomo is exploring the use of Ag alloy sheaths for improved performance in high magnetic fields. The main technical issue is that well-annealed Ag, which is the state of the conductor sheath after the final thermal process step, is mechanically very weak, having a low yield stress, and it has a large mechanical mismatch to the Bi-2223 core. To withstand the large Lorentz forces, which produce hoop stresses in a solenoid at high magnetic fields, doped Ag sheaths with higher mechanical strength are desirable. However, this advantage must be balanced against the effect of oxidation of the dopant and possible reaction with the HTS core. Alloys of Ag with Mn, Sb, and Ni have been manufactured, and the mechanical performance has been substantially improved, as can be seen in Fig. 5.4.

Fig. 5.4 Normalized critical current vs. tensile stress for alloyed sheath tapes. Data on the right is for different outer and inner sheath alloys, as indicated by "inner alloy/outer alloy" compositions for each curve (Hayashi et al. 1996).

Bi-2212/Ag Sheath Conductors

Hitachi Research Laboratory and Hitachi Cable, Ltd., have made major advances within the last several years to produce very high critical current and critical current density Bi-2212/Ag-sheathed tape. This has been achieved by improving the geometric uniformity of the conductor's HTS filaments by a "continuous pressing" deformation process. In this process, multifilament (55) tape is first conventionally drawn to 0.2-2 mm diameter, then it is continuously pressed to a thickness of 0.1-0.2 mm. Finally, it is partially melt processed and then given a final anneal to adjust the oxygen stoichiometry to maximize J_c. Compared with conventionally rolled Bi-2212 multifilament tape, the Hitachi process results in much better geometric uniformity, or much less "sausaging." This improved cross-sectional uniformity results in a higher n value for the current-voltage characteristic (V = I ⁿ) and a higher J_c for a long piece length. Conductor performance at 4.2 K is 470 kA/cm² at zero field, 171 kA/cm² at 30 T parallel to the tape, and 110 kA/cm² at 23 T perpendicular to the tape. The data are shown in Fig. 5.5. This wire has been wound into a coil to produce 22.76 T with the aid of a backup field.

Fig. 5.5. Performance of Hitachi continuous pressed Bi-2212/Ag tape conductor at 4.2 K. The application for this wire is high field insert magnets (1000 A/mm² = 100 kA/cm²) (Okada et al. 1995).

Kobe Steel has also been very active in Bi-2212/Ag sheath conductor development, also with high field NMR (nuclear magnetic resonance) magnets as the target application. Its researchers have investigated two features that they expect to lead to better quality magnets. The first is the use of round wire, rather than flat tape, as the conductor geometry. Round wire has the advantage that it can be wound into a solenoid with very good control over the position of each winding for the generation of a highly homogeneous field. In contrast, flat tape must generally be wound into pancake coils with gaps between the pancakes that generate spatial inhomogeneities. The second feature is the use of alloy sheath materials to improve the mechanical performance of the conductor. In particular, the "double sheath" configuration (Fig. 5.6) for multifilament round wire or tape has several benefits: the outer sheath of Ag alloy with Ni or Mg has high strength, while at the same time the pure Ag inner sheaths eliminate any chemical incompatibilities between the Bi-2212 filaments and the alloying element. The double sheath round wire has been made into a small test solenoid.

Fig. 5.6. Schematic diagram of a transverse section of a "double sheath" round wire showing the Bi-2212 filament core, the inner Ag sheath, with well textured material at the core/Ag interface, and the outer Ag-alloy sheath for high strength (Hase et al. 1997).

Bi-2212/Ag Coated Conductor

Showa Electric Wire and Cable Company, in collaboration with the National Research Institute for Metals (NRIM), has developed a continuous heat treatment furnace system for processing Bi-2212 coated conductors. In this process, shown schematically in Fig. 5.7, Ag tapes are first coated with a Bi-2212 slurry by dip coating. Three such tapes are stacked and then covered with Ag foil. This composite is then transported through a long oven with an empirically optimized temperature profile. Showa Electric researchers have experimented with many process parameters, including tape speed and reel tension. They have processed up to 100 m lengths at a transport speed of 0.2 m/h to produce a conductor with a J_c of >120 kA/cm² at 4.2 K and 12 T with a uniformity of ~20% over the entire length.

Fig. 5.7. Schematic of the furnace developed by Showa Electric for continuous heat treatment of Bi-2212/Ag coated conductors (Hasegawa et al 1995).

