In the near future, all power cables will be made of superconducting materials

The principle of superconductivity. Magnetic field effect

The flow of current in conductors is always associated with energy losses, i.e. with the transition of energy from electrical to thermal. This transition is irreversible, the reverse transition is associated only with the completion of work, as thermodynamics speaks of this. There is, however, the possibility of converting thermal energy into electrical energy and using the so-called thermoelectric effect, when two contacts of two conductors are used, one of which is heated and the other is cooled.

In fact, and this fact is surprising, there are a number of conductors in which, under certain conditions, there is no energy loss during the flow of current! In classical physics, this effect is inexplicable.

According to the classical electronic theory, the motion of a charge carrier occurs in an electric field uniformly accelerated until it collides with a structural defect or with a lattice vibration. After a collision, if it is inelastic, like a collision of two plasticine balls, an electron loses energy, transferring it to a lattice of metal atoms. In this case, in principle, there can be no superconductivity.

It turns out that superconductivity appears only when quantum effects are taken into account. It’s hard to imagine it. Some weak idea of ​​the superconductivity mechanism can be obtained from the following considerations.

It turns out, given that the electron can polarize the atom of the lattice closest to it, i.e. pull it slightly toward you due to the action of the Coulomb force, then this lattice atom will slightly shift the next electron. A bond of a pair of electrons is formed.

When the electron moves, the second component of the pair, as it were, perceives the energy that the electron transfers to the atom of the lattice. It turns out that if we take into account the energy of a pair of electrons, then it does not change during a collision, i.e. electron energy loss does not occur! Such electron pairs are called Cooper pairs.

In general, it is difficult to understand for a person with established physical ideas. It’s easier for you to understand, at least you can take it for granted.

Superconductivityas well superfluiditywere found in experiments at ultra-low temperatures, near absolute zero temperatures. As you approach absolute zero, the lattice vibrations freeze. The resistance to current flow decreases even according to the classical theory, but to zero at a certain critical temperature T_from, it decreases only according to quantum laws.

Superconductivity was discovered by two phenomena: firstly, on the fact of the disappearance of electrical resistance, and secondly, on diamagnetism. The first phenomenon is clear - if you pass a certain current I through the conductor, then by the voltage drop U on the conductor you can determine the resistance R = U / I. The disappearance of tension means the disappearance of resistance as such.

The second phenomenon requires more detailed consideration. Logically, the lack of resistance is identical to the absolute diamagnetic nature of the material. Indeed, imagine a little experience. We will introduce superconducting material into the region of the magnetic field. According to the Joule-Lenz law, a current must occur in the conductor that completely compensates for the change in magnetic flux, i.e. the magnetic flux through the superconductor was both zero and remains zero. In a conventional conductor, this current decays, because the conductor has a resistance. Only then does a magnetic field penetrate the conductor. In a superconductor, it does not fade.This means that the flowing current leads to a complete compensation of the magnetic field inside itself, i.e. the field does not penetrate into it. From a formal standpoint, a zero field means that the magnetic permeability of the material is zero, m = 0 i.e. the body manifests itself as an absolute diamagnet.

However, these phenomena are characteristic only for weak magnetic fields. It turns out that a strong magnetic field can penetrate into the material, moreover, it destroys superconductivity itself! Introduce the concept of critical field B_fromwhich destroys a superconductor. It depends on the temperature: maximum at a temperature close to zero, disappears upon transition to a critical temperature T_from. Why is it important for us to know the tension (or induction) at which superconductivity disappears? The fact is that when a current flows through a superconductor, a magnetic field is physically created around the conductor, which should act on the conductor.

For example, for a cylindrical conductor of radius r placed in a medium with magnetic permeability m, magnetic induction on the surface in accordance with the law of Bio-Savard-Laplace will be

B = m₀× m ×I / 2pr (1)

The larger the current, the larger the field. Thus, with some induction (or tension), superconductivity disappears, and therefore, only a current less than that which creates critical induction can be passed through the conductor.

