High-temperature superconductors are brittle oxide ceramics, yet they have been made into wire that has been wrapped into solenoids and used in demonstration magnets and motors. Fabricating wires from these ceramics is an extremely challenging materials science process that requires a precisely engineered microstructure with the correct chemical, mechanical, and electromagnetic properties if these wires are to transport large current densities (Jc) in high magnetic fields. Heine et al. first demonstrated that wires of these materials could carry high Jc in very high magnetic fields. At 4.2 K, the oxide superconducting wires can carry higher Jc at higher magnetic fields than conventional Nb-Ti or Nb3Sn wires (Figure 1), and as shown in the companion article in this issue by Kato et al. they can also have high Jc at 77 K.Of the three major families of high-temperature superconductors, YBa2Cu3O7-x, Bi-Sr-Ca-Cu-O (BSCCO), and Tl-Ba-Ca-Cu-O, the best wires to date have been made in the BSCCO system. At present, all YBa2Cu3O7-x wires are weak linked and have only small Jc in magnetic fields. In the Tl-based system, the superconducting properties are potentially very interesting, but the toxicity of Tl and the system's complex processing have limited conductor development. For the Bi-based system, the basic processing steps are becoming known, the grains are well connected, and the weak link problem can be controlled. This permits applications in the temperature range 4–77 K, depending on the field and current density requirements of the particular use.
