Oxide superconductors

5.7.5 Oxide superconductors

In 1986 a new class of ‘warm’ superconductors, based on mixed ceramic oxides, was discovered by J. G. Bednorz and K. A. Müller. These lanthanum–copper oxide superconductors had a T c around

35 K, well above liquid hydrogen temperature. Since then, three mixed oxide families have been developed with much higher T c values, all around 100 K. Such materials give rise to optimism for superconductor technology; first, in the use of liquid nitrogen rather than liquid hydrogen and, second, in the prospect of producing a room temperature superconductor.

The first oxide family was developed by mixing and heating the three oxides Y 2 O 3 , BaO and CuO. This gives rise to the mixed oxide YBa 2 Cu 3 O 7 −x , sometimes referred to as 1–2–3 compound or YBCO. The structure is shown in Figure 5.29 and is basically made by stacking three perovskite-type unit cells one above the other; the top and bottom cells have barium ions at the center and copper ions at the corners, the middle cell has yttrium at the center. Oxygen ions sit halfway along the cell edges but planes, other than those containing barium, have some missing oxygen ions (i.e. vacancies denoted by x in the oxide formula). This structure therefore has planes of copper and oxygen ions

containing vacancies, and copper–oxygen ion chains perpendicular to them. YBCO has a T c value of about 90 K, which is virtually unchanged when yttrium is replaced by other rare earth elements. The second family of oxides are Bi–Ca–Sr–Cu–O x materials with the metal ions in the ratio of 2:1:1:1, 2:1:2:2 or 2:2:2:3, respectively. The 2:1:1:1 oxide has only one copper–oxygen layer between the

bismuth–oxygen layers, the 2:1:2:2 two and the 2:2:2:3 three, giving rise to an increasing T c up to about 105 K. The third family is based on Tl–Ca–Ba–Cu–O with a 2:2:2:3 structure having three

copper–oxygen layers and a T c of about 125 K.

While these oxide superconductors have high T c values and high critical magnetic field (H c ) values, they unfortunately have very low values of J c , the critical current density. A high J c is required if they are to be used for powerful superconducting magnets. Electrical applications are therefore unlikely until the J c value can be raised by several orders of magnitude comparable to those of conventional superconductors, i.e. 10 6 A cm −2 . The reason for the low J c is thought to be largely due to the grain boundaries in polycrystalline materials, together with dislocations, voids and impurity particles. Single crystals show J c values around 10 5 A cm −2 and textured materials, produced by melt growth

Physical properties 273

Yttrium Barium

Copper Oxygen

cba

Figure 5.29 Structure of 1–2–3 compound; the unit cell of the 90 K superconducting perovskite, YBa 2 Cu 3 O 7 −x , where x ∼ 0 (by courtesy of P. J. Hirst, Superconductivity Research Group, University of Birmingham, UK).

techniques, about 10 4 A cm −2 , but both processes have limited commercial application. Electronic applications appear to be more promising, since it is in the area of thin (1 µm) films that high J c values have been obtained. By careful deposition control, epitaxial and single-crystal films having

J c ≥ 10 6 A cm −2 with low magnetic field dependence have been produced.