October 17, 2014 at 13:15High temperature superconductors
Today I saw this comment and discussion under it. Given 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 want to note that the term "high-temperature superconductor" means superconductors with a critical temperature above 77 K (-196 ° C) - the boiling point of cheap liquid nitrogen. It is not uncommon for them to include superconductors with a critical temperature of about 35 K, because 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 are still very low.
The main distribution was gained 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). Materials similar to YBCO are also used, in which yttrium is replaced by another rare-earth element, for example gadolinium, their common designation is ReBCO.
Produced by YBCO, and other ReBCOs, have a critical temperature of 90-95 K. Manufactured by BSCCO reach a critical temperature of 108 K.
In addition to the high critical temperature, ReBCO and BSCCO differ in large values of the critical magnetic field (in liquid helium - more than 100 T) and critical current. However, the latter is not so simple ...
In a superconductor, electrons do not move independently, but in pairs (Cooper pairs). If we want the current to pass from one superconductor to another, then the gap between them should 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 fractions of a nanometer, depending on the direction of movement. Even the gaps between the individual grains of the polycrystal are already quite a tangible obstacle, not to mention the gaps between the individual pieces of the superconductor. As a result, superconducting ceramics, if you do not take special tricks, can pass through it only a relatively small current.
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 critical current value. This made it possible to quickly and simply create the first generation of high-temperature superconducting cables, or rather, high-temperature superconducting tapes. They are a silver matrix in which there are many thin tubes filled with BSCCO. This matrix is flattened, while the grains of the superconductor acquire the desired order. We get a thin flexible tape containing many separate flat superconducting veins.
Alas, BSCCO material is far from ideal: its critical current drops very quickly with increasing external magnetic field. His critical magnetic field is large enough, but long before this limit is reached, he loses the ability to transmit any large currents. This greatly limited the use of high-temperature superconducting tapes; they could not replace the good old niobium-titanium and niobium-tin alloys working in liquid helium.
A completely different thing is ReBCO. But to create the correct grain orientation in it is very difficult. Only relatively recently have learned to make superconducting tapes based on this material. Such tapes, called the second generation, are obtained by sputtering superconducting material on a substrate having a special texture that sets the direction of crystal growth. The texture, as you might guess, has nanometer sizes, so these are real nanotechnologies. In the Moscow company SuperOx, in which I actually was, 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 incident at a certain angle. As a result, crystals of this layer grow in only one direction, in which it is most difficult for ions to spray them. Other manufacturers and there are four of them in the world, other technologies can use. By the way, domestic tapes use gadolinium instead of yttrium, it turned out to be more technologically advanced.
Second-generation superconducting tapes with a width of 12 mm and a thickness of 0.1 mm in liquid nitrogen in the absence of an external magnetic field pass current 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 still do not even go over to the superconducting state), then the same tape can pass 500 A in a field of 8 T, and “some” 200-300 A in a field of the level of a couple of tens of tesla (the 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 T! True, the problem of mechanical strength already arises in full growth: a magnetic field always seeks to break the electromagnet, but when this field reaches tens of tesla, its aspirations are easily realized ...
However, all these wonderful technologies do not solve the problem of connecting two pieces of a superconductor: although the crystals are oriented in the same direction, there is no question of polishing the outer surface to a subnanometer size. Koreans have a technology for sintering individual ribbons with each other, but it is still, to put it mildly, far from perfect. Typically, the tapes are connected to each other by conventional soldering using conventional tin-lead solder or other classic method. Of course, in this case, a finite resistance appears on the contact, so to create a superconducting magnet from such tapes that does not require power for many years, and simply does not work with power lines with exactly zero losses. But the contact resistance is small fractions of a microohm, so that even at 500 A current only fractions of a milliwatt are released 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 of a comment on YouTube, in which the author argued that superconductivity does not exist, and the magnet levitation is a completely independent effect, inviting everyone to verify its correctness by directly measuring the resistance. As you can see, superconductivity still exists.
