Bismuth strontium calcium copper oxide

It is a cuprate superconductor, an important category of high-temperature superconductors sharing a two-dimensional layered (perovskite) structure (see figure at right) with superconductivity taking place in a copper-oxide plane. BSCCO and yttrium barium copper oxide (YBCO) are the most studied cuprate superconductors.

Specific types of BSCCO are usually referred to using the sequence of the numbers of the metallic ions. Thus Bi-2201 is the n = 1 compound (Bi₂Sr₂CuO_6+x), Bi-2212 is the n = 2 compound (Bi₂Sr₂CaCu₂O_8+x), and Bi-2223 is the n = 3 compound (Bi₂Sr₂Ca₂Cu₃O_10+x).

The BSCCO family is analogous to a thallium family of high-temperature superconductors referred to as TBCCO and having the general formula Tl₂Ba₂Ca_n−1Cu_nO_2n+4+x, and a mercury family HBCCO of formula HgBa₂Ca_n−1Cu_nO_2n+2+x. There are a number of other variants of these superconducting families. In general, their critical temperature at which they become superconducting rises for the first few members and then falls. Thus Bi-2201 has T_c ≈ 33 K, Bi-2212 has T_c ≈ 96 K, Bi-2223 has T_c ≈ 108 K, and Bi-2234 has T_c ≈ 104 K. This last member is very difficult to synthesize.

Wires and tapes

BSCCO was the first HTS material to be used for making practical superconducting wires. All HTS have an extremely short coherence length, of the order of 1.6 nm. This means that the grains in a polycrystalline wire must be in extremely good contact – they must be atomically smooth. Further, because the superconductivity resides substantially only in the copper-oxygen planes, the grains must be crystallographically aligned. BSCCO is therefore a good candidate because its grains can be aligned either by melt processing or by mechanical deformation. The double bismuth-oxide layer is only weakly bonded by van der Waals forces. So like graphite or mica, deformation causes slip on these BiO planes, and grains tend to deform into aligned plates. Further, because BSCCO has n = 1, 2 and 3 members, these naturally tend to accommodate low angle grain boundaries, so that indeed they remain atomically smooth. Thus, first-generation HTS wires (referred to as 1G) have been manufactured for many years now by companies such as American Superconductor Corporation (AMSC) in the USA and Sumitomo in Japan, though AMSC has now abandoned BSCCO wire in favour of 2G wire based on YBCO.

Typically, precursor powders are packed into a silver tube, which is then extruded down in diameter. These are then repacked as multiple tubes in a silver tube and again extruded down in diameter, then drawn down further in size and rolled into a flat tape. The last step ensures grain alignment. The tapes are then reacted at high temperature to form dense, crystallographically aligned Bi-2223 multifilamentary conducting tape suitable for winding cables or coils for transformers, magnets, motors and generators. Typical tapes of 4 mm width and 0.2 mm thickness support a current of 200 A at 77 K, giving a critical current density in the Bi-2223 filaments of 5 kA/mm². This rises markedly with decreasing temperature so that many applications are implemented at 30–35 K, even though T_c is 108 K.