High temperature superconductors
Today I saw this comment and discussion under it. Considering that today I was in the production of superconducting cables, I wanted to insert a couple of comments, but read-only ... In the end, I decided to write a short article about high-temperature superconductors.
To begin with, just in case, I would like to note that the term “high-temperature superconductor” itself means superconductors with a critical temperature above 77 K (-196 ° C) - the boiling point of cheap liquid nitrogen. Not rarely they include superconductors with a critical temperature of about 35 K, since such a temperature had the first superconducting cuprate La 2-x Ba x CuO 4 (a substance of variable composition, hence x). Those. “High” temperatures here are still very low.
The main distribution was received by two high-temperature superconductors - YBa 2 Cu 3 O 7-x (YBCO, Y123) and Bi 2 Sr 2 Ca 2 Cu 3 O 10 + x (BSCCO, Bi-2223). Also, materials similar to YBCO are used in which yttrium is replaced by another rare-earth element, for example, gadolinium, their general designation is ReBCO.
Produced by YBCO, and others ReBCO, have a critical temperature at the level of 90-95 K. The produced BSCCO reach a critical temperature of 108 K.
In addition to the high critical temperature, ReBCO and BSCCO are distinguished by large values ​​of the critical magnetic field (more than 100 T in liquid helium) and critical current. However, the latter is not so simple ...
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In a superconductor, electrons move not independently, but in pairs (Cooper pairs). If we want the current to pass from one superconductor to another, then the gap between them must be less than the characteristic size of this pair. For metals and alloys, this size is tens or even hundreds of nanometers. But in YBCO and BSCCO, it is only a couple of nanometers and a fraction of a nanometer, depending on the direction of movement. Even the gaps between the individual grains of a polycrystal are already quite tangible obstacles, not to mention the gaps between the individual pieces of the superconductor. As a result, superconducting ceramics, if not to make special tricks, is able to pass only a relatively small current through itself.
The easiest way was to solve the problem in BSCCO: its grains naturally have smooth edges, and the simplest mechanical compression allows these grains to be ordered to obtain a high value of the critical current. This made it possible to quickly and easily create the first generation of high-temperature superconducting cables, or rather, high-temperature superconducting tapes. They represent a silver matrix, in which there are many thin tubes filled with BSCCO. This matrix is ​​flattened out, and the grains of the superconductor acquire the necessary order. We get a thin flexible tape containing a lot of individual flat superconducting cores.
Alas, the BSCCO material is far from perfect: its critical current drops very rapidly with increasing external magnetic field. His critical magnetic field is large enough, but long before reaching this limit, he loses the ability to pass any large currents. This greatly limited the use of high-temperature superconducting ribbons; they could not replace the good old niobium-titanium and niobium-tin alloys operating in liquid helium.
Quite another thing - ReBCO. But to create in it the correct orientation of the grains is very difficult. Only relatively recently have they learned how to make superconducting tapes based on this material. Such tapes, called the second generation, are obtained by sputtering a superconducting material onto a substrate having a special texture that sets the direction of crystal growth. The texture, as it is not difficult to guess, has nanometer sizes, so these are real nanotechnologies. In the Moscow company SuperOx, in which I actually was, in order to obtain such a structure, five intermediate layers are sprayed onto a metal substrate, one of which is simultaneously sprayed with a stream of fast ions falling at a certain angle. As a result, the crystals of this layer grow only in one direction, in which it is most difficult for ions to spray them. Other manufacturers, and there are four of them in the world, can use other technologies. By the way, domestic tapes use gadolinium instead of yttrium, it turned out to be more technological.
Second-generation superconducting ribbons 12 mm wide and 0.1 mm thick in liquid nitrogen in the absence of an external magnetic field pass a current of up to 500 A. In an external magnetic field of 1 T, the critical current still reaches 100 A, and at 5 T - up to 5 A If the tape is cooled to the temperature of liquid hydrogen (niobium alloys at this temperature have not even become superconducting), then the same tape can skip 500 A in a field of 8 T, and “some” 200-300 A in a field on a couple dozen tesla level (a frog flies). There is no need to talk about liquid helium: there are projects of magnets on these tapes with a field at the level of 100 Tl! True, the problem of mechanical strength arises at full height: the magnetic field always seeks to break the electromagnet, but when this field reaches dozens of tesla, its aspirations are easily realized ...
However, all these excellent technologies do not solve the problem of connecting two pieces of a superconductor: even though the crystals are oriented in one direction, there is no talk about polishing the outer surface to the subnanometer size of the roughness. Koreans have the technology of sintering separate ribbons with each other, but, to put it mildly, it is still far from perfect. Usually tapes are connected to each other by ordinary soldering with ordinary tin-lead solder or by another classical method. Of course, at the same time, a final resistance appears on the contact, so it is impossible to create a superconducting magnet from such tapes that does not require power for many years, and just a power line with exactly zero loss does not work. But the contact resistance is a small fraction of the microsome, so that even at 500 A current, only milliwatt fractions stand out there.
Of course, in a popular science article, the reader is looking for more entertainment ... Here are some videos of my experiments with a second-generation high-temperature superconducting tape:
The last video was recorded under the impression from a comment on YouTube, in which the author argued that superconductivity does not exist, and the levitation of the magnet - a completely independent effect, suggested that everyone should be convinced that he was right by measuring the resistance directly. As we see, superconductivity still exists.