Tl-1223 Coated Conductors

Although many companies, including Hitachi Cable and Sumitomo Electric, have worked with thallium-based HTS materials, for the last several years, except for the interesting fluorine doping work by Tachikawa et al. (1996) at Tokai University, there has been only one strong effort on this material, that based at the Hitachi Research Laboratory. Research there has concentrated on producing Tl-1223 superconductor on thermomechanically textured Ag substrates. In this process, an Ag slab is given a large rolling deformation at a temperature of 130°C. This is followed by a recrystallization heat treatment of 700°C for 1 h and 850°C for 10 h. This results in a highly oriented Ag substrate with good in-plane alignment. Fig. 5.8 shows that the X-ray diffraction phi scan peak width (full width at half maximum, FWHM) is about 6°.

Fig. 5.8. Phi scan of thermomechanically textured Ag substrate (FWHM ~ 6°) (Doi et al. 1996).

Spray pyrolysis is then used to deposit Tl-0223 precursor directly on the Ag substrate, followed by heating the sample in a Tl-oxide atmosphere to form the Tl-1223 film. The film is c-axis- and mostly in-plane- aligned; however, some grain colonies are oriented at 27° and 45° from the preferred orientation. Fig. 5.9 shows a (103) pole figure of the Tl-1223 film on a {100}<100> textured Ag substrate.

Fig. 5.9. (103) Pole figure of Tl-1223 on the {100}<100> textured Ag substrate (Doi et al. 1996).

The misoriented colonies result in a fraction of the grains being poorly connected (i.e., having weak links) and thus result in degraded critical current density. Nevertheless, comparing the transport properties of these films that have c-axis and partial in-plane alignment with those that have only c-axis alignment (i.e., no in-plane alignment), the transport properties of the former are much better. Figure 5.10 shows that the in-plane aligned sample shows a much smaller decrease in J_c at low field compared to the c-axis aligned sample, indicating a lower level of weak links in the former. The best films grown to date have a J_c at 77 K, 0 T of 100 kA/cm² and 15 kA/cm² at 1 T B||c. Hitachi is also examining scale-up processes for longer lengths (~10 m) using multiple spray nozzles.

Fig. 5.10. Critical current density of the Tl-1223 film of Fig. 5.9 and a reference film with only c-axis alignment (Doi and Higashiyama 1995).

Y-123 Coated Conductors

Fujikura, Ltd., was one of the pioneers in the development of the Y-123 coated conductor technology. Other organizations in Japan, notably Sumitomo Electric Industries, have been working on developing Y-123 for conductor use almost since the discovery of this material. The major limitation for this material had been the inability to obtain both c-axis and in-plane alignment, which are required to eliminate weak links and obtain high critical current densities, on anything but single-crystal substrates. This problem was overcome by Fujikura, Ltd. in 1992 by the use of ion beam assisted deposition (IBAD) to form a textured buffer layer of YSZ as a template for the Y-123. Polycrystalline nickel alloy (Hastelloy) tape is used as substrate material. The YSZ buffer layer has an in-plane misorientation, characterized by the FWHM of an X-ray diffraction f scan of the (202) peak of 25-30°. The Y-123 film, deposited by pulsed laser deposition (PLD) has an FWHM of the (103) peaks of only 15-20°. This group achieved short sample J_c values of 1.1 MA/cm² at 77 K, 0 T in 1995. The sample shows very good in-field performance, as shown in Fig. 5.11.

Fig. 5.11. Magnetic field performance of J_c for a high quality Y-123 tape produced by PLD on an IBAD-YSZ buffer on Hastelloy (Iijima et al. 1996).

Fujikura is scaling up its process to longer lengths using IBAD for the YSZ template, but using either PLD or metallorganic chemical vapor deposition (MOCVD) for the Y-123 layer. Fig. 5.12 shows a schematic of the IBAD chamber. Recent results at 77 K and self field for a 0.8 m tape produced by PLD are 0.17 MA/cm² (Iijima et al. 1996); under the same measurement conditions, a 0.16 m tape made by MOCVD showed a J_c of 0.21 MA/cm² (Onabe et al. 1997).

Fig. 5.12. Schematic of IBAD apparatus for deposition on a long length of tape (Iijima et al. 1996).

In May 1996, Sumitomo Electric announced details of a new, non-IBAD technique for producing in-plane aligned YSZ using pulsed laser deposition of the IBAD with the plume directed at an angle to the substrate, as shown in Fig. 5.13 (Hasegawa et al. 1997). This technique has resulted in a YSZ buffer layer with a FWHM of 12.8° when deposited at 0.5 µm/min. In contrast, IBAD buffer layers with FWHM as small as 6° have been reported, but the deposition rate is more than an order of magnitude slower. Sumitomo Electric has produced a tape with 0.5 µm thick YBCO and 0.55 m long with a J_c of 0.2 MA/cm² at 77 K, self-field using this technique.