Thus, for a superconducting material, we have two parameters: critical magnetic field induction B_from and critical temperature T_from.  

For metals, critical temperatures are close to absolute zero temperatures. This is the area of ​​so-called “Helium” temperatures, comparable with the boiling point of helium (4.2 K). Regarding critical induction, we can say that it is relatively small. It can be compared with induction in transformers (1-1.5 T). Or for example with induction near the wire. For example, we calculate induction in air near a wire with a radius of 1 cm with a current of 100 A.

m₀ = 4p 10^-7 GN / m
m = 1, I = 100 A,
r = 10^-2m

Substituting into expression (1) we obtain B = 2 mT, i.e., a value approximately corresponding to critical. This means that if such a conductor is put into a power line, for example 6 kV, then the maximum power that can be transmitted through each phase will be P_m = U_f· I = 600 kW. The example considered shows that the intrinsic magnetic field limits the ability to transfer power through a cryogenic wire. Moreover, the closer the temperature to the critical temperature, the lower the critical induction value.

Low temperature superconductors

Above, I have already focused on some specific superconducting materials. In principle, the property of superconductivity is characteristic of almost all materials. Only for the most electrically conductive - copper, silver (paradox?) Superconductivity is not detected. The specific application of superconductivity in the energy sector is tempting: having lossless power lines would be wonderful. Another application is a generator with superconducting windings. A sample of such a generator was developed in St. Petersburg, and successful tests were conducted. The third option is an electromagnet, the induction of which can be controlled in a controlled manner depending on the current strength.

Another example is a superconducting inductive storage. Imagine a huge coil of superconducting conductor. If you inject current into it in some way and close the input and output wires, then the current in the coil will flow indefinitely. In accordance with the known law, energy will be enclosed in a coil

W = l× I²/2

Where L- coil inductance. Hypothetically, one can imagine that at some point in time there is excess energy in the energy system, energy is taken from it into such a storage device. Here it is stored for as long as necessary until the need for energy. Then it is gradually, controllably pumped back into the power system.

In physics and the technology of superconductivity, there are also low-current analogs of the radio elements of conventional electronics. For example, in the systems "superconductor - a thin layer of resistive metal (or dielectric) - superconductor" a number of new physical effects are possible that are already used in electronics. This is the quantization of the magnetic flux in a ring containing such an element, the possibility of an abrupt change in current depending on voltage when a weak radiation is applied to the system, and standard voltage sources built on this principle with an accuracy of 10^-10 B. In addition, there are storage elements, analog-to-digital converters, etc. There are even a few superconductor computer designs.

The urgency of the problem of microminiaturization using semiconductors is that even a small energy release in a very small volume can lead to significant overheating and the problem of heat dissipation is acute.

This problem is especially relevant for supercomputers. It turns out that microchips local heat fluxes can reach kilowatts per square centimeter. It is not possible to remove heat in the usual way, by blowing air. They suggested removing the case of microcircuits and blowing directly the microcrystal. Here the problem of poor heat transfer to the air arose. The next step was to fill everything with liquid and remove heat by boiling the liquid on these elements. The liquid should be very clean, not contain microparticles, not wash out any of the many elements of the computer. So far, these issues have not been fully resolved. Research is conducted with organofluorine fluids.

In superconducting computers, there are no such problems, because no loss. However, cooling the equipment to cryogenic temperatures requires a lot of costs. Moreover, the closer to absolute zero - the greater the cost. Moreover, the dependence is nonlinear, it is even stronger than the inversely proportional dependence.

The temperature scale in the cryogenic region is conventionally divided into several areas according to the boiling points of liquefied gases: helium (below 4.2 K), hydrogen 20.5 K, nitrogen 77 K, oxygen 90 K, ammonia (-33 °FROM). If we could find a material with a boiling point near or above hydrogen, the cost of maintaining the cable in working condition would be ten times less than for helium temperatures. Upon transition to nitrogen temperatures, there would be a gain by several orders of magnitude. Therefore, superconducting materials operating at helium temperatures, although they were discovered more than 80 years ago, still have not found application in the energy sector.