To begin with, just in case, I would like to note that the term “high-temperature superconductor” itself means superconductors with a critical temperature above 77 K (-196 ° C) - the boiling point of cheap liquid nitrogen. Not rarely they include superconductors with a critical temperature of about 35 K, since such a temperature had the first superconducting cuprate La 2-x Ba x CuO 4 (a substance of variable composition, hence x). Those. “High” temperatures here are still very low.
The main distribution was received by two high-temperature superconductors - YBa 2 Cu 3 O 7-x (YBCO, Y123) and Bi 2 Sr 2 Ca 2 Cu 3 O 10 + x (BSCCO, Bi-2223). Also, materials similar to YBCO are used in which yttrium is replaced by another rare-earth element, for example, gadolinium, their general designation is ReBCO.
Produced by YBCO, and others ReBCO, have a critical temperature at the level of 90-95 K. The produced BSCCO reach a critical temperature of 108 K.
In addition to the high critical temperature, ReBCO and BSCCO are distinguished by large values ​​of the critical magnetic field (more than 100 T in liquid helium) and critical current. However, the latter is not so simple ...
')
In a superconductor, electrons move not independently, but in pairs (Cooper pairs). If we want the current to pass from one superconductor to another, then the gap between them must be less than the characteristic size of this pair. For metals and alloys, this size is tens or even hundreds of nanometers. But in YBCO and BSCCO, it is only a couple of nanometers and a fraction of a nanometer, depending on the direction of movement. Even the gaps between the individual grains of a polycrystal are already quite tangible obstacles, not to mention the gaps between the individual pieces of the superconductor. As a result, superconducting ceramics, if not to make special tricks, is able to pass only a relatively small current through itself.
The easiest way was to solve the problem in BSCCO: its grains naturally have smooth edges, and the simplest mechanical compression allows these grains to be ordered to obtain a high value of the critical current. This made it possible to quickly and easily create the first generation of high-temperature superconducting cables, or rather, high-temperature superconducting tapes. They represent a silver matrix, in which there are many thin tubes filled with BSCCO. This matrix is ​​flattened out, and the grains of the superconductor acquire the necessary order. We get a thin flexible tape containing a lot of individual flat superconducting cores.
Alas, the BSCCO material is far from perfect: its critical current drops very rapidly with increasing external magnetic field. His critical magnetic field is large enough, but long before reaching this limit, he loses the ability to pass any large currents. This greatly limited the use of high-temperature superconducting ribbons; they could not replace the good old niobium-titanium and niobium-tin alloys operating in liquid helium.
Quite another thing - ReBCO. But to create in it the correct orientation of the grains is very difficult. Only relatively recently have they learned how to make superconducting tapes based on this material. Such tapes, called the second generation, are obtained by sputtering a superconducting material onto a substrate having a special texture that sets the direction of crystal growth. The texture, as it is not difficult to guess, has nanometer sizes, so these are real nanotechnologies. In the Moscow company SuperOx, in which I actually was, in order to obtain such a structure, five intermediate layers are sprayed onto a metal substrate, one of which is simultaneously sprayed with a stream of fast ions falling at a certain angle. As a result, the crystals of this layer grow only in one direction, in which it is most difficult for ions to spray them. Other manufacturers, and there are four of them in the world, can use other technologies. By the way, domestic tapes use gadolinium instead of yttrium, it turned out to be more technological.
Second-generation superconducting ribbons 12 mm wide and 0.1 mm thick in liquid nitrogen in the absence of an external magnetic field pass a current of up to 500 A. In an external magnetic field of 1 T, the critical current still reaches 100 A, and at 5 T - up to 5 A If the tape is cooled to the temperature of liquid hydrogen (niobium alloys at this temperature have not even become superconducting), then the same tape can skip 500 A in a field of 8 T, and “some” 200-300 A in a field on a couple dozen tesla level (a frog flies). There is no need to talk about liquid helium: there are projects of magnets on these tapes with a field at the level of 100 Tl! True, the problem of mechanical strength arises at full height: the magnetic field always seeks to break the electromagnet, but when this field reaches dozens of tesla, its aspirations are easily realized ...
However, all these excellent technologies do not solve the problem of connecting two pieces of a superconductor: even though the crystals are oriented in one direction, there is no talk about polishing the outer surface to the subnanometer size of the roughness. Koreans have the technology of sintering separate ribbons with each other, but, to put it mildly, it is still far from perfect. Usually tapes are connected to each other by ordinary soldering with ordinary tin-lead solder or by another classical method. Of course, at the same time, a final resistance appears on the contact, so it is impossible to create a superconducting magnet from such tapes that does not require power for many years, and just a power line with exactly zero loss does not work. But the contact resistance is a small fraction of the microsome, so that even at 500 A current, only milliwatt fractions stand out there.
Of course, in a popular science article, the reader is looking for more entertainment ... Here are some videos of my experiments with a second-generation high-temperature superconducting tape:
The last video was recorded under the impression from a comment on YouTube, in which the author argued that superconductivity does not exist, and the levitation of the magnet - a completely independent effect, suggested that everyone should be convinced that he was right by measuring the resistance directly. As we see, superconductivity still exists.
Source: https://habr.com/ru/post/361999/
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