To begin with, just in case, I want to note that the term "high-temperature superconductor" means superconductors with a critical temperature above 77 K (-196 ° C) - the boiling point of cheap liquid nitrogen. It is not uncommon for them to include superconductors with a critical temperature of about 35 K, because 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 are still very low.
The main distribution was gained 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). Materials similar to YBCO are also used, in which yttrium is replaced by another rare-earth element, for example gadolinium, their common designation is ReBCO.
Produced by YBCO, and other ReBCOs, have a critical temperature of 90-95 K. Manufactured by BSCCO reach a critical temperature of 108 K.
In addition to the high critical temperature, ReBCO and BSCCO differ in large values of the critical magnetic field (in liquid helium - more than 100 T) and critical current. However, the latter is not so simple ...
In a superconductor, electrons do not move independently, but in pairs (Cooper pairs). If we want the current to pass from one superconductor to another, then the gap between them should 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 fractions of a nanometer, depending on the direction of movement. Even the gaps between the individual grains of the polycrystal are already quite a tangible obstacle, not to mention the gaps between the individual pieces of the superconductor. As a result, superconducting ceramics, if you do not take special tricks, can pass through it only a relatively small current.
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 critical current value. This made it possible to quickly and simply create the first generation of high-temperature superconducting cables, or rather, high-temperature superconducting tapes. They are a silver matrix in which there are many thin tubes filled with BSCCO. This matrix is flattened, while the grains of the superconductor acquire the desired order. We get a thin flexible tape containing many separate flat superconducting veins.
Alas, BSCCO material is far from ideal: its critical current drops very quickly with increasing external magnetic field. His critical magnetic field is large enough, but long before this limit is reached, he loses the ability to transmit any large currents. This greatly limited the use of high-temperature superconducting tapes; they could not replace the good old niobium-titanium and niobium-tin alloys working in liquid helium.
A completely different thing is ReBCO. But to create the correct grain orientation in it is very difficult. Only relatively recently have learned to make superconducting tapes based on this material. Such tapes, called the second generation, are obtained by sputtering superconducting material on a substrate having a special texture that sets the direction of crystal growth. The texture, as you might guess, has nanometer sizes, so these are real nanotechnologies. In the Moscow company SuperOx, in which I actually was, 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 incident at a certain angle. As a result, crystals of this layer grow in only one direction, in which it is most difficult for ions to spray them. Other manufacturers and there are four of them in the world, other technologies can use. By the way, domestic tapes use gadolinium instead of yttrium, it turned out to be more technologically advanced.
Second-generation superconducting tapes with a width of 12 mm and a thickness of 0.1 mm in liquid nitrogen in the absence of an external magnetic field pass current 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 still do not even go over to the superconducting state), then the same tape can pass 500 A in a field of 8 T, and “some” 200-300 A in a field of the level of a couple of tens of tesla (the 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 T! True, the problem of mechanical strength already arises in full growth: a magnetic field always seeks to break the electromagnet, but when this field reaches tens of tesla, its aspirations are easily realized ...
However, all these wonderful technologies do not solve the problem of connecting two pieces of a superconductor: although the crystals are oriented in the same direction, there is no question of polishing the outer surface to a subnanometer size. Koreans have a technology for sintering individual ribbons with each other, but it is still, to put it mildly, far from perfect. Typically, the tapes are connected to each other by conventional soldering using conventional tin-lead solder or other classic method. Of course, in this case, a finite resistance appears on the contact, so to create a superconducting magnet from such tapes that does not require power for many years, and simply does not work with power lines with exactly zero losses. But the contact resistance is small fractions of a microohm, so that even at 500 A current only fractions of a milliwatt are released 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 of a comment on YouTube, in which the author argued that superconductivity does not exist, and the magnet levitation is a completely independent effect, inviting everyone to verify its correctness by directly measuring the resistance. As you can see, superconductivity still exists.