It may be noted that further attempts to develop an operating cryogenic device are made after each of the breakthroughs in technology. Advances in technology have led to alloys that have the best critical induction and temperature characteristics.

So in the early 70s there was a boom in the study of stannide niobium Nb₃Sn. He has B_from = 22 T, and T_from= 18 K. However, in these superconductors, in contrast to metals, the effect of superconductivity is more complicated. It turns out that they have two values ​​of the critical tension B_c0 and B_s1.  

In the gap between them, the material has no resistance to direct current, but has a finite resistance to alternating current. And although In_c0 large enough, but the values ​​of the second critical induction B_s1 differs little from the corresponding values ​​for metals. “Simple” superconductors are called superconductors of the first kind, and “complex” superconductors of the second kind.

New intermetallic compounds do not have the ductility of metals, so the question was simultaneously solved how to make extended elements such as wires from brittle materials.Several options have been developed, including the creation of composites such as a layer cake with plastic metals, such as copper, the deposition of intermetals on a copper substrate, etc., which was useful in the development of superconducting ceramics.

Superconducting ceramics

The next radical step in the study of superconductivity was an attempt to find superconductivity in oxide systems. The vague idea of ​​the developers was that in systems containing substances with variable valency superconductivity is possible, and at higher temperatures. Binary systems, i.e. consisting of two different oxides. It was not possible to find superconductivity. And only in triple systems Bao-la₂O₃-CuO in 1986, superconductivity was detected at a temperature of 30-35 K. For this work, Bednorts and Muller received the Nobel Prize in the following, (!!) 1987

Intensive studies of related compounds during the year led to the discovery of superconductivity in the system Bao-y₂O₃-CuO at a temperature of 90 K. In fact, superconductivity is obtained in an even more complex system, the formula of which can be represented as Yba₂Cu₃O_7-d. Value d for the highest temperature superconducting material is 0.2. This means not only a certain percentage of the starting oxides, but also a reduced oxygen content.

Indeed, if you calculate by valency, then yttrium - 3, barium - two, copper 1 or 2. Then the metals have a total valency of 10 or 13, and oxygen has a little less than 14. Therefore, in this ceramic there is an excess of oxygen relative to the stoichiometric correlation.

Ceramics are produced using conventional ceramic technology. How to make wires from a fragile substance? One way, a suspension of the powder is made in a suitable solvent, then the solution is forced through a die, dried and wound onto a drum. The final removal of the ligament is carried out by burning, the wire is ready. Properties of such fibers: critical temperatures 90-82 K, at 100 K r= 12 mOhm · cm, (approximately like graphite), critical current density 4000 A / m².

Let us dwell on the last digit. This value is extremely low for use in the energy sector. Comparing with economic current density (~1 A / mm²), it is seen that in ceramics the current density is 250 times lower. Scientists investigated this issue and came to the conclusion that contacts that are not superconducting are to blame. Indeed, single crystals have obtained current densities that reach the economic current density. And in the last two or three years, ceramic wires have been obtained whose current density exceeds the economic current density.

In 1999, a superconducting cable connecting two metro stations was commissioned in Japan. The cable is made using the technology of "sandwich", i.e. fragile ceramics in it is located between two layers of elastic and ductile copper. The insulation and, at the same time, the refrigerant is liquid nitrogen.

What do you think is one of the main problems with this cable? You can guess these issues were previously discussed in relation to isolation. It turns out that the dielectric loss in such a wonderful dielectric as liquid nitrogen warms it up, which requires constant care for additional cooling.

But Idon’t give up, and according to news agencies in Japan, TEPCO intends to create the first superconducting networks for delivering electricity to residential buildings. At the first stage, approximately 300 kilometers of such cables will be laid in Yokohama, which will cover about half a million buildings!