Superconductors and cryoconductors
Superconductors and cryoconductors
Known 27 pure metals and more than a thousand different alloys and compounds in which a transition to a superconducting state is possible. These include pure metals, alloys, intermetallic compounds, and some dielectric materials.
Superconductors
When the temperature drops specific electrical resistance of metals decreases and at very low (cryogenic) temperatures, the electrical conductivity of metals approaches absolute zero.
In 1911, when cooling a ring of frozen mercury to a temperature of 4.2 K, the Dutch scientist G. Kamerling-Onnes found that the electrical resistance of the rings suddenly dropped to a very small value that could not be measured. Such disappearance of electrical resistance, i.e. the appearance of infinite conductivity in a material is called superconductivity.
Materials with the ability to pass into a superconducting state when cooled to a sufficiently low temperature level began to be called superconductors.The critical cooling temperature at which there is a transition of matter into a superconducting state is called the superconducting transition temperature or critical transition temperature Tcr.
A superconducting transition is reversible. When the temperature rises to Tc, the material returns to its normal (non-conducting) state.
A characteristic of superconductors is that once induced in a superconducting circuit, the electric current will circulate for a long time (years) along this circuit without appreciable reduction in its strength and, moreover, without an additional supply of energy from the outside. Like a permanent magnet, such a circuit creates in the surrounding space magnetic field.
In 1933, the German physicists V. Meissner and R. Oxenfeld established that superconductors during the transition to the superconducting state become ideal diamagnets. Therefore, the external magnetic field does not penetrate a superconducting body. If the transition of the material to a superconducting state occurs in a magnetic field, then the field is "pushed" out of the superconductor.
Known superconductors have very low critical transition temperatures Tc. Therefore, devices in which they use superconductors must operate under liquid helium cooling conditions (the liquefaction temperature of helium at normal pressure is about 4.2 DA SE). This complicates and increases the cost of manufacturing and operating superconducting materials.
Besides mercury, superconductivity is inherent in other pure metals (chemical elements) and various alloys and chemical compounds. However, at most metals such as silver and copper, the low temperatures reached at the moment become superconducting if the condition fails.
Possibilities of using the phenomenon of superconductivity are determined by the values of the temperature of the transition to the superconducting state of Tc and the critical strength of the magnetic field.
Superconducting materials divided into soft and hard. Soft superconductors include pure metals, except niobium, vanadium, tellurium. The main disadvantage of soft superconductors is the low value of the critical magnetic field strength.
In electrical engineering, soft superconductors are not used, because the superconducting state in them disappears already in weak magnetic fields at low current densities.
Solid superconductors include alloys with distorted crystal lattices. They retain superconductivity even at relatively high current densities and strong magnetic fields.
Properties of solid superconductors were discovered in the middle of this century, and until now the problem of their research and application is one of the most important problems of modern science and technology.
Solid superconductors have a number of functions:
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on cooling, the transition to the superconducting state does not occur abruptly, as in soft superconductors and for a certain temperature interval;
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some of solid superconductors have not only relatively high values critical transition temperature Tc, but also relatively high values critical magnetic induction Vkr;
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in changes in magnetic induction, intermediate states between superconducting and normal can be observed;
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have a tendency to dissipate energy when passing alternating current through them;
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addictive properties of superconductivity from technological methods of production, material purity and the perfection of its crystal structure.
According to technological properties, solid superconductors are divided into the following types:
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relatively easily deformable of which wire and strips [niobium, niobium-titanium alloys (Nb-Ti), vanadium-gallium (V-Ga)];
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difficult to deform due to fragility, from which products are obtained by powder metallurgy methods (intermetallic materials such as niobium stanide Nb3Sn).
Often superconducting wires covered with a "stabilizing" sheath made of copper or other highly conductive material electricity and the heat of the metal, which makes it possible to avoid damaging the base material of the superconductor with an accidental increase in temperature.
In some cases, composite superconducting wires are used, in which a large number of thin filaments of superconducting material are enclosed in a solid sheath of copper or other non-conducting material.
Superconducting film materials have special properties:
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critical transition temperature Tcr in some cases significantly exceeds Tcr bulk materials;
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large values of the limiting currents passed through the superconductor;
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less temperature range of the transition to the superconducting state.
Superconductors are used when creating: electrical machines and transformers with small mass and dimensions with a high efficiency factor; large cable lines for power transmission over long distances; especially low attenuation waveguides; drives power and memory devices; magnetic lenses of electron microscopes; inductance coils with printed wiring.
Based on film superconductors created a number of storage devices and automation elements and computing technology.
Electromagnetic coils from superconductors make it possible to obtain the maximum possible values of magnetic field strength.
Cryoprobes
Some metals can reach at low (cryogenic) temperatures a very small value of the specific electrical resistance p, which is hundreds and thousands of times less than the electrical resistance at normal temperature. Materials with these properties are called cryoconductors (hyperconductors).
Physically, the phenomenon of cryoconductivity is not similar to the phenomenon of superconductivity. Current density in cryoconductors at operating temperatures is thousands of times higher than the current density in them at normal temperature, which determines their use in high-current electrical devices that are subject to high requirements for reliability and explosion safety.
Application of cryoconductors in electrical machines, cables, etc. has a significant advantage over superconductors.
If liquid helium is used in superconducting devices, the operation of cryoconductors is ensured due to the higher boiling point and cheap refrigerants — liquid hydrogen or even liquid nitrogen. This simplifies and reduces the cost of manufacturing and operating the device. However, it is necessary to consider technical difficulties that arise when using liquid hydrogen, forming, at a certain ratio of components, an explosive mixture with air.
As cryoprocessors use copper, aluminum, silver, gold.
Source Information: "Electromaterials" Zhuravleva L. V.