-[HIGH-TEMPERATURE] [SUPER-CONDUCTIVITY]-

.
*bismuth strontium calcium copper oxide*

A sample of BSCCO, which currently is one of the most practical high-temperature superconductors
.
(notably, it does not contain ‘rare-earths’)

BSCCO is a cuprate superconductor based on bismuth and strontium.

(thanks to its higher ‘operating temperature’, ‘cuprates’ are now becoming competitors for more ordinary niobium-based superconductors, as well as ‘magnesium di-boride superconductors’)

*abbreviations* –>

high-T_c

HTS*

-‘high’-temperature ‘super’-conductors’ are ‘operatively defined as materials that behave as ‘super-conductors’ at temperatures above nearly -200°C (-320°F)–

This is in fact the lowest temperature reachable by ‘liquid nitrogen’, one of the simplest coolant in ‘cryo-genics’*

the opposite of ‘euthanasia’

All superconducting materials known at ordinary pressures currently work far below ambient temperatures and therefore require cooling.

The majority of high-temperature superconductors are ceramics materials.

On the other hand, Metallic superconductors usually work below -200°C: they are then called low-temperature superconductors.

Metallic superconductors are also ordinary superconductors, since they were discovered and used before the high-temperature ones.

Ceramic superconductors are now becoming suitable for some practical use, but they still have many manufacturing issues and there are very few successful practical examples of employment.

Most ceramics are brittle which makes the fabrication of wires from them very problematic.

The major advantage of high-temperature ceramic superconductors is that they can be cooled by using liquid nitrogen.

On the other hand, metallic superconductors require usually more difficult coolants – mostly liquid helium.

Unfortunately, none of high-temperature superconductors are coolable using only dry ice, and none of them works at room temperature and pressure (they work well below the lowest temperature recorded on Earth).

All high-temperature superconductors require some kind of cooling systems.

The main class of high-temperature superconductors are in the class of copper oxides (only some particular copper oxides).

The second class of high-temperature superconductors in the practical classification is the class of iron-based compounds.

Magnesium diboride is sometimes included in high-temperature superconductors: it is relatively simple to manufacture, but it superconducts only below -230°C, which makes it unsuitable for liquid nitrogen cooling (30°C below liquid nitrogen limit).

For example, it can be cooled with liquid helium, which works at much lower temperatures.

Many ceramic superconductors physically behave as superconductors of the second type.

(the first high temperature superconductor was discovered in 1986, by IBM researchers Bednorz and Müller, who were awarded the Nobel Prize in Physics in 1987 “for their important break-through in the discovery of superconductivity in ceramic materials”)

Some extremely-high pressure gases are usually categorized as high-temperature superconductors by theoretical physicists

(in fact, many articles on ‘high-temperature super-conductors’ can be found on this ‘theoretical research’ on ‘high pressure gases’, which is not suitable for ‘practical applications’ but only for the theoretical mathematical explanation of the ‘super-conductivity phenomenon’)

Bi2Sr2Can−1CunO2n+4+x,

with n = 2 being the most commonly studied compound

(though n = 1 and n = 3 have also received significant attention).

en.wikipedia.org /wiki/Bismuth_strontium_calcium_copper_oxide

Bismuth strontium calcium copper oxide

Contributors to Wikimedia projects

8-10 minutes

A piece of cuprate of bismuth and strontium: this piece is a cube with an edge of nearly 1 mm.

Bismuth strontium calcium copper oxide (BSCCO, pronounced bisko), is a type of cuprate superconductor having the generalized chemical formula

Discovered as a general class in 1988,[1] BSCCO was the first high-temperature superconductor which did not contain a rare-earth element.

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 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 (Bi2Sr2CuO6+x), Bi-2212 is the n = 2 compound (Bi2Sr2CaCu2O8+x), and Bi-2223 is the n = 3 compound (Bi2Sr2Ca2Cu3O10+x).

The BSCCO family is analogous to a thallium family of high-temperature superconductors referred to as TBCCO and having the general formula Tl2Ba2Can−1CunO2n+4+x, and a mercury family HBCCO of formula HgBa2Can−1CunO2n+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 Tc ≈ 33 K, Bi-2212 has Tc ≈ 96 K, Bi-2223 has Tc ≈ 108 K, and Bi-2234 has Tc ≈ 104 K. This last member is very difficult to synthesize.

Wires and tapes[edit]

For practical applications, BSCCO is compressed with silver metal into tape by the powder-in-tube 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.[2][3] 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.

For making superconducting chips, it was suggested that due to advances in blue laser technology notably 445, 450 and 405nm single mode diodes it may be possible to selectively nudge the Sr atoms in Bi-2223 to preferentially form high-Tc materials designed for computer chips. If so then the setup for making these in bulk may be very simple such as a Mott insulator pellet surface modified with BSCCO via MOVCD and then laser annealed under oxygen at a very specific set of electrostatic fields, temperatures and wavelengths sequentially, with the polarization aligned to the grain boundaries. If the 2223 variant is used then Tc may substantially increase and thus make the material suitable for a quantum sensor, SQUID and other applications needing these parameters. This idea was suggested on Twitter and a paper is being compiled with further information for release around 3Q 2018 and further discussions on 4HV.org. The original idea was inspired by the single glowing strontium atom sitting between electrostatic plates in a vacuum which incidentally won the PhD student responsible an award.[4]

Applications[edit]
1G conductors made from Bi-2223 multifilamentary tapes. eg. :
Holbrook Superconductor Project
Testing BSCCO tapes at CERN[5]
Discovery[edit]
BSCCO as a new class of superconductor was discovered around 1988 by Hiroshi Maeda and colleagues[1] at the National Research Institute for Metals in Japan, though at the time they were unable to determine its precise composition and structure. Almost immediately several groups, and most notably Subramanian[6] et al. at Dupont and Cava[7] 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[8] 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.

The crystal unit cell of BSCCO-2212, comprising two repeat units offset by (1/2,0,0). The other BSCCO family members have very similar structures: 2201 has one less CuO2 in its top and bottom half and no Ca layer, while 2223 has an extra CuO2 and Ca layer in each half.

Properties[edit]
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.[9][10] This is referred to as optimal doping. Samples with lower doping (and hence lower Tc) are generally referred to as underdoped, while those with excess doping (also lower Tc) are overdoped. By changing the oxygen content, Tc can thus be altered at will. By many measures, overdoped HTS are strong superconductors, even if their Tc is less than optimal, but underdoped HTS become extremely weak. The application of external pressure generally raises Tc in underdoped samples to values that well exceed the maximum at ambient pressure. This is not fully understood, though a secondary effect is that pressure increases the doping. Bi-2223 is complicated in that it has three distinct copper-oxygen planes. The two outer copper-oxygen layers are typically close to optimal doping, while the remaining inner layer is markedly underdoped. Thus the application of pressure in Bi-2223 results in Tc rising to a maximum of about 123 K due to optimization of the two outer planes. Following an extended decline, Tc then rises again towards 140 K due to optimization of the inner plane. A key challenge therefore is to determine how to optimize all copper-oxygen layers simultaneously. Considerable improvements in superconducting properties could yet be achieved using such strategies.

BSCCO is a Type-II superconductor. The upper critical field Hc2 in Bi-2212 polycrystalline samples at 4.2 K has been measured as 200 ± 25 T (cf 168 ± 26 T for YBCO polycrystalline samples).[11] 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)[12] 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.