Bismuth Strontium Calcium Copper Oxide - Wires and Tapes

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 1.6nm. This means that the grains in a polycrystalline wire must be 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 (ASC) in the USA and Sumitomo in Japan – though ASC has now abandoned BSCCO wire in favour of 2G wire based on YBCO.

Typically, precursor powders are packed into a silver tube which is 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 4mm width and 0.2mm thickness support a current at 77K of 200 A, giving a critical current density (maximal amperes per square metre of cross-sectional area) in the Bi-2223 filaments of 5×105 Amps/cm2. This rises markedly with decreasing temperature so that many applications are implemented at 30-35 K, even though T_c is 108 K.