Bismuth strontium calcium copper oxide

process 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 Tc is 108 K. == Applications ==

Electrical power transmission : • 1G conductors made from Bi-2223 multifilamentary tapes. e.g. : • Holbrook Superconductor Project Electromagnets and their current leads : • Testing BSCCO tapes at CERN == Discovery ==

BSCCO as a new class of superconductor was discovered around 1988 by Hiroshi Maeda and colleagues et al. at Dupont and Cava et al. at AT&T Bell Labs, identified Bi-2212. The n = 3 member proved quite elusive and was not identified until a month or so later by Tallon et al. in a government research lab in New Zealand. There have been only minor improvements to these materials since. A key early development was to replace about 15% of the Bi by Pb, which greatly accelerated the formation and quality of Bi-2223. == Properties ==

BSCCO needs to be hole-doped by an excess of oxygen atoms (x in the formula) in order to superconduct. As in all high-temperature superconductors (HTS) Tc is sensitive to the exact doping level: the maximal Tc for Bi-2212 (as for most HTS) is achieved with an excess of about 0.16 holes per Cu atom. In practice, HTS are limited by the irreversibility field H*, above which magnetic vortices melt or decouple. Even though BSCCO has a higher upper critical field than YBCO it has a much lower H* (typically smaller by a factor of 100) thus limiting its use for making high-field magnets. It is for this reason that conductors of YBCO are preferred to BSCCO, though they are much more difficult to fabricate. == See also ==