CRITICAL ISSUES

Discussions with hosts at the organizations the WTEC panel visited indicate that there is a high degree of consensus on which technical issues are limiting commercial applications of HTS materials and also on some of the approaches to overcome these limitations.

Bi-2223

The main limitation for Bi-2223 PIT conductors is that achievable critical current density is lower than what is needed for applications. That value is widely believed to be 100 kA/cm² at 77 K and self field. Solutions to reach this goal include improved powder uniformity and density, better grain alignment, and better thermomechanical process control. The silver sheath widely used also has several drawbacks, such as its poor mechanical strength, which makes handling and Lorentz stresses serious issues, and its low resistivity, which results in extra ac losses. These problems are being attacked by doping the Ag sheath with elements such as Mn to both raise its resistivity and increase its mechanical strength. At least one host cited poor connectivity and low pinning energy as problems, whose solution is to examine other HTS systems.

Bi-2212

For Bi-2212, the problem of low J_c is not considered as serious an issue: operation is typically at 4.2 K, where this material performs better than Bi-2223 and the J_c is already satisfactory. A more severe problem is the lack of uniformity and reproducibility of J_c along the length of a conductor. Here, the solution envisaged is better process control to eliminate second-phase inclusions, which tend to disrupt grain alignment and current flow. The mechanical strength is also too low, especially for the very high field (>20 T) insert magnets manufactured from this conductor. The solution here is similar to that for Bi-2223, i.e., doping the Ag sheath with Mn and other impurities. A "double sheath" arrangement is also of benefit, such that the pure Ag sheath is in direct contact with the HTS material and the more chemically reactive Ag alloy is used as the outer sheath, the alloy material being applied in the restacking operation. A further difficulty with these conductors is the lack of truly superconducting joints and persistent current switches, both important for NMR magnet operation.

Tl-1223

The major issue for Tl-1223 is the less than optimal in-plane alignment of the HTS material on the thermomechanically textured Ag substrate; this limits the performance to about 100 kA/cm². Scale-up to long-length production is also an issue, both for deposition of the precursor powder and the thallination, when the Tl vapor pressure and the oxygen pressure must both be controlled.

Table 5.3
Present State of the Art for HTS Conductors -- Japan

Table 5.4
Present State of the Art for HTS Conductors -- Germany

Table 5.5
Present State of the Art for HTS Conductors -- United States

Y-123

For Y-123 coated conductors the major issue is the speed of production of the buffer layers and the Y-123 film. For use of ion beam assisted deposition of the biaxially textured (usually YSZ) buffer layer, this is the rate-limiting step. Several non-IBAD processes are being explored to address this issue, such as pulsed laser deposition with an inclined substrate and shaped electrode magnetron sputtering. In long-length scale-up, it will be important to maintain good biaxial alignment of the buffer layer. Other important issues for MOCVD deposition of the Y-123 are thickness and J_c uniformity, reaction layers, oxygen pressure control, and speed.

Bulk Materials

The critical issues for bulk HTS materials, primarily Y-123, are the needs for larger and better aligned grains with improved pinning properties. One or more temperature gradients have been employed to achieve larger, better aligned grains, and doping with excess Y and small amounts of Pt have been used to improve pinning. Work on the RE (Nd or Sm)-123 systems focuses on control of the partial pressure of oxygen to achieve the best superconducting transition temperature and on better understanding of the source of pinning in this material. Currently, better in-field performance can be achieved in the RE-123 materials, but the overall sample size that can be grown is smaller than for Y-123.