There are several ways to catalog conductor technologies: by material, by application, or by manufacturing technique. The materials method is used here. Four HTS compounds are used for all the large-scale power applications discussed in this report:
Tl-1223 may also have some Bi substituted for the Tl or Pb, and the last compound can be formed with many rare earths; in particular, Nd-123 is being exploited as a bulk material for bearings and magnetic levitation.
The four major techniques used to manufacture HTS conductors are the (1) powder in tube method, (2) dip coating and other ceramic coating methods, (3) deposition of biaxially textured thin films on textured buffer layers or substrates, and (4) bulk growth techniques.
The sheathed, or powder-in-tube (PIT), process was one of the first to be developed for making HTS conductors. The PIT process is now in use in many laboratories and companies around the world. It is sometimes used for Bi-2212 and Tl-1223 and is almost always used for processing Bi-2223 into conductor. Fig. 5.1 shows the steps in the process.
Fig. 5.1. Schematic diagram of the powder in tube process for Bi-2223, Bi-2212, and Tl-1223 wire or tape.
The material of choice for the tube is silver or a silver alloy. Silver is permeable to oxygen, is nonreactive with the HTS core material, lowers the melting point of Bi-based HTS materials during thermal processing, and forms a template upon which the HTS material can grow. Typically, the tube is filled with HTS powder, then extruded or drawn to wire about 1-2 mm in diameter. For multifilament conductor, the wire is drawn in a hexagonal shape, cut into shorter lengths, and formed into a stack of 7, 19, 37, 55, 61, 85, or higher numbers of filaments. This stack is then inserted in another tube, and the composite is extruded or drawn to wire. The restacking and redrawing steps are omitted for monofilament wire. For round wire, the final step is heat treatment, but most conductors are made in a flat "tape" format achieved by rolling the wire to an aspect ratio of ~10:1.
Bi-2212 is subjected to a partial melt process at 800-900°C to form large grains of that compound with the crystallographic a-b planes (i.e., the Cu-O planes that have a high critical current density) oriented parallel to the current flow direction of the tape and to the wide face of the tape. Both Bi-2223 and Bi-2212 are highly anisotropic materials, and the Jc within the a-b plane may be several orders of magnitude larger than Jc along the c axis. Obtaining good uniaxial (c-axis) orientation of the grains in these two materials is necessary to achieve high Jc. Bi-2223 also undergoes a heat treatment (800-840°C) at the tape stage, but then usually goes through one or more additional rolling/heat treatment cycles. Tl-1223 is processed similarly to Bi-2223.
The second process used for manufacturing superconductors employs typical ceramic techniques such as dip coating or doctor-blade coating of the HTS material (Bi-2212) mixed into a slurry with an organic binder and a solvent onto a substrate, typically silver or silver alloy. Fig. 5.2 shows the process schematically for coating the conductor, coiling into a form, heating to remove the organic materials in the slurry, and then using a partial melt process step to form large-grain, oriented Bi-2212 superconductor. This process has mainly been used to make conductors for magnet applications.
Fig. 5.2. Schematic of typical Bi-2212 coated conductor processing (NRIM 1995).
Depositing a superconductor film epitaxially onto a textured buffer layer or textured substrate so as to achieve biaxial texture is the process used to make superconductors of Y-123 and sometimes of Tl-1223. These two materials are more isotropic than the two Bi compounds, and they require both good c-axis grain orientation and also good in-plane orientation. The classic experiment by Dimos et al. (1988) showed that Jc drops by an order of magnitude for current transfer between two grains with parallel c axes, but with the a axis misoriented by only 14°. The best Jc values for Y-123 are obtained for thin films grown epitaxially on single crystal substrates, such as SrTiO3 or LaAlO3, which are impractical for conductors due to high cost, small size, and poor mechanical properties. Instead, Ni, Ni alloys, and Ag are used as substrate materials in one of two processes.
In the first process (Fig 5.3), a buffer layer, of yttrium-stabilized zirconia (YSZ) for example, is deposited on an untextured polycrystalline substrate by one of several techniques to achieve biaxial texture. The Y-123 is then deposited directly on this template or on top of one or more intermediate buffer layers that are grown epitaxially on the YSZ. In the second process, the substrate itself is biaxially textured by thermomechanical processing, typically a very large rolling reduction followed by heating to obtain the preferred grain orientation needed to form a template for the HTS film and any intermediate buffer layers. A protective coating may be made over the HTS film to protect it from the environment and to facilitate attachment of electrical contacts.
Fig. 5.3. Schematic view of a Y-123 coated conductor (Iijima et al. 1996).
The processing of bulk pieces of superconductor makes use of two different techniques. For Bi-2212 and Bi-2223, the HTS material is cast to form a rod or cylinder to be used as a current lead; it then undergoes partial melt processing. The product is not nearly as highly textured as that from the powder-in-tube or dip-coating processes, but since it has a much larger cross-sectional area, it is able to carry a substantial current. There are a variety of related growth techniques for obtaining bulk pieces of Y-123 that rely on one or more thermal gradients to provide large (>1 cm) oriented grains. Seeding with single crystals of Sm-123 has also been effective in nucleating the growth of large Y-123 grains. Various additions, such as Pt and Y2BaCuO5 (Y-211), result in enhanced pinning for Y-123. Pieces approximately 10 cm in diameter and 1-2 cm thick can be grown by this technique. Research on rare earth-123 (RE-123) compounds has begun, and now centimeter-size grains of Nd-123 can also be obtained. This material must be grown under controlled low oxygen pressure to prevent site exchange of Nd and Ba that would result in low Tc. Processing to control site exchange and to produce precipitates of Nd4Ba2Cu2Ox (Nd-422) phase yields higher critical current density values at 1-3 T and 77 K than can be achieved with Y-123.