Fig. 5.13. Schematic of the non-IBAD process used by Sumitomo Electric to produce biaxially textured YSZ buffer layers for Y-123 coated conductors (Hasegawa et al. 1997).

M. Fukutomi's group at NRIM is investigating another non-IBAD process for producing a textured buffer layer. This technique makes use of two electrodes installed in a magnetron sputtering system to define the shape of the plasma edge (parabolic) and the direction of the ion flux on the tilted substrate. Fig. 5.14 shows a schematic of the apparatus. The tilted substrate and extra electrodes result in a biaxially aligned YSZ layer.

Fig. 5.14. Schematic of the magnetron sputtering apparatus at NRIM with two electrodes to deposit a biaxially textured YSZ buffer layer (Fukutomi, Kumagai, and Maeda 1997).

The two main efforts in Germany are in the groups of H. Kinder at the Technische Universität München and H.C. Freyhardt at the Universität Göttingen. Kinder has been concentrating on producing large-area Y-123 films on single-crystal substrates by thermal evaporation of the three cation constituents from individual boats. The sample is rotated inside a cylindrical box, within which the temperature can be controlled very well and which has a modest oxygen partial pressure for oxidation of the film. A sector of the box is removed to expose the sample to the evaporants. With this apparatus the group has succeeding in fabricating films up to 9 inches (~23 cm) in diameter (Semerand et al. 1997). Freyhardt's group uses IBAD to produce biaxially textured YSZ buffer layers on flat and curved Ni alloy substrates. Large-area (10 x 10 cm and 20 x 20 cm) flat surfaces have been coated by rastering and rotating the substrates during both buffer layer and Y-123 film deposition, which is done by PLD. YSZ FWHM values of 10.8° have been achieved on flat substrates; for curved surfaces, the best values are above 20°. J_c values of 1 MA/cm² have been achieved for films on small-area flat substrates, and values > 0.1 MA/cm² have been achieved on curved substrates and small tubes (15 mm diameter) (Freyhardt 1997).

Bulk Superconductor Components

Bulk current leads are for sale and are supplied in many cryocooled magnets to substantially reduce the heat leak between room temperature and the magnet operating temperature, typically ~10 K, and thus increase the efficiency of the system. These applications are discussed in greater detail in Chapter 4.

Work in Japan on producing bulk Y-123 HTS parts for bearing, levitation, trapped field magnets, and magnetic shielding applications is going on at Nippon Steel, Chubu Electric Power, Fujikura, and ISTEC, among others. Using the quench melt growth (QMG) process Nippon Steel has produced bulk parts and small (12-turn) coils. Chubu Electric is working on bearings, and Fujikura on current leads. ISTEC has the largest effort to produce Y-123 bulk pieces for levitation by the melt powder melt growth (MPMG) technique (Sakai, Yoo, and Murakami 1995). The Y-123 powder contains excess Y that results in Y-211 inclusions during processing, which result in fine precipitates that act as pinning centers and enhance J_c. Small amounts of Pt from the crucible used in the first melt stage help refine the grain size of the Y-211 inclusions. The ISTEC group has produced single disks 8 cm in diameter and 2.5 cm thick that have a repulsive force of 200 N and can levitate 10 kg. A set of 100 smaller-diameter Y-123 pieces assembled on a large plate levitated a 140 kg sumo wrestler and 60 kg support plate to a height of 2.5 cm. To summarize these highlights and to compare recent results, Tables 5.3 through 5.5 show the state of the art for the different HTS conductor materials and configurations in Japan, Germany, and the United States.

It is clear from these tables that there is rough parity between results achieved in the United States and Japan for Bi-2223 and Bi-2212 conductors, with Germany slightly lagging in production of long lengths. For Tl-1223, the group at Hitachi in Japan is the clear leader. For Y-123 coated conductors, the United States is ahead in J_c and I_c for short lengths, and several Japanese groups lead in the 1 m length category; Germany has good short sample results and has several projects on large area deposition. Japan has the lead in bulk Y-123 and Nd-123, mainly because of the work at ISTEC; the University of Houston also has good results but is concentrating on improving reproducibility in smaller-size disks. Thus, overall, the United States and Japan are producing materials with comparable performance, and Germany is a